An Approach to Study Band-gap and Interface States in a-Si:H p-i-n Solar Cells

Abstract

The defect states in bulk of i-layer and at p/i interface and their energy locations in a-Si:H p-i-n solar cells have been studied by using dark reverse current–voltage ( J–V ), capacitance–voltage–temperature–frequency (C–V–T–ω) and deep level transient spectroscopy (DLTS) measurements. The dark reverse currents as a function of voltage have been analyzed on the basis of thermal generation of the carriers. In current–voltage characteristics, it has been observed that at low reverse voltages (V<5 V) the defect states in the bulk of the i-layer and at high reverse voltages (V∼24 V) the defect states at p/i interface are contributing to the reverse current. Reverse bias annealing (RBA) treatment has been performed on these cells. It has been observed that RBA reduces the defect states more in the i-region near to the p-layer and at the p/i interface as compared to the deep regions in the bulk of i-layer. The calculated defect state density (DOS) is varying from its intrinsic value of 4.8×1016 cm-3 in the bulk of i-layer upto 1.8×1018 cm-3 near and at p/i interface. These values decrease to 1.8×1016 cm-3 and 6.6×1016 cm-3, respectively, in the samples annealed under reverse bias at 2 V. The capacitance as a function of temperature under fixed voltage of 0.5 V and frequency of 100 Hz is divided into three regions depend on the capacitance behavior. The RBA treatment has showed prominent changes only in the third region (T>350 K) which is due to the defects at p/i interface. The DOS in the bulk of i-layer and at p/i interface have been determined. The hole and electron trap states densities have been estimated to be 1.8×1016 eV-1 cm-3 and 1.6×1016 eV-1 cm-3, respectively, at 0.8 eV and 1.12 eV above the valence band from the DLTS measurements. The existence of interface states at 0.55 eV and 1.2 eV above the valence band, respectively, for p/i and i/n interfaces have also been observed. A good correlation has been observed in DOS estimation and their affect by RBA treatment in all the studies.