Electron selective TiOx contact for ultrathin amorphous germanium solar cells

Ultrathin amorphous germanium (a-Ge:H) absorbers enable innovative concepts like fully amorphous multi-quantum well solar cells, switchable PV windows or spectrally selective solar cells. Like in other solar cell technologies, TiOx is a suitable electron-selective and high-bandgap contact, enabling low parasitic absorption. However, due to quantum confinement effect the thickness of the ultrathin a-Ge:H absorber layer becomes an important parameter for the proper band alignment. We show that an absorber thickness of 5 nm leads to an s-shape in the J-V behavior, which can be eliminated by reducing the absorber size to 2 nm.