Hydrogenated amorphous silicon p¿i¿n solar cells deposited under well controlled ion bombardment using pulse-shaped substrate biasing

We applied pulse-shaped biasing (PSB) to the expanding thermal plasma deposition of intrinsic hydrogenated amorphous silicon layers at substrate temperatures of 200¿°C and growth rates of about 1¿nm/s. Fourier transform infrared spectroscopy of intrinsic films showed a densification with increasing deposited energy and a reduction in void content, whereas dual-beam photoconductivity measurements showed an increase in Urbach energy above 4.8¿eV/Si atom. From dark conductivity and photoconductivity measurements, we determined a maximum photoresponse of 2¿×¿106 at 3¿eV/Si atom, which decreased at higher deposited energies because of a higher dark conductivity as a result of a lower band gap. p¿i¿n solar cells with PSB applied during the intrinsic layer deposition showed initial energy conversion efficiencies of 7.4% at around 1¿eV/Si atom. Decreasing open-circuit voltage at >1¿eV/Si atom can be related to a lower band gap, whereas the short-circuit current drops at >4.8¿eV/Si atom, predominantly because of hole collection losses as determined from quantum efficiency measurements. The reduced fill factor for >1¿eV/Si atom was presumably related to a decrease in mobility-lifetime product because of an increase in defect density.