Trap assisted tunneling modelling for aSi contact stacks in IBC-SHJ

Interdigitated back contact silicon heterojunction (IBC-SHJ) solar cells have demonstrated 26.6\% world record efficiency by combining outstanding passivation of Si thin film layers with the absence of front shading contact in the crystalline silicon (c-Si) absorber bulk. Furthermore, this type of solar cell merges advantages of c-Si with Si thin film technology that allows for adjustment and tuning of bandgap and Fermi energy in deposited layers, and thus, modifying material properties that also impact the carrier collection. As heterointerfaces tailor to band offset and potential barriers for collecting carriers, the transport is described by thermionic emission and tunneling mechanisms. Moreover, Si thin film layers typically exhibit a defective matrix with characteristic traps and charge distribution that affect solar cell external parameters. Such phenomena involves the so called trap assisted tunneling (TAT) together with trap recombination mechanisms in a complex physical system.In this work, the effect of TAT mechanisms on IBC-SHJ is studied by means of numerical simulations based on TCAD Sentaurus. Firstly, the TAT model is implemented as non-local process. It was found that TAT exhibits a negligible effect on electron collection, but dominates transport mechanisms in case of hole contact. Such an effect depends on band alignment at the doped layer/transparent conductive oxide (TCO) interface, where band-to-band tunneling in combination with TAT describe the transport of holes. In particular, TCO carrier concentration in combination with activation energy of deposited layer allows the collection by either TAT or band-to-band tunneling. In more detail, the density of traps described typically as dangling bond improves the transport of collecting carriers. Thus, the collection of carriers is evaluated in terms of energy and trap concentration, demonstrating that traps with an energy level of 0.5 to 0.7eV over the valence band enhances the FF, and thus, the solar cell performance. Finally, the complete model is calibrated by comparing measured with simulated external parameters from a reference solar cell with and without TAT mechanisms. From this analysis, it is demonstrated that solar cell external parameters deploys more consistent values for Shockley-Read-Hall (SRH) recombination associated to bulk lifetime and surface recombination velocity using TAT model.Electrical Engineering | Sustainable Energy Technolog