Modelling and Evaluation of Contact Resistance in High-Efficiency c-Si Solar Cells Featuring Carrier-Selective Passivating Contacts

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Abstract

In this thesis, contact resistivity for carrier-selective contacts (CSCs) is evaluated by using finite element simulations TCAD Sentaurus. First, the process of transmission line measurement (TLM) is modelled and validated based on current-voltage (I-V) data comparison between reference experiment and simulation results on polycrystalline silicon (poly-Si) based CSCs. Simulation and experimental data are in a good agreement, thus confirming that the modeling method accurately describes the main physical mechanism. Therefore, the simulation approach is used to evaluate the resistivity of complete contact stack for poly-Si and silicon heterojunction (SHJ) based CSCs. Simulation results reveals that the contact resistivity exhibits a clear dependence on tunneling mechanisms in terms of potential barrier size and band alignment. For poly-Si based CSCs, SiO2 thickness (potential barrier size) is the prevalent parameter impacting on the contact resistivity. Additionally, proper doping in poly-Si and buried region in c-Si can improve the band alignment, thus the contact resistivity becomes more resilient to the effect of the tunneling barrier. For SHJ based CSCs, low contact resistivity values are achieved with high carrier concentration in TCO and low activation energy in doped thin film silicon layer. In general, low activation energy reduces the potential barrier for carrier transport while high TCO carrier concentration allows a better band alignment. In particular, for p-type contact, high carrier concentration in TCO is crucial to ensure an efficient band alignment for band-to-band tunneling at TCO/doped-layer interface. Additionally, the contact resistance depends also on the bandgap and the thickness of the passivating intrinsic amorphous silicon (i-a-Si:H) as they impact on band alignment and also energy barrier size. Indeed, lower values of contact resistance are calculated for thinner i-a-Si:H and narrow bandgap because the reduction of the potential barrier opposing to hole collection. Finally, the presented simulation platform has the potential and flexibility of predicting the contact resistance for any type of CSC stack in terms of materials and number of layers.