Dual-junction-based near-field thermophotovoltaic converter robust to emitter temperature drop using thermophotonic emission

Near-field thermophotovoltaic (NFTPV) converters can efficiently convert thermal radiation from mid-grade heat sources below 1000 K to electricity without moving parts. Among them, dual-junction-based NFTPV (DJ-NFTPV) converters demonstrate higher performance compared to single-junction-based NFTPV converters by efficiently converting broadband near-field thermal radiation at the two sub-cells. However, below the emitter temperature of 1000 K, the DJ-NFTPV converter suffers from sharp performance degradation when the emitter temperature drops from the design temperature due to the disruption of the current mismatch condition between the two sub-cells. Unfortunately, temperature drops commonly happen in real-world heat sources such as thermal energy storage and waste heat. To facilitate the application of DJ-NFTPV converters, this study presents an approach that can effectively recover the performance of a DJ-NFTPV converter at an emitter temperature drop. It includes the integration of a thin LED layer on a bulk emitter, which can be powered for thermophotonic emission above its bandgap energy. Upon emitter temperature drop, the photocurrent of the top sub-cell is selectively enhanced by the thermophotonic emission, retrieving the current matching condition and regaining the performance. At temperature drops of several tens of Kelvin from a design temperature of 800 K, recovery rates for the damaged efficiency from 17% to 38% are achievable. By our approach, the DJ-NFTPV converter becomes even more robust to emitter temperature drops than single-junction-based NFTPV converters, and its most significant disadvantage disappears. Interestingly, the highest recovery rate is achieved at a gap distance of 100 nm rather than 10μm (i.e., far field) or 10 nm (i.e., extreme near field). Furthermore, the effects of thickness and bandgap energy of the LED are comprehensively analyzed. In accordance, the proposed design ensures the robust performance of DJ-NFTPV converters across a wide range of emitter temperatures, presenting a new paradigm for reliable and high-performance electrical power generation from medium-grade heat sources.