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.