Silicon quantum dot (Si QD) tandem solar cell is a promising cell structure for realising high efficiency at low cost. The tandem solar cell effectively harnesses energy from the solar spectrum by stacking two or more cells together in the order of descending band gaps. Due to quantum confinement, the band gap of silicon based nanostructures such as Si QDs can be tailored by varying the size of the QDs. Solar cells and light emitting diodes based on Si QDs have been realised in experiments. However, current crowding due to high lateral resistance remains to be a major problem for Si QD devices grown on quartz substrates. Annealed silicon rich carbide (SRC), owing to its electrical conductivity, thermal stability and energy band gap compatible with Si QD cell fabrication, has the potential to overcome this problem. Further, this quasi-transparent thin-film can be used as either substrate or superstrate of a Si QD solar cell and therefore provides flexibility in cell structure design. Here, we investigate the physical, optical and electrical properties of the new material as functions of silicon concentration and doping conditions via a number of characterisation techniques including X-ray diffraction, Raman spectroscopy, ultraviolet-visible-infrared spectroscopy and four-point probe measurement. Some discoveries, including the lower crystallisation temperature of SiC within SRC, are also discussed. The research may provide some insight into the optimisation of annealed SRC as the new conductive material for Si QD solar cell and may boost the final arrival of all-silicon tandem solar cell.
A vertical structure with a back contact layer is suggested for silicon quantum dots (Si QDs) solar cells to overcome the current crowding effect arising from the high lateral resistance in the emitter layer of the existing mesa-structured Si QDs solar cells on quartz substrates. Molybdenum (Mo) is widely used as the back contact layer in CIGS solar cells due to its high electrical conductivity, good optical reflectance and chemical stability. This paper will focus on the feasibility of Mo as a back contact layer deposited between a quartz substrate and a sputtered silicon rich oxide (SRO) bilayers structure to obtain a fully vertical Si QDs solar cell. In this structure, the desired previously mentioned electrical and optical properties of the Mo thin film have to be maintained during and after a high temperature annealing process. This high temperature process is unavoidable in this structure as it is required to form the Si QDs. This paper aims to study factors that have impacts on critical properties of the Mo thin films processed in contact with Si and SiO2 at high temperatures. Characterizations including film thickness, microstructure, sheet resistance and optical reflectance measurements are also performed. Furthermore, interfacial properties between the Mo layer and the upper SRO bilayers are investigated.