Copper sulfide (Cu_2-xS) is potentially a good candidate for an extremely thin absorber (ETA) nanoporous solar cell. While the material has a relatively high recombination rate compared to most photovoltaic semiconductors, the effectively thin layers of Cu_2-xS (much thinner than those used in early Cu₂S/CdS thin film cells) in a three-component ETA cell should minimize this disadvantage. The availability and low toxicity of the material are other potential benefits. However, up to now, no experimental results have been published on such a cell; the closest being flat cell of Cu_1.8S/TiO₂ giving ca. 0.002% conversion efficiency.¹
Here we demonstrate such porous cells wherein the Cu_2-xS light-absorber, prepared by Cu⁺ ion exchange from CdS or ZnS, is applied as an ultra-thin coating on a nanoporous n-type metal oxide substrate, coated with a buffer layer. The pores are filled with p-type copper (I) thiocyanate, CuSCN, as a hole conductor allowing charge separation from Cu_2-xS into the electron- and hole-conducting phases.
Two main factors currently limiting such a cell have been identified: low efficacy of the buffer layer (at present, In-OH-S) and poor long-wavelength light absorption of the Cu_2-xS.
Combined current-voltage (I-V), transient photovoltage decay and contact potential difference (CPD) measurements are used to examine loss mechanisms and the efficiency of various buffer layers, informing the approach to realizing much improved ETA solar cells.

1. L. Reijnen et al., Mater. Sci. Eng C 2002, 19, p311.