MODELING FREE-CARRIER ABSORPTION IN SOLAR-CELLS

Free-carrier absorption can be a significant parasitic optical absorpt ion process in solar cells, Although estimates of its influence have b een made in the past, it has nor previously been incorporated into a n umerical semiconductor device semiconductor and studied ill conjunctio n with other effects. A finite element model of free-carrier absorptio n is presented that incorporates the dependency of the absorption coef ficient on the carrier concentration profile, including the change in carrier density that occurs across a single finite element. This model has been implemented in the semiconductor modelling program PC1D for Windows, and used to simulate the effects of free-carrier absorption o n several types of silicon solar cells. It was found to have only a ve ry small effect on cell efficiency, but can significantly affect the l ong-wavelength spectral response, which has implications for device ch aracterization. Empirical equations for the behaviour of free-carrier absorption in a variety of materials are presented. (C) 1997 John Wile y & Sons, Ltd.