CO₂ Plasma Treatment To Prepare the Rear Emitter with a Boron-Doped Hydrogenated Amorphous/Nanocrystalline Silicon Stack for a High-Efficiency Silicon Heterojunction Solar Cell

A rear emitter with a p-type boron-doped hydrogenated amorphous silicon/nanocrystalline silicon [a-Si:H(p)/nc-Si:H(p)] stack was prepared for the silicon heterojunction (SHJ) solar cell to improve its short-circuit current density (J_SC). CO₂ plasma treatment (CO₂ PT) was applied to a-Si:H(p) to facilitate the crystallization of the subsequently deposited nc-Si:H(p). To evaluate the effect of the CO₂ PT, two different nc-Si:H(p) layers with low and high crystallinity (χ_C) were investigated. For the emitter with low crystallinity (χ_C = 21%), the solar cell efficiency could boost from 23.6 to 25.0% with a 10 s short-time CO₂ PT primarily due to the significant increase in fill factor (FF) and J_SC. χ_C of the emitter increased to 41%. For the emitter with high crystallinity (χ_C = 60%), the solar cell efficiency increased from 25.1 to 25.2% with a 7 s short-time CO₂ PT. χ_C of the emitter increased to 70%. The corresponding improvement mechanisms were analyzed by combining high-resolution transmission electron microscopy (HRTEM) and external quantum efficiency (EQE) measurements. It is considered that an appropriate short-time CO₂ PT to a-Si:H(p) can make the subsequent nc-Si:H(p) more transparent and conductive by facilitating its crystallization without deteriorating the underneath passivation layer. Thus, the solar cell efficiency can be improved, especially with the enhanced FF and J_SC. As a demonstration, the SHJ solar cell with the CO₂ PT-treated a-Si:H(p)/nc-Si:H(p) stack emitter achieved an efficiency of up to 25.27%.