Silicon Solar Cell Passivation using Heterostructures

Standard surface passivation schemes for crystalline silicon solar cells use SiO2 or SiNx. The c-Si surface passivation mechanisms related with these schemes have been elucidated within the framework of interface recombination modeled by an extended SRH formalism: interface recombination centers characteristic of the SiO2 passivation have larger electron (e) than hole (h) capture cross sections and SiO2 yields thus poorer passivation of lower resistivity p-type c-Si, while SiNx layers lead to the occurrence of a large fixed charge density leading, for instance, to parasitic leakage currents that adversely affect the performance of SiNx rear passivated p-type cSi solar cells. An alternative to these two major schemes is hydrogenated amorphous silicon (aSi:H). In this paper, we determine first differences between the natures of the electronic recombination centers. To understand the passivation properties of a-Si:H, we then introduce an interface recombination model based on the amphoteric nature of silicon dangling bonds. We show experimentally and theoretically that a-Si:H is a high performance and broad range surface passivation material as its e and h capture cross sections on neutral defects are similar, while additional field effect passivation can be tuned by further growth of doped a-Si:H layers. The quality of the passivation is illustrated with a lifetime of 7ms reached on lightly p-type doped c-Si passivated with intrinsic a-Si:H, which is to the best of our knowledge the highest ever measured value for a-Si:H passivated wafers. By using our modeling of the a-Si:H/c-Si interface recombination as a guidance for interface and solar cell improvement, we are able to fabricate full a-Si:H/c-Si heterojunction devices reaching a maximal open-circuit voltage of 713mV and a maximal efficiency of 19.1% on flat c-Si wafers, using very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD).