Co-bifacial zero-vacuum-gap thermophotonic stack providing high-performance scaled-up thermophotonic conversion

Zero-vacuum-gap thermophotonic (TPX) systems provide a practical route to efficient energy recovery from mid-range temperature heat sources. A solid spacer replacing the vacuum gap introduces additional high-wavevector propagating channels under far-field conditions. It also provides structural stability by connecting the LED and PV cell, which is beneficial for integrated, scaled-up systems aiming for large-scale waste heat recovery. This stability makes the zero-vacuum-gap concept attractive for bifacial TPX configurations, which introduce gaps on both sides of the LED or PV cell for minimizing photon loss. However, the potential of bifacial zero-vacuum-gap TPX systems remains unexplored. Additionally, heat conduction through the solid spacer requires effective thermal management of the PV cell. Therefore, this work proposes a co-bifacial zero-vacuum-gap TPX (CoBi-z-TPX) stack enabled by an internal cooling channel that maximizes the potential of the bifacial configuration. Benefiting from the zero-vacuum-gap concept, a single bifacial zero-vacuum-gap TPX (Bi-z-TPX) system with a 10-cm-thick fused silica spacer achieves a power density comparable to that of a system with 90 nm vacuum gaps, while delivering 2.0 times greater efficiency. Furthermore, internal cooling enables unique scale-up of single systems to a CoBi-z-TPX stack by allowing the removal of the PV cell backside reflector, resulting in 64% higher system power density and 40% higher system efficiency than the single-system configuration. Internal cooling also improves robustness against heat conduction through thin spacers in compact configuration. Finally, the CoBi-z-TPX stack with optimized cooling channel thicknesses exhibits a maximum net power density of 0.39 W/cm2 and a net efficiency of 11.1%.