| Sol-Gel Preparation of M+-Doped CuO (M+ = Li+, Na+, K+) Thin Films and Their Photocathodic Properties |
| Kota ISOBE |
| Kansai University |
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Wet-type solar cells using semiconductor photoelectrodes have been widely investigated since the discovery of the Honda-Fujishima effect in 1972 [1]. CuO, a p-type semiconductor, is receiving much attention as a candidate of an photocathode, because CuO has a relatively small band gap of ca. 1.4 eV that provides photoresponse to the visible light. However, CuO normally shows low energy conversion efficiency, and thus the improvement of the photocathodic performance is required. In this work, we prepared M+-doped porous CuO films (M+ = Li+, Na+, K+) by sol-gel method, and investigated the effect of the porous structure and the doping with M+ ions on the photocathodic performance.
CuO films were prepared by dip-coating from the solutions consisting of Cu(NO3)2, LiNO3, NaCl, KNO3, H2O and poly(vinylpyrrolidone) (PVP, viscosity average molecular weight of 960,000) ([Cu(NO3)2] = 0.45‒0.50 M, [LiNO3], [NaCl] or [KNO3] = 0‒0.05 M, and [PVP] = 0.05‒0.40 M). Precursor films were deposited on silica glass substrates coated with FTO at a withdrawal speed of 3 cm min-1. The films thus obtained were heated at 600 ¨¬C for 30 min in air. The dip-coating and the heat treatment were repeated 2‒10 times so that the resultant film thickness was ca. 220 nm. The photocathodic properties were studied in a three-electrode cell with an aqueous buffer solution of pH = 7 as the supporting electrolyte.
Pure CuO films were prepared from the solutions of [Cu(NO3)2] = 0.50 M and [PVP] = 0.05‒0.40 M. The diffraction peaks attributed to CuO were detected in the XRD patterns for all the films. The CuO films prepared at [PVP] = 0.05‒0.25 M had porous structure, and the porosity increased with increasing [PVP] (Figure 1 (a) - (c)). On the other hand, homogeneous films were not obtained at [PVP] = 0.40 M (Figure 1 (d)). The CuO films exhibited photocathodic response under UV and visible light. The photocathodic performance was enhanced with increasing [PVP] from 0.05 M to 0.25 M, and the CuO film prepared at [PVP] = 0.25 M exhibited the maximum IPCE (incident photon-to-current efficiency) of 8.7 % and 4.9 % at the wavelengths of 300 and 500 nm, respectively (Figure 2). On the other hand, IPCE decreased with increasing [PVP] from 0.25 M to 0.40 M (Figure 2).
M+-doped porous CuO films (M+ = Li+, Na+, K+) were prepared from the solutions of [Cu(NO3)2] = 0.45 M, [LiNO3], [NaCl] or [KNO3] = 0.05 M and [PVP] = 0.25 M. The M+-doped CuO films exhibited photocathodic response under UV and visible light, while IPCE decreased by the doping with M+ ions.
References:
[1] A. Fujishima and K. Honda, Nature, 238, 37 (1971)
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