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2009, 113, 4254–4256 Published on Web 02/20/2009
Sb2S3-Sensitized Nanoporous TiO2 Solar Cells Yafit Itzhaik, Olivia Niitsoo, Miles Page, and Gary Hodes* Department of Materials and Interfaces, Weizmann Institute of Science, RehoVot 76100, Israel ReceiVed: January 12, 2009; ReVised Manuscript ReceiVed: February 10, 2009
Extremely Thin Absorber (ETA) solar cells were made using chemical-bath-deposited Sb2S3 as the absorber and TiO2/CuSCN as the interpenetrating electron/hole conductors. A solar conversion efficiency of 3.37% at 1 sun illumination was obtained. Surface oxidation of the Sb2S3 formed a passivation layer on Sb2S3: without this oxidation, much poorer cells were obtained. Preliminary stability measurements showed good stability over 3 days of illumination (at 60 mW/cm2) under load. Semiconductor-sensitized nanoporous solar cells are an offshoot of the dye-sensitized cell (DSC)1 where a solid-state semiconductor absorber is used in place of the molecular dye of the DSC. Semiconductor-sensitized cells can be either all solid state or liquid junction. The solid-state versions can be further divided into two-phase cells (the best example being the CuInS2/TiO2 cell, where CuInS2 acts both as the absorber and the hole conductor2,3) and three-phase cells, better known as the ETA (Extremely Thin Absorber) cells, where the semiconductor absorber, sandwiched between interpenetrating electron and hole conductors, has a typical thickness ranging between several nanometers and several tens of nanometers.4 Both TiO2 (most common) and ZnO have been used as electron conductors in ETA cells, and the most successful hole conductors until now have been CuSCN and the PEDOT/PSS copolymer.5 A range of absorbing semiconductors have been employed, including Se,4 PbS,6 CdSe,7 CdS,8 In2S3,9 and Cu2- xS.10 The best cell to date used In2S3 with a solar conversion efficiency of 3.4%.9 Sb2S3 is a semiconductor that has been used on a number of occasions for novel solar cells. In its crystalline form (stibnite), it has a band gap of 1.7-1.8 eV (like CdSe). Both solid-state cells and photoelectrochemical cells have been reported,11-13 where Sb2S3 was usually formed by chemical bath deposition (CBD). We find only one report where (probably amorphous) Sb2S3 was used in a nanoporous (liquid junction) cell. An incident photon-to-current quantum efficiency of nearly 30% (a calculated absorbed photon-to-current efficiency of 88%) was measured, but the photoelectrode was unstable in the electrolyte; therefore, power efficiency measurements were not made.14 A cell using crystalline Sb2S3 deposited on flat TiO2 was also described recently.15 Although the conversion efficiency of this cell was not explicitly given, it appeared to be less than 10-3%. In this report, we describe ETA cells using CBD Sb2S3 as the ultrathin absorber and nanoporous TiO2 and CuSCN as the electron and hole conducting phases, respectively. Solar-toelectric conversion efficiencies up to 3.37% in full sun and monochromatic external quantum efficiencies up to nearly 80% have been obtained from these cells. * Corresponding author. E-mail:
[email protected].
10.1021/jp900302b CCC: $40.75
Cell Preparation. An ∼120 nm thick, dense TiO2 layer was deposited onto a SnO2:F conducting glass substrate (FTO) by spin coating five layers of a titanium isopropoxide sol (0.125 M)16 and annealing in air at 250 °C for 10 min, followed by annealing at 500 °C for 30 min after each coating. A porous TiO2 film (1 to 2 µm thick) was spin-coated from a paste made of commercially available P25 powder (Degussa, 25 nm particles) in ethanol on the FTO coated with dense TiO2 and annealed as for the dense layer.10 Inx(OH)ySz (In-OH-S) was deposited on the above film by CBD using a solution of In2(SO4)3 and thioacetamide at 69 °C for typically 1 h.17 The average layer thickness was ca. 1 nm,10 although it was not very evenly distributed. Sb2S3 was then deposited on the P25/ In-OH-S substrates by CBD from a solution of SbCl3 and Na2S2O3 as described in ref 12. A rough estimated average thickness from SEM imaging was 5-10 nm. The as-deposited orange films of amorphous Sb2S3 were annealed under N2 at 300 °C for 30 min to give dark-brown crystalline stibnite. The samples were removed from the oven immediately after annealing and were allowed to cool in air. SEM images of various steps in the film preparation are shown in the Supporting Information. The deposit often covered several TiO2 particles in the as-deposited film, and this is seen much more so in the annealed film (Figure 1c in the Supporting Information), where at least some melting of the low-melting-point (550 °C) Sb2S3 clearly occurred. (The melting point of the nanoscale material will be lower, and even for bulk material, surface melting could occur.) The sample was then dipped into 0.5 M aqueous KSCN solution for 5 min, and the excess solution wicked off. After drying, a saturated solution of CuSCN in di-n-propyl sulfide (PrS) diluted 1:1 with PrS was slowly infiltrated18 into the porous film on a hot plate at a sample surface temperature of 65 °C. A back contact of ca. 80 nm thick gold was evaporated onto the CuSCN. Finally, a defined area (0.15 cm2 unless otherwise stated) was scribed to separate the measured cell from the surrounding deposit. This step was essential to prevent artifacts arising from the relatively large lateral current flow from areas where there was no back contact. Cells normally required aging (normal storage, no special conditions) for several days, sometimes even weeks, after preparation to reach their maximum efficiency. This improvement with storage time was seen in both the short circuit current and fill factor; the open circuit voltage 2009 American Chemical Society
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Figure 1. (-) External quantum efficiency of a cell (including the In-OH-S and KSCN-treated layers) as a function of wavelength. (---) Raw transmission data (measured with an integrating sphere) for a complete cell similar to the one used for the spectral response, but without the Au back contact. Most of the transmission loss in the nonabsorbing long-wavelength region is due to scattering not picked up by the sphere.
did not change appreciably. A similar increase in cell performance on storage or vacuum treatment has been reported for solid-state dye cells using CuSCN as a hole conductor.18 Several possible explanations have been given in that paper, and all are based on the slow evaporation of the propyl sulfide solvent. The external quantum efficiencies (measured using a calibrated Si photodiode) of these cells were very high, typically 60-80% of peak efficiency between 450 and 520 nm (solid curve in Figure 1). Taking into account the optical losses in the conducting glass (ca. 20% total loss), this translates to 75-100% absorbed light (internal) peak quantum efficiency. The photocurrent onset is at ca. 750 nm, similar to the optical spectra of the cells (shown in Figure 1 as a dashed curve). This is consistent with the ca. 1.75 eV band gap of crystalline Sb2S3, although we do note that all ETA cells employing CuSCN as a hole conductor show a rather similar spectral response, even using considerably higher band gap absorbers, and it was suggested10 that this might be due to solid-state reaction between CuSCN and the metal chalcogenide (or chalcogen itself) to give a copper sulfide phase (or possibly a ternary phase). The (photo)current-voltage characteristics of our best cell are shown in Figure 2. Under full sun illumination (see legend to Figure 2 for details of sample illumination), we measure a short circuit current (JSC) of 14.1 mAcm-2, an open circuit voltage (VOC) of 490 mV, and a fill factor (FF) of 48.8%, giving a conversion efficiency of 3.37%. Efficiencies at lower illumination are somewhat higher but somewhat less reliable because of the nonhomogeneous transmission spectra of the neutral density filters used to make these measurements. The dashed curve in Figure 2 shows the I-V characteristic at 10% sun (actually measured between 10.1 and 10.2% transmission over the relevant spectral range of the neutral density filters). The relative JSC (i.e., JSC/light intensity) appears to be a little greater than for the full sun measurements. The photocurrent versus light intensity is approximately linear over the range of our measurements. However, the increase in FF is very real; unlike JSC, this parameter is not very dependent on the accuracy of the light intensity measurements. We measure a conversion efficiency of nearly 3.8% at 10% sun, although as noted above, with some level of uncertainty in the low-lightlevel JSC measurement. There are some characteristics of these cells that are worth mentioning qualitatively and indicate the various directions of further research needed. A very important one concerns the air cooling of annealed Sb2S3. We noted in the cell preparation
J. Phys. Chem. C, Vol. 113, No. 11, 2009 4255
Figure 2. (Photo)current-voltage measurements for a P25/In-OH-S/ Sb2S3/KSCN treatment/CuSCN/Au cell. Scribed area ) 0.15 cm2. (-) Full sun intensity (see below). (---) 10% of full sun intensity (×10 scaling factor). Dark characteristics corresponding to the scaling of the full sun (- · · -) and 10% sun ( · · · ), respectively. A tungsten halogen lamp was used for the I-V measurements. The light was calibrated by measuring the short circuit currents of the cells outside on a clear day, together with a measure of the solar intensity using an Eppley pyranometer. The cells were then measured inside, and the lamp intensity (at a fixed lamp-to-sample distance) was adjusted to give the same short circuit current. The lamp voltage was then adjusted by the small amount needed to convert from the measured solar intensity (typically around 900 W m-2) to full sun of 1000 W m-2 based on the linear correlation between photocurrent and light intensity.
procedure that after annealing Sb2S3 we remove the samples from the furnace immediately and allow them to cool in air. If they are allowed to cool under N2, much poorer performance (ca. 30% of the air-cooled samples) is obtained. XRD showed the presence of some Sb2O3 when Sb2S3 was removed hot (300 °C) from the oven. This surface oxide presumably acts as a passivation layer. Possible causes for this are the reduction of electron-hole recombination in Sb2S3 or, maybe more likely, the reduction of recombination between electrons in the Sb2S3 and holes in the CuSCN (i.e., electron injection from the Sb2S3 to the CuSCN). If the samples are allowed to cool under N2 and then heated in air (10 min at 200 °C), good cells can be obtained. Another is that relatively large-area cells (0.7 cm2 was typically used in our earlier experiments) give low efficiencies due to both lower JSC/light intensity and FF. Decreases in either cell size (by scribing) or light intensity increased both of these parameters. Thus, the values of JSC were not linearly dependent on light intensity for larger-area cells. These trends suggest resistance losses in the cells. In fact, preliminary experiments show that the series resistance per unit area of these cells (estimated from the slope of the forward I-V characteristics), both in the dark and under illumination, decreases when the area is reduced. The cell structure we used, based on P25/In-OH-S/Sb2S3/ KSCN treatment/CuSCN, was found to be the optimum one taking into account both efficiency and stability. The simplest cell (P25/Sb2S3/CuSCN) was found to discolor with time. Whereas good cells were not obtained with this architecture in full sun (although we did not carry out many experiments on these cells), it is notable that fairly good cells (over 2% conversion efficiency) were obtained at low light intensities. The discoloration did not visibly occur for considerably thicker layers of Sb2S3 on TiO2 and is probably due to oxidation of Sb2S3 near the interface to colorless Sb2O3, catalyzed by TiO2.
4256 J. Phys. Chem. C, Vol. 113, No. 11, 2009 The main purpose of the In-OH-S layer was to prevent the discoloration (Sb2S3 oxidation) that occurred when thin Sb2S3 layers are in direct contact with porous TiO2. Other than this increase in stability, the In-OH-S layer did not improve the output characteristics. The use of an In-OH-S buffer layer (or other relatively high band gap semiconductor) is often important in TiO2-based ETA cells, particularly for relatively low band gap absorbers. The effect of the buffer layer is usually attributed, in a general way, to the reduction of interface recombination. We believe that the buffer layer is not of fundamental importance to the present cells, and its main purpose is to impart stability to Sb2S3. Using a larger particle size TiO2, and therefore a larger pore size, should allow thicker layers of Sb2S3 to be used, and the buffer layer may not be needed. The KSCN treatment of the cell prior to deposition of CuSCN was found to be beneficial, as reported in ref 8 for CdS-based ETA cells using CuSCN. This treatment improved the JSC (although less dramatically than the order of magnitude reported for the CdS cells in ref 8), Voc, and often also the FF. Several other cations were also tried for the thiocyanate. There was little difference between LiSCN and KSCN (as reported also in ref 8) in terms of performance. However, the KSCN-treated cells were more stable than those treated by LiSCN: the latter slowly became discolored, often over several days, in spite of the presence of the In-OH-S buffer layer. This discoloration did not occur for treatment with KSCN. Other thiocyanate cations (Na, Cs, and NH4) were not found to give comparable cell performance. Also, as reported in ref 8, other Li salts did not activate the cells. Of the possible reasons given in ref 8 for the beneficial effect of the thiocyanate treatment, doping of the CuSCN by excess SCN- is the one that seems most likely to us. In agreement with the results of ref 8, we also find that the treatment reduces the resistance of our cells. We also note that the shapes of the spectral responses of the various cell configurations were all similar. The measured quantum efficiencies did vary over a moderate range (peak values of 40-80%). However, it should be remembered that these quantum efficiency measurements were made under low light conditions where all of the cells perform at least fairly well (ignoring stability issues). The differences between the various types of cells were more marked at full sun intensity. Finally, whereas CuSCN has been commonly used for ETA cells, we find no reports on the operational stability (as opposed to shelf life) of such cells (there is one report18 of operational stability for CuSCN-based dye cells). We carried out some preliminary experiments on our cells, both LiSCN- and KSCNtreated. Not surprisingly, in view of our earlier remarks on the discoloration of the LiSCN-treated cells, we found a slow but continuous decrease in output when illuminated (at ca. 60% of full sun intensity) at initial maximum power. A loss of 40%
Letters power output occurred after 3 days of continuous illumination. The KSCN-treated cells were much more stable. When run under the same conditions (at maximum power with an occasional change to operation under short circuit current and open circuit voltage), no more than 10% loss occurred after 3 days, and most of this loss was regained after standing in the dark for a few hours (as would occur naturally in any terrestrial solar cell). To summarize, we showed that Sb2S3 is one of the best absorbers to date for ETA cells as long as the Sb2S3 is passivated by air heating, with full sun efficiencies of up to 3.37% for cells with 0.15 cm2 area. The cells were shown to be stable over 3 days of operation. A decrease in the resistivity of the cell and possibly more controlled passivation of Sb2S3 can be expected to lead to even better cells. Acknowledgment. We acknowledge the Harold Perlman family’s historic generosity and support from the GMJ Schmidt Minerva Center for Supramolecular Chemistry. Supporting Information Available: SEM images of films. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Gra¨tzel, M. J. Photochem. Photobiol. A 2004, 164, 3. (2) Nanu, M.; Schoonman, J.; Goossens, A. Nano Lett. 2005, 5, 1716. (3) Goossens, A.; Hofhuis, J. Nanotechnology 2008, 19, 424018. (4) Tennakone, K.; Kumara, G. R. R. A.; Kottegoda, I. R. M.; Perera, V. P. S.; Aponsu, G. M. L. P. J. Phys. D: Appl. Phys. 1998, 31, 2326. (5) Levy-Clement, C. Nanostructured ETA-Solar Cells in Nanostructured Materials for Solar Energy ConVersion; Soga, T., Ed.; Elsevier: Amsterdam, 2006. (6) Oja, I.; Belaidi, A.; Dloczik, L.; Lux-Steiner, M-Ch.; Dittrich, Th. Semicond. Sci. Technol. 2006, 21, 520. (7) Le´vy-Cle´ment, C.; Tena-Zaera, R.; Ryan, M. A.; Katty, A.; Hodes, G. AdV. Mater. 2005, 17, 1512. (8) Larramona, G.; Chone, C.; Jacob, A.; Sakakura, D.; Delatouche, B.; Pere, D.; Cieren, X.; Nagino, M.; Bayon, R. Chem. Mater. 2006, 18, 1688. (9) Belaidi, A.; Dittrich, T.; Kieven, D.; Tornow, J.; Schwarzburg, K.; Lux-Steiner, M. Phys. Status Solidi (RRL) 2008, 2, 172. (10) Page, M.; Niitsoo, O.; Itzhaik, Y.; Cahen, D.; Hodes, G. Energy EnViron. Sci. 2009, 2, 220. (11) Savadogo, O.; Mandal, K. C. Appl. Phys. Lett. 1993, 63, 228. (12) Messina, S.; Nair, M. T. S.; Nair, P. K. Thin Solid Films 2007, 515, 5777. (13) Savadogo, O.; Mandal, K. C. J. Electrochem. Soc. 1992, 139, L16. (14) Vogel, R.; Hoyer, P.; Weller, H. J. Phys. Chem. 1994, 98, 3183. (15) Manolache, S. A.; Duta, A. Romanian J. Information Sci. Tech. 2008, 11, 109. (16) Takahashi, Y.; Matsuoka, Y. J. Mater. Sci. 1988, 23, 2259. (17) Bayon, R.; Guillen, C.; Martinez, M. A.; Gutierrez, M. T.; Herrero, J. J. Electrochem. Soc. 1998, 145, 2775. (18) O’Regan, B.; Lenzmann, F.; Muis, R.; Wienke, J. Chem. Mater. 2002, 14, 5023.
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