a Study of High Efficiency Thin Thermophotovoltaic Solar Cells.

High conversion efficiency of solar energy into electrical energy is possible if the incident radiation is first absorbed by an intermediate absorber and then re-emitted onto a photovoltaic (PV) solar cell. This mode of operation is known as solar thermophotovoltaic (TPV) energy conversion. This thesis explores the limits on performance of TPV systems based on germanium in which the source temperature and the opto-electronic structure of the germanium PV cell are varied and optimized with respect to overall radiant energy conversion efficiency. The principal characteristic of the optimized high efficiency TPV germanium cells is that they are thin p-n junction solar cells which incorporate minority carrier mirrors (MCM) and optical mirrors (OM) at the front and back surfaces of the device examined. In this study, the role of MCM and OM is studied theoretically by solving the minority carrier diffusion equation in the n- and p-type quasineutral regions of the cell with the appropriate boundary conditions at the end of these regions and an appropriate minority carrier generation function. The high theoretical efficiency calculated for these thin structures derives from the simultaneous use of optical and electronic reflection. The calculations presented here determine the theoretical upper limit to TPV conversion efficiency and show the dependence of this limit on cell geometry, resistivity, surface recombination and input density. In addition, TPV systems based on more than one PV cell, each utilizing a different photovoltaically active semiconductor are also considered. A number of possible TPV systems are treated within this theoretical framework. When blackbody thermal radiation sources having temperatures in the range 1500-2000 C are considered, the upper limit efficiency is found to be about 22% for an optimum design germanium cell 90 microns thick and about 26% for a two-junction silicon-germanium tandem cell arrangement 50 and 90 microns thick, respectively, both systems receiving an overall input power density of 25 W/cm('2) from a 2000 C blackbody source. The upper limits to the conversion efficiency can be further enhanced if the longer wavelength photons which are not absorbed by the solar cells are recycles (i.e., returning them to the radiator by making the solar cell highly reflective at these wavelengths). The performance of TPV systems in which the radiation emitters are coated with selective absorber-emitters of erbium (Er(,2)O(,3)) and and ytterbium (Yb(,2)O(,3)) oxides is also explored.