Simulation of radiation effects in solar cells: DLTS vs. SRIM for trap data

We present a predictive computational approach that may reduce the need for extensive inputs from Deep Level Transient Spectroscopy (DLTS) experiments. Three-dimensional NanoTCAD simulations are used for physics-based prediction of space radiation effects in a p+n GaAs solar cell with AlGaAs window, and validated with experimental data. The computed dark and illuminated I-V curves, as well as corresponding performance parameters, matched experimental data very well for 2 MeV proton irradiation at various fluence levels. We analyze the role of majority vs. minority and deep vs. shallow carrier traps in the solar cell performance degradation. The defects level parameters used in the simulations were taken from DLTS data obtained at NRL. It was determined from numerical simulations that the degradation of the photovoltaic parameters could be modeled and showed similar trends when a only a single deep level defect was considered compared to a spectrum of defect levels. This led to the development of an alternate method to simulate the degradation of a solar cell by using only a single deep level defect whose density is calculated by the Stopping and Range of Ions in Matter (SRIM) code. Using SRIM, we calculated the number of vacancies produced by 2 MeV proton irradiation for fluence levels ranging from 6x1010 cm-2 to 5x1012 cm-2. Based on the SRIM results, we applied trap models in NanoTCAD and performed I-V simulations from which the degradation of the photovoltaic parameters (Isc, Voc, FF, Pmax) was calculated. The simulations using SRIM-derived defect concentrations showed reasonable agreement with simulations using parameters determined from DLTS.