Band tail states in microcrystalline silicon solar cells probed by photoluminescence and open circuit voltage

Thin film p-i-n microcrystalline silicon (mu c-Si:H) solar cells were studied by photoluminescence (PL) spectroscopy and open circuit voltage, V-oc. By this means, the cause of the commonly observed increase of V-oc when the silane concentration in the gas phase for the preparation of the intrinsic absorber layers is raised is addressed. The hot wire chemical vapour deposition (HW-CVD) technique was used for the preparation of films and intrinsic absorber layers of solar cells with efficiencies higher than 9%. By monitoring V-oc and the PL energy, E-PL, as a function of temperature, information on the photogenerated carrier distributions and the splitting of the quasi-Fermi energies was gained and the effect of states in the gap was studied by comparison with results from a crystalline p-i-n diode. An increase of E-PL and V-oc for increasing SC and decreasing temperature is attributed to the shift of the excess carrier distributions to higher energies. It is proposed that this shift is limited by band tail states. It is proposed that increasing SC leads to a reduction of the density of band tail states, due to structural relaxation of the mu c-Si:H network by the presence of hydrogen or hydrogenated amorphous silicon