Abstract
Cd-free buffer/CIGSe heterojunction solar cells have attracted much attention due to their non-toxicity and potential to enhance the photovoltaic performance. However, the heterointerface may contain a high density of defects, including the vacancies, antisites and defect complexes, leading the severe interface recombination, metastability behavior and the energy barrier blocking carrier transport, lowering the energy conversion efficiency. This dissertation aims to ameliorate the Cd-free buffer/CIGSe heterojunction properties and improves the device performance. We point out the main challenges in designing the heterojunction and propose three approaches to resolving these issues: In the first part, we demonstrate an effective room-temperature chemical solution treatment, by using thioacetamide (S treatment) or thioacetamide-InCl3 (In-S treatment) solution, on CIGSe surface to engineer the ZnS(O,OH)/CIGSe heterojunction. With treatments, the absolute average efficiency is significantly enhanced over 2 %, and the metastability, in terms of light soaking time, is minimized by 48%. The influences of chemical treatments on defect passivation at the interface are studied carefully. In the second part, we propose a novel approach to ameliorate the sputtered Inx(O,S)y/selenized CIGSe heterojunction, in terms of band alignment and interface properties. The band alignment was tailored by tuning the base pressure of the sputtering process to incorporate oxygen into deposited In2S3 layers. The interface properties were ameliorated by optimizing the air-annealing temperature on Inx(O,S)y /CIGSe stacked layers. Our approach enables the average efficiency improved from 2.30 % to 10.93 %. The mechanisms responsible for the improvements are investigated. In the third part, we successfully develop a full sputtered Inx(O,S)y/CIGSe solar cell technology, and investigate the impacts of Na and Se doping on the defect mechanism and device performance. As the Na-doped CIGSe absorbers buffered with Inx(O,S)y layer, the (VSe-VCu) deep acceptor defects were induced, acting as a transport barrier (p+ layer) at interface, decreasing the FF and JSC. By Se-doping, the (VSe-VCu) deep defects could be eliminated, improving the average efficiency to 11.13 %. We also proposed an approach to reducing the (VSe-VCu) defects in Na-doped CIGSe device. By increasing the Cu content and reducing the amount of Na doping, the average efficiency of the corresponding device could be improved from 4.64 % to 9.04 %.