A Unified Chemical Bonding Model for Defect Generation in a-SiH: Photo-Induced Defects in Photovoltaic Devices and Current-Induced Defects in TFTs

Abstract

Generation of metastable neutral and charged defects in hydrogenated amorphous silicon (a-Si:H) by reactions involving changes in H-atom bonding arrangements has been studied by ab initio calculations applied to small molecules. A unified defect generation model has been developed in which reactions pathways for photo-induced defect generation in PV devices are initiated by hole-electron pair recombination and/or deep trapping, and in thin film transistor (TFT) devices by electron trapping. Reactions pathways are proposed for intrinsic processes involving only Si and H-atoms, as well as extrinsic processes involving O and NH impurities as well. In photovoltaic (PV) devices, defect generation includes the creation of neutral and charged defects and involves displacive rather than diffusive hydrogen motion, thereby ensuing metastability. The positively charged defects are associated with over-coordinated H-, O- and N-atoms. In TFT devices, defect generation results from electron trapping in anti-bonding orbitals of Si–H groups, and includes the formation of neutral and negatively charged Si-atom dangling bonds.