Combining strong interface recombination with bandgap narrowing and short diffusion length in Cu2ZnSnS4 device modeling

In this work we establish a device model in SCAPS, incorporating bandgap narrowing, short minority carrier diffusion length and interface recombination. The model is based on a reference device with standard structure; sputtered Mo on soda lime glass, a reactively sputtered and annealed Cu2ZnSnS4 (CZTS) absorber layer, chemical bath deposited CdS and sputtered i-ZnO buffer layers, and front contact formed with sputtered ZnO:Al and an evaporated Ni/Al/Ni grid. The efficiency of the reference device is 6.7%. Model parameter values of the absorber layer are based on the analysis of temperature dependent current-voltage (I-V-T) measurements, capacitance-voltage (C-V) and drive-level capacitance profiling (DLCP) measurements, performed on the reference device, and on the comparison of simulated and measured quantum efficiency (QE) and current-voltage (J-V) performance. Additional parameters are taken from literature. The key elements, electron-hole pair generation and recombination in the absorber layer, are the main focus in this study. Reported values of the absorption coefficient of a is vary around one order of magnitude when comparing data from reflectance transmission (R-7) measurements with ellipsometry measurements, and calculations. Therefore, a modified semi-empirical absorption coefficient, extracted from R-T and QE measurements, with the depletion width from CV and DLCP, is presented and used in this study. The dominating recombination path is evaluated with J-V-T analysis and the zero Kelvin activation energy (E-A,E-0) is extracted from both temperature dependent open circuit voltage (V-OC) and from modified Arrhenius plots. In each case, E-A,E-0 is found to be substantially smaller than the bandgap energy, even when considering bandgap narrowing due to disorder, which is an indication that the V-OC deficit observed in our CZTS device dominated by interface recombination. Finally, a complete device model is established, with J-V and QE simulations in good agreement with corresponding measurements, where the interface has the biggest impact on the V-OC deficit, but with clear contribution from bulk recombination, with minority carrier diffusion length 250 nm, and from bandgap narrowing, giving a lower than nominal bandgap energy of 1.35 eV. (C) 2015 Elsevier B.V. All rights reserved.