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Thermophotovoltaic, or TPV, is an energy conversion technology to harvest the energy of photons from a high-temperature blackbody source and directly transform it to electricity. One of the main applications is the collection of waste energy from high-temperature industrial operations. The process is identical to the one used in photovoltaic solar cells: a semiconductor with a determined bandgap Eg will absorb photons with an energy E higher than the bandgap (E>Eg), inducing the formation of an electron-hole pair. This electron-hole pair can then be separated by a built-in electric field in the device, usually through a p-n junction. The two main differences between classic photovoltaic solar cell and TPV devices reside in the spectrum and the amplitude of the incoming radiation. For photovoltaics, the source of radiation is the Sun, which can be approximated as a blackbody source at 5778K located 1.5×105m from the device. As a result the peak of radiation is in the visible, with wavelengths between 400 and 800nm. The optimal device bandgap is thus between 1.1 and 1.5eV. Moreover the incoming power is pretty diluted at about 1kW.m-2, resulting in relatively low output currents in the tens of mA.cm-2. For TPVs the source is much closer, typically a few centimeters from the device, and presents a lower temperature, between 1000K and 1800K. Consequently the current produced by a TPV device is much higher and the emission spectrum of the source is peaking at longer wavelengths, in the infrared between 1500nm and 3500nm, resulting in a lower optimal bandgap for the device (between 0.35 and 0.75eV). The goal of the thermophotovoltaics project is to develop high efficiency thermophotovoltaic devices using the cutting-edge MBE growth facilities available at UCL. First a model has been developed to assess the theoretical efficiency of InGaAs TPV devices grown on mismatched substrates such as Si or GaAs [1]. In particular the model takes into account the impact of non-radiative recombination processes in the cell such as intrinsic Auger recombinations, important for such low bandgap materials, and extrinsic Shockley-Read-Hall recombinations, caused by threading dislocations due to the lattice mismatch between the substrate and the epilayers. InGaAs TPV devices are now being grown by MBE using UCL unique experience in mismatched growth and 0D, 1D and 2D growth. Thanks to the new group IV MBE recently delivered, low bandgap SiGeSn compound TPV devices will also be grown on top of classic III-V-based TPVs.
Schematic diagram of Thermophotovoltaic devices
[1] Jurczak P, Onno A, Sablon K, Liu H. “Efficiency of GaInAs thermophotovoltaic cells: the effects of incident radiation, light trapping and recombinations”. Optics Express 2015 23(19): A1208–A1219, DOI: 10.1364/OE.23.0A1208.