Photoabsorption enhancement in thin-silicon photovoltaics using opaline photonic crystal back-reflectors

Two attributes of opaline photonic crystal (PC) back-reflectors optically coupled to thin semiconductor films contribute to enhanced optical absorption in the semiconductor. Namely, (i) the PC back-reflector behaves as a perfect mirror (PM), exhibiting complete reflection over stop-gap frequencies; and (ii) the PC-semiconductor film interface couples incident light into resonant states that propagate along the plane of the film, thereby further enhancing absorption. In order to fully realize these benefits the PC must be placed directly adjacent to the absorbing medium. However, in conventional thin-film silicon photovoltaic (PV) cells this is where a nontransparent back contact is typically located. Herein we perform finite-difference-time-domain (FDTD) based calculations in order to investigate the benefits of utilizing an existing electrically conducting PC, namely a ZnO inverted opal, as the back contact for an a-Si:H PV cell. FDTD calculations show that, under normally incident light, the addition of a ZnO inverted opal back-reflector comprising 12 layers enhances the average absorption in a 300nm thick a-Si:H cell by a factor of 4.7 in the spectral region slightly above the a-Si:H band gap. In comparison, absorption is enhanced by just a factor of 2.3 for the case in which a PM is used as a back-reflector. Considering that the efficiency of commercially available a-Si:H cells is typically ~6% the increased absorption due to a 12layered ZnO inverted opal back-reflector represents a potential increase in the relative efficiency of the a-Si:H cell of more than 10% compared to the case in which the a-Si:H cell is backed by a PM.