Poly-Si passivating contacts formed via spin-on doping for c-Si solar cells

Abstract

With the constant improvement of crystalline silicon solar cells over the last decades, the carrier recombination and transportation at the contacts have become limiting factors to efficiency improvement for crystalline silicon solar cells. A potential solution is to use passivating contact, which suppresses carrier recombination while simultaneously ensures efficient majority carrier transportation. The polycrystalline silicon (poly-Si) passivating contact has shown a low carrier recombination current density, a low contact resistivity and a high thermal stability which can be easily integrated with the current high efficiency industrial silicon solar cell production lines. A wide range of doping methods have been employed to fabricate poly-Si passivating contacts, but only a small amount of work has been presented on spin-on doping process. The spin-on doping technique involves less dangerous dopant precursors than other doping techniques like dopant gas diffusion and a simpler process tool than e.g. ion implantation, and it may promote the doping process versatility based on its features of diverse dopant elements, wide dopant concentration range, and the ability of single-side doping. However, more comprehensive and detailed investigations are required for the poly-Si passivating contacts fabrication by using spin-on doping. Thus, in this thesis we focus on the application of spin-on doping process on industrially processed poly-Si passivating contacts. We performed detailed investigations on phosphorus and boron spin-on dopants. For both phosphorus and boron spin-on doping, the various spin speed and acceleration during spin coating process show slight impacts on passivation quality. Besides, a longer baking time leads to better passivation and lower sheet resistance. In addition, the drive-in process yields large impacts on passivation and electrical properties. Implied open-circuit voltages of ~730 or 705 mV are obtained after 975 or 950 centigrade drive-in process and a forming gas annealing (FGA), for phosphorus or boron case respectively. A low contact resistivity below 5 mOhmcm2 for both, phosphorus or boron, spin-on doping cases were achieved. These results are to our knowledge the highest iVOC values achieved by spin-on doping to date and are close to the performance of gas diffused or ion implanted poly-Si contacts. This shows the potential of spin-on doping process as alternative to traditional gas diffusion or ion implantation doping methods. Show more