Photovoltaic effect in Ge nanocrystals/c-silicon heterojunctions devices

Heterojunction (HJ) devices have been realized by depositing gallium-doped Ge nanocrystals (Ge-ncs) on an n-type crystalline silicon substrate. Transmission electron microscopy (TEM) and Raman spectroscopy have revealed that the Ge-ncs were formed in SiO2 matrix, with sizes in the range of 3.5–6.2 nm. The rapid thermal annealing for dopants activation of Ge-ncs in SiO2 matrix was investigated by four point probe measurements. The decrease of sheet resistance was observed at temperatures higher than 600 °C and was related to annealing temperatures. The electrical and photovoltaic properties of Ge-ncs:SiO2/c-Si HJ devices were characterized by dark and illuminated I–V and quasi-steady-state open-circuit voltage (Suns-Voc) measurements. The HJ devices showed a good rectification effect and clear photovoltaic effect. However, fairly large ideality factor and series resistance, which were obtained from numerical fitting of I–V curves, significantly limited the diode performance. Models were given to explain the experimental findings. This study is very helpful for a preliminary understanding of the electrical material quality of the Ge-ncs:SiO2 films for potential applications in thin film tandem solar cells.