Nanoscale charge transport and local surface potential distribution to probe the defect passivation in Cr-substituted earth abundant CZTS absorber layer

Abstract

The non-stochiometric deviations in CZTS often results in certain detrimental point defects that act as trap centers causing non-radiative recombination and potential fluctuations, eventually results in large open circuit voltage (VOC) deficit in CZTS solar cells. Cationic substitution in CZTS layer is one of the leading approaches to capture the great potential of kesterite solar cells for future cost-effective photovoltaic technology. Here, chromium incorporation in CZTS is studied for cationic substitution in CZTS layer, as it renders multiple chemical states including +1, +2 and + 4 of Cu, Zn and Sn cations in CZTS. The CZTS films determine to have good crystallinity and morphology with no significant disparity among pristine and Cr-doped CZTS films, apart from slight shift of XRD (112) peak indicating partial cation substitution by smaller Cr atoms. However, the Zn content is found to be decreased with increasing Cr content in CZTS layer. Substantial enhancement in the absorption of Cr-doped CZTS films indicate intermediate band (IB) within the band gap. Photoluminescence (PL) emission spectra strongly suggests reduced non-radiative recombination and potential fluctuations in Cr-CZTS films. X-ray photoelectron spectroscopy investigations reveal Zn substitution by Cr in CZTS crystal geometry. Using nanoscale Kelvin Probe Force Microscopy (KPFM) and Conducting Atomic Force Microscopy (CAFM), we verify the surface potential variation and nanoscale electrical conductivity in pristine and Cr-CZTS films. Additionally, Zn substitution by Cr eventually lead to suppression of ZnSn deep level defects segregated at grain boundaries (GBs). CAFM strongly confirms the GB passivation in Cr-CZTS films. This work widens the opportunity of exploring potential cationic substitution in CZTS for developing high efficiency CZTS solar cells.