4-PhCz is a self-assembled monolayer material designed for high-efficiency perovskite solar cells and perovskite-silicon tandem solar cells. 4-PhCz features an aromatic asymmetric 11H-benzo[a]carbazole terminal group, a butyl linker and a phosphonic acid anchoring group. 4-PhCz severs as hole selection layer between the substrate and the perovskite active layer. The asymmetric, extended π-conjugated structure of the benzo[a]carbazole terminal not only minimizes aggregation to ensure uniform surface coverage on the substrate but also strengthens the interactions between the self-assembled monolayer and perovskites, a bifacial reinforcement. The larger dipole moment of 2.04 D presented by the terminal group facilitates efficient hole extraction while the smaller contact angle of 11.06° with perovskite precursors enhances film crystallinity. This results a great VOC of 1.273 V with a low voltage loss of 0.397 V relative to the wide bandgap (1.67 eV) of the perovskite solar cell.
Monolithic perovskite/silicon tandem based on 4-PhCz as the self-assembled hole transport layer demonstrates a certified efficiency of 31.26% with an impressive VOC of 1.957 V. Moreover, 92% of its initial efficiency after 1000 h of maximum power point tracking (MPPT) is achieved under 1-sun illumination in a nitrogen atmosphere at 25 °C, owing to the suppressed light-induced halide segregation presented by the self-assembled monolayer material 4-PhCz.
Serving as hole selective contact for organic solar cells and perovskite solar cells, 4-PhCz is an alternative to PEDOT:PSS with superior performance and the convenience of solution deposition at low concentration, i.e. 1 mM.