Spontaneous Emission, Free Energy, and Relaxation-Limited Processes in Setting Limits on Solar Energy Conversion Efficiency

TL;DR

This paper develops a thermodynamic framework to evaluate the maximum efficiency of solar energy conversion, incorporating light-matter interactions and processes like spontaneous emission, estimating an upper limit of about 74%.

Contribution

It introduces a simplified approach to assess free energy of radiation and provides a theoretical maximum efficiency estimate for solar energy conversion, validated against the Shockley-Queisser limit.

Findings

The thermodynamic maximum limit for light-to-usable-energy conversion is approximately 74%.

The model reproduces the Shockley-Queisser limit (~33%) accurately.

Maximum efficiency can reach about 48% with multijunction cells or photon upconversion.

Abstract

Understanding the thermodynamics of radiation and the quantum-mechanical interactions between light and matter is important both for theoretical purposes and for technological advances, such as determining the limits of key processes like light-to-usable-energy conversion efficiencies. In this report, we discuss the physics of these two aspects, considering spontaneous emission as a pathway, and highlight the limitations of such descriptions in assessing energy-harvesting efficiency. In view of these limitations, we adopt a simplified approach to evaluate the free energy of radiation, providing a framework to assess various aspects of light-to-usable-energy conversion efficiencies. Our approach allows a theoretical estimate of the thermodynamic maximum limit for light-to-usable-energy conversion, which is approximately 74%. We validate this free energy estimate by modeling and accurately reproducing the Shockley-Queisser limit (~ 33%), which imposes a practical constraint on solar-to-usable-energy conversion efficiency. Beyond free-energy considerations, our model incorporates various processes, such as spontaneous emission, nonradiative thermal losses, and photon upconversion, allowing us to evaluate their roles. The model further suggests that, under certain conditions, the maximum conversion efficiency can reach approximately 48%, for example with multijunction solar cells or via photon upconversion. These findings further suggest that the true thermodynamic limit for light-to-usable-energy conversion may be much higher (approximately 74%). However, accurately estimating this limit requires a more complete understanding of the thermodynamics of light, light-matter interactions, and the connection between them.