Infrared quenching of photocapacitance in CuxS/CdS solar cells

Copper diffusion into CdS substrates normally occurs in CuxS/CdS solar cells during fabrication, and may cause degradation of solar efficiency because of a decreased proportion of copper in the CuxS. In order to obtain information about deep Cu-acceptor levels in CuxS/CdS solar cells prepared by both the wet and dry methods on CdS single crystals, an infrared quenching of photocapacitance (PHCAP IR quenching) method has been used. A theory for the description of the PHCAP-IR quenching, and also for a doping profile by a modified C-V method using PHCAP-IR quenching, has been developed to obtain energy levels, photoionization cross sections, and concentration profiles of Cu levels in CdS. Two transitions of 1.08 and 0.86 eV, corresponding to the transition (3d9)Cu2+→(3d10)Cu+, and an internal transition in (3d9)Cu2+, respectively, were measured at room temperature. The photocapacitance quenching level of 0.86 eV disappears at low temperature, and only a 1.20-eV transition was measured at 91 K, corresponding to the internal transition (3d9)Cu2+[E5/2(2T2)]→(3d9)Cu2+[G( 2E)]. Spectral distributions of photoionization cross sections σ0pv corresponding to the transition (3d9)Cu2+→(3d10)Cu+ were obtained; the peak value was 1.1×10−16 cm2. The doping profile of Cu shows that a density of Cu up to one half of the majority-carrier density diffused into CdS after heat treatments at 200 °C for a few minutes.