Steady-state distributions of carrier concentration and recombination rates in a solar cell under operating conditions

Abstract

The steady-state distribution of carrier concentrations in a solar cell under operating conditions is a key source of carrier recombination and directly influences the output current density. In this study, we investigated the effects of illumination and bias voltage on the steady-state distributions of carrier concentrations and recombination rates in a homo-pn junction solar cell using one-dimensional numerical simulations to explore passivation strategies driven by the reduction of carrier concentrations. Simulations under open circuit conditions revealed that controlling the standard states of carriers can enhance the open circuit voltage without changing the carrier concentration. Furthermore, distributing recombination rates conventionally concentrated in the vicinity of the interface into the bulk region, such as within the absorbing layer, improves the open circuit voltage. Our results also showed that changing the carrier distribution in the dark to that under illumination, in other words, transition from an equilibrium to a steady state, is governed by drift rather than diffusion. This means that the electric field, which induces drift, is a primary driving force for carrier separation. Consequently, by optimizing the electric field distribution depending on properties of recombination-causing defects, a higher short circuit current density can be achieved, even without chemical passivation of defects. This work offers fundamental insights into reduction of steady-state recombination rates through precise control of carrier concentration, chemical potential, and electric field distributions.