Kesterite Thin Film Solar Cell Fabricated By Sulphurising Sputtered Metallic/Metal-Sulphide Precursor

摘要

Photovoltaics is regarded as one of the most potentially important clean energy sources in the future providing it is able to meet the demands of a lower production cost. It requires cheap materials, equipment and processes as well as a high energy conversion efficiency. The focus of this thesis is on kesterite thin film solar cells, mainly Cu2ZnSnS4 (CZTS) and Ge-alloyed Cu2ZnSnxGe1-xS4 (CZTGS), prepared by a cost-effective two-step approach of magnetron sputtering and post-sulphurisation –an emerging thin film photovoltaic technology with a potential to meet the requirements stated above. It combines the advantages of the mature and high-performance Copper-Indium-Gallium-Selenide/Sulphide (CIGS) photovoltaic technology with cheaper and non-toxic materials. Besides, it allows a large-area production of thin film solar cell with the cheap and controllable deposition approach of magnetron sputtering.This thesis presents the author’s research results on kesterite thin film solar cell prepared by sulphurising sputtered metallic/metal-sulphide precursor. It covers a wide range of important topics: the influence of precursor parameters and sulphurisation conditions on kesterite thin film quality and the associated photovoltaic performance, complete cell fabrication and characterisation, thorough investigation of kesterite crystallization process and the influence of Ge alloying on the property of CZTGS thin film. It can be categorised into two parts according to the different kesterite materials studied. The first part focuses on fabricating the CZTS thin film absorber and the associate CZTS thin film solar cell. A novel precursor type of Zn/(Cu&Sn) with Zn as the bottom layer and Cu-Sn co-sputtered as the top layer is designed for synthesizing the CZTS thin film absorber. A best device efficiency of 4.47% is achieved by optimizing the sulphurisation conditions. The crystallization process of CZTS is studied to reveal the reaction sequence at different temperature ranges. Other important issues are also addressed, including the formation mechanism of voids, the influence of different precursor stacking orders and the influence of Zn concentration. The second part studies the Ge-alloyed CZTGS thin films. A systematic study is presented on the variation of structural, optical and electrical properties of CZTGS thin film caused by Ge alloying. Additionally, the fabrication of CZGS thin film by sulphurising sputtered metal-sulphide precursor is developed. The first working CZGS thin film device with an efficiency of 0.56% is manufactured. Related issues like the influence of sulphurisation conditions and the influence of sodium diffusion on the CZGS grain growth are also discussed.