Electron selective TiOx contact for ultrathin amorphous germanium solar cells

Norbert Osterthun, Hosni Meddeb, Nils Neugebohrn, Kai Gehrke, Martin Vehse German Aerospace Centre (DLR) Institute of Networked Energy Systems, Oldenburg, Germany

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 highbandgap 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 removed by reducing the absorber size to 2 nm.