Semitransparent thermophotovoltaics for efficient utilization of moderate temperature thermal radiation

Recent advances in thermophotovoltaic (TPV) power generation have produced notable gains in efficiency, particularly at very high emitter temperatures. However, there remains substantial room for improving TPV conversion of waste, solar, and nuclear heat streams at temperatures below 1,100 degrees C. Here, we demonstrate the concept of transmissive spectral control that enables efficient recuperation of below-bandgap photons by allowing them to transmit through the cell to be absorbed by a secondary emitter. We fabricate a semitransparent TPV cell consisting of a thin InGaAs-InP heterojunction membrane supported by an infrared-transparent heat-conducting substrate. The device absorbs less than 1% of below-bandgap radiation, resulting in a TPV efficiency of 32.5% at an emitter temperature of 1,036 degrees C. To our knowledge, this represents an 8% absolute improvement (similar to 33% relative) in efficiency relative to the best TPV devices at such low temperatures. By enabling near-zero photon loss, the semitransparent architecture facilitates high TPV efficiencies over a wide range of applications.