Progress in all-silicon tandem solar cells with silicon quantum dot in silicon dioxide matrix

Abstract

Global warming and environment pollution are very serious issues facing the world. Renewable energy is currently the fastest growing energy source in the world due to these concerns. The main effort of research and industrial technology development is directed towards reducing the production cost. “Third generation” photovoltics involves the investigation of ideas that may achieve this goal. Therefore an all-silicon tandem solar cell concept is a promising approach because this uses abundant, non-toxic silicon in a nanostructure within a matrix of oxide, nitride, or carbide to engineer the bandgap of solar cell material. In this thesis, a wide-bandgap silicon based thin film material formed by using quantum-confinement in silicon quantum dots (Si QDs) was investigated. Uniformly sized quantum dots dispersed in a silicon dioxide (SiO2) matrix with dopants (phosphorus and boron) were fabricated by precipitation from silicon rich oxide (SRO) deposited by RF co-sputtering. The Si QDs in SiO2 matrix films were investigated by optical and electrical characterisation techniques such as cross-sectional transmission electron microscopy (TEM), Raman spectroscopy, glancing incidence X-ray diffraction (GIXD), double-beam UV/visible/IR spectrophotometry, X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS). Further, Si QD heteroface devices and homojunction devices were fabricated to investigate carrier properties and conduction behaviour. To compare the quantum confinement effect, different sizes (3 nm, 4 nm, 5 nm and 8 nm diameter) of Si QD were fabricated, whose optical energy bandgaps are in the range of 1.3 ~ 1.65 eV. The electrical and photovoltaic properties of heterojunction devices were characterized by illuminated and dark I-V measurements, C-V measurements and spectral response measurements. Temperature dependent dark I-V measurements suggest that the carrier transport in the devices is controlled by recombination in the space charge region. The open circuit voltage was found to increase proportionally with reductions in QD size, which may relate to a bandgap widening effect in the Si QD or an improved heterojunction field allowing a greater split of the Fermi levels in the Si substrate. Successful demonstration of Si QD photovoltaic devices is an encouraging step towards realisation of all-silicon tandem solar cells based on Si QD materials.