The role of bulk and interface states on performance of a-Si : H p-i-n solar cells using reverse current–voltage technique

Abstract

The defect state densities in the bulk of the i-layer and at the p/i interface have been studied in hydrogenated amorphous silicon (a-Si : H) solar cells using reverse current–voltage (J–V) measurements. In this work the cells have been soaked with blue and red lights prior to measurements. The voltage-dependent reverse current has been analysed on the basis of thermal generation of the carriers from midgap states in the i-layer and the carrier injection through the p/i interface. Based on the reverse current behaviour, it has been analysed that at lower reverse bias (reverse voltage, Vr < 5 V) the defect states in the bulk of the i-layer and at higher bias (Vr ∼ 25 V) the defect states at the p/i interface are contributing to the reverse currents. The applied reverse bias annealing (RBA) treatment on these cells shows more significant annihilation of defect states at the p/i interface as compared with the bulk of the i-layer. An analytical model is developed to explain the observed behaviour. There is good agreement between the theory and the experimental observations. The fitted defect state densities are 9.1 × 1015 cm−3 and 8 × 1018 cm−3 in the bulk of the i-layer and near the p/i interface, respectively. These values decrease to 2.5 × 1015 cm−3 and 6 × 1017 cm−3, respectively, in the samples annealed under reverse bias at 2 V.