Modelling free-carrier absorption in solar cells

Free-carrier absorption can be a significant parasitic optical absorption process in solar cells. Although estimates of its influence have been made in the past, it has not previously been incorporated into a numerical semiconductor device simulator and studied in conjunction with other effects. A finite element model of free-carrier absorption is presented that incorporates the dependency of the absorption coefficient 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 on several types of silicon solar cells. It was found to have only a very small effect on cell efficiency, but can significantly affect the long-wavelength spectral response, which has implications for device characterization. Empirical equations for the behaviour of free-carrier absorption in a variety of materials are presented. © 1997 John Wiley & Sons, Ltd.