Silicon Vertical Multijunction Cell for Thermophotovoltaic Conversion

A new class of thermophotovoltaiccells converting thermal radiationpower into electrical power from sources at very high temperature(>1800 & DEG;C) is currently emerging. Like concentrating solarcells,these cells are subject to resistive losses due to high current densities.Hence, tandem cells with horizontally stacked junctions have beenrecently developed and have achieved record pairwise efficienciesand electrical power densities. Here, we employ alternatively a siliconvertical multijunction cell as a means of reducing current densitywhile operating at high voltage. Both under 1-Sun illumination andthat of a thermal source at 2100 & DEG;C, the cell kept at 25 & DEG;Cexhibits open-circuit voltages above 25 V and short-circuit currentsbelow 6 mA. With a series resistance below a few milliohms, Joulelosses are negligible. The cell receiving 2.01 W/cm(2) andabsorbing 1.24 W/cm(2) generates an electrical power densityof 0.14 W/cm(2), leading to a pairwise efficiency of only11.4% because of substantial out-of-band absorption. With photonicengineering of the emitter ensuring no out-of-band emission, thisefficiency could reach 52.9% under the same illumination conditions.The cell could also be used under bifacial conditions, which woulddouble the electrical power. However, due to the heat generated inthe cell, its temperature can exceed 25 & DEG;C. Advantageously, amoderate temperature coefficient of the electrical power of (-0.309 & PLUSMN; 0.005)% / & DEG;C is measured under 1-Sun illumination andit becomes much smaller, (-0.18 & PLUSMN; 0.01)%/ & DEG;C, inthermophotovoltaic conditions. These results suggest reopening thepath of research aimed at optimizing the design and fabrication ofvertical multijunction cells for thermophotovoltaic conversion.