Determination of Effective Lifetime and Light Trapping Enhancement in Silicon using Free Carrier Absorption

Abstract

A novel experimental technique has been developed for measuring the light trapping enhancement and the carrier recombination lifetime in silicon wafers. The technique is based on the pump/probe modulated free carrier absorption (MFCA) method, where the probe beam, attenuated by generated free carriers, carries information about the effective lifetime and the average light path enhancement in a textured silicon wafer. For the first time, a reflection mode MFCA technique is presented where the reflected part of the probe beam is used to perform measurements, while the conventional technique is based on measurements of the transmitted part of the probe beam. A theoretical model is presented to explain the behavior of the light beam in double-side polished and double-side textured silicon wafers. The model yields good agreement with the experimental results and explains the difference in the amplitudes of the reflected and transmitted signals. The results of the experimental measurements of the light path enhancement in a double-side textured sample are analyzed and the reasons for their deviation from the Lambertian limit are discussed. This work presents new applications of the MFCA technique and shows how it can be used for the simultaneous determination of more than one crucial characteristic of silicon solar cells.