Thermophotovoltaic (TPV) energy conversion enables millimeter scale power generation required for portable microelectronics, robotics, etc. In a TPV system, a heat source heats a selective emitter to incandescence, the radiation from which is incident on a low bandgap TPV cell. The selective emitter tailors the photonic density of states to produce spectrally confined selective emission of light matching the bandgap of the photovoltaic cell, enabling high heat-to-electricity conversion efficiency. The selective emitter requires: thermal stability at high-temperatures for long operational lifetimes, simple and relatively low-cost fabrication, as well as spectrally selective emission over a large uniform area. Generally, the selective emission can either originate from the natural material properties, such as in ytterbia or erbia emitters, or can be engineered through microstructuring. Our approach, the 2D photonic crystal fabricated in refractory metals, offers high spectral selectivity and high-temperature stability while being fabricated by standard semiconductor processes. In this work, we present a brief comparison of TPV system efficiencies using these different emitter technologies. We then focus on the design, fabrication, and characterization of our current 2D photonic crystal, which is a square lattice of cylindrical holes fabricated in a refractory metal substrate. The spectral performance and thermal stability of the fabricated photonic crystal thermal emitters are demonstrated and the efficiency gain of our model TPV system is characterized.

The increasing power demands of portable electronics and micro robotics has driven recent interest in millimeter-scale microgenerators. Many technologies (fuel cells, Stirling, thermoelectric, etc.) that potentially enable a portable hydrocarbon microgenerator are under active investigation. Hydrocarbon fuels have specific energies fifty times those of batteries, thus even a relatively inefficient generator can exceed the specific energy of batteries. We proposed, designed, and demonstrated a first-of-a-kind millimeter-scale thermophotovoltaic (TPV) system with a photonic crystal emitter. In a TPV system, combustion heats an emitter to incandescence and the resulting thermal radiation is converted to electricity by photovoltaic cells. Our approach uses a moderate temperature (1000–1200°C) metallic microburner coupled to a high emissivity, high selectivity photonic crystal selective emitter and low bandgap PV cells. This approach is predicted to be capable of up to 30% efficient fuel-to-electricity conversion within a millimeter-scale form factor. We have performed a robust experimental demonstration that validates the theoretical framework and the key system components, and present our results in the context of a TPV microgenerator. Although considerable technological barriers need to be overcome to realize a TPV microgenerator, we predict that 700–900 Wh/kg is possible with the current technology.

Solar thermophotovoltaic (STPV) systems convert solar energy into electricity via thermally radiated photons at tailored wavelength to increase energy conversion efficiency. In this work we report the design and analysis of a STPV using a high-fidelity 2D axisymmetric thermal-electrical hybrid model that includes thermal coupling between the absorber/emitter/PV cell and accounts for non-idealities such as temperature gradients and parasitic thermal losses. The radiative spectra of the absorber and emitter are engineered by using two-dimensional periodic square array of cylindrical holes on a tantalum (Ta) substrate. The optimal solar concentration and resulting temperature are determined by considering the energy losses associated with re-emission at the absorber, low energy (below band gap) emission at the emitter, and carrier thermalization/recombination in the PV cell. The modeling results suggest that the overall efficiency of a realistic planar STPV consisting of Ta PhCs and existing InGaAsSb PV cells with a filter can be as high as ∼8%. The use of high performance PhCs allows us to simplify the system layout and operate STPVs at a significantly lower optical concentration level and operating temperature compared with STPVs using metallic cavity receivers. This work shows the importance of photon engineering for the development of high efficiency STPVs and offers design guidelines for both the PhC absorber/emitter and the overall system.