Theory of amorphous silicon solar cell (b): a five layer analytical model

An analytical model for amorphous silicon p–i–n solar cell is developed by dividing the i-layer into five regions. The region close to p+ layer is dominated by charge in the tail states, whereas the region next to it has a space charge due to positively charged dangling bonds. This is followed by a quasi-neutral region with a nearly constant electric field. Then, there is a space charge arising due to negatively charged dangling bonds with a conduction band tail dominated space charge region adjacent to n+ layer. Poissonʹs equation and continuity equation for electrons and holes are solved to obtain the current–voltage characteristic of the cell. An interesting finding of the analysis is that the electric field in the quasi-neutral region increases significantly in short circuit condition when illuminated by light. This is due to neutralization of dangling bonds in the space charge region. When the voltage across the cell is allowed to develop as one moves from short circuit to open-circuit condition, the electric field in the central part of the cell reduces resulting in a loss of collection efficiency and hence a poorer fill factor.