Enhancement of solar absorption with black Cu2O Nanostructures*

Cu$_{2}$O is a direct gap semiconductor with a band gap of 2.1 eV. It was considered to be a solar absorber material, while the application is hindered by its large band gap and weak stability. Here we report an electrochemical synthesis of Cu$_{2}$O. By rationally control the synthetic parameters, we achieved two types of Cu$_{2}$O: one of black color and the other ``normal'' red Cu$_{2}$O. Both Cu$_{2}$O films were in cubic phase and their crystal structures are almost identical as seen by X-ray diffraction. This is further corroborated by their nearly identical Raman spectra. The scanning tunneling spectrum (STS) revealed a gap in the red Cu$_{2}$O around 2.1 eV and a significantly lowered gap of $\sim$ 1.7 eV in the black Cu$_{2}$O, indicating that the black color is caused by a change in the electronic structure. The reflectance and transmittance indicated a band gap of $\sim$ 1.7 eV for the black Cu$_{2}$O, with a significantly broadened absorption spectrum. While further effort is needed to understand the mechanism for the lowering of the band gap, we believe that our approach demonstrated means to promote earth abundant and nontoxic materials for potential photovoltaic applications through band gap engineering.