Enhancing the photovoltaic effect in the infrared region by germanium quantum dots inserted in the intrinsic region of a silicon p-i-n diode with nanostructure

We show that a strong photovoltaic response in the infrared region of the solar spectrum (1.1–1.4 μm wavelength) is obtained by inserting a multilayer structure of germanium quantum dots and silicon spacer layers into the intrinsic region of a silicon p-i-n diode. The multilayer structure (active layer) is deposited on an n-type silicon wafer using the technique of ultra-high vacuum chemical vapor deposition. Reflection high-energy electron diffraction has been used to in situ monitor the transition from the two-dimensional to three-dimensional growth mode of germanium on silicon. The p-type layer of the diode is formed in situ by doping a layer of silicon with boron. Prototype solar cells have been fabricated in situ to measure the energy conversion efficiency. Photoluminescence spectroscopy has been used to probe the presence of any defect-related energy levels within the band gap, and the quality of the diode is determined from measurement of dark current. Scanning electron microscopy, atomic force microscopy, and transmission/scanning transmission electron microscopy have been used to characterize the structure of the active layer. It is demonstrated that by optimizing the structure of the active layer to minimize recombination of charge carriers in the quantum dots, a short-circuit current of 24 mA/cm² and an open-circuit voltage of 0.6 V could be achieved leading to an energy conversion efficiency of about 11.5% corresponding to an active layer with a thickness of 300 nm.