Toward silicon quantum dot junction “all-silicon” tandem solar cells

Silicon quantum dot-based "all-silicon" tandem solar cells have been proposed as a third-generation technology with features of high energy conversion efficiency and low cost. They combine inexpensive silicon thin-film technology with a high efficiency multi-bandgap tandem cell approach. Cells with different bandgap in the tandem stack can be made by taking advantage of the quantum confinement effect in silicon.Previous studies related to the essentials for implementing this concept are reviewed. The review reveals that two key requirements for implementation of "all-silicon" tandem solar cells are control of silicon quantum dot size distribution and fabrication of a nano-scale p( i)-n junction possibly via doping for dissociating the photo-generated electron-hole pairs. Both are challenging in terms of both theoretical and experimental aspects. Such full silicon quantum dot junctions have never previously been reported.This thesis consists of two parts: (J) fundamental studies of intrinsic silicon quantum dots (for bandgap engineering) and boron- and phosphorus-doped silicon quantum dots (for subsequent fabrication of quantum dot p-n junction); (2) fabrication and characterization of silicon quantum dot p-(i)-n junction devices.Firstly, for intrinsic silicon quantum dots films, size control of the silicon quantum dot and thereby bandgap control is investigated. A method of forming silicon quantum dots by silicon-rich oxide/silicon dioxide multilayers giving features of narrow size-distribution and easily-tuneable dot density is discussed. The effects of the thickness and stoichiometry of silicon-rich oxide layers, as the two most important factors determining the dot size distribution, are investigated. The size distribution of the silicon quantum dots is found to be affected by both the stoichiometry and thickness of these layers.Secondly, boron- and phosphorus-doped silicon quantum dots are investigated, in the sequence of monolayer and the multilayer structures: I) Effects of boron- and phosphorus-doping on the properties of silicon quantum dots formed as a relatively wide monolayer are investigated. Boron and phosphorus show different effects on silicon quantum dot properties and subsequently on optical properties. The optimized parameters regarding doping of monolayers are obtained for subsequent work with boron- and phosphorus-doped silicon quantum dots formed as multilayers; 2) Effects of boron- and phosphorus-doping on the properties of silicon quantum dots formed as multi layers are investigated. The addition of phosphorus or boron has little effect on quantum dot size, whereas phosphorus and boron have different influences on crystallinity. Conductivity of silicon quantum dots multilayers can be altered by adding either phosphorus or boron, whereas the conductivity shows a different trend upon increasing doping concentration between phosphorus and boron. The results provide optimized parameters for the subsequent realization of silicon quantum dot junctions.Finally, based on the optimized parameters obtained in the above fundamental studies, silicon quantum dot junction devices are fabricated and characterized. Structural parameters of silicon quantum dot multi layers are optimized to improve the conductivity of silicon quantum dots film towards the realization of full quantum dot junction devices. Full silicon quantum dot junction solar cells formed as multilayers are then successfully fabricated for the first-time by a co-sputtering technique. Then the properties of these silicon quantum dot p-i-n junctions are investigated in terms of structural, optical, electrical and photovoltaic aspects. The structure of these junctions is optimized towards demonstrating a high open circuit voltage. The open circuit voltage of quantum dot junction devices, the most critical parameter at the present stage of development, is increased from 0.078 V to -0.5 V during this work. The mechanism of improvement is investigated and discussed. Suggestions are also brought forward to further improve the performance of full silicon quantum dot junction solar cells in the future