Hydrogen plasma treatment on a-Si:H thin films for c-Si surface passivation

The most attracting way to fabricate high efficiency solar cells is the amorphous/crystalline silicon technology, because of the high Voc obtainable as a consequence of excellent c-Si surface passivation offered by a-Si:H films. This passivation is obtained by saturation of silicon dangling bonds at c-Si/a-Si interface and can be influenced by the hydrogen inclusion within the a-Si:H layer. In this work we propose the use of hydrogen plasma treatments subsequently performed on the thin amorphous silicon layer deposited to passivate the c-Si surface. We compare the hydrogen effect on the interface with that of thermal annealing of the interface to identify how the hydrogen evolves inside the a-Si:H network modifying the defect density and the a-Si:H/c-Si interface. To monitor the hydrogen effect on the heterointerface we propose the use of surface photovoltage technique as a contact-less tool for the evaluation of the energetic distribution of the state density at amorphous/crystalline silicon interface. This technique results to be very sensitive to the different experimental treatments, and therefore it can be considered a precious tool to monitor and improve the interface electronic quality. These characteristics are compared with FTIR spectra measurements performed on both hydrogen plasma post-treated and thermal annealed a-Si:H/c-Si samples. We find out that hydrogen plasma post treatment is effective to stably reduce interface state density.