Nonequilibrium theory of the conversion-efficiency limit of solar cells including thermalization and extraction of carriers

TL;DR

This paper develops a nonequilibrium theoretical framework for solar cell efficiency limits, including effects of thermalization and carrier extraction, to understand losses and device behavior beyond traditional detailed balance assumptions.

Contribution

It introduces a model that incorporates carrier relaxation and extraction dynamics, extending the Shockley-Queisser limit to nonequilibrium regimes and analyzing conditions where conventional theory applies or fails.

Findings

Reformulation of the solar cell efficiency theory to include nonequilibrium effects.

Identification of parameter regimes where traditional SQ theory is valid or breaks down.

Insights into the impact of carrier extraction time on efficiency limits.

Abstract

The ideal solar cell conversion efficiency limit known as the Shockley-Queisser (SQ) limit, which is based on a detailed balance between absorption and radiation, has long been a target for solar cell researchers. While the theory for this limit uses several assumptions, the requirements in real devices have not been discussed fully. Given the current situation in which research-level cell efficiencies are approaching the SQ limit, a quantitative argument with regard to these requirements is worthwhile in terms of understanding of the remaining loss mechanisms in current devices and the device characteristics of solar cells that are operating outside the detailed balance conditions. Here we examine two basic assumptions: (1) that the photo-generated carriers lose their kinetic energy via phonon emission in a moment (fast thermalization), and (2) that the photo-generated carriers are extracted into carrier reservoirs in a moment (fast extraction). Using a model that accounts for the carrier relaxation and extraction dynamics, we reformulate the nonequilibrium theory for solar cells in a manner that covers both the equilibrium and nonequilibrium regimes. Using a simple planar solar cell as an example, we address the parameter regime in terms of the carrier extraction time and then consider where the conventional SQ theory applies and what could happen outside the applicable range.