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J. Phys. Chem. C 2010, 114, 6854–6859
Light Soaking and Gas Effect on Nanocrystalline TiO2/Sb2S3/CuSCN Photovoltaic Cells following Extremely Thin Absorber Concept Shinji Nezu,* Gerardo Larramona, Christophe Chone´, Alain Jacob, Bruno Delatouche, Daniel Pe´re´, and Camille Moisan IMRA Europe S.A.S., 220 rue Albert Caquot, B.P.213, 06904 Sophia Antipolis Cedex, France ReceiVed: January 15, 2010; ReVised Manuscript ReceiVed: March 3, 2010
Photovoltaic cells with the structure of nanocrystalline TiO2/Sb2S3/(LiSCN)CuSCN were prepared following the extremely thin absorber cell concept and characterized under different gas atmospheres. The cells showed an important light soaking effect in which the cell performance typically evolved from an initial lower efficiency into a higher one in contact with ambient air. Oxygen was shown to be involved with the effect. The best cell efficiency was 3.7% at ∼1 sun, with a photocurrent Jsc normalized at 1 sun of 11.6 mA/cm2, an open-circuit voltage Voc of 0.56 V, and a filling factor, ff, of 0.58. The cells are shown to run more than 300 days, keeping the efficiency higher than 1.5% at ∼1 sun. The need for taking into account the influence of oxygen and humidity during photovoltaic measurement is emphasized for further investigation of this type of cell. Introduction Inorganic absorber-sensitized nanoporous solar cell comprising a porous heterojunction of “n-a-p” type (“a” meaning absorber, also known as n-i-p type, “i” meaning intrinsic) has been of interest because of the expectation for overcoming the drawback of instability originated from use of organic dye and liquid electrolyte in a conventional dye-sensitized nanoporous solar cell (DSC).1 In such a cell known as extremely thin absorber cells (ETA cells), a thin layer of an inorganic absorber is deposited over the internal surface of a porous film made of a transparent (nonabsorbing or wide bandgap) n-type inorganic semiconductor, the rest of the pore volume being filled with a transparent p-type inorganic semiconductor.2-9 Photons are only absorbed in the absorber layer, and the generated electrons and holes are injected into the two separated phases, n-type and p-type, respectively. The expected advantage for this kind of cell is that a fast charge separation will occur due to the low thickness of the absorber, and charge recombination of opposite carriers traveling in separate phases will be highly reduced. Consequently, the absorber material will need lower restrictions concerning the number of defects, allowing a low fabrication cost. TiO2 or ZnO as n-type materials and CuSCN as p-type materials have been commonly used. Le´vy-Cle´ment’s team5 has reported 2.3% at 0.36 sun with a ZnO/CdSe/CuSCN cell. Dittrich’s team6 has reported 3.4% at 1 sun and cell area of 0.03 cm2 with a ZnO/In2S3/CuSCN cell. We7 have also reported 1.3% at 1 sun with a TiO2/CdS/CuSCN cell of 0.54 cm2 (using a high internal surface nanocrystalline TiO2 and a CdS coating ∼5-10 nm thick). Subsequently, we have patented TiO2/Sb2S3/ CuSCN cells8 which can provide 3.4% efficiency with the band gap of ∼1.65 eV (∼750 nm). The best external quantum efficiency (EQE) attained to a maximum of 80%,8 which is approximately the maximum value that can be reached due to at least ∼15% loss by absorption/reflection by the TCO glass substrate. Recently, Hode’s team9 also reported 3.37% for similar Sb2S3 nanoporous solar cells in the configuration TiO2/ * To whom ai-i.aisin.co.jp.
correspondence
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Figure 1. Schematic of the nanostructured solar cell.
In-OH-S/Sb2S3/(KSCN)CuSCN with a cell area of 0.15 cm2. In their report, they9 also confirmed the beneficial effect of thiocyanate pretreatment before CuSCN impregnation shown in our previous report.7 Following these results with CdS and Sb2S3 cells, we have been intensively studying them in order to optimize and to confirm the stability. In the course of such study, we noticed significant effects of gas in contact with the cells. In this report, the newest results following our patent disclosure regarding Sb2S3 nanocrystalline photovoltaic cells are presented first in order to outline the photovoltaic behavior such as the light soaking effect. Then the gas effect is described in detail to show the difference in light soaking behavior between air and a N2 atmosphere. We also studied cells which underwent heat treatment under different atmospheres. The latest results of cell stability tests based on the acquired knowledge are presented. Finally, morphology on the cell nanostructure using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis is given. Experimental Section Figure 1 shows the solar cell structure. Commercial F-doped SnO2 transparent conductive oxide (TCO) glasses from Asahi Glass were used as front contact. A hole-barrier compact TiO2 layer of ∼20 nm thickness (confirmed by SEM and TEM) was deposited by spray pyrolysis.10 Porous nanocrystalline TiO2 films of ∼3 µm thickness, ∼40-50 nm average particle size, and
10.1021/jp100401e 2010 American Chemical Society Published on Web 03/18/2010
Nanocrystalline TiO2/Sb2S3/CuSCN Photovoltaic Cells 150-300 roughness factor were fabricated as described in our previous publication.7 After the antimony sulfide deposition described below, the samples were dipped into 0.5 M aqueous solution of LiSCN salt. Non-salt-treatment samples were also prepared for comparison. Then they were dried by N2 gas blowing to the surface for several seconds. CuSCN filling was made by impregnation and evaporation of a CuSCN solution in propyl sulfide,7 a method similar to that reported for solid DSC cells of the type TiO2/dye/CuSCN.11,12 Just after the CuSCN filling, a back contact of a gold layer of ∼40 nm thickness was deposited by thermal evaporation using an Edwards 306 evaporator. The completed cells were stored in a N2 glovebox until measurement (typically 1 night). Some cells were heat pretreated under N2 or synthetic dry air at 80 °C for 15 h before measurement. Deposition of antimony sulfide on the nanocrystalline TiO2 films was done by chemical bath deposition (CBD). Several methods have been reported for depositing thin films of Sb2S3 on flat substrates, due to its interesting properties for different applications. Such methods included wet chemistry processes (such as CBD13-18 and electrodeposition19), thermal evaporation, radio frequency magnetron sputtering, or MO-CVD. For deposition inside nanocrystalline porous TiO2 films, we used a CBD method based on thiosulfate precursor, similar to some procedures reported for deposition of thin films on a flat substrate.16-18 The CBD bath was done by preparing an initial ∼1 M solution of SbCl3 in acetone and by diluting this one with ∼1 M Na2S2O3 cold aqueous solution and cold water, so as to have final concentrations of Sb(III) and S2O32- of ∼0.025 and ∼0.25 M, respectively. The thiosulfate anion acts as complexing agent and sulfide source. Samples were immersed vertically in the bath and placed in a refrigerator at 5-10 °C. The CBD reaction was left for a typical period of 2 h. Then samples were rinsed with water and dried by flowing nitrogen. Afterward, samples were annealed in a N2 glovebox at 300-320 °C for about 15-30 min. Among the different methods that we tried, this one allowed the deposit of a coating of antimony sulfide inside the porous TiO2 films without producing an outerlayer or blocking the internal pores. The following apparatus and setups were used for the characterization of components and cells. Current-voltage I-V characterization and quantum efficiency (QE, also known as IPCE) were recorded with in-house setups.7 I-V curve measurements were made in ambient air (room air) or in controlled atmospheres of synthetic dry air flow or N2 gas flow. An unsealed cell was placed in a sample holder which had a mask to have an exact and fixed illuminated surface of 0.54 cm2 (0.6 × 0.9 cm). For measurements in controlled atmospheres, it was put in a cell container made of a metal box with a glass window for light illumination and gas inlet and outlet to allow controlled gas environment. I-V plots were recorded typically at irradiance of ∼1 sun ()1000 W/m2). Series resistance of solar cells was estimated from a single I-V plot under illumination. That consists in making the derivative of the I-V curve (at a single irradiance) of a scan up to sufficiently high voltage forward bias, so as to achieve a plateau on the plot of (-dV/dI) versus V, which directly gives the value of Rs. SEM images were acquired with a field-emission scanning electron microscope, Hitachi S-4700, to which an EDX (energy-dispersive X-ray microanalysis) Noran System SIX was coupled. TEM analysis was carried out with a JEOL 2100F FEG-200 kV microscope, having a STEM accessory, and an integrated JEOL JED-2300T EDX analyzer.
J. Phys. Chem. C, Vol. 114, No. 14, 2010 6855
Figure 2. I-V curves of a TiO2/Sb2S3/(LiSCN)CuSCN cell at 1 sun (1006 W/m2) measured at different time t after starting light soaking under ambient air at open circuit (cell active area of 0.54 cm2): (a) t ) 0; (b) t ) 10 min; (c) t ) 20 min; (d) t ) 100 min.
Results and Discussion Cell Performance with Light Soaking in Ambient Air. Experiments were carried out using cells treated by LiSCN for this purpose, since they provides higher performance as pointed out for CuSCN containing cells by our previous report.7 Unsealed cells exposed to ambient air (room air) were used for photovoltaic measurement. Cells were stored overnight in a N2 glovebox before measurement. Higher cell efficiencies were obtained after the cells were held under light soaking at 1 sun in ambient air and open circuit condition for ∼20-200 min. Before light soaking, cells showed an efficiency typically
