Figure 1. Schematic of ZnO/CdS/CIGS solar cell structure (a),(b) ZnO/CdS/CIGS-Si solar cell CIGS structure

Where ψ the electrostatic potential, the free electron n and free hole p, trapped electron n_t and trapped hole p_t as well as ionized donor-like doping N_D⁺ and ionized acceptor-like doping N_A^- concentrations are all given in terms of the different band gap energy levels by the Boltzmann and Fermi-Dirac distributions, and are a function of the position “x”. J_n and J_p are the electron and hole current densities respectively. The term G_op is the optical carrier generation rate due to illumination and R is the total recombination rate accounting for direct (band to band) and indirect (Shockley-Read-Hall) recombination traffics.