ZnO-Based Dye-Sensitized Solar Cells - The ... - ACS Publications

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ZnO-Based Dye-Sensitized Solar Cells Juan A. Anta,*,† Elena Guillén,† and Ramón Tena-Zaera*,‡ †

Department of Physical, Chemical and Natural Systems, Universidad Pablo de Olavide, 41013 Sevilla, Spain Energy Department, IK4-CIDETEC, Centre for Electrochemical Technologies, Parque Tecnológico de San Sebastián, Paseo Miramón 196, Donostia-San Sebastián 20009, Spain

‡

ABSTRACT: ZnO was one of the first metal oxides used in dye-sensitized solar cells (DSCs). It exhibits a unique combination of potentially interesting properties such as high bulk electron mobility and probably the richest variety of nanostructures based on a very wide range of synthesis routes. However, in spite of the huge amount of literature produced in the past few years, the reported efficiencies of ZnO-based solar cells are still far from their TiO2 counterparts. The origin of this striking difference in performance is analyzed and discussed with the perspective of future applications of ZnO in dye-sensitized solar cells and related devices. In this regard, a change of focus of the current research on ZnO-based DSCs (from morphology to surface control) is suggested.

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INTRODUCTION More than two decades after O’Regan and Grätzel’s pioneering work1 on dye-sensitized solar cells (DSCs), these devices still attract growing interest due to their appealing properties2 (i.e., transparency and/or multicolor options, easy integration into building architecture, short energy payback time,3,4 etc., as well as potentially low production costs5) and recently reported exciting improvements, reaching a power conversion efficiency (PCE) of 12.3% under simulated air mass 1.5 global sunlight.6 The DSC concecpt (see Figure 1) is based on the optical excitation of a dye, which injects an electron into the conduction band of a nanostructured wide bandgap metal oxide. The oxidized dye molecule is subsequently regenerated back to its ground state by accepting one electron from a redox couple present in an electrolyte surrounding the sensitized nanostructured metal oxide film. The light harvesting (i.e., charge generation) and transport of charge carriers (electrons in the n-type metal oxide and holes in the electrolyte) are thus separated by a proper combination of three materials: metal oxide, dye, and electrolyte. The most used ruthenium-based dyes and the iodide/triiodide redox couple, for which a long-standing record of 11.1% was obtained,7 have been replaced recently by porphyrin dyes and a cobalt-based redox couple, respectively, reaching a new record power conversion efficiency (PCE) in DSCs (i.e., 12.3%).6 As a common feature with the pioneering work,1 a randomly packed TiO2 nanoparticle network is still © 2012 American Chemical Society

used as an electron transport building block. However, due to the relatively slow ionic diffusion of cobalt-based redox couples with respect to those based on iodide/triiodide species,8,9 some room for improvement might be expected for other metal oxide architectures, as better electron transport properties may alleviate the mass transport limitations. Among all the wide band gap semiconductors explored as alternatives to TiO2 as an electron conductor, ZnO presents excellent bulk electron mobility (more than 1 order of magnitude larger than anatase TiO2)10,11 and the richest family of nanostructures.12,13 This unique combination of properties opens, in principle, wide possibilities in terms of DSC design. ZnO showed, indeed, the first experimental evidence of irreversible electron injection from organic molecules into the conduction band of a wide bandgap semiconductor.14 Nowadays, it is emerging as an efficient electron transport material in technologies similar to DSC such as inverted polymer solar cells,15−17 quantum dot solar cells,18,19 and hybrid light emitting diodes.20 However, although a large variety of ZnO nanostructures (i.e., nanoparticles,21,22 hierarchical aggregates,23,24 porous films,25 nanosheets,26 nanowires,27,28 tetrapods,29 etc., see Table 1) obtained by different synthetic routes, such as chemical bath Received: January 31, 2012 Revised: March 8, 2012 Published: March 29, 2012 11413

dx.doi.org/10.1021/jp3010025 | J. Phys. Chem. C 2012, 116, 11413−11425

The Journal of Physical Chemistry C

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Figure 1. Illustration of the molecular mechanisms underlying the functioning of a ZnO-based dye-sensitized solar cell.

Table 1. Photovoltaic Parameters of Some Reported ZnO-Based DSCsa ref

ZnO nanostructure

dye

cell area (cm2)

Jsc (mA cm−2)

Voc (V)

FF

η (%)

24 28 21 29 137 138 26 25 139 22 23 59 140 141 119 142 121 143 144 118 55 145 112 115 115

hierarchical aggregates ZnO NWs/TiO2 shell commercial nanopowders ZnO tetrapods/SnO2/ZnO core−shell NPs ZnO aggregates/TiO2 shell hierarchical aggregates ZnO nanosheet oriented porous films nanoparticles + scattering hollow cavities (HCs) nanoparticles hierarchical aggregates hierarchical aggregates ZnO/Nb2O5 shell tetrapod-like ZnO nanopowders nanosheet/NWs self-assembled nanostructures ZnO NWs/NPs hierarchical structure ZnO NWs/nanoporous layer mesoporous film hierarchical NWs NWs nanorods NWs NTs NTs

N719 N719 N719 N719 N3 N3 D149 D149 N719 D102 N3 D205 N719 D149 N719 N719 N3 D419 N719 N719 D102 Z907 N719 N719 N719

0.25 0.2 0.25 (open cells) 0.25 0.49 no data 0.16 no data 0.2 0.25 no data 0.28 0.25 0.28 0.64 3.2 no data 0.12 0.5 no data no data no data 0.5 0.28 0.2

19.8 ∼15.5 18.1 16.3 15.8 21 18.0 12.2 15.7 17.4 18.7 12.2 12.4 12.4 10.9 10.7 15.2 12.3 11.8 8.8 14.1 6.4 9.3 3.3 5.9

0.64 ∼0.77 0.621 0.656 0.709 0.660 0.53 0.69 0.563 0.63 0.635 0.653 0.712 0.607 0.68 0.71 0.61 0.57 0.65 0.68 0.55 0.72 0.67 0.739 0.71

0.59 − 0.58 0.59 0.56 0.44 0.63 0.65 0.62 0.48 0.45 0.67 0.59 0.65 0.65 0.62 0.46 0.58 0.52 0.53 0.34 0.49 0.34 0.64 0.38

7.5 7 6.6 6.3 6.3 6.1 6.1 5.6 5.5 5.4 5.4 5.3 5.2 4.9 4.8 4.7 4.2 4.1 4 2.6 2.6 2.3 2.1 1.6 1.5

a

The dye code and ZnO nanostructure are also given. NWs, NTs and NPS mean nanowires, nanotubes and nanoparticles respectively. Additionally to the power conversion efficiency, other parameters such as ZnO nanostructure and dye nature have been considered in the selection criterion to give a “quick” global view of the ZnO-based DSC literature.

deposition, including hydrothermal,28 electrodeposition,25 spray pyrolysis,24 polyol hydrolysis,23 and chemical vapor deposition,30 have been tested, the highest PCE reported for ZnO-based DSC up to now is 7.5%24 far from the record efficiency for TiO2. More importantly, it is not possible to draw clearly defined strategies to improve the present scenario, even considering relatively recent review articles where a nice and accurate state-of-the-art DSC based on one-dimensional31 and other ZnO nanostructures is presented.32 Thus, the research on ZnO-based DSCs needs especially “better focused research” as suggested by Prof. Peter for DSCs, in general, in a recent perspective article.33 A highlight article by Prof. Bisquert also stressed recently the need of a better understanding of the mechanisms determining DSC performance.34

In this feature article, we focus on the physical mechanisms involved in the performance of ZnO-based DSCs. The correlation between these mechanisms and the physicochemical properties of ZnO is also described. The discussion deals mainly with devices built from randomly packed ZnO nanoparticle networks (i.e., directly comparable to the most common TiO2 architecture). However, some results on arrays of one-dimensional ZnO nanostructures will also be presented, emphasizing the differences with respect to randomly packed nanocrystalline building blocks. Keeping in mind that the incident-photon-to-current-efficiency (IPCE), also called external quantum efficiency, of a DSC can be expressed according to eq 1,35 the relatively lower performance of ZnO-based DSCs 11414



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Figure 2. Sensitization of ZnO electrodes. (a) Incident Photon to Current Efficiency at 520 nm for ZnO DSCs as a function of immersion time of the electrodes in N3-dye solution for three different dye concentrations: 0.5, 0.25, and 0.125 mM. It is observed that by decreasing the dye concentration it took a longer time to reach the maximum efficiency, and the decrease of the photocurrent is observed at shorter immersion times. Reprinted (adapted) with permission from ref 46. Copyright 2000 American Chemical Society. (b) Short circuit photocurrents as a function of immersion time in dye solution for DSCs based on ZnO electrodes sensitized with the common N719 dye and two xanthene derivatives, Eosin Y and Eosin B. For N719 there is a critical time of immersion beyond which the photocurrent diminishes. On the contrary, the photocurrents for both eosin dyes are independent of the sensitization time of the electrodes.

found out that the immersion time in dye solution of the ZnO electrodes was critical for the good performance of DSCs based on this metal oxide. Hence, they demonstrated that, for too long immersion times in ruthenium-based dye solution, the photocurrent of the solar cell becomes substantially diminished (see Figure 2a). The optimum immersion time depends on dye concentration, in such a way that concentrated solutions lead to a quicker sensitization but also to a more rapid decrease of the photocurrent. This behavior was attributed to aggregation of dye molecules in the semiconductor surface, which prevents efficient injection of electrons. Alternatively, Chou and coworkers explained this behavior by the formation of Zn2+ complexes with the dye ligands. Zn2+−dye complexes have indeed been detected by transient absorption UV/vis spectroscopy.48,49 This undesired chemical reaction deteriorates the metal oxide surface and hinders charge separation, as dye molecules forming complexes are not capable of injecting electrons into the ZnO conduction band. In previous work, it has been found50 that xanthene dyes like eosin do not show this behavior (see Figure 2b), probably due to the lower acidity of these dyes and their lack of complexing agents (like the pyridine of the typical ruthenium-based dyes). As a matter of fact, ZnO-based solar cells sensitized with xanthene dyes showed quantum efficiency at the maximum of the action spectra similar to that for the ruthenium-based dye solar cell. This finding suggests that only a wider absorption spectrum is responsible for the better performance of ruthenium-based dyes in ZnO devices and that a dye with a panchromatic absorption and which does not react with ZnO would be optimal to make cells based on this metal oxide. The isoelectric point of ZnO is ∼9, whereas that of TiO2 is ∼6.51 This means that the former is more basic than the latter and explains the poor chemical stability of ZnO electrodes in the presence of acidic dyes. Taking this into account, ruthenium dyes with only one carboxylic group have been studied. In this case, a lower formation of complexes was found, a fact that was

should come from one or several of these contributions, which are light harvesting efficiency (ηLHE), injection quantum yield (ηinj), and collection efficiency (ηcol) IPCE = ηLHE × ηinj × ηcol

(1)

The two first contributions, ηLHE and ηinj, will be discussed in the section entitled “sensitization of ZnO nanostructures”. Meanwhile, ηcol will be the key parameter in the “transport and recombination” section.

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SENSITIZATION OF ZnO Nanostructures The efficacy of the ZnO sensitization with organic and metal− organic dyes is determined by a good light absorption and an efficient electron injection. As mentioned in the Introduction, ZnO was one of the first metal oxides utilized in sensitization studies.14,36−38 These early studies, prior to the advent of DSCs, showed that electron injection from organic molecules into the conduction band of the semiconductor was possible and irreversible if an electrochemical gradient was sustained in the electrode. Furthermore, the works of Pettinger et al.37,38 revealed that the degree of doping in ZnO is crucial to achieve an efficient sensitization. The first dyes investigated to sensitize ZnO were natural dyes14 and small molecules such as xanthenes.39 After the appearance of the DSCs in 1991, several reports demonstrated the concept for ZnO electrodes using xanthene derivatives such as rhodamine,40 mercurochrome,41,42 and eosin.43 Ruthenium-based dyes were also tested in the early stages of DSC research in combination with ZnO nanostructured electrodes.44,45 The first studies with ruthenium-based dyes already pointed to a problematic dye uptake with the most common rutheniumbased sensitizers as the most probable reason for the poorer performance of ZnO DSC when compared with TiO2 ones. The sensitization process of ZnO electrodes has been carefully investigated by Keis et al.46 and Chou et al.47 These authors 11415



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accompanied with a relative improvement in efficiency.52 Reaction with acids is not the only cause of ZnO deterioration. Dyes containing complexing agents can remove Zn2+ ions from the ZnO lattice. Near Edge X-ray Absorption Fine Structure (NEXAFS) experiments have recently showed that protoporphyrin molecules may become “metalated” by acquiring zinc atoms from the ZnO surface.53 This was proven by the appearance of characteristic features of the Zn-porphyrin in the N 1s absorption edge of the protoporphyrin after its attachment to ZnO nanorods (see Figure 3). These results provide

surface (as is the case of TBP, dicyanamide, and thiocyanate anions, see Figure 4b). DSCs fabricated with ionic-liquid electrolytes including 1-methyl-3-ethylimidazolium dicyanamide (emiDCN), one of the most widely used ionic-liquid electrolytes in DSCs due to its low viscosity, exhibit a noticeable deterioration of the photocurrent at high concentrations of dicyanamide ions (Figure 4c).62 In general, species that tend to attach strongly to the metal oxide surface and that are helpful to reduce recombination and, thus, to improve voltage (Figure 4d) may have detrimental effects when the dye−metal oxide interaction is weak. Some examples of the effect of the interaction between the dye and ZnO in the presence of different electrolyte species can be found in Figure 4. The troublesome sensitization of ZnO with the most common dyes (at least those that proved to be the most successful for TiO2) is also complicated by the poor electron injection quantum yield, as pointed out by several studies. Asbury and co-workers63 used femtosecond IR spectroscopy to investigate the kinetics of the injection of electrons upon visible excitation. The signal detected in the mid-IR range is attributed to injected electrons, so that the time-dependent change in absorbance can be used to extract kinetic parameters. These authors found a multiexponential time dependence, indicative of a distribution of injection times. More importantly, they found that, for the same sensitizer, the injection of electrons is much slower than for TiO2. The reasons for the slower injection kinetics in sensitized ZnO were investigated by Furube and co-workers64,65 who identified intermediate states by ultrafast mid-IR spectroscopy in the photoexcitation process, for ZnO nanoparticles functionalized both with rutheniumbased N3 dye and with the NKX-2311 coumarin dye. These intermediate states, which occur at the semiconductor/dye interface, appear before the production of conduction band electrons (detected a longer wavelengths). Stockwell et al.66 also used ultrafast mid-IR spectroscopy to study the injection process for coumarin 343-sensitized TiO2 and ZnO nanoparticles. In this work, long-lived “interface-bound chargeseparated pairs” (IBCSP) were detected for ZnO but not for TiO2. The bound pairs play the role of an intermediate state prior to generation of free electrons. The dissociation of the IBCSP is found to be much slower in the case of ZnO. These authors attributed the different behavior of this metal oxide with respect to TiO2 to the lower dielectric constant of ZnO (∼8 versus ∼30−170 67−69), which makes the exciton Coulombic bound energy relatively large, hence preventing fast photogeneration of electrons. Long-lived IBCSPs were also reported for N3 on ZnO.64 Recenty, Tiwana et al.70detected “slow” components in the injection kinetics of ZnO sensitized with the Z907 ruthenium-based dye, using time-resolved terahertz photoconductivity measurements. As a bonus with respect to previous studies, the slow injection kinetics is correlated with a lower performance of solar cells fabricated with the same material. Although the dielectric constant issue is mentioned as a major reason for the slower injection in the case of ZnO, other factors such as low density of states in the conduction band and, especially, a high density of intraband defect states in specific nanostructures58,70 have also been suggested as responsible for the lower performance.

Figure 3. N 1s absorption edge of H2− and Zn-Protoporphyrin IX attached to ZnO nanorods compared to the molecular bulk powders (dotted lines). A strong change in the spectrum of the H 2 − Protoporphyrin IX after attachment to ZnO was detected due to the displacement of H2 by a Zn atom from the ZnO surface, i.e., a metalation (see the vertical lines) of the H2−Protoporphyrin IX. Reprinted (adapted) with permission from ref 53. Copyright 2010 American Chemical Society.

additional evidence of the poor chemical stability of this metal oxide in certain environments. Indoline dyes have also been successfully tested with ZnO.54−58 In previous work,57 a study of the sensitization process of ZnO commercial powder with the indoline dye coded D149 indicated that, in this case, no undesirable chemical reactions occur between dye and ZnO. Furthermore, due to its high extinction coefficient and panchromatic absorption spectrum, good light harvesting is ensured, so that efficiencies above 5% can be achieved (see Table 1).59 The good performance of this dye in ZnO-based DSCs is explained by its lower acidity and the lack of complexing agents. Moreover, coadsorbents such as chenodeoxycholic acid are well-known to prevent efficiently dye aggregation when working with indoline derivative dyes.60 However, the use of indoline dyes poses an additional problem, which has been pointed out by several authors.57,61 The lower interaction between dye and ZnO, which seems to avoid complex formation, provokes a weak and unstable sensitization effect. This effect has been noticed as bleaching and dye desorption in solar cells fabricated with ionic liquid electrolytes (Figure 4a)57 or when common additives such as tert-butylpyridine (TBP) are added to the electrolyte.61 In fact, it has been found that this desorption occurs for standard (i.e., N719) ruthenium-based dyes as well. The weakness of the dye−ZnO interaction provokes that the dye gets detached from the surface when the electrolyte acts as a good solvent or when there are adsorbing species competing for the adsorption sites at the semiconductor

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TRANSPORT AND RECOMBINATION IN ZnO-Based DSC A good performing DSC requires good collection of charges in the external circuit, i.e., a collection efficiency ηcol approaching 11416



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Figure 4. (a) Dotted line represents the UV−vis absorption spectrum of ionic liquid 1-propyl-3-methyl imidazolium iodide (pmiI) in tert-butanol/ acetonitrile (1:1). The thick line represents the UV−vis spectrum of pmiI after contact with a ZnO film sensitized with D149 dye and afterward dissolved in tert-butanol/acetonitrile. The presence of the characteristic D149 dye absorption peak at 530 nm reveals that small amounts of dye get detached from the ZnO surface. (b) Photocurrent versus pmiI content for ZnO cells sensitized with N719 and with an electrolyte based on pmiI and 1-ethyl-3-methylimidazolium thiocyanate (emiSCN). Inset shows the sensitized photoanodes at 100% pmiI content (left) and 20% pmiI content (right). Bleaching effects are clearly observed when the sensitized photoanode is in contact with an electrolyte containing emiSCN. (c) Photocurrent versus the pmiI content in an electrolyte based on a mixture of this ionic liquid with the low viscosity ionic liquid 1-ethyl-3-methylimidazolium dycianamide (emiDCN) and 0.05 M I2, for ZnO cells sensitized with N719 dye. Although the reduction of the pmiI ratio initially provokes an increase of photocurrent, it is deteriorated as the content of emiDCN increases as a consequence of dye desorption from the ZnO surface. (d) Open circuit voltage versus pmiI content in an electrolyte based on a mixture of this ionic liquid with one of the following low viscosity ionic liquids: 1-ethyl-3-methylimidazolium dycianamide (emiDCN), 1-ethyl-3-methylimidazolium tetracyanoborate (emiBCN), and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide (emiTFSI) and 0.05 M I2, for ZnO cells sensitized with N719 dye. It is found that the photovoltage is incremented by more than 100 mV when the electrolyte is based on a mixture of emiDCN and pmiI ionic liquids. On the contrary to DCN− anions, BCN− and TFSI− anions do not produce a positive effect on the photovoltage. However, the stability of ZnO cells is improved because dye desorption is not observed. The nature of the interaction between the metal oxide surface and the anion influences crucially the recombination kinetics.

encountered for nanostructured TiO2. For instance, Solbrand and co-workers74 reported values ranging between 10−4 and 10−6 cm2 s−1 (depending on voltage as it is characteristic of a multiple-trapping mechanism of transport). Noack et al.84 also used photocurrent transients at short-circuit conditions to estimate a diffusion coefficient of 1.7 × 10−4 cm2 s−1. Quintana et al.85 obtained 1.2 × 10−4 cm2 s−1 from Intensity-Modulated Photocurrent Experiments (IMPS), very similar to values found for TiO2-based solar cells for a similar illumination intensity at short-circuit conditions. Recently, Tiwana et al.70 measured a “device diffusion coefficient” of 1.1 × 10−4 cm2 s−1 at short circuit, several orders of magnitude below the value obtained from terahertz spectroscopy, which is attributed to bulk transport. The combination of techniques utilized by these authors confirms that ZnO produces more rapid transport than TiO2, as long as trapping effects are not taking place. However, in the full device and in the long time scale, where charge transport is limited by electronic defects and by the presence of the electrolyte, both metal oxides perform basically the same. In recent work,86 the electron diffusion coefficient of ZnO cells fabricated with different dyes and electrolytes has been extracted from IMPS measurements at varying light intensities.

100% in eq 1. This is achieved by a combination of fast transport and slow recombination in the photoanode, which means that the electron diffusion length is substantially longer than the film thickness.71,72 Early studies on electron transport in nanostructured ZnO/electrolyte systems were performed by Hoyer and Weller73 and by Solbrand et al.74These studies showed that a trap-limited mechanism of transport, analogous to that occurring in TiO2,71,75−78 was also characteristic of randomly packed ZnO nanoparticle networks. It is important to stress here that time scales of different processes are relevant when studying transport. In this connection, it is necessary to distinguish between bulk or free-electron movement,79 probed with techniques such as terahertz spectroscopy70,80 and traplimited transport, probed by photoelectrochemical experiments performed and analyzed in the long time scale (photocurrent transients, electrochemical impedance spectroscopy, and intensity-modulated spectroscopies).81−83 In fact, as pointed out in the introduction, ZnO is superior to TiO2 as regards bulk conductivities, which are not affected by traps, defects, and grain boundaries typical of randomly packed nanoparticle-based electrodes. In contrast, the electron diffusion coefficients measured for nanostructured ZnO devices are very similar to those 11417



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Values ranging between 10−4 and 10−6 cm2 s−1 are obtained, and a multiple-trapping behavior with respect to the position of the quasi-Fermi level, analogous to the typical behavior of standard TiO2 solar cells, is clearly observed (see Figure 5).

energy trap distribution in the semiconductor. Quintana et al.85 have measured the accumulated charge in the ZnO electrodes used for DSCs from charge extraction measurements. They found indeed an exponential dependence with α ∼ 0.14. Similar results have been obtained,57,58,62,86 from capacitances extracted from Electrochemical Impedance Spectroscopy (EIS)82,89,90 experiments for different dyes and electrolytes. The coincidence between charge extraction and capacitances obtained in EIS measurements strongly suggests that an exponential distribution of trap energies is characteristic of ZnO, as well as of TiO2. However, as discussed in the next section, the doping level in the semiconductor seems to have a strong influence on the voltage dependence of the capacitance,58,91,92 indicating that other, not chemical, contributions to the capacitance should be taken into account.93 In spite of the analogies with TiO2-based DSCs, anomalous features in the transport properties of ZnO-based devices have been detected. These can be attributed to a nonideal behavior of conducting electrons in this metal oxide.86 ZnO-based DSCs do not show58,86 Warburg feature in the impedance spectrum (detected as a 45° straight line in the Nyquist plot82), which is attributed to electron transport in the semiconductor. A new strategy combining EIS and IMPS measurements was used to obtain the transport resistance in ZnO solar cells.86 The results indicated that this resistance does not exhibit an ideal slope with respect to Fermi level variation. The departure from ideal statistics can be related to interactions between free electrons.94 These interactions may be more important in the case of ZnO due to its lower dielectric constant. The effect of the lower dielectric constant in the transport of sensitized ZnO nanocrystals has also been studied by Nemec et al.80 who found a deterioration of the short time scale mobility (terahertz mobility) for ZnO with respect to the bare material. On the contrary, deterioration of the current was not observed in the case of sensitized TiO2. These authors explained this behavior by a stronger interaction between electrons and dye cations in the case of ZnO. Recombination losses in DSCs are critical for photoconversion efficiency. They affect the maximum photovoltage attainable and the shape of the current−voltage curve. Very few studies have performed a direct comparison between TiO2 and ZnO-based DSCs, utilizing the same dye and electrolyte70 and a similar particle size for both kind of electrodes.85 As a common feature, significantly lower short-circuit photocurrents and fill factors were detected in ZnO-based solar cells and also a

Figure 5. Diffusion coefficients obtained from the IMPS response of ZnO-based dye-sensitized solar cells with three different configurations involving two dyes and two types of electrolytes including the iodide/ iodine redox couple: C1 (N719 dye, 0.5 M LiI, 0.05 M I2, and 0.5 M TBP in 3-methoxypropionitrile), C2 (D149, 0.5 M TBAI, 0.05 M I2, in acetonitrile/ethylenecarbonate, 1:4), and C3 (0.05 M I2 in 1-ethyl3-methylimidazolium tetracyanoborate (emiBCN) and and 1-propyl-3methylimidazolium iodide (pmiI) (35:65 v/v)). Values ranging between 10−4 and 10−6 cm2 s−1 are obtained, and a multiple-trapping behavior with respect to the position of the quasi-Fermi level, analogous to the typical behavior of standard TiO2 solar cells, is clearly observed. Reprinted (adapted) with permission from ref 86.

These findings, in agreement with previous literature,74,85 suggest that an exponential distribution of trap energies is also occurring in nanostructured ZnO, which would explain the observed voltage dependence.87 The trap distribution in nanostructured electrodes is commonly obtained in charge extraction experiments as a function of applied voltage or light intensity. Alternatively, it can be detected in the form of a chemical capacitance Cμ88 that corresponds to the accumulation of electrons in localized states in the semiconductor. The exponential dependence is expressed as ⎛ αeV ⎞ Cμ(V ) = C0 exp⎜ ⎟ ⎝ kBT ⎠

(2)

where e is the elementary charge, kB the Boltzmann constant, T the absolute temperature, and α a parameter that depends on the

Figure 6. (a) Current−voltage characteristics under simulated solar conditions of solar cells based on ZnO and TiO2 electrodes with a similar particle size, sensitized with N719 and using the same electrolyte composition. Illumination: 1/10 sun. (b) Current−voltage characteristics under simulated solar conditions measured for three typical liquid-electrolyte photovoltaic cells fabricated from either TiO2, ZnO, or SnO2 nanoporous films sensitized with Z907 dye and the same electrolyte composition. Reprinted (adapted) with permission from refs 85 (Copyright 2007 American Chemical Society) and 70 (Copyright 2011 American Chemical Society). 11418



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The collection efficiency ηcol has typically been analyzed in the literature in terms of the diffusion length,71,72,98,102 which represents the average distance that carriers can travel in the material. For small perturbations of the Fermi level, the electron diffusion length is shown to be equal to98

tendency to a lower photovoltage (see Figure 6 and Table 1). The reasons for this behavior are discussed below, summarizing the basic equations that allow for a systematic analysis of our and other literature results. The role of recombination in the shape of the current− voltage (IV) curve is determined by the voltage dependence of the recombination current density Jrec. The IV curve can be described, in its simplest form, by the diode equation. Assuming no series or shunt resistances, this equation can be written as95 J = Jsc − Jrec = Jsc − J0 (exp(eV/mkBT ) − 1)

Ln = (Dn τn)1/2

where Dn is the apparent electron diffusion coefficient. Measurements of the diffusion coefficient and the lifetime at coincident positions of the Fermi level76 permit us to obtain the diffusion length at different illumination intensities or voltage bias. The diffusion lengths of ZnO-based DSCs86 have been measured with two different procedures and using a combination of the IMVS/IMPS and EIS techniques. In this work, the fact that the Fermi level (directly related to total charge density accumulated in the semiconductor) is not the same at open- and short-circuit has been taken care of, as well as the occurrence of band-edge displacements when additives such as TBP are present in the electrolyte. Thus, by extracting Dn and τn at the same position of the Fermi level with respect to the conduction band edge, it is found that Ln is significantly longer than the film thickness (see Figure 7). The data are

(3)

where Jsc is the short-circuit photocurrent density; J0 is the exchange-current density; and m is the ideality factor. The recombination current can be written in terms of the electron density in the semiconductor and recombination rate constant (inverse of an electron lifetime).71,96−98 The voltage dependence of the recombination current defines a recombination resistance, which typically takes the form97 −1 R rec =

∂Jrec ∂V

−1 exp[βV /kBT ] = R rec,0

(6)

(4)

where β is an empirical parameter that defines the voltage dependence of the recombination resistance. Equation 4 shows that the recombination resistance is diminished as the voltage of the cell is increased. This can be observed in an impedance spectrum,58,86 where the recombination resistance corresponds to the width of the midfrequency semicircle. Integration of the voltage-dependent recombination resistance leads to eq 3 with m = 1/β. Hence, the parameter β determines directly the fill factor of the current−voltage curve. A recombination parameter β < 1, meaning a nonideal diode with a low fill factor, has been related to nonlinear recombination kinetics with respect to free electrons.98,99However, other contributions such as shunt and series resistances do have an influence in the fill factor. A small-perturbation electron lifetime τn can be defined for small variations of the Fermi level.97,100 This parameter can be measured by open-circuit photovoltage decay experiments (OCVDs),100 Intensity Modulated photovoltage spectroscopy (IMVS),101 or EIS measurements.89,97 Assuming that there is no significant band bending in the nanostructured material, the small-perturbation lifetime can be related to the recombination resistance and the chemical capacitance of the semiconductor film (5)

Figure 7. Small-perturbation electron diffusion lengths obtained from EIS + IMPS ((Rct/Rt)1/2, dashed lines) and IMVS + IMPS ((τnDn)1/2, symbols) for the same three solar cell configurations detailed in Figure 5. Rt is the electron transport resistance obtained by combining capacitance data from EIS and transport times from IMPS (see ref 86). Very good agreement is found between the two methods used to estimate the diffusion length. The diffusion lengths are found to be much longer than the film thickness, indicating 100% collection efficiency. Adapted by permission from ref 86.

Quintana et al.85 measured the small-perturbation lifetime by IMVS as a function of light intensity and found trends similar to those of TiO2, with slightly shorter values for ZnO. This work has been extended58,86 by measuring the lifetime by OCVD, EIS, and IMVS for nanoparticulated ZnO electrodes sensitized with the dyes N719 and D149 and two types of electrolytes, organic solvent and ionic liquid-based electrolytes. The lifetime was found to be independent of the measurement technique, with values ranging between 1 and 100 ms depending on the electrolyte used, at 1 sun open-circuit conditions. The recombination resistances extracted from EIS measurements gave an exponential dependence as predicted by eq 4, with β ∼ 0.6−0.7. This is in fact a behavior very similar to that found on typical TiO2 cells.90 Hence, the reasons of the lower fill factor of ZnO devices with respect to TiO2 do not seem to be related with the nonlinear character of the recombination reaction, and other factors should be considered. These will be discussed in connection with the also lower photocurrent, as described below.

supported by the fact that the diffusion length extracted from an independent strategy (combining the transport and the recombination resistance: Ln2 = Rct/Rt) leads essentially to the same results. The results obtained for the diffusion length show then that ηcol approaches 100% in all cases. Furthermore, the diffusion length is observed to increase with Fermi level as predicted for a recombination parameter β < 1, which means that the recombination reaction is not linear with respect to free electron concentration, an analogous result to that reported for standard TiO2 DSCs.98,99 These results demonstrate that it is possible to make ZnO-based DSCs with a good collection of electrons in the semiconductor, and therefore, this does not appear to be the reason for the low performance of this metal oxide in DSC applications, at least for nanoparticulated electrodes. The previous discussion led us to go back to the other two factors in eq 1 as the origin of the low photocurrent and low fill factor in ZnO-based DSCs. To analyze this, it is important to

τn = R recCμ

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co-workers,105 who have recently discussed the effect of poor dye regeneration in the IV curve at near open-circuit conditions. The raise of the Fermi level as the cell is forward biased may hinder the injection of electrons from the dye and enhance electron−dye geminate recombination.106,107These unfavorable effects can be much more important in ZnO-based devices due to the low dielectric constant of ZnO, which produces the formation of longlived surface-bound dye-electron states, as discussed in the previous section. The results obtained in our group58,86 suggest that mechanisms related to poor injection and/or poor dye regeneration are the most likely candidates to explain the poor performance of nanoparticulated ZnO devices with respect to TiO2 DSCs.

look at the photocurrent that can theoretically be obtained at short-circuit conditions. In previous works,58,86 ∼10 mA cm−2 has been obtained for ZnO-based devices sensitized with the indoline dye D149, very similar to the value obtained by Chen and co-workers for sensitized hierarchical ZnO nanostructurebased solar cells.59However, the light harvesting efficiency of nanoparticulated electrodes sensitized with this dye is very efficient (see Figure 8), which predicts ∼14 mA cm−2. If the

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ARRAYS OF ONE-DIMENSIONAL NANOSTRUCTURES The suitability of ZnO to prepare a variety of nanostructures has led to a plethora of studies where vertically aligned, onedimensional (1-D) ZnO nanostructures are utilized as photoanodes in DSCs. The pioneering work in this approach was published by Law et al.27 who showed that ZnO nanowires were superior to randomly packed nanoparticle networks when the photocurrent is compared at the same value of the roughness factor. These authors explained the promising properties of the nanowires by assuming that a 1-D nanostructure favors transport and minimizes recombination, hence improving electron collection, due to the existence of a grain boundary-free direct pathway toward the external circuit.108,109 The conclusions of this work were confirmed by Galoppini and co-workers,30 who measured the transport time of ZnO nanowires and found it significantly shorter than in mesoporous ZnO films. However, the photoconversion efficiency of nanowires remained low, a fact that was attributed to the low surface area of the nanowire electrodes, which limits the dye uptake. The findings of these early works triggered the publication of many papers where the alleged advantages of ZnO nanowires and related nanostructures were exploited.30,31,55,110−117The issue of the low surface area has been addressed by means of hierarchical structures, using, for instance, “arborescent” nanostructures.118−121This kind of nanostructure achieves a reasonable good dye uptake while ensuring, in principle, good collection and good light absorption due to scattering effects. However, no clear progress has been achieved as the best efficiencies reported so far still correspond to electrodes without 1-D nanostructures (Table 1), a reality that is also common to TiO2-based DSCs. It is worth noting that the comparison of the different investigated electrodes is difficult, due to the variety of preparation methods and experimental conditions. These can affect the chemical nature of the ZnO surface, the degree of doping, etc. The chemical instability of ZnO with certain dyes discussed above is also an additional source of uncertainty, which affects the reproducibility of many literature results. The lack of significant progress in performance for 1-D nanostructures suggests that other factors apart from low surface area and low dye uptake might be responsible for the low efficiencies reported. Furthermore, it has to be noted that a 1-D nanostructure on its own is not capable of improving the efficiency of a DSC if the charge collection efficiency is already close to 100% in a disordered electrode. As shown in the previous section, this is the case in randomly packed ZnO electrodes combined with standard dyes and electrolytes. Hence, only for architectures where recombination may be very rapid (e.g., devices using Co-based redox couples6 or solid hole collectors122), it might be interesting to move to arrays of aligned and/or single-crystal nanostructures.123

Figure 8. (a) SEM micrographs of nanowire arrays (left) and nanoparticles (right). (b) Amount of loaded dye for the different ZnO morphology samples and an estimation of the maximum theoretical short-circuit current density obtained by integrating the AM1.5 photon flux with 100% quantum yield and 100% collection efficiency. (c) Absorptance spectra of glass/SnO2: F/ZnO/D149 samples (left y-axis) and optical density of solutions containing the desorbed dye (right y-axis). Reprinted (adapted) with permission from ref 58.

assumption of nearly quantitative collection efficiency is correct, this result suggests that there is a significant current loss due to poor injection or poor dye regeneration (which results in poor injection). In ref 86, the IV curve of the DSC devices has been simulated by solving the continuity equation for accumulated electrons in the semiconductor.96,103,104 In this numerical calculation, a nonlinear recombination kinetics with the β parameter obtained experimentally has been introduced. In addition, the zero-bias lifetime, the dye-loading, and the series resistance were taken as fitting parameters. The most remarkable result is that, whereas the lifetime was found to be in agreement with the values obtained by small-perturbation experiments, the fitted values of the dye loading and the series resistance differed from the experimental data. With respect to the dye loading, the fitted value was significantly larger than that obtained from dye-loading measurements for the same configuration.58 This indicates that poor injection is causing an important reduction in the short-circuit photocurrent. As regards the series resistance needed to fit the experimental fill factor, this is found to be larger than the values estimated from impedance spectra, even taking into account the fill factor reduction due to nonlinear recombination. In this regard, it is important to cite the recent work by Jennings and 11420



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commercial ZnO nanoparticles seems to be thus the main candidate to explain the poorer performance of ZnO nanowire based solar cells. Although our study58 was limited to electrodeposited nanowires, a similar scenario seems to occur in ZnO nanowires deposited by other wet-chemistry routes such as chemical bath deposition. Similar free-carrier densities have been reported in those structures,91,128,129 and also a limited performance was found when they were integrated to DSCs.130 The passivation of surface states in ZnO nanostructures seems to be thus necessary to exploit the appealing properties of one-dimensional ZnO nanostructures in DSCs. In this framework, after some reports from different research groups,131−133 Xu et al.28 reported recently DSC based on ZnO/TiO2 core−shell NW arrays with PCE of 7%. This value results in a significant step forward with respect to efficiencies obtained for unmodified ZnO nanowires in the 1.5−2.5% range (Table 1), pointing out the surface modification of ZnO nanowires as a very promising strategy to enhance the performance of ZnO-based DSCs. Other ways of nanowire surface modification are also possible. As an example, electrodeposited ZnO nanowires coated with a thin shell of colloidal ZnO nanoparticles have shown themselves as efficient electron collector building blocks for polymer solar cells, reaching an increase of 45% in photocurrent with respect to the planar configuration.134 The deposition of ZnO thin films from emerging nonaqueous media such as aprotic ionic liquids,135 which allow avoiding the formation of Zn(OH)2 as an intermediate phase,136 opens wide possibilities for building nanowire-based core−shell nanostructures with innovative surface properties. The extremely long diffusion lengths (i.e., 103 μm) reported for 63 μm length nanotube-like peculiar ZnO nanostructures92 deposited by the atomic layer deposition technique (i.e., gas phase technique) are supporting the relevance of surface properties to diminish recombination losses.

With the original aim of studying the real contribution of the low surface area on efficiency, the performance of nanowire arrays prepared by electrodeposition and commercial ZnO nanoparticles, both sensitized with the indoline dye D149, has been compared.58 It has been found that, in spite of the lower dye loading, the nanowire arrays are very efficient in collecting light when sensitized with this dye (see Figure 8). As a matter of fact, the measured absorption efficiencies would lead to photocurrents of ∼11 mA cm−2 for nanowires of 5 μm in length assuming 100% collection efficiency. This was only 2 mA below the theoretical photocurrent obtained for nanoparticulated electrodes in this study. As shown in the original work, the good light harvesting properties of the nanowires are not translated in high photoconversion efficiencies. The electron lifetimes obtained from EIS and OCVD experiments at variable illumination indicate that recombination is stronger in the case of the nanowires, which would explain the low photocurrent and photovoltage.58 The reasons for the poorer performance of the electrodeposited ZnO nanowires appear to be related to the lowtemperature, wet-chemistry based synthesis procedure, which differs from the vapor phase method used to accomplish the synthesis of ZnO commercial nanoparticles.124 Although the free-carrier density of ZnO electrodeposited nanowires can be tailored in a relatively wide range (i.e., from 6 × 1018 to 4 × 1020 cm−3, see Figure 9)125 by varying the chloride concen-

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CONCLUSIONS AND OUTLOOK In this feature article, we have intended to cast some light on the origin of the lower performance of ZnO as photoanode material in DSCs as compared with TiO2. This limitation in photoconversion efficiency stubbornly remains in the current state of the art, in spite of the huge amount of literature produced for ZnO-based DSCs. The abundance of studies is a consequence of the facility of ZnO for the preparation of a great variety of exciting morphologies. In addition, electron transport in the bulk is much more rapid in ZnO than in TiO2, which seems a great advantage for its use as an electron conductor in nanostructured solar cells. Hence, ZnO is an especially propitious material to contribute with something new but strikingly elusive to produce real progress in the field up to date. Going beyond the simple measurement of the current− voltage curve, which shows that ZnO yields systematically lower efficiencies than TiO2, the origin of the performance limitation should be studied by looking at the factors affecting light absorption and electron injection, as well as the transport and recombination rates in the operating device. As regards light absorption, most of the dyes commonly used in DSCs exhibit complications when they are used to sensitize ZnO. Molecules with acidic groups or that include complexing moieties should be used with a careful control of the concentration and sensitization time, to avoid reaction with the metal oxide surface and formation of Zn2+ complexes. On the other hand, dyes that do not show reaction with ZnO tend to get detached from the metal oxide surface in the presence of certain

Figure 9. Donor density of ZnO nanowire arrays synthesized by electrodeposition in aqueous media as a function of the chloride concentration. Reprinted (adapted) by permission from ref 125. Copyright 2008 American Chemical Society.

tration in the electrolyte, the obtained values are significantly higher with respect to that of nanowires grown from vapor phase techniques (i.e., ∼ 1017 cm−3, see ref 126). A postdeposition thermal annealing is required to get comparable values (i.e., in the 1017−1018 cm−3 range, Figure 9). The physical characterization of electrodeposited ZnO nanowire arrays, by using EIS and photoluminescence (PL), pointed out the presence of donor states below the conduction band. Indeed, the formation of a donor band was claimed in the asdeposited nanowires due to the overlap of the wave functions of the neighboring donors. 127 Although the nanoscale resolution mapping of the free carriers along the radial direction of the nanowires remains still as an ambitious technical challenge, the dependence of the PL spectra with the nanowire diameter suggests that a significant amount of donors are located at the nanowire surface.127 The latter may perturb the injection quantum yield, due to the creation of an intermediate state and, as a consequence, slow injection kinetics. The collection efficiency can also be affected by the presence of additional recombination pathways. A higher density of surface states in the electrodeposited ZnO nanowires than in 11421



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electrolytes and additives. Hence, a dye that is not too “strong” and not too “weak” to interact with the ZnO surface remains to be found. Apart from the sensitization failures, a clear injection limitation has been identified in ZnO-based DSCs, as demonstrated by several reports reviewed in this feature article. A good performing DSC requires efficient charge-separation across the metal oxide surface. This separation is hindered in ZnO, as a consequence of the formation of electrically bound electron−cation pairs after dye excitation. As ZnO has a lower dielectric constant than TiO2, we can conclude that this intrinsic property of the material seems thus to pose a fundamental limitation on device performance, at least for the dyes studied up to present. The formation of the surface-bound state not only leads to lower photocurrents but also deteriorates the fill factor, as the build-up of negative charge in the semiconductor enhances progressively the geminate dye cation−electron recombination and complicates dye regeneration. An indirect evidence of the injection limitation in ZnO-based DSCs is found when collection efficiency is carefully analyzed. Although several anomalies are detected, electron transport and electron recombination in nanoparticle ZnO networks are found to behave quite similarly to their TiO2 counterparts. Thus, a multiple-trapping mechanism of transport can be identified, with a voltage-dependent diffusion coefficient that shows values similar to those found in mesoporous TiO2. This means that the good bulk transport properties of ZnO have little effect on fully operating DSC devices, at least for those based on disordered nanocrystalline electrodes, where electron trapping is limiting. With regard to recombination of photogenerated electrons with electron acceptors in the electrolyte, the typical nonlinear features recently described for TiO2-based DSCs are also observed. Careful measurement of the lifetime, the diffusion coefficient, and the transport and charge transfer resistances, by combination of three different techniques and at the same position of the quasi-Fermi level, clearly demonstrates that ZnO electrodes can be used to make DSCs with a collection efficiency close to 100%. As light harvesting can also be very efficient with the use of highly absorptive dyes, the conclusion is that electron injection is the main loss source in ZnO-based DSCs. The good collection efficiencies found for DSCs based on randomly packed ZnO nanoparticle-based electrodes seem to suggest that 1-D nanostructures are not the way to break the efficiency limitation. Although this point cannot be ignored, it is important to note that the most recent advances involved the use of device designs with slow diffusion and/or rapid recombination, in which the implementation of 1-D nanostructures can provide a definitive advantage. However, 1-D ZnO nanostructures, especially those obtained from wet-chemistry, low-temperature methods, suffer from a high degree of doping. The appearance of localized states in the band gap leads to increased recombination and, possibly, slower injection. However, no great limitation appears in light harvesting in spite of the lower surface area, if high extinction coefficient dyes are used. Hence, the disadvantage of many 1-D nanostructures appears to be more an issue of poor injection/ collection, rather than dye uptake, as commonly believed. In summary, as a general conclusion, it can be established that further improvements in performance of ZnO-based DSCs should be achieved by means of a better control of the surface chemistry of ZnO as regards its interaction with dyes and electrolytes. This can even be more important than the actual morphology, i.e., the photoanode architecture. ZnO shows itself

as an excellent backbone material due to its good bulk electron transport but inadequate to produce charge separation via sensitization with dyes. In this context, the use of core−shell architectures, either with 1-D or disordered morphologies, appears to be especially appealing. A combination of TiO2 and ZnO, for instance, can provide a way to ensure robust dye adsorption and charge separation, at the same time that rapid electron collection can be achieved for especially fastrecombining electrolytes (i.e., cobalt-based complexes or solid-hole conductors). In any case, a change of focus of the current research on ZnO-based DSCs (from morphology to surface control) is desirable. Investigations addressing new dyes particularly designed for ZnO are also highly convenient.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Notes

The authors declare no competing financial interest. Biographies

Juan A. Anta received his Ph.D. in Physical Chemistry from the University Complutense de Madrid in 1997. He was a postdoctoral fellow in the Department of Theoretical Chemistry of the University of Oxford where he worked on numerical simulation of quantum fluids. From mid 1999 to mid 2000 he was a research assistant at the Department of Chemistry of the Imperial College, London. At present, he is an Associate Professor in the Physical Chemistry Section of the University Pablo de Olavide, Seville. His research interest focuses on solar cell modeling and characterization, computer simulation, and nanostructured solar cells.

Elena Guillén received her Ph.D. in 2011 at University Pablo de Olavide, Seville. Her thesis work was focused on Dye Sensitized 11422



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Solar Cells based on ZnO substrates. She is currently a postdoctoral fellow in the Department of Physical Chemistry at University of Alicante, and her research interest is focused on quantum dot and nanostructured solar cells.

Ramon Tena-Zaera received his Ph.D. degree in Applied Physics at University of Valencia in 2004. After 4 years at the Institute de Chimie et des Matériaux Paris Est (ICMPE-CNRS), he joined IK4-CIDETEC in November 2008. He was then awarded with a Ramon y Cajal fellowship, and he is now leading the Photovoltaics Unit of IK4CIDETEC. His current research is focused mainly on nanomaterials and their integration in solar cells and (opto-)electronic devices.

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ACKNOWLEDGMENTS We thank the Ministerio de Educación and Ciencia of Spain for project HOPE CSD2007-00007 (Consolider-Ingenio 2010), CTQ2009-10477 (TRANSLIGHT), and a FPU studentship and Junta de Andaluciá (Andalusian Regional Government) under projects P07-FQM-02595, P07-FQM-02600, and P09FQM-04938, European Union under project ORION CP-IP 229036-2, and the Basque Regional Government through Etortek Program. R.T.-Z. acknowledges the support from the Program “Ramón and Cajal” of the MICINN. We would like to thank all our collaborators on previous investigations on ZnObased solar cells.

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