Efficient Dye-Sensitized Solar Cells Made from ZnO Nanostructure

Article pubs.acs.org/JPCC

Efficient Dye-Sensitized Solar Cells Made from ZnO Nanostructure Composites Etienne Puyoo,*,† Germain Rey,† Estelle Appert,†,‡ Vincent Consonni,† and Daniel Bellet† †

Laboratoire des Matériaux et du Génie Physique, CNRS−Grenoble INP, 3 parvis Louis Néel 38016 Grenoble, France Laboratoire de Science et Ingénierie des Matériaux et Procédés, CNRS−Grenoble INP, 1130 rue de la Piscine 38402 Saint-Martin d’Hères, France

‡

ABSTRACT: We describe a simple and low-cost wetchemical route for the synthesis of ZnO nanowire/nanoparticle composite electrodes. The integration of such nanocomposite photoanodes in dye-sensitized solar cells (DSSCs) leads to much better photovoltaic properties than for bare nanowire or nanoparticle ensembles with a photoconversion efficiency as high as 4.7%. Importantly, beneficial effects of thermal heat treatments are investigated in terms of ZnO nanoparticle formation and DSSC photovoltaic properties. In particular, we reveal the presence of a zinc oxoacetate intermediate phase from 120 °C, which drastically reduces the electron leakages at the ZnO/dye/electrolyte interface, while retaining a large specific surface area and a relatively good electron injection efficiency.

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fluorine-doped SnO2 (FTO) substrates. The NW arrays are subsequently filled with ZnO NPs deposited by CBD. First, the photovoltaic properties of three different DSSCs made from ZnO NPs, ZnO NWs, and ZnO NW/NP hybrid photoanodes are compared. Second, the beneficial effects of thermal heat treatments are investigated regarding the ZnO NP formation mechanisms. It is revealed that the annealing temperature is crucial and affects the composition and morphological properties of the NP aggregates and, therefore, the ZnO NP/ dye/electrolyte interface quality. Eventually, we investigate in detail the effect of the annealing temperature on the photovoltaic properties of the resulting NW/NP compositebased DSSCs.

INTRODUCTION Dye-sensitized solar cells (DSSCs) can convert solar light into electrical energy and may be used in mobile applications for which inexpensive, nonrare and nontoxic materials are required. The initial DSSC structure was first designed in 1991 by Grätzel and O’Regan1 with the use of a nanoporous TiO2-based photoanode exhibiting a high specific surface area. This type of DSSC can achieve a conversion efficiency up to 12.3%.2 Owing to its very similar energy band levels and superior electron mobility,3 ZnO has been extensively explored as an alternative to TiO2.3−11 However, the best conversion efficiency so far for ZnO does not exceed 7.5%.6−9 ZnO is regardless promising due to its ability to grow in a wide variety of nanostructures,12−14 presenting unique physical properties for optics and electronics In the present paper, the potential of ZnO nanocomposite photoanodes is investigated by hybridizing ZnO nanowires (NWs) with ZnO nanoparticles (NPs). The key point is to combine a large specific surface area, as provided by NPs, with direct conduction paths, as allowed by NWs.15−17 In 2005, Baxter et al.18 first hybridized ZnO NWs with ZnO NPs and achieved DSSCs with a conversion efficiency up to 1.1%. Subsequently, Ku et al.15 used a full chemical bath deposition (CBD) process to synthesize ZnO nanocomposite photoanodes. By notably optimizing the NP amount and postannealing conditions, a DSSC photoconversion efficiency of 3.2% was reached.15 More recently, two systematic studies based on the same concept, but using different deposition techniques, have been reported with an interesting power conversion efficiency close to 4%.19,20 Here, we report the synthesis of ZnO nanocomposite electrodes via a simple and low-cost wet-chemical route. Initially, ZnO NW arrays are grown by CBD on preseeded © 2012 American Chemical Society

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EXPERIMENTAL SECTION

Growth and Structural Characterization of ZnO Nanostructures. ZnO nanostructures were grown on FTO thin films acting as transparent electrodes. FTO layers were deposited on glass substrates (Corning C1737) by ultrasonic spray pyrolysis under atmospheric pressure. FTO growth was performed at 420 °C from a precursor solution consisting of 0.16 M tin chloride pentahydrate 98% (Sigma-Aldrich) and 0.04 M ammonium fluoride 98% (Sigma-Aldrich) in methanol.21 The as-grown FTO transparent electrodes present typical square sheet resistances of 10 Ω/sq and overall transmittance, including a glass substrate effect of nearly 85% in the visible range. We used the growth conditions described in ref 22 to synthesize dense networks of vertically aligned ZnO NWs. Received: June 22, 2012 Revised: July 25, 2012 Published: August 9, 2012 18117

dx.doi.org/10.1021/jp306174f | J. Phys. Chem. C 2012, 116, 18117−18123

The Journal of Physical Chemistry C

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Prior to the ZnO NW growth, FTO thin films were seeded with ZnO NPs by dip-coating the substrate in a 37.5 mM equimolar ethanolic solution of zinc acetate (99.99%, Sigma-Aldrich) and monoethanolamine (J.T. Baker).23 An annealing process was carried out for 20 min at 450 °C in order to decompose zinc acetate into oriented ZnO NPs. ZnO NW growth was then performed by immersing the preseeded FTO substrates in an aqueous solution kept at 90 °C for 5 h and composed of 0.05 M zinc nitrate hexahydrate (99%, Sigma-Aldrich), 0.025 M hexamethylene tetramine (HMTA) (Sigma-Aldrich), 0.4 M ammonium hydroxyde (Sigma-Aldrich), and 5 mM polyethylenimine (PEI) (Sigma-Aldrich). It was found that the addition of PEI and NH4OH in the growth solution inhibits the homogeneous nucleation of ZnO while allowing a rapid growth of ZnO NWs from the seeds.22 To synthesize composite electrodes, ZnO NPs are directly grown on top of the ZnO NW arrays according to the growth conditions described in ref 12. The first step consists of growing hydroxide zinc acetate (HZA) NPs by CBD in a methanolic solution of 0.15 M zinc acetate dihydrate (ZnAc2·2H2O) (99.5%, Merck) kept at 60 °C for 22 h. Annealing in air at 120 °C for 15 min is then necessary to convert HZA NPs to ZnO NPs. In parallel, a ZnO NP reference sample was created by directly growing ZnO NPs on the FTO substrate. The morphology of ZnO nanostructures was investigated by scanning electron microscopy (SEM) imaging recorded with an FEI Quanta Plus equipment. The structural properties of different NP samples were investigated by XRD with a Bruker D8 Advance diffractometer using a Bragg−Brentano θ/2θ configuration and Cu Kα1 radiation. DSSC Fabrication and Physical Characterization. To fabricate DSSCs, ZnO nanostructured photoanodes presenting a typical size of 1 cm × 2 cm were immersed in an ethanolic solution of 0.25 mM N719 dye (Solaronix) at 60 °C for different durations ranging from 30 to 150 min. Two 25 μm thick plastic ribbons were then sandwiched between the sensitized ZnO electrode and a 100 nm thick Pt counter electrode that was previously deposited by sputtering on a glass substrate. A solution of 0.1 M LiI, 0.6 M 1,2-dimethyl-3propylimidazolium iodide, 0.05 M I2, 1.0 M 4-tert-butylpyridine, and 3-methoxypropionitrile7 was introduced between the electrodes by capillarity. The DSSC photocurrent density (J) and photovoltage (V) were then immediately tested under 100 mW/cm2 AM 1.5G simulated sunlight (model 96000, Oriel Instruments). J(V) characteristics were recorded over a device active area of 0.25 cm2, regulated by a metallic mask. It should be noted that the solar simulator was previously calibrated by using an NREL certified solar cell (Spectra Nova). The impedance spectra of the operating photovoltaic devices were also recorded under 100mW/cm2 AM 1.5G simulated sunlight by using a multichannel potentiostat equipped with an impedance module (VMP2 from Bio-Logic SAS) operating in a two-electrode configuration and potentiostatic mode: a 10 mV amplitude sinusoidal signal was applied upon open-circuit voltage with the frequency ranging from 1 Hz to 200 kHz. A UV−visible-NIR spectrophotometer (Lambda 950 PerkinElmer) was employed to determine dye loading on each device. Dye desorption was realized in 0.01 M NaOH ethanol−water (1:1) solution. The effective dye loading was calculated according to the Beer’s law. The N719 molar extinction coefficient, ε, was previously measured with a maximum value of 10 051 M−1·cm−1 at 525 nm. The spectrophotometer

equipped with an integrating sphere was also used to measure the light-harvesting efficiency (LHE) of the dye-sensitized nanoporous films. LHE spectra were determined by comparing the total transmission of the nanoporous films before and after dye sensitization.

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RESULTS AND DISCUSSION Characterization of DSSCs Made from Different ZnO Nanostructures. To investigate the potential of NW/NP composite photoanodes, we have compared three devices made from ZnO NP, ZnO NW, and ZnO NW/NP electrodes. An SEM image of a 10 μm long ZnO NW array grown for 5 h from a single solution bath is presented in Figure 1a. The NW diameters lie in the range from 200 to 600 nm with a mean surface density of 1.3 μm−2, as shown in the top-view SEM image presented in Figure 1b. Figure 1c presents a side-view SEM image of a 10 μm thick ZnO NW/NP composite

Figure 1. Tilted-view (a) and top-view (b) SEM images of a ZnO NW array grown for 5 h. Note that the scale bar is 5 μm. Tilted-view (c) and cross-sectional (d) SEM images of a ZnO NW/NP composite layer. Note that the scale bars are 5 and 1 μm, respectively. Crosssectional (e) and top-view (f) SEM images of a ZnO NP layer. Note that the scale bar is 5 μm. 18118



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Figure 2. J(V) characteristics (a), LHE spectra (b), and impedance spectra (c) of DSSCs made from ZnO NW ensembles, ZnO NP ensembles, and ZnO NW/NP photoanodes.

Table 1. Photovoltaic Properties and Dye Loading, LHE, and Electron Recombination Data of DSSCs Made from ZnO NW, ZnO NP, and ZnO NW/NP Composite Photoanodes photoanode structure

Jsc (mA·cm−2)

Voc (mV)

FF (%)

η (%)

dye loading (10−8 mol·cm−2)

LHE(525 nm) (%)

τk (ms)

Rk (Ω)

NW NP NW/NP

2.4 4.7 10.1

570 594 666

46.1 74.7 69.2

0.6 2.1 4.7

0.26 7.62 7.94

29.8 79.3 97.0

18.3 9.9 2.1

153 37.5 21.5

electrode postannealed at 120 °C. As shown by these SEM images, the technique used here is effective for covering NW ensembles from the top to their bottom with a uniform layer of ZnO NPs. In Figure 1d, we can observe an individual ZnO NW tip surrounded by ZnO NPs with diameters approximately ranging from 10 to 20 nm. In parallel, a 10 μm thick ZnO NP reference sample was grown with postannealing conditions similar to those of the NW/NP sample (Figure 1e,f). The resulting NP diameters were also in the range of 10−20 nm.16,17 As expected and revealed in Figure 1e,f, the NP electrode presents a layered morphology coming from the heteronucleation of layered hydroxide zinc acetate (LHZA) on the planar FTO substrate during the CBD process.12,16 The dye sensitization of each ZnO nanostructured electrode has been optimized in terms of photovoltaic properties. ZnO NW-, ZnO NP- and ZnO NW/NP-based photoanodes have, respectively, been sensitized for 30, 120, and 150 min in an ethanolic solution of N719 dye at 60 °C. The J(V) characteristics and LHE spectra of the as-obtained DSSCs are presented in Figure 2a,b, respectively. The short-circuit current density (Jsc), open-circuit voltage (Voc), fill factor (FF), and power conversion efficiency (η) are summarized in Table 1. The dye loading values and LHE at 525 nm measured on each nanoporous film are also presented in Table 1. A maximum power conversion efficiency of 4.7% is determined for the ZnO NW/NP-based device, whereas the ZnO NW- and ZnO NPbased DSSCs exhibit a power conversion efficiency of only 0.6% and 2.1%, respectively. Jsc increases by combining NPs

with NWs, as reported in refs 16 and 17. As shown in Table 1, the dye loading increases with the NP addition from 0.26 × 10−8 mol·cm−2 for the NW film to 7.94 × 10−8 mol·cm−2 for the NW/NP composite. Indeed, the NP addition within the NW network enhances the photoanode specific surface area, which, therefore, can load a large amount of dye. Concerning the ZnO NP reference sample, its corresponding dye loading is evaluated to 7.62 × 10−8 mol·cm−2, which is only 4% below the ZnO NW/NP one. However, Jsc obtained for the NP reference sample is divided by a factor of 2 with respect to the NW/NP composite sample. This effect points out two important beneficial roles of ZnO NWs, which enhance both the lighttrapping and the charge collection efficiency of the NW/NP composite photoanode.20 In fact, the LHE of the NW/NP sample is 18% higher than the LHE of the NP sample. Because their size is comparable to the wavelength of visible light, NWs act as effective scattering centers, which, in turn, results in the increase in the optical path length.20,24 Moreover, because of their high electron mobility, NWs also act as preferential electron pathways for efficient charge collection, which leads to high Jsc values.15−20 To elucidate the variations observed for Voc and FF, we have measured the impedance spectra of these three operating DSSCs. The corresponding Nyquist plots are presented in Figure 2c. From these plots and according to models from refs 25 and 26, we determined the characteristic time constants, τk, and charge-transfer resistances, Rk, related to electron recombination phenomena at the ZnO/dye/electrolyte inter18119



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face. τk, also called electron lifetime, and Rk data evaluated on each device are reported in Table 1. Under constant illumination at open circuit, the solar cell reaches a photostationary state in which the free electron density, n, is given by27 U (n) = G

distinct structural properties and morphologies. It is expected that the surface and near-surface chemistry may vary from the NP sample to the NW/NP sample, leading to a different injection efficiency. Moreover, it is revealed in Table 1 that the two devices containing ZnO NPs result in the highest fill factor, FF. FFs obtained for the NP and NW/NP samples are indeed close to 70%, whereas the one obtained for the NW device is below 50%. This decrease in FF observed for the NW-based device can be associated with a lower shunt resistance, Rsh, value. In fact, in solar cells, low shunt resistances cause power losses by providing an alternate current path for the light-generated current. Here, we assume one single alternate current pathway corresponding to electron recombination at the ZnO/dye/ electrolyte interface. From impedance spectroscopy measurements, the charge-transfer resistances Rk related to electron recombination phenomena at the ZnO/dye/electrolyte interface have been determined. These Rk data are inversely proportional to the surface area, S, developed by the nanostructured photoanode. From this point, the shunt resistance Rsh can be expressed as

(1)

where U(n) and G are electron recombination and photogeneration rates at the ZnO/dye/electrolyte interface per unit volume of ZnO, respectively. The free electron density n in ZnO nanostructures is affected by two main physical processes: the electron injection from the photoexcited dye molecule into the semiconductor conduction band and the recombination of photogenerated electrons by reaction with electrolyte or dyeoxidized species. The photogeneration rate, G, is the number of absorbed photons per unit time, nϕ, and per unit ZnO volume, VZnO, times the photoinjection efficiency, ηinj. The number of absorbed photons per unit time is nϕ = ϕ0A·LHE, where ϕ0 is the incident photon flux, A is the surface area of incident light, and LHE is the light-harvesting efficiency. Additionally, the volume of ZnO is VZnO = Ad(1 − p), where d is the ZnO film thickness and p is the sample porosity. From these assumptions, the photogeneration rate G is expressed as G=

R sh = R kS

We assume that the adsorbed dye molecules completely cover the surface area S developed by the nanostructured photoanode. S corresponds to the number, nN719, of adsorbed dye molecules multiplied by the surface area, s, occupied by one dye molecule: S = nN719s. The number nN719 can be expressed as nN719 = DANA, where D is the dye loading, A is the effective surface area of the sample, and NA is the Avogadro's number. Finally, S can be estimated as

ϕ0 LHEηinj d(1 − p)

Alternately, U(n) can be expressed as n − n0 U (n) = τk(n)

(2)

(3)

where n0 is the free electron density in the dark. It should be noted that the electron lifetime, τk(n), extensively varies with the free electron density n. Moreover, under constant illumination, Voc corresponds to the increase in the quasi-Fermi level value of the semiconductor (EFn) with respect to its value under dark conditions (EF0), which is equal to the electrolyte redox energy. Therefore, Voc can be expressed as27 Voc =

E Fn − E F0 kT ⎛n⎞ = B ln⎜ ⎟ e e ⎝ n0 ⎠

S = DNAsA

τk(n)ϕ0 LHEηinj ⎞ kBT ⎛ ⎟ ln⎜⎜1 + e n0d(1 − p) ⎟⎠ ⎝

(7) 2 28

By taking s = 1.46 nm , the shunt resistances of 874, 6279, and 3751 Ω·cm2 are determined for the NW-, NP-, and NW/ NP-based devices, respectively. The shunt resistance evaluated for the NW structure is far below the one evaluated for the NP and NW/NP structures, contributing to a low FF. The higher Rsh observed for the NP and NW/NP samples can be due to a better interface quality between ZnO NPs and the electrolyte, which can limit electron leakages. It should be noted that the chemical composition and morphology of ZnO NPs are mainly controlled by the thermal heat treatment of HZA NPs. The annealing temperature of HZA NPs is consequently supposed to influence the ZnO NP/dye/electrolyte interface quality and thus the device photovoltaic properties. Hereafter, we investigate in detail the effects of the annealing temperature on the NP formation mechanisms and on the photovoltaic properties of the NW/NP composite DSSCs. Effects of Annealing Temperature on Photovoltaic Performances. To study the impact of thermal heat treatments on the ZnO NP formation, HZA nanopowders have been synthesized by CBD and then annealed for 15 min at different temperatures. Basically, the synthesis of ZnO NPs involves two consecutive steps. First, HZA NPs are grown by CBD in a methanolic solution of ZnAc2·2H2O at 60 °C for 22 h. The hydrolysis of ZnAc2·2H2O, which leads to HZA NP formation, can be described by the following chemical reaction:8

(4)

where kBT is the thermal energy and e is the elementary charge. Finally, by combining eqs 1,2, and 3, Voc can be rewritten as Voc =

(6)

(5)

From Table 1, we can notice a clear correlation between Voc and τk values obtained with the three different devices. Typically, Voc is increased while τk is decreased. From this observation and eq 5, it can be deduced that Voc does not solely depend on τk but exhibits in these three devices a drastic dependence on the light-harvesting efficiency LHE, the photoelectron injection efficiency ηinj, and the sample porosity p. For example, concerning the NW-based DSSC presenting relatively high porosity p and τk values, its Voc is expected to be limited mainly by its low LHE. As regards to the NW/NPbased DSSC presenting the lowest τk value, its Voc may be enhanced by a combination of higher LHE and photoinjection efficiency as compared to the NW- and NP-based DSSCs. It should be noted that these three different samples present

5Zn(CH3COO)2 ·2H 2O → Zn5(OH)8 (CH3COO)2 ·2H 2O + 8CH3COOH 18120



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The annealing is then necessary to decompose HZA NPs so as to form ZnO NPs. Figure 3 presents XRD diagrams of five

3Zn5(OH)8 (CH3COO)2 → Zn4O(CH3COO)6 + 11ZnO + 12H 2O

Above 120 °C, the zinc oxoacetate phase undergoes a transformation with the decomposition of acetate groups that leads to a single ZnO phase. The intensity of zinc oxoacetate characteristic peaks decreases at 140 °C until complete decomposition at 450 °C. At the same time, the characteristic peaks of the ZnO phase become more intense and narrower, which points out that the ZnO NP crystallinity is enhanced with an increase of the crystallite size according to Debye− Scherrer’s formula. Subsequently, we have fabricated three different NW/NP composite DSSCs by varying the annealing temperature of HZA NPs. Figure 4a presents the J(V) characteristics of these three devices made from untreated and 120 and 450 °C annealed photoanodes. The corresponding LHE and impedance spectra are also presented in Figure 4b,c, respectively. The photovoltaic properties (η, Jsc, Voc, FF), dye loadings, LHE at 525 nm, and recombination time constants τk and resistances Rk are summarized in Table 2. The nanocomposite photoanode that has been annealed at 120 °C leads to the highest power conversion efficiency of 4.7%. The as-grown and 450 °C annealed nanocomposite electrodes show power conversion efficiencies of only 1.7% and 2.5%, respectively. As reported in Table 2, the amount of dye adsorbed on the untreated (6.53 × 10−8 mol·cm−2) and 120 °C annealed (7.94 × 10−8 mol·cm−2) electrodes are very similar, but a strong difference turns out for Jsc. In fact, the short-circuit current density of the as-grown photoanode is almost half that obtained for the 120 °C annealed one. This reduction of Jsc observed for the untreated device can probably be attributed to the large amount of the HZA phase in the NP network, which might limit or even block the current of injected photoelectrons.15 On the contrary, the 450 °C annealed device presents a dye loading of only 1.29 × 10−8 mol·cm−2 and, at the same time, a relatively high Jsc of 9.21

Figure 3. XRD diagrams of HZA nanopowder samples annealed at different temperatures in the range from 20 to 450 °C.

nanopowder samples annealed at different temperatures from 20 to 450 °C. First, the as-grown sample presents two phases: HZA and ZnO. In contrast to the results described in ref 12, a ZnO phase appears directly after the CBD process without any additional thermal heat treatment, which is probably due to aging effects. By annealing up to 100 °C, the intensity of HZA characteristic peaks gets weaker until vanishing at 120 °C. According to Poul et al.,29 this decomposition follows water evaporation, including that due to the dehydroxylation process. This decomposition leads to the formation of a mixture of ZnO and a zinc oxoacetate secondary phase, as described by the following chemical reaction:

Figure 4. J(V) characteristics (a), LHE spectra (b), and impedance spectra (c) of DSSCs made from NW/NP composite photoanodes annealed at different temperatures in the range from 20 to 450 °C. 18121



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Table 2. Photovoltaic Properties and Dye Loading, LHE, and Electron Recombination Data of DSSCs Made from NW/NP Composite Photoanodes Annealed at Different Temperatures annealing temperature

Jsc (mA·cm−2)

Voc (mV)

FF (%)

η (%)

dye loading (10−8 mol·cm−2)

LHE(525 nm) (%)

τk (ms)

Rk (Ω)

as-grown 120 °C 450 °C

4.3 10.1 9.2

582 666 514

70.2 69.2 52.8

1.7 4.7 2.5

6.53 7.94 1.29

87.6 97.0 72.1

5.0 2.1 9.9

35.0 21.5 32.0

mA·cm−2. The lower dye loading obtained here is attributed to a sintering effect of ZnO NPs, which decreases the electrode surface area available for dye adsorption; a change in chemical composition of NP surfaces could also play a role in the dye loading variations. Its relatively high short-circuit current density is related to the NPs' composition, which are solely made of ZnO. Indeed, no secondary phase, such as HZA or zinc oxoacetate, is assumed here to dominate the current of photoinjected electrons; thus, no limitation of Jsc is expected. As previously mentioned, we observe again a strong correlation between open-circuit voltage Voc and electron lifetime τk in these three devices. In addition to τk, Voc is mainly dependent upon the light-harvesting efficiency LHE, the photoelectron injection efficiency ηinj, and the sample porosity p (eq 5). In the case of the 450 °C annealed device that presents the lowest Voc and highest τk values, its Voc is probably limited by its low LHE of 72% at 525 nm and also by porosity reduction. Indeed, sintering effects in the 450 °C annealed sample are supposed to reduce the porosity and, consequently, Voc. Concerning the untreated device that presents a relatively high dye loading and large LHE, its Voc is most likely dominated by the presence of the HZA phase, which can inhibit the injection of photogenerated electrons.15 The 120 °C annealed sample reaches a good compromise with a large LHE, a large porosity, and a rather efficient electron injection, leading to the highest Voc value. Finally, we can notice from Table 2 that the untreated and 120 °C annealed samples present relatively high FFs as compared to the 450 °C treated sample. FFs obtained for the two first devices are indeed about 70%, whereas the one obtained for the 450 °C annealed is close to 50%. From eqs 6 and 7 and Rk values, we can determine shunt resistances of 5022, 3751, and 907 Ω·cm2 for the untreated and 120 and 450 °C annealed devices, respectively. The shunt resistance evaluated for the 450 °C treated sample is far below those corresponding to the untreated and 120 °C annealed specimen, which contribute to a low FF. The highest Rsh values observed for the untreated and 120 °C treated samples can be associated with a better interface quality between the NPs and the electrolyte. According to Ku et al.,15 the HZA phase appearing in the untreated electrode can act as an efficient blocking barrier that limits electron recombinations at the ZnO/dye/ electrolyte interface. The same trend is thus expected for the zinc oxoacetate phase appearing in the 120 °C annealed electrode. Therefore, the presence of HZA or zinc oxoacetate phases in the NW/NP composite electrodes paves the way to an enhancement of the charge recombination resistance Rk and, as a consequence, to an increase in the shunt resistance Rsh and in the fill factor FF. These results indicate that the photovoltaic properties of ZnO NW/NP composite DSSCs can significantly be improved by optimizing the HZA NP postannealing conditions. Indeed, a low-temperature heat treatment is necessary to limit sintering effects in order to maintain a relatively high surface area for dye adsorption. The presence of an HZA or a zinc oxoacetate phase

in the ZnO NP network is essential to block electron leakages at the ZnO/dye/electrolyte interface. However, we suggest that the fraction of HZA or zinc oxoacetate should be controlled and optimized since it dominates the photoelectron injection and collection.

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CONCLUSION The synthesis of ZnO NW/NP composite electrodes has been achieved by using a simple and low-cost wet-chemical route. The integration of such ZnO nanocomposite photoanodes in DSSCs leads to a promising power conversion efficiency of 4.7%. By comparing these photovoltaic properties with the one obtained for NW- and NP-based DSSCs, we demonstrate the advantages of NW/NP composite electrodes, which combine the large specific surface area of NPs with the better electron transport properties of NWs. The impact of annealing temperature on the NP formation has been investigated as well as the performances of the as-obtained NW/NP composite DSSCs. We have revealed the presence of a zinc oxoacetate intermediate phase at 120 °C that enables maintaining a large specific surface area and a relatively good electron injection efficiency and also reducing the electron leakage at the ZnO/ dye/electrolyte interface. Future works will address the long-term stability of the zinc oxoacetate phase and its effect on DSSC performances.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +33 (0)4 56 52 93 54. Fax: +33 (0)4 56 52 93 01. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank A. Muthukumar, A. Bionaz, and G. Giusti for the FTO thin films elaboration, H. Roussel for the XRD facilities, and D. Langley for the manuscript reading. This work has been supported by the French Research National Agency (ANR) ̈ through the Habitat Intelligent et Solaire Photovoltaique program (project ASYSCOL no. ANR-08-HABISOL-002) as well as by the Carnot Energies du Futur (Grenoble).

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REFERENCES

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