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  • Open Access

Zero-Vacuum-Gap Thermophotonics

Mohammad Habibi1 and Longji Cui1,2,*

  • *Contact author: longji.cui@colorado.edu
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PRX Energy 5, 013005 – Published 29 January, 2026

DOI: https://doi.org/10.1103/ff2l-d85y

Abstract

Thermophotonic (TPX) devices offer a promising pathway to overcome the power density and efficiency trade-offs of thermophotovoltaics (TPVs) by using biased light-emitting diodes (LEDs) as active infrared photon emitters. However, practical implementation of TPXs has been limited by stringent demands requiring the LED to operate at high temperatures, as well as the need for near-perfect external quantum efficiency (EQE) to enable self-sustaining (i.e., no external power supply) operation. Here, we introduce a zero-vacuum-gap thermophotonic (zTPX) architecture that integrates a high-refractive-index solid-state spacer between the LED and the photovoltaic cells, which forms an optical cavity that minimizes index mismatch and enables high photon concentration. This design enhances both power density and efficiency, over 40-fold power improvement, and up to doubled efficiency enhancement compared to conventional far-field TPX, allowing the operation of TPXs at relatively lower temperatures, at which high-performance LEDs are available. Furthermore, we show that the zTPX structure achieves a record-breaking EQE of approximately 98%, which enables a self-sustaining TPX circuit with a sizable power output. These combined performance gains position zTPX as a highly practical thermophotonic solution for thermal energy conversion and utilization at low to moderate temperatures (<1000 °C), including industrial waste-heat recovery, distributed power generation, and portable heat-to-electricity converters.

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Popular Summary

Up to two-thirds of conventionally generated electricity ends up as waste heat, and much of this waste heat is at low to moderate temperatures below 1000 ∘C. Capturing and converting this lost heat into usable power is a key opportunity to boost energy efficiency and mitigate carbon emissions. Emerging solid-state technologies that directly turn heat into electricity must operate efficiently at these moderate temperatures to compete with traditional energy technologies. However, thermophotovoltaic (TPV) systems mostly work at extremely high temperatures, and thermophotonic (TPX) systems, which offer great promise at moderate temperatures, exhibit poor efficiency and power density. Here, the authors present a new zero-vacuum-gap TPX (zTPX) design that swaps the gap in conventional TPX devices for an infrared transparent, high-index solid material placed between a light-emitting diode and a photovoltaic cell. This scalable and compact device architecture dramatically improves energy conversion, resulting in near-perfect external quantum efficiency, over 40 times higher power output, and lower working temperature for thermal emitters. This approach offers a viable route to harness untapped heat sources for power generation in numerous industrial applications.

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Outline

Information

PRX Energy 5, 013005– Published 29 January, 2026
  • Received 4 July 2025
  • Revised 9 November 2025
  • Accepted 6 January 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

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