1. Field
Embodiments discussed herein relate to a back surface reflector (BSR) for thermophotovoltaic (TPV) systems.
2. Description of the Related Art
TPV systems include a radiator, optical cavity and TPV device, with the radiator and TPV device being on opposite sides of the optical cavity. A 20% heat-to-electric conversion efficiency may be achieved by using an InGaAs/InP TPV device and an etched radiator, at temperatures of ˜1050° C. and ˜26° C., respectively. To achieve this high conversion efficiency, both the diode and spectral efficiencies of the system must be high. Relatively high spectral efficiencies can be achieved by using either transmissive or reflective spectral control strategies. Each of these strategies requires a highly reflective (non-absorbing) back surface reflector (BSR).
For the transmissive spectral control strategy, most of the light is allowed into the TPV device. The spectral efficiency is highly dependent upon the ability of the back surface reflector to reflect low energy (less than the active region bandgap energy) photons back out of the TPV device. For the reflective spectral control scheme, a highly reflective BSR is needed to increase the diode efficiency by increasing the photon recycling. Therefore, specular, highly reflective BSRs are needed to maximize the efficiency of either type of TPV system.
A related BSR 100 for InGaAs/InP based TPV devices is shown in FIG. 1. The BSR 100 includes a reflector 110, an adhesion layer 120, a diffusion barrier 130 and a substrate 140. The reflector HQ is a 2000 Å Au reflector, the adhesion layer 120 is 50-200 Å and made of a transition metal, such as Ti, and the substrate 140 is made of InP or InGaAs. The diffusion barrier 130 is made of SiO2 or Si3N4 and prevents the diffusion of in from the InP substrate 140 into the Au reflector 110. The major loss of reflectivity is in the adhesion layer 120 due to the high characteristic absorption of this material (as well as other transition metals) for wavelengths <7 μm. The absorption of the Ti can be reduced by making the adhesion layer 120 thinner, but this may result in incomplete coverage of the adhesion layer 120, which may result in inadequate adhesion. In order to achieve proper adhesion, the standard Ti adhesion layer for InGaAs/InP TPV devices is 125 Å.
One way to eliminate the trade-off between reflectivity and thickness is to use a single monolayer of adhesive material that has minimal absorption. However, this is difficult to achieve with standard deposition techniques (i.e., evaporation, sputtering, CVD), because film growth is determined by the flux of the depositing species and the adatom mobility of that species on the substrate surface. These two considerations prevent perfect monolayer growth, and lead to island formation and roughening that makes it necessary to deposit even more material to ensure complete coverage of the substrate.
In light of the foregoing, there is a need for a BSR having an adhesive material with good coverage and high reflectivity/minimal absorption, particularly in the 1-10 μm spectral range.