Role of gap states in discharge produced a-SiHx in determining carrier transport and recombination

The role of gap states in determining the transport of recombination of carriers in hydrogenated amorphous silicon (a-SiHx) is examined for discharge produced films such as are used in efficient solar cell structures. Junction, photovoltaic and photoconductivity characteristics, are presented and discussed for these films. Free carrier transport is found to be present over the temperature range of 400 to 100 K, with the role of states near the free carrier bands being limited to trapping. This is in direct contrast to the previously reported transition from carrier transport to hopping through these localized states and with direct carrier recombination between these states at about 250 K. As a result the recombination kinetics can be directly related to the different states present in the gap of a-SiHx, and examples of this are given for both electrons and holes. The recently published reversible conductivity changes must be taken into account if the ambiguities in the interpretation of the kinetics data are to be avoided.