Enhancing Optical and Electrical Performances via Nanocrystalline Si-Based Thin Films for Si Heterojunction Solar Cells

Silicon heterojunction (SHJ) solar cells, as one of the most promising passivated contact solar cell technologies of the next generation, have the advantages of high conversion efficiency, high open-circuit voltage, low-temperature coefficient, and no potential-induced degradation. For the single-side rear-emitter SHJ solar cells, the n-type carrier selective layer, which serves as the light-incident side, plays a pivotal role in determining the performance of heterojunction devices. Consequently, a superior n-doped layer should exhibit high optical transmittance and minimal optical absorption, along with a substantial effective doping level to guarantee the formation of dark conductivity (σ_d) and electron-transport capacity. In this work, we investigated the optical and electrical properties of different n-type monolayers and stacked gradient multilayers, including monolayer, bilayer, and trilayer Si-based thin films, acting as electron-transport layers (ETL) prepared by plasma-enhanced chemical vapor deposition, and studied the influences of these above layers on the performance of SHJ solar cells. The experimental results demonstrate that the ETL with an n-nc-Si:H/n-nc-SiOx:H/n⁺-nc-Si:H trilayer structure exhibits the potential to boost highly efficient solar cells. The bottom highly crystallized, lightly phosphorus-doped n-nc-Si:H film promotes rapid nucleation of the intermediate n-nc-SiOx:H film and thus reduces the thickness of the incubation layer, as well as improves the passivation contact. The n-nc-SiOx:H film in the middle layer provides excellent optical properties and reduces parasitic absorption, thereby increasing the short-circuit current density. Furthermore, the highly doped n⁺-nc-Si:H at the top offers an optimal ohmic contact with the reactive plasma deposition-grown TCO layer, which ultimately enhances the fill factor. Ultimately, a conversion efficiency of 20.41%, with an open-circuit voltage of 720 mV, a short-circuit current density of 39.34 mA/cm², and a filling factor of 72.05%, was achieved in the SHJ solar cell using a typical trilayer structure. This kind of trilayer structure has a particular significance for potential industrialized applications as it allows for efficient utilization of solar energy.