# The Critical Role of Substrates in Mitigating the Power–Efficiency Trade-Off in Near-Field Thermophotovoltaics

## TL;DR

This paper shows how substrate engineering can improve both power and efficiency in thermophotovoltaic systems.

## Contribution

The novel use of gradient-based optimization to design substrates that enhance power and efficiency in near-field thermophotovoltaics.

## Key findings

- Thin lossless metallic films with matched plasma frequencies improve radiative power density.
- Optimized substrates increase power density by over an order of magnitude compared to noble metals.
- Spectral confinement enhances efficiency while maintaining high power output.

## Abstract

Near-field thermophotovoltaic systems can achieve ultrahigh
power
densities, however, this often comes at the cost of reduced efficiency.
We show that this power–efficiency trade-off can be mitigated
through substrate engineering. We exploit gradient-based optimization
and show that thin lossless metallic films with plasma frequencies
resonantly matched to the plasmonic emitter can yield high power and
spectral efficiency by spectrally enhancing and confining radiative
heat transfer to a narrow spectral range just above the photovoltaic
bandgap. Compared to noble metals and air-bridged structures, designs
deriving from such optimization yield more than an order-of-magnitude
increase in radiative power density while maintaining high efficiency.
Our results highlight the critical role of the substrate and the potential
of substrate optimization for overcoming fundamental limitations of
near-field thermophotovoltaic systems.