Introducing Bi ₂ S ₃ Interlayer to Synergistically Modify Mo Back Contact and Regulate Bulk Defects in Kesterite Solar Cells

A suitable interlayer between the Mo back electrode and kesterite absorber layer has been proven to have a positive effect on limiting the bulk defects of the absorber by the constitute diffusion. Here, a thin Bi₂S₃ layer is used as the back-interface intermediate layer for the first time, this innovative approach allows for simultaneous modification of the back contact and reduction of bulk defects, resulting in improving the power conversion efficiency of the kesterite device from 9.66% to 11.8%. The evaporated Bi₂O₃ thin films turn into the Bi₂S₃ interlayers after sintering the Cu₂ZnSnS₄ precursor thin films. The Bi₂S₃ interlayer can inhibit the decomposition reaction of back contact and suppress the formation of the secondary phases. It can also optimize the Fermi level offset and promote the separation of the photoinduced carriers, resulting from its characteristic of high work function. Besides, a small part of the Bi element can diffuse into Cu₂ZnSn(S, Se)₄ film and induce the crystal growth and restrain Zn-related defects, which is attributed to forming the low melting-point liquid BiSe_x phase during the high-temperature selenization process. The conclusions highlight the bifunction of the thin Bi₂S₃ intermediate layer, which can provide a new approach to improve the photoelectric conversion efficiency of kesterite solar cells.