The microstructure and electrical properties of grain boundaries in polycrystalline thin film photovoltaic devices
Recombination of charge carriers occurring at defects and grain boundaries (GBs) are detrimental to the electrical performance of thin film photovoltaic devices. However, recent studies of thin film CdTe devices have shown that polycrystalline films outperform the single crystal PV counterparts after the activation treatment with cadmium chloride at 400֯C. The treatment removes stacking faults and the chlorine passivates the GBs, nonetheless, the precise mechanism that lead to the enhanced electrical performance of the treated CdTe thin film remains unclear. To further improve the efficiency of the PV films, we need to understand the grain boundary activities during CdCl treatment. We will use in-situ heating SEM-EBSD and TEM to obtain the real time evidence of the boundary migration and re-crystallisation. On the other hand, activation treatment is not required to achieve high performance CIGS and perovskite devices. The structure and chemistry of GB’s in these devices will be investigated for comparison. The focus of this project is to obtain an extensive characterisation of the GBs. FIB-assisted EBSD and EBIC will be used to reconstruct the GB networks and current collection activity in 3D. High resolution TEM and AC-STEM and EELS will be applied to understand the GB atomic and electronic structure. Along with complementary DFT studies in the Department of Chemistry, a systematic and wide range of GBs (not limited to ∑3 and ∑9) will be investigated to understand the passivation mechanisms of GBs in thin film solar cells. The ultimate aim of this project is to optimize the cell efficiency via GB engineering, building on the detailed 3D microstructural and chemical information obtained from the proposed work.