Introducing Bi2S3 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 BiSex 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.