Highly efficient CIGS solar cells based on a new CIGS bandgap gradient design characterized by numerical simulation

Optimization of Cu(In,Ga)Se2 (CIGS) bandgap gradient by modifying Ga gradient is an important approach to get highly efficient CIGS solar cells. In this work, we propose a new type of CIGS bandgap gradient, tetra-gradient or W-shaped gradient bandgap (TGB), and compare it with other three existed types, including flat bandgap (FB), single gradient bandgap (SGB) and double gradient/V-shaped bandgap (DGB). In TGB of CIGS layer, the bandgap distribution contains two bandgap minima from the front surface to the back side, and the bandgap gradient distribution presents a 'W' shape. The device performance was characterized through wxAMPS software. It is found that two narrowed bandgap regions of the TGB increase the absorption probability of photons at long wavelengths, thereby improving the short circuit current of CIGS devices. In addition, TGB can effectively increase the open circuit voltage and the resultant photoelectric conversion efficiency of CIGS solar cells. The maximum efficiency of the CIGS solar cells based on FB, SGB, DGB and TGB are simulated to be 20.49%, 22.91%, 23.00% and 23.35%, respectively. The new bandgap gradient proposed in this work can be achieved experimentally by adjusting Ga gradient. This work reveals that further optimization of the bandgap gradient of CIGS layer is possible to achieve a further breakthrough in the efficiency of CIGS solar cells.