Space charge limited current effect on photoconductive decay in silicon at high injection levels

Abstract

The space charge limited current (SCLC) effect was analyzed on undoped crystalline silicon wafers at high injection levels. Space charge limited currents develop when the electric field from the injected carriers exceeds that of the background doping. This becomes a relevant phenomenon when applied to materials that use undoped wafers such as back-contact solar cells. It may also be applicable to silicon and non-silicon concentrator cells. The SCLC effect may be significant in the epitaxial thin film materials used in space photovoltaics. Our study uses Resonant Coupled Photoconductive Decay (RCPCD) to analyze and view SCLC via photoconductive carrier lifetime measurements. An undoped Si sample was subjected to the high injection of excess carries by using a pulsed laser source. The excess carrier densities were well above background doping, creating an excess carrier density of approximately 3 × 1017 cm−3. This is well above the background carrier density of 1.73 × 1013 cm−3 (measured by Capacitance-Voltage techniques). The SCLC is detected by observing irregularities, such as positive slope of the photoconductive decay curve in the initial, very high injection portion of the process. A theory of SCLC will be developed and data will be presented.