Investigation of buffer layer process on CIGS solar cells by dual beam optical modulation technique

A contactless, nondestructive reflection mode dual beam optical modulation (DBOM) technique has been employed to investigate the effects of different CdS buffer layer processings (i.e., CBD, MOCVD, and sputtering) on the properties of CIS/CIGS films. The results show a significant increase in the DBOM signal (ΔI/I) which is attributed to change of the free carrier absorption and of excess carrier lifetime in the absorber layer, after the buffer layer processing. In order to explain the observed DBOM results, an analytical model for the CIGS P-N junction cell is derived which contains the functional dependence of ΔI/I on the surface/interface recombination velocities, excess carrier lifetimes, diffusion length and depletion layer width in the CIS or CIGS cells. A detailed comparison of the DBOM data versus different processing is presented in this paper