Inhomogeneous transport in microcrystalline p-i-n devices

Entirely microcrystalline hydrogenated silicon p-i-n structures presenting an enhanced sensitivity to the near-infrared region (greater than 1000 nm) are analysed under different external voltage biases and light illumination conditions. A two-phase model to explain the transport properties is proposed using as input parameters the measured experimental data. The role played by the boundary regions between the crystalline grains and the amorphous tissue is treated similarly to a junction interface and leads to the presence of local electric fields. The influence of the local electric field on the transport mechanism is outlined. The results show that the transport is preferentially concentrated in the crystalline grains. The conduction within the amorphous regions is poor and it contributes to the transport only by allowing a percolation path of the carriers through the crystalline grains. The percolation paths are different for electrons and holes and are determined by the local fields at the boundaries. These local fields are independent of the externally applied condition, and they can be related to the presence of the small positive photocurrent observed when a bias voltage is applied, which is higher than the open-circuit voltage.