Intrinsic layer modification in silicon heterojunctions: Balancing transport and surface passivation
In this work, the intrinsic hydrogenated amorphous silicon (a-Si:H) layer of silicon heterojunction (SHJ) solar cells was modified to improve carrier transport while maintaining excellent passivation of the c-Si absorber surface. The microstructure of different multilayer intrinsic a-Si:H films was measured and its influence on contact resistance and passivation quality was investigated thoroughly. We show that a pronounced trade-off between passivation and transport exists and that this trade-off is governed by the a-Si:H properties close to the c-Si surface. The fact that the same trend was observed for hole and electron contact suggests that the transport barrier formed by the interfacial a-Si:H layer is governed by a higher resistivity of the void-rich interfacial layer or a less pronounced induced junction and not by asymmetric hole or electron barriers (band offsets, tunnel efficiencies, EL). Modified intrinsic layers have been tested on cell level, resulting in a series resistance (Rs) reduction by about 0.3 Ocm2 and an increase in fill factor (FF) by roughly 1.0 %abs. The power conversion efficiency (i) was improved by about 0.3 %abs with respect to our baseline. Further, the beneficial effect of a hydrogen plasma treatment (HPT) on passivation and transport of the hole contact was shown on device level.