Thin Film Polycrystalline Silicon Solar Cells Studied by Transient Terahertz Probe Spectroscopy

Thin film polycrystalline silicon (poly-Si) solar cells were annealed in hydrogen plasma or in water vapour to suppress electrical activity of defects and impurities (passivation). The hydrogen plasma passivation procedure was optimized and a modification of several hydrogenation parameters was suggested, e.g. higher hydrogen pressure of 300 Pa and keeping bias voltage Vbias constant during the whole passivation process until the plasma was switched off. Since the plasma hydrogenation process was run as a closed system (without hydrogen flow) and the achieved passivation results (up to 497 mV) were close to the limit for poly-Si (500 mV), the generally accepted necessity to operate the process with a hydrogen flow was called into question. Samples passivated in a hydrogen plasma at different processing conditions were analyzed by transient terahertz probe spectroscopy and Suns-VOC method to measure a solar cell's charge carrier lifetime and its open-circuit voltage VOC, respectively. A correlation of these measured parameters was observed even though both these techniques characterize solar cells in completely different states. The transient terahertz probe spectroscopy appears to be a useful tool for a contactless investigation of materials where ultrafast processes of ns-long lifetimes play a crucial role, e.g. nanostructured objects and materials.