Appropriate materials and preparation techniques for polycrystalline-thin-film thermophotovoltaic cells

Abstract

Polycrystalline-thin-film thermophotovoltaic (TPV) cells have excellent potential for reducing the cost of TPV generators so as to address the hitherto inaccessible and highly competitive markets such as self-powered gas-fired residential warm air furnaces and energy-efficient electric cars, etc. Recent progress in polycrystalline-thin-film solar cells have made it possible to satisfy the diffusion length and intrinsic junction rectification criteria for TPV cells operating at high fluences. Continuous ranges of direct bandgaps of the ternary and pseudoternary compounds such as Hg1−xCdxTe, Pb1−xCdxTe, Hg1−xZnxTe, and Pb1−xZnxS cover the region of interest of 0.50–0.75 eV for efficient TPV conversion. Other ternary and pseudoternary compounds which show direct bandgaps in most of or all of the 0.50–0.75 eV range are Pb1−xZnxTe, Sn1−xCd2xTe2, Pb1−xCdxSe, Pb1−xZnxSe, and Pb1−xCdxS. Hg1−xCdxTe (with x≈0.21) has been studied extensively for infrared detectors. PbTe and Pb1−xSnxTe have also been studied for infrared detectors. Not much work has been carried out on Hg1−xZnxTe thin films. Hg1−xCdxTe and Pb1−xCdxTe alloys cover a wide range of cut-off wavelengths from the far infrared to the near visible. Acceptors and donors are introduced in these materials by excess non-metal (Te) and excess metal (Hg and Pb) respectively. Extrinsic acceptor impurities are Cu, Au, and As while and In and Al are donor impurities. Hg1−xCdxTe thin films have been deposited by isothermal vapor-phase epitaxy (VPE), liquid phase epitaxy (LPE), hot-wall metalorganic chemical vapor deposition (MOCVD), electrodeposition, sputtering, molecular beam epitaxy (MBE), laser-assisted evaporation, and vacuum evaporation with or without hot-wall enclosure. The challenge in the preparation of Hg1−xCdxTe is to provide excess mercury incidence rate, to optimize the deposition parameters for enhanced mercury incorporation, and to achieve the requisite stoichiometry, grain size, and doping. MBE and MOCVD techniques have paved the way for obtaining epitaxial Hg1−xCdxTe thin films at substrate temperatures of ∼180 °C with the desired crystalline perfection, stoichiometry, and doping without the necessity of further annealing for improving either the crystalline quality or dopant activity. Retaining larger mercury proportions during annealing would require heated enclosures as in isothermal VPE, hot-wall technique, vacuum evaporation, hot-wall MOCVD, or close-space sublimation. Pb1−xCdxTe thin films can be prepared by magnetron sputtering from cooled Pb1−xCdxTe targets on heated substrates. Hot-wall technique is suitable for the deposition of Pb1−xCdxTe thin films. Hg1−xCdxTe and Pb1−xCdxTe TPV cells will benefit from the substantial work on CdTe thin film solar cells. The paper reviews work on thin films of ternary and pseudoternary compounds of interest for TPV conversion and methods of their preparation with a view to choosing the appropriate materials and fabrication techniques for polycrystalline-thin-film TPV cells.