Dark I-V characteristics and defect states in amorphous silicon solar cells

One of the biggest challenges to improve the performance and stability of hydrogenated amorphous silicon (a-Si:H) based solar cells is to minimize the density of defect states in the intrinsic amorphous silicon layers. These defect states determine the lifetime of photo-generated carriers and thus the efficiency of solar cells. However, the understanding of defect states is still very limited due to a lack of accurate and reliable methods for characterizing their densities and distribution in the gap of a-Si:H. In this study a novel characterization method for the distributions of defect states in the intrinsic amorphous silicon materials has been developed which is based on the detailed analysis of dark forward bias current-voltage (I-V) characteristics of p-i-n solar cells. In these dark I-V characteristics, the contributions from different part of the cell have been identified and separated so that the courier recombination in the bulk of the i-layer can be directly characterized. Through this new method the defect state distributions in different i-layer materials are directly obtained from the dark I-V characteristics which lead to the first quantitative explanation on the differences between these materials. Dark currents at small forward biases have also been used to characterize the creation kinetics of metastable defect states in the midgap induced both by illumination and carrier injection which yield results similar to those reported previously. More detailed analysis however clearly suggests the presence of fundamentally different mechanism for the creation of the metastable states than those currently being proposed. Dark currents have also been used in characterizing the annealing of light induced defect states which leads to the first observation of annealing at room temperature. Finally, the defect state distributions obtained in this study are successfully correlated with the degradation under illumination of solar cell parameters having different intrinsic layers.