Optical absorption by gap states in amorphous silicon

Measurements of the steady state primary photocurrent for photon energies between 0.58 and 2.0 eV have been made on a-Si:H solar cell structures by means of blocking contacts. From these, the absorption coefficient for weakly absorbing gap states has been determined. In addition, transient secondary photocurrents have been measured to determine the drift mobility of the majority carriers. It is shown that photon absorption by gap states produces electrons in the same state as absorption by band states. This result leads to the conclusion that holes are mobile in gap states above the valence band.