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Solar Cells
Metal Oxide Semiconductors for Dye- and Quantum-Dot-Sensitized Solar Cells Isabella Concina* and Alberto Vomiero*
From the Contents 1. Introduction .............................................. 2
T
his Review provides a brief summary of the most recent research developments in the synthesis and application of nanostructured metal oxide semiconductors for dye sensitized Metal Oxides Alternative and quantum dot sensitized solar cells. In these devices, the wide to TiO2 ....................................................... 7 bandgap semiconducting oxide acts as the photoanode, which provides the scaffold for light harvesters (either dye molecules Composite Systems ..................................24 or quantum dots) and electron collection. For this reason, proper tailoring of the optical and electronic properties of the Perovskite Solar Cells............................... 26 photoanode can significantly boost the functionalities of the Conclusions and Perspectives...................28 operating device. Optimization of the functional properties relies with modulation of the shape and structure of the photoanode, as well as on application of different materials (TiO2, ZnO, SnO2) and/or composite systems, which allow fine tuning of electronic band structure. This aspect is critical because it determines exciton and charge dynamics in the photoelectrochemical system and is strictly connected to the photoconversion efficiency of the solar cell. The different strategies for increasing light harvesting and charge collection, inhibiting charge losses due to recombination phenomena, are reviewed thoroughly, highlighting the benefits of proper photoanode preparation, and its crucial role in the development of high efficiency dye sensitized and quantum dot sensitized solar cells.
2. TiO2 Photoanodes...................................... 3 3.
4. 5. 6.
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1. Introduction Metal oxide semiconductors have been exploited for decades in advanced application fields like microelectronics, gas sensing, energy conversion, and storage.[1–3] At the basis of their success is the possibility of fine tuning their electronic, optical, and charge transport properties in a broad range of values, targeting their functional features for specific applications. In the past few years, the ability of shaping metal oxide semiconductors at the nanoscale opened up new perspectives for their exploitation, significantly improving the performances of end-user devices.[4,5] One of the most successful applications for metal oxide semiconductors are excitonic solar cells.[6,7] Very recently, a renaissance of this kind of devices occurred,[8] thanks to significant breakthroughs that led to unprecedented photoconversion efficiencies (PCE) in dye-sensitized (DSSCs),[9,10] quantum dot-sensitized (QDSSCs)[11] and perovskite-sensitized[12] solar cells. In this kind of cells, metal oxide semiconductors have a critical importance, since they are one of the main constituents (the anode) of the photoelectrochemical system.[13,14] In several cases proper modulation of the structure and morphology of the oxide led to unprecedented PCE thanks to induced new optical and charge transport properties: it is the case, for instance, of ZnO-based dye sensitized solar cells, in which record PCE (7.5%)[15] was obtained by applying a hierarchically self-assembled thick film in combination with a compact blocking layer or of SnO2 hollow microspheres.[16] Another example relies with application of single crystal structures in dye sensitized solar cells:[13] since several years increased electron transport was claimed to improve PCE in such devices by applying single crystalline one-dimensional nanostructures like nanowires (NWs), nanorods, nanobelts,[17–19] but only recently substantially higher conductivities and electron mobilities than does nanocrystalline TiO2 led to record PCE (7.3%) in all-solidstate perovskite sensitized solar cells. Furthermore, the advent of graphene pursued the research on composite systems based on metal oxide semiconductors and carbon-based nanostructures.[20] The main effect of carbon nanotubes and/ or graphene addiction is to significantly boost the collection of photogenerated charges,[21] leading to increase of shortcircuit photocurrent to more than 20%[22] and opening promising perspectives for further advancement in the field. The present review focuses on progress in application of different metal oxide semiconductors (TiO2, ZnO, SnO2) as photoanodes in excitonic solar cells, with a forward look at the development of high efficiency and low cost photovoltaic devices. The basic processes taking place in an excitonic solar cell are (Figure 1):[23] 1) light absorption and exciton generation; 2) exciton separation and electron injection from the dye to the oxide; (3) Electron transport to the anode; 4) dye regeneration as a consequence of hole reduction by the I3− species; 5) hole transport to the cathode, where a catalytic reaction occurs (typically mediated by the presence of Pt electrode). Other competing processes tend to inhibit fast exciton separation, charge transport and collection, resulting in decrease of photoconversion efficiency; 6) non radiative exciton
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Figure 1. Schematic representation of the operating principle of a DSSC. In blue the processes contributing to light-to-electric power conversion: in red the processes degrading the performances of the device.
recombination before charge separation and injection; 7) charge recombination between electrons at the oxide photoanode and I3− in the electrolyte; 8) fast electron–hole recombination for dye regeneration. As reported in consolidated literature,[9,23–26] processes 2 and 4 are much faster than processes 6 and 8. For this reason, the competing processes regulating the photoconversion efficiency in DSSCs and QDSSCs are mainly charge transport through the photoanode (3) and charge recombination between electron at the oxide photoanode and I3− in the electrolyte (7). As for charge generation, injection, transfer and collection in the previously reported scheme, these processes can take place only under certain conditions of band alignment among the various components of the system. In particular, the position of the LUMO of the dye has to be above the conduction band of the oxide devoted to charge transport in order to make electron injection possible from the excited state of the dye to the oxide. At the same time, favorable band alignment has to be present between the electrolyte and the oxide forming the photoanode, in order to induce hole transport through the electrolyte and to obtain as fast as possible regeneration of the dye/QD. It is clear that proper band alignment requires a potential drop from one stage to the next one, which provides the driving force for the process to occur, but that is detrimental in terms of cell performances, since that potential drop does not reflect at the output of the solar cell. Dr. I. Concina, Dr. A. Vomiero CNR-INO SENSOR Lab Via Branze 45, 25123 Brescia, Italy E-mail: [email protected]; [email protected] Dr. I. Concina University of Brescia Via Valotti 9, 25133, Brescia, Italy Dr. A. Vomiero Luleå University of Technology 971 98, Luleå, Sweden DOI: 10.1002/smll.201402334
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Light absorption is the first phenomenon occurring in a solar cell. Light management is of utmost importance, since maximizing photon to charge conversion is one of the main goals (if not the main) in third generation solar cells. When light interacts with a solar cell, several processes can occur: i) partial reflection at the surface of the glass; ii) absorption of photons from light harvesters; iii) light scattering inside the solar cell; iv) partial transmission of light, if light absorption at the anode and/or in the other parts of the cell is not complete. The main process relying with light absorption is, of course, the absorption of solar radiation by the light harvester contained in the photoanode. The probability of absorption relies with many features: the optical density of the photoanode, the extinction coefficient of the light harvester, the time spent by the light inside the photoanode. Most of these factors are dependent on the wavelength of the incident radiation. For these reasons huge amount of research addressed these three issues in excitonic solar cells: 1. Maximize light absorbance by light harvester in the widest possible wavelength interval; 2. Minimize charge recombination leading to loss of photogenerated charges; 3. Modulate band alignment in order to obtain as fast as possible charge injection processes limiting voltage drop to the minimum necessary to induce charge transfer. Intense research has been carried out to optimize all the aforementioned aspects of the solar cells. In this review we will focus on the various kinds, morphologies, structures and shapes of the oxides forming the photoanode that have been applied to improve the functionalities of excitonic solar cells. Different aspects will be covered, including light management, band alignment and charge transport, i.e., most of the open issues in DSSCs and QDSSCs. Of course the transaction is not exhaustive of the research efforts in these kinds of cells, including also the design of new dyes, and application of different electrolytes and counter-electrodes, which will not be considered here. In particular we will focus on three oxides, i.e. TiO2, ZnO and SnO2 that represent the most widely investigated materials as photoanodes for DSSCs.
2. TiO2 Photoanodes Possibility of using dye molecules for light harvesting came before the pioneering paper of O’Regan and Gratzel in 1991.[27,28] However, the main drawback of the proof of concept was the very limited photoconversion efficiency, mainly due to the poor optical density of the layer, determined by the low specific surface of the investigated materials. The revolutionary concept proposed by O’Regan and Gratzel was to apply a mesoporous film composed of small nanoparticles (NPs) (20 nm in size) as scaffold for light harvesters. The dramatically increased optical density allowed to boost photoconversion efficiency up to 7%.[29] Since then, optimization of TiO2 network occurred,[30] trying to reduce small 2014, DOI: 10.1002/smll.201402334
Isabella Concina is assistant professor in experimental physics at the University of Brescia, Italy. She earned her Ph.D. in chemical sciences in 2006, as well as her degree in chemistry in 2002, from the University of Padova, Italy. Her main research interests focus on the engineering of semiconductor micro- and nano-structures for energy application by applying green chemistry concept for the enhancement of device performances.
Alberto Vomiero is chaired professor in experimental physics at the Luleå University of Technology, Sweden. He was awarded his Ph.D. in electronic engineering from the University of Trento in 2003 and his degree in physics from the University of Padova in 1999. His main interests are in composite nanomaterials (wide bandgap semiconductors, semiconducting nanocrystals, and hybrid systems) for gas sensors and excitonic solar cells. He is Marie Curie International Outgoing Fellow of the European Commission, Fellow of the Institute of Physics (UK) and of the Institute of Nanotechnology (UK), chair of the Italian section of the American Nano Society and member of the Global Young Academy.
charge losses during charge collection and increase light harnessing. Several strategies were proposed for the purpose. One possibility is the treatment of TiO2 surface with TiCl4 during photoanode preparation.[31] In 2007, O’Regan and co-workers demonstrated that the main effect of TiCl4 treatment of TiO2 photoanode is a downward shift in the TiO2 conduction band edge potential and a strong decrease (up to 20-fold) in the electron/electrolyte recombination rate constant (Figure 2). The combination of these two effects is a neat increase in the efficiency of charge separation and the consequent boost of photocurrent. It has to be noted that, in this case, the detrimental loss in terms of Voc of the operating device is compensated by the increased accumulation of electrons due to the reduced recombination rate, which offsets the Voc loss otherwise expected from the conduction band edge shift. Another useful strategy to increase PCE is the application of single crystalline nanowires and nanotubes to fasten charge transport.[33] In mesoporous photoanodes the electrons transport through a slow trap-limited diffusion process:[34] in an anatase TiO2 nanoparticle film, the electron diffusion coefficient is more than two orders of magnitude lower than in single crystals.[35,36] In fact, while offering extremely high specific surface for dye loading, mesoporous networks suffer for high density of grain boundaries, which foster charge recombination in operating devices. Application of one dimensional (1D) nanostructures can offer more straightforward path to photogenerated electrons, minimizing
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Figure 2. Recombination rate constant at Voc vs charge density at Voc for TiO2 NP cells with and without TiCl4 treatment. Reproduced with permission.[32] Copyright 2007, American Chemical Society.
the time inside the photoanode, and then the recombination probability.[17,18,37] Among various 1D nanostructures, both polycrystalline and single crystalline nanowires/nanotubes have been applied. 1D polycrystals in general offer the benefit of straightforward path and an almost mesoporous structure with quite acceptable specific surface. 1D single crystals, instead, typically present much higher electron mobility than polycrystals, thanks to absence of grain boundaries and their single crystalline assembly, but suffer of very poor specific surface, significantly limiting the possibility of dye loading. Application of single-crystal anatase TiO2 nanorod film was reported in 2005 and 2006.[38,39] In the specific case, quite short rods were applied (100–300 nm in length). The authors found that the decreased number of contact barriers between the rods, limiting electron traps, was the main reason for increased PCE compared to standard solar cell based on P25 NPs. Application of nanorods was highly beneficial in increasing the FF of the solar cell for quite thick photoanodes (above 10 µm), where the effect of charge recombination starts limiting cell performance. PCE as high as 7.29% was obtained for the best cell using nanorods, while 4.5% was achieved in P25-based cells. However, in the case of nanorods, the length of fast electron transfer was in the order of hundreds of nanometers (the dimensions of the rod), which is around 50 times lower than the typical thickness of the best photoanodes in DSSCs. The rapid charge transport in DSSCs made of vertically aligned single-crystal rutileTiO2 nanowires was directly measured recently.[40] Quantitative evaluation of charge collection was estimated through the electron diffusion length Ln by applying intensity-modulated photocurrent and photovoltage spectroscopies.[34,41] A longer diffusion length is typically associated to a higher charge collection efficiency: Ln = 60 µm was found for the NW-based DSSC and only 13 µm for the NPbased DSSC, giving direct evidence of the improved charge transport of NWs in an operating device. However, NW-based cells suffered of low dye loading and corresponding lower photocurrent density due to decreased optical density. An elegant solution for exploiting quite long nanowires in a system capable of high dye loading was proposed by Tan and Wu in 2006,[36] by mixing anatase TiO2 nanoparticle and single crystal TiO2 anatase nanowires (Figure 3).
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Figure 3. Top: cross-section SEM images for photoanodes of pure NPs (left) and NPs/NWs (right). Bottom: Left) Picture of NP (transparent) and NPs/NWs (opaque) samples before (white) and after (red) dye sensitization. Right) PCE as a function of photoanode thickness for different NP/NW ratios. Reproduced with permission.[36] Copyright 2006, American Chemical Society.
They clearly demonstrated that NP/NW composites possess the advantages of both the building blocks, i.e., the high specific surface of mesoporous nanoparticle films, and the fast electron transport and the light scattering effect of single-crystalline nanowires. An optimized ratio exists between NPs and NWs, which systematically outperforms NPs for all photoanode thickness (Figure 3). In the best device, PCE was 8.6% compared to 6.8% PCE for the best photoanode composed of NP-only. An interesting feature is that in the optimized device the highest efficiency is obtained for a 13 µm thick photoanode, while for NP film 10 µm is the optimum thickness. This behavior can be explained with increased electron diffusion length in composite network, as expected for the beneficial influence of single crystal nanowires on electron transport. Similar results were then confirmed by other research groups using almost the same photoanode structure composed of NPs and NWs mixed together.[42] An alternative strategy based on application of NWs exploited the one-dimensional (1D) assembly of TiO2 nanoparticles (NPs) by electrospinning.[43] Compared to the traditional disordered TiO2 NPs, the 1D structures resulted in faster charge transport and longer electron lifetime, as well as a higher light scattering ability (especially in the wavelength range from 500 nm to 650 nm, corresponding to the strong absorption from N719 dye). An increase of efficiency as high as 15% in comparison to the reference cell made of standard TiO2 NPs was achieved. An alternative methodology was recently developed by applying porous Rutile TiO2 nanorod arrays.[44] The photoanode was prepared by a combined bottom-up approach for the growth of thick nanorod array films (up to 30 µm)
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followed by a top-down etching of the compact rods through a hydrothermal treatment, to obtain the porous rods offering high specific surface. It is clearly demonstrated that the enhanced PCE (7.91% compared to 6.59% obtained on standard P25 NP photoanode) comes from increased dye loading in the optimized network. A strategy to both increase electron collection and have a high specific surface is application of TiO2 nanosheets. A very recent review from Prof. Wang on application of this material[45] foresees all the potential benefits of the new geometry offered by very thin (below 1 nm in certain cases) TiO2 sheets. In addition to high dye loading and fast electron transport, a composite material formed of TiO2 nanosheets and TiO2 NPs exhibits also light scattering ability. Optimized photoanode resulted in PCE = 10.1%, with a significant increase compared to standard P25 (7.8%).[46] Nanotubes are another geometry, which was deeply investigated for both DSSCs[4] and QDSCs.[47] A step forward exploitation of TiO2 nanotubes was the ability to grow nanotubes on transparent conducting oxide glass with the lengths needed for high-efficiency device.[48] Grimes and co-workers were able to grow very long nanotubes on FTO glass (film thickness 33 µm, see Figure 4) yielding a power conversion efficiency of 6.9% and an incident photon-to-current conversion efficiency ranging from 70 to 80% for wavelengths between 450 and 650 nm, which was unprecedented for tube-like structures in DSSCs. The main drawback in preparation of such long tubes is the required anodization time. For this reason, a technologically relevant advancement in the field was proposed by Diau and co-workers,[49,50] who combined a sequential potentiostatic and galvanostatic anodization. The potentiostatic mode at the beginning of the process defines the pattern for nanotube growth, including the main tube parameters (external and internal tube diameter), while the galvanostatic mode induces fast tube formation based on the preformed pattern. Tubes up to 57 µm long were produced with growth rates as high as 20.3 µm h−1. We recently demonstrated the possibility of the growth of TiO2 nanotubes on different substrates.[51] The modification of anodization conditions on specific substrates such as polyethylene terephthalate (PET), conducting glass and granular alumina was found to affect the morphology of TiO2 nanotubes. These findings are critical to application of nanotubes in DSSCs and QDSCs, since tube morphology affects both electron transport and specific surface. On the other side, application of this technology enables exploitation of a
Figure 4. Left: Ti film before anodization. Right: TiO2 nanotubes after anodization. Reproduced with permission.[48] Copyright 2009, Macmillan Publishers Ltd. small 2014, DOI: 10.1002/smll.201402334
Figure 5. Left) Surface finishing of the surface of nanotube arrays (left) with and (right) without ink pen pre-treatment of Ti layer before anodization. Ink pen coverage results in a debris-free surface of the nanotube array. Right) j–V curves of a DSC under different simulated sunlight intensities. Back-irradiation geometry has been applied, as illustrated in the inset. Reproduced with permission.[54] Copyright 2011, Royal Society of Chemistry.
bunch of different substrates for DSSCs, which can be technologically relevant in the panorama of industrial scale up of this kind of solar cells. In addition, we demonstrated the possibility of growing nanotubes on heat-resistant Kapton HN substrates (Figure 5). In fact, anodized TiO2 nanotubes are amorphous after anodization, and so unsuitable for use in DSSCs, where anatase TiO2 is best suited. Amorphous to anatase transition is typically induced after anodization through annealing in ambient atmosphere in the temperature range 350–500 °C. However, most of plastic substrates are unable to undergo this kind of treatment without being deeply damaged by heat. For this reason, Kapton HN was applied, which is a polyimide able to bear controlled annealing without loss of its mechanical properties. Amorphous to anatase transition was obtained for TiO2 nanotubes grown on Kapton HN and flexible solar cells were demonstrated.[52] The main drawback of Kapton HN substrate is that it is colored (cutting off all solar radiation at wavelength below 500 nm), coercing back illuminated solar cell geometry. However, the advantage of the proposed solution is the obtained PCE (3.5%), which is in line with similar results obtained on titanium substrate (3.58%).[53] In the latter case other features have to be considered, like the high weight and the limited flexibility of titanium foil compared to Kapton HN. For this reason the proposed solution represents a very promising novelty in the panorama of flexible DSSCs. In addition, we demonstrated a straightforward methodology to obtain debris-free nanotube surface, simply coating Ti surface with a felt-tip alcohol-based ink before anodization.[54]
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Figure 6. Influence of the blocking layer on the physical and chemical processes taking place in a DSSC under operation. Reprinted with permission.[58] Copyright 2012, American Chemical Society.
This procedure prevents application of post anodization treatments (typically sonication) that can damage the nanotube array. Another solution, which entered in the standard fabrication process of DSSC photoanode was application of TiO2 thin film acting as blocking layer for electron back reaction from the FTO front contact to the electrolyte (Figure 6).[55–58] Two mechanisms were claimed to explain the increased PCE, namely, a decrease in the electronhole recombination at FTO/electrolyte interface,[59] or the increased mechanical contact between the mesoporous TiO2 network and the FTO.[58,60] In literature it is usually found that a layer around 50–90 nm thick guarantees the best performances, boosting photoconversion efficiency up to 15–20% compared to the cell without the blocking layer for optimized photoanode thickness (10–15 microns).[61] A very recent paper[62] pointed out that very thin layer (as thick as 5 nm) is much more effective, due to the reduced series resistance it induces in the operating solar cell. However, in that paper, very thin photoanodes were considered (2.5 micron), well below the electron diffusion length. And in fact limited PCE was reported (3.12% maximum). For this reason the conclusions cannot be readily extended to much thicker photoanodes, where electron recombination inside TiO2 mesoporous network is supposed to be much higher than in very thin layers. However, a complete understanding of the role of the blocking layer is still missing, and different hypotheses are formulated.[56,58] In fact, Cameron and Peter demonstrated the ability of the blocking layer to prevent the back reaction of electrons with tri-iodide ions in the electrolyte under short circuit conditions, but found the presence of electron accumulation at the surface of the titanium dioxide blocking layer under open circuit conditions, which would negatively affect the operating device (Figure 7).[56] On the opposite, Fabregat and co-workers demonstrated an increase of the functional properties of DSSCs by applying a TiO2 blocking layer. In particular, they found that, after discarding conduction band
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Figure 7. Photovoltage decays for DSSCs with and without blocking layer under LED illumination. Similar decay is observed at the beginning, while the large difference at longer times indicates that electron transfer occurs at the uncoated FTO electrode. Reproduced with permission.[56] Copyright 2003, American Chemical Society.
shift and charge collection limitations associated to diffusion length, the improvement of the functional properties can be attributed to a better contact between the coated FTO substrate and the TiO2 film, even if they did not give any quantification or direct evidence of such contact.[58] One of the most remarkable conclusions of this work is that the blocking layer introduces a non negligible internal series resistance, which prevents from application of thick blocking layers, and, finally, limits the outcomes of this approach for TiO2-DSSCs. Completely different conclusion can be drawn, instead, for application of blocking layer in ZnO DSSCs, as extensively reported in the next section of the paper.[15] The research to fasten electron transport in DSSCs and QDSCs paralleled intense investigation on light managing, with the aim of increasing the residence time of solar photons inside the photoanode. In fact, in the original photoanode design, a mesoporous transparent film of small NPs (around 20 nm in size) was used. In this configuration unabsorbed light after travelling once through the sensitized layer was completely lost, affecting PCE. A proposed solution was addition of a layer composed by large particles (in the range between 200–400 nm) that induces light scattering inside the photoanode and increases probability of photon absorption. The use of opaque films for light managing demonstrated of critical importance for the increase of PCE (PCE up to 11.1% was obtained by using the so-called “haze concept”).[63] The haze is defined as the ratio of diffused transmittance to total optical transmittance. It was found that the IPCE of DSSCs increases with increase in the haze of the TiO2 electrodes, especially in the near infrared wavelength region (Figure 8). A drawback in the use of haze is that large particles, which allow enhanced light scattering in the visible range, are characterized by rather low specific surface compared to the typical nanoparticles applied in photoanodes. For this reason research addressed to application of hierarchically assembled structures, able to effectively scatter visible radiation and maintaining the high specific surface of a mesoporous layer (Figure 9).[64]
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Figure 8. Dependence of IPCE spectra on haze of TiO2 electrodes. Haze in the figure was measured at 800 nm. Reproduced with permission.[63] Copyright 2006, Japan Society of Applied Physics.
R. Caruso and co-workers[64] demonstrated the application of nanocrystalline, mesoporous TiO2 beads with surface areas up to 108.0 m2 g−1 and tunable pore sizes through a combination of sol-gel and solvothermal processes. The mesoporous beads are composed of anatase TiO2 nanocrystals. The functional properties of DSSCs fabricated using these beads are reported in Table 1 and compared to Degussa P25 TiO2 photoanodes. An impressive increase in IPCE was achieved (Figure 9) thanks to the highly scattering photoanode and its high specific surface.
Almost similar PCE (7.40%) was achieved by applying an analogous concept through TiO2 hollow spheres (150–200 nm in diameter) produced by selective etching of Au@TiO2 coreshell nanoparticles and applied as scattering layer atop commercial nanoparticle layer.[65] The increment in efficiency was related to efficient light scattering, electrolyte diffusing feasibility for better electron transport, and a high surface area allowing higher dye loading. In an effort to capitalize both on the light harvesting and charge collection, a layered system was recently proposed, composed of three mesoporous stacks made of shape-tailored TiO2 anatase nanocrystals, which have been ad hoc synthesized by suitable colloidal routes.[66] The increased efficiency of this multilayered DSSC was primarily due to the enhancement in Jsc related to the larger amount of adsorbed dye due to increased surface area (2.7 × 10−7 mol cm−2 compared to 2.0 × 10−7 mol cm−2 for the reference DSSC). In addition, the system demonstrated significant improvement in charge collection efficiency, as deduced from the measurement of electron diffusion length, which was 30% larger in the multilayered cell, compared to benchmarking photoanode. In Table 2 we report the efficiencies and functional parameters of various kinds of DSSCs based on different TiO2 photoanodes under one sun irradiation. As a concluding remark on DSSCs based on TiO2, we highlight the efforts of the scientific community to develop technologically relevant platforms to reduce the production times of this kind of cells. In particular, a very recent paper from O’Regan and co-workers demonstrated the reliability of an ultrafast sintering process based on near infrared (NIR) radiation, which results in an operating photoanode after 12.5 seconds sintering. The photovoltaic performance of devices made with NIR sintered films match those devices made with conventionally sintered films prepared by heating for 1800 seconds. The authors demonstrated that in NIR processing of TiO2 the rapid heating (to temperatures of up to 785 °C) does not lead to a large scale rutile phase transition and consequent loss of photocurrent.[69]
3. Metal Oxides Alternative to TiO2
Figure 9. Top and bottom left: SEM images of calcined mesoporous TiO2 beads obtained after a solvothermal process with different amounts of ammonia. Top right: Diffuse reflectance spectra of the TiO2 films prepared from P25 nanoparticles and mesoporous TiO2 beads (sample S4) of varying thickness. Bottom right: Incident photon to current conversion efficiency (IPCE) curves of the TiO2 electrodes prepared from P25 nanoparticles and mesoporous TiO2 beads (sample S4) of two film thicknesses. Reproduced with permission.[64] small 2014, DOI: 10.1002/smll.201402334
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Other semiconductor metal oxides have raised a wide interest as potential alternatives to TiO2. Among them, particular attention is being paid to ZnO and SnO2: both these materials present higher electron mobility as compared with TiO2 as well as specific advantages. ZnO has indeed a band structure very similar to that of TiO2 and the possibilities of synthesis are almost infinite, as for shapes and sizes, being this oxide probably the most studied one for several
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reviews www.MaterialsViews.com Table 1. Photovoltaic properties of the dye-sensitized solar cells assembled by using electrodes made from Degussa P25 particles and mesoporous beads of different thickness. From Ref. [64] Sample
Thickness (µm)
Voc (mV)
P25
TiO2 beads
Jsc (mA cm−2)
FF
PCE (%)
5.5
781
9.99
0.66
5.11
10.1
758
10.98
0.69
5.73
12.9
748
10.60
0.71
5.66
5.4
797
9.84
0.72
5.65
9.8
769
11.25
0.70
6.06
12.3
777
12.79
0.72
7.20
applications, spanning from photovoltaics, to catalysis, gas sensing and batteries.[70–76] On the other hand, SnO2 has a wider band gap (about 3.8 eV) that is believed to guarantee for a better stability under UV illumination. Moreover, the position of SnO2 conduction band at higher energy would allow the successful application as light harvester of dyes able to collect light in the near infrared (NIR). However, it should be remarked that, as we will see, most of studies applying SnO2 as photoanode in DSSCs exploit dye N719 as sensitizer, despite an unfavorable band alignment between the two partners: this approach typically results in low photovoltages and consequently in overall low performances.
3.1. ZnO Photoanodes ZnO is a wide bang gap semiconductor (3.37 eV) that has attracted attention since very long ago, due to interesting optoelectronic features and a well-established polymorphism, which has enabled researchers to obtain ZnO in countless morphologies. It would be almost impossible providing the reader with a comprehensive introduction on synthetic approaches, morphologies and possible applications of this fascinating and
versatile material. However, a review on this amazing semiconductor metals oxide can be found in Ali and Winterer.[77] In the frame of possible exploitation in energy application, ZnO has been the first semiconductor metal oxide for which the possibility of electron injection from a molecular dye was demonstrated.[78] Since then, a huge amount a literature on the exploitation of ZnO in solar devices, and more generally in photoelectrochemical cells, has been published. As for DSSCs, most papers focus on the application of (more or less) new ZnO morphologies as photoanodes and a large variety of micro- and nano- ZnO structures have been tested. A comprehensive review of the topic has been published in 2012 by Anta and coworkers[79] and the reader is mainly referred to this paper for the works published up to 2011. Literature results over the years demonstrate, as we are going to highlight and as already pointed out by Anta and coworkers, that application of ZnO as anode in DSSCs is no trivial challenge, due to several reasons. It has to be remarked that the application of ZnO poses issues even from the viewpoint of dye uptake, which is, on the contrary, a well-established procedure for TiO2-based electrodes. Early studies carried out by photochemists, dating before the advent of DSSCs, had demonstrated that effective dye uptake can be realized with ZnO and irreversible charge injection from the molecular dye is possible if an electrochemical gradient was operating inside the electrode,[78] but the application of ruthenium complexes appeared critical since the beginning, especially related to the acidic environment determined by the dyes, in which ZnO is suffering. Evidences of dye agglomeration on ZnO porous films have been found:[80] after surface coverage from dye, “extra” molecules accumulate on the film, which are not only useless from injection viewpoint, but behave as a shield for light absorption towards dye molecules directly anchored to the metal oxide, thus decreasing the overall injection efficiency and IPCE. Differences in chemical bonding of classical N719 and N3 dyes have been observed by several the authors, which may affect the injection processes under irradiation and, eventually, the device performances.
Table 2. Functional properties of DSSCs based on various shapes/structures of TiO2 photoanodes sensitized with different dye molecules under AM 1.5 G irradiation, 100 mW cm−2. Shape/Structure 7 um mesoporous TiO2 TiO2 NP (5 µm T + 5 µm scattering
Dye
Voc [mV]
Jsc
FF
PCE [%]
Ref.
SM315
910
18.1
0.78
13.0
[10]
YD2-o-C8 / Y123
935
17.66
0.74
12.3
[9]
11 µm T TiO2 NP + 5 µm layer of 400 nm reflecting particles
YD-2
770
18.6
0.764
11.0
[67]
Multi-stack nanocrystals
N719
810
18.10
0.70
10.26
[66]
Anatase TiO2 NP/NW Composites
N719
610
25.5
0.55
8.6
[36]
ZnO NW/TiO2 NP network
N719
763
16.08
0.688
8.44
[68]
TiO2 beads
N719
777
12.79
0.72
7.20
[64]
Nanotubes
N719
730
15.8
0.59
6.9
[48]
Rutile TiO2 nanorod arrays
N719
710
20.49
0.55
7.91
[44]
Porous Titania Nanosheet/NP
N719
750
19.2
0.70
10.1
[46]
Nanotubes on flexible plastic substratea)
N719
759
8.1
0.55
3.5
[54]
Nanotubes on flexible Ti substratea)
D205
709
8.99
0.561
3.58
[53]
a)Comparison
of two kinds of flexible solar cells illustrating the possibility of substituting the flexible, but heavy and expensive Ti foil with a plastic and highly versatile substrate.
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Figure 10. a,b,d–f) SEM analysis of the hierarchical ZnO photoanode reported in Memarian et al.[15] a) Cross-sectional SEM image showing the layered structure of the photoanode: the active layer, composed of hierarchically structured ZnO nanocrystals, the compact buffer layer, and the FTO conducting layer. b) Top view of the absorbing layer, showing a highly disordered structure. c) Size distribution of the aggregates, as estimated by SEM. Inset: shape analysis of the aggregates from the SEM image in (b), with elliptical shapes predominant. d,e) High-resolution SEM images of two aggregates, showing the closely packed assembly of the nanoparticles. f) High-resolution SEM image of the buffer layer, indicating its compact morphology with respect to the polydispersed aggregates. Reproduced with permission.[15]
The nature of applied dye is then something to pay attention to and specific studies would be still needed in order to improve the capability of solar light conversion. For those devices exploiting the usual N710 dye, dye loading time becomes a critical parameter to take into account in order to especially maximize the photocurrent inside the device.[80–83] Another critical issue, apparently still open, concerns the efficiency of electron injection process from the excited dye to the ZnO conduction band. In their review, Anta and coworkers identified this as the most critical topic to address, suggesting that a limited number of studies have been carried out especially devoted to investigate this fundamental process. We fully agree with this statement and we have to remark that today this point would still deserve attention. Some authors have tried to address the issue, with different success rates. An interesting paper has been published by Hagfeldt in 2001,[84] focusing on dynamics of electron injection and recombination in N719-ZnO nanostructures. By applying femto- and nano-second spectroscopies, the authors found ultrafast (
Solar Cells
Metal Oxide Semiconductors for Dye- and Quantum-Dot-Sensitized Solar Cells Isabella Concina* and Alberto Vomiero*
From the Contents 1. Introduction .............................................. 2
T
his Review provides a brief summary of the most recent research developments in the synthesis and application of nanostructured metal oxide semiconductors for dye sensitized Metal Oxides Alternative and quantum dot sensitized solar cells. In these devices, the wide to TiO2 ....................................................... 7 bandgap semiconducting oxide acts as the photoanode, which provides the scaffold for light harvesters (either dye molecules Composite Systems ..................................24 or quantum dots) and electron collection. For this reason, proper tailoring of the optical and electronic properties of the Perovskite Solar Cells............................... 26 photoanode can significantly boost the functionalities of the Conclusions and Perspectives...................28 operating device. Optimization of the functional properties relies with modulation of the shape and structure of the photoanode, as well as on application of different materials (TiO2, ZnO, SnO2) and/or composite systems, which allow fine tuning of electronic band structure. This aspect is critical because it determines exciton and charge dynamics in the photoelectrochemical system and is strictly connected to the photoconversion efficiency of the solar cell. The different strategies for increasing light harvesting and charge collection, inhibiting charge losses due to recombination phenomena, are reviewed thoroughly, highlighting the benefits of proper photoanode preparation, and its crucial role in the development of high efficiency dye sensitized and quantum dot sensitized solar cells.
2. TiO2 Photoanodes...................................... 3 3.
4. 5. 6.
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1. Introduction Metal oxide semiconductors have been exploited for decades in advanced application fields like microelectronics, gas sensing, energy conversion, and storage.[1–3] At the basis of their success is the possibility of fine tuning their electronic, optical, and charge transport properties in a broad range of values, targeting their functional features for specific applications. In the past few years, the ability of shaping metal oxide semiconductors at the nanoscale opened up new perspectives for their exploitation, significantly improving the performances of end-user devices.[4,5] One of the most successful applications for metal oxide semiconductors are excitonic solar cells.[6,7] Very recently, a renaissance of this kind of devices occurred,[8] thanks to significant breakthroughs that led to unprecedented photoconversion efficiencies (PCE) in dye-sensitized (DSSCs),[9,10] quantum dot-sensitized (QDSSCs)[11] and perovskite-sensitized[12] solar cells. In this kind of cells, metal oxide semiconductors have a critical importance, since they are one of the main constituents (the anode) of the photoelectrochemical system.[13,14] In several cases proper modulation of the structure and morphology of the oxide led to unprecedented PCE thanks to induced new optical and charge transport properties: it is the case, for instance, of ZnO-based dye sensitized solar cells, in which record PCE (7.5%)[15] was obtained by applying a hierarchically self-assembled thick film in combination with a compact blocking layer or of SnO2 hollow microspheres.[16] Another example relies with application of single crystal structures in dye sensitized solar cells:[13] since several years increased electron transport was claimed to improve PCE in such devices by applying single crystalline one-dimensional nanostructures like nanowires (NWs), nanorods, nanobelts,[17–19] but only recently substantially higher conductivities and electron mobilities than does nanocrystalline TiO2 led to record PCE (7.3%) in all-solidstate perovskite sensitized solar cells. Furthermore, the advent of graphene pursued the research on composite systems based on metal oxide semiconductors and carbon-based nanostructures.[20] The main effect of carbon nanotubes and/ or graphene addiction is to significantly boost the collection of photogenerated charges,[21] leading to increase of shortcircuit photocurrent to more than 20%[22] and opening promising perspectives for further advancement in the field. The present review focuses on progress in application of different metal oxide semiconductors (TiO2, ZnO, SnO2) as photoanodes in excitonic solar cells, with a forward look at the development of high efficiency and low cost photovoltaic devices. The basic processes taking place in an excitonic solar cell are (Figure 1):[23] 1) light absorption and exciton generation; 2) exciton separation and electron injection from the dye to the oxide; (3) Electron transport to the anode; 4) dye regeneration as a consequence of hole reduction by the I3− species; 5) hole transport to the cathode, where a catalytic reaction occurs (typically mediated by the presence of Pt electrode). Other competing processes tend to inhibit fast exciton separation, charge transport and collection, resulting in decrease of photoconversion efficiency; 6) non radiative exciton
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Figure 1. Schematic representation of the operating principle of a DSSC. In blue the processes contributing to light-to-electric power conversion: in red the processes degrading the performances of the device.
recombination before charge separation and injection; 7) charge recombination between electrons at the oxide photoanode and I3− in the electrolyte; 8) fast electron–hole recombination for dye regeneration. As reported in consolidated literature,[9,23–26] processes 2 and 4 are much faster than processes 6 and 8. For this reason, the competing processes regulating the photoconversion efficiency in DSSCs and QDSSCs are mainly charge transport through the photoanode (3) and charge recombination between electron at the oxide photoanode and I3− in the electrolyte (7). As for charge generation, injection, transfer and collection in the previously reported scheme, these processes can take place only under certain conditions of band alignment among the various components of the system. In particular, the position of the LUMO of the dye has to be above the conduction band of the oxide devoted to charge transport in order to make electron injection possible from the excited state of the dye to the oxide. At the same time, favorable band alignment has to be present between the electrolyte and the oxide forming the photoanode, in order to induce hole transport through the electrolyte and to obtain as fast as possible regeneration of the dye/QD. It is clear that proper band alignment requires a potential drop from one stage to the next one, which provides the driving force for the process to occur, but that is detrimental in terms of cell performances, since that potential drop does not reflect at the output of the solar cell. Dr. I. Concina, Dr. A. Vomiero CNR-INO SENSOR Lab Via Branze 45, 25123 Brescia, Italy E-mail: [email protected]; [email protected] Dr. I. Concina University of Brescia Via Valotti 9, 25133, Brescia, Italy Dr. A. Vomiero Luleå University of Technology 971 98, Luleå, Sweden DOI: 10.1002/smll.201402334
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Light absorption is the first phenomenon occurring in a solar cell. Light management is of utmost importance, since maximizing photon to charge conversion is one of the main goals (if not the main) in third generation solar cells. When light interacts with a solar cell, several processes can occur: i) partial reflection at the surface of the glass; ii) absorption of photons from light harvesters; iii) light scattering inside the solar cell; iv) partial transmission of light, if light absorption at the anode and/or in the other parts of the cell is not complete. The main process relying with light absorption is, of course, the absorption of solar radiation by the light harvester contained in the photoanode. The probability of absorption relies with many features: the optical density of the photoanode, the extinction coefficient of the light harvester, the time spent by the light inside the photoanode. Most of these factors are dependent on the wavelength of the incident radiation. For these reasons huge amount of research addressed these three issues in excitonic solar cells: 1. Maximize light absorbance by light harvester in the widest possible wavelength interval; 2. Minimize charge recombination leading to loss of photogenerated charges; 3. Modulate band alignment in order to obtain as fast as possible charge injection processes limiting voltage drop to the minimum necessary to induce charge transfer. Intense research has been carried out to optimize all the aforementioned aspects of the solar cells. In this review we will focus on the various kinds, morphologies, structures and shapes of the oxides forming the photoanode that have been applied to improve the functionalities of excitonic solar cells. Different aspects will be covered, including light management, band alignment and charge transport, i.e., most of the open issues in DSSCs and QDSSCs. Of course the transaction is not exhaustive of the research efforts in these kinds of cells, including also the design of new dyes, and application of different electrolytes and counter-electrodes, which will not be considered here. In particular we will focus on three oxides, i.e. TiO2, ZnO and SnO2 that represent the most widely investigated materials as photoanodes for DSSCs.
2. TiO2 Photoanodes Possibility of using dye molecules for light harvesting came before the pioneering paper of O’Regan and Gratzel in 1991.[27,28] However, the main drawback of the proof of concept was the very limited photoconversion efficiency, mainly due to the poor optical density of the layer, determined by the low specific surface of the investigated materials. The revolutionary concept proposed by O’Regan and Gratzel was to apply a mesoporous film composed of small nanoparticles (NPs) (20 nm in size) as scaffold for light harvesters. The dramatically increased optical density allowed to boost photoconversion efficiency up to 7%.[29] Since then, optimization of TiO2 network occurred,[30] trying to reduce small 2014, DOI: 10.1002/smll.201402334
Isabella Concina is assistant professor in experimental physics at the University of Brescia, Italy. She earned her Ph.D. in chemical sciences in 2006, as well as her degree in chemistry in 2002, from the University of Padova, Italy. Her main research interests focus on the engineering of semiconductor micro- and nano-structures for energy application by applying green chemistry concept for the enhancement of device performances.
Alberto Vomiero is chaired professor in experimental physics at the Luleå University of Technology, Sweden. He was awarded his Ph.D. in electronic engineering from the University of Trento in 2003 and his degree in physics from the University of Padova in 1999. His main interests are in composite nanomaterials (wide bandgap semiconductors, semiconducting nanocrystals, and hybrid systems) for gas sensors and excitonic solar cells. He is Marie Curie International Outgoing Fellow of the European Commission, Fellow of the Institute of Physics (UK) and of the Institute of Nanotechnology (UK), chair of the Italian section of the American Nano Society and member of the Global Young Academy.
charge losses during charge collection and increase light harnessing. Several strategies were proposed for the purpose. One possibility is the treatment of TiO2 surface with TiCl4 during photoanode preparation.[31] In 2007, O’Regan and co-workers demonstrated that the main effect of TiCl4 treatment of TiO2 photoanode is a downward shift in the TiO2 conduction band edge potential and a strong decrease (up to 20-fold) in the electron/electrolyte recombination rate constant (Figure 2). The combination of these two effects is a neat increase in the efficiency of charge separation and the consequent boost of photocurrent. It has to be noted that, in this case, the detrimental loss in terms of Voc of the operating device is compensated by the increased accumulation of electrons due to the reduced recombination rate, which offsets the Voc loss otherwise expected from the conduction band edge shift. Another useful strategy to increase PCE is the application of single crystalline nanowires and nanotubes to fasten charge transport.[33] In mesoporous photoanodes the electrons transport through a slow trap-limited diffusion process:[34] in an anatase TiO2 nanoparticle film, the electron diffusion coefficient is more than two orders of magnitude lower than in single crystals.[35,36] In fact, while offering extremely high specific surface for dye loading, mesoporous networks suffer for high density of grain boundaries, which foster charge recombination in operating devices. Application of one dimensional (1D) nanostructures can offer more straightforward path to photogenerated electrons, minimizing
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Figure 2. Recombination rate constant at Voc vs charge density at Voc for TiO2 NP cells with and without TiCl4 treatment. Reproduced with permission.[32] Copyright 2007, American Chemical Society.
the time inside the photoanode, and then the recombination probability.[17,18,37] Among various 1D nanostructures, both polycrystalline and single crystalline nanowires/nanotubes have been applied. 1D polycrystals in general offer the benefit of straightforward path and an almost mesoporous structure with quite acceptable specific surface. 1D single crystals, instead, typically present much higher electron mobility than polycrystals, thanks to absence of grain boundaries and their single crystalline assembly, but suffer of very poor specific surface, significantly limiting the possibility of dye loading. Application of single-crystal anatase TiO2 nanorod film was reported in 2005 and 2006.[38,39] In the specific case, quite short rods were applied (100–300 nm in length). The authors found that the decreased number of contact barriers between the rods, limiting electron traps, was the main reason for increased PCE compared to standard solar cell based on P25 NPs. Application of nanorods was highly beneficial in increasing the FF of the solar cell for quite thick photoanodes (above 10 µm), where the effect of charge recombination starts limiting cell performance. PCE as high as 7.29% was obtained for the best cell using nanorods, while 4.5% was achieved in P25-based cells. However, in the case of nanorods, the length of fast electron transfer was in the order of hundreds of nanometers (the dimensions of the rod), which is around 50 times lower than the typical thickness of the best photoanodes in DSSCs. The rapid charge transport in DSSCs made of vertically aligned single-crystal rutileTiO2 nanowires was directly measured recently.[40] Quantitative evaluation of charge collection was estimated through the electron diffusion length Ln by applying intensity-modulated photocurrent and photovoltage spectroscopies.[34,41] A longer diffusion length is typically associated to a higher charge collection efficiency: Ln = 60 µm was found for the NW-based DSSC and only 13 µm for the NPbased DSSC, giving direct evidence of the improved charge transport of NWs in an operating device. However, NW-based cells suffered of low dye loading and corresponding lower photocurrent density due to decreased optical density. An elegant solution for exploiting quite long nanowires in a system capable of high dye loading was proposed by Tan and Wu in 2006,[36] by mixing anatase TiO2 nanoparticle and single crystal TiO2 anatase nanowires (Figure 3).
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Figure 3. Top: cross-section SEM images for photoanodes of pure NPs (left) and NPs/NWs (right). Bottom: Left) Picture of NP (transparent) and NPs/NWs (opaque) samples before (white) and after (red) dye sensitization. Right) PCE as a function of photoanode thickness for different NP/NW ratios. Reproduced with permission.[36] Copyright 2006, American Chemical Society.
They clearly demonstrated that NP/NW composites possess the advantages of both the building blocks, i.e., the high specific surface of mesoporous nanoparticle films, and the fast electron transport and the light scattering effect of single-crystalline nanowires. An optimized ratio exists between NPs and NWs, which systematically outperforms NPs for all photoanode thickness (Figure 3). In the best device, PCE was 8.6% compared to 6.8% PCE for the best photoanode composed of NP-only. An interesting feature is that in the optimized device the highest efficiency is obtained for a 13 µm thick photoanode, while for NP film 10 µm is the optimum thickness. This behavior can be explained with increased electron diffusion length in composite network, as expected for the beneficial influence of single crystal nanowires on electron transport. Similar results were then confirmed by other research groups using almost the same photoanode structure composed of NPs and NWs mixed together.[42] An alternative strategy based on application of NWs exploited the one-dimensional (1D) assembly of TiO2 nanoparticles (NPs) by electrospinning.[43] Compared to the traditional disordered TiO2 NPs, the 1D structures resulted in faster charge transport and longer electron lifetime, as well as a higher light scattering ability (especially in the wavelength range from 500 nm to 650 nm, corresponding to the strong absorption from N719 dye). An increase of efficiency as high as 15% in comparison to the reference cell made of standard TiO2 NPs was achieved. An alternative methodology was recently developed by applying porous Rutile TiO2 nanorod arrays.[44] The photoanode was prepared by a combined bottom-up approach for the growth of thick nanorod array films (up to 30 µm)
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followed by a top-down etching of the compact rods through a hydrothermal treatment, to obtain the porous rods offering high specific surface. It is clearly demonstrated that the enhanced PCE (7.91% compared to 6.59% obtained on standard P25 NP photoanode) comes from increased dye loading in the optimized network. A strategy to both increase electron collection and have a high specific surface is application of TiO2 nanosheets. A very recent review from Prof. Wang on application of this material[45] foresees all the potential benefits of the new geometry offered by very thin (below 1 nm in certain cases) TiO2 sheets. In addition to high dye loading and fast electron transport, a composite material formed of TiO2 nanosheets and TiO2 NPs exhibits also light scattering ability. Optimized photoanode resulted in PCE = 10.1%, with a significant increase compared to standard P25 (7.8%).[46] Nanotubes are another geometry, which was deeply investigated for both DSSCs[4] and QDSCs.[47] A step forward exploitation of TiO2 nanotubes was the ability to grow nanotubes on transparent conducting oxide glass with the lengths needed for high-efficiency device.[48] Grimes and co-workers were able to grow very long nanotubes on FTO glass (film thickness 33 µm, see Figure 4) yielding a power conversion efficiency of 6.9% and an incident photon-to-current conversion efficiency ranging from 70 to 80% for wavelengths between 450 and 650 nm, which was unprecedented for tube-like structures in DSSCs. The main drawback in preparation of such long tubes is the required anodization time. For this reason, a technologically relevant advancement in the field was proposed by Diau and co-workers,[49,50] who combined a sequential potentiostatic and galvanostatic anodization. The potentiostatic mode at the beginning of the process defines the pattern for nanotube growth, including the main tube parameters (external and internal tube diameter), while the galvanostatic mode induces fast tube formation based on the preformed pattern. Tubes up to 57 µm long were produced with growth rates as high as 20.3 µm h−1. We recently demonstrated the possibility of the growth of TiO2 nanotubes on different substrates.[51] The modification of anodization conditions on specific substrates such as polyethylene terephthalate (PET), conducting glass and granular alumina was found to affect the morphology of TiO2 nanotubes. These findings are critical to application of nanotubes in DSSCs and QDSCs, since tube morphology affects both electron transport and specific surface. On the other side, application of this technology enables exploitation of a
Figure 4. Left: Ti film before anodization. Right: TiO2 nanotubes after anodization. Reproduced with permission.[48] Copyright 2009, Macmillan Publishers Ltd. small 2014, DOI: 10.1002/smll.201402334
Figure 5. Left) Surface finishing of the surface of nanotube arrays (left) with and (right) without ink pen pre-treatment of Ti layer before anodization. Ink pen coverage results in a debris-free surface of the nanotube array. Right) j–V curves of a DSC under different simulated sunlight intensities. Back-irradiation geometry has been applied, as illustrated in the inset. Reproduced with permission.[54] Copyright 2011, Royal Society of Chemistry.
bunch of different substrates for DSSCs, which can be technologically relevant in the panorama of industrial scale up of this kind of solar cells. In addition, we demonstrated the possibility of growing nanotubes on heat-resistant Kapton HN substrates (Figure 5). In fact, anodized TiO2 nanotubes are amorphous after anodization, and so unsuitable for use in DSSCs, where anatase TiO2 is best suited. Amorphous to anatase transition is typically induced after anodization through annealing in ambient atmosphere in the temperature range 350–500 °C. However, most of plastic substrates are unable to undergo this kind of treatment without being deeply damaged by heat. For this reason, Kapton HN was applied, which is a polyimide able to bear controlled annealing without loss of its mechanical properties. Amorphous to anatase transition was obtained for TiO2 nanotubes grown on Kapton HN and flexible solar cells were demonstrated.[52] The main drawback of Kapton HN substrate is that it is colored (cutting off all solar radiation at wavelength below 500 nm), coercing back illuminated solar cell geometry. However, the advantage of the proposed solution is the obtained PCE (3.5%), which is in line with similar results obtained on titanium substrate (3.58%).[53] In the latter case other features have to be considered, like the high weight and the limited flexibility of titanium foil compared to Kapton HN. For this reason the proposed solution represents a very promising novelty in the panorama of flexible DSSCs. In addition, we demonstrated a straightforward methodology to obtain debris-free nanotube surface, simply coating Ti surface with a felt-tip alcohol-based ink before anodization.[54]
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Figure 6. Influence of the blocking layer on the physical and chemical processes taking place in a DSSC under operation. Reprinted with permission.[58] Copyright 2012, American Chemical Society.
This procedure prevents application of post anodization treatments (typically sonication) that can damage the nanotube array. Another solution, which entered in the standard fabrication process of DSSC photoanode was application of TiO2 thin film acting as blocking layer for electron back reaction from the FTO front contact to the electrolyte (Figure 6).[55–58] Two mechanisms were claimed to explain the increased PCE, namely, a decrease in the electronhole recombination at FTO/electrolyte interface,[59] or the increased mechanical contact between the mesoporous TiO2 network and the FTO.[58,60] In literature it is usually found that a layer around 50–90 nm thick guarantees the best performances, boosting photoconversion efficiency up to 15–20% compared to the cell without the blocking layer for optimized photoanode thickness (10–15 microns).[61] A very recent paper[62] pointed out that very thin layer (as thick as 5 nm) is much more effective, due to the reduced series resistance it induces in the operating solar cell. However, in that paper, very thin photoanodes were considered (2.5 micron), well below the electron diffusion length. And in fact limited PCE was reported (3.12% maximum). For this reason the conclusions cannot be readily extended to much thicker photoanodes, where electron recombination inside TiO2 mesoporous network is supposed to be much higher than in very thin layers. However, a complete understanding of the role of the blocking layer is still missing, and different hypotheses are formulated.[56,58] In fact, Cameron and Peter demonstrated the ability of the blocking layer to prevent the back reaction of electrons with tri-iodide ions in the electrolyte under short circuit conditions, but found the presence of electron accumulation at the surface of the titanium dioxide blocking layer under open circuit conditions, which would negatively affect the operating device (Figure 7).[56] On the opposite, Fabregat and co-workers demonstrated an increase of the functional properties of DSSCs by applying a TiO2 blocking layer. In particular, they found that, after discarding conduction band
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Figure 7. Photovoltage decays for DSSCs with and without blocking layer under LED illumination. Similar decay is observed at the beginning, while the large difference at longer times indicates that electron transfer occurs at the uncoated FTO electrode. Reproduced with permission.[56] Copyright 2003, American Chemical Society.
shift and charge collection limitations associated to diffusion length, the improvement of the functional properties can be attributed to a better contact between the coated FTO substrate and the TiO2 film, even if they did not give any quantification or direct evidence of such contact.[58] One of the most remarkable conclusions of this work is that the blocking layer introduces a non negligible internal series resistance, which prevents from application of thick blocking layers, and, finally, limits the outcomes of this approach for TiO2-DSSCs. Completely different conclusion can be drawn, instead, for application of blocking layer in ZnO DSSCs, as extensively reported in the next section of the paper.[15] The research to fasten electron transport in DSSCs and QDSCs paralleled intense investigation on light managing, with the aim of increasing the residence time of solar photons inside the photoanode. In fact, in the original photoanode design, a mesoporous transparent film of small NPs (around 20 nm in size) was used. In this configuration unabsorbed light after travelling once through the sensitized layer was completely lost, affecting PCE. A proposed solution was addition of a layer composed by large particles (in the range between 200–400 nm) that induces light scattering inside the photoanode and increases probability of photon absorption. The use of opaque films for light managing demonstrated of critical importance for the increase of PCE (PCE up to 11.1% was obtained by using the so-called “haze concept”).[63] The haze is defined as the ratio of diffused transmittance to total optical transmittance. It was found that the IPCE of DSSCs increases with increase in the haze of the TiO2 electrodes, especially in the near infrared wavelength region (Figure 8). A drawback in the use of haze is that large particles, which allow enhanced light scattering in the visible range, are characterized by rather low specific surface compared to the typical nanoparticles applied in photoanodes. For this reason research addressed to application of hierarchically assembled structures, able to effectively scatter visible radiation and maintaining the high specific surface of a mesoporous layer (Figure 9).[64]
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Figure 8. Dependence of IPCE spectra on haze of TiO2 electrodes. Haze in the figure was measured at 800 nm. Reproduced with permission.[63] Copyright 2006, Japan Society of Applied Physics.
R. Caruso and co-workers[64] demonstrated the application of nanocrystalline, mesoporous TiO2 beads with surface areas up to 108.0 m2 g−1 and tunable pore sizes through a combination of sol-gel and solvothermal processes. The mesoporous beads are composed of anatase TiO2 nanocrystals. The functional properties of DSSCs fabricated using these beads are reported in Table 1 and compared to Degussa P25 TiO2 photoanodes. An impressive increase in IPCE was achieved (Figure 9) thanks to the highly scattering photoanode and its high specific surface.
Almost similar PCE (7.40%) was achieved by applying an analogous concept through TiO2 hollow spheres (150–200 nm in diameter) produced by selective etching of Au@TiO2 coreshell nanoparticles and applied as scattering layer atop commercial nanoparticle layer.[65] The increment in efficiency was related to efficient light scattering, electrolyte diffusing feasibility for better electron transport, and a high surface area allowing higher dye loading. In an effort to capitalize both on the light harvesting and charge collection, a layered system was recently proposed, composed of three mesoporous stacks made of shape-tailored TiO2 anatase nanocrystals, which have been ad hoc synthesized by suitable colloidal routes.[66] The increased efficiency of this multilayered DSSC was primarily due to the enhancement in Jsc related to the larger amount of adsorbed dye due to increased surface area (2.7 × 10−7 mol cm−2 compared to 2.0 × 10−7 mol cm−2 for the reference DSSC). In addition, the system demonstrated significant improvement in charge collection efficiency, as deduced from the measurement of electron diffusion length, which was 30% larger in the multilayered cell, compared to benchmarking photoanode. In Table 2 we report the efficiencies and functional parameters of various kinds of DSSCs based on different TiO2 photoanodes under one sun irradiation. As a concluding remark on DSSCs based on TiO2, we highlight the efforts of the scientific community to develop technologically relevant platforms to reduce the production times of this kind of cells. In particular, a very recent paper from O’Regan and co-workers demonstrated the reliability of an ultrafast sintering process based on near infrared (NIR) radiation, which results in an operating photoanode after 12.5 seconds sintering. The photovoltaic performance of devices made with NIR sintered films match those devices made with conventionally sintered films prepared by heating for 1800 seconds. The authors demonstrated that in NIR processing of TiO2 the rapid heating (to temperatures of up to 785 °C) does not lead to a large scale rutile phase transition and consequent loss of photocurrent.[69]
3. Metal Oxides Alternative to TiO2
Figure 9. Top and bottom left: SEM images of calcined mesoporous TiO2 beads obtained after a solvothermal process with different amounts of ammonia. Top right: Diffuse reflectance spectra of the TiO2 films prepared from P25 nanoparticles and mesoporous TiO2 beads (sample S4) of varying thickness. Bottom right: Incident photon to current conversion efficiency (IPCE) curves of the TiO2 electrodes prepared from P25 nanoparticles and mesoporous TiO2 beads (sample S4) of two film thicknesses. Reproduced with permission.[64] small 2014, DOI: 10.1002/smll.201402334
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Other semiconductor metal oxides have raised a wide interest as potential alternatives to TiO2. Among them, particular attention is being paid to ZnO and SnO2: both these materials present higher electron mobility as compared with TiO2 as well as specific advantages. ZnO has indeed a band structure very similar to that of TiO2 and the possibilities of synthesis are almost infinite, as for shapes and sizes, being this oxide probably the most studied one for several
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reviews www.MaterialsViews.com Table 1. Photovoltaic properties of the dye-sensitized solar cells assembled by using electrodes made from Degussa P25 particles and mesoporous beads of different thickness. From Ref. [64] Sample
Thickness (µm)
Voc (mV)
P25
TiO2 beads
Jsc (mA cm−2)
FF
PCE (%)
5.5
781
9.99
0.66
5.11
10.1
758
10.98
0.69
5.73
12.9
748
10.60
0.71
5.66
5.4
797
9.84
0.72
5.65
9.8
769
11.25
0.70
6.06
12.3
777
12.79
0.72
7.20
applications, spanning from photovoltaics, to catalysis, gas sensing and batteries.[70–76] On the other hand, SnO2 has a wider band gap (about 3.8 eV) that is believed to guarantee for a better stability under UV illumination. Moreover, the position of SnO2 conduction band at higher energy would allow the successful application as light harvester of dyes able to collect light in the near infrared (NIR). However, it should be remarked that, as we will see, most of studies applying SnO2 as photoanode in DSSCs exploit dye N719 as sensitizer, despite an unfavorable band alignment between the two partners: this approach typically results in low photovoltages and consequently in overall low performances.
3.1. ZnO Photoanodes ZnO is a wide bang gap semiconductor (3.37 eV) that has attracted attention since very long ago, due to interesting optoelectronic features and a well-established polymorphism, which has enabled researchers to obtain ZnO in countless morphologies. It would be almost impossible providing the reader with a comprehensive introduction on synthetic approaches, morphologies and possible applications of this fascinating and
versatile material. However, a review on this amazing semiconductor metals oxide can be found in Ali and Winterer.[77] In the frame of possible exploitation in energy application, ZnO has been the first semiconductor metal oxide for which the possibility of electron injection from a molecular dye was demonstrated.[78] Since then, a huge amount a literature on the exploitation of ZnO in solar devices, and more generally in photoelectrochemical cells, has been published. As for DSSCs, most papers focus on the application of (more or less) new ZnO morphologies as photoanodes and a large variety of micro- and nano- ZnO structures have been tested. A comprehensive review of the topic has been published in 2012 by Anta and coworkers[79] and the reader is mainly referred to this paper for the works published up to 2011. Literature results over the years demonstrate, as we are going to highlight and as already pointed out by Anta and coworkers, that application of ZnO as anode in DSSCs is no trivial challenge, due to several reasons. It has to be remarked that the application of ZnO poses issues even from the viewpoint of dye uptake, which is, on the contrary, a well-established procedure for TiO2-based electrodes. Early studies carried out by photochemists, dating before the advent of DSSCs, had demonstrated that effective dye uptake can be realized with ZnO and irreversible charge injection from the molecular dye is possible if an electrochemical gradient was operating inside the electrode,[78] but the application of ruthenium complexes appeared critical since the beginning, especially related to the acidic environment determined by the dyes, in which ZnO is suffering. Evidences of dye agglomeration on ZnO porous films have been found:[80] after surface coverage from dye, “extra” molecules accumulate on the film, which are not only useless from injection viewpoint, but behave as a shield for light absorption towards dye molecules directly anchored to the metal oxide, thus decreasing the overall injection efficiency and IPCE. Differences in chemical bonding of classical N719 and N3 dyes have been observed by several the authors, which may affect the injection processes under irradiation and, eventually, the device performances.
Table 2. Functional properties of DSSCs based on various shapes/structures of TiO2 photoanodes sensitized with different dye molecules under AM 1.5 G irradiation, 100 mW cm−2. Shape/Structure 7 um mesoporous TiO2 TiO2 NP (5 µm T + 5 µm scattering
Dye
Voc [mV]
Jsc
FF
PCE [%]
Ref.
SM315
910
18.1
0.78
13.0
[10]
YD2-o-C8 / Y123
935
17.66
0.74
12.3
[9]
11 µm T TiO2 NP + 5 µm layer of 400 nm reflecting particles
YD-2
770
18.6
0.764
11.0
[67]
Multi-stack nanocrystals
N719
810
18.10
0.70
10.26
[66]
Anatase TiO2 NP/NW Composites
N719
610
25.5
0.55
8.6
[36]
ZnO NW/TiO2 NP network
N719
763
16.08
0.688
8.44
[68]
TiO2 beads
N719
777
12.79
0.72
7.20
[64]
Nanotubes
N719
730
15.8
0.59
6.9
[48]
Rutile TiO2 nanorod arrays
N719
710
20.49
0.55
7.91
[44]
Porous Titania Nanosheet/NP
N719
750
19.2
0.70
10.1
[46]
Nanotubes on flexible plastic substratea)
N719
759
8.1
0.55
3.5
[54]
Nanotubes on flexible Ti substratea)
D205
709
8.99
0.561
3.58
[53]
a)Comparison
of two kinds of flexible solar cells illustrating the possibility of substituting the flexible, but heavy and expensive Ti foil with a plastic and highly versatile substrate.
8 www.small-journal.com
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small 2014, DOI: 10.1002/smll.201402334
www.MaterialsViews.com
Figure 10. a,b,d–f) SEM analysis of the hierarchical ZnO photoanode reported in Memarian et al.[15] a) Cross-sectional SEM image showing the layered structure of the photoanode: the active layer, composed of hierarchically structured ZnO nanocrystals, the compact buffer layer, and the FTO conducting layer. b) Top view of the absorbing layer, showing a highly disordered structure. c) Size distribution of the aggregates, as estimated by SEM. Inset: shape analysis of the aggregates from the SEM image in (b), with elliptical shapes predominant. d,e) High-resolution SEM images of two aggregates, showing the closely packed assembly of the nanoparticles. f) High-resolution SEM image of the buffer layer, indicating its compact morphology with respect to the polydispersed aggregates. Reproduced with permission.[15]
The nature of applied dye is then something to pay attention to and specific studies would be still needed in order to improve the capability of solar light conversion. For those devices exploiting the usual N710 dye, dye loading time becomes a critical parameter to take into account in order to especially maximize the photocurrent inside the device.[80–83] Another critical issue, apparently still open, concerns the efficiency of electron injection process from the excited dye to the ZnO conduction band. In their review, Anta and coworkers identified this as the most critical topic to address, suggesting that a limited number of studies have been carried out especially devoted to investigate this fundamental process. We fully agree with this statement and we have to remark that today this point would still deserve attention. Some authors have tried to address the issue, with different success rates. An interesting paper has been published by Hagfeldt in 2001,[84] focusing on dynamics of electron injection and recombination in N719-ZnO nanostructures. By applying femto- and nano-second spectroscopies, the authors found ultrafast (
