Enhanced performance of self-assembled Ge-dots in Si thin film solar cells

Germanium (Ge)-islands have been deposited by MBE on silicon (Si)-substrates to enhance the efficiency of Si thin film solar cells. The islands are deposited in the Stranski-Krastanow growth mode on a standard 10Ωcm, p-type Si substrate and form the base of the solar cell. We employed Sb as surfactant to achieve rather high island densities (>10¹¹ cm^-2, typical lateral width 20nm and heights 3nm). Up to 75 Ge layers with 8 monolayers (ML) Ge each separated by a thin (9nm-18nm) undoped Si buffer layer have been stacked on top of each other. On top of it we deposited 200nm undoped Si layer into which we performed an n⁺ emitter diffusion. Preliminary measurements show a reduced open circuit voltage but an enhancement of the short circuit current in the infrared regime with an efficiency of about 12%.