Thermally Stable 2-D Pgotonic Crystals for Solar Thermophotovoltaics

H.J. Lee, K. Smyth, S.G. Kim
Massachusetts Institute of Technology, US

Keywords: solar thermophotovoltaics, photonic crystal, thermal stability, selective emitters

Summary:

A fundamental challenge in solar-thermal-electrical energy conversion is the thermal stability of materials and devices at high operational temperatures. This study focuses on the thermal stability of selective emitters for solar thermophotovoltaic (STPV) systems to enhance the conversion efficiency. 2-D photonic crystals are periodic micro/nano-scale structures that are designed to affect the motion of photons at certain wavelengths. The structured patterns, however, lose their structural integrity at high temperature, which disrupts the tight tolerances required for spectral control of the thermal emitters. Through analytical studies and experimental observations, the four major mechanisms of thermal degradation of 2-D photonic crystal are identified: oxidation, grain growth and re-crystallization, surface diffusion, and evaporation and re-condensation. In this work, the design of a flat surface photonic crystal (FSPC) is proposed and experimental validations are performed. The FSPC design includes a thin diffusion barrier layer to prevent oxidation and evaporation while maintaining optical performance. Pre-annealing or the use of single crystalline tungsten is further employed to prevent grain growth. By replacing the air in micro/nano-scale holes with a damascened IR transparent ceramic material, the surface of the emitter will have negligible second derivative of the curvature, which minimizes the surface diffusion even at high temperatures.