Photoelectrochemical Cells With Single-Crystal Cadmium Telluride and Alkaline Telluride, Ditelluride Redox Couple

Abstract

In the photoelectrochemical cell Ga -In /n- CdTe (single crystal) 0.1 M Te2- , Te22-, OH- /Pt the efficiency of energy conversion of solar radiation (AM2 spectral distribution, 58 mW cm-2 was increased from the previously reported 5% to 9.0%, by etching the polished crystal surface, by making [Te2-] as large as practicable by using KOH as the supporting electrolyte, and by optimizing [Te22- At reduced irradiances the energy conversion efficiency was higher. At AM2, an efficiency of 14% is predicted to be attainable. Etching with chromic acid (Na2Cr2O7 in 3M HNO3) increased the open-circuit voltage in 0.1 M partly oxidized Na2Te, 5 M KOH, to 0.74 V from 0.48 V as observed on the polished but unetched crystal, and increased the short-circuit photocurrent density Jsc from 4 mA cm-2 to 13 mA cm-2 (AM2 spectral distribution, 58 mW cm-2. Etching reduced the surface defect concentration as recorded by two-beam photocurrent spectroscopy. For redox potentials Eredox more negative than - 1.12 V v . s.c.e . the crystal potential at open circuit Ec,oc was independent of the value of Eradox which was varied by changing [Te22-]; a change in Eredox therefore produced a quantitatively similar change in the open-circuit photovoltage Uoc (both at 1 mWcm-2 of 633 nm radiation and at AM2 solar radiation) and so Fermi-level pinning did not occur. Uoc was proportional to In Jsc . The very negative value of - 1.86 V v . s.c.e . observed for Ec,oc was attributed to adsorbed anionic charge, estimated as c.1013 elementary charges cm-2, which gave rise to a potential drop of 1.5 V across the Helmholtz layer.