Development, characterization, and modeling of copper gallium diselenide/copper(indium,gallium)diselenide thin film tandem solar cells

Device modeling and numerical simulation of mechanically-stacked CuGaSe 2/Cu(In1-xGax)Se2 (CGS/CIGS) tandem solar cells were carried out in this work using AMPS-ID (analysis of microelectronic and photonic structures) device simulation program. The CGS/CIGS two junction tandem cell consists of a CIGS bottom cell using a double graded band gap CIGS absorber layer and a CGS top cell with a uniform band gap CGS absorber layer. Simulated photo-current-voltage and spectral response (quantum efficiency) characteristics for both single junction CIGS and CGS cells were found in good agreement with the experimental data. Simulation results revealed that a CGS/CIGS tandem cell with a conversion efficiency of 25% under AM1.5G condition could be achieved using a CGS top cell with an efficiency of over 16% and an optimized CIGS bottom cell structure.; To determine the effect of the relative content of zinc, x, in the chemical bath deposition (CBD) bath on the Cd1-xZn xS (CdZnS) thin-film properties, the structural, surface morphological, optical, and electrical characterizations are conducted by using XRD, SEM, spectrophotometer, and resistivity measurements. The degree of crystallinity in the film decreases with increasing relative Zn content x, leading to an amorphous structure at a concentration level of x = 0.5. CdZnS films with film thickness less than 50 nm and relative Zn composition x = 0.3 show better than 80% transmittance for wavelengths longer than 600 nm. Band gap increases with increasing Zn content x. The results reveal that a CdZnS film with a relative Zn content of x = 0.3 in the CBD bath is an excellent candidate as a buffer-layer material for the CIGS and CGS cells.; In this study Cd1-xZnxS buffer layers were deposited on CIGS absorber films with relative Zn content x = 0 (CdS), 0.1, 0.2, 0.3, 0.4, and 0.5 in the CBD bath and on CGS absorber films with x = 0 and 0.3. In both the CIGS and CGS cases, the experimental results show that it is beneficial to pre-treat the absorber layer with a 10% KCN solution before CBD growth. A CdZnS/CIGS cell with relative Zn composition of x = 0.2 achieves an efficiency of approximately 13% under AM1.5G condition, showing improved Voc, Jsc, and FF values as compared to the CdS/CIGS cells and other CdZnS/CIGS cells with different Zn content. For CGS cells, it is demonstrated that the CdZnS buffer layer improves V oc value which is attributed to the reduced conduction band discontinuity at the junction. It is also observed that the CdZnS/CGS cell has increased the photocurrent generation at shorter wavelengths (lambda < 500 nm), resulting in a higher Jsc value than the CdS/CGS cells due to higher quantum efficiency.