The Concentration Limit for Solar Cells Based on Entropy Production

TL;DR

This paper revises the maximum concentration limit for solar cells by incorporating entropy generation during photovoltaic conversion, revealing a bandgap dependence and lower limits for low-bandgap materials, challenging traditional assumptions.

Contribution

It introduces a bandgap-dependent concentration limit for solar cells considering entropy production, contrasting with the classical invariant limit derived from thermodynamics.

Findings

Maximal concentration limit depends on the semiconductor bandgap.

Entropy generation reduces the concentration limit for low-bandgap materials.

Traditional models assuming invariant limits are challenged by this analysis.

Abstract

The maximal concentration limit of solar radiation for photovoltaic applications is assumed to be constant, at Cmax\approx46,000 suns. This limit is easily found via a geometrical application of the 2nd law of thermodynamics to a radiation transfer system [A. Rabl, Sol. Energy vol. 18, pp. 93-111, 1976]. However, previous analysis did not include the generation of entropy in photovoltaic conversion. Here, we show a bandgap dependence of the maximal concentration limit for a semiconductor solar cell when taking this entropy generation into account, and show that the limit is reduced for low bandgap materials. This new concentration limit lies in contrast to the assumed invariance of the concentration limit, and we attribute this difference to a breakdown in the assumptions used to derive the traditional detailed-balance model.