Interference Filters for Thermo-photovoltaic Applications

Abstract

The overall intention is to use a source of black-body radiation, perhaps some object being heated inside by burning fuel, to produce electricity from a thermophotovoltaic (TPV) cell by the correct sort of radiation arriving at the cell. Only light below a certain wavelength λ_g will generate a voltage in the cell; such high-frequency radiation may be termed “useful” in contrast to the “useless” lower-frequency radiation. To increase efficiency, ideally the otherwise useless long-wavelength light is returned to the radiator by reflecting back from the TPV cell.

A key part of the device currently under investigation is an “interference filter”. This consists of a large number (say around 60) of thin layers (the whole device is about 2 μm thick) with differing refractive indices. The combination of multiple reflections and refractions, with associated change of phase, results in interference so that light of certain frequencies is mostly reflected back while other light is mostly transmitted through to the other side of the filter. In the ideal case, all the good frequencies will pass through the filter to the TPV cell lying behind it while the bad ones are reflected away:
- overall reflection coefficient (by power) = 1 for λ < λ_g , 0 for λ > λ_g ;
- overall transmission coefficient (by power) = 0 for λ < λg , 1 for λ > λg .
Unfortunately, because of the effect of direction on optical lengths and hence on the level of interference, the transmission and reflection coefficients depend strongly upon the angle of incidence. For black-body radiation the dependence of the normal component of power upon the angle of incidence α exhibits a maximum at α = π/4, although, of course, the power density (with respect to angle) is positive for all 0 < α < π/2. This means that – assuming that black-body radiation is impinging on the TPV cell – it makes sense to try to optimize the filter’s performance for α = π/4, but then most radiation will be incident along sub-optimal directions. Indeed, because the optical lengths inside the filter are essentially given by cos α (or sec α), the effectivenesss of the filter depends least strongly upon angle for α near 0 (normal incidence). For this reason attention must also be focused on trying to direct the light from the radiator. With light travelling in (roughly) one direction, it is then possible to orient the cells so that light falls on them normally and the interference filters should be designed to work optimally with α = 0 (or small).

A second design improvement is also worthy of mention. It was raised during our deliberations, but constraints on time prevented us from further examining this research topic. The black-body radiation spectrum is not immutable; it is modified by using a multiple dielectric layer design for the emitter. This has the advantage of controlling both the angular emission pattern and the emission spectrum.