Wide-angle, polarization-independent ultrathin broadband visible absorbers

A capability of absorbing a broad range of visible lights is essential to boost the performance of various applications, such as photovoltaics (PV), photodetectors, and thermal emitters. Here, we present an angle-insensitive, polarization-independent ultrathin (<150 nm) broadband absorber in the visible regime exploiting strong interference behaviors in highly absorbing semiconductor materials. A proposed structure simply has four layers composed of two stacks of a metal and a semiconductor demonstrating a remarkably enhanced absorption property as compared with the device without a top semiconductor film. This is attributed to multi-cavity resonance effects in each cavity, which is obviously elucidated with phase calculations and electric field distributions. The maximum absorption efficiency of the device is 95.5% at a resonance and its absorption characteristic can be maintained over a wide angle of incidence up to ±70° regardless of the incident light polarization. Finally, we investigate how our approach can be utilized to achieve a tandem PV cell with high efficiency. Our strategy can be applied to other material systems and can be useful in diverse applications, including thermal emitters and PV.