Innovative Charge-Selective Contacts for Simplified Crystalline Silicon Heterojunction Solar Cells

This study investigates the implementation of dopant-free charge-selective contacts—i.e., the replacement of conventional doped hydrogenated amorphous silicon (a-Si:H) layers—for silicon heterojunction (SHJ) solar cells as a simplified alternative to conventional architectures. Although standard in the industry, doped a-Si:H layers induce significant parasitic absorption and require complex PECVD processing. By utilizing wide-bandgap transition metal oxides and alkali metal salts, this approach is expected to reduce optical losses, potentially offering a route toward simplified device architectures and reduced fabrication complexity. For the electron transport layers (ETLs), our investigation focuses on lithium fluoride (LiF), alongside phosphonic acid-based self-assembled monolayers (SAMs) such as 2PACz and MeO-2PACz. Through specifically designed test structures, we show the work-function shift induced by these layers at the silicon/metal boundary, enhancing selective electron collection. For the hole transport layers, high-work-function molybdenum oxide (MoOx) is employed to facilitate efficient hole extraction. The optoelectronic performance was evaluated across semi-dopant-free architectures—retaining a conventional front side based on doped silicon and delivering nearly 20% efficiency with a 6 nm MoOx HTL—and fully dopant-free configurations based on silicon wafers with industry-standard texturing combining MoOx and LiF/Al contacts, delivering 16.6% efficiency. Strategies for mitigating known metal migration issues in the LiF/Al contact were also explored to improve the long-term stability of the solar cells.