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 Te^2- , Te₂^2-, 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 [Te^2-] as large as practicable by using KOH as the supporting electrolyte, and by optimizing [Te₂^2- 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 J_sc 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 E_redox more negative than - 1.12 V v . s.c.e . the crystal potential at open circuit E_c,oc was independent of the value of E_radox which was varied by changing [Te₂^2-]; a change in E_redox therefore produced a quantitatively similar change in the open-circuit photovoltage U_oc (both at 1 mW^cm-2 of 633 nm radiation and at AM2 solar radiation) and so Fermi-level pinning did not occur. U_oc was proportional to In J_sc . The very negative value of - 1.86 V v . s.c.e . observed for E_c,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.