Alternative buffer layers, device modeling and characterization of copper-indium-diselenide-based thin-film solar cells

The goal of this research is aimed at developing Cd-free alternative buffer layers and performing the device modeling and numerical simulations of optimized device structures and parameters for enhancing the performance of copper-indium-diselenide (CIS)-based thin-film solar cells. To facilitate the characterization of CIS-based cells a reflection mode dual beam optical modulation (DBOM) technique was developed for determining the excess carrier lifetimes in the CIS and Cu(In,Ga)Se2 (CIGS) films. Additionally, the computer-controlled automatic current-voltage (I-V) and spectral response (quantum efficiency) measurement systems with LabVIEW programs for controlling the measurement procedures and data acquisition were developed.; Several buffer-layer materials including CdS, (Cd,Zn)S, ZnS, and In(OH) _xS_y deposited by the chemical-bath-deposition (CBD) process were investigated for the cell fabrication. Among the CIS-based cells with CdS, (Cd,Zn)S, ZnS, or In(OH)_xS_y buffer layers studied in this research, the CIGS/CdS cell gave the best performance. A higher short-circuit current density was obtained for the CIGS/ZnS cell than for the CIGS/CdS cell.; A comprehensive study of the device modeling and simulation of CIGS solar cells with an emphasis on the band-gap engineering of the CIGS absorber layers was carried out. The device physics and performance parameters of the CIGS absorber layers with different band-gap profiles were analyzed. Additionally, the dependence of performance parameters of the CIGS cells on the material parameters of the Cu-poor surface defect layer in the CIGS absorber layers was investigated.; A reflection mode DBOM technique was developed to investigate the effects of various CdS-buffer-layer processes on the excess carrier lifetimes in the CIGS films. To explain these DBOM results, a modified analytical DBOM formulation for the carrier lifetime of CIGS p-n junction cells was derived.; To sum up, the major contributions of this research include an assessment of Cd-free buffer layers for improving the performance of CIS-based cells, analysis of impacts of band-gap profiles and junction parameters on device performance, and development of the computer-controlled device characterization systems for measuring the performance parameters of the cells. The optimization of buffer-layer process and enhancement of junction quality by laser annealing process to further improve the performance of CIS-based cells are suggested for future studies.