Thermally Stable Emitters Selected by Pareto Optimization for Solar Thermophotovoltaics

A central idea of solar thermophotovoltaics (STPV) is to spectrally tune the solar spectrum to better match the bandgap of a target PV cell in order to exceed the Shockley-Queisser limit of ~33%. We propose a new design of the selective emitters to tailor thermal emission by exploiting critical coupling between reflection in a Bragg reflector and absorption in optically tunable layer. After Pareto optimization was applied to computationally identify a small number of record-setting emitter structures, we fabricate several multilayer stacks in which the Bragg reflector is composed of SiO₂ and TiO₂ layers and the tunable absorber is a W-Al₂O₃ alloy with variable volume fraction. Plasma-enhanced atomic layer deposition (PE-ALD) allows precise control over each layer’s thickness and density as well as the alloy composition. We evaluate the spectral efficiency and spectral density of each structure and compare it to computational models. We further investigate the stabilizing effect of ALD oxide overlayers on underlying layers that are otherwise prone to oxidation. The multilayer stacks exhibit excellent thermal stability upon repetitive annealing at 1000 °C under inert atmosphere.