Amorphous hydrogenated silicon modified by femtosecond laser radiation for photovoltaics

Thin film technology based on hydrogenated amorphous silicon (a-Si:H) has been playing a significant role in thin-film photovoltaics. However, a-Si:H based solar cells suffer from a low carrier mobility and light-induced degradation. In order to diminish these effects, tandem solar cells based on amorphous and nanocrystalline silicon (nc-Si) are developed. One of the technologically attractive methods for incorporating nc-Si into a-Si:H is laser induced crystallization.Most of the studies on laser crystallization of a-Si:H employ picosecond and nanosecond laser pulses with radiation photon energies higher than the band gap in a-Si:H of 1.6-1.8 eV. In the last few years, femtosecond laser driven crystallization of a-Si:H films attracts considerable interest. Intense femtosecond laser pulses can alter the structure of the film at photon energies smaller than the band gap. Numerous studies on the structural transformation of a-Si:H induced by femtosecond laser pulses have been reported. However there is a lack of data on the electric and photoelectric properties which are vital for the application of femtosecond laser introduced structures in solar cells.Here we present the results of optical, electric and photoelectric properties of a-Si:H films treated by femtosecond laser radiation with different wavelengths and laser fluencies. Specifically the correlation between changes of these properties and the film structure is shown and the problem of hydrogen out-diffusion is discussed