Interfacial layer formations between Cu(In,Ga)Se2 and InxSy layers

Cu ( In , Ga ) Se 2 (CIGS) thin-film solar cells with InxSy buffer layers deposited by physical vapor deposition yield efficiencies of up to 14.8%. For substrate temperatures during the InxSy deposition ranging from 23to200°C, air annealing of the completed solar cells leads to an improvement of the photovoltaic performance. However, at substrate temperatures of 300°C, the efficiencies are practically zero, and air annealing does not improve this value. To understand the effects of substrate temperature and air annealing on the CIGS/InxSy interfaces of the solar cells, these interfaces have been studied by means of bright-field and high-resolution transmission electron microscopy, selected-area electron diffraction (SAED), and energy-dispersive x-ray spectrometry (EDX). It is shown that air annealing leads to a substantial Cu depletion on the CIGS side of the CIGS/InxSy interface, probably inducing the formation of a compositionally graded interface between the buffer and CIGS. For the 300°C sample, CuIn5S8 formed instead of InxSy, as identified by means of SAED and EDX. The large density of vacancies and defects in the spinel-type cubic crystal structure of CuIn5S8 provides a large number of recombination centers at the heterojunction of the solar cell, thus deteriorating considerably its photovoltaic performance.