OPTIMIZATION OF THE SPACING EFFECT OF THERMAL RADIATION FOR MICROSCALE THERMOPHOTOVOLTAIC DEVICES

The optimization of the performance of microscale thermophotovoltaic devices is investigated by exploiting the interference region and by using heterojunctions. We examine the influence of interference effects in the near thermal radiation field arising from a small vacuum gap between the hot emitter and the cold receiver. By simplifying the fluctuational electrodynamic approach to thermal energy transfer, we derive relations, which permit the structure of the near field to be analyzed. By positioning the peaks and valleys in the thermal field relative to the maximum in Planck's distribution and the band-gap energy of the receiver material, efficiency can be optimized, although gains are not greater than 10 % of the value of the efficiency. By using our model of a microscale thermophotovoltaic device that includes recombination mechanisms, we investigate the influence of multiple band gaps on performance. We expand our model to include a single heterojunction. The results indicate that varying the band gap in each layer of the junction can permit an increase in power output concomitant with an increase in efficiency for carefully selected band gaps and surface separations.