Intrinsic amorphous silicon bilayers for surface passivation in silicon heterojunction solar cells

Abstract

The performance of silicon heterojunction (SHJ) solar cells is highly influenced by the microstructural characteristics of hydrogenated amorphous silicon (a-Si:H) layers used for surface passivation. This study investigates the optimization of intrinsic amorphous silicon (i-a-Si:H) bilayers, in which two intrinsic layers are sequentially deposited under distinct plasma conditions to achieve improved interface passivation. While high porosity in the first intrinsic layer (i1) is conventionally achieved using pure silane plasma, we demonstrate that a comparable porous buffer layer can also be realized by employing hydrogen-diluted plasma at elevated power, providing an alternative process route for porous layer formation. Through systematic variation of deposition parameters, the optical properties and Si-H bonding configuration of the resulting thin films are characterized using ellipsometry and ATR spectroscopy, respectively. Microstructure factors are derived from ATR data to interpret the porosity of the layers. Bilayers (10 nm) comprising a porous interfacial layer and a dense overlying layer were applied on c-Si surfaces. Optimal passivation was achieved with a first layer thickness of 2-4 nm and a microstructure factor of about 0.4. Longer post-HPT durations led to an increase in minority carrier lifetime, with the effect being more pronounced for thinner i1 layers. Integrating these bilayers on either the n-or p-side of the device revealed that tuning the first intrinsic layer thickness improves open-circuit voltage (VOC), and overall efficiency. These findings demonstrate that precise interface engineering can simultaneously enhance both surface passivation and device performance of SHJ solar cells.