J. Phys. Chem. C 2007, 111, 5549-5556
5549
Nanostructured Zinc Stannate as Semiconductor Working Electrodes for Dye-Sensitized Solar Cells Teresa Lana-Villarreal,* Gerrit Boschloo, and Anders Hagfeldt Center of Molecular DeVices, Department of Chemistry, Royal Institute of Technology, Teknikringen 30, SE-10044 Stockholm, Sweden ReceiVed: NoVember 28, 2006; In Final Form: February 2, 2007
Zinc stannate (Zn2SnO4) particles with 27-nm size were synthesized by hydrothermal treatment. Nanoporous Zn2SnO4 thin films were prepared on conducting glass substrates and used as working electrodes in dyesensitized solar cells, DSSC. Their behavior was compared with standard TiO2 cells, using (TBA)2-cis-Ru(Hdcbpy)2(NCS)2 (known as N719) as a dye and an electrolyte containing 0.7 M LiI and 0.05 M I2 in 3-methoxypropionitrile. Under the same working conditions, Zn2SnO4 DSSC showed higher open-circuit potential, but their overall efficiency was lower due to their lower incident photon-to-current conversion efficiency. The properties of electrons in DSSC have been studied by measuring their transport time and lifetime by photocurrent and photovoltage transient measurements, respectively. The electron diffusion length is similar in both oxides, demonstrating the possible use of Zn2SnO4 as an electron collector in DSSC applications. On the other hand, photoinduced absorption measurements reveal problems in the electron injection from the dye to Zn2SnO4, owing to the higher position of its conduction band, in agreement with the higher open-circuit potential measured. Zinc stannate will be an interesting mesoporous material for DSSC, provided the use of dyes with a higher position of the LUMO compared to that of N719, as it will permit attaining higher photovoltages without affecting the photocurrent.
1. Introduction In the past decade, dye-sensitized solar cells (DSSC) have attracted much attention as low-cost alternatives to harvest solar energy. Power efficiencies of up to 10% have been reported1,2 for systems with cis-di(thiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylate)ruthenium(II) as dye, nanostructured titanium dioxide films, and I-/I3- electrolyte. Most research is focused on systems using nanoporous titanium dioxide as the electron collector, although other semiconductor films such as ZnO3 or SnO24 have also shown promising properties. Interestingly, Tennakone et al.5 achieved a dramatic enhancement in the conversion efficiency of DSSC using as working electrode a mixture of ZnO and SnO2 particles. The improvement in current-voltage characteristics was mainly due to larger opencircuit voltages.6 Hence, it seems interesting to study a mixed tin and zinc oxide, i.e., zinc stannate as working electrode. Zinc stannate describes two different oxides with different crystallographic structures and Zn-to-Sn ratios: the orthorhombic phase with a stoichiometry of ZnSnO3, and Zn2SnO4 which forms a cubic spinel crystal structure. ZnSnO3 is thermally less stable than Zn2SnO4, and their properties remain quite unknown, although a few publications can be found in the literature.7,8 Studies on Zn2SnO4 are also scarce despite expectations of a wide variety of applications. Most of them focus on its preparation and electrical properties as it can be employed as photocatalyst9 and as a sensor of humidity,10 NO2,11 CO,12,13 or ethanol.14 Zn2SnO4 has also been studied as a transparent conducting oxide because of the combination of visual light transparency and high electrical conductivity.15,16 Recently, Rong et al.17 have also demonstrated its potential use as anode material for Li-ion batteries. However, no references can be * To whom correspondence
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found about their possible use in DSSC. Interestingly, it has been used in polycrystalline thin-film solar cells of CdS/CdTe as a buffer layer between CdS and the conducting substrate (Cd2SnO4 or SnO2), improving the device performance and reaching a world record efficiency of 15.8% for this type of cells.18 In the literature Zn2SnO4 has been prepared by spray pyrolysis,19 radio frequency magnetron sputtering,15,20,21 hydrothermal treatment, and thermal evaporation.22 With the latter method, different powder morphologies have been obtained as nanobelts, nanorings,23 nanocones,24 and nanowires.25-27 It seems that as in the case of ZnO, different morphologies can be prepared with relative ease. In our case, Zn2SnO4 was synthesized via a hydrothermal method on the basis of previous published methods.9,17,28,29 In this report, we present for the first time results of DSSC based on nanoporous Zn2SnO4 films sensitized with (TBA)2cisRu(Hdcbpy)2(NCS)2 (also called N719) as working electrodes. The results are compared with standard cells based on titanium dioxide in the absence of additives in the electrolyte. Lower short-circuit density currents (JSC) but higher open-circuit potentials (EOC) are measured for Zn2SnO4, under the same conditions. However, both oxides show similar electron diffusion lengths. The different location of the conduction band is proposed as the origin for the enhancement of EOC and for the problems in the electron injection that induce lower JSC. 2. Experimental Zn2SnO4 Synthesis and Characterization. (CH3)4NOH (51.2 mmol, from a 25% aqueous solution, Alfa) dissolved in 50 mL of MilliQ MilliPore water was slowly added to a 100 mL solution of ZnCl2 (12.8 mmol, Merck), and SnCl4 (6.4 mmol, Acros Organics) under constant stirring. The final mole ratio of Zn2+/Sn4+/(CH3)4NOH was 2:1:8. Upon addition of (CH3)4NOH, Zn2+ and Sn4+ reacted, leading to the formation
10.1021/jp0678756 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/22/2007
5550 J. Phys. Chem. C, Vol. 111, No. 14, 2007 of a white precipitate. After 10 min of stirring the solution was transferred to an autoclave, heated at 250 °C during 24 h, and cooled naturally to room temperature. The precipitated product was centrifuged, rinsed several times with MilliQ water, and rinsed finally with ethanol. Zn2SnO4 particles were characterized with a Zeiss EM 902A transmission electron microscope (Carl Zeiss NTS, Oberkochen, Germany). The instrument was operated at 80 kV and in zero loss bright-field mode. Digital images were recorded with a BioVision Pro-SM Slow Scan CCD camera (Proscan GmbH, Scheuring, Germany) and analySIS software. To verify its composition, X-ray diffractograms were recorded with a PANalytical X’PertPro equipment. Zn2SnO4 and TiO2 Dye-Sensitized Solar Cells Preparation. Zn2SnO4 particles were suspended, mixing 0.4 g with 0.2 g of polyethyleneglycol (20000, Fluka) in 2 mL of water. Thin porous films were prepared by spreading the suspension onto conducting glass substrates (TEC8, Pilkington), followed by heating at 450 °C for 30 min in air to eliminate the polyethyleneglycol. To improve the electrode mechanical properties, the samples were pressed at 68.75 kN cm-2. Transparent, nanostructured TiO2 electrodes were prepared by spreading a concentrated colloidal paste onto the conducting glass substrate, followed by drying and heating to 450 °C in air for 30 min. Preparation of the TiO2 paste (APS 20 nm) is described elsewhere.30 The film thickness determined by profilometry (Dektak 3 profilometer, Veeco) was about 6 µm for all samples. (Specifically, 5.8 ( 0.1 µm for TiO2 and 5.7 ( 0.2 µm for Zn2SnO4.) Semiconductor working electrodes were sensitized in an ethanolic (99.5%, Kemetyl) 0.5 mM N719 ((TBA)2-cis-Ru(Hdcbpy)2(NCS)2, Solaronix) solution for 6 h. The dyed electrodes were rinsed with ethanol, dried, and finally assembled with a platinized counter electrode using a thermoplastic frame (Surlyn 1702). The cells were filled with 0.7 M LiI (Aldrich) and 0.05M I2 in 3-methoxypropionitrile (Fluka). The active area of the cells was 0.32 cm2. Dye loading was determined by detaching the dye from the working electrode surface with 0.1 M NaOH solution and measuring the optical absorbance with a HR 2000 Ocean Optics spectrometer. Characterization of Dye-Sensitized Solar Cells. DSSC efficiency was evaluated from current-voltage characteristics measured with a PC-controlled Keithley 2400 meter. The incident light intensity coming from a Xe arc lamp (300 W Cemax, ILC Technology) was 1000 W m-2 measured with a silicon diode (BPW21R: photodiode, color corrected). Incident photon-to-current efficiencies (IPCE), and opencircuit potentials at different wavelengths were recorded using a computerized setup composed of a Xe arc lamp (300 W Cemax, ILC Technology), a 1/8 m monochromator (CVI Digikro¨m CM 110), a Keithley 2400 source/meter, and a Newport 1830-C power meter with 818-UV detector head. Electron lifetimes, transport times, and electron concentrations in the DSSC were measured in a PC-controlled setup using a red-light-emitting-diode (Luxeon Star 1W, λmax ) 640 nm). The photocurrent and photovoltage were measured using a 16-bit resolution data acquisition board (National Instruments) in combination with a current amplifier (Stanford Research Systems SR570). Both electron lifetime and electron transport time were obtained by monitoring the transient response of the cells when a small square-wave modulation (typically 0.1-1 Hz,
