Chemical bath process for highly efficient Cd-free chalcopyrite thin-film-based solar cells

The highest efficiency for Cu(Ga,In)Se-2 (CIGS) thin-film-based solar cells has been achieved with CdS buffer layers prepared by a solution growth met hod known as the chemical bath deposition (CBD). With the aim of developing Cd-free chalcopyrite-based thin-film solar cells, we describe the basic co ncepts involved in the CBD technique. The recipes developed in our laborato ry for the heterogeneous deposition of good-quality thin films of ZnO, ZnSe , and MnS are presented. In view of device optimization, the initial format ion of chemical-bath-deposited ZnSe thin films on Cu(Ga,In)(S,Se)(2) (CIGSS ) and the subsequent development of the ZnSe/CIGSS heterojunctions were inv estigated by X-ray photoelectron spectroscopy (XPS). The good surface cover age was controlled by measuring changes in the valence-band electronic stru cture as well as changes in the In4d, Zn3d core lines. From these measureme nts, the growth rate was found to be around 3.6 nm/min. The valence band an d the conduction band-offsets Delta E-V and Delta E-C between the layers we re determined to be 0.60 and 1.27 eV, respectively for the CIGSS/ZnSe inter face. The energy-band diagram is discussed in connection with the band-offs ets 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 w ith a total area efficiency of 13.7%.