Germanium nanocrystals towards tandem solar cell applications: physics and technology

摘要

Germanium nanocrystals (Ge-ncs) embedded in amorphous SiO2 matrix have attracted much attention as a promising material for optoelectronic applications, such as thin film tandem solar cells. The successful implementation of this nanostructure requires the development of fabrication techniques and the understanding of structural, optical and electrical properties of the produced nanocrystals.A comprehensive study of Ge-ncs grown in a superlattice structure is presented to demonstrate the feasibility of the superlattice for controlling nanocrystal size and engineering the electronic band gap of thin films. The structural properties of Ge-ncs were extensively studied by a series of characterization techniques including Raman spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). A kinetic model for the crystallization process in a superlattice has been proposed to explain the size control effect. Band gap engineering by tuning the superlattice structure was studied using optical absorption and photoluminescence measurements, and the experimental results were further compared with the results from theoretical calculation. Although a clear shift towards higher energies was observed in the absorption edge and luminescence peak, it remains ambiguous whether this could be attributed to the quantum confinement effect in Ge-ncs.A simple and silicon process-compatible technique is reported for the synthesis of Ge-ncs at temperatures below 400 oC, which is much lower than the typical growth temperatures. The Ge-ncs were found to form only within a temperature window between 350 oC and 420 oC. The underlying growth mechanism can be explained by a competitive process between Volmer-Weber growth and oxidation reaction. This technique has also been shown to be suitable for the fabrication of superlattice structure. Case studies on the stress development and optical absorption properties of the low temperature grown Ge-ncs are presented as well.Thin films composed of Ge-ncs in SiO2 matrix were prepared using the low temperature growth technique and their electrical properties were comprehensively studied. P-type behaviour was observed in the undoped thin films, which is attributed to the hole accumulation in Ge-ncs caused by the acceptor-like surface states. The charge transport is a thermally activated process involving charge hopping from one nanocrystal to its nearest neighbors. The p-type conductivity was further improved by the post-growth rapid thermal annealing and this can be explained by the modification of Ge-ncs’ surface structure and the reduction of defects in SiO2 matrix. The effects of impurities on the electrical conduction properties of Ge-nc thin films were studied as a starting point of future work on making n-type thin films and thereby the homojunction devices. Moreover, the electrical and photovoltaic properties of the heterojunction diodes employing the p-type Ge-nc thin films were characterized to demonstrate their functionality in device implementation.The findings in this thesis provide initial insight into the Ge-ncs embedded in SiO2 matrix and indicate that this kind of nanomaterial is very promising for producing low cost thin film tandem solar cells, but further research is still required.