Surface engineering of ZnO nanostructures for semiconductor-sensitized solar cells.

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Surface Engineering of ZnO Nanostructures for Semiconductor-Sensitized Solar Cells Jun Xu, Zhenhua Chen, Juan Antonio Zapien, Chun-Sing Lee,* and Wenjun Zhang*

via quantum confinement effects and surface-area effects.[18–20] These advantages offer new possibilities for a variety of new solar cell structures with reduced cost and improved efficiency. It is expected that the nanostructured cells, through band structure engineering of the nanomaterials and new device design concepts, could achieve a power conversion efficiency (PCE) even greater than the thermodynamic limit of bulk single junction solar cells (33% under 1 Sun illumination).[21] Among the nanostructured solar cells, the DSSC has shown to be an important solar cell design with considerable superiority given its simple device structure as well as facile, scalable, and low cost fabrication.[7–10] However, the advances of DSSC techniques have been seriously obstructed by stability and lifetime issues often caused by degradation of organic dyes.[22,23] Semiconductor sensitizers, in particular semiconductor quantum dots (QDs), have been regarded as a superb alternative to replace dye sensitizers. Compared to organic dyes, QDs in SSCs have: i) better stabilities; ii) higher optical absorption coefficients; iii) lower costs and iv) more tunable properties as they can be easily prepared with controllable size, shape and composition at low costs.[23–26] Furthermore, QDs may also enable utilization of hot electrons or generating multiple charge carriers with a single photon, which could boost the theoretical PCE of SSCs by up to 44% higher than the Shockley and Queisser limit (33%).[27–30] So far, various QDs with their bandgaps covering a wide spectrum range such as CdS,[31–34] CdSe,[35–38] CdTe,[39,40] CdSxSe1−x,[41,42] CdSexTe1−x,[43] ZnxCd1−xSe,[44] PbS,[45–47] PbSe,[48,49] Bi2S3,[50,51] In2S3,[52–55] InP,[56,57] InAs,[58] and CuInS2[59–61] have been employed in SSCs. A number of metal oxide (MO) semiconductors such as TiO2,[35,43,62–64] ZnO,[32,36] SnO2,[65,66] Zn2SnO4,[67,68] Nb2O5,[69] W2O3,[70] and In2O3,[54,55] have also been used as scaffolding for hosting semiconductor sensitizers and to provide efficient electron transport in SSCs. TiO2-nanostructure electron transporter has been studied most comprehensively following its successful application in DSSCs, and a record PCE beyond 6% has been achieved in TiO2-based SSCs very recently.[43,71] However, as an electron transporter, ZnO presents attractive properties which in some aspects are superior to those of TiO2 and has received increasing research interest in the past few years. ZnO is a direct bandgap semiconductor with similar band structure and physical properties as those of TiO2. Significantly, ZnO has the

Semiconductor-sensitized solar cells (SSCs) are emerging as promising devices for achieving efficient and low-cost solar-energy conversion. The recent progress in the development of ZnO-nanostructure-based SSCs is reviewed here, and the key issues for their efficiency improvement, such as enhancing light harvesting and increasing carrier generation, separation, and collection, are highlighted from aspects of surface-engineering techniques. The impact of other factors such as electrolyte and counter electrodes on the photovoltaic performance is also addressed. The current challenges and perspectives for the further advance of ZnO-based SSCs are discussed.

1. Introduction With the advance of nanotechnology, a variety of novel photovoltaic (PV) devices based on nanostructures have been developed in recent years.[1–7] These include dye-sensitized solar cells (DSSCs),[8–11] colloidal nanocrystal thin-film solar cells,[12,13] and three-dimensional (3D) nanostructured semiconductor junction solar cells,[14–17] among others. These new types of devices can be classified as the third-generation solar cells, following the first generation of crystalline silicon bulk solar cells that followed by a second generation of thin films cells based on a variety of materials including amorphous Si, polycrystalline cadmium telluride, or copper indium gallium selenide (CIGS). As compared with their bulk and thin-film counterparts, nanomaterials often have unique and, importantly, tunable electronic and optical properties resulting from their sizes

Dr. J. Xu, Dr. Z. Chen, Dr. J. A. Zapien, Prof. C.-S. Lee, Prof. W. J. Zhang Center of Super-Diamond and Advanced Films (COSDAF) Department of Physics and Materials Science City University of Hong Kong Hong Kong SAR, P. R. China; Shenzhen Research Institute City University of Hong Kong Shenzhen P. R. China E-mail: [email protected]; [email protected] Dr. J. Xu School of Electronic Science and Applied Physics Hefei University of Technology Hefei 230009, P. R. China

DOI: 10.1002/adma.201400403

Adv. Mater. 2014, DOI: 10.1002/adma.201400403

© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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www.MaterialsViews.com Table 1. Structural and electronic characteristics of ZnO, SnO2 and TiO2. ZnO

TiO2

SnO2

Crystal Structure

Wurtzite

Rutile, Anatase

Rutile

[72–75]

Bandgap [eV]

3.2–3.4

3.0–3.2

3.6–3.8

[72–76]

Conduction Band Minimum [eV]

−4.36

−4.41

−4.88

[77]

Electron Effective Mass

0.26

9

0.275

[78–80]

Static Dielectric Constant (ε⊥,//)

9.26, 8.2

86, 170

14, 9

[81]

Electron Mobility [cm2 V−1 s−1]

130–200

0.1–4

200–250

[72,80,82]

1.1 × 10−4 (nanoporous film)



Effective Electron Diffusion Coefficient [cm2 s−1]

Reference

[83]

highest reported electron mobility and the highest conduction band edge among various candidates to electron transporters (Table 1) that would respectively benefit electron transport with less recombination and enable the possibility for a larger opencircuit voltage (VOC). As-synthesized ZnO shows n-type conductivity due to oxygen vacancies and interstitial Zn which can be further tuned by substituting Zn with Al, Ga, and In.[84–86] Moreover, the ease of crystallization and anisotropic growth of ZnO allows the preparation of ZnO nanostructures in various morphologies. In addition to its electronic, optoelectronic, and photocatalytic applications, recent studies on ZnO-nanostructure-based SSCs have demonstrated some new concepts and led to a better understanding of photo-electrochemical energy conversion. In this paper, we start with a brief discussion of the working principle of SSCs and the factors determining the performance of SSCs. Then we review the approaches for growing ZnO nanostructures with controlled morphologies on transparent conductive oxide (TCO) substrates, their post-treatments for crystallinity and conductivity enhancements, and the recently developed techniques for tuning the band structure of ZnO nanostructures and surface sensitization with QDs and noble metal nanoparticles by different chemical and physical methods. Next, we highlight a number of strategies for improving the device performance including optical engineering of ZnO morphologies for enhanced light absorption, bandgap engineering and co-sensitization of QDs for improved photoelectron injection and transport, and suppression of charge recombination by surface passivation. We also address the impacts of counter electrodes (CEs) and electrolyte on the photovoltaic performance of SSCs. Finally, the challenges and perspectives of SSCs based on ZnO nanostructures for future practical applications are discussed.

2. Working Principle of SSCs

Jun Xu obtained his Ph.D. degree from the City University of Hong Kong in 2012, and then continued as a senior research associate at the Center of Super-Diamond and Advanced Films (COSDAF), City University of Hong Kong. He joined Hefei University of Technology in 2013, and currently works as a Professor at the School of Electronic Science and Applied Physics. His research interest focuses on multinary chalcogenide photovoltaic materials, semiconductor sensitized solar cells, and optoelectronic devices.

Chun-Sing Lee obtained his Doctor of Philosophy degree in 1991 from the University of Hong Kong. He then moved to the University of Birmingham to carry out postdoctoral research with the support of a Croucher Foundation Fellowship. He joined the faculty of the City University of Hong Kong in 1994 and is currently a Chair Professor in materials science. He co-founded COSDAF in 1998 and is currently the center’s Director. Prof. Lee’s main research interest is on surface and interface physics, organic electronics and nanomaterials.

Wenjun Zhang obtained his Doctor of Philosophy degree in 1994 from Lanzhou University. He was a postdoc at the Fraunhofer Institute for Surface Engineering and Thin Films (1995 to 1997) and at the City University of Hong Kong (1997 to 1998). From 1998 to 2000, he worked as a Science and Technology Agency Fellow at National Institute for Research in Inorganic Materials. He joined CityU in 2000 again as a Senior Research Fellow. He is currently a Professor in Department of Physics and Materials Science; and he is also a core member of COSDAF. His research focuses on thin films, semiconducting nanomaterials, surface science and modification, and ions/materials interactions.

A typical SSC consists of three major components: a photoanode, a counter electrode (CE), and electrolyte with redox couples. A

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FF × JSC × VOC Pin

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PCE =

(1)

JSC can also be calculated using the equation: JSC = q ∫ F ( λ ) IPCE ( λ ) dλ

(2)

where q is the electron charge, F(λ) is the incident photon flux density. In Equation 2, IPCE(λ) is the incident photon to charge carrier efficiency, also known as the external quantum efficiency (EQE), which relates the ratio charge carriers collected at electrodes to the number of incident photons, and IPCE(λ) can be given by: IPCE ( % ) = LHE × φinj × ηcol = LHE × ΦET

Figure 1. A) Schematic diagram showing the working principle of an SSC. B) A simplified equivalent circuit model of an SSC. RTCO include the sheet resistance of the TCO and the TCO/MO contact resistance; RCT(P) and Cµ are the back electron transfer resistance and the capacitance at the photoanode/electrolyte interface, respectively; RCT(CE) and CCE are the charge transfer resistance and the capacitance at the CE/electrolyte interface, respectively. Zd is the Warburg diffusion impedance of ions transport in electrolyte. The sum of RTCO, RCT(CE) and Zd corresponds to the series resistance (Rs) of the SSC.

schematic diagram showing the working principle of an ideal SSC is shown in Figure 1A. The photoanode in an SSC is typically constructed with a wide bandgap MO semiconductor (such as TiO2, SnO2, and ZnO) scaffold coated with a layer of semiconductor sensitizer typically in thin film or QDs formats. The MO scaffolds also act as electron acceptor and transporter. Upon photoirradiation, electron-hole pairs (excitons) are generated in QDs. The excited electrons are then injected into the conduction band (CB) of MO, leaving the QDs in their oxidized states. The injected electrons in MO are collected by the TCO substrate (collector) and transported through the external load to the CE. The oxidized QDs are restored to their ground state through hole scavenging by reduced species (e.g., S2− in the redox couples of S2−/Sn2−) in the electrolyte. The oxidized species (Sn2−) diffuse to the CE where they are reduced by electrons from the external circuit, resulting in electrolyte regeneration.[7,87,88] Photovoltaic performance of a solar cell is typically gauged with its PCE, short-circuit current density (JSC), open-circuit voltage (VOC), and fill factor (FF). The PCE of a solar cell is defined as the ratio of the actual maximum electrical power generated to the incident optical power (Pin):


(3)

LHE is the light harvesting efficiency by the photoanode in an SSC, which depends on the extinction coefficient of QDs, the amount of QDs loaded on MO surface, the optical absorption range of QDs, and the optical path length of the incident light within the photoanode. ϕinj is the electron injection efficiency from the photoexcited QDs to the MO, and ηcol is the charge collection efficiency at the electrodes. A high ϕinj requires appropriate energy band alignment at the MO/QD interfaces. It has been reported that in DSSCs an over-potential (−ΔG) of approximately 0.2 V between the conduction band minimum (CBM) of the MO and the lowest unoccupied molecular orbital (LUMO) level of the dye, and a −ΔG of 0.3 V between the highest occupied molecular orbital (HOMO) level of the dye and the redox potential of the electrolyte are needed for efficient electron injection from dye to MO and regeneration of the oxidized dye by hole scavenging.[89] Due to the similarity in structure and working mechanism of DSSCs and SSCs, these data can be considered as a reference for SSCs. ΦET is the electron transfer yield, which is the product of ϕinj and ηcol. In a SSC system, ΦET is seriously influenced by various recombination processes, including direct recombination of photogenerated electron-hole pairs within the QD, interfacial recombination of electrons in the CB of MO with holes in the valence band (VB) of QD and oxidized species in the electrolyte (electron capture), and interfacial recombination of electrons in the CB of QD by electron capture in electrolyte.[90,91] Traps in QDs and MO also play an important role in carrier recombination. In an SSC, the charge-transfer and -transport processes must be much faster than recombination to obtain efficient photovoltaic performance. VOC is determined by the potential different between the quasi-Fermi level (EF*) of electrons in the MO under illumination and the Fermi level (EF) of the photoanode in dark (being equal to the redox potential (Eredox) of the electrolyte),[92,93] as indicated in Figure 1A. It can be expressed as: VOC =

kBT kT n (E C − E redox ) + B ln c q q NC

(4)

where kB is the Boltzmann constant, T is temperature, EC is the CBM of the MO, nc is the free electron density in the CB of the MO under illumination, and NC is the density of accessible states in the CB of the MO. According to Equation 4, either an



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upward shift of EC or an increase in nc would give rise to an enlarged VOC. It should be noted that nc is not only determined by the photoelectron generation yield in the QDs, but also by the electron injection rate from the QDs to the MO, which, however, is strongly influenced by photoelectron recombination in the QDs, and in particular at the MO/QDs, the QDs/electrolyte, and the MO/electrolyte interfaces. To further analyze the dynamic information on charge transport and recombination, Figure 1B shows a simplified electrochemical impedance equivalent circuit of an SSC.[94–97] In such an equivalent circuit, RCT(P) is the back electron transfer resistance at the MO/QDs/electrolyte interfaces which display a diode-like behavior. Combination of the sheet resistance of TCO (RTCO), the charge transfer resistance at the CE/electrolyte interface (RCT(CE)), and diffusion resistance of the redox species in the electrolyte (Zd) gives the series resistance (Rs), i.e., Rs = RTCO + RCT(CE) + Zd.[98,99] The shunt resistance (Rsh) represents the effects that divert photogenerated carriers from flowing in the external circuit. RCT(P), which is associated with the trapping and recombination of photogenerated carriers at interfaces of MO/QDs, MO/electrolyte, and QDs/electrolyte, could be considered as a part of Rsh. An ideal solar cell should have a large Rsh and a small Rs to attain high values of JSC, VOC and FF. Therefore, a larger RCT(P) and smaller RCT(CE) and RTCO are desirable. RCT(CE) can be regarded as an indicator to reveal the electrocatalytic activities of CE materials, which has significant influences on the JSC and the FF. A smaller RCT(CE) facilitates electron transfer from the CE to the electrolyte for catalyzing electrolyte regeneration, consequently results in less interfacial recombination. Overall, the performance (JSC, VOC and FF) of an SSC associated with ΦCT, nc, Rsh, and Rs is strongly influenced by the surface trap states and the recombination of photoelectrons in QDs and MO, and at MO/QDs, QDs/electrolyte, MO/electrolyte

and CE/electrolyte interfaces. The least charge recombination in the processes of photoelectron generation, and charge separation and transport is desired for pursuing high photovoltaic performance. Therefore, suppression of carrier recombination by surface/interface engineering is an important key for improving the PCE of SSCs.

3. Controllable Synthesis and Post-Treatments of ZnO Nanostructures on TCO Substrates ZnO has been shown both experimentally and theoretically to be a promising electron transfer semiconductor for low-cost and high-performance SSCs.[16] ZnO nanostructures offer a scaffold for effective loading of QDs, and they play important roles in light scatting, charge separation and transportation in SSCs. These particular behaviors require growing ZnO nanostructures with controllable and tunable morphologies, sizes and crystallinity which have direct effects on carrier transport and photon trapping and scattering.[100,101] Various ZnO nanostructures have thus far been developed, including particles,[102,103] rods,[104,105] wires,[106,107] belts,[108,109] tubes,[110–112] rings,[113,114] sheets,[115,116] combs,[117,118] nails,[119,120] tetrapods,[121,122] branched structures[123–126] and hierarchical structures.[127–129] ZnO nanostructures have been synthesized by a variety of approaches, e.g., the sol-gel method,[130–132] hydrothermal/ solvothermal growth,[104–106] physical or chemical vapor deposition,[107,108,122] and electrochemical deposition.[110,111] For applications in solar cells, two approaches, i.e., deposition of pre-synthesized ZnO nanostructures and direct growth of ZnO nanostructures on TCO substrates, have been mostly used. Figure 2A shows different types of ZnO nanostructures on TCO substrates, including disordered nanostructures (i and ii), 1D nanoarrays (iii and iv), and hierarchical nanostructures

Figure 2. A) Scheme of ZnO nanostructures deposited on TCO substrates including disordered nanostructures (i and ii), 1D nanoarrays (iii and iv) and hierarchical structures based on 1D nanoarrays (v and vi). B) Typical SEM images of the corresponding ZnO nanostructures on TCO substrates: (i) Nanoparticles. Reproduced with permission.[140] Copyright 2013, The Royal Society of Chemistry. (ii) Disordered nanorods. Reproduced with permission.[105] Copyright 2013, The Royal Society of Chemistry. (iii) Array of nanorods. Reproduced with permission.[110] Copyright 2007, Elsevier. (iv) Array of nanotubes. Reproduced with permission.[110] Copyright 2007, Elsevier. (v) Array of nanoforests. Reproduced with permission.[125] Copyright 2008, American Chemical Society. (vi) Bilayer structures. Reproduced with permission.[156] Copyright 2009, Elsevier.

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3.1. Coating of Pre-synthesized ZnO Nanostructures on TCO Substrates Various ZnO nanostructures (nanoparticles, nanorods, nanotetrapods and nanospheres, etc.) have been prepared ex situ by different chemical and physical methods. The pre-synthesized ZnO nanostructures were deposited on TCO substrates for SSC applications (Panel i and ii of Figure 2A) by the methods including spin coating,[131,132] screen printing,[133,134] spray coating,[135,136] and doctor-blade methods.[137–141] As a typical example, ZnO nanoparticles can be synthesized by preparing ZnO sols in a homogeneous alcoholic solution (such as methanol, ethanol, propanol and butanol) containing zinc acetate precursor and additives (such as alkali metal hydroxides, carboxylic acids, alkanolamines, alkylamines, acetylacetone and polyalcohols).[132] Such sol containing ZnO nanoparticles, with average diameter in the ten to several tens nanometers, have been coated onto the TCO substrates by spin or dip coating resulting in a change from liquid sol into solid wet gel. Drying and heat treatments were then used to generate a porous-structure film on the TCO glass substrate.[132] Besides the spin/dip coating, screen printing and doctor-blade methods are also used to deposit pre-synthesized ZnO nanostructures onto the TCO substrates. In general, a sufficiently viscous paste of ZnO nanostructures can be prepared by mixing the nanostructures with organic binders, such as polyethylene glycol,[137] acetyl acetone,[138] butanol[139] or mixture of ethyl cellulose and terpineol.[140,141] The ZnO paste can then be spread onto the TCO substrates by the screen printing process or the doctor-blade method. Uniform films of ZnO nanostructures with controlled thickness and pore size on TCO substrates can be obtained by suitable heat treatment to remove the residual organic binders and solvents. These deposition approaches have advantages on manipulating the morphology and size of the pre-synthesized ZnO nanostructures. However, an additional deposition process is required, which might also cause organic contamination and affect the quality of the ZnO/TCO contacts. Moreover, the electron transport pathway in photoanodes of such ex situ prepared ZnO nanostructures is random and winding, which increase the probability of carrier recombination due to the increased grain boundaries and diffusion length.

3.2. Direct Growth of 1D ZnO Nanoarrays on TCO Substrates Compared with the coating of pre-synthesized ZnO nanostructures, direct growth of 1D ZnO arrays (nanorods, nanowires, and nanotubes) on TCO substrates has obvious advantages for photovoltaic applications. Firstly, the 1D ZnO nanorods/nanowires (Panel iii of Figure 2A) can provide a direct conduction path in the interior of a crystal bulk for electron transport, reducing their scattering at grain-boundaries. It has been shown that the electron diffusivity in ZnO nanowires (Dn = 0.05–0.5 cm2 s−1) which is several hundred times larger than that (Dn ≤ 10−4 cm2 s−1) in semiconductor nanoparticle films.[9,142] On the other hand,


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based on 1D nanoarrays (v and vi). The typical SEM images of these ZnO nanostructures are presented in Figure 2B.

the array structure can also enhance optical absorption due to light scattering and trapping.[143] The direct growth of ZnO nanowire arrays on TCO substrates is commonly performed by seed-assisted hydrothermal process.[9,144] Compared with vapor based methods, the hydrothermal process can be conducted at low temperatures, which decrease the possibility of film cracking and nanowires separation from the substrates, and enables ZnO nanowire growth even on flexible plastic substrates.[145–147] Also, it is possible to control the density, length and diameter of the ZnO nanowires via manipulating the reaction duration, precursor concentration, and number of repeated growth cycles.[9] In addition, ZnO nanowires prepared by hydrothermal growth are generally free of metal catalyst and other possible contaminants, which is beneficial for applications in electronic and optoelectronic devices. The use of 1D ZnO nanowire arrays provide several advantages related to charge separation and transfer as follows. The formation of ZnO/QDs core/shell nanocables with type II staggered energy band structure gives a stepwise energy band alignment.[148] Electrons and holes would be preferably transferred across the interface in opposite directions to achieve the formation of an excitonic charge separation state. The shells in the nanocables can also provide effective passivation that inhibits non-radiative recombination of percolated electrons in 1D ZnO with electrolyte and suppresses corrosion of the ZnO cores by electrolyte. More significantly, the core/shell nanocables have large-area interfacial heterojunction. As a result, efficient carrier separation occurs in the radial, instead of the long axial direction, leading to a smaller carrier collection distance comparable to the minority carrier diffusion length.[1,17,149] While 1D ZnO nanowire arrays provide a base for efficient loading of QDs, the loading of QDs and the junction area can be further increased by the use of arrays of ZnO nanotubes (panel iv of Figure 2A). In this case not only the outer surface but also the inner surface of the tubes could be coated with sensitizers for promoting light absorption.[150–152] She et al. reported the synthesis of ZnO nanotube arrays using a two-step process, i.e., electrodeposition of ZnO nanorod arrays on TCO substrates, followed by selective etching of ZnO nanorods to form ZnO nanotubes.[110,111] The formation of ZnO nanotubes was proposed to be due to the defect-selective etching of the core of the ZnO nanorods along the c axis by high concentration OH− or H+ in solutions. Enhanced photo-electrochemical properties were demonstrated after ZnO nanorods were converted to ZnO nanotubes. While the CdS sensitized ZnO nanorods array showed a photocurrent density of 7.00 mA cm−2 at 0 V vs saturated calomel electrode (SCE), the CdS sensitized ZnO nanotube arrays increased the photocurrent density to 10.64 mA cm−2.[150] Yang et al. also observed an obvious increase of JSC from 1.86 mA cm−2 for the CdS sensitized ZnO nanorod SSC to 4.07 mA cm−2 for the CdS sensitized ZnO nanotube SSC under 1 Sun illumination, correspondingly PCE increased from 0.33% to 0.87%.[151]

3.3. Growth of 3D Hierarchical ZnO Nanostructures on TCO Substrates To maintain the merit of 1D nanostructure for providing direct electron conduction pathway and meanwhile to further



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increase the surface area of ZnO nanostructure for light harvesting and QD loading, a vertically-aligned branched-nanowire “forest” (Panel v of Figure 2A) has been synthesized. The ZnO “nanoforests” were typically obtained by following three steps: i) growth of ZnO nanowires arrays on TCO substrates, ii) deposition of a ZnO seeding layer on the ZnO nanowire surface, and iii) growth of branched nanorods on the surface of ZnO nanowires.[123–126,153,154] Bilayer architectures have also been used for SSC aplications; these consist of ZnO nanorod array as the bottom layer and ZnO nanostructures (such as nanoflowers, nanospheres, and nano-tetrapods) as the upper layer.[155–157] In the bilayer flower-rod structure illustrated in Panel vi of Figure 2A, an array of ZnO nanorods with a uniform density is first prepared on a TCO substrate followed by the growth of ZnO nanoflowers on the top surfaces of the nanorods. The top layer increases the roughness factor (RF) of the ZnO photoanode and consequently improves the effective loading density of QDs and overall PCE of the fabricated SSCs.

3.4. Post-Treatment of ZnO Nanostructures Among the mentioned synthesis methods, the solution approach is of considerable interest since it is environmentally friendly, and has low production cost and low synthesis temperature. However, the ZnO nanostructures prepared at low temperatures especially by the solution methods are typically featured with high defect densities, low conductivities, and probable residual organic contamination on their surfaces. Various post-treatments, such as plasma modification,[36,158,159] UV irradiation,[160] and in particular annealing under different conditions,[161–165] have been demonstrated to be feasible approaches enabling improved crystallinity, increased conductivity, and/or enhanced stability of ZnO nanostructures. For example, exposure of ZnO nanowire arrays to oxygen plasma was shown to be effective for removing the surface contamination and thus enhancing the QDs adsorption (as discussed in more details in Section 4.1.1).[36] The donor density (5.19 × 1019 cm−3) of the as-grown ZnO nanorods could be increased to 1.79 × 1020 cm−3 by hydrogen plasma treatment and decreased to 1.65 × 1019 cm−3 by oxygen plasma treatment.[158] Annealing has been demonstrated to be a powerful tool for improving the crystallinity and thermal stability of asgrown ZnO nanostructures; and annealing parameters, e.g., atmosphere, temperature, and duration, have been shown to have significant influences on the properties of ZnO nanostructures.[161–165] Zhang and Li et al. reported that annealing in air could significantly improve the crystal structure and reduce defects but had little effect on hole-trapping. In contrast, annealing in hydrogen atmosphere leads to a reduction in hole-trapping due to the passivation of Zn vacancy trap states. As a consequence, samples first annealed in air followed by hydrogen treatment showed decreased hole-trapping and increased conductivity.[163] The shape and intensity of defect photoluminescence emission from ZnO were founded to depend strongly on the annealing atmosphere and temperature.[161,164] Cabot and co-workers reported recently that the ZnO nanowires annealed in Ar exhibited a four-fold decrease

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in electrical resistivity (15.6 Ω cm down to 3.6 Ω cm). The improved conductivity was attributed to the reduced negatively charged oxygen-containing species (CO2, O2−, O2−, O−, OH−, or H2O) adsorbed on the ZnO surface and the higher concentration of oxygen vacancies induced during argon Ar annealing. As a result, the DSSCs composed of Ar-annealed ZnO nanowires exhibited 50% increase in JSC, and yielded 30% enhancement in PCE as compared with the cells based on air-annealed ZnO nanowires.[165] Furthermore, doping of ZnO nanostructures could be achieved by annealing in atmospheres containing gases such as NH3.[166,167] Controllable N concentrations (atomic ratio of N to Zn) up to ca. 4% was achieved by varying the annealing time. IPCE studies revealed that the ZnO:N nanowire arrays yielded an obvious increase of photoresponse in the visible region compared to the undoped ZnO nanowires. An increase of photocurrent density by one order of magnitude and a photoconversion efficiency of 0.15% at an applied potential of +0.5 V (vs Ag/ AgCl) were obtained for the ZnO:N nanowires in the application for water splitting.[166] It should be noted that the post-treatments of ZnO and their impact on the applications of ZnO in electronic and optoelectronic devices,[159] DSSCs,[165] and water splitting[166,167] have been extensively studied. However, there have been only limited reports for the ZnO nanostructures employed in SSCs. Further studies are still needed to explore the beneficial effects of post-treatments on the performance improvement of ZnO nanostructure based SSCs.

4. Surface Sensitization of ZnO Nanostructures While ZnO is an excellent electron transporting material, it cannot effectively harvest visible light due to its wide bandgap. Therefore, surface sensitization of ZnO nanostructures is essential to enhance the light absorption capability, and carrier generation and separation of ZnO-based photovoltaic devices. Thus far, various chemical and physical technologies have been developed to modify the surface of ZnO nanostructures with QDs and noble metal nanoparticles.

4.1. Sensitization with QDs Using Solution Methods Loading of suitable narrow bandgap QDs on the surfaces of ZnO nanostructures is an effective way to enable harvesting of visible light. Two main strategies are mostly employed to decorate nanostructured ZnO with QDs: i) ex situ growth of colloidal QDs and subsequent attachment of the presynthesized QDs to the surface of ZnO nanostructures via bifunctional linker molecules;[36,143,168–172] ii) in situ growth of QDs on the ZnO surface by chemical reaction of ionic species using the methods including chemical bath deposition (CBD),[32,152,173–175] successive ionic layer adsorption and reaction (SILAR),[115,151,176–181] ion-exchange,[48,182–187] and electrochemical deposition.[146,150,188–194] In comparison with ex situ process, the in situ approach involve direct nucleation and growth of QDs on ZnO surface, typically leading to improvements in effective loading and uniform coverage of QDs;




however, it increases the difficulties in controlling the size distribution of the deposited QDs. 4.1.1. Attachment of Pre-synthesized QDs by Molecular Linkers The pre-synthesized QDs are typically attached to the surface of ZnO nanostructures using bifunctional molecular linkers. Commonly used linkers include thioglycolic acid (TGA), mercaptopropionic acid (MPA), mercaptoalkanoic acid (MAA), methoxybenzoic acid (MBA), and cysteine (CYS), as shown in the right column of Figure 3. A typical feature of these linkers is that they bear simultaneously carboxylate and thiol functional groups.[6] The carboxylic acid group (–COOH) and the thiol group (–SH) can respectively bind to ZnO and metal chalcogenide QDs, respectively. Other linker molecules such as oxalic acid (OA), malonic acid (MA), hexandithiol (HDT), thioacetic acid (TAA), and thiolactic acid (TLA) have also been reported for decorating QDs on ZnO nanostructure surface.[6,168,169] The effect of molecular linkers has been studied taking the assembly of pre-synthesized PbS QDs on ZnO porous films as an example.[168] The ZnO films, with a thickness between 300 and 400 nm, were prepared by spin coating ZnO nanoparticles onto ITO (ITO-ZnO) substrates. After annealing, the ITO-ZnO substrates were put into a solution of molecule linker (e.g., OA, MA, TAA, TGA, MPA, and HDT) in tetrahydrofuran (THF) for surface treatment. Then the linker modified ITO-ZnO substrates were immersed in a THF solution containing presynthesized PbS QDs. A clear color change from almost transparent to a distinct brown coloration was observed, while there was no change discernible by eye on the ITO-ZnO substrates without linker modification. The degree of the coloration could provide a visual aid to evaluate the amount of PbS adsorbed on the surface. The gained absorption spectrum of the ITO-ZnOlinker-PbS substrate showed a weak absorption shoulder in the NIR, which matched well with the solution phase absorption of PbS nanoparticles. The attachment of colloidal QDs through molecular linkers enables the use of QDs with precise control of their shape and


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Figure 3. Schematic illustration of QDs sensitized ZnO by molecular linker.

size of the QDs. This technique has achieved great success in high performance SSCs using TiO2 mesoporous films as photoanodes.[37,43,71] However, it still faces difficulties in achieving uniform coverage and sufficient loading of QDs onto the ZnO nanostructured photoanode, probably due to the large dimension and different surface chemical states of ZnO nanostructures, which limits their light harvesting and corresponding photovoltaic performance.[36,169,170] On the other hand, surface states of ZnO, such as surface charging, dangling bonds, and surface contamination, seriously affect the attachment of colloidal QDs. Therefore, treatment of ZnO surface is generally required to improve QD loading. Aydil et al. reported that enhanced coverage of colloidal MPA-capped CdSe QDs on ZnO nanowire surface can be achieved by exposing the ZnO nanowires to oxygen plasma.[36] The treatment removed the surfacebound contaminants (surface hydroxyl and hydrocarbon groups) which prevented the colloidal QDs from attaching to the ZnO nanowire surface through the carboxyl group. It was demonstrated that oxygen plasma treatment of ZnO nanowires increased JSC to 2.1 mA cm−2 and PCE to 0.4%, which were more than one order of magnitude higher as compared with those of the SSC assembled using untreated ZnO nanowires. The molecular linkers serve as a binding bridge between ZnO and QDs; however, they also act as in-series component in the charge transfer processes (Figure 3). The linker molecules impose a barrier potential between ZnO and QDs, which has to be overcome for electron transfer.[195] Therefore, the nature of the molecular bridges is an important issue to be concerned for electron transfer processes. Much effort has been devoted to optimizing the photoelectron injection rates and photoelectrochemical responses of the cells by changing the linker molecules, particularly by varying the alkyl chain length and by selecting molecules ending with different acid and/or thiol groups as the attachment moieties.[169,195]

4.1.2. CBD of QDs on ZnO Surface CBD is one of the most commonly used methods for direct growth of QDs onto ZnO nanostructures. In this one-pot synthesis method, the ZnO nanostructures are immersed in an intended QD precursor solution for certain duration. For SSC applications, effective loading and homogeneous coverage of QDs on ZnO surface are desired, but aggregation of QDs should be minimized to enhance light absorption and reduce charge recombination (Figure 4A,B). Aggregation of QDs on ZnO surface (Figure 4A) increases the diffusion length and the probability of recombination of photogenerated electrons, and thus results in a reduced injection rate of photoelectrons into ZnO.[172,173] CdS QDs have been deposited on ZnO nanowires in a chemical bath solution of CdSO4, thiourea, and ammonia. It was shown that the quality of QDs depends strongly on the pH value of the solution, the precursor concentration, its reaction temperature, and the reaction duration in CBD process. Reaction in dilute solutions improved the coverage of CdS QDs on ZnO surface, but led to reduced QDs loading. On the other hand, prolonging the reaction duration was revealed to induce aggregation of the CdS QDs.[175]



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cycles, solvents and precursor concentrations. For CdS QD deposition, the SILAR method involves successive immersion of ZnO nanostructure in solutions of Cd2+ and S2− ions and rinsing between dips (Figure 5A) while the desired CdS thickness is obtained by repeating the processes as needed. The correlations between the number of SILAR cycles and the thickness of CdS QD layer can be demonstrated through high-resolution TEM analyses, as shown in Figure 5B. In an ideal SILAR process, the thickness should increase with cycling number, regardless of the sample surface area and dipping time.[176] The absorption spectra of the ZnO/CdS core/shell nanowire arrays with different CdS shell thicknesses are presented in Figure 5C. An absorption edge shorter than 420 nm is observed for the bare ZnO nanowire array, and it continuously red shifted to the visible light region with increasing number of SILAR cycles. Such cycle-dependent layer-by-layer growth in the solutionphase SILAR process has been shown to be a powerful thinfilm growth technique in current semiconductor processing. During the SILAR process, the ZnO surface is first converted to ZnS by ion exchange in sulfide solution. Therefore, similar to the CBD approach on a sulfide-treated ZnO surface,[175,185] the SILAR method typically is characteristic with an even coverage of QDs on ZnO nanostructures. Comparing with CBD, the SILAR process typically gives a better control on the thickness uniformity of the QD layer, but many repeating cycles are required to achieve sufficient QD layer thickness.

4.1.4. Ion Exchange for ZnO Surface Sensitization Figure 4. Schematic transport path of photogenerated electron in ZnO nanorod-based photoanodes with (A) aggregated, and (B) uniformly covered QD sensitization layer. SEM images showing the effect of sulfide treatment on ZnO surface coverage by (C,D) CdS from ammonia/thiourea bath, and (E,F) CBD CdSe. Left column images (C,E) are untreated ZnO rods, right column images (D,F) show sulfide-treated ZnO rods. The insets are higher magnification backscattered images. A–F) Reproduced with permission.[175] Copyright 2010, American Chemical Society.

Sulfurization of ZnO nanostructure surface has been shown to enable a significant improvement of CdS QDs coverage.[175,185] The SEM images in Figure 4C and D depict CdS QDs synthesized in ammonia/thiourea bath on the surfaces of ZnO nanorods without and with sulfide treatment, respectively. By converting the surfaces of ZnO to ZnS with an alkaline sulfide solution treatment, a CdS QD layer with thicknesses of ca. 10 nm was uniformly covered on ZnO nanorod surface. Similar results were also observed in the deposition of CdSe QDs on ZnO nanorods by CBD. By incorporating surface sulfurization, the coverage of CdSe QD layer was obviously improved, as shown in Figure 4E and F.

Surface sensitization of ZnO nanostructures by ion exchange technique is based on the large difference in solubility product constant (Ksp) between the precursor and the target semiconductors. The Ksp is the equilibrium constant for a chemical reaction in which an ionic compound dissolves to produce its ions in a solution. An ionic compound with a smaller Ksp is more difficult to be dissolved in a solution than that with a lager Ksp. For example, in an ion exchange reaction, AB + C− = AC + B−, when the Ksp value of target semiconductor (AC) is sufficiently smaller than that of precursor semiconductor (AB), the C− ions in solution are driven to replace the B− ions in precursor semiconductor, leading to the formation of target semiconductor (AC) on the surface of precursor semiconductor (AB).[196] In ion exchange reactions, ZnO can be easily converted to ZnS by surface sulfurization or to ZnSe by surface selenization due to the much larger Ksp value of Zn(OH)2 (10−16.5) with respect to those of ZnS (10−23.8) and ZnSe (10−25.4).[197–200] For example, the following reaction takes place in the surface selenization process: K

ZnO + Se 2− + H2O ZnSe + 2OH−

8

(5)

4.1.3. SILAR Method for Depositing QDs on ZnO Surface

A large equilibrium constant of the reaction:

The SILAR method is another approach used for in situ deposition of QDs on nanostructured ZnO surfaces by alternative adsorption of cations and anions in respective solutions.[176,177] The growth of QDs is controlled by tuning the number of

K=


[OH− ]2 [Zn 2+ ][OH− ]2 K sp (Zn(OH)2 ) = = = 10 8.9 [Se 2− ] [Zn 2+ ][Se 2− ] K sp (ZnSe)

(6)

indicates that the reaction is spontaneous.




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The ion exchange method could generate a continuous and uniform layer of QD shell on ZnO surface. Figure 6A illustrates the synthesis process of copper indium selenide (CIS) shells on ZnO nanorod surfaces by successive ion exchange.[182] In the first stage, the ZnO nanorods arrays were grown on TCO substrates by the seedassisted growth method as discussed above. The Se2− solution is prepared by reducing Se powder with NaBH4 in distilled water. As the Ksp of Zn(OH)2 is much larger than that of ZnSe, the ZnO nanorod array can be used as a sacrificial template to synthesize more stable ZnSe by anion exchange (Equation 5). Upon immersing a ZnO nanorod array into a Se2− ion solution, ion exchange reaction between Se2− and ZnO takes place, which produces a continuous ZnSe layer on the surface of ZnO nanorods resulting in ZnO/ZnSe core/shell nanocables. The ZnO/ZnSe core/shell nanocable arrays are then immersed in a Cu2+ ion solution. Due to the smaller Ksp value of CuSe compared to that of ZnSe, Cu2+ ions replace Zn2+ ions in ZnSe shells to form CuSe shells, leading to the formation of ZnO/ CuSe core/shell nanocables. Finally, CIS is synthesized by reacting CuSe shells with In3+ via a polyol reduction process. In this step, the ZnO/CuSe core/shell nanocable array is immersed in In3+ ion contained triethylene glycol (TEG) solvent. The growth of CIS shell is accompanied with the gradual dissolution of ZnO cores, and prolonging the reaction time may lead to complete etching of ZnO cores and the formation of CIS nanotube array. The TEM image and corresponding electron energy loss spectroFigure 5. A) Schematic diagram showing the SILAR deposition processes of CdS QDs on ZnO scopic (EELS) elemental mappings in Figure [176] nanowires. A) Reproduced with permission. Copyright 2013, Elsevier. B) HRTEM observa6B further confirm the uniform thickness of tions of CdS QD layer thickness upon cycle numbers in SILAR process. C) UV–vis absorption the nanotube and homogeneous distribution spectra of the as-prepared ZnO nanowire and the ZnO/CdS core/shell nanowire arrays, where the shell thickness increases with the number of SILAR cycles (5, 10, 15, 30, 60, 90, 120 cycles). of Cu, In and Se throughout the tube wall. The photoanodes were grown on Ti foil substrates. B,C) Reproduced with permission.[177] Copy- It is interesting to note that subjecting ZnO/ right 2009, The Royal Society of Chemistry. ZnSe and ZnO/CuSe nanocable arrays to acidic etching could be used to prepare ZnSe and CuSe nanotube arrays, respectively. ZnSe (or ZnS) has a relatively larger Ksp value as compared with some other metal chalcogenides (selenides and sulfides), such as CdS (10−26.1), CdSe (10−35.2), Ag2S (10−49.2), Ag2Se (10−63.7), CuS (10−35.2), CuSe (10−48.1), PbS (10−27.1), 4.1.5. Electrochemical Deposition of Semiconductor Sensitizers PbSe (10−42.1), HgS (10−52.4), HgSe (10−59), CoS (10−24.7), CoSe Various semiconductors such as CdS,[188–190] CdSe,[191,192] (10−31.2), NiS (10−24), NiSe (10−32.7), In2S3 (10−73.24) and Sb2S3 (10−92.8). Therefore, ZnSe (or ZnS) can further act as precursors CdTe,[193,194] and PbSe,[49] have been deposited on ZnO to prepare more stable metal chalcogenides, obtaining a series nanowire/nanorod arrays using electrochemical deposition. of chalcogenide semiconductor sensitized ZnO photoanodes. The electrochemical deposition is usually carried out in a threeBy successive anion and cation exchange reactions, single or electrode electrochemical workstation. Standard saturated double shelled semiconductor sensitizers could be coated on calomel electrode (SCE) and Pt foil are used normally as the ZnO surface, e.g., arrays of ZnO/ZnSe/CdSe trilayer nanoreference and counter electrodes, respectively, while the ZnO cables[183] and bilayer ZnO/ZnxCd1–xSe nanocables.[184] nanostructures grown on TCO substrate used as the working




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Figure 6. A) Ion exchange processes for the formation of ZnO-based nanocables and corresponding nanotubes. B) TEM image of a CIS nanotube and the corresponding Cu, In, and Se elemental EELS mappings of the same region. A,B) Reproduced with permission.[182] Copyright 2010, American Chemical Society.

electrode. The electrolyte selection is a key factor for the electrochemical deposition of QDs and a critical prerequisite is that the electrolyte does not etch the ZnO nanostructures. The electrochemical deposition is usually performed in galvanostatic or potentiostatic mode. In contrast to the deposition methods mentioned above, electrical current is the driving force for processing the deposition; the deposition rate and quality of QDs, however, are also controlled by the operation mode, precursor concentration in electrolyte, and deposition duration. Li et al. reported the electrochemical synthesis of ZnO/CdTe core-shell nanocables.[193] The deposition of CdTe was performed at a fixed potential of −1.0 V vs SCE. Figure 7 shows a single ZnO/CdTe nanocable, revealing a clear ZnO/CdTe core-shell structure. It should be pointed out that complete coverage of the ZnO core by a CdTe shell was achieved without any interfacial void formation. CdTe is a promising photovoltaic material with advantages of high optical absorption coefficient (ca. 104 cm−1) and a band gap of ca. 1.5 eV. The ideal absorption properties of the CdTe shell and the type II staggered band alignment (Figure 7E) would make the ZnO/CdTe core/shell nanocables a promising photoelectrode for solar energy conversion.

4.2. Sensitization of ZnO Using Vapor Phase Methods In addition to the above chemical solution approaches, various vapor phase methods, including chemical vapor deposition (CVD), pulsed laser deposition (PLD), thermal evaporation,

10


Figure 7. A) TEM image of a single ZnO/CdTe nanocable. B) Elemental profile obtained from STEM-EDX showing the distribution of the compositional elements (Zn, O, Te, and Cd) along the radial direction of the nanocable (indicated by the red arrow in panel (A)). C,D) HRTEM image and SAED pattern taken from the same ZnO/CdTe nanocable. E) Schematic of the operation of ZnO/CdTe nanocable grown on ITO substrate for SSC application. A–E) Reproduced with permission.[193] Copyright 2010, American Chemical Society.

and sputtering, have been employed to deposit narrow bandgap semiconductors onto ZnO nanostructures.[201–207] As compared with the chemical solution methods, vapor phase methods typically need elevated temperatures to grow the semiconductor sensitization layers resulting in high crystalline quality and even epitaxial growth of the sensitizers on the ZnO nanostructure surfaces. Due to the reduced defect density, epitaxial growth of high quality semiconductors on ZnO surface could decrease the extent of non-radiative recombination and carrier scattering loss particularly at the ZnO/sensitizer interface, and benefit the charge separation and transport.[205,207] Nevertheless, much less work has been reported on gas phase synthesis of sensitizer on




REVIEW

the (001) planes parallel to the ZnO (001) planes, as shown in Figure 8B; and the Fast Fourier Transform (FFT) and electron diffraction (ED) pattern also verified the alignment of [0001] CdS0.5Se0.5 with [0001] ZnO (insets). Energy dispersive spectroscopic (EDS) mapping in Figure 8C and the line-scanning in Figure 8D demonstrated the formation of ZnO/CdS0.5Se0.5 core/shell structure with Zn confined in the core only. The stoichiometry of the ternary CdSxSe1–x shell could be well controlled by changing the ratio of CdS and CdSe source powder. Measurements on the composition-dependent optical absorption reveal a decrease in bandgap with increasing Se content.[201] The ZnO/CdS nanocables exhibited a bandgap of 2.35 eV with CdS shell thickness of ca. 50 nm, and the ZnO/ CdS0.5Se0.5 nanocables showed a red shift to 1.95 eV, matching well to that of the bulk CdS0.5Se0.5. The bandgap of ZnO/CdSe nanocables was estimated to be 1.66 eV. Pulsed laser deposition (PLD) is a physical deposition technique that uses a high power, short-pulse laser beam focused on the surface of the source material (target). This material is thus vaporized from the target in the form of a plasma plume which can then deposit as a thin film onto a substrate in an ultra-high Figure 8. A) TEM image of a ZnO/CdSxSe1−x (x = 0.5) core/shell nanorod. B) HRTEM image vacuum or in the presence of a back-filled, of the ZnO/CdSxSe1−x interface region, showing the epitaxial growth single-crystalline wurtzite inert or reactive gas. The PLD processing CdSxSe1−x (x = 0.5) on ZnO. The corresponding FFT and ED patterns confirm the parallel align- parameters include the target-to-substrate ment of the [0001] CdSxSe1−x (x = 0.5) with [0001] ZnO (insets). C) EDS mapping of the ZnO/ distance, deposition duration, pulse repCdSxSe1−x (x = 0.5) core/shell nanocable. D) Line-scan of the ZnO/CdSxSe1−x (x = 0.5) core/shell etition frequency, and laser energy density. nanocable, showing Zn and O elements in the core, and Cd, S, and Se elements in the shell. Wang et al. reported the use of PLD techA–D) Reproduced with permission.[201] Copyright 2010, American Chemical Society. nique for coating ZnSe on ZnO nanowire arrays.[205] The TEM image of the resulting ZnO nanostructures, mainly due to the more expensive and complicated high-vacuum deposition facilities usually required ZnO/ZnSe core/shell nanocable, Figure 9A, indicates the ZnSe by vapor phase methods. shell can grow on the ZnO nanowire surface with a thickness CVD technology has been widely used for synthesizing nanoof about 5–8 nm in the radial direction. A sharp interface of the materials and surface coating for electronic and optoelectronic ZnO/ZnSe core/shell nanocable is confirmed by the HRTEM applications. This method provides a great controllability on image in Figure 9B, which reveals that ZnO and ZnSe present the composition, morphology, and crystallinity of the materials Wurtzite (WZ) and zinc blende (ZB) crystalline structures, deposited by tuning the reactive gas composition, pressure, and respectively. Figure 9C and D show the FFT patterns of the (WZ) substrate temperature. Recently, much effort has been devoted ZnO core and the (ZB) ZnSe shell, with zone axes [2–1–10] to synthesizing nanostructured ternary chalcogenide alloys and [011], respectively, which further confirms the epitaxial with controllable composition using the CVD approach. ZnO/ growth. The spatial distributions of the atomic composition CdSxSe1−x,[201] ZnO/ZnxCd1−xSe,[44,202] and ZnO/ZnSxSe1−x[204] across the ZnO/ZnSe core/shell nanocable are shown in the EDS line-scan analysis (marked by a line in Figure 9A), showing core/shell nanocables with tunable shell composition have been the homogeneous coating of the ZnO nanowire (Figure 9E). successfully synthesized. Park et al. reported CVD synthesis of ZnO/CdSxSe1–x core/shell nanocables with tunable shell composition in a full range (0 ≤ x ≤ 1) where the ZnO nanorod substrates were placed downstream apart from the CdS/CdSe 4.3. Sensitization with Noble Metal Nanoparticles mixed powder precursors in a CVD reactor.[201] Thickness of Noble metal nanoparticles (NPs), such as Au and Ag, have also the deposited CdSxSe1−x shell was then controlled by adjusting been decorated on ZnO NWs to enhance light absorption based the growth temperature or duration. Figure 8A shows a TEM on localized surface plasmon resonance (LSPR) effects.[208–210] image of a ZnO/CdS0.5Se0.5 core/shell nanocable with a shell thickness of 50 (±10) nm. The lattice-resolved image of the LSPR is originated from the interaction of incident light with interface region revealed a single-crystalline wurtzite shell with electrons in the metal NPs, and it has been extensively studied




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Figure 9. A) Low-magnification TEM image of a ZnO/ZnSe core/shell nanowire. A thin layer of ZnSe was coated on the ZnO nanowire. B) Highresolution TEM image of the interface of the core/shell heterostructure, enlarged from the rectangular area outlined in (A), showing the epitaxial growth relationship of the ZnO WZ core and ZnSe ZB shell. C,D) FFT patterns of the rectangular areas outlined in (B). E) EDS nanoprobe line-scan of the elements Zn, Se, and O, across the ZnO/ZnSe core/shell nanowire as indicated by the line in (A). A–E) Reproduced with permission.[205] Copyright 2008, Wiley-VCH.

in enhancement of Raman scattering, biomedicine, and solar cells.[211–213] The plasmon resonance wavelength depends strongly on the size and composition of the material as well as on its local dielectric environment,[211] which give us opportunity to design and tailor the optical properties of the noble metal NPs sensitized photoanodes.

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There have been two major approaches for chemical coating of noble metal NPs on ZnO nanostructure: ex situ growth by assembling the pre-prepared metal nanoparticles and in situ growth of the metal nanoparticles by chemical deposition. Au NPs typically served as the most commonly used plasmonic material to reveal plasmon induced effects on ZnO, because its resonant wavelength is in the visible region,[211,212] which extended the wavelengths region absorbed by ZnO. Some research groups reported Au NPs decorated ZnO nanostructures by directly reducing HAuCl4 solution.[208,214,215] On the other hand, pre-prepared Au NPs with controlled size and shape could be attached to ZnO nanostructures by using a bifunctional molecular linker.[209,216] Recent studies showed that Au NPs coated ZnO nanorod arrays present distinct chemical and physical properties, as compared with uncoated ZnO nanorods arrays, due to enhanced separation of excited electron-hole pairs. For example, a photovoltaic device with a single ZnO nanorod decorated with Au NPs has been reported to show a high photocurrent of 22.6 µA at a bias of 1.0 V under UV illumination, showing the photocurrent increased nearly 1.5 times in comparison with a device using a pristine ZnO nanorod.[209] ZnO nanorod arrays decorated with Au NPs have been reported to show approximately 8× increase in photocatalytic activity under UV irradiation compared to bare ZnO.[209] Plasmonic enhancement is a useful and important approach for development of high performance photovoltaic devices. Introducing Au plasmonic material onto ZnO photoanodes has been reported to markedly enhance their photovoltaic performance, which was proposed to involve the coupling of hot electrons formed by plasmons and the electromagnetic field.[216] Figure 10A shows the UV–vis absorption spectra of ZnO nanorod arrays coated with different amounts of Au NPs. Other than the strong ultraviolet absorption, the bare ZnO nanorods showed no absorption between 400 and 800 nm. In contrast, the Au-ZnO composite arrays show obvious absorption band in the visible region due to the LSPR of Au NPs. The LSPRrelated absorbance increases with the increasing loading of Au NPs, which was controlled by varying the deposition duration and conditions. Figure 10B is a schematic diagram showing the mechanism of photocurrent enhancement by LSPR in the Au-ZnO nanostructure. Upon irradiation, electrons in the VB of ZnO rod will be excited to the CB. Simultaneously, upon irradiation, plasmon will be induced on the surface of Au NPs that in turn generate hot electrons and a secondary electromagnetic field. The plasmon-induced hot electrons would also be injected into the CB of ZnO leading to an increase in photocurrent. On the other hand, the LSPR can generate a strong electromagnetic field close to the surfaces of the Au NPs. The electromagnetic field can modify the band structure at the Au-ZnO interface and create more vacancies at the bottom of the ZnO CB. It would further facilitate the generation of photoelectrons by photoexcitation. Chen et al. reported enhanced photovoltaic performance of solar cell based on Au NPs sensitized ZnO nanorod arrays.[217] Figure 10C showed the J–V curves of the cells before and after Au NP sensitization using iodide-based electrolyte. While the device with bare ZnO nanorods did not show measurable photocurrent, the cell with Au NPs coated ZnO nanorod array




REVIEW Figure 10. A) UV–vis absorption spectra of ZnO nanorod arrays decorated with Au nanoparticles prepared with various durations. B) Schematic illustration of the plasmon-induced effects on Au-ZnO photoelectrode. A,B) Reproduced with permission.[216] Copyright 2012, American Chemical Society. C) J–V characteristics of solar cell devices with bare ZnO nanorod array and Au-coated ZnO nanorod array under illumination. D) A schematic of a band diagram corresponding to the ZnO/Au/electrolyte cell structure. C,D) Reproduced with permission.[217] Copyright 2009, American Chemical Society.

presented a JSC of 1.72 mA cm−2, a VOC of 0.37 V, and a FF of 0.46, and yielded a PCE of 0.30%. The photovoltaic performance enhancement was due to the increased optical absorption in the visible light caused by the LSPR effects from the Au NPs. The hot electrons excited at the Au NP surfaces could be separated and transported to the CB of the ZnO nanorods with subsequent drift to the conductive TCO electrode (Figure 10D). The Schottky Au-ZnO contact enabled the injection of electrons from Au NPs to ZnO nanorods while blocking the reverse flow. The excited Au ions would capture electrons donated from the redox species in the electrolyte to compensate for their lost electrons. The oxidized redox species were then regenerated by taking electrons from the outside circuit via the counter electrode. Reaction involved in the photocurrent generation process in the Au-ZnO Schottky barrier solar cell can be summarized as follows: Photoanode : Au + hv → Au ⊕ + e − ( Au )

(7)

2Au ⊕ + 3I− → 2Au + I3−

(8)

Counter Electrode : I3− + 2e − → 3I−

(9)

5. SSCs Based on ZnO Nanostructures As discussed in Section 2, the power conversion efficiency (PCE) of an SSC is determined by its current density–


voltage (J–V) characteristics, which includes three important operational parameters, the short-circuit current density (JSC), the open-circuit voltage (VOC), and fill factor (FF). In this part, we review recent efforts to enhance the performance of ZnOnanostructure-based SSCs by various approaches including improvements on optical absorption, charge separation, transportation, and recombination processes, as well as optimizing energy levels and gaps of the QDs. Recent advances of photovoltaic performance of high efficiency ZnO nanostructures based SSCs are summarized in Table 2. However, it should be noted that efficiencies in these reports have not been verified by national laboratories nor other recognized third parties. While the table has to be read with caution, the rapid progress achieved in recent years are unarguable. 5.1. Improvement of Short-Circuit Current (JSC) by Enhancing Light Absorption and Charge-Injection Efficiency The JSC in a SSC is determined by its IPCE which depends on the light harvesting efficiency (LHE) and the electron injection yield (ϕinj) from the photoexcited QDs into ZnO film. To achieve a high JSC in SSCs, some basic features are generally required. These include wide optical absorption over the visible and the near-infrared regions, efficient injection of photogenerated electrons into the CB of the ZnO electrode, and efficiently regeneration of oxidized QDs. Herein, we review recent progress in SSCs with specific emphasis on the strategies for tailoring optical absorption, charge injection, and transfer.



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www.MaterialsViews.com Table 2. Recent photovoltaic performance of ZnO nanostructures based SSCs. No.

Morphology of ZnO

Sensitizer

1

ZnO nano-tetrapods

2

Branched double-layer ZnO nanorod-nano-tetrapods

CdS/CdSe

3

ZnO nanowire array

4

Method

Electrolyte

Counter electrode

JSC [mA/cm2]

VOC [V]

FF

PCE [%]

Reference

Na2S+S

GO/Cu2S; Pt-coated FTO glass

17.3 17.8

0.761 0.741

0.471 0.398

6.2 5.25

[218]

SILAR

Na2S+S

Cu2S on brass

16.56

0.703

0.45

5.24

[157]

ZnxCd1–xSe

Ion exchange

Na2S+S

Cu2S on brass

18.05

0.65

0.40

4.74

[184]

ZnO nanoparticles passivated with TiO2

CdS/CdSe

SILAR and CBD

Na2S+S

Cu2S on brass

15.42

0.62

0.49

4.68

[219]

5

ZnO nanowire array

ZnSe/CdSe

Ion exchange

Na2S+S

Cu2S on brass

11.96

0.836

0.45

4.54

[183]

6

ZnO nanoparticles

CdS/CdSe

SILAR and CBD

Na2S+S

Cu2S on brass

10.48

0.683

0.623

4.463

[140]

7

ZnO nano-tetrapods

CdS/CdSe

SILAR and CBD

Na2S+S

Pt-coated FTO glass

13.85

0.722

0.424

4.24

[134]

8

ZnO nanowire array

CdS/CdSe

SILAR and CBD

Na2S+S

Au-coated FTO glass

17.3

0.627

0.383

4.15

[220]

9

ZnO nanowire array

ZnSe/CdSe

Ion exchange

Na2S+S

Cu2SnS3 on FTO glass

11.46

0.810

0.437

4.06

[221]

10

ZnO nanowire array

ZnSe/CdSe

Ion exchange

Na2S+S

CZTS on FTO glass

11.06

0.822

0.410

3.73

[222]

11

ZnO nanowire array

CdS/CdSe

SILAR and CBD

Na2S+S

Mesocellular carbon foam on FTO glass

12.6

0.685

0.42

3.60

[223]

12

ZnO nanowire array

CdS/CdSe

spin-SILAR

Na2S+S

Pt-coated FTO glass

9.38

0.663

0.56

3.45

[224]

13

ZnO nanowire array

CdS/CdSe

CBD and Ion exchange

Na2S+S

Cu2S on brass

14.49

0.62

0.36

3.23

[185]

14

Arrays of ZnO nanorods passivated with TiO2

CdS/CdSe

SILAR and CBD

Na2S+S

Cu2S on brass

9.93

0.61

0.52

3.14

[225]

15

ZnO nanowire array

CdS/CdSe

SILAR

Na2S+S

Au-coated FTO glass

18.63

0.48

0.342

3.06

[176]

16

Branched n-Si NW/ZnO NR

CdS/CdSe

SILAR and CBD

Na2S+S

Pt-coated FTO glass

11.0

0.71

0.38

3.00

[226]

17

Disordered ZnO nanorods passivated with TiO2

CdS/CdSe

SILAR and CBD

Na2S+S

Cu2S on brass

8.17

0.64

0.52

2.72

[105]

18

ZnO nanosheets

CdS/CdSe

SILAR

Na2S+S

Cu2S on copper foil

19.3

0.49

0.28

2.67

[115]

19

sol-modified ZnO

CdS/CdSe

SILAR and CBD

Na2S+S

Pt on FTO glass

13.3

0.600

0.331

2.64

[227]

20

ZnO NWs

CdS/CdSe

SILAR and CBD

Na2S+S

Pt on FTO glass

8.36

0.555

0.51

2.36

[228]

21

Hierarchical ZnO nanowire array

CdS

Electrochemical Deposition

LiI+I2

Pt-coated FTO glass

7.38

0.70

0.31

1.62

[188]

22

ZnO nanowire array

ZnxCd1–xSe

CVD

LiI+I2

Pt-coated ITO

6.7

0.64

0.35

1.5

[44]

23

ZnO nanorod array

CdS/CdTe

Electrodeposition

LiI+I2

Pt-coated CE

4.93

0.58

0.37

1.05

[229]

24

Array of Al2O3 passivated ZnO nanorods

CdSe

Molecular link

LiI+I2

Pt-coated FTO glass

2.72

0.66

0.55

0.99

[171]

ZnSe/CdSe/ Ion exchange and ZnSe SILAR

5.1.1. Optical Engineering by Tailoring ZnO Morphologies Good optical absorption is an obvious basic requirement for any solar cell design. The size and morphology of the ZnO nanostructured photoanode have important influence on its QD loading as

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well as light scattering and trapping. Although a larger surface area might augment surface recombination losses, it could also enhance light harvesting by enabling more effective QD loading. Vertically aligned nanowire arrays have been demonstrated to have good light scattering and trapping properties resulting in




REVIEW Figure 11. A) Layout of the double-layer-assembly branching processes for fabrication of the branched ZnO double-layer NR−TP film photoanode. Top view (B) and cross sectional view (C) SEM images of the branched ZnO double-layer NR−TP film photoanode. D–F) UV−vis absorbance spectra (D), IPCE curves (E), and illuminated J−V curves (F) of the ZnO/CdS/CdSe TP film (black short-dashed line), the ZnO/CdS/CdSe NR−TP film (orange dashed line), and the branched ZnO/CdS/CdSe branched NR−TP film (violet solid line). A–F) Reproduced with permission.[157] Copyright 2013, American Chemical Society.

absorption of most incident light with a relatively thin absorber layer.[9,143] Tena-Zaera et al. have investigated the influence of the dimensions of ZnO nanowires on light scattering.[230] These authors reported that for ZnO nanowires with a constant diameter, the maximum of the total reflectance increases from 8.6% to 20.4% for lengths of 0.5 and 2.0 µm, respectively, with no significant spectral shift observed. For ZnO nanowires with a constant length, there was an obvious red shift in the reflectance peak with increasing diameter (from 105 to 330 nm) because the total reflectance in the long wavelength region increases with the nanorods diameter. Therefore, optical engineering of nanowire arrays for enhanced scattering for wavelengths where QDs exhibit a relatively low absorption coefficient can result in increased light absorption. The rational design of the ZnO nanostructures for sufficient QD loading and efficient electron transport is another important approach. An obvious approach to increase QD loading is to increase the length and decrease the diameter of the ZnO nanowire. However, it is still a challenge to grow ZnO nanowires longer than 10 µm with diameters smaller than 100 nm. While ZnO nanowires with longer lengths have been synthesized, their diameters became correspondingly larger, often also resulting diminished roughness factors and poor photovoltaic performance.


Yang’s group has recently reported a branched doublelayer architecture of ZnO nanorods (NRs) and nano-tetrapods (TPs) as an efficient photoanode, achieving a PCE as high as 5.24%.[157] In the NR-TP film, the ZnO TPs prepared via vapor transport growth were coated by a doctor blade technique onto a 2-µm-thick ZnO NR array to form a double-layer architecture. Additional branched structures were further put onto the double-layer to improve the roughness factor and network connectivity (Figure 11A). Such 3D structures (Figure 11B and C) effectively increase the surface area for efficient QD loading. CdS- and CdSe-cosensitized ZnO photoanodes of single-layer TP film, double layer NR−TP film, and branched double layer NR−TP film were prepared by SILAR method, respectively. The UV–vis spectra in Figure 11D show that the ZnO/ CdS/CdSe TP film, the ZnO/CdS/CdSe NR−TP film and the branched ZnO/CdS/CdSe NR−TP film have a similar absorption onset at around 720 nm. However, in the visible region from 400 to 700 nm, the absorbance of the ZnO/CdS/CdSe branched NR−TP film is higher than those for the other two films. The enhanced absorbance is attributed to the secondary branching which allows larger QD loading. The IPCE spectra of the three photoanodes in Figure 11E show significant increase in photocurrent density from the ZnO/CdS/CdSe single-layer



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Figure 12. A) SEM images of the ZnO nanowires and ZnO/ZnxCd1–xSe nanocables, B) UV–vis spectra of the ZnO/ZnxCd1–xSe nanocables (solid curves) and the corresponding ZnxCd1–xSe nanotubes (dashed curves), and C) J–V characteristics of the solar cells using ZnO/ZnxCd1–xSe nanocable arrays as photoanodes (measured under AM 1.5G simulated sunlight with an intensity of 100 mW cm−2). Curves of (i) ZnO/ZnSe nanocables, (ii) ZnO/ Zn0.7Cd0.3Se nanocables, (iii) ZnO/Zn0.33Cd0.67Se nanocables, and (iv) ZnO/CdSe nanocables. Inset of Panel B: Photographs of the arrays of ZnO/ ZnxCd1−xSe nanocables. A–C) Reproduced with permission.[184] Copyright 2011, American Chemical Society.

TP film to the double layer NR−TP film, and even further increase for the branched double layer NR−TP film reaching IPCE values of up to ca. 80% at ca. 590 nm. The corresponding J–V curves of cells assembled using the three photoanodes (Figure 11F) show that the branched double layer structure increases JSC significantly suggesting that the double-layer design and branching strategies are effective means for developing high-efficiency SSCs.

5.1.2. Bandgap Engineering of QDs by Composition Tuning Besides the quantum confinement effect for optical tuning, multinary metal chalcogenide alloyed QDs offer an additional possibility for bandgap tuning by composition variations. Bandgap tuning of multinary alloys from composition effects originate from changes in the effective exciton mass that result from the strong dependence of the electronic energies with composition.[231,232] Several ZnO/ZnxCd1–xSe core/shell nanocables with tunable shell composition (0 ≤ x ≤ 1) have been synthesized on fluorine-doped tin oxide (FTO) glass substrates via a simple and facile ion exchange method using ZnO nanowire array as sacrificial templates.[184] The ZnO/ZnSe/CdSe nanocable arrays were prepared by anion exchange of ZnO nanowire with Se2− ions to form ZnO/ZnSe nanocables. This was followed by the partial conversion of ZnSe to CdSe by replacing Zn2+ ions with Cd2+ ions in the ZnSe shells, where the ratio of ZnSe/CdSe in the bilayer shells was controlled by adjusting the reaction temperature of ZnO/ZnSe nanocables with Cd2+ ions. Ternary ZnxCd1−xSe shells were then obtained by annealing 16


the trilayer ZnO/ZnSe/CdSe nanocables. The bilayer ZnSe/ CdSe shells would easily intermix to form a ZnxCd1−xSe alloy upon annealing due to rapid diffusion of the smaller sized cations as compared to the anions. It was reported that the ZnO/ ZnxCd1−xSe nanocables had a rougher surface with increasing Cd content in the shells (Figure 12A). The optical absorption of the ZnO/ZnxCd1−xSe nanocables can then be tuned almost over the entire visible spectrum by changing the composition of the ternary ZnxCd1–xSe shells. It was observed a continuous red shift of the absorption edges of ZnO/ZnxCd1–xSe nanocables from longer wavelength of 535 nm (2.32 eV) for ZnSe, 643 nm (1.93 eV) for Zn0.7Cd0.3Se, and 725 nm (1.71 eV) for Zn0.33Cd0.67Se, to 776 nm (1.60 eV) for ZnSe with increasing Cd content in the ternary ZnxCd1–xSe shells (Figure 12B), demonstrating bandgap tuning by composition variation. While the lattice parameter of the alloys show a linear relationship with the Zn content (x), the corresponding bandgap of the ternary ZnxCd1−xSe shells follow a quadratic dependence. The photovoltaic performance of the ZnO/ZnxCd1–xSe nanocables is shown in Figure 12C. The solar cell based on an array of ZnO/ZnSe nanocables presents a JSC of 2.94 mA cm−2, a VOC of 0.67 V, and a FF of 0.34, yielding a PCE of 0.68% (curve i). A significant increase in JSC was observed with increasing Cd content in the ZnxCd1–xSe shells. The value of JSC drastically increased from 9.54 mA cm−2 for ZnO/Zn0.7Cd0.3Se nanocables (curve ii) to 14.07 mA cm−2 for ZnO/Zn0.33Cd0.67Se nanocables (curve iii), and further to 18.05 mA cm−2 for ZnO/CdSe nanocables (curve iv) while the VOC remained almost constant (ca. 0.65 V). The cell with the ZnO/CdSe nanocable array showed a PCE as high as 4.74%. The performance improvement was demonstrated to be due to the tunable bandgaps of the ZnxCd1–xSe shells, which




5.1.3. Co-sensitization of QDs with Cascade Band Structure To further enhance the light harvesting and electron injection capability of QDs, two different kinds of QDs have been sequentially assembled onto the surfaces of ZnO nanostructures, forming a cascade co-sensitized structure. It has been illustrated that co-sensitized SSCs with suitable energy band alignment can show a superior performance compared to SSCs using single sensitizer.[62,220,233] Among various sensitizers, CdS/CdSe co-sensitizers are the most employed co-sensitized system and shows impressive performance when used with polysulfide electrolyte probably due to appropriate energy band alignment between CdS and CdSe for fast electron injection and favorable optical response over the visible region.[234–236] A series of high performance SSCs co-sensitized with CdS/CdSe based on either TiO2[62–64,234,237] or ZnO[134,140,157,219,220] photoanodes have been reported to achieve PCEs over 4%. For example, Yong et al. employed a novel CdSe/CdS/ZnO nanowire array as the photoanode of SSCs with a Au coated FTO as counter electrode, achieving a PCE as high as 4.15% with a VOC of 627 mV.[220] Recently, CdS/CdSe co-sensitized SSCs based on 20 nm sized ZnO nanoparticles mesoporous film prepared by a simple doctor blade technology has been developed, yielding a PCE as high as 4.463% with a high FF of 0.623 and VOC of 0.623 V.[140] Cheng et al. reported high-efficiency cascade CdS/CdSe SSCs based on hierarchical tetrapod-like ZnO nanoparticles.[134] The ZnO mesoporous films were prepared by screen-printing ZnO nano-tetrapods on FTO substrates. CdS/CdSe-sensitized ZnO photoelectrodes were then prepared by sequentially coating with CdS QDs by SILAR and CdSe QDs by CBD on the ZnO films (Figure 13A). Compared with the as-prepared ZnO nano-tetrapods with an absorption peak at 368 nm, the coating of CdS shell and CdSe shifted the optical absorption edge to about 500 and 670 nm, respectively (Figure 13B). The bandgaps of CdS and CdSe were estimated to be 2.48 and 1.85 eV, respectively. These values are slightly higher than the reported bandgaps for bulk CdS (2.4 eV) and CdSe (1.7 eV), due to quantum confinement effect. In comparison with the ZnO/CdSe photoanode, the ZnO/CdS/CdSe photoanode shows similar absorption edge, but slightly higher absorbance in the visible light region. Photographs of the corresponding samples (inset in Figure 13B) show color change from gray for the bare ZnO electrode, to orange after deposition of a CdS shell by the SILAR process, and eventually to dark brown after the deposition of CdSe QDs by CBD. The IPCE spectra of the solar cells based on the four photoanodes described above are shown in Figure 13C. The observed IPCE results are consistent with the corresponding absorption spectra. The ZnO/CdSe photoanode shows a wider IPCE spectrum with higher efficiency than that of the ZnO/CdS photoanode due to its better light harvesting, and even higher values are obtained when CdSe QDs are deposited on a CdS-coated ZnO to form the ZnO/CdS/CdSe photoelectrode. This result suggests that the CdS interlayer promotes the charge transport from CdSe to ZnO, yielding a maximum IPCE close to 80%.


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resulted in broader ranges of light absorption with increasing Cd content in the ZnO/ZnxCd1–xSe nanocables.

Figure 13D depicts J–V characteristics for the SSCs assembled from various photoanodes with a thickness of ca. 14 µm under 1 Sun illumination. The CdS-sensitized solar cell was characterized with a JSC of 1.4 mA cm−2, a VOC of 0.229 V, a FF of 0.238, with a PCE of 0.078%, presenting much better performance than the cell based on bare ZnO photoanode. The device based on a CdSe-sensitized ZnO photoanode with a 2 h CBD process delivered a much higher JSC of 8.94 mA cm−2, a VOC of 0.287 V, a FF of 0.203, and a PCE of 0.52%. The enhanced JSC and PCE are owing to the broadened light absorption range of ZnO/ CdSe photoanode with respect to that of the ZnO/CdS photoanode. When CdSe QDs were coated on the CdS-sensitized ZnO photoanode to form a ZnO/CdS/CdSe photoanode, the photovoltaic performance was significantly enhanced. The cell of ZnO/CdS/CdSe photoanode prepared with a 1 h CdSe CBD process presented a JSC of 9.43 mA cm−2, a VOC of 0.685 V, a FF of 0.397, giving a PCE of 2.56%. By carefully modulating the CdSe CBD deposition time, the best photovoltaic performance could be achieved when ZnO/CdS/CdSe photoanode prepared by depositing CdSe QDs with a 2 h CBD process, showing a JSC of 13.85 mA cm−2, a FF of 0.424, a VOC as high as 0.722 V, a maximum PCE of 4.24%. Figure 13E shows the time-resolved photoluminescence emission decay of the CdSe QDs grown on bare glass, bare ZnO photoelectrode, and the CdS coated ZnO photoelectrode, respectively. A decrease of the radiative decay time from 2.84 ns for SiO2/CdSe to 1.98 ns for on the ZnO/ CdSe and further to 1.53 ns for ZnO/CdS/CdSe indicates a faster electron injection from CdSe QDs into ZnO after coating with a CdS interlayer. The electron-transfer rates for CdSe QDs anchored onto bare ZnO and CdS-sensitized ZnO were estimated to be 3.02 × 108 s−1 and 1.52 × 108 s−1, respectively. It should be noted that the relative band edges of bulk CdS and CdSe typically show a type-I band structure. However, when CdS and CdSe are brought into contact in a polysulfide electrolyte, a cascade band structure is obtained because the band edges are rearranged due to Fermi level alignment by electron transfer from CdS to CdSe.[62,234–236,238] Furthermore, the polysulfide electrolyte shifts the conduction band energies of CdSe QDs toward negative potentials (vs NHE).[40] Thus, the ZnO/CdS/CdSe system can have a stepwise type-II band structure as shown in Figure 13A. In this system, CdS acts mainly as a buffer layer, assisting electron injection from CdSe to the ZnO photoanode by providing a cascading energy ladder. The cascade co-sensitized SSCs manifested good electron transfer dynamics and overall power conversion efficiency. Furthermore, the CdS interlayer also behaves as an effective passivation layer to suppress the recombination of the injected electrons with the redox electrolyte and holes in CdSe QDs.[220,238] The incorporation of CdS interlayer not only prolongs the electron lifetime but also increases electron-transfer rate constant leading to much enhanced performance.

5.1.4. Energy-Band Alignment of QDs by Quantum Confinement A novel feature of QDs that open an additional way for optimizing its optical and electronic properties is the quantum confinement effect. When the dimension of a QD is comparable to or smaller than the size of its exciton Bohr radius, its



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Figure 13. A) Schematic illustration of the CdS/CdSe-cosensitized ZnO nano-tetrapod photoelectrode with cascade band structure and the possible electron transport pathway. B) UV–vis absorption spectra of ca. 2 µm thick films made of (a) a bare ZnO electrode, (b) a CdS-sensitized ZnO electrode, (c) a CdSe-sensitized ZnO electrode, and (d) a CdSe/CdS-sensitized ZnO electrode. Insets are photographs of the corresponding samples. C) IPCE spectra of various SSCs composed of various photoanodes (ca. 14 µm thick). D) J–V characteristics for SSCs assembled with various photoanodes (ca. 14 µm thick) under AM 1.5 illumination at intensity of 100 mW cm−2). E) Emission (at 655 nm) decay of CdSe QDs deposited on glasses, bare ZnO photoelectrodes and CdS-sensitized ZnO photoelectrodes. The excitation wavelength is 532 nm. A–E) Reproduced with permission.[134] Copyright 2012, The Royal Society of Chemistry.

energy levels will be shifted resulting in a widened bandgap. In a MO/QDs system, the energy difference between their conduction band levels has significant influences on the efficiency of electron transfer across their interface.[77,239] One can thus optimize the electron transfer via adjusting the energy level offset through controlling the size of the QDs.[240–242] Furthermore, through the same effect, absorption spectra of QDs can also be finely tuned such that they have maximum overlap with the solar spectrum for better light harvesting. Size-dependent bandgap of CdSe QDs has been revealed in many reports.[240–243] The widened bandgaps are mainly due to

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the up-shifted CBM of the QDs, while the VBM of the QDs are nearly independent of their sizes,[77,243] as shown in Figure 14A. The up-shifted CBM contributes to enhancing the driving force for electron-transfer from CdSe QDs to ZnO semiconductor. This phenomenon was highlighted in the time-resolved transient absorption (TA) kinetic decay spectra of the arrays of CdSe-sensitized ZnO nanowires with different QD sizes, revealing a much faster TA decay of the ZnO nanowires decorated with smaller size CdSe QDs (Figure 14B).[242] The electron-transfer rate constants showed a significant increase with decreased QD size, leading to an enhanced driving force for electron transfer. The same result




REVIEW Figure 14. A) Diagram of the relative electronic energy differences between CdSe donating species and MO accepting species. A) Reproduced with permission.[77] Copyright 2011, the National Academy of Sciences. B) (Left) Normalized TA kinetics of pure QDs (dotted lines) and sensitized ZnO NWs (solid lines) with different QD sizes. (Top right) Scheme of reversible initial electron transfer from CdSe QD (1) to ZnO nanowire (2). (Bottom right) Dependence of fast TA decay rate (solid squares) described by Marcus theory (gray line). B) Reproduced with permission.[242] Copyright 2012, American Chemical Society.

has also been observed in TiO2 photoanodes sensitized with CdSe QDs of different sizes.[240,241] QDs with a decreasing size show negatively shifted (on an NHE scale) conduction band edge but a widened bandgap; for this reason, the ZnO/QDs system faces a dilemma regarding optical-absorption range and charge-injection-rate optimization. As a consequence, understanding and tailoring the energy band level of QDs is critical for optimizing the electron transfer and light harvesting simultaneously.

5.2. Improvement of Photovoltaic Performance (VOC and FF) by Surface Passivation Both VOC and FF of SSCs are strongly influenced by the recombination loss of electrons at the ZnO/QDs/electrolyte interface.


Low VOC (typically < 0.75 V) and FF (typically < 0.5) are two main factors limiting the PCE of SSCs. It has been recognized that charge recombination by back electrons transfer through the ZnO/QDs/electrolyte interface is a main issue to deteriorate the VOC and FF of SSCs. Trapping states at the surfaces of ZnO and QDs within the photoanode often serve as recombination centers. In addition, poor chemical stability of ZnO makes it easy to react with the electrolyte and decreases the performance of the SSCs. To suppress the surface trap states and surface recombination, surface passivation by coating the photoanode with a thin layer of wide band-gap material, such as ZnSe,[183,218] TiO2,[105,219,225,244] Al2O3,[171,245–247] ZnS,[248–250] and CdS,[251] has been investigated. The coating is typically chemical stable in the electrolyte, and also has a more negative conduction band edge than that of



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Figure 15. A) Cross sectional SEM image of a trilayered ZnO/ZnSe/CdSe nanocable array. B) High-resolution TEM image of a trilayered ZnO/ZnSe/ CdSe nanocable. C) UV–vis absorption spectra of the arrays of (i) bare ZnO nanowires; (ii) ZnO/ZnSe nanocables; (iii) ZnO/ZnSe/CdSe (55 °C) nanocables; and (iv) ZnO/ZnSe/CdSe (90 °C) nanocables. Insets are photographs of the corresponding samples. D) Schematic illustration of the cell configuration based on the trilayered ZnO/ZnSe/CdSe nanocables. E) Current density–voltage (J–V) characteristics under illumination of the SSCs based on (a) the array of ZnO/ZnSe/CdSe (55 °C) nanocables with a Cu2S counter-electrode, (b) the array of ZnO/ZnSe/CdSe (55 °C) nanocables with a Pt/FTO counter-electrode, (c) the array of ZnO/ZnSe/CdSe (90 °C) nanocables with a Cu2S counter-electrode, and (d) the array of ZnO/ZnSe/CdSe (90 °C) nanocables with a Pt/FTO counter-electrode. A–E) Reproduced with permission.[183] Copyright 2012, The Royal Society of Chemistry.

ZnO or creates a dipole at the interface to upshift the band edge so as to block trap states and suppress surface recombination, leading to a drastic enhancement of photovoltaic performance. The coating layer acts as surface energy barrier or tunneling layer depending on the energy level of the coating materials and/or passivated recombination sites on the photoanode’s surface to suppress ZnO/QDs/electrolyte interfacial recombination. 5.2.1. Surface Passivation of Photoanode by ZnSe ZnSe with a bulk bandgap of 2.7 eV has a much higher conduction band edge than those of ZnO and CdSe QD. These suggest that upon forming a heterojunction with ZnO, electrons from ZnSe can be more favorably transferred to the lower conduction band of ZnO. The redistribution of electrons in a ZnO/ ZnSe type-II heterojunction via Fermi level alignment would induce significant upward shift of the conduction band edges for ZnO, which will be beneficial for VOC enhancement. ZnSe 20


shell with a high conduction band edge can serve as blocking layer and energy barrier to shield the ZnO core from the outer CdSe QDs and the electrolyte, and provides physical separation of the injected electrons from the CB of ZnO from the positively charged CdSe nanoparticles and the redox electrolyte, thereby retarding their interfacial recombination rate. Xu et al. has reported ZnSe passivated ZnO to pursue large VOC for high efficiency CdSe-based SSCs.[183] Arrays of trilayered ZnO/ZnSe/CdSe nanocables (Figure 15A and B) were prepared by surface selenization of ZnO nanowires with Se2− ions to form ZnO/ZnSe nanocables, followed by partial conversion of the as-obtained ZnSe shell to CdSe through cation replacement of Zn2+ by Cd2+. Extended absorption over the visible light region was demonstrated upon formation of nanocables as shown in Figure 15C. While the bare ZnO nanowire array absorbed only over the UV region at a wavelength shorter than 390 nm, the absorption edges of ZnO/ZnSe nanocables, ZnO/ ZnSe/CdSe nanocables could be red shifted up to 695 nm (1.78 eV). The content of CdSe and its diameter were adjusted




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by controlling the reaction temperature. CdSe crystallites with larger sizes in the nanocables prepared at a higher temperature had an absorption edge of longer wavelength. Arrays of trilayered ZnO/ZnSe/CdSe core/shell nanocables grown on FTO glass substrates were used as efficient photoelectrodes for SSCs as shown in Figure 15D. Figure 15E shows the current density– voltage (J–V) characteristics of the cells. A cell based on ZnO/ ZnSe/CdSe (55 °C) nanocables with a thicker ZnSe shell gave a JSC of 11.96 mA cm−2, a FF of 0.45 and a VOC as high as 0.836 V, yielding a PCE of 4.54% by using a CE of nanostructured Cu2S on brass substrate under AM 1.5G illumination with an intensity of 100 mW cm−2. Notably, a record VOC up to 0.855 V was achieved for the same cell using a typical platinized FTO (Pt/ FTO) CE (curve b in Figure 15E). The photoanode of ZnO/ZnSe/ CdSe (90 °C) nanocable array with thinner ZnSe shells but thicker CdSe shells showed an improved JSC of 13.57 mA cm−2, but a lower VOC of 0.652 V and a FF of 0.40. Due to the larger CdSe crystallite size in the ZnO/ZnSe/CdSe (90 °C) nanocables, the corresponding cell shows enhanced light absorption and JSC. The lower VOC and FF were believed to be related

mainly with the poor passivation effect of thin ZnSe shells, which consequently leaded to a lower PCE of 3.56%. Enhancement of photovoltaic performance by introducing a ZnSe passivation layer at the photoanode/electrolyte interfaces has also been demonstrated. Yang’s group have prepared ZnO nano-tetrapods with diameters of 50−200 nm and lengths of 400−1000 nm, which were then coated with sensitizers to form ZnO/ZnSe/CdSe and ZnO/ZnSe/CdSe/ZnSe nanostructures (Figure 16A).[218] The J–V characteristics of the two cells are shown in Figure 16B. The ZnO/ZnSe/CdSe nano-tetrapod solar cell delivers a JSC of 15.2 mA cm−2, a VOC of 0.703 V, and a FF of 37.4%, yielding a PCE of 4.02% using the graphene oxide (GO)/Cu2S CE and polyethylene glycol (PEG) 2 000 000 PEG gelled polysulfide electrolyte. Notably, after coating an additional layer of ZnSe to form the ZnO/ZnSe/CdSe/ZnSe structure, the corresponding cell shows a JSC of 17.3 mA cm−2, VOC of 0.761 V, and FF of 47.1%, leading to a record PCE of 6.20%. Figure 16C shows that after the outermost ZnSe coating transformed the ZnSe/CdSe heterojunction (HJ) to a ZnSe/CdSe/ ZnSe quantum well (QW), the IPCE of the ZnO/QW is higher

Figure 16. A) Schematic diagram of the formation process of the ZnSe/CdSe/ZnSe QW sensitizer: (i) Place the photoanode in freshly prepared NaHSe; (ii) dip in Cd2+ and NaHSe solution successively for four cycles; (iii) dip in Zn2+ and NaHSe solution successively for two cycles. B) The J–V characteristics of the cells based on the ZnO/QW and ZnO/HJ solar cells. C) IPCE curves of the cells based on the ZnO/QW and ZnO/HJ solar cells: pink hatched region indicates the photoconversion improvement from ZnO/HJ to ZnO/QW. D) Nyquist plots of ZnO/HJ and ZnO/QW solar cells at 0.8 V forward bias. E) Schematic illustration of the two channel transport model and the proposed energy band diagram of ZnO/HJ and ZnO/QW based on the measured Fermi level (J2 of HJ flows along the surface whereas J2 of QW flows along the middle trench). A–E)Reproduced with permission.[218] Copyright 2013, American Chemical Society.




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than that of the ZnO/HJ for wavelengths >500 nm (highlighted by the hatched pink area), while the UV–vis absorption edge of ZnO/QW is blue-shifted (ca. 15 nm) from 729 to 714 nm. This result reveals that the outmost ZnSe shell contributes to electron collection enhancement and/or electron injection from the CdSe shell particularly at long wavelengths. Figure 16D shows EIS results of the ZnO/QW and the ZnO/ HJ solar cells at 0.8 V in dark. It shows a much smaller electron transport resistance Rt for ZnO/QW (4.2 ± 0.2 Ω) than that for ZnO/HJ (18.5 ± 0.5 Ω), demonstrating more efficient electron transport in ZnO/QW. Furthermore, the recombination resistance (Rr) in the ZnO/QW cell (105 ± 3 Ω) is two times higher than that in the ZnO/QW cell (51 ± 2 Ω). This suggests that the outermost ZnSe coating serves as an effective surface energy barrier to reduce photoanode/electrolyte interfacial recombination for performance enhancement. Because of its smaller transport resistance and larger recombination resistance, the ZnO/QW solar cell has a much larger electron diffusion length (125.0 µm) than that of the ZnO/HJ cell (41.5 µm). The Fermi level of the electrodes, as measured with ultraviolet photoelectron spectroscopy (UPS), are −4.38, −3.83, −4.13 and −3.86 eV, respectively, for ZnO, ZnO/ZnSe, ZnO/ZnSe/ CdSe and ZnO/ZnSe/CdSe/ZnSe (Figure 16E). The uplifting of the Fermi level from −4.13 eV of ZnO/HJ to −3.86 eV of ZnO/ QW also supported the enhanced VOC. Since EF and ECB have a relationship as: E CB = E F −

kBT ⎛ C μkBT ⎞ ln ⎜ ⎝ e 2 ⎟⎠ α

(10)

where kB is Boltzmann’s constant, α is the electron-state distribution parameter, T is the temperature, and Cµ is the chemical capacitance. Given the similar Cµ values extracted from EIS, (0.50 ± 0.01 mF for ZnO/HJ, 0.58 ± 0.01 mF for ZnO/QW), it follows that ECB is only determined by EF. Therefore, it was estimated that ECB of ZnO/QW was 0.27 eV higher than that of ZnO/HJ. These results further confirmed that coating of ZnSe raised the CB of the ZnO photoanode via Fermi level re-alignment and thus enhance the VOC. It was proposed that the electron can be transported via two channels (Figure 16E): the CdSe channel plus the conventional ZnO channel. “J1” and “J2” in the figure represent, respectively, the photocurrents transported along the ZnO and the HJ and QW. Since electrons tend to transfer to the outermost CdSe layer in the HJ, due to the lower CdSe CB edge, compared to that of ZnSe, electron transport along the outer surface of the HJ can be lost to trap states due to surface defects or electrolyte redox couples. In contrast, electron transport in the potential well (ZnSe/CdSe/ZnSe) benefits from a smaller transport resistance and a larger recombination resistance resulted from the energy barrier formed by ZnSe. This yields a more efficient charge collection with larger JSC and VOC.

5.2.2. Surface Passivation of Photoanode by TiO2 TiO2 has similar VB and CB energy levels as those of ZnO, but better chemical stability, and is widely used as electron acceptor in SSCs. It can also act as an effective passivation material in

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ZnO-based photoanodes. Unlike the ZnSe case,[183,218] using a TiO2 shell onto ZnO cannot form an intrinsic surface energy barrier due to the similar energy levels of TiO2 and ZnO. However, it is possible to form a n-n+ heterojunction at the ZnO/ TiO2 interface due to differences in their electron concentrations (1018 cm−3 in ZnO vs 1010 cm−3 in TiO2).[252] This heterojunction induces a built-in voltage: V=

kT ⎛ N d+ ⎞ ln ⎜ ⎝ N d ⎟⎠ q

(11)

where k is the Boltzmann’s constant, T is the temperature, q is the electron charge, Nd+ and Nd are the electron concentrations in the ZnO core and the TiO2 shell. The built-in voltage gives rise to a strong electric field against electron flow from ZnO to TiO2. This enhances the VOC by reducing recombination. Tian and Cao et al. have investigated the effects of TiO2 passivation on ZnO based SSCs.[219] A facile passivation strategy for a ZnO nanoparticle mesoporous photoelectrode was reported. Formation of a thin TiO2 passivation layer on the surface of the ZnO nanoparticles was completed by immersing the ZnO mesoporous film in an aqueous solution containing H3BO3 and (NH4)2TiF6 at room temperature, followed by annealing. While the ZnO nanoparticles were etched in the mixed solution, TiO2 particles deposited on the fresh surface and combined with the newly broken chemical bonds to form an effective passivation layer. The simultaneous deposition of TiO2 nanoparticles not only changed the surface chemistry of the photoelectrode to favor high loading of QDs, but also functioned as an energy barrier layer to suppress surface charge recombination. There was an increase in the specific surface area and the mesopore volume from 57.7 m2 g−1 and 0.342 cm3 g−1 for the unpassivated film to 68.6 m2 g−1 and 0.401 cm3 g−1 for the passivated one, respectively. However, the average mesopore diameter of the passivated film (29.7 nm) was slightly smaller than that of the unpassivated film (31.3 nm). The photovoltaic performance of CdS/CdSe co-sensitized solar cells assembled from unpassivated ZnO nanoparticle mesoporous film and TiO2 passivated ZnO nanoparticle mesoporous film are reproduced in Figure 17A. The solar cell assembled from unpassivated film using polysulfide electrolyte shows a JSC of 11.61 mA cm−2, a VOC of 0.57V, and a FF of 0.36, yielding a PCE of 2.38%. However, the cell with TiO2 passivation delivers a JSC of 15.42 mA cm−2, a VOC of 0.62 V, and a FF of 0.49, leading to a PCE as high as 4.68%. Measurements of dark current under positive bias, where electrons flow from the photoanode into the electrolyte, have been used to provide information about the electron transfer process. While the dark current itself cannot be directly related to recombination because of potential differences in electrolyte concentration and potential distribution in the photoanode in the dark and under illumination, dark current measurement of SSCs can be used to interpret the extent of back electron transfer. Figure 17B shows the J–V characteristic curves reported for the two SSCs assembled with ZnO mesoporous photoanode with and without TiO2 passivation under dark. Under the same positive potential bias, the dark current for the TiO2 passivated photoanode was much smaller than that in the unpassivated photoanode, indicating smaller back electron




REVIEW Figure 17. A) J–V curves of SSCs under illumination. B) J–V curves of SSCs under dark condition. C) Nyquist plot curves of SSCs under forward bias (−0.6 V) and dark conditions. D) Bode plot curves of SSCs under forward bias (−0.6 V) and dark conditions. E) Schemes of the energy band structure of ZnO/TiO2/CdS/CdSe. F) Schematic illustration of charge recombination pathways in the SSC. A–F) Reproduced with permission.[219] Copyright 2013, The Royal Society of Chemistry.

transfer and lower charge recombination from the conduction band of ZnO to the redox couple (S2−/Sn2−) in the electrolyte. The electrochemical impedance spectroscopy (EIS) results of the SSCs measured under dark with a forward bias of −0.6 V is reproduced in Figure 17C. The semicircle represents the back electron transfer at the photoelectrode/QDs/electrolyte interface and transport in the photoelectrode (Rct). It was observed that the Rct increases from 131.6 to 470.3 Ω upon


TiO2 passivation. The increase of Rct may also suggest that the surface defects of ZnO are reduced by the TiO2 passivation process, which accounted for obvious enhancement of VOC and FF. Figure 17D shows Bode plots of the photoelectrodes with and without TiO2 passivation. The peaks of the spectra can be used to determine the electron lifetime in the ZnO according to τ = 1 , resulting in estimated electron lifetime in the pas2π f sivated photoelectrode device of ca. 317.9 ms, which was much n

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longer than that of the unpassivated ZnO (50.4 ms). These results clearly demonstrate that the TiO2 passivating layer can enhance the charge recombination resistance and thus increase the electron lifetime in the device. Furthermore, the passivated SSC was also shown to have much better chemical stability. Figure 17E shows values of the conduction bands (CB): CdSe > CdS > ZnO > TiO2. The CdS/CdSe sensitizers have a type-II band alignment in the SSCs with polysulfide electrolyte. Under the operating conditions, photons are captured by the QDs, yielding electron–hole pairs that are rapidly separated. The electrons first inject into the CB of TiO2 and then transfer into the ZnO CB. The holes in the QDs are reduced by redox couples (S2−/Sn2−) in the electrolyte. Finally it is noted that, in addition to the previously discussed passivation effects, the TiO2 coated ZnO mesoporous film has a much higher BET surface area and pore volume than those of the unpassivated ZnO mesoporous film. This allows the TiO2 coated ZnO mesoporous film to load more QDs and leads to a higher JSC. On the other hand, the thin TiO2 passivation layer serves as an energy barrier suppressing back electron recombination (as shown in Figure 17F) that prevents back electrons transfer from the ZnO to the electrolyte (Process C) and from the ZnO to the QDs (Process D). The reduced charge recombination rate results in larger recombination resistance, prolonged electron lifetime, and enhanced FF and VOC. Furthermore, the SSCs also showed good stability in ambient conditions after 1 week measurement. Tian and Cao et al. have further shown that the TiO2 passivation layer can also be applied to SSCs with ZnO nanorods of different morphologies with good results.[105,225] On the other hand, Que et al. also reported dramatic enhancement of photovoltaic performance by introducing a TiO2 layer as energy barrier on the surface of ZnO nanowire arrays for CdS sensitized solar cells.[244]

5.2.3. Surface Passivation of Photoanode by Al2O3 Insulating metal oxide Al2O3 has also been used as a surface passivation material to enhance the performance of ZnO based SSCs. Luan et al. reported that the performance (VOC, FF, PCE) of CdSe QDs sensitized ZnO solar cells can be improved by using a ultrathin Al2O3 layer (ca. 2 nm) grown by atomic layer deposition (ALD) on ZnO nanorods before CdSe QDs sensitization.[171] The use of Al2O3 shell resulted in an obvious enhancement of FF from 0.24 to 0.55 and a slight increase of VOC from 0.65 to 0.66 V, and but a decrease of JSC from 2.94 to 2.72 mA cm−2. The significantly improved FF resulted in about 50% enhancement of PCE from 0.46 to 0.99%. The increases in VOC and FF of Al2O3 passivated SSCs were attributed to lower electron recombination at the photoanode/electrolyte interface due to passivation of ZnO recombination sites by the Al2O3 coating. However, due to its insulating nature, the Al2O3 impose an additional barrier for electron injection from the CdSe QDs to the ZnO nanorods and thus slightly reduces the JSC. They also investigated that the effects of an ultrathin Al2O3 passivation layer (ca. 2 nm) on the performance of the CdS nanorods sensitized ZnO nanowires solar cells.[245] The Al2O3 passivated solar cell showed a 51% increase in VOC (from 0.43 to 0.65 V) and a 42% increase in JSC (from 4.03 to 5.63 mA cm−2), consequently 24


resulting in a ca. 50% increase in PCE from 0.55% to 1.15%. It is noted that JSC in the passivated cell is actually larger. This indicates that the benefits of reducing recombination can outbalance the penalty caused by an increase in serial resistance due to the Al2O3 layer.

5.2.4. Surface Passivation of Photoanode by ZnS As a wide bandgap semiconductor ZnS is commonly used for QDs surface passivation in SSCs since its CB edge potential is more negative than those of QDs. It is usually coated onto the photoanode surface by a SILAR method. Many groups have found that ZnS coating significantly improves the performance of SSCs especially its JSC, VOC, and stability.[248–250,253–256] The beneficial roles of ZnS are generally attributed to: i) surface states passivation of QDs by suppressing surface trapping of photoexcited carriers in the QDs; ii) barrier layer formation preventing back electron transfer from QDs to the electrolyte to inhibit interfacial charge recombination; and iii) formation of an effective tunneling channel for hole transfers into the electrolyte. For example, after coating a ZnS passivation layer onto the surface of CdS QDs, the VOC, JSC and FF of the CdS-sensitized ZnO/Zn2SnO4 core/shell nanocable array solar cells were found to improve from 0.74 to 0.76 V, 3.44 to 3.68 mA cm−2 and 43.65% to 44.35%, respectively, thus resulting in an increase in PEC from 1.10% to 1.24%.[250] The thickness of the ZnS passivation layer typically plays an important role for the photovoltaic performance; Toyoda et al. found that efficiency of PbS-sensitized TiO2 solar cell was related to the thickness of the ZnS layer by changing the SILAR cycle number.[255] The PCE first showed an increase followed by a decrease with the increasing number of SILAR cycles. Similar phenomenon was also found in ZnS passivated ZnO-based SSCs. Sun et al. reported that the solar cell of CdS-sensitized ZnO nanorod array showed low performance with a JSC of 1.23 mA cm−2, a VOC of 0.52, a FF of 0.29, and a PCE of 0.19%. Upon the deposition of a 7.5 nm thick ZnS layer on the ZnO nanorod surface using 10 SILAR cycles, PCE was increased to 0.62% with a JSC of 3.59 mA cm−2, a VOC of 0.59 V, and a FF of 0.37.[248] Similarly, Wang et al. found increased photoanode saturated photocurrent density from 6.5 mA cm−2 for ZnO/CdTe nanocables to maximum 13.8 mA cm−2 for ZnO/CdTe/ZnS nanocables after coating with an ultrathin layer of ZnS (ca. 2 nm) by 10 SILAR cycles.[194] Optimum photocurrent enhancement of the ZnO/ CdTe photoanode was achieved by the ZnS passivation with thickness between 2 and 5 nm.

5.3. Effects of Counter Electrode in SSCs The counter electrode (CE) of a SSC plays a major role in the regeneration of the oxidized species in the electrolyte to their reduced state by transferring electrons from the external circuit to the electrolyte. The development of improved CE materials with good conductivities, large effective surface areas, and high catalytic activities is important for pursuing high performance SSCs. Recently reported CE materials include noble metals (Pt, Au),[62,134,176,220,224,257] carbon based materials,[223] binary metal





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(Cu, Co, Pb) sulfides,[183,184,219,258–263] multinary metal sulfides (Cu2ZnSnS4, CuInS2, Cu2SnS3),[221,223,264–267] and carbon materials – metal sulfides composite materials.[268–271] While platinized CEs have high electrocatalytic activity in DSSCs using I−/I3− redox couple, their performance in SSCs using polysulfide electrolyte is less satisfactory. In particular, it has been reported that Pt CE is not active for S2−/Sn2− couple regeneration.[257] This is mainly caused by poisoning of the Pt electrode due to sulfur compounds adsorption resulting in a larger Rct and consequently poor electrocatalytic activity. Au CE has been reported to have much better performance in SSCs using polysulfide electrolyte.[62,220] For example, Yong et al. reported efficient SSCs using a CdSe/CdS/ZnO NW array as a photoanode and a Au-based CE with a maximum PCE of 4.15%.[220] However, both Pt and Au are rare and high-cost materials and exploration for low-cost and noble metal-free CE (such as metal sulfides and carbon-based materials.) with high electrocatalytic activity is highly desirable. Due to their excellent electron transport properties, carbon materials (graphene, reduced graphene oxide, and carbon nanotubes, etc.) are promising CE materials in SSCs. In addition to Pt and Au CEs, Yong et al. have also examined the photovoltaic properties of SSCs using mesocellular carbon foam (MSU-F-C) CE.[223] While nearly the same VOC (ca. 0.68 V) were obtained for various cells, both the FF and JSC were, respectively, significant increased from 0.25 and 9.9 mA cm−2 for Pt, 0.39 and 11.8 mA cm−2 for Au, and to 0.42 and 12.6 mA cm−2 for MSU-F-C. The cell of CdS/CdSe co-sensitized ZnO nanowire anode using MSU-F-C CE yielded the highest PCE of 3.60%. The MSU-F-C CE has an extremely high surface area (ca. 900 m2 g−1) as well as ordered and interconnected pores thus facilitating diffusion of redox relay in the electrolyte and providing more active sites for redox species reduction. Furthermore, the MSU-F-C CE exhibits good durability in the polysulfide electrolyte as well as remarkable stability for a variety of solvents used in the electrolyte solution.[223] Binary metal sulfides, such as Cu2S, CoS and PbS, have been demonstrated as promising CEs and exhibit superior activities in polysulfide electrolyte. These metal sulfides can be easily prepared by exposing metal foils of Cu, Co, or Pb to a sulfide solution. As presented in Table 2, the large majority of highly-efficient SSCs reported use Cu2S-based CE due to its outstanding catalytic activity and good electrical properties. For example, curves a and b in Figure 15E show the J–V characteristics of solar cells using the same ZnO/ZnSe/CdSe nanocable photoanode and counter electrodes of Pt-FTO and Cu2S-brass, respectively. After replacing Pt-FTO counter electrode with Cu2S-brass, the cell showed obvious increases in FF from 0.32 to 0.45, and JSC from 10.79 to 11.96 mA cm−2, resulting in significant enhancement of PCE from 2.93% to 4.54%.[183] However, one of the key issues in SSCs is the CE’s stability; in particular, brass based Cu2S CE suffers from continual corrosion and ultimately mechanical instability, which also consumed and contaminated the polysulfide electrolyte.[268] On other hand, copper sulfides in the polysulfide electrolyte can undergo phase transformation under long-term illumination. Copper sulfide (CuxS) can exist in several stoichiometries and crystallographic structures and the resulting complex structures and valence states (Cu+ and Cu2+) result in large electrocatalytic activity variations upon regeneration of the polysulfide electrolyte.

Incorporation of alloying elements into copper chalcogenides to form multinary semiconductors is a promising approach to improve their electrocatalytic and electrical properties. Copper-based multinary materials such as Cu–In–Ga–S– Se and Cu–Zn–Sn–S–Se are themselves excellent photovoltaic materials. Some recent reports also demonstrated their potentials for applications as effective CE materials in SSCs. For example, Cu1.8S and Cu2SnS3 (CTS) hierarchical microspheres have been synthesized (Figure 18A and B), and used as efficient CE materials for ZnO-based SSCs.[221] Figure 18C shows J–V characteristic of three cells using ZnO/ZnSe/CdSe nanocable photoanode different CEs. The solar cell with a CTS/FTO CE achieved a PCE of 4.06% with a JSC of 11.46 mA cm−2 and a FF of 0.437, which were much better than the performances of the Cu1.8S/FTO based cell with a PCE of 3.65%, a JSC of 10.51 mA cm−2 and a FF of 0.423. Nyquist plots of the Cu1.8S/ FTO–Cu1.8S/FTO and the CTS/FTO–CTS/FTO symmetric cells containing polysulfide redox electrolyte show that the Rct decreases from 11.4 Ω cm2 for the Cu1.8S/FTO counter electrode to 6.2 Ω cm2 for the CTS/FTO counter electrode (Figure 18D). Quaternary Cu2ZnSnS4 (CZTS) hierarchical microspheres (ca. 2 µm in diameter) have also shown to be effective counter electrode materials in SSCs.[222] The photovoltaic performance of ZnO/ZnSe/CdSe nanocables SSCs using various CEs were examined as shown in Figure 18E. The device with a bare FTO glass CE gave a JSC of 3.46 mA cm−2, a VOC of 0.745 V, and a FF of 0.128, yielding a PCE of 0.33%. The cell using a Pt/ FTO CE acquired a JSC of 9.11 mA cm−2, a VOC of 0.816 V, a FF of 0.305, and a PCE of 2.27%. Whereas for the case of CZTS microspheres coated FTO (CZTS/FTO) CE, the photovoltaic performance of the cell, using the same photoanode, improves further. The JSC, VOC, and FF of the cell improve significantly to 11.06 mA cm−2, 0.822 V, and 0.410, respectively, for a PCE ca. 3.73%. It was found that the CZTS microspheres show higher electrocatalytic activity compared to Pt for the reduction of the polysulfide electrolyte. The Rct value decreased from 600 Ω cm2 for the FTO-based cell to 270 Ω cm2 for the Pt/FTO-based cell, and further to 105 Ω cm2 for the CZTS/FTO-based cell. Such results indicate that CZTS microspheres can act as effective CE material in SSCs based on polysulfide electrolyte. On the other hand, carbon materials, such as graphene, reduced graphene oxide (rGO), carbon nanotubes, and carbon black, have been shown to be effective additives to metal chalcogenides (CuxS, CoS, PbS and Cu2ZnSnSe4) for forming high performance composite (e.g., rGO-Cu2S, multiwalled carbon nanotube (MWCNT)-CZTSe, and carbon black-PbS) counter electrodes. For example, the 2D structure of rGO with high surface area scaffold are considered to increase the number of the Cu2S reactive sites.[268] The rGO in composite CE serve as electron transport pathway and shuttles electrons across the 2D structure to the active Cu2S catalyst sites where the electrons are used to reduce the oxidized polysulfide. Yang et al. reported a solar cell employing a ZnO/ZnSe/CdSe/ZnSe photoanode and a GO/Cu2S CE with a FF of 47.1% and a PCE of 6.20%. When Pt CE was used, however, a smaller FF of 39.8% was obtained, leading to a deteriorated PCE of 5.25% due to the increased Rct between CE and the polysulfide electrolyte.[218] Zeng et al. have prepared MWCNTs-CZTSe composite CEs.[269] It was found that blending ratio of MWCNTs and CZTSe in the



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Figure 18. A) SEM image of Cu1.8S microspheres. B) SEM image of Cu2SnS3 microspheres. C) J–V characteristics of SSCs using Cu1.8S/FTO and CTS/ FTO counter electrodes. D) Nyquist plots of Cu1.8S/FTO–Cu1.8S/FTO and CTS/FTO–CTS/FTO symmetric cells containing polysulfide electrolyte. A–D) Reproduced with permission.[221] Copyright 2012, The Royal Society of Chemistry. E) SEM image of Cu2ZnSnS4 microspheres. F) J–V characteristics of SSCs using FTO, Pt/FTO and CZTS/FTO counter electrodes. E,F) Reproduced with permission.[222] Copyright 2012, American Chemical Society.

composite CEs played a critical role in controlling the photovoltaic performance. The composite CE with a weight ratio of 0.1 (MWCNT:CZTSe) shows the best performance, resulting in a smallest Rct of 2.5 Ω, a maximum JSC of 17.04 mA cm−2, a maximum FF of 0.51, and an optimal solar cell efficiency of 4.60%. These parameters are much superior to those of the cells using CEs of the individual constituting components. The higher catalytic activities of the composite CEs were ascribed to the combination of the fast electron transport of the MWCNTs and the high catalytic activity of CZTSe NPs.[269]

5.4. Effects of Electrolyte in SSCs While the iodide/triiodide (I−/I3−) redox has been widely used, the polysulfide (S2−/Sn2−) redox couple has attracted more recent attention for applications in SSCs. The I−/I3− redox 26


couple is an ideal electrolyte for regeneration of the oxidized dye and has beneficial effects on suppressing the recombination of excited electrons with electrolyte in DSSCs. However, it is often corrosive to QDs in SSCs, leading to poor operation performance and stability.[272,273] In comparison with the I−/I3− redox couple, the polysulfide electrode usually gives a much higher JSC, but lower FF and smaller VOC. Lee et al. have reported CdS SSCs using S2−/Sn2− electrolyte with much higher IPCE (as high as 80%) and JSC (6.16 mA cm−2) than those (

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