External luminescence and photon recycling in near-field thermophotovoltaics

TL;DR

This paper investigates how near-field effects influence photon recycling and external luminescence in thermophotovoltaic devices, revealing that close emitter proximity enhances performance by modifying emission and recombination processes.

Contribution

It provides a detailed analysis of near-field effects on photon recycling and external luminescence, highlighting the importance of emitter proximity and surface polariton modes in TPV efficiency.

Findings

External luminescence increases as vacuum gap decreases below 400 nm.

Photon recycling is strongly affected by near-field tunneling and surface polaritons.

Enhanced external luminescence improves open-circuit voltage and power density.

Abstract

The importance of considering near-field effects on photon recycling and spontaneous emission in a thermophotovoltaic device is investigated. Fluctuational electrodynamics is used to calculate external luminescence from a photovoltaic cell as a function of emitter type, vacuum gap thickness between emitter and cell, and cell thickness. The observed changes in external luminescence suggest strong modifications of photon recycling caused by the presence of the emitter. Photon recycling for propagating modes is affected by reflection at the vacuum-emitter interface and is substantially decreased by the leakage towards the emitter through tunneling of frustrated modes. In addition, spontaneous emission by the cell can be strongly enhanced by the presence of an emitter supporting surface polariton modes. It follows that using a radiative recombination model with a spatially uniform radiative lifetime, even corrected by a photon recycling factor, is inappropriate. Applying the principles of detailed balance, and accounting for non-radiative recombination mechanisms, the impact of external luminescence enhancement in the near field on thermophotovoltaic performance is investigated. It is shown that unlike isolated cells, the external luminescence efficiency is not solely dependent on cell quality, but significantly increases as the vacuum gap thickness decreases below 400 nm for the case of an intrinsic silicon emitter. In turn, the open-circuit voltage and power density benefit from this enhanced external luminescence toward the emitter. This benefit is larger as cell quality, characterized by the contribution of non-radiative recombination, decreases.