Earth-abundant Cu-based chalcogenide semiconductors as photovoltaic absorbers

Abstract

The materials Cu₃PQ₄ (Q = S, Se) of the enargite structure are studied as photovoltaic (PV) absorbers. Optical band gaps in the series Cu₃PS_4−xSe_x (0 ≤ x ≤ 4) are found to range from 2.36 eV (x = 0) to 1.35 eV (x = 4). Seebeck measurements on powder samples at room temperature yield large positive values (>100 μV K⁻¹) indicating p-type behavior. Hole carrier concentrations are found in the range of 10¹⁶–10¹⁷ cm⁻³. Crystal structures of Cu₃PS_1.89Se_2.11 and Cu₃PS_0.71Se_3.29 are refined in the orthorhombic space group Pmn2₁ with the unit-cell parameters – Cu₃PS_1.89Se_2.11: a = 7.5034(5) Å, b = 6.4951(5) Å, c = 6.2174(4) Å, and Cu₃PS_0.71Se_3.29: a = 7.6164(6) Å, b = 6.5945(6) Å, c = 6.3107(5) Å. Photoelectrodes, fabricated from Cu₃PSe₄ single crystals, exhibit p-type photoresponse and yield open circuit voltages V_oc = 0.12 V and short circuit currents J_sc = 0.25 mA cm⁻² under 100 mW cm⁻² of 660 nm illumination in a non-aqueous cobaltocene/cobaltocenium cell.