Optimization of a New ZnO Nanorods Hydrothermal Synthesis Method

Article pubs.acs.org/JPCC

Optimization of a New ZnO Nanorods Hydrothermal Synthesis Method for Solid State Dye Sensitized Solar Cells Applications Laurent Schlur,† Anne Carton,† Patrick Lévêque,‡ Daniel Guillon,† and Geneviève Pourroy*,† †

Institut de Physique et Chimie des Matériaux de Strasbourg IPCMS, UMR 7504, CNRS-Université de Strasbourg -ECPM, 23 rue du Loess BP 43, 67034 Strasbourg cedex 2, France ‡ Institut d’Électronique du Solide et des Systèmes InESS, UMR 7163, CNRS-Université de Strasbourg, 23 rue du Loess BP 20, 67037 Strasbourg cedex 2, France ABSTRACT: We report on the growth control of zinc oxide nanorods to point out the effect of the ZnO nanorods quality on the power conversion efficiency (PCE) of transparent conductive oxide (TCO)/ZnO nanorods/dye/spiro-OMeTAD/metal electrode photovoltaic devices. A promising PCE of 0.61% was measured for the best nanorods growth conditions. A careful control of all the growth parameters during the seeds layer deposition and the hydrothermal synthesis was necessary to reach such a high PCE for this kind of device. A regular nanorod layer with a flat upper surface was obtained for ethylenediamine to zinc acetate dihydrate molar ratio equal to 1.74 and a pH of 8.2. The growth was performed at 65 °C for 2 h to avoid zinc oxide brushes deposition on the surface, arising from zinc hydroxyacetate decomposition during the hydrothermal treatment. The effect of ZnO nanorods length (ranging from 1 to 3 μm) on solar cell efficiency was tested. Although the UV−vis absorption increases when the nanorods length increases, the best photovoltaic parameters were measured for the shortest nanorods length studied (1 μm). very close to that of TiO2 (ca. −4.4 eV), and ZnO presents a higher bulk electron mobility (200−300 cm2 V−1 s−1) than TiO2 (0.1 cm2 V−1 s−1).7 The electron lifetime in ZnO is increased compared to TiO2 with reduced recombination losses.8 Furthermore, numerous hierarchical ZnO structures have been fabricated, and large arrays of single-crystalline nanorods were obtained at low temperatures (about 90 °C) on several substrates such as glass or silica.9−11 They were generally obtained by reaction of zinc nitrate hexahydrate (Zn(NO3)2·6H2O) in methenamine (hexamethylenetetramine, C6H12N4).9,12,13 However, the geometries have to be improved and controlled to reveal the real potential applications of this material. Thus, this paper focuses on the control of ZnO seeds and nanorods elaboration by a low-cost process using zinc acetate dihydrate and ethylenediamine as reactants in order to get the best photovoltaic properties. We have previously proved that the use of these reactants allows the growth of singlecrystalline ZnO nanorods, having very few structural defects.14 The effects of ZnO morphology on the cell efficiency are tested in TCO/ZnO/dye/spiro-OMeTAD/gold devices. Our goal is to estimate in what extent the precise control of ZnO elaboration parameters will allow the power conversion efficiency to be improved. The dye is the organic D102 dye often used in solid state hybrid solar cells15 and the solid holes

1. INTRODUCTION The use of quasi-infinite solar energy through the development of photovoltaic conversion is one of the most promising solutions within the global context of increasing demand for stable, renewable, and sustainable energy sources. Besides the conventional silicon-based solar cells and the promising organic cells, dye-sensitized solar cells (DSSCs) incorporating both inorganic and organic semiconductor materials give rise to an increasing interest.1−4 The key components of a DSSC are a photoanode made of a film of semiconductor nanoparticles (generally TiO2), coated onto a transparent conducting oxide substrate (usually fluorinedoped tin oxide, FTO), a dye, and a liquid electrolyte, which was initially based on the iodide/triiodide couple.5 The nanoparticles film provides a large internal surface area for the chromophore anchoring to maximize light absorption. The photoelectrons generated by the dye excitation are injected into the semiconductor and undertake a random walk through the nanoparticles network. They may interact with a distribution of traps before reaching the collecting electrode. A promising strategy for improving electron transport in DSSCs is to replace the nanoparticles photoelectrode by a single-crystalline nanorods photoelectrode. Electrons can then be conducted within a nanorod instead of by multiplescattering transport within the nanoparticles network.6 ZnO is considered as one of the most promising materials to grow up nanorods photoelectrode in DSSCs. Indeed, ZnO has a wide band gap (ca. 3.37 eV), its conduction band edge is found to be © 2013 American Chemical Society

Received: June 13, 2012 Revised: January 11, 2013 Published: January 16, 2013 2993

dx.doi.org/10.1021/jp305787r | J. Phys. Chem. C 2013, 117, 2993−3001

The Journal of Physical Chemistry C

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conductor, the spiro-OMeTAD.16,17 The traditional liquid electrolytes used in DSSC18,19 are corrosive20 and require careful packaging procedures which are not necessary with a solid hole transporter.

mM) dissolved in 1:1 acetonitrile/tert-butyl alcohol and rinsed in anhydrous acetonitrile. The molecular glass 2,2′,7,7′tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) was dissolved in chlorobenzene at different concentrations depending on the length of ZnO nanorods. For nanorods length of 1, 2, and 3 μm, concentrations were 210, 300, and 370 mg/mL, respectively. tert-Butylpyridine (tbp) and lithium trifluoromethyl sulfonylimide (Li-TFSI) were used as spiro-OMeTAD dopants: tbp was added to the spiro-OMeTAD solution (3.2:40 w/w tbp/spiro-OMeTAD). Li-TFSI, previously dissolved in acetonitrile at 170 mg/mL, was added to the hole transporting material (1.3:40 w/w Li-TFSI/spiro-OMeTAD). Then 70 μL of spiro-OMeTAD was deposited on the top of the substrate before spin-coating at a speed of 1500 rpm for a nanorods length of 1 μm and 2000 rpm for 2 or 3 μm nanorods length. Finally, a 120 nm thick gold electrode was evaporated on top of the cell under low vacuum pressure, in order to complete the device with two 0.16 cm2 diodes. 2.4. Characterization. The seeds layer and nanorods morphologies were observed by scanning electron microscopy (SEM) using a JEOL electron microscope 6700. XRD measurements were performed on a Siemens D5000 X-ray diffraction system in reflection mode with a quartz monochromator (Cu Kα1 = 1.540 56 Å). The roughness of the seeds layer was determined by atomic force microscopy (AFM) using a Digital Instrument 3100 microscope coupled to a Nanoscope IIIa recorder. Measurements were done in the tapping mode. TG and DT analysis were performed using a TA Instruments apparatus SDT Q600 with a heating rate of 2 °C/min in air. The absorbance of the solar cells was measured with a PerkinElmer Lambda 950 spectrophotometer. The filling fraction of spiro-OMeTAD between the nanorods was determined by using the method developed by I-Kang Ding et al.23 The filling fraction is calculated by dividing the volume of spiro-OMeTAD by the free volume between the rods. The volume of spiro-OMeTAD and ZnO nanorods has been determined by absorbance measurements and by ICPAES, respectively. The presence of the dye has been neglected. The total volume (ZnO nanorods and vacuum between the rods) has been calculated by knowing the surface of the substrate covered by the rods and their length, which has been determined by SEM images. The current−voltage (I−V) characteristics of the solar cells were measured using a Keithley 2400 source measurement unit. The Oriel 150 W AM1.5G solar simulator using a filtered Xe lamp was calibrated before each measurement with a THORLABS optical power meter to obtain standard 100 mW/cm2 illumination conditions.

2. EXPERIMENTAL SECTION Zinc acetate dihydrate (Zn(CH3COO)2·2H2O, 98%) and tertbutyl alcohol were purchased from Alfa Aesar, ethylenediamine (C2H4(NH2)2, ≥99%), tert-butylpyridine (tBP), and chlorobenzene were from Sigma-Aldrich, anhydrous ethanol and acetonitrile were from Carlo Erba, spiro-OMeTAD (2,2′,7,7′tetrakis(N,N-di-p-methoxyphenylamine)9,9′-spirobifluorene) was from Merck KGaA, and lithium trifluoromethyl sulfonylimide (Li-TFSI) was from Tokyo Chemical Industry (TCI). Indium tin oxide (ITO) (≤20 Ω/square) and F-doped SnO2 (FTO) (7 Ω/square) coated glass substrates were purchased from Precision Glass Optics and Solaronix, respectively. Their rms roughnesses are 2.7 and 36.7 nm, respectively. The D102 dye was purchased from Mitsubishi Paper Mills Limited Japan. 2.1. ZnO Seeds Layer on Substrates. A solution of zinc acetate dihydrate (Zn(CH3COO)2·2H2O) 5 × 10−3 M in anhydrous ethanol was prepared and stirred in a glovebag filled with N2 gas. The solution was deposited on the transparent conducting oxide (TCO) coated glass substrates according to the method developed by Greene et al.21 The substrates (2 × 1 cm) were first cleaned under sonication successively in acetone and ethanol, then washed in water, and finally dried in an oven at 50 °C for 15 min. The substrates were oxygen plasma treated for 5 min in order to improve their wettability and to eliminate last organic traces before the zinc acetate solution deposition.22 Then, the substrates were placed in the glovebag under N2 gas. 40 μL of the prepared solution was deposited on the substrate. Thanks to the surface treatment, the drop covers all the substrate. After the solvent evaporation, the substrate was annealed at 400 °C for 20 min in air in order to transform zinc acetate into zinc oxide. The deposition of the solution and annealing steps were repeated four times again to get a sufficient ZnO layer thickness. 2.2. ZnO Hydrothermal Growth. ZnO nanorods were obtained after submitting the previous substrate to a hydrothermal treatment. In a typical procedure, an aqueous solution of 10 mL of ethylenediamine (C2H4(NH2)2, 20 vol %) and 24 mL of Zn(CH3COO)2·2H2O (0.72 M) was prepared and then poured into a homemade Teflon-lined stainless autoclave (V = 110 mL); i.e., zinc acetate dihydrate and ethylenediamine concentrations in the autoclave are equal to CrefZn = 5.05 × 10−1 mol/L and CrefEn = 8.8 × 10−1 mol/L, respectively. The seedcoated substrate was suspended horizontally upside-down in the autoclave. The later is sealed and placed in an oven at 110 °C for 2 h. Then, it was immersed for 10 min in a water bath in order to cool it. The substrate was finally rinsed with distilled water and dried in an oven at 50 °C for 30 min. Temperature, reaction time, ethylenediamine, and zinc acetate dihydrate concentrations were varied in order to describe the synthesis mechanism and to determine the most adapted parameters to the realization of the solar cell. The temperature was varied from 65 to 130 °C and the reaction time from 30 min up to 18 h, and the concentrations of reactants were reduced and increased. 2.3. Preparation of the Cells. Solid-state DSSCs were built using the following procedure. Substrates covered with nanorods were immersed for 1 h at 50 °C in D102 dye (0.5

3. RESULTS AND DISCUSSION The morphology of ZnO nanorods, the distribution of the rods on the whole substrate surface, and the flatness of the upper surface, i.e., the homogeneity of the rods length and the lack of sediments, are very significant for the quality and the efficiencies of solar cells. Indeed, the hole conductor has to be homogeneously spread out inside and over the ZnO layer. The thickness of spiro-OMeTAD at the top of the rods must be homogeneous and as thin as possible. ZnO nanorods elaboration occurs in two steps: the seeds deposition and annealing on the substrate and the nanorods growth from the seeds in hydrothermal conditions. Both steps must be accurately controlled to get homogeneous distribution of nanorods on all the substrate and homogeneous nanorods 2994



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lengths. Reactants concentrations, growth time, and temperature were varied in order to have several rod lengths and consequently several solar cells thicknesses. 3.1. Step 1: The ZnO Seeds Layer. The quality of the seeds layer is critical to obtain homogeneous and oriented nanorods but also to prevent any contact between the transparent conducting glass and the dye/spiro-OMeTAD. Seeds have to be spread out on the whole surface, and the layer must be homogeneous after the solvent evaporation and the annealing at 400 °C. The solution is prepared by mixing zinc acetate dihydrate with ethanol and may involve adsorbed water.24 As ethanol evaporates faster than water,24,25 droplets of water/zinc acetate dihydrate may be formed on the substrate, leading to an inhomogeneous seeds repartition after annealing. This hypothesis has been checked by comparing seeds layers obtained after addition of water to the solution (1 vol %) and obtained in a dried N2 atmosphere (Figure 1). In the first case,

Figure 2. X-ray diffraction pattern of ZnO seeds layer on an ITO substrate, dried under N2 gas (glovebag), and air calcined at 400 °C.

3.2. Step 2: ZnO Nanorods Morphology According to the Hydrothermal Reaction Parameters. Let us consider the evolution of ZnO nanorods when the zinc acetate dihydrate volume VZn (0.72 M) is varied while the ethylenediamine volume (20 vol %) is kept equal to 10 mL. For these samples, the hydrothermal growth was performed at 110 °C for 2 h. The morphologies of ZnO either on the substrate or in the solution are presented in Figure 3. Below 18 mL, no ZnO is recovered from the solution by centrifugation, and no nanorods growth is

Figure 1. SEM images of ZnO seeds layer on an ITO substrate, dried under N2 gas (glovebag) and calcination at 400 °C: (a) top view when 1 vol % of water is added in the seed solution, (b) top view, and (c) cross section without additional water.

the seeds are not regularly distributed on the surface. Zinc acetate dihydrate is concentrated in the droplets, leading to the formation of uneven aggregates after the annealing. Similar behaviors are observed when a solution of ethanol/zinc acetate dihydrate without water was deposited on the substrate in ambient air due to the adsorption of water from the external environment.24 Under N2 gas, the thickness of zinc oxide seeds layer is homogeneous and was estimated to be 55 ± 5 nm (Figure 1b,c). A rms roughness of 2.45 ± 0.2 nm has also been determined thanks to AFM measurements on seeds formed on ITO substrate (figure not shown). These two values indicate that the ZnO seeds layer will prevent the contact between the transparent conducting oxide on the substrate and the dye/ spiro-OMeTAD. The diffraction pattern recorded on the seeds layer exhibits the (0002) diffraction peak of the hexagonal wurtzite (JCPDS Card No. [36-1451]), showing that the (000l) planes are parallel to the substrate surface (Figure 2). This orientation will allow ZnO nanorods to grow perpendicularly to the substrate: the bottom plane of ZnO nanorods is (000−1) considering the fact that the (000−1) plane of the rods will grow on the (0001) plane of the seeds. Although the (000l) planes have a high surface energy, they are largely developed. According to Greene et al.,21 it can be due to acetates which can adsorb onto nascent (0001) planes, to a decrease of their surface energy for very thin seeds, or to a different configuration of the first few atomic layers that can convert into (000l) orientation thanks to a minor structural transformation.

Figure 3. SEM images of ZnO nanorods grown in the solution and on the substrate using 10 mL of ethylenediamine (20 vol %) solution with added zinc acetate dihydrate (0.72 M) volume of (a) 18, (b) 19, (c) 20, (d) 21, (e) 22, (f) 23, and (g) 24 mL corresponding to initial pH of 9.95, 9.80, 9.55, 9.25, 8.80, 8.55, and 8.20, respectively (each scale bar corresponds to 1 μm). 2995



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observed on the substrate. The ZnO seeds layer deposited on the TCO surface was totally dissolved. When VZn ≥ 18 mL, i.e., when ethylenediamine concentration in the autoclave [En] divided by zinc acetate dihydrate concentration in the autoclave [Zn] is less than or equal to 2.32, nanorods are observed on the substrate, but their length decreases as VZn increases from 19 to 24 mL. Except for 18 mL, ZnO precipitate is present in the solution and drop down on the nanorod layer. As VZn increases, the ZnO quantity recovered in the solution increases according to the variation presented in Figure 4. Zinc oxide rods are

NH 2(CH 2)2 NH 2 + 2H 2O ↔ [NH3(CH 2)2 NH3] 2 + + 2OH−

(2)

NH 2(CH 2)2 NH3 + + H 2O ↔ [NH3(CH 2)2 NH3]2 + + OH−

(3)

[Zn(NH 2(CH 2)2 NH3)g (NH 2(CH 2)2 NH 2)r ](2 + g ) + ↔ Zn 2 + + r NH 2(CH 2)2 NH 2 + g NH 2(CH 2)2 NH3+ (4)

Zn 2 + + 2OH− ↔ ZnO + H 2O

(5)

For VZn > 21 mL, a precipitate is formed immediately when ethylenediamine and zinc acetate dihydrate are mixed together. The precipitate was recovered by centrifugation and washed three times in water. It was shown to be isomorphous to zinc hydroxyacetate (Zn5(OH)8(CH3COO)2·nH2O) (Figure 5, JCPDS Card No. [56-0569]), and the pattern was indexed in the hexagonal system.30

Figure 4. Evolution of zinc oxide mass present in the hydrothermal solution after reaction with the volume of zinc acetate dihydrate (0.72 M) added to 10 mL of ethylenediamine (20 vol %).

observed in the solution for 19 mL ≤ VZn ≤ 21 mL and rods and brushes for 21 mL ≤ VZn ≤ 24 mL, showing that two different phenomena are involved. [En]/[Zn] = 2.32 (VZn = 18 mL) is a limit for nanorods growth as no zinc oxide is visible in solution, and nanorods length and density are lower than for VZn = 19 mL. It corresponds certainly to the limit between formation and dissolution of ZnO, in other words, to a pH at which zincate species are soluble at 110 °C. As explained in previous studies, several chemical equilibriums are involved in the formation of ZnO nanorods. When mixing the reactants at room temperature, the majority of Zn2+ cations are chelated by ethylenediamine.26,27 Shapnik et al.28,29 proved that ethylenediamine and Zn2+ cations do not form only bis- and tris-ethylenediamine complexes (g = 0 and r = 2 or 3 in eq 1) but also protonated ethylenediamine zinc complexes (g ≠ 0). The ethylenediamine hydrolysis provides OH− anions in the solution according to eqs 2 and 3. During the hydrothermal treatment, zinc ethylenediamine complex formed in eq 1 partly decomposes, resulting in the increase of ethylenediamine, protonated ethlyenediamine, and Zn2+ in solution (eq 4). The presence of OH− anions (eqs 1, 2, and 3) and the condensation reaction (eq 5) result in the formation of ZnO either as nanorods on the substrate or in solution.

Figure 5. XRD pattern of the Zn5(OH)7.6(CH3COO)2.4·3.7H2O precipitate formed immediately after mixing ethylenediamine and zinc acetate dihydrate.

Thanks to the elementary analysis and TG analysis, the formula of the precipitate was determined to be Zn5(OH)7.6(CH3COO)2.4·3.7H2O. It forms according to eq 6 and decomposes into zinc oxide (eq 7) when heated. Similarly to previous studies showing that forced hydrolysis on cloudlike phase Zn3(OH)8(NO3)2·2H2O allowed ZnO nanowhisker films or twinlike ZnO nanoarrays assembly to crystallize, the decomposition of zinc hydroxyacetate provides nucleation sites which lead to ZnO brushes according to eqs 1−5.31,32 However, part of hydroxyacetate can be dissolved during heating participating to the nanorods growth. 5Zn 2 + + 7.6OH− + 2.4CH3COO− + 3.7H 2O ↔ Zn5 (OH)7.6 (CH3COO)2.4 ·3.7H 2O Zn5(OH)7.6 (CH3COO)2.4 ·3.7H 2O → 5ZnO

Zn 2 + + (g + r )NH 2(CH 2)2 NH 2 + g H 2O

+ 2.4CH3COOH + 6.3H 2O

↔ [Zn(NH 2(CH 2)2 NH3)g (NH 2(CH 2)2 NH 2)r ](2 + g ) + + g OH−

(6)

(7)

The decrease of nanorods length on the substrate and the increasing mass of ZnO in suspension observed when VZn

(1) 2996



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Figure 6. Evolution of ZnO nanorods length and diameter as a function of (a) [Zn]/CrefZn ratio with [En] to [Zn] ratio fixed at 1.74, (b) the growth time, (c) the synthesis temperature for a growth time of 2 h, and (d) the synthesis temperature for a growth time of 18 h.

34 mL. [Zn] to CrefZn ratios varied between 0.25 and 2.0. The pH of the starting mixture varies between 8.1 and 8.4. Figure 6a represents the evolution of ZnO nanorods length and diameter after the hydrothermal reaction at 110 °C for 2 h for several [Zn]/CrefZn ratios ([En]/[Zn] = 1.74). The length of the rods increases when the reactants concentration increases, the maximal length 8.08 ± 0.27 μm is obtained for a ratio of 2.0, and the minimal length is 3.09 ± 0.25 μm for a ratio of 0.25. Diameters around 110 ± 40 nm are generally encountered except for the 0.25 ratio, for which the diameter is equal to 77 ± 16 nm. The nanorods length and diameter variations versus the reaction time are illustrated in Figure 6b for a reaction at 110 °C. In the first 90 min of hydrothermal growth, the nanorods length increases linearly versus the time and reaches up to 4.77 ± 0.20 μm at 90 min. For times longer than 90 min, the length increase is slowed down. This behavior has been already observed by Wang et al.34 At the beginning of the reaction, the growth was rapid as a lot of matter (Zn2+, OH−) was available. When the equilibrium is reached, the growth stops. The nanorods diameter increases during the first 60 min and stabilizes around 102 nm for a longer reaction time. For a very long growth time, the diameter seems to increase slightly up to 131 ± 26 nm. For a reaction time of 30 min, the rods length was 0.13 ± 0.03 μm. They are very short, so it can be supposed that for a growth time of 30 min (approximately the time the autoclave needs to reach the reaction temperature) the rods have only begun to grow. This can also explain the fact that the diameter is smaller than for other reaction times. The reaction time was so short that the rods have not the time to reach a diameter around 102 nm, which seems to be the

increases appear to be inconsistent. That means that a competition takes place between ZnO nucleation and growth in the solution on one side and growth on the substrate on the other side. The zinc concentration increase seems to favor the growth of ZnO in solution rather than the growth of nanorods on the substrate. For 19 mL ≤ VZn ≤ 21 mL, zinc cations were released in solution according to eq 4. According to La Mer’s theory,33 when the supersaturation reaches the critical supersaturation, the formation of stable nuclei in solution begins, and when the supersaturation becomes lower than the critical supersaturation, the nucleation stops and the growth starts. Increasing Zn2+ amounts are available for the nucleation and the growth of ZnO in solution. In addition, for VZn > 21 mL, ZnO crystallizes by decomposition of zinc hydroxyacetate. Therefore, growth of rods on the substrate and growth of ZnO brushes from nuclei in solution occur simultaneously leading to smaller nanorods. The most regular ZnO nanorods layer and flattest upper surface was observed when VZn = 24 mL (Figure 3), i.e., [En]/ [Zn] = 1.74 and pH = 8.2. In that case, [En] = CrefEn = 8.8 × 10−1 mol/L and [Zn] = CrefZn = 5.05 × 10−1 mol/L. For VZn > 24 mL, the length decrease is too high. As explained previously, the flatness of the surface is significant for the photovoltaic devices efficiency, so the effect of matter concentration, reaction temperature, and time on nanorods length, nanorods diameter, and ZnO in suspension (rods or brushes) were studied starting from these conditions. The concentrations of zinc acetate dihydrate and etylenediamine were varied in the same range; i.e., [En]/[Zn] was kept constant and equal to 1.74, and the total volume was equal to 2997



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month later. Between the two measurements, the devices were stored without encapsulation at room temperature in the dark. Nanorods of 3 μm length and covered with brushes were also obtained with a growth temperature of 80 °C with [Zn] to CrefZn = 1.0 (called S3b). Figure 9 exhibits the current−voltage curve for devices which were not aged, built with nanorods of 1 μm (S1), 2 μm (S2), 3

equilibrium diameter. The effect of temperature synthesis on the rods length and diameter is illustrated in Figures 6c and 6d for two reaction times of 2 and 18 h, respectively. A quasi-linear relationship between the length of ZnO nanorods and the synthesis temperature for a reaction time of 2 h is observed. The rods length varies from 1.09 ± 0.10 μm at 65 °C to 7.33 ± 0.49 μm at 130 °C. For a longer reaction time (18 h), the rods length is around 7 μm for every synthesis temperature. Therefore, the growth rate depends on the temperature. For a lower synthesis temperature, the growth rate is smaller. In both cases, the rods diameter does not change significantly with an average diameter of 106 and 125 nm for 2 and 18 h, respectively. However, ZnO brushes are observed on the surface for a temperature of 110 °C while they are absent when the reaction is performed at 65 °C (Figure 7). After 2 h at 65 °C, zinc hydroxyacetate is not decomposed yet into ZnO, and it does not fix on the surface of ZnO nanorods (Figure 7b).

Figure 9. J−V curve of solid state solar cells having nanorods length of 1 μm (black), 2 μm (red), 3 μm (blue), and 3 μm with brushes (green) (light intensity 100 mW/cm2, AM 1.5G illumination conditions). These devices were not aged. Figure 7. SEM tilted view (85°) of the top of the substrate for a reaction time of 2 h and a synthesis temperature of (a) 110 °C and (b) 65 °C.

μm (S3), and 3 μm with brushes (S3b). The photovoltaic parameters are tabulated in Table 1. An important decrease of the photovoltaic parameters is observed when impurities cover the surface (S3b). Figure 10 shows that ZnO brushes pass over the blocking spiroOMeTAD layer. The presence of brushes is responsible of the formation of cracks in the spiro-OMeTAD when chlorobenzene evaporates. Nano short circuits occur between ZnO brushes or rods (below the cracks) and the gold electrode. These short circuits induce a strong increase of the leakage current (100-fold higher in the case of S3b compared to the S3 counterpart), which can be at the origin of the slight decrease of the shunt resistance observed when brushes are present. Furthermore, a dramatic reduction of the open circuit voltage (VOC) is also observed for the S3b device. This strong VOC reduction could be due to enhance charge carrier recombination at the surface in S3b devices. The S1 sample exhibits the best photovoltaic characteristics with an open circuit voltage (VOC) of 557 mV, a short circuit current density (JSC) of 2.0 mA/cm2, and a fill factor (FF) of 0.38 leading to an overall power conversion efficiency (PCE) of 0.42%. For the S2 and S3 samples, the VOC value is similar to S1. On the other hand, JSC, FF, and consequently the PCE decrease when the nanorods length increases. Looking at the UV−vis absorbance spectra, we can see that the absorbance increases when the rods length varies from 1 to 3 μm as shown in Figure 11. An absorbance increase is expected to lead to a current increase contrary to what is observed. Several hypotheses can be put forward to explain this behavior. First, it has already been observed that for solid state dye sensitized solar cells made of nanoporous electrodes the pore filling fraction is getting smaller when the device thickness increases.35,36 Consequently, the power conversion efficiency

3.3. Solar Cells Characteristics Using Optimized ZnO Nanorods Growth. The effect of ZnO morphology on solar cell efficiency was tested for various nanorods length. According to the previous observations, [En]/[Zn] = 1.74 was chosen since the upper surface of nanorods layer is flat for every matter concentration reaction temperature and reaction time. The reaction was performed at 65 °C for 2 h in order to avoid the zinc hydroxyacetate decomposition into ZnO and the deposition of rods and brushes on top of the nanorods. [Zn]/CrefZn ratio = 0.5 was chosen in order to get a nanorods length of exactly 1.0 μm. The reaction was repeated on the same substrate with a new hydrothermal solution 2 or 3 times in order to get 2.0 or 3.0 μm rods, respectively (Figure 8). The photovoltaic characteristics of these solar cells were measured two times, namely immediately after their fabrication and one

Figure 8. SEM images of ZnO nanorods grown by an hydrothermal treatment of 2 h at 65 °C with a ratio of 1.0. These growth conditions have been repeated: (a) once, (b) twice, and (c) three times. 2998



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Table 1. Photovoltaic Parameters of Solid State Solar Cells Sensitized with D102 as a Function of the Nanorods Length, Growth Conditions, Annealing Temperature, and Solar Cell Ageinga sample S1 S2 S3 S3b S1old S2old S3old

ZnO nanorods length (μm) 1.04 2.01 3.22 3.04 1.04 2.01 3.22

± ± ± ± ± ± ±

0.08 0.13 0.20 0.11 0.08 0.13 0.20

spiro-OMeTAD overlayer thickness (μm)

JSC (mA/cm2)

VOC (mV)

FF

PCE (%)

filling fraction (%)

1 month aging

presence of brushes

± ± ± ± ± ± ±

2.0 1.8 1.4 1.2 2.4 2.0 1.9

557 546 546 223 604 615 626

0.38 0.33 0.32 0.37 0.43 0.32 0.30

0.42 0.33 0.25 0.10 0.61 0.40 0.35

97 96 95 b 97 96 95

no no no no yes yes yes

no no no yes no no no

0.22 0.35 0.64 0.72 0.22 0.35 0.64

0.04 0.09 0.11 0.27b 0.04 0.09 0.11

a

The relative PCE uncertainty estimated from diode to diode variations is below 15%. bSpiro-OMeTAD thickness determined far away from the brushes. The filling fraction was therefore not calculated.

thickness could not be decreased because of the increasing length inhomogeneity (Table 1) due to the repetition of hydrothermal syntheses at 65 °C. Third, increasing the nanorods length (S1, S2, and S3) leads to a slight increase of the series resistance (calculated from the J−V curve in the dark) that is not sufficient to explain the measured decrease of both JSC and FF when the nanorods length increases. On the other hand, the hole mobility in doped spiro-OMeTAD is around 10−3 cm2 V−1 s−138 while the electron mobility in ZnO is more than 3-fold higher, so that the charge carrier mobility is highly unbalanced. This may be the main reason for the observed inefficient charge extraction (the highest fill factor measured for S1 diodes is as low as 0.38).10,39 As the nanorods length is increased, the charge extraction limitations become more and more critical, and consequently, both JSC and FF decrease when the nanorods length increases. We strongly believe that the third hypothesis is the main explanation of the photovoltaic parameters evolution as a function of the nanorods length even though the second one cannot be completely ruled out. All these results show that despite a more direct pathway for the charges and a better filling fraction for a nanorods structure compared to a nanoporous one, the thickness of the solar cells cannot be increased more than for the nanoporous structure for which the best thickness is around 2 μm (for TiO2 particles). The aging effect on the J−V response of the solar cells has also been tested. Figure 12 shows the evolution of the

Figure 10. SEM top view of the S3b solar cell.

Figure 11. UV−vis absorption spectra of ZnO nanorods sensitized with D102 and infiltrated by spiro-OMeTAD. The rods have a length of 1 μm (black), 2 μm (red), and 3 μm (blue).

decrease observed when the device thickness increases can be at least partially attributed to the pore filling fraction evolution. In our case, for samples S1, S2, and S3, the filling fraction between the rods are almost equal and close to 100% (Table 1). Therefore, the decrease of the power conversion efficiency is not linked to the pore filling fraction. Interestingly, the modification of the semiconductor oxide morphology from a nanoporous structure to a nanorod structure can increase the value of the filling fraction which was estimated to be 60−65% for a TiO2 nanoporous structure of 2.8 μm thick by Ding et al.23 Second, the increase of the spiro-OMeTAD overlayer thickness (Table 1) with the increasing ZnO nanorods length can also be partially responsible of the decrease of the JSC, FF, and PCE.37 The problem is that the spiro-OMeTAD overlayer

Figure 12. J−V curve of solid state solar cells stored in the dark without encapsulation for 1 month and having nanorods length of 1 μm (black), 2 μm (red), and 3 μm (blue) (light intensity 100 mW/ cm2, AM 1.5G illumination conditions). 2999



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photovoltaic characteristics of the solar cells after an aging time of 1 month for nanorods having a length of 1 μm (S1old), 2 μm (S2old), and 3 μm (S3old). After aging, a VOC and a JSC increase are visible, compared to the samples without aging and having the same nanorods length. The S1old sample exhibit the best photovoltaic performances with an open circuit potential (VOC) of 604 mV, a short circuit current density (JSC) of 2.4 mA/cm2, a fill factor (FF) of 0.43, and a power conversion efficiency (PCE) of 0.61%. To our knowledge, a PCE of 0.61% is the highest reported to date for (TCO/ZnO nanorods/dye/ spiro-OMeTAD/metal electrode) devices. For the same device architecture, Plank et al. reported an efficiency of 0.25%. The PCE increase after aging has already been observed for hybrid solar cells including a ZnO or TiO2 layers.13,40,16 This increase has not been explained yet. Two different hypotheses are often suggested. First, the aging can improve the contact between spiro-MeOTAD and gold. Second, ZnO can spontaneously lose oxygen at the surface when it is in contact with some organic material or when the cell is placed under vacuum (as for the top electrode evaporation). The ZnO surface is then electron-rich, converting the oxide into an electron donor, and disrupting the device. Therefore, an air exposure (like during the aging) allows the ZnO surface to be regenerated.

4. CONCLUSION The elaboration of the oxide is one of the key parameters for increasing the hybrid solar cells efficiency. We have pointed out the conditions for having a homogeneous ZnO seeds layer regularly spread out on the whole substrate surface. Nanorods perpendicular to the substrate surface have to be grown at 65 °C for 2 h with an ethylenediamine to zinc acetate dihydrate molar ratio equal to 1.74 and a pH of 8.2 to avoid impurities deposition and to obtain a regular upper surface. As a consequence of elaboration control, the power conversion efficiency has been increased in TCO/ZnO nanorods/dye/ spiro-OMeTAD/metal electrode devices to reach an overall power conversion efficiency of 0.61%. The efforts have now to be carried on the hole conductor to further improve the photovoltaic parameters.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given their approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support of CNRS and Region Alsace. REFERENCES

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Optimization of a New ZnO Nanorods Hydrothermal Synthesis Method

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