Thermophotovoltaic cells containing multiple quantum wells have been invented as improved means of conversion of thermal to electrical energy. The semiconductor bandgaps of the quantum wells can be tailored to be narrower than those of prior thermophotovoltaic cells, thereby enabling the cells to convert energy from longer-wavelength photons that dominate the infrared-rich spectra of typical thermal sources with which these cells would be used. Moreover, in comparison with a conventional single-junction thermophotovoltaic cell, a cell containing multiple narrow-bandgap quantum wells according to the invention can convert energy from a wider range of wavelengths. Hence, the invention increases the achievable thermal-to-electrical energy-conversion efficiency. These thermophotovoltaic cells are expected to be especially useful for extracting electrical energy from combustion, waste-heat, and nuclear sources having temperatures in the approximate range from 1,000 to 1,500 °C.
In its original form, the invention applies to the InxGa1–xAs (0<1)-and-InP material system. In principle, it is equally applicable to any narrow-bandgap semiconductor material system that is amenable to the growth of lattice-matched multiple quantum wells on suitable substrates. A cell according to the invention is best described with reference to the corresponding conventional InxGa1–xAs thermophotovoltaic cell, which is an electron-acceptor-doped/ intrinsic/electron-donor-doped (p/i/n) In0.47Ga0.53As cell lattice-matched to an InP substrate. In the cell according to the invention, instead of the intrinsic (undoped) region, there are multiple strained, lattice-matched, narrow-bandgap quantum wells comprising layers of InxGa1–xAs (x>0.6) interspersed with layers of In0.47Ga0.53As. It has been estimated that for black-body thermal sources having temperatures between 1,000 and 1,500 °C, the energy-conversion efficiencies of thermophotovoltaic cells according to the invention can be more than twice those of the corresponding conventional InxGa1–xAs thermophotovoltaic cells.
An appropriate choice of the number of quantum wells and the thicknesses of the individual quantum-well layers (typically of the order of a few nanometers) in conjunction with the selection of the quantum-well materials makes it possible to prevent the generation of lattice-mismatch crystal defects in the quantum-well layers. Thus, it is possible to prevent the degradation of crystalline quality and thereby prevent the consequent degradation of electronic performance associated with the fabrication of thicker conventional lattice- mismatched devices. Inasmuch as conventional InxGa1–xAs thermophotovoltaic cells are already manufactured by techniques compatible with the growth of multiple quantum wells, little additional expense would be incurred by adding quantum-well-growth steps to conventional manufacturing processes.