Transport considerations in hydrogenated amorphous silicon materials with widely varying mobilities and the consequences on device performance

The transport properties of device quality hydrogenated amorphous silicon (a-Si:H) materials deposited at different substrate temperatures, T_s, are investigated. The mobilities are found to vary continuously from about 1 cm²/Vsec to 30 cm² /Vsec as T_s increases from 220°C to 350°C and, in some cases, the mobilities are temperature activated. These results provide an explanation for the Meyer-Neldel rule in a-Si:H films with dark conductivity activation energies greater than 0.4 eV where it cannot be explained by the statistical shift of the Fermi level. The solar cells formed with the different intrinsic layers exhibit decreasing short-circuit currents, open circuit voltages and fill factors despite the increased mobilities with T_s. These results need to be carefully modeled in order to obtain further insights into the operation of a-Si:H p-i-n solar cell operation