Development of Cu(In,Ga)Se2 superstrate devices with alternative buffer layers

Superstrate Cu(In,Ga)Se2 (CIGS) solar cells are prepared in the structure SLG/TCO/buffer/CIGS/Au with CIGS deposited onto the buffer layer by a single stage co-evaporation process. Three buffer materials - CdS deposited by chemical surface deposition, ZnSe and ZnO by RF magnetron sputtering - are tested in the superstrate structure. The best cell achieves 8.6% efficiency with the ZnO buffer after light soaking and forward bias treatments. The efficiency of devices with CdS or ZnSe buffers are less than 4%. The junction formation between the absorber and buffer layers are investigated by x-ray photoelectron spectroscopy, scanning electron microscopy and transmission electron microscopy. For CdS/CIGS and ZnSe/CIGS superstrate devices extensive inter-diffusion between the absorber and buffer layer under CIGS growth condition is the critical problem. For ZnO/CIGS superstrate cells GaxOy formation at the junction interface and unfavorable conduction band alignment are the main factors that limit the device performance.