Effect of calcium incorporation on properties of Cu(InGa)Se2 thin film and solar cell

Abstract

Cu(InGa)Se2 (CIGS) thin films with Ca doping from CaF2 film synthesized onto quartz glass substrates via a two-step process were studied. XRD peak splitting ascribed to graded compositional distribution existed in pristine CIGS film but disappeared in those with Ca doping. Depth profiles verified that the Ca element mainly existed in the near surface region of CIGS film with CaF2 post-deposition treatment (PDT) while diffused throughout the absorber with CaF2 pre-deposition. The grain sizes in surface of CIGS films without doping and with CaF2 PDT were much larger than those with CaF2 pre-deposition. The Fermi level of film with pre-deposited CaF2 showed no change while that of film with CaF2 PDT exhibited lower energy compared to the pristine film. Performance degradation due to inferior Mo/CIGS interface was observed in device with CaF2 pre-deposition. Inversely, PDT of CaF2 enhanced the device performance with an absolute efficiency of 2.2 %. SCAPS-1D was used to simulate the CIGS devices for further study. It showed a shallower defect position in solar cell with CaF2 PDT, giving explanation for its better performance.