Modeling of Transport Mechanisms in Silicon Heterojunction Solar Cells: Trap-Assisted Tunneling at the Emitter Interface

Silicon heterojunction (SHJ) solar cells exhibit outstanding performance, but the physical transport mechanism at the emitter contact remains a subject of debate. This study investigates the hole extraction process at the TCO/a-Si:H interface by means of numerical simulations. Efficient carrier collection is essential for device operation, requiring photogenerated electrons and holes to be selectively extracted. At the emitter contact, holes accumulate at the a-Si:H/c-Si interface and can be extracted by means of two possible mechanisms. The first, thermionic emission over the c-Si/a-Si:H valence band barrier, is unlikely at room temperature due to the significant barrier at the a-Si:H/TCO interface, which limits the hole emission. Instead, a more plausible mechanism is trap-assisted tunneling (TAT) or trap-to-trap tunneling, where defect states within the a-Si:H band gap facilitate hole transport toward the TCO. This mechanism effectively explains the efficient hole extraction observed in SHJ devices. To validate these hypotheses, the transport mechanisms were implemented in a Sentaurus TCAD simulation. Electron collection at the base contact was a less critical process, since it is sufficient to allow direct tunneling, based on the WKB approximation, through the thin a-Si:H barrier. At the emitter contact instead, the TAT mechanism was modeled using a non-local mesh (NLM) approach, where the holes are coupled to the defects inside the a-Si barrier from the a-Si:H/c-Si interface and the electrons tunnel from the TCO. Once both carriers reach the traps, they recombine, generating the output current. This model incorporates a Shockley-Read-Hall-like recombination model, where capture and emission rates account for both elastic and phonon-assisted processes. The strong agreement between simulated and experimental electrical curves supports the proposed transport model and provides further insight into the physical mechanisms governing hole extraction in SiHJ solar cells.