Defect Pool Numerical Model in Amorphous Semiconductor Device Modeling Program

Amorphous Semiconductor Device Modeling Program (ASDMP), developed by Professor P. Chatterjee and widely validated by experimental results, is a detailed program where the Poisson’s equation and the electron and hole continuity equations are simultaneously solved without any simplifying assumption. It takes into account the trapping and recombination kinetic through the gap states. In this program, the density of states is modeled using the standard model (SM). Such a model describes the defects by two Gaussians near the center of the gap and two tails exponentially distributed in energy, and assumes the density of states homogenous in space. The defect pool model (DPM) is an improved model for defect formation in hydrogenated amorphous silicon based on the idea that the a-Si:H network has a large spectrum of local environments at which a defect could be formed. So, these defects choose the sits where their formation energy is minimal and this becomes possible with hydrogen motion.Using the defect pool approach, we have developed a numerical DPM and inserted it in ASDMP at thermodynamic equilibrium. We have used ASDMP to get the density of states in each position of a solar cell based on a standard p-i-n structure. We have shown the effect of doping on defect concentration and studied the impact of the position of Fermi level on the distribution of the density of states. We recognized using ASDMP the key result that negatively charged defects in n-type material are situated lower in energy than positively charged defects in p-type material even when the correlation energy is positive.We calculated the electric field and the band diagrams at thermodynamic equilibrium both with the DPM and the SM. We showed that the electric field obtained from the DPM is stronger near the interfaces and lower in the bulk where the band diagrams are flatter. This behavior of the electric field calculated with this model is accentuated with the increase of the slope of the valence band tails.