2, Stanford University, Stanford, California, United States
Achieving efficient solar capture and optical emission requires the ability to distinguish between desired and unwanted wavelengths, which is known as spectral selectivity. Particularly for semiconductor materials operating above ambient, many of these properties can be temperature dependent. At high enough temperatures, spectral selectivity can be degraded greatly through multiple mechanisms. This gives rise to a need for developing new materials less susceptible to spectral degradation. At the same time, it is preferred that they be earth-abundant to provide scalable solutions to solve energy generation and energy efficiency challenges at a global scale.
To improve spectral selectivity, an earth-abundant thin-film semiconductor-metal tandem structure based on thin-film single-crystalline silicon and silicon nitride (Si3N4) is designed and tested here. It is first modeled, studied experimentally at high temperatures in a custom-designed test chamber, and then evaluated for applications in selective solar absorption and efficient optical sources. The thin-film Si with 10-20 μm thickness is fabricated through wet etching on commercial single-crystalline Si wafers. Earth abundant Si3N4 is then sputtered on top as an anti-reflection coating. Characterization in visible-near infrared (IR) shows that Si3N4 can strongly enhance above-bandgap absorption and therefore the spectral selectivity. At high temperatures, the emittance spectra measured by FTIR show significantly lower parasitic emission compared with a wafer based absorber. For a Si wafer based absorber at 535°C, the spectral-average solar absorptance αave is 0.72 and the spectral-average emittance εave is 0.60. At a higher temperature (595°C), the thin-film selective absorber has a similar αave of 0.75 and a lower εave of only 0.24. The physical basis of the improvement is that the reduced thickness of Si lowers the intrinsic carrier concentration per unit area. Furthermore, these structures can survive repeated thermal cycling with negligible loss of performance. In addition, single-crystalline Si based thin films exhibit strong mechanical flexibility, allowing conformal coating on various surfaces in real applications. Such properties make these earth-abundant material structures attractive candidates for high-temperature solar thermal, efficient incandescent lighting, infrared sources, and other applications.