Intrinsic layer modification in silicon heterojunctions: Balancing transport and surface passivation

Abstract

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, …). Modified intrinsic layers have been tested on cell level, resulting in a series resistance ( R s ) reduction by about 0.3 Ωcm 2 and an increase in fill factor ( FF ) by roughly 1.0 % abs . The power conversion efficiency ( η ) 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. • Void-rich a-Si(i) is essential to prevent epitaxial growth for good surface passivation, but is also the main contributor to a-Si:H(i)-induced transport losses for both electron and hole contact. • Both thickness and microstructure of the void-rich a-Si:H(i) are decisive for the cell’s series resistance, small differences can have a huge impact on transport and surface passivation. • The microstructure of void-rich a-Si:H(i) is carefully tailored towards denser films via a-Si:H deposition parameters and a post-deposition hydrogen plasma treatment to significantly improve transport and the conversion efficiency of SHJ solar cells.