Study of the Parallel Vertical-Junction Silicon Solar Cell Under External Electrical Bias: Determination of Dark Capacitance and Diffusion Potential

Abstract

This study presents a theoretical analysis of a parallel vertical-junction silicon solar cell under external electrical bias. The focus is on the influence of a biasing electric field on the solar cell's performance under constant multispectral illumination at steady state. The analytical solution of the continuity equation, incorporating the quasi-neutral base approximation and boundary conditions that account for the recombination velocity at each junction, has enabled the derivation of an expression for the density of photogenerated minority charge carriers in the base. Consequently, expressions for photocurrent, photovoltage, electrical power, and capacitance have been derived and analysed as functions of the electric field and the minority-charge-carrier recombination velocity at the junction. Analysis of the results shows that applying an external electric field enhances the movement of minority charge carriers toward the junctions, thereby reducing recombination losses and promoting photocurrent generation by improving carrier collection. This movement of a large number of minority charge carriers toward the junctions under the influence of the electric field increases their density near the junctions. Consequently, the short-circuit photocurrent, open-circuit photovoltage, junction capacitance near open circuit, and maximum electrical power delivered by the solar cell all increase. The study also reveals a decrease in the diffusion potential as the electric field increases. Finally, the capacitance-photovoltage characteristic enables determination of the dark capacitance and the diffusion potential of the solar cell. The results show a relatively small increase in open-circuit voltage (Voc), from 0.58613 V at E = 0 V/cm to 0.58619 V at E = 6 V/cm. Short-circuit current density (Jsc) increases from 0.1032 A/cm² at E = 0 to 0.4168 A/cm² at E = 6 V/cm, representing a significant change. The diffusion potential (VD) decreases from 0.73 V to 0.42 V. The maximum power (Pmax) increases from 0.0507 W to 0.2348 W. Applying an external electric field to the parallel vertical-junction silicon solar cell is therefore an effective way to improve its electrical performance, which may prove useful for optimisation purposes. This work will be extended to include an experimental study of these phenomena and an analysis of the simultaneous influence of the electric field and temperature on the solar cell's electrical parameters.