Relation of defects and grain boundaries to transport and photo-transport: Solved and unsolved problems in microcrystalline silicon

Defects and grain boundaries play a crucial role in the dark and photo-transport of charge carriers. Surprisingly, the transport (trapping and recombination) in microcrystalline silicon is better understood at low temperatures, while room-temperature operation is of interest for real-life devices. In the first part of this review, the advantages of photo-transport techniques, used for the defect density evaluation, will be recapitulated and commented on. The second part is devoted to the present understanding of the specific features of transport in microcrystalline silicon like anisotropy, dominant transport path and the role of H and O in the grain boundary formation. The results of macroscopic measurements on series of samples will be confronted with the results of local conductivity studies on a nanometer scale and finally, the influence of oxygen and the ability to explain it by our model of transport will be illustrated.