Abstract :
The highest efficiency for Cu(Ga,ln)Se2 (CIGS) thin-film-based solar cells has been achieved with CdS buffer layers prepared by a solution growth method known as the chemical bath deposition (CBD). With the aim of developing Cd-free chalcopyrite-based thin-film solar cells, we describe the basic concepts involved in the CBD technique. The recipes developed in our laboratory for the heterogeneous deposition of good-quality thin films of ZnO, ZnSe, and MnS are presented. In view of device optimization, the initial formation of chemical-bathdeposited ZnSe thin films on Cu(Ga,ln)(S,Se)2 (CIGSS) and the subsequent development of the ZnSe/CIGSS heterojunctions were investigated by X-ray photoelectron spectroscopy (XPS). The good surface coverage was controlled by measuring changes in the valence-band electronic structure as well as changes in the ln4d, Zn3d core lines. From these measurements, the growth rate was found to be around 3.6 nm/min. The valence band and the conduction band-offsets (delta)E(v) and (delta)E(C) between the layers were determined to be 0.60 and 1.27 eV, respectively for the CIGSS/ZnSe interface. The energy-band diagram is discussed in connection with the band-offsets detemined from XPS data. A ZnSe thickness below 10 nm has been found to be optimum for achieving a homogeneous and compact buffer layer on CIGSS with a total area efficiency of 13.7%.
