Modeling the charge density profile in thin film devices using SIMS profiling data

To obtain an insight into the degradation kinetics of thin film solar cells, we developed a theory to model the variation of total charge density across the absorber layer of CdS/CdTe thin film solar cells. The variation in total charge density has been generally ignored in the literature despite being the original reason of instability in device parameters. We used SIMS profiling data reported in the literature to track the Cu concentration across CdTe layer and it's variation under short- & open-circuit conditions. Cu migration under the stress of elevated temperature and prolonged irradiation is known to be the major degradation source in CdTe layer by increasing the charge accumulation at the junction and creating a barrier to carrier passivation. We obtained the Cu distribution functions before/after stressing the cell and imported these fitting functions in our model to calculate the real total charge density and electric field variation at both front/back regions. Our simulation indicates that stressing the cell cause charge accumulation at interface/junction of CdTe/metal and CdS/CdTe and reduces the electric field and leads to the relevant degradation trend. The depletion width is also reduced by increased Cu concentration under the stress. This model is capable to be developed to include the real distribution of vacancies, defects, doping and ions (obtained by SIMS or other spectroscopy methods) to calculate the total charge density, electric field, resistivity of solar cells stressed under various light, temperature and biasing conditions. The worsen stress condition is predicted to be the open-circuit condition.