Micro-structural defects in thin film photovoltaic devices
The efficiency of thin-film PV is limited by atomic-scale defects in the material (grain boundaries, planar defects, point defects etc). The manufacture of a high efficiency cell typically requires some form of defect passivation treatment. Examples include activation of CdTe by annealing in CdCl2, sodium doping in CIGS and the use of mixed-halide perovskites. Recent work at Loughborough using HRTEM has shown that as deposited CdTe contains high densities of stacking faults and that these are removed by the CdCl2 activation process, see Fig . DFT modelling at Bath University has confirmed that certain types of stacking fault are ‘hole killers’. This work has shown that the combination of HRTEM and advanced DFT modelling can provide vital insights into our understanding of the role of defects and their effect on device performance. Previous characterisation has focused on only one type of defect, rather than the material as a whole. It has therefore not been possible to draw conclusions on the relative impact of different defects on cell efficiency or identify the dominant defect passivation mechanism(s). Still less work has been done comparing defects in different materials systems, although they share common properties. In this project, the student will use advanced characterization techniques to study defects in thin film CdTe, CIGS, CZTS and perovskites deposited using different techniques. The project aims to determine how atomic-scale defects collectively degrade cell efficiency and the fundamental mechanisms underpinning their passivation. From this we can develop strategies for higher efficiencies. Electron microscopy is the primary characterisation tool, since it enables simultaneous structural, chemical and electrical property measurement on nearly every class of defect. Supporting this is the emergence of advanced electron microscopy instrumentation, which has created unprecedented opportunities for exploring materials at the atomic scale. Their application in PV is still to be fully exploited. Results will be shared with collaborators working on DFT at Bath University (Prof Walsh). Ideally, a project would also be supported at Bath thereby exploiting the multi-institutional nature of the CDT and its ethos of creating a cohort culture. The student will also be exposed to International and Industrial engagement with samples supplied from leading thin film PV laboratories at Colorado State University, Colorado School of Mines, NREL and First Solar Inc.
Prof. Mike Walls