Thermophotovoltaics (TPVs) are solid-state devices that may enable scalable electricity generation from a variety of high-temperature heat sources in applications such as grid-scale electricity storage and distributed co-generation of heat and power. These systems consist of a thermal emitter and a photovoltaic (PV) cell in close proximity. Spectrally selective techniques, categorized as either emission control or absorption control, have led to improved performance in TPVs. In particular, suppression of sub-bandgap radiative transfer is essential for improving efficiency. However, the spectral-selectivity of absorption control strategies in conventional cells has been limited by parasitic absorption of sub-bandgap radiation due to a variety of possible mechanisms including absorption in the growth substrate, the thickest layer of the cell. Thin-film TPV cells have the potential to enable selective radiative transfer by reducing the optical path length through the cell and leveraging thin-film interference. Here, we demonstrate high spectral-selectivity in thin-film InGaAs-based TPV cells with back-surface-reflectors and optimized dielectric coatings. Selective absorption using thin semiconductor layers has been investigated for solar absorbers to minimize thermal re-radiation, but has not been demonstrated in the context of TPV cells. The fabricated TPV cells exhibit high absorption of radiation above the semiconductor bandgap and high reflectance below the bandgap, particularly when dielectric layers surrounding the InGaAs are optimized. Furthermore, thin-film devices have the potential for significant economic improvements over conventional TPVs that use expensive growth substrates in the operating device. Fabrication of thin-film group III-V semiconductor cells through non-destructive epitaxial lift-off has enabled wafer reuse. Considering the high cost of III-V wafers, reuse can be expected to significantly reduce the cost of TPV generators.
Ideal thermophotovoltaic (TPV) converters approach Carnot efficiency in the limit of monochromatic radiative transfer, motivating the design of narrowband thermal emitters and absorbers. Here, we investigate whether this trend holds for converters with realistic losses by studying the effects of transmission bandwidth on the performance of far-field and near-field TPV and thermophotonic (TPX) converters. To bridge the near-field and far-field regimes, the analysis relies on an approximation that the near-field spectral energy flux is a scaled version of the far-field flux, which is validated for weakly absorbing materials using a rigorous near-field simulation. We show that the optimal bandwidth depends on the type of converter. For far-field TPVs with realistic heat losses, narrowband transport is typically detrimental to the efficiency because the converter becomes more susceptible to parasitic loss. However, narrowband transport boosts efficiencies in TPX converters and near-field TPVs as long as the excitation barrier is comparable with the thermal energy. Given the same excitation energy barrier as TPVs, TPX converters benefit from a larger available photon density of states due to the applied bias. This study suggests that near-field TPX converters with a large applied bias have the largest ratio of useful energy flux to parasitic loss. Leveraging this mechanism for actual improvement is contingent on large near-field enhancements improving photon extraction.