PCCP View Article Online
Published on 07 January 2014. Downloaded by New York University on 09/10/2014 21:11:53.
PERSPECTIVE
Cite this: Phys. Chem. Chem. Phys., 2014, 16, 5026
View Journal | View Issue
Cu(I)-based delafossite compounds as photocathodes in p-type dye-sensitized solar cells Mingzhe Yu, Thomas I. Draskovic and Yiying Wu* The research of p-type dye-sensitized solar cells (p-DSSCs) has attracted growing attention because of the potential for integration with conventional n-type DSSCs (n-DSSCs) into the more efficient tandemDSSCs. However, to date the performance of p-DSSCs is lagging behind that of n-DSSCs. One main reason is the lack of optimal photocathode materials. This article reviews the most recent progress in utilizing Cu(I)-based delafossite compounds, CuMO2 (M = Al, Ga or Cr), as photocathodes in p-DSSCs.
Received 11th November 2013, Accepted 6th January 2014
As alternative materials to the commonly used NiO, the CuMO2 compounds have their intrinsic advantages
DOI: 10.1039/c3cp55457k
such as lower valence band edge, larger optical bandgap and higher conductivity. By providing an insight into these materials and their applications in p-DSSCs, this perspective aims to stimulate more exciting
www.rsc.org/pccp
research in the development of p-DSSCs as well as of tandem-DSSCs.
1. Introduction Dye-sensitized solar cells (DSSCs) are regarded as among the most promising photovoltaic technologies because of their low fabrication cost and high energy conversion efficiency.1–2 Recently the idea of tandem-DSSCs, with a theoretical photon to energy conversion efficiency (PCE) over 40%, has been raised.2–4 By integrating a conventional n-type TiO2-DSSC with a p-type DSSC, the tandem-DSSC is expected to have a higher open-circuit voltage (Voc) and achieve broader light absorption of the solar Department of Chemistry & Biochemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210, USA. E-mail: [email protected]; Fax: +1-614-292-1685; Tel: +1-614-247-7810
Mingzhe Yu is currently a PhD candidate in the Department of Chemistry and Biochemistry at the Ohio State University. He obtained his BSc in the Materials Science and Engineering Department at the University of Science and Technology of China (USTC) in 2010 and then joined Dr Wu’s group as a graduate student. His research is focused on the synthesis and characterization of delafossite semiconductors with Mingzhe Yu controlled-structures and their applications in the energy conversion and storage fields, including dye-sensitized solar cells and metal-air batteries.
5026 | Phys. Chem. Chem. Phys., 2014, 16, 5026--5033
spectrum5–7 (Fig. 1a). The construction of a highly efficient tandem-DSSC requires the efficient p-DSSC that has a comparable performance to the n-DSSC. However, the research on p-DSSCs is a relatively young field and the efficiency of p-DSSCs still has much room to improve: the first NiO-based p-DSSC was reported by He et al. in 19998 and the highest PCE value to date is 1.30% under AM 1.5 illumination.9 The p-DSSCs are based on the cathodic sensitization of p-type semiconductors. Fig. 1b shows the operation principle of a p-DSSC: upon illumination, dye molecules which are chemisorbed on the photocathode will be excited from its highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). This triggers the electron transfer from the photocathode’s valence band (VB) to the dye. The hole
Thomas Draskovic
Thomas is a graduate student in the Department of Chemistry and Biochemistry at the Ohio State University. He received his BS in chemistry in 2011 from the Pennsylvania State University. His current research interests include the synthesis, characterization, and doping of delafossite structured semiconductors with the desire to utilize nanoparticle films of these materials in photovoltaic and photocatalytic cells.
This journal is © the Owner Societies 2014
View Article Online
Published on 07 January 2014. Downloaded by New York University on 09/10/2014 21:11:53.
Perspective
Fig. 1 Schematic structures of (a) the tandem-DSSCs and (b) the p-DSSCs.
left in the VB of the photocathode then diffuses to the back contact which is linked to the outside-circuit. Meanwhile, by reducing the redox mediator, the dye gets regenerated. And the
Professor Yiying Wu received his BS in chemical physics from the University of Science and Technology of China in 1998, and his PhD in chemistry from the University of California at Berkeley in 2003 with Professor Peidong Yang. He then did his postdoctoral research with Professor Galen D. Stucky at the University of California, Santa Barbara, and joined the chemistry faculty at The Ohio Yiying Wu State University in the summer of 2005. He was promoted to associate professor with tenure in 2011. His group focuses on materials chemistry for energy conversion and storage.
This journal is © the Owner Societies 2014
PCCP
redox mediator is recovered on the counter-electrode by giving away the electron to the outside-circuit. NiO is the most extensively investigated photocathode material in p-DSSCs.10–11 A lot of research has been done to improve the efficiency of NiO-based solar cells, including improving the quality of NiO films,12–14 molecular engineering of specific sensitizers,15–16 exploring novel redox mediators,9,17 etc. Despite all the efforts, the record efficiency for p-DSSCs (1.30%)9 is still lower than that of n-DSSCs (12.3%).2 One crucial reason is the missing of optimal wide bandgap p-type semiconductor which is equivalent to the anatase TiO2 as in n-DSSCs.10,18 Though widely employed, in fact NiO is not an optimal material as photocathodes in p-DSSCs for the following reasons: (1) although reported as a wide bandgap material (Eg B 3.55 eV), NiO films have a significant light absorption in the visible range (30–40% for a 2.3 mm thick film5) likely due to the d–d electron transition, and possibly a residual amount of nickel metal left in the NiO films.19 The sensitizers have to compete with NiO for light absorption and therefore the incident photon-to-current efficiency (IPCE) is limited. (2) In p-DSSCs, the Voc is governed by the difference between the quasi-Fermi level of the photocathode (at best, close to the VB edge of the p-type semiconductor) and the potential of the redox mediator (Fig. 1b). The VB edge of NiO is +0.54 V (vs. NHE, pH = 7),3 close to the potential of the commonly used I3/I redox mediator (i.e. +0.35 V vs. NHE). As a result, the maximum obtainable Voc is small. (3) Due to the low conductivity, NiO films have a low hole diffusion coefficient (i.e. 4 108 cm2 s1),20 which is 2–3 orders lower than that of TiO2, and this limits the diffusion length of hole carriers. Considering the drawbacks of NiO, alternative p-type semiconductors with better optical transparency, lower VB edge position and higher hole mobility are desired for p-DSSCs. Several materials have been investigated as photocathodes in p-DSSCs, including CuO,21 CuSCN,22–23 GaP,24 boron-doped diamond25 and the Cu(I)-based delafossite compounds, CuMO2 (M = Al, Ga or Cr).26–32 Among them, the CuMO2 series materials have shown very promising performances in p-DSSCs because of their intrinsic advantages over NiO. This perspective will provide an insight and discussion on these delafossite CuMO2 compounds and their applications as photocathodes in p-DSSCs. Delafossite, named in honor of the mineralogist Gabriel Delafosse, is a copper iron oxide compound with the formula CuIFeIIIO2. The Cu(I)-based delafossite compounds refer to a group of metal oxides which have the same crystal structure as the delafossite CuFeO2. They have the formula CuIMIIIO2, where the trivalent M atom can be B, Al, Ga, In, Sc, Y, Cr, Fe, Co or Ni.33 As shown in Fig. 2a, the crystal has a layered structure with one layer as the close-packed monovalent Cu atoms and the other as edge-shared MIIIO6 octahedra. The CuI atoms are coordinated to two oxygen atoms with the linear O–Cu–O bonds. In the MIIIO6 octahedra layer, each MIII atom is coordinated with six O atoms while each O atom is coordinated with three MIII atoms and one CuI atom. By adopting an ABCABC stacking or ABABAB stacking, the crystal can have either a rhombohedral (3R) or hexagonal (2H) symmetry.
Phys. Chem. Chem. Phys., 2014, 16, 5026--5033 | 5027
View Article Online
PCCP
Perspective Table 1 The optical bandgaps, VB edge positions and conductivity of the CuMO2 compounds (M = Al, Ga or Cr)3,35,38,41–45
VB edge position (pH = 9.2)
Published on 07 January 2014. Downloaded by New York University on 09/10/2014 21:11:53.
CuAlO2 CuGaO2 CuCrO2
Optical bandgap (eV)
(eV, vs. vacuum)
(V, vs. NHE)
Conductivity (S m1)
3.5 3.4–3.7 2.95–3.30
B5.1 B5.1 B5.3
B+0.6 B+0.6 B+0.8
B2 101 B6 101 B1 102
eliminated. As a result, in addition to the p-type conductivity, the Cu(I)-based delafossite compounds also have a large optical bandgap with the high transparency (Table 1).35,38,41–42 In the 1980’s, Benko et al. did a series of photoelectrochemical studies and revealed that the VB edges of CuMO2 (M = Al, Ga or Cr) locate at +0.6–0.8 V (vs. NHE, pH = 9.2), which is lower than that of NiO.3,43–45 As mentioned above, the lower VB edge indicates a higher obtainable photovoltage when applied in p-DSSCs. Moreover, the CuMO2 compounds also have significantly higher hole mobilities than NiO. The hole mobility for CuMO2 (M = Al or Ga) is reported to be 101–102 cm2 V1 s1.35,43–44 While for NiO, the hole mobility is only B2 106 cm2 V1 s1 due to the localized Ni 3d orbitals.20 The higher hole mobility of CuMO2 is expected to favor the faster hole transport kinetics in p-DSSCs. Therefore, the research of utilizing Cu(I)-based delafossite compounds as photocathodes in p-DSSCs has been initialized during the past few years. And the p-DSSCs based on delafossite CuAlO2,26,46 CuGaO228–29,32 and CuCrO230–31 are reported with very promising performance. Fig. 2 (a) The crystal structure of the delafossite compounds. (b) The calculated band-structures of the CuMO2 compounds from the sX-LDA method. Reprinted with permission from ref. 37. Copyright (2011) by The American Physical Society.
2. p-DSSCs based on delafossite CuMO2 (M = Al, Ga or Cr) 2.1.
Benefiting from the crystal structure, the Cu(I)-based delafossite compounds are found to be promising p-type transparent conductive oxides (p-TCO).34–36 For most metal oxides, due to the strong electronegativity of oxygen, the valence band edge is localized on the O 2p orbitals. The positive holes in the band edge are deeply ‘‘trapped’’ and cannot migrate freely in the lattice. Hence, it is difficult for metal oxides to achieve p-type conductivity through acceptor doping. However, in the Cu(I)-based delafossite compounds, the Cu(I) has a fully filled 3d orbital with an energy close to the O 2p orbital. By forming the occupied O 2p–Cu 3d antibond, the valence band edge is now a hybrid of O 2p and Cu 3d orbitals and delocalized from the oxygen.37 Therefore, the holes introduced by defects of Cu vacancies and O interstitials can migrate freely in the valence band edge and the compounds exhibit good p-type conductivity.35,38–40 At the same time, in the layered –MO2–Cu–MO2– structure, the crosslinking of Cu to Cu is only two dimensional. The direct interaction between 3d10 electrons on neighboring CuI atoms, which may reduce the bandgap, is weak. Moreover, since the CuI has a d10 closed-shell configuration, the coloration due to intra-atomic excitation is also
5028 | Phys. Chem. Chem. Phys., 2014, 16, 5026--5033
CuAlO2-based p-DSSCs
In 2006, Yasomanee’s group demonstrated the concept of applying CuAlO2 in solid-state DSSCs for the first time.26 The solar cell with a configuration of TiO2/dye/CuAlO2 was fabricated by dropping the CuAlO2 suspension on the Ru-dye sensitized TiO2 film to form a hole-conducting layer. The I–V performance confirmed that CuAlO2 is a suitable material as a hole-conductor: under 100 mW cm2 visible band illumination, the short-circuit current ( Jsc) and Voc were recorded as 0.08 mA cm2 and 524 mV, respectively. Being consistent with the material’s lower VB position, the CuAlO2-based solar cells exhibit higher Voc compared to the analogous TiO2/dye/NiO device (Voc = 510 mV). However, the photocurrent of the CuAlO2-based solar cells was low. The author attributed this to the poor contact between the large-sized CuAlO2 particles (B5 mm in size) and the dye. More recently, Cheng’s group reported the fabrication of CuAlO2-based p-DSSCs with an improved Voc value in 2011.46 In their work, CuAlO2, with the majority of particles around 1–2 mm in size, was synthesized through a solid-state reaction. By photoelectron spectroscopy in air (PESA) measurements, the author confirmed that the VB position for CuAlO2 is at B5.3 eV below vacuum (i.e. B+0.8 V vs. NHE), which is about 200 mV
This journal is © the Owner Societies 2014
View Article Online
Published on 07 January 2014. Downloaded by New York University on 09/10/2014 21:11:53.
Perspective
PCCP
Fig. 3 The I–V curve of the CuAlO2/PMI-6T-TPA p-DSSCs. Reprinted with permission from ref. 27. Copyright (2011) by the International Society of Optics and Photonics.
lower than NiO (B5.1 eV below vacuum, +0.6 V vs. NHE). The solar cells were fabricated by sensitizing the CuAlO2 films with the highly efficient PMI-6T-TPA sensitizer5 and an I3/I-based electrolyte. The I–V curve of the solar cell is shown in Fig. 3. Promisingly, the author observed a Voc of 333 mV under AM 1.5 illumination, which is substantially higher than the analogous NiO-based solar cells (Voc = 218 mV).5 The B115 mV Voc improvement can explained by the B200 mV lower VB position of CuAlO2 than NiO according to the PESA results. Along with the improvement of Voc, the fill factor (FF) for CuAlO2 cells is around 40%, also being higher than that of NiO devices. However, due to the large particle size, the limited film surface area (1.7 m2 g1) for dye-adsorption results in a low photocurrent ( Jsc = 0.3 mA cm2 under AM 1.5 illumination). Despite the low photocurrent, a peak IPCE value of 4.0% was still recorded at 430 nm, where the dye absorption spectra reaches its maximum, and confirmed that the photocurrent is generated by the dye. 2.2.
CuGaO2-based p-DSSCs
In 2012, Wu’s group and Jobic’s group independently reported the application of hydrothermally synthesized CuGaO2 nanoplates as photocathodes in p-DSSCs.28–29 In contrast to the dark-colored NiO, the CuGaO2 was white with a pale yellow tinge. It was found that CuGaO2 nanoplates are thermokinetically stable below 350 1C in air and therefore compatible for the annealing treatment in the solar cell fabrication process. Wu and co-workers fabricated the solar cell with a donor–acceptor P1 dye47 and the I3/I or Co3+/2+ (dtb-bpy) electrolyte.28 The dependence of Voc on the illumination intensity was measured to estimate the maximum obtainable Voc of the CuGaO2-based solar cells. The author recorded a saturation photovoltage of 464 mV in the CuGaO2based p-DSSCs with the Co3+/2+ (dtb-bpy) electrolyte, which is an increase of more than 100% compared to that of NiO-based solar cells under the same conditions (Fig. 4). This substantial increase also remained when the I3/I electrolyte was applied (243 mV vs.132 mV for CuGaO2 and NiO, respectively).
This journal is © the Owner Societies 2014
Fig. 4 Dependence of Voc on light intensity: (a) CuGaO2-DSSC and NiODSSC with the simple I3/I electrolyte; (b) CuGaO2-DSSC and NiO-DSSC with the Co3+/2+ (dtb-bpy) electrolyte. Reprinted with permission from ref. 28. Copyright (2012) American Chemical Society.
The enhancement of Voc was explained by the lower VB edge of CuGaO2, and confirmed by the Mott–Schottky analyses: Jobic and co-workers obtained the flatband potential (Efb) of CuGaO2 and NiO of B+0.73 V and B+0.57 V, respectively (vs. NHE, pH = 6.3) (Fig. 5). Under AM 1.5 illumination, for the CuGaO2based solar cells with the Co3+/2+ (dtb-bpy) electrolyte, the two groups obtained an impressive Voc of 357 mV with the P1 sensitizer and 375 mV with the PMI-NDI sensitizer. The major
Fig. 5 The comparison of the flat-band potential and Voc between NiO and CuGaO2. Reprinted from ref. 29 with permission from The Royal Society of Chemistry.
Phys. Chem. Chem. Phys., 2014, 16, 5026--5033 | 5029
View Article Online
Published on 07 January 2014. Downloaded by New York University on 09/10/2014 21:11:53.
PCCP
Perspective
limitation of the CuGaO2 solar cells is the modest current density: the short-circuit current ( Jsc) is smaller than 0.4 mA cm2 with the I3/I electrolyte under AM 1.5 illumination, and even lower when the Co3+/2+ (dtb-bpy) electrolyte was applied. By carrying out the dye adsorption isotherm tests and Brunauer–Emmett– Teller (BET) measurements, both groups attributed the limited photocurrent to the much lower surface area of the CuGaO2 nanoplates (B30 m2 g1 and B158 m2 g1 for CuGaO2 and NiO, respectively), which limits the dye-loading amount and the light harvesting efficiency. Most recently, a short-circuit current of 2.05 mA cm2 was reported using a mechanically pressed filmed of slightly smaller CuGaO2 plates (25 nm 100–200 nm) with the P1 dye and the I3/I electrolyte. This greatly promising result further indicates the potential of and need for high-quality nanoparticle films.32 2.3.
CuCrO2-based p-DSSCs
Among the Cu(I)-based delafossite compounds, CuCrO2 is reported with a high electrical conductivity up to 1.0 S cm1.40 Due to the d–d transition at the Cr3+ site, the CuCrO2 exhibits a relatively small bandgap of 2.95–3.30 eV.41 In 2012, Chen’s group reported the hydrothermal synthesis of ultrasmall CuCrO2 nanocrystals and applied them as photocathodes in p-DSSCs.30 The synthesized CuCrO2 had a typical size of 15 nm 5 nm (Fig. 6a) and a surface area of 87.86 m2 g1. The nanocrystals are thermally stable up to 400 1C. By the Mott–Schottky analyses, it was confirmed that CuCrO2 had a lower VB edge than NiO (Efb of +0.84 V and +0.56 V vs. NHE, for CuCrO2 and NiO, respectively). The optimized CuCrO2-based solar cell was fabricated with the coumarin 343 dye (C343) and the I3/I electrolyte, and achieved a Voc value of 102 mV under AM 1.5 illumination, being higher than its analogous NiO device (Voc = 90 mV). By carrying out the photocurrent–photovoltage transient decay measurements, the author highlighted that CuCrO2 cells have a better transport property than the analogous NiO cells with a remarkably longer hole lifetime tr (B101 vs. B102 s), higher diffusion coefficient D (B105 vs. B107–106 cm2 s1) and longer diffusion length L (B10 mm vs. 1–2 mm). These enhancements were attributed to the better conductivity of CuCrO2. It is worth mentioning that although the CuCrO2 nanocrystals have a comparable
Fig. 6 (a) The transmission electron microscopy (TEM) image of the hydrothermally synthesized CuCrO2. Reprinted from ref. 30 with permission from the Royal Society of Chemistry. (b) The molecular structures of 1-methyl-1H-tetrazole-5-thiolate (T) and its disulfide dimer (T2). Reprinted from ref. 31 with permission from The Royal Society of Chemistry.
5030 | Phys. Chem. Chem. Phys., 2014, 16, 5026--5033
surface area to the TiO2 in n-DSSCs, the CuCrO2 films are way thinner than the conventional 10–20 mm-thick TiO2 films. (Thicker CuCrO2 films would absorb light significantly due to their dark color.) As a result, the photocurrent of CuCrO2DSSCs is still limited ( Jsc o 1 mA cm2 under AM 1.5 illumination). Very recently in 2013, the same group reported that by utilizing the 1-methyl-1H-tetrazole-5-thiolate (T) and its disulfide dimer (T2) as a redox mediator, together with a P1 sensitizer and a CoS counter-electrode, the CuCrO2-based p-DSSCs can achieve an improved performance with Voc of 304 mV, Jsc of 1.73 mA cm2 and PCE of 0.23% under AM 1.5 illumination.31 The T2/T electrolyte has a high transparency compared to the I3/I electrolyte and therefore allows better light utilization. And through the scanning electrochemical microscopy (SECM) and the electrochemical impedance study (EIS) analyses, the author observed the faster charge transfer kinetics in the dye regeneration process with the T2/T redox mediator, which is apparently another advantage. Therefore, the author concluded that integrated with the CoS counter-electrode, the new T2/T can be a more suitable redox mediator for the CuCrO2-based p-DSSCs as well as for tandem-DSSCs.
3. Future prospects As summarized in Table 2, with the substantially higher photovoltages over the analogous NiO devices, the great potential of CuMO2-based p-DSSCs has been clearly demonstrated in the recent few studies. Though the door of utilizing more efficient photocathode materials in p-DSSCs has been opened, the modest photocurrent is still hindering these solar cells to achieve higher efficiency. Towards the more efficient p-DSSCs, progresses on the following aspects are expected. 3.1. The synthesis of CuMO2 nanoparticles with controlled size and morphology Firstly and most importantly, a breakthrough at the synthetic bottleneck of getting CuMO2 nanostructures with controlled size and morphology is highly anticipated. Ideally, the spherical CuMO2 nanocrystals with the size around 20–40 nm (which is the typical size of TiO2 nanoparticles in n-DSSCs,) are desired to support a sufficient dye-loading on the photocathodes in p-DSSCs. In the current stage, apparently the large particle-size and small surface area of CuMO2 particles are the main reasons that limit the light-harvesting efficiency and photocurrent of the solar cells. The conventional synthetic strategy for ternary metal oxide is solid-state reaction, which usually yields large particle sizes (i.e. >1 mm) due to the high-temperature agglomeration. The recently developed hydrothermal approach provides chances of getting smaller particles (e.g. B200 nm for CuGaO2 and B15 nm for CuCrO2)30,48 and is also appealing to the costefficient concept with the fabrication of DSSCs. The anisotropic delafossite crystal structure results in the c-plane growth less favored than the a,b-plane and the final CuMO2 nanoparticles having a plate-like morphology.28,30 When fabricated into
This journal is © the Owner Societies 2014
View Article Online
Perspective Table 2
PCCP
The Voc, Jsc, FF and PCE(Z) for Cu(I)-based delafossite p-DSSCs and the Voc of analogous NiO cells under AM 1.5 illumination
Published on 07 January 2014. Downloaded by New York University on 09/10/2014 21:11:53.
Redox mediator
Sensitizer
Voc (mV)
0 b Voc (mV)
Jsc (mA cm2)
FF (%)
Z (%)
Ref.
CuAlO2a
I3 /I
Ru-dye
524
510
0.08
54.3
0.046
26
CuAlO2
I3/I
PMI-6T-TPA
333
218
0.33
40
0.041
46
CuGaO2
I3/I Co3+/2+ I3/I I3/I Co3+/2+
P1
180 357 199.3 187 375
105 149 108.4 130 285
0.384 0.165 2.05 0.29 0.12
37.6 30.8 44.5 41 33.2
0.026 0.018 0.182 0.023 0.015
28
I3/I T2/T
Coumarin 343 P1
90 —
0.584 1.73
31.8 44
0.016 0.23
CuCrO2
PMI-NDI
102 304
32 29 30 31c
a The CuAlO2 acted as the hole conductor for the sensitized TiO2 solar cell. b The Voc of analogous NiO solar cells under the same conditions for comparison. c The catalyst on the counter-electrode used here is CoS instead of Pt.
films, the nanoplates prefer a dense stacking along the basal planes. This eventually results in an even more limited exposed surface area available for sensitizer molecules adsorption. Therefore, in addition to achieving smaller size, controlling of the product’s morphology is also essential. The successful hydrothermal syntheses of delaffosite CuGaO2 and CuCrO2 nanoparticles are definitely encouraging. However, the chemistry of hydrothermal syntheses of CuMO2 is complicated and varies with different M cations. Briefly speaking, the stabilization of Cu(I) and olation of the M cations in aqueous solvents are two key factors that should be considered carefully. There has been no specific study on the phase formation and particle growth mechanism of these nanocrystals reported. To achieve a better control on the size and morphology of the delafossite nanocrystals, more research efforts on the deeper understanding of the basic chemistry and crystallization process in the hydrothermal synthesis of these delafossite nanocrystals are anticipated. Meanwhile, it is worth pointing out that focusing on film fabrication techniques can also provide significant improvements in device performance, even with particles of traditionally undesirable size or morphology.32 3.2. The thermal and chemical stability of the CuMO2 compounds Secondly, investigations on the stability of the nanostructured CuMO2 compounds are also of considerable importance. The Cu(I) is thermodynamically unstable at low temperatures and can be oxidized into Cu(II) through the following reaction: 2CuIMO2 + 1/2O2 - CuIIM2O4 + CuO This raises the concern that whether the CuMO2 is capable for the annealing treatment in the solar cell fabrication process. In the previous work, CuGaO2 and CuCrO2 nanocrystals have been reported to be thermokinetically stable with the annealing treatment in air up to 350 1C and 400 1C, respectively.28,30 Therefore, to prevent the thermodecomposition of the delafossite phase, caution needs to be taken when deciding the annealing temperature for CuMO2 films. Also, using an inert atmosphere such as flowing argon is expected to improve the stability of these compounds.
This journal is © the Owner Societies 2014
The chemical stability of CuMO2 is another aspect that should be carefully evaluated. In a real solar cell, the semiconductor surface is in direct contact with the chemisorbed dye molecules. Therefore, the reactivity between the CuMO2 nanocrystals and the acidic dye molecules is important. Cu(I) has a rich coordination chemistry and in the CuMO2, the Cu(I) atoms are unsaturated, coordinated by only two oxygen atoms (Fig. 2a). Therefore, there are chances for the Cu(I)+–H+ ion exchange reaction to happen between the surface Cu(I) and the anchoring group (usually carboxylic) of the sensitizers, and this may lead to the phase change of the CuMO2 photocathodes. Though so far there is no specific research reported on this topic, we notice that Jobic and co-workers emphasized that the CuGaO2–C343 solar cells failed to give any photocurrent in their study.29 This may be due to an ion-exchange reaction that happened between the surface Cu(I) of CuGaO2 and the carboxyl group of the C343 molecule. Interestingly, Chen’s group employed C343 as sensitizers for the CuCrO2 p-DSSCs and recorded reasonable I–V performance.30 One possible explanation may be that the reactivity of CuMO2 differs from one to another since the different MO6 octahedra can affect O–CuI–O bonding strength. Therefore, a careful investigation on the chemical stability study of CuMO2 compounds in different acidic dye solutions is essential and should provide crucial information for the sensitizer designing. Moreover, surface passivation and protection of the CuMO2 surface by atomic layer deposition (ALD) coating of a stable metal oxide such as Al2O3 can also help to improve the chemical stability of the compounds. 3.3.
The device physics study of CuMO2-based p-DSSCs
The device physics study of CuMO2-based p-DSSCs is essential for obtaining a deeper understanding of the materials as well as the solar cell performance. Being different from the isotropic rock-salt structure of NiO, the delafossite structure gives CuMO2 many anisotropic properties along the a,b-plane and along the c-axis, such as conductivity, dielectric constant, absorption, etc.37,42 These properties have a direct influence on the charge transport and recombination processes in the CuMO2-based p-DSSCs. The preliminary work by Chen and co-workers revealed the superior charge transport property of
Phys. Chem. Chem. Phys., 2014, 16, 5026--5033 | 5031
View Article Online
Published on 07 January 2014. Downloaded by New York University on 09/10/2014 21:11:53.
PCCP
Perspective
CuCrO2-based p-DSSCs and also provided a good example of studying the charge carrier dynamics in the new CuMO2 system.30 However, so far there is no specific study documented on the fundamental charge transfer kinetics at the delafossite/dye/ electrolyte interface. A symmetrical study with techniques such as EIS and photocurrent/photovoltage transient decay techniques shall benefit the deeper understanding of the materials’ physiochemical properties and also provide the diagnostic information for further improving the efficiency of the CuMO2-based p-DSSCs. There are also other aspects in the device physics field that could be of interest and deserve more research attention. For instance, the solid-state physics community can focus on how to push the valence band of the delafossite compounds even more positive with the aim of achieving higher Voc. While beyond the scope of this review, electrochemists can look closer into finding an electrolyte with more negative redox potential. In addition, the dye–electrolyte interaction plays a critical role in the n-DSSCs’ high performance. It is important to investigate if it is the similar case in the delafossite-based p-DSSCs. 3.4.
MIII cations beyond Al, Ga or Cr
As mentioned in Section 2.1, the Cu(I)-based delafossite compounds CuMO2 can have the trivalent M atom as B, Al, Ga, In, Sc, Y, Cr, Fe, Co or Ni. While only AlIII, GaIII and CrIII based CuMO2 compounds have been explored as photocathodes in p-DSSCs. In fact, some other cations that have either empty or fully filled d orbitals (with which no d–d transition absorptions will be introduced) such as BIII, ScIII or YIII should also be suitable candidates as photocathodes in p-DSSCs with their high carrier mobility and transparency. For example, the CuScO2 has a hole mobility of 2.0 101 cm2 V1 s1 and a wide optical bandgap of around 3.3–3.7 eV.49–51 Considering that the Cu(I)-based delafossite compounds are one group of promising p-TCO materials, the research of exploring novel CuMO2 compounds with different MIII cations and their applications in p-DSSCs can be of both practical and fundamental interest.
4. Conclusion To sum up, the recent work of applying the delafossite structure CuMO2 (M = Al, Ga or Cr) compounds in p-DSSCs has opened up the door for improving the performance of p-DSSCs by replacing the NiO photocathodes with the more efficient p-type semiconductors. By producing a substantially higher photovoltage, absorbing less incident light and having better transport properties, the CuMO2 compounds have clearly demonstrated their advantages over the NiO in p-DSSCs. However, more revolutionary findings are anticipated to improve the CuMO2-based DSSCs. Currently the main limitation lies in the modest photocurrent of the solar cells, which is due to the limited surface area of the CuMO2 films that are available for the dye molecule adsorption. To improve the photocurrent, research efforts on the synthetic study of getting smaller CuMO2 nanostructures with a controlled morphology are highly anticipated. The investigations on the stability of CuMO2 species will provide valuable information for the solar cell
5032 | Phys. Chem. Chem. Phys., 2014, 16, 5026--5033
fabrication as well as sensitizer designing. The device physics study of the CuMO2-based solar cells will lead to a better understanding of the fundamental charge transport and transfer processes at the CuMO2/dye/electrolyte interface and help in further improvement of the efficiency. Exploring new MIII in CuMO2 is also an interesting direction for the development of p-DSSCs. Moreover, research efforts for obtaining a better and deeper understanding of the delafossite CuMO2 compounds shall not only benefit the development of DSSCs but related fields such as the applications of the CuMO2 compounds in transparent electronics and photoelectrochemical cells for solar fuels.
Acknowledgements The authors acknowledge the funding support from the U.S. Department of Energy (Award no. DE-FG02-07ER46427).
Notes and references 1 B. O’Regan and M. Gratzel, Nature, 1991, 353, 737. 2 A. Yella, H.-W. Lee, H. N. Tsao, C. Yi, A. K. Chandiran, M. K. Nazeeruddin, E. W.-G. Diau, C.-Y. Yeh, S. M. Zakeeruddin and ¨tzel, Science, 2011, 334, 629. M. Gra ¨m, A. Hagfeldt and S.-E. Lindquist, Sol. 3 J. He, H. Lindstro Energy Mater. Sol. Cells, 2000, 62, 265. 4 D. Xiong and W. Chen, Front. Optoelectron., 2012, 5, 371. 5 A. Nattestad, A. Mozer, M. K. Fischer, Y.-B. Cheng, ¨uerle and U. Bach, Nat. Mater., 2009, 9, 31. A. Mishra, P. Ba 6 E. A. Gibson, A. L. Smeigh, L. Le Pleux, J. Fortage, G. Boschloo, E. Blart, Y. Pellegrin, F. Odobel, A. Hagfeldt ¨m, Angew. Chem., Int. Ed., 2009, 48, 4402. and L. Hammarstro 7 A. Nakasa, H. Usami, S. Sumikura, S. Hasegawa, T. Koyama and E. Suzuki, Chem. Lett., 2005, 500. 8 J. He, H. Lindstrom, A. Hagfeldt and S.-E. Lindquist, J. Phys. Chem. B, 1999, 103, 8940. ¨tz, 9 S. Powar, T. Daeneke, M. T. Ma, D. Fu, N. W. Duffy, G. Go ¨uerle and L. Spiccia, Angew. M. Weidelener, A. Mishra, P. Ba Chem., 2013, 125, 630. 10 F. Odobel, L. c. Le Pleux, Y. Pellegrin and E. Blart, Acc. Chem. Res., 2010, 43, 1063. 11 F. Odobel and Y. Pellegrin, J. Phys. Chem. Lett., 2013, 4, 2551. 12 S. Sumikura, S. Mori, S. Shimizu, H. Usami and E. Suzuki, J. Photochem. Photobiol., A, 2008, 199, 1. 13 L. Li, E. A. Gibson, P. Qin, G. Boschloo, M. Gorlov, A. Hagfeldt and L. Sun, Adv. Mater., 2010, 22, 1759. 14 X. L. Z. X. L. Zhang, F. Huang, A. Nattestad, K. Wang, D. Fu, ¨uerle, U. Bach and Y.-B. Cheng, Chem. A. Mishra, P. Ba Commun., 2011, 47, 4808. 15 Z. Ji, G. Natu, Z. Huang and Y. Wu, Energy Environ. Sci., 2011, 4, 2818. 16 Z. Huang, G. Natu, Z. Ji, M. He, M. Yu and Y. Wu, J. Phys. Chem. C, 2012, 116, 26239. ¨m, 17 E. A. Gibson, A. L. Smeigh, L. c. Le Pleux, L. Hammarstro F. Odobel, G. Boschloo and A. Hagfeldt, J. Phys. Chem. C, 2011, 115, 9772.
This journal is © the Owner Societies 2014
View Article Online
Published on 07 January 2014. Downloaded by New York University on 09/10/2014 21:11:53.
Perspective
18 F. Odobel, Y. Pellegrin, E. A. Gibson, A. Hagfeldt, ¨m, Coord. Chem. Rev., A. L. Smeigh and L. Hammarstro 2012, 256, 2414. 19 A. Renaud, B. Chavillon, L. Cario, L. L. Pleux, N. Szuwarski, Y. Pellegrin, E. Blart, E. Gautron, F. Odobel and S. Jobic, J. Phys. Chem. C, 2013, 117, 22478. 20 S. Mori, S. Fukuda, S. Sumikura, Y. Takeda, Y. Tamaki, E. Suzuki and T. Abe, J. Phys. Chem. C, 2008, 112, 16134. 21 S. Sumikura, S. Mori, S. Shimizu, H. Usami and E. Suzuki, J. Photochem. Photobiol., A, 2008, 194, 143. 22 C. A. N. Fernando, A. Kitagawa, M. Suzuki, K. Takahashi and T. Komura, Sol. Energy Mater. Sol. Cells, 1994, 33, 301. 23 B. O’Regan and D. T. Schwartz, Chem. Mater., 1995, 7, 1349. 24 M. Chitambar, Z. Wang, Y. Liu, A. Rockett and S. Maldonado, J. Am. Chem. Soc., 2012, 134, 10670. 25 S. Nakabayashi, N. Ohta and A. Fujishima, Phys. Chem. Chem. Phys., 1999, 1, 3993. 26 J. Bandara and J. Yasomanee, Semicond. Sci. Technol., 2007, 22, 20. 27 A. Nattestad, X. Zhang, U. Bach and Y. Cheng, J. Photonics Energy, 2011, 1, 011103. 28 M. Yu, G. Natu, Z. Ji and Y. Wu, J. Phys. Chem. Lett., 2012, 3, 1074. 29 A. Renaud, B. Chavillon, L. Le Pleux, Y. Pellegrin, E. Blart, ´, L. Cario, S. Jobic and F. Odobel, M. Boujtita, T. Pauporte J. Mater. Chem., 2012, 22, 14353. 30 D. Xiong, Z. Xu, X. Zeng, W. Zhang, W. Chen, X. Xu, M. Wang and Y.-B. Cheng, J. Mater. Chem., 2012, 22, 24760. 31 X. Xu, B. Zhang, J. Cui, D. Xiong, Y. Shen, W. Chen, L. Sun, Y.-B. Cheng and M. Wang, Nanoscale, 2013, 5, 7963. 32 Z. Xu, D. Xiong, H. Wang, W. Zhang, X. Zeng, L. Ming, W. Chen, X. Xu, J. Cui, M. Wang, S. Powar, U. Bach and Y.-B. Cheng, J. Mater. Chem. A, 2014, advance article. 33 W. C. Sheets, E. Mugnier, A. Barnabe, T. J. Marks and K. R. Poeppelmeier, Chem. Mater., 2006, 18, 7.
This journal is © the Owner Societies 2014
PCCP
34 S. Sheng, G. Fang, C. Li, S. Xu and X. Zhao, Phys. Status Solidi A, 2006, 203, 1891. 35 H. Kawazoe, M. Yasukawa, H. Hyodo, M. Kurita, H. Yanagi and H. Hosono, Nature, 1997, 389, 939. 36 H. Yanagi, H. Kawazoe, A. Kudo, M. Yasukawa and H. Hosono, J. Electroceram., 2000, 4, 407. 37 R. Gillen and J. Robertson, Phys. Rev. B: Condens. Matter Mater. Phys., 2011, 84, 035125. 38 K. Ueda, T. Hase, H. Yanagi, H. Kawazoe, H. Hosono, H. Ohta, M. Orita and M. Hirano, J. Appl. Phys., 2001, 89, 1790. 39 V. Varadarajan and D. Norton, Appl. Phys. A: Mater. Sci. Process., 2006, 85, 117. 40 R. Nagarajan, A. D. Draeseke, A. W. Sleight and J. Tate, J. Appl. Phys., 2001, 89, 8022. 41 D. Li, X. Fang, Z. Deng, S. Zhou, R. Tao, W. Dong, T. Wang, Y. Zhao, G. Meng and X. Zhu, J. Phys. D: Appl. Phys., 2007, 40, 4910. 42 X. Nie, S.-H. Wei and S. Zhang, Phys. Rev. Lett., 2002, 88, 066405. 43 F. Benko and F. Koffyberg, J. Phys. Chem. Solids, 1984, 45, 57. 44 F. Benko and F. Koffyberg, Phys. Status Solidi A, 1986, 94, 231. 45 F. Benko and F. Koffyberg, Mater. Res. Bull., 1986, 21, 753. 46 A. Nattestad, J. Photonics Energy, 2011, 1, 011103. 47 P. Qin, H. Zhu, T. Edvinsson, G. Boschloo, A. Hagfeldt and L. Sun, J. Am. Chem. Soc., 2008, 130, 8570. 48 R. Srinivasan, B. Chavillon, C. Doussier-Brochard, L. Cario, M. Paris, E. Gautron, P. Deniard, F. Odobel and S. Jobic, J. Mater. Chem., 2008, 18, 5647. 49 Y. Kakehi, K. Satoh, T. Yotsuya, K. Masuko and A. Ashida, Jpn. J. Appl. Phys., 2007, 46, 4228. 50 N. Duan, A. Sleight, M. Jayaraj and J. Tate, Appl. Phys. Lett., 2000, 77, 1325. 51 Y. Kakehi, S. Nakao, K. Satoh and T. Yotsuya, Thin Solid Films, 2003, 445, 294.
Phys. Chem. Chem. Phys., 2014, 16, 5026--5033 | 5033
Published on 07 January 2014. Downloaded by New York University on 09/10/2014 21:11:53.
PERSPECTIVE
Cite this: Phys. Chem. Chem. Phys., 2014, 16, 5026
View Journal | View Issue
Cu(I)-based delafossite compounds as photocathodes in p-type dye-sensitized solar cells Mingzhe Yu, Thomas I. Draskovic and Yiying Wu* The research of p-type dye-sensitized solar cells (p-DSSCs) has attracted growing attention because of the potential for integration with conventional n-type DSSCs (n-DSSCs) into the more efficient tandemDSSCs. However, to date the performance of p-DSSCs is lagging behind that of n-DSSCs. One main reason is the lack of optimal photocathode materials. This article reviews the most recent progress in utilizing Cu(I)-based delafossite compounds, CuMO2 (M = Al, Ga or Cr), as photocathodes in p-DSSCs.
Received 11th November 2013, Accepted 6th January 2014
As alternative materials to the commonly used NiO, the CuMO2 compounds have their intrinsic advantages
DOI: 10.1039/c3cp55457k
such as lower valence band edge, larger optical bandgap and higher conductivity. By providing an insight into these materials and their applications in p-DSSCs, this perspective aims to stimulate more exciting
www.rsc.org/pccp
research in the development of p-DSSCs as well as of tandem-DSSCs.
1. Introduction Dye-sensitized solar cells (DSSCs) are regarded as among the most promising photovoltaic technologies because of their low fabrication cost and high energy conversion efficiency.1–2 Recently the idea of tandem-DSSCs, with a theoretical photon to energy conversion efficiency (PCE) over 40%, has been raised.2–4 By integrating a conventional n-type TiO2-DSSC with a p-type DSSC, the tandem-DSSC is expected to have a higher open-circuit voltage (Voc) and achieve broader light absorption of the solar Department of Chemistry & Biochemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210, USA. E-mail: [email protected]; Fax: +1-614-292-1685; Tel: +1-614-247-7810
Mingzhe Yu is currently a PhD candidate in the Department of Chemistry and Biochemistry at the Ohio State University. He obtained his BSc in the Materials Science and Engineering Department at the University of Science and Technology of China (USTC) in 2010 and then joined Dr Wu’s group as a graduate student. His research is focused on the synthesis and characterization of delafossite semiconductors with Mingzhe Yu controlled-structures and their applications in the energy conversion and storage fields, including dye-sensitized solar cells and metal-air batteries.
5026 | Phys. Chem. Chem. Phys., 2014, 16, 5026--5033
spectrum5–7 (Fig. 1a). The construction of a highly efficient tandem-DSSC requires the efficient p-DSSC that has a comparable performance to the n-DSSC. However, the research on p-DSSCs is a relatively young field and the efficiency of p-DSSCs still has much room to improve: the first NiO-based p-DSSC was reported by He et al. in 19998 and the highest PCE value to date is 1.30% under AM 1.5 illumination.9 The p-DSSCs are based on the cathodic sensitization of p-type semiconductors. Fig. 1b shows the operation principle of a p-DSSC: upon illumination, dye molecules which are chemisorbed on the photocathode will be excited from its highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). This triggers the electron transfer from the photocathode’s valence band (VB) to the dye. The hole
Thomas Draskovic
Thomas is a graduate student in the Department of Chemistry and Biochemistry at the Ohio State University. He received his BS in chemistry in 2011 from the Pennsylvania State University. His current research interests include the synthesis, characterization, and doping of delafossite structured semiconductors with the desire to utilize nanoparticle films of these materials in photovoltaic and photocatalytic cells.
This journal is © the Owner Societies 2014
View Article Online
Published on 07 January 2014. Downloaded by New York University on 09/10/2014 21:11:53.
Perspective
Fig. 1 Schematic structures of (a) the tandem-DSSCs and (b) the p-DSSCs.
left in the VB of the photocathode then diffuses to the back contact which is linked to the outside-circuit. Meanwhile, by reducing the redox mediator, the dye gets regenerated. And the
Professor Yiying Wu received his BS in chemical physics from the University of Science and Technology of China in 1998, and his PhD in chemistry from the University of California at Berkeley in 2003 with Professor Peidong Yang. He then did his postdoctoral research with Professor Galen D. Stucky at the University of California, Santa Barbara, and joined the chemistry faculty at The Ohio Yiying Wu State University in the summer of 2005. He was promoted to associate professor with tenure in 2011. His group focuses on materials chemistry for energy conversion and storage.
This journal is © the Owner Societies 2014
PCCP
redox mediator is recovered on the counter-electrode by giving away the electron to the outside-circuit. NiO is the most extensively investigated photocathode material in p-DSSCs.10–11 A lot of research has been done to improve the efficiency of NiO-based solar cells, including improving the quality of NiO films,12–14 molecular engineering of specific sensitizers,15–16 exploring novel redox mediators,9,17 etc. Despite all the efforts, the record efficiency for p-DSSCs (1.30%)9 is still lower than that of n-DSSCs (12.3%).2 One crucial reason is the missing of optimal wide bandgap p-type semiconductor which is equivalent to the anatase TiO2 as in n-DSSCs.10,18 Though widely employed, in fact NiO is not an optimal material as photocathodes in p-DSSCs for the following reasons: (1) although reported as a wide bandgap material (Eg B 3.55 eV), NiO films have a significant light absorption in the visible range (30–40% for a 2.3 mm thick film5) likely due to the d–d electron transition, and possibly a residual amount of nickel metal left in the NiO films.19 The sensitizers have to compete with NiO for light absorption and therefore the incident photon-to-current efficiency (IPCE) is limited. (2) In p-DSSCs, the Voc is governed by the difference between the quasi-Fermi level of the photocathode (at best, close to the VB edge of the p-type semiconductor) and the potential of the redox mediator (Fig. 1b). The VB edge of NiO is +0.54 V (vs. NHE, pH = 7),3 close to the potential of the commonly used I3/I redox mediator (i.e. +0.35 V vs. NHE). As a result, the maximum obtainable Voc is small. (3) Due to the low conductivity, NiO films have a low hole diffusion coefficient (i.e. 4 108 cm2 s1),20 which is 2–3 orders lower than that of TiO2, and this limits the diffusion length of hole carriers. Considering the drawbacks of NiO, alternative p-type semiconductors with better optical transparency, lower VB edge position and higher hole mobility are desired for p-DSSCs. Several materials have been investigated as photocathodes in p-DSSCs, including CuO,21 CuSCN,22–23 GaP,24 boron-doped diamond25 and the Cu(I)-based delafossite compounds, CuMO2 (M = Al, Ga or Cr).26–32 Among them, the CuMO2 series materials have shown very promising performances in p-DSSCs because of their intrinsic advantages over NiO. This perspective will provide an insight and discussion on these delafossite CuMO2 compounds and their applications as photocathodes in p-DSSCs. Delafossite, named in honor of the mineralogist Gabriel Delafosse, is a copper iron oxide compound with the formula CuIFeIIIO2. The Cu(I)-based delafossite compounds refer to a group of metal oxides which have the same crystal structure as the delafossite CuFeO2. They have the formula CuIMIIIO2, where the trivalent M atom can be B, Al, Ga, In, Sc, Y, Cr, Fe, Co or Ni.33 As shown in Fig. 2a, the crystal has a layered structure with one layer as the close-packed monovalent Cu atoms and the other as edge-shared MIIIO6 octahedra. The CuI atoms are coordinated to two oxygen atoms with the linear O–Cu–O bonds. In the MIIIO6 octahedra layer, each MIII atom is coordinated with six O atoms while each O atom is coordinated with three MIII atoms and one CuI atom. By adopting an ABCABC stacking or ABABAB stacking, the crystal can have either a rhombohedral (3R) or hexagonal (2H) symmetry.
Phys. Chem. Chem. Phys., 2014, 16, 5026--5033 | 5027
View Article Online
PCCP
Perspective Table 1 The optical bandgaps, VB edge positions and conductivity of the CuMO2 compounds (M = Al, Ga or Cr)3,35,38,41–45
VB edge position (pH = 9.2)
Published on 07 January 2014. Downloaded by New York University on 09/10/2014 21:11:53.
CuAlO2 CuGaO2 CuCrO2
Optical bandgap (eV)
(eV, vs. vacuum)
(V, vs. NHE)
Conductivity (S m1)
3.5 3.4–3.7 2.95–3.30
B5.1 B5.1 B5.3
B+0.6 B+0.6 B+0.8
B2 101 B6 101 B1 102
eliminated. As a result, in addition to the p-type conductivity, the Cu(I)-based delafossite compounds also have a large optical bandgap with the high transparency (Table 1).35,38,41–42 In the 1980’s, Benko et al. did a series of photoelectrochemical studies and revealed that the VB edges of CuMO2 (M = Al, Ga or Cr) locate at +0.6–0.8 V (vs. NHE, pH = 9.2), which is lower than that of NiO.3,43–45 As mentioned above, the lower VB edge indicates a higher obtainable photovoltage when applied in p-DSSCs. Moreover, the CuMO2 compounds also have significantly higher hole mobilities than NiO. The hole mobility for CuMO2 (M = Al or Ga) is reported to be 101–102 cm2 V1 s1.35,43–44 While for NiO, the hole mobility is only B2 106 cm2 V1 s1 due to the localized Ni 3d orbitals.20 The higher hole mobility of CuMO2 is expected to favor the faster hole transport kinetics in p-DSSCs. Therefore, the research of utilizing Cu(I)-based delafossite compounds as photocathodes in p-DSSCs has been initialized during the past few years. And the p-DSSCs based on delafossite CuAlO2,26,46 CuGaO228–29,32 and CuCrO230–31 are reported with very promising performance. Fig. 2 (a) The crystal structure of the delafossite compounds. (b) The calculated band-structures of the CuMO2 compounds from the sX-LDA method. Reprinted with permission from ref. 37. Copyright (2011) by The American Physical Society.
2. p-DSSCs based on delafossite CuMO2 (M = Al, Ga or Cr) 2.1.
Benefiting from the crystal structure, the Cu(I)-based delafossite compounds are found to be promising p-type transparent conductive oxides (p-TCO).34–36 For most metal oxides, due to the strong electronegativity of oxygen, the valence band edge is localized on the O 2p orbitals. The positive holes in the band edge are deeply ‘‘trapped’’ and cannot migrate freely in the lattice. Hence, it is difficult for metal oxides to achieve p-type conductivity through acceptor doping. However, in the Cu(I)-based delafossite compounds, the Cu(I) has a fully filled 3d orbital with an energy close to the O 2p orbital. By forming the occupied O 2p–Cu 3d antibond, the valence band edge is now a hybrid of O 2p and Cu 3d orbitals and delocalized from the oxygen.37 Therefore, the holes introduced by defects of Cu vacancies and O interstitials can migrate freely in the valence band edge and the compounds exhibit good p-type conductivity.35,38–40 At the same time, in the layered –MO2–Cu–MO2– structure, the crosslinking of Cu to Cu is only two dimensional. The direct interaction between 3d10 electrons on neighboring CuI atoms, which may reduce the bandgap, is weak. Moreover, since the CuI has a d10 closed-shell configuration, the coloration due to intra-atomic excitation is also
5028 | Phys. Chem. Chem. Phys., 2014, 16, 5026--5033
CuAlO2-based p-DSSCs
In 2006, Yasomanee’s group demonstrated the concept of applying CuAlO2 in solid-state DSSCs for the first time.26 The solar cell with a configuration of TiO2/dye/CuAlO2 was fabricated by dropping the CuAlO2 suspension on the Ru-dye sensitized TiO2 film to form a hole-conducting layer. The I–V performance confirmed that CuAlO2 is a suitable material as a hole-conductor: under 100 mW cm2 visible band illumination, the short-circuit current ( Jsc) and Voc were recorded as 0.08 mA cm2 and 524 mV, respectively. Being consistent with the material’s lower VB position, the CuAlO2-based solar cells exhibit higher Voc compared to the analogous TiO2/dye/NiO device (Voc = 510 mV). However, the photocurrent of the CuAlO2-based solar cells was low. The author attributed this to the poor contact between the large-sized CuAlO2 particles (B5 mm in size) and the dye. More recently, Cheng’s group reported the fabrication of CuAlO2-based p-DSSCs with an improved Voc value in 2011.46 In their work, CuAlO2, with the majority of particles around 1–2 mm in size, was synthesized through a solid-state reaction. By photoelectron spectroscopy in air (PESA) measurements, the author confirmed that the VB position for CuAlO2 is at B5.3 eV below vacuum (i.e. B+0.8 V vs. NHE), which is about 200 mV
This journal is © the Owner Societies 2014
View Article Online
Published on 07 January 2014. Downloaded by New York University on 09/10/2014 21:11:53.
Perspective
PCCP
Fig. 3 The I–V curve of the CuAlO2/PMI-6T-TPA p-DSSCs. Reprinted with permission from ref. 27. Copyright (2011) by the International Society of Optics and Photonics.
lower than NiO (B5.1 eV below vacuum, +0.6 V vs. NHE). The solar cells were fabricated by sensitizing the CuAlO2 films with the highly efficient PMI-6T-TPA sensitizer5 and an I3/I-based electrolyte. The I–V curve of the solar cell is shown in Fig. 3. Promisingly, the author observed a Voc of 333 mV under AM 1.5 illumination, which is substantially higher than the analogous NiO-based solar cells (Voc = 218 mV).5 The B115 mV Voc improvement can explained by the B200 mV lower VB position of CuAlO2 than NiO according to the PESA results. Along with the improvement of Voc, the fill factor (FF) for CuAlO2 cells is around 40%, also being higher than that of NiO devices. However, due to the large particle size, the limited film surface area (1.7 m2 g1) for dye-adsorption results in a low photocurrent ( Jsc = 0.3 mA cm2 under AM 1.5 illumination). Despite the low photocurrent, a peak IPCE value of 4.0% was still recorded at 430 nm, where the dye absorption spectra reaches its maximum, and confirmed that the photocurrent is generated by the dye. 2.2.
CuGaO2-based p-DSSCs
In 2012, Wu’s group and Jobic’s group independently reported the application of hydrothermally synthesized CuGaO2 nanoplates as photocathodes in p-DSSCs.28–29 In contrast to the dark-colored NiO, the CuGaO2 was white with a pale yellow tinge. It was found that CuGaO2 nanoplates are thermokinetically stable below 350 1C in air and therefore compatible for the annealing treatment in the solar cell fabrication process. Wu and co-workers fabricated the solar cell with a donor–acceptor P1 dye47 and the I3/I or Co3+/2+ (dtb-bpy) electrolyte.28 The dependence of Voc on the illumination intensity was measured to estimate the maximum obtainable Voc of the CuGaO2-based solar cells. The author recorded a saturation photovoltage of 464 mV in the CuGaO2based p-DSSCs with the Co3+/2+ (dtb-bpy) electrolyte, which is an increase of more than 100% compared to that of NiO-based solar cells under the same conditions (Fig. 4). This substantial increase also remained when the I3/I electrolyte was applied (243 mV vs.132 mV for CuGaO2 and NiO, respectively).
This journal is © the Owner Societies 2014
Fig. 4 Dependence of Voc on light intensity: (a) CuGaO2-DSSC and NiODSSC with the simple I3/I electrolyte; (b) CuGaO2-DSSC and NiO-DSSC with the Co3+/2+ (dtb-bpy) electrolyte. Reprinted with permission from ref. 28. Copyright (2012) American Chemical Society.
The enhancement of Voc was explained by the lower VB edge of CuGaO2, and confirmed by the Mott–Schottky analyses: Jobic and co-workers obtained the flatband potential (Efb) of CuGaO2 and NiO of B+0.73 V and B+0.57 V, respectively (vs. NHE, pH = 6.3) (Fig. 5). Under AM 1.5 illumination, for the CuGaO2based solar cells with the Co3+/2+ (dtb-bpy) electrolyte, the two groups obtained an impressive Voc of 357 mV with the P1 sensitizer and 375 mV with the PMI-NDI sensitizer. The major
Fig. 5 The comparison of the flat-band potential and Voc between NiO and CuGaO2. Reprinted from ref. 29 with permission from The Royal Society of Chemistry.
Phys. Chem. Chem. Phys., 2014, 16, 5026--5033 | 5029
View Article Online
Published on 07 January 2014. Downloaded by New York University on 09/10/2014 21:11:53.
PCCP
Perspective
limitation of the CuGaO2 solar cells is the modest current density: the short-circuit current ( Jsc) is smaller than 0.4 mA cm2 with the I3/I electrolyte under AM 1.5 illumination, and even lower when the Co3+/2+ (dtb-bpy) electrolyte was applied. By carrying out the dye adsorption isotherm tests and Brunauer–Emmett– Teller (BET) measurements, both groups attributed the limited photocurrent to the much lower surface area of the CuGaO2 nanoplates (B30 m2 g1 and B158 m2 g1 for CuGaO2 and NiO, respectively), which limits the dye-loading amount and the light harvesting efficiency. Most recently, a short-circuit current of 2.05 mA cm2 was reported using a mechanically pressed filmed of slightly smaller CuGaO2 plates (25 nm 100–200 nm) with the P1 dye and the I3/I electrolyte. This greatly promising result further indicates the potential of and need for high-quality nanoparticle films.32 2.3.
CuCrO2-based p-DSSCs
Among the Cu(I)-based delafossite compounds, CuCrO2 is reported with a high electrical conductivity up to 1.0 S cm1.40 Due to the d–d transition at the Cr3+ site, the CuCrO2 exhibits a relatively small bandgap of 2.95–3.30 eV.41 In 2012, Chen’s group reported the hydrothermal synthesis of ultrasmall CuCrO2 nanocrystals and applied them as photocathodes in p-DSSCs.30 The synthesized CuCrO2 had a typical size of 15 nm 5 nm (Fig. 6a) and a surface area of 87.86 m2 g1. The nanocrystals are thermally stable up to 400 1C. By the Mott–Schottky analyses, it was confirmed that CuCrO2 had a lower VB edge than NiO (Efb of +0.84 V and +0.56 V vs. NHE, for CuCrO2 and NiO, respectively). The optimized CuCrO2-based solar cell was fabricated with the coumarin 343 dye (C343) and the I3/I electrolyte, and achieved a Voc value of 102 mV under AM 1.5 illumination, being higher than its analogous NiO device (Voc = 90 mV). By carrying out the photocurrent–photovoltage transient decay measurements, the author highlighted that CuCrO2 cells have a better transport property than the analogous NiO cells with a remarkably longer hole lifetime tr (B101 vs. B102 s), higher diffusion coefficient D (B105 vs. B107–106 cm2 s1) and longer diffusion length L (B10 mm vs. 1–2 mm). These enhancements were attributed to the better conductivity of CuCrO2. It is worth mentioning that although the CuCrO2 nanocrystals have a comparable
Fig. 6 (a) The transmission electron microscopy (TEM) image of the hydrothermally synthesized CuCrO2. Reprinted from ref. 30 with permission from the Royal Society of Chemistry. (b) The molecular structures of 1-methyl-1H-tetrazole-5-thiolate (T) and its disulfide dimer (T2). Reprinted from ref. 31 with permission from The Royal Society of Chemistry.
5030 | Phys. Chem. Chem. Phys., 2014, 16, 5026--5033
surface area to the TiO2 in n-DSSCs, the CuCrO2 films are way thinner than the conventional 10–20 mm-thick TiO2 films. (Thicker CuCrO2 films would absorb light significantly due to their dark color.) As a result, the photocurrent of CuCrO2DSSCs is still limited ( Jsc o 1 mA cm2 under AM 1.5 illumination). Very recently in 2013, the same group reported that by utilizing the 1-methyl-1H-tetrazole-5-thiolate (T) and its disulfide dimer (T2) as a redox mediator, together with a P1 sensitizer and a CoS counter-electrode, the CuCrO2-based p-DSSCs can achieve an improved performance with Voc of 304 mV, Jsc of 1.73 mA cm2 and PCE of 0.23% under AM 1.5 illumination.31 The T2/T electrolyte has a high transparency compared to the I3/I electrolyte and therefore allows better light utilization. And through the scanning electrochemical microscopy (SECM) and the electrochemical impedance study (EIS) analyses, the author observed the faster charge transfer kinetics in the dye regeneration process with the T2/T redox mediator, which is apparently another advantage. Therefore, the author concluded that integrated with the CoS counter-electrode, the new T2/T can be a more suitable redox mediator for the CuCrO2-based p-DSSCs as well as for tandem-DSSCs.
3. Future prospects As summarized in Table 2, with the substantially higher photovoltages over the analogous NiO devices, the great potential of CuMO2-based p-DSSCs has been clearly demonstrated in the recent few studies. Though the door of utilizing more efficient photocathode materials in p-DSSCs has been opened, the modest photocurrent is still hindering these solar cells to achieve higher efficiency. Towards the more efficient p-DSSCs, progresses on the following aspects are expected. 3.1. The synthesis of CuMO2 nanoparticles with controlled size and morphology Firstly and most importantly, a breakthrough at the synthetic bottleneck of getting CuMO2 nanostructures with controlled size and morphology is highly anticipated. Ideally, the spherical CuMO2 nanocrystals with the size around 20–40 nm (which is the typical size of TiO2 nanoparticles in n-DSSCs,) are desired to support a sufficient dye-loading on the photocathodes in p-DSSCs. In the current stage, apparently the large particle-size and small surface area of CuMO2 particles are the main reasons that limit the light-harvesting efficiency and photocurrent of the solar cells. The conventional synthetic strategy for ternary metal oxide is solid-state reaction, which usually yields large particle sizes (i.e. >1 mm) due to the high-temperature agglomeration. The recently developed hydrothermal approach provides chances of getting smaller particles (e.g. B200 nm for CuGaO2 and B15 nm for CuCrO2)30,48 and is also appealing to the costefficient concept with the fabrication of DSSCs. The anisotropic delafossite crystal structure results in the c-plane growth less favored than the a,b-plane and the final CuMO2 nanoparticles having a plate-like morphology.28,30 When fabricated into
This journal is © the Owner Societies 2014
View Article Online
Perspective Table 2
PCCP
The Voc, Jsc, FF and PCE(Z) for Cu(I)-based delafossite p-DSSCs and the Voc of analogous NiO cells under AM 1.5 illumination
Published on 07 January 2014. Downloaded by New York University on 09/10/2014 21:11:53.
Redox mediator
Sensitizer
Voc (mV)
0 b Voc (mV)
Jsc (mA cm2)
FF (%)
Z (%)
Ref.
CuAlO2a
I3 /I
Ru-dye
524
510
0.08
54.3
0.046
26
CuAlO2
I3/I
PMI-6T-TPA
333
218
0.33
40
0.041
46
CuGaO2
I3/I Co3+/2+ I3/I I3/I Co3+/2+
P1
180 357 199.3 187 375
105 149 108.4 130 285
0.384 0.165 2.05 0.29 0.12
37.6 30.8 44.5 41 33.2
0.026 0.018 0.182 0.023 0.015
28
I3/I T2/T
Coumarin 343 P1
90 —
0.584 1.73
31.8 44
0.016 0.23
CuCrO2
PMI-NDI
102 304
32 29 30 31c
a The CuAlO2 acted as the hole conductor for the sensitized TiO2 solar cell. b The Voc of analogous NiO solar cells under the same conditions for comparison. c The catalyst on the counter-electrode used here is CoS instead of Pt.
films, the nanoplates prefer a dense stacking along the basal planes. This eventually results in an even more limited exposed surface area available for sensitizer molecules adsorption. Therefore, in addition to achieving smaller size, controlling of the product’s morphology is also essential. The successful hydrothermal syntheses of delaffosite CuGaO2 and CuCrO2 nanoparticles are definitely encouraging. However, the chemistry of hydrothermal syntheses of CuMO2 is complicated and varies with different M cations. Briefly speaking, the stabilization of Cu(I) and olation of the M cations in aqueous solvents are two key factors that should be considered carefully. There has been no specific study on the phase formation and particle growth mechanism of these nanocrystals reported. To achieve a better control on the size and morphology of the delafossite nanocrystals, more research efforts on the deeper understanding of the basic chemistry and crystallization process in the hydrothermal synthesis of these delafossite nanocrystals are anticipated. Meanwhile, it is worth pointing out that focusing on film fabrication techniques can also provide significant improvements in device performance, even with particles of traditionally undesirable size or morphology.32 3.2. The thermal and chemical stability of the CuMO2 compounds Secondly, investigations on the stability of the nanostructured CuMO2 compounds are also of considerable importance. The Cu(I) is thermodynamically unstable at low temperatures and can be oxidized into Cu(II) through the following reaction: 2CuIMO2 + 1/2O2 - CuIIM2O4 + CuO This raises the concern that whether the CuMO2 is capable for the annealing treatment in the solar cell fabrication process. In the previous work, CuGaO2 and CuCrO2 nanocrystals have been reported to be thermokinetically stable with the annealing treatment in air up to 350 1C and 400 1C, respectively.28,30 Therefore, to prevent the thermodecomposition of the delafossite phase, caution needs to be taken when deciding the annealing temperature for CuMO2 films. Also, using an inert atmosphere such as flowing argon is expected to improve the stability of these compounds.
This journal is © the Owner Societies 2014
The chemical stability of CuMO2 is another aspect that should be carefully evaluated. In a real solar cell, the semiconductor surface is in direct contact with the chemisorbed dye molecules. Therefore, the reactivity between the CuMO2 nanocrystals and the acidic dye molecules is important. Cu(I) has a rich coordination chemistry and in the CuMO2, the Cu(I) atoms are unsaturated, coordinated by only two oxygen atoms (Fig. 2a). Therefore, there are chances for the Cu(I)+–H+ ion exchange reaction to happen between the surface Cu(I) and the anchoring group (usually carboxylic) of the sensitizers, and this may lead to the phase change of the CuMO2 photocathodes. Though so far there is no specific research reported on this topic, we notice that Jobic and co-workers emphasized that the CuGaO2–C343 solar cells failed to give any photocurrent in their study.29 This may be due to an ion-exchange reaction that happened between the surface Cu(I) of CuGaO2 and the carboxyl group of the C343 molecule. Interestingly, Chen’s group employed C343 as sensitizers for the CuCrO2 p-DSSCs and recorded reasonable I–V performance.30 One possible explanation may be that the reactivity of CuMO2 differs from one to another since the different MO6 octahedra can affect O–CuI–O bonding strength. Therefore, a careful investigation on the chemical stability study of CuMO2 compounds in different acidic dye solutions is essential and should provide crucial information for the sensitizer designing. Moreover, surface passivation and protection of the CuMO2 surface by atomic layer deposition (ALD) coating of a stable metal oxide such as Al2O3 can also help to improve the chemical stability of the compounds. 3.3.
The device physics study of CuMO2-based p-DSSCs
The device physics study of CuMO2-based p-DSSCs is essential for obtaining a deeper understanding of the materials as well as the solar cell performance. Being different from the isotropic rock-salt structure of NiO, the delafossite structure gives CuMO2 many anisotropic properties along the a,b-plane and along the c-axis, such as conductivity, dielectric constant, absorption, etc.37,42 These properties have a direct influence on the charge transport and recombination processes in the CuMO2-based p-DSSCs. The preliminary work by Chen and co-workers revealed the superior charge transport property of
Phys. Chem. Chem. Phys., 2014, 16, 5026--5033 | 5031
View Article Online
Published on 07 January 2014. Downloaded by New York University on 09/10/2014 21:11:53.
PCCP
Perspective
CuCrO2-based p-DSSCs and also provided a good example of studying the charge carrier dynamics in the new CuMO2 system.30 However, so far there is no specific study documented on the fundamental charge transfer kinetics at the delafossite/dye/ electrolyte interface. A symmetrical study with techniques such as EIS and photocurrent/photovoltage transient decay techniques shall benefit the deeper understanding of the materials’ physiochemical properties and also provide the diagnostic information for further improving the efficiency of the CuMO2-based p-DSSCs. There are also other aspects in the device physics field that could be of interest and deserve more research attention. For instance, the solid-state physics community can focus on how to push the valence band of the delafossite compounds even more positive with the aim of achieving higher Voc. While beyond the scope of this review, electrochemists can look closer into finding an electrolyte with more negative redox potential. In addition, the dye–electrolyte interaction plays a critical role in the n-DSSCs’ high performance. It is important to investigate if it is the similar case in the delafossite-based p-DSSCs. 3.4.
MIII cations beyond Al, Ga or Cr
As mentioned in Section 2.1, the Cu(I)-based delafossite compounds CuMO2 can have the trivalent M atom as B, Al, Ga, In, Sc, Y, Cr, Fe, Co or Ni. While only AlIII, GaIII and CrIII based CuMO2 compounds have been explored as photocathodes in p-DSSCs. In fact, some other cations that have either empty or fully filled d orbitals (with which no d–d transition absorptions will be introduced) such as BIII, ScIII or YIII should also be suitable candidates as photocathodes in p-DSSCs with their high carrier mobility and transparency. For example, the CuScO2 has a hole mobility of 2.0 101 cm2 V1 s1 and a wide optical bandgap of around 3.3–3.7 eV.49–51 Considering that the Cu(I)-based delafossite compounds are one group of promising p-TCO materials, the research of exploring novel CuMO2 compounds with different MIII cations and their applications in p-DSSCs can be of both practical and fundamental interest.
4. Conclusion To sum up, the recent work of applying the delafossite structure CuMO2 (M = Al, Ga or Cr) compounds in p-DSSCs has opened up the door for improving the performance of p-DSSCs by replacing the NiO photocathodes with the more efficient p-type semiconductors. By producing a substantially higher photovoltage, absorbing less incident light and having better transport properties, the CuMO2 compounds have clearly demonstrated their advantages over the NiO in p-DSSCs. However, more revolutionary findings are anticipated to improve the CuMO2-based DSSCs. Currently the main limitation lies in the modest photocurrent of the solar cells, which is due to the limited surface area of the CuMO2 films that are available for the dye molecule adsorption. To improve the photocurrent, research efforts on the synthetic study of getting smaller CuMO2 nanostructures with a controlled morphology are highly anticipated. The investigations on the stability of CuMO2 species will provide valuable information for the solar cell
5032 | Phys. Chem. Chem. Phys., 2014, 16, 5026--5033
fabrication as well as sensitizer designing. The device physics study of the CuMO2-based solar cells will lead to a better understanding of the fundamental charge transport and transfer processes at the CuMO2/dye/electrolyte interface and help in further improvement of the efficiency. Exploring new MIII in CuMO2 is also an interesting direction for the development of p-DSSCs. Moreover, research efforts for obtaining a better and deeper understanding of the delafossite CuMO2 compounds shall not only benefit the development of DSSCs but related fields such as the applications of the CuMO2 compounds in transparent electronics and photoelectrochemical cells for solar fuels.
Acknowledgements The authors acknowledge the funding support from the U.S. Department of Energy (Award no. DE-FG02-07ER46427).
Notes and references 1 B. O’Regan and M. Gratzel, Nature, 1991, 353, 737. 2 A. Yella, H.-W. Lee, H. N. Tsao, C. Yi, A. K. Chandiran, M. K. Nazeeruddin, E. W.-G. Diau, C.-Y. Yeh, S. M. Zakeeruddin and ¨tzel, Science, 2011, 334, 629. M. Gra ¨m, A. Hagfeldt and S.-E. Lindquist, Sol. 3 J. He, H. Lindstro Energy Mater. Sol. Cells, 2000, 62, 265. 4 D. Xiong and W. Chen, Front. Optoelectron., 2012, 5, 371. 5 A. Nattestad, A. Mozer, M. K. Fischer, Y.-B. Cheng, ¨uerle and U. Bach, Nat. Mater., 2009, 9, 31. A. Mishra, P. Ba 6 E. A. Gibson, A. L. Smeigh, L. Le Pleux, J. Fortage, G. Boschloo, E. Blart, Y. Pellegrin, F. Odobel, A. Hagfeldt ¨m, Angew. Chem., Int. Ed., 2009, 48, 4402. and L. Hammarstro 7 A. Nakasa, H. Usami, S. Sumikura, S. Hasegawa, T. Koyama and E. Suzuki, Chem. Lett., 2005, 500. 8 J. He, H. Lindstrom, A. Hagfeldt and S.-E. Lindquist, J. Phys. Chem. B, 1999, 103, 8940. ¨tz, 9 S. Powar, T. Daeneke, M. T. Ma, D. Fu, N. W. Duffy, G. Go ¨uerle and L. Spiccia, Angew. M. Weidelener, A. Mishra, P. Ba Chem., 2013, 125, 630. 10 F. Odobel, L. c. Le Pleux, Y. Pellegrin and E. Blart, Acc. Chem. Res., 2010, 43, 1063. 11 F. Odobel and Y. Pellegrin, J. Phys. Chem. Lett., 2013, 4, 2551. 12 S. Sumikura, S. Mori, S. Shimizu, H. Usami and E. Suzuki, J. Photochem. Photobiol., A, 2008, 199, 1. 13 L. Li, E. A. Gibson, P. Qin, G. Boschloo, M. Gorlov, A. Hagfeldt and L. Sun, Adv. Mater., 2010, 22, 1759. 14 X. L. Z. X. L. Zhang, F. Huang, A. Nattestad, K. Wang, D. Fu, ¨uerle, U. Bach and Y.-B. Cheng, Chem. A. Mishra, P. Ba Commun., 2011, 47, 4808. 15 Z. Ji, G. Natu, Z. Huang and Y. Wu, Energy Environ. Sci., 2011, 4, 2818. 16 Z. Huang, G. Natu, Z. Ji, M. He, M. Yu and Y. Wu, J. Phys. Chem. C, 2012, 116, 26239. ¨m, 17 E. A. Gibson, A. L. Smeigh, L. c. Le Pleux, L. Hammarstro F. Odobel, G. Boschloo and A. Hagfeldt, J. Phys. Chem. C, 2011, 115, 9772.
This journal is © the Owner Societies 2014
View Article Online
Published on 07 January 2014. Downloaded by New York University on 09/10/2014 21:11:53.
Perspective
18 F. Odobel, Y. Pellegrin, E. A. Gibson, A. Hagfeldt, ¨m, Coord. Chem. Rev., A. L. Smeigh and L. Hammarstro 2012, 256, 2414. 19 A. Renaud, B. Chavillon, L. Cario, L. L. Pleux, N. Szuwarski, Y. Pellegrin, E. Blart, E. Gautron, F. Odobel and S. Jobic, J. Phys. Chem. C, 2013, 117, 22478. 20 S. Mori, S. Fukuda, S. Sumikura, Y. Takeda, Y. Tamaki, E. Suzuki and T. Abe, J. Phys. Chem. C, 2008, 112, 16134. 21 S. Sumikura, S. Mori, S. Shimizu, H. Usami and E. Suzuki, J. Photochem. Photobiol., A, 2008, 194, 143. 22 C. A. N. Fernando, A. Kitagawa, M. Suzuki, K. Takahashi and T. Komura, Sol. Energy Mater. Sol. Cells, 1994, 33, 301. 23 B. O’Regan and D. T. Schwartz, Chem. Mater., 1995, 7, 1349. 24 M. Chitambar, Z. Wang, Y. Liu, A. Rockett and S. Maldonado, J. Am. Chem. Soc., 2012, 134, 10670. 25 S. Nakabayashi, N. Ohta and A. Fujishima, Phys. Chem. Chem. Phys., 1999, 1, 3993. 26 J. Bandara and J. Yasomanee, Semicond. Sci. Technol., 2007, 22, 20. 27 A. Nattestad, X. Zhang, U. Bach and Y. Cheng, J. Photonics Energy, 2011, 1, 011103. 28 M. Yu, G. Natu, Z. Ji and Y. Wu, J. Phys. Chem. Lett., 2012, 3, 1074. 29 A. Renaud, B. Chavillon, L. Le Pleux, Y. Pellegrin, E. Blart, ´, L. Cario, S. Jobic and F. Odobel, M. Boujtita, T. Pauporte J. Mater. Chem., 2012, 22, 14353. 30 D. Xiong, Z. Xu, X. Zeng, W. Zhang, W. Chen, X. Xu, M. Wang and Y.-B. Cheng, J. Mater. Chem., 2012, 22, 24760. 31 X. Xu, B. Zhang, J. Cui, D. Xiong, Y. Shen, W. Chen, L. Sun, Y.-B. Cheng and M. Wang, Nanoscale, 2013, 5, 7963. 32 Z. Xu, D. Xiong, H. Wang, W. Zhang, X. Zeng, L. Ming, W. Chen, X. Xu, J. Cui, M. Wang, S. Powar, U. Bach and Y.-B. Cheng, J. Mater. Chem. A, 2014, advance article. 33 W. C. Sheets, E. Mugnier, A. Barnabe, T. J. Marks and K. R. Poeppelmeier, Chem. Mater., 2006, 18, 7.
This journal is © the Owner Societies 2014
PCCP
34 S. Sheng, G. Fang, C. Li, S. Xu and X. Zhao, Phys. Status Solidi A, 2006, 203, 1891. 35 H. Kawazoe, M. Yasukawa, H. Hyodo, M. Kurita, H. Yanagi and H. Hosono, Nature, 1997, 389, 939. 36 H. Yanagi, H. Kawazoe, A. Kudo, M. Yasukawa and H. Hosono, J. Electroceram., 2000, 4, 407. 37 R. Gillen and J. Robertson, Phys. Rev. B: Condens. Matter Mater. Phys., 2011, 84, 035125. 38 K. Ueda, T. Hase, H. Yanagi, H. Kawazoe, H. Hosono, H. Ohta, M. Orita and M. Hirano, J. Appl. Phys., 2001, 89, 1790. 39 V. Varadarajan and D. Norton, Appl. Phys. A: Mater. Sci. Process., 2006, 85, 117. 40 R. Nagarajan, A. D. Draeseke, A. W. Sleight and J. Tate, J. Appl. Phys., 2001, 89, 8022. 41 D. Li, X. Fang, Z. Deng, S. Zhou, R. Tao, W. Dong, T. Wang, Y. Zhao, G. Meng and X. Zhu, J. Phys. D: Appl. Phys., 2007, 40, 4910. 42 X. Nie, S.-H. Wei and S. Zhang, Phys. Rev. Lett., 2002, 88, 066405. 43 F. Benko and F. Koffyberg, J. Phys. Chem. Solids, 1984, 45, 57. 44 F. Benko and F. Koffyberg, Phys. Status Solidi A, 1986, 94, 231. 45 F. Benko and F. Koffyberg, Mater. Res. Bull., 1986, 21, 753. 46 A. Nattestad, J. Photonics Energy, 2011, 1, 011103. 47 P. Qin, H. Zhu, T. Edvinsson, G. Boschloo, A. Hagfeldt and L. Sun, J. Am. Chem. Soc., 2008, 130, 8570. 48 R. Srinivasan, B. Chavillon, C. Doussier-Brochard, L. Cario, M. Paris, E. Gautron, P. Deniard, F. Odobel and S. Jobic, J. Mater. Chem., 2008, 18, 5647. 49 Y. Kakehi, K. Satoh, T. Yotsuya, K. Masuko and A. Ashida, Jpn. J. Appl. Phys., 2007, 46, 4228. 50 N. Duan, A. Sleight, M. Jayaraj and J. Tate, Appl. Phys. Lett., 2000, 77, 1325. 51 Y. Kakehi, S. Nakao, K. Satoh and T. Yotsuya, Thin Solid Films, 2003, 445, 294.
Phys. Chem. Chem. Phys., 2014, 16, 5026--5033 | 5033
