Titanium‐Oxide‐Based Electron‐Selective Contact for Ultrathin Germanium Quantum Well Solar Cell

Abstract

In solar cells with an optical cavity as a light trapping mechanism, parasitic absorption is among the main detrimental optical losses. One approach to reducing these losses is the implementation of charge carrier‐selective contacts with wide bandgaps using thin metal oxides like titanium oxide (TiOx). Herein, it is proved that TiOx is a suitable alternative for lossy n‐doped hydrogenated amorphous silicon (n a‐Si:H) as electron selective contact in ultrathin solar cells based on intrinsic hydrogenated amorphous germanium (a‐Ge:H). The integration of the TiOx layer leads to an improved photocurrent generation in the blue light wavelengths. Furthermore, a substantial extraction of charge carriers is demonstrated from the a‐Ge:H quantum well nanoabsorber embedded between intrinsic a‐Si barrier layers. However, the substitution of n a‐Si:H by TiOx results in limited photovoltaic performance due to a lower open‐circuit voltage. This effect is analyzed via electrical simulation considering a variation in the electron affinity and the doping of TiOx as well as the defect density in the silicon buffer. Moreover, the TiOx‐based solar cell exhibits improved light transmittance when the opaque back contact is omitted, which is beneficial for semi‐transparent applications.