Charge Transport Layer Capacitance Contribution to Si Solar Cell Optoelectronic Properties Investigated Using Photocarrier Radiometry

The question of identifying the dominant capacitance in Si solar cell optoelectronic transport phenomena under open circuit conditions is explored using quantitative noncontacting laser photocarrier radiometry (PCR) as a dynamic spectrally gated photoluminescence method. The combined theoretical and experimental approach addresses the dependence of the PCR signal on the capacitance of the charge transport layer (CTL) and of the base layer of the p-n junction which, in conjunction with the solar cell series resistance, form RC diffusive time constants shown to be sensitively measurable using PCR frequency scans. The experimental strategy is based on the dependence of layer capacitance on the laser-illuminated area and involves frequency responses under partial or total surface area illumination. It is shown that the recombination lifetime and CTL diffusion lifetime ${{\tau }_{RC}}$ are mainly responsible for the kinetics and extraction of nonequilibrium optically generated carrier density waves in three types of photovoltaic Si solar cells. It is concluded that ${{\tau }_{RC}},$which is related to the thin upper CTL and the associated recombination lifetime, plays the main role in the dynamic optoelectronic PCR frequency response of all tested devices. ${\rm{Through\ the\ capacitance\ of\ this\ layer}},\ {{\tau }_{RC}}$ strongly affects free photocarrier transport across the p-n junction, surface distribution, and electrode collection, therefore CTL capacitance should become a major focus of solar efficiency enhancement designs and studies.