Effect of doping concentration on conversion efficiency of silicon based nano-gap near-field thermophotovoltaic cells

We studied the doping dependence of a nano-gap thermophotovoltaic device that utilises a p-on-n Si photovoltaic cell. The concentration of acceptor dopants, Na in the emitter region was varied from 1023 m-3 to 1025 m-3 while the donor concentration, Nd in the absorber region was varied from 1021 m-3 to 1024 m-3. The combination of Na and Nd that produces the highest conversion efficiency is Na = 1023 m-3 and Nd = 1022 m-3. However, the absorber doping concentration that produces the greatest output power is around 1023 m-3 at all Na values. It is discovered that at higher Na values, the optical response of the device is less sensitive to Nd. It is shown that a decreasing diffusion length does not necessarily jeopardise performance especially when it already exceeds the absorber region thickness. This is due to the positive effects of a decreasing diffusion coefficient.