When incident light is absorbed by a plate semiconducting material like silicon, negatively charged elec­trons are excited from the lower-energy valence band to the higher-energy conduction band and similar for positively charged holes (that represent the lack of electrons). By attaching electrodes to the plate, the electrons and holes are harvested and sent into an electric circuit to drive a useful appliance. This process notably occurs inside a solar cell, see Figure 1, where the harvested current serves to power an LED for ambient lighting. While thick silicon plates are widely used, thin silicon films are enjoying a rising popularity on account of their obvious sustainability, since they require much less material, less re­sources, and lower cost. Unfortunately, however, thin and ultrathin silicon films hardly absorb light, especially at long wavelengths in the visible spectrum where the sun radiates a lot. In other words, thin silicon films are not “black”. Therefore the team set out to study how a back reflector could recycle un­absorbed light, and become highly absorbing, or “black”.

As a back reflector, the Twente team studied a diamond-like photonic crystal composed of two sets of perpendicular pores, shown in Figure 1. Such photonic crystals are known to have a record-wide 3D photonic band gap. As a result, the team indeed finds that this crystal is a truly omnidirectional, broadband, and polarization-robust back reflector. Lead author Devashish effuses: "Our extensive computations reveal that the photonic back reflector yields a striking 9.15 times enhanced absorption even for a 80 nanometer ultrathin film (see Figure 2). Our devices are up to 80% lighter than bulk silicon, due to the porosity of the photonic structure, jokingly referred to as “holeyness”".

 

 

Group leader Vos explains: "Such a strong absorption in a thin silicon film (see Figure 3) can also be in­terpreted in a quantum physical picture, namely that the photonic crystal acts as a colored electromag­netic vacuum below the absorbing film. The absorption of incident light is so strongly boosted that ul­trathin silicon would effectively turn black". The Twente team also projects that their holey 3D inverse wood­pile structures offer application potential for compact on-chip sensors, photodiodes, and charge-coupled devices (CCD) for cameras (see Figure 4).

Previously, the team reported enhanced optical absorption for realistic and finite 3D silicon photonic band gap crystals with an embedded resonant cavity (https://nano-cops.com/publications/article/three-dimensional-photonic-band-gap-cavity-with-finite-support-enhanced-energy-density-and-optical-absorption/). The absorption was found to be substantially enhanced, however, only within the tiny cavity volume as opposed to the present case, where the absorption occurs throughout the whole film.