Photogating effect as a defect probe in nanocrystalline silicon cells

Abstract

The measurement of the spectrally resolved collection efficiency is of great importance in solar cell characterization. Under standard conditions the bias light is a solar simulator or a light source with a similar broadband irradiation spectrum. When a colored blue or red bias light is used instead, an enhanced collection efficiency effect, in the literature known as the photogating effect, can be observed under certain conditions. While most of the published reports on such effect were on solar cells with amorphous silicon based absorber layers, we have shown that the enhanced collection efficiency could be also present in thin film silicon solar cells where hydrogenated nanocrystalline silicon (nc-Si:H) is used as the absorber layer. In this article we present detailed experimental results and simulations aiming at a better understanding of this phenomenon. We show that the collection efficiency is strongly dependent on the intensity of bias light and the intensity of the monochromatic light. These experimental results are consistent with the computer predictions made by our code. We also show that the photogating effect is greatly enhanced when nanocrystalline silicon cells are built with an improperly doped p-layer or with a defective p/i interface region due to the reduced internal electric field present in such cells. The existence of this effect further proves that carrier transport in a nc-Si:H solar cell with an i-layer made close to the phase transition regime is influenced to a large extent by drift transport. The study of this effect is proposed as an alternative approach to gain a deeper understanding about the carrier transport scenarios in thin film solar cells, especially nanocrystalline silicon solar cells.