Influence of defects and band offsets on carrier transport mechanisms in amorphous silicon/crystalline silicon heterojunction solar cells

We have investigated the carrier transport mechanisms in undoped a-Si:H/p-type c-Si heterojunctions with and without a μc-Si buffer layer, as well as their effects on the photovoltaic properties of the junction. The conduction behavior of the junction is strongly affected by the defect state distribution and band offset at the hetero-interface. The recombination process involving the interface states on the thin film silicon (a-Si:H/μc-Si) side dominates at low forward bias (V < 0.3 V), whereas multistep tunneling capture emission (MTCE) dominates in the higher bias region (0.3 < V < 0.55 V) until the conduction becomes space charge limited (V > 0.55 V). The MTCE process seems to be more closely related to the bulk defects in the thin film silicon than the interface states. In addition, the position of a trapping level, where the tunneling process occurs, seems to be determined by the hole energy at the edge of the c-Si and the trap distribution in the thin film silicon. Despite the domination of MTCE in the indicated voltage range, the reduced band offset at the interface increases current levels by the enhanced diffusion and/or emission processes. The insertion of a 200 angstrom thick μc-Si buffer layer between the a-Si:H (700 angstrom)/c-Si increases the solar cell efficiency to 10%, without an antireflective coating, by improving both the carrier transport and the red response of the cell.