I-III-VI 2 (Copper Chalcopyrite-based) Thin Films for Photoelectrochemical Water-Splitting Tandem-Hybrid Photocathode
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I-III-VI2 (Copper Chalcopyrite-based) Thin Films for Photoelectrochemical WaterSplitting Tandem-Hybrid Photocathode Jess M. Kaneshiro1, Alexander Deangelis1, Xi Song1, Nicolas Gaillard1, Eric L. Miller2 1
HNEI, U. of Hawai`i at Manoa, 1680 East-West Rd. POST109, Honolulu, HI 96822
2
U.S. Department of Energy, EE-2H 1000 Independence Ave. SW, Washington, D.C. 20585
ABSTRACT This presentation will investigate various parameters regarding the use of I-III-VI2 Copper Chalcopyrite-based materials for use in tandem-hybrid photocathodes capable of splitting water into hydrogen and oxygen gases in an acidic electrolyte. Constituent parts (fabricated at HNEI) of a proposed monolithically integrated hybrid photovoltaic/photoelectrochemical (PV/PEC) device were characterized separately and combined theoretically using electronic and optical models to simulate tandem operation to first indicate feasibility of matching existing materials. Robust CGSe2 photocathodes were focused on for the PEC cells and CIGSe2 and CISe2 devices were evaluated for the PV cells. Simulation suggested the hybrid PV/PEC system could pass enough light to produce up to 15.87mA/cm2, validating the feasibility and warranting the fabrication of stacked PV/PEC devices. INTRODUCTION Materials in the I-III-VI2 ternary chalcopyrite class (most popularly CuInGaSe2) are highly suitable for use in solar cells because of very favorable optical and electronic properties as well as bandgap-tuneability based on alloy composition [1]. Tandem photovoltaic (PV) cells utilizing these materials have not yet achieved their theoretical performances, partially due to difficulty in utilizing the full potential of wide-bandgap top cells, mitigation of the shadowing of the bottom cells, and lattice matching between the many layers being incorporated [2,3,4,5]. Wide-bandgap thin films like CGSe2 have been found to be very efficient solar energy conversion devices in a PEC setup (shown in Figure 1A), where the solar energy is used to directly split water into oxygen and hydrogen gases; the latter of which being a useful, convenient, and storable form of energy [6]. However, photoelectrochemical water-splitting with current materials require an applied voltage bias that can be very elegantly fulfilled by an underlying PV device to create a tandem-hybrid photocathode [7]. Two distinct advantages a PV/PEC tandem water-splitting cell would have over a tandem photovoltaic cell are higher top-cell current densities (CGSe2 has demonstrated up to 20mA/cm2 saturated photocurrent) and less shadowing of an underlying cell. The PEC cell eliminates the need for a TCO window and grids an overlying PV cell would, transmitting more light to an underlying PV cell (as shown in Figures 1B and 1C). It should be kept in mind that for a PEC device in operation, the sun will still have pass through a container wall (quartz, glass, plastic) as well as the electrolyte itself. Piece-wise simulation using characterization data from J-V measurements, UV/visible spectrophotometry, quantum efficiency (QE)
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