Copper Chalcopyrite Film Photocathodes for Direct Solar-Powered Water Splitting
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0974-CC09-05
Copper Chalcopyrite Film Photocathodes for Direct Solar-Powered Water Splitting Bjorn Marsen, Susanne Dorn, Brian Cole, Richard E. Rocheleau, and Eric L. Miller Hawaii Natural Energy Institute, University of Hawaii at Manoa, 1680 East-West Road, Honolulu, HI, 96822
ABSTRACT In search of an efficient semiconductor material for direct photoelectrochemical (PEC) hydrogen production, chalcopyrite films in the Cu(In,Ga)Se2 system (CIGS) with bandgaps of 1.3-1.65 eV have been evaluated. The films have been fabricated by 2-stage and 3-stage coevaporation processes. Film samples have been fabricated into CIGS/CdS solar cells for evaluation of solid-state device properties, and into CIGS photocathodes for evaluation of the photoelectrochemical hydrogen-production characteristics. The PEC current-potential scans of the photocathodes in 0.5M sulfuric acid show photocurrents of 18-27 mA/cm2 under simulated AM1.5 global light (100 mA/cm2) at sufficient cathodic potential bias. In terms of fill factor of the photocurrent curves, electrodes with molybdenum back contact are superior to SnO2:F back contact because of better conductivity. The morphology as seen in scanning electron micrographs is unchanged after initial PEC testing in the cathodic regime, suggesting films are stable. INTRODUCTION More than 30 years after the first account of water splitting by semiconductor photoelectrolysis[1] suitable materials and devices are still lacking that would present an efficient, stable and economic solution of this appealing hydrogen production technology. It has become clear that tandem junctions will be necessary in order to supply the necessary voltage at practical current densities[2]. A functional photoelectrochemical (PEC) top junction with a bandgap in the range from 1.7-2.2 eV[2] is the most urgent ingredient to implement such a tandem device. A wide variety of semiconductor materials has been researched for this purpose, including silicon, III-V and II-VI[3] compounds, and transition metal oxides[4-7]. Because of the three simultaneous requirements of efficiency, stability, and cost, an ideal candidate material has yet to be found. Copper chalcopyrites exhibit a number of properties that renders them particularly suitable for solar energy conversion applications, including: (1) a direct band gap; (2) good electronic transport properties ; and (3) tunable band edges (by composition). In this context, the Cu(In,Ga)Se2 system (1.0-1.7 eV bandgap) has been well studied for photovoltaic applications[8]. The highest-efficiency thin film (solid state) solar cells are based on polycrystalline Cu(In,Ga)Se2 and interestingly outperform even their single-crystalline equivalents[9].
The earliest photoelectrochemical studies of copper chalcopyrite compounds have been focused on regenerative photoelectrochemical solar cells, based on n-type CuInS2[10,11] or nCuInSe2 [12-14]. P-type CuInSe2 and Cu(In,Ga)Se2 have been studied by electrochemical means in the context of solid-state photovoltaic devices, utilizing photoelectrochem
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