Most modern solar cell architectures utilize electron/hole collecting contacts in order to efficiently collect photogenerated carriers. In the case of Si heterojunction cells, the carrier collecting contacts are heterojunctions stacks which block the collection of one carrier while optimizing the collection of the other. However, the resistive losses incurred by the heterojunction stacks are not been completely understood, as the description of transport requires further investigation. These resistive losses are detrimental to the overall fill factor (FF) of the solar cell.
The transmission line method (TLM) is often used to characterize the resistive losses of contacts by measuring the contact resistivity. This method allows us to analyze a simpler device structure whilst capturing all the representative physics of the contact stack. Previously, we conducted experiments using the TLM technique to measure the contact resistivity of a contact stack that consisted of n-type indium tin oxide (ITO), p-type amorphous silicon (a-Si(H):p) and intrinsic amorphous silicon (a-Si:(H):i) on top of an n-type crystalline silicon (c-Si:n) wafer . In this work we perform numerical simulation using Silvaco ATLAS of the TLM method and calculate the contact resistivity for the ITO:n/a-Si(H):p/a-Si(H):i/c-Si:n contact stack in comparison to the experimentally extracted values. By recreating the TLM structure in simulations, we can explore the contributions of the different layers and interfaces that are present in the contact stack. This methodology can also be applied to study contact stacks comprised of different materials.
We simulated the contact resistance for a contact stack with increasing a-Si(H):i layer thickness and obtained a good match with experiments when we explicitly model the ITO as a semiconductor (as opposed to a simple metal contact). For an increase in intrinsic layer thickness from 4 to 16 nm, we observed an increase in contact resistivity from 0.5 to 0.98 Ω cm2. The contact resistance increased with a decrease in emitter doping as low dopings caused the emitter to deplete; hence increasing the resistance of the layer. Simulations treating the ITO as a metallic contact gave poor agreement with experiment in contrast. The treatment of ITO as a degenerate widebandgap semiconductor adds another layer of detail to the transport picture as the ITO:n/a-Si(H):p forms a reverse biased pn junction when the cell is forward biased. Our simulations indicate that tunneling is a dominant mechanism at ITO/emitter interface. Finally, we also explore the behavior of contact resistance under different temperatures and illuminations in comparison with experiment.
 M. Leilaeioun, W. Weigland, P. Muralidharan, M. Boccard, D. Vasileska, S.M. Goodnick and Z. Holman, "TLM measurements varying the intrinsic a-Si:H layer thickness in silicon heterojunction solar cells", PVSC 2017.