Dipoles and defects caused by CO₂ plasma improve carrier transport of silicon solar cells

Carrier-selective contact is a fundamental issue for solar cells. For silicon heterojunction (SHJ) solar cells, it is important to improve hole transport because of the low doping efficiency of boron in amorphous silicon and the barrier stemming from valence band offset. Here, we develop a carbon dioxide (CO2) plasma treatment (PT) process to form dipoles and defect states. We find a dipole moment caused by longitudinal distribution of H and O atoms. It improves hole transport and blocks electron transport and thus suppresses carrier recombination. In the meantime, the CO2 PT process also results in defect states, which reduce passivation performance but improve hole hopping in the intrinsic amorphous layer. As a balance, an appropriate CO2 PT process at the i/p interface increases fill factor and power conversion efficiency of SHJ solar cells. We emphasize, based on sufficient evidences, this work finds a distinct role of the CO2 plasma in SHJ solar cells opposed to reported mechanisms.