Contactless determination of the injection dependent carrier mobility sum in silicon

Abstract

The carrier mobility is a key parameter in determining the performance of silicon solar cells. The carrier mobility decreases with both injection level and temperature. Since solar cells often operate under a wide range of injection levels and temperatures, it is thus fundamental to determine the impact of those parameters on the carrier mobility. Numerous techniques exist to measure mobility, and the impact of dopant concentration and temperature is thus well known in crystalline silicon. Nevertheless, data on the impact of injection or the simultaneous impact of injection and temperature on the carrier mobility are scarce. Furthermore, the effect of dopant compensation on carrier mobilities, especially for minority carriers, remains unclear. Dannhauser and Krausse used a combination of infrared radiation and voltage measurements across a pin diode to extract the injection dependence of the mobility sum. Similarly, Neuhaus et al. used a combination of Quasi-steady-state photoconductance (QSS-PC) and Quasi-steady-state open-circuit-voltage (QSS-Voc) measurements on a solar cell to determine the injection dependence of the mobility sum. However, both of these methods require a complicated structure and need contacting. This can make it more difficult to perform temperature dependent measurements. In this paper we present a new method to determine the simultaneous injection and temperature dependence of the sum of the majority and minority carrier mobilities. Contrary to the method proposed by Neuhaus et al., this method doesn't require a full cell structure, only surface passivation. The mobility sum is determined by combined measurement of transient and Quasi-steady-state photoconductance measurements. The mobility dependence on both injection and temperature is discussed and compared to several models. The potential of this method to measure the impact of dopant concentration, compensation ratio, injection level and temperature on the mobility is demonstrated.