Br-4PhCz is a carbazole-based self-assembled monolayer material designed for high-efficiency organic, perovskite and all-perovskite tandem solar cells. Br-4PhCz features an asymmetric structure with a brominated 11H-benzo[a]carbazole terminal, a butyl linker and a phosphonic acid anchor. Br-4PhCz shows a large dipole moment 4.32 D which is critical for efficient hole extraction to speed up carrier transport and reduce interfacial recombination. Br-4PhCz adopts a face-on molecular orientation to the active layer with uniform substrate coverage to suppresses interfacial non-radiative recombination. Also, Br-4PhCz gives a suitable HOMO energy level of −5.32 eV with an exceptionally high hole mobility of 4.16 × 10−4 cm2V-1s−1.
As a result, binary OSC based on PM6:eC9 active layer using Br-4PhCz as the hole selection layer (HSL) exhibits a champion efficiency of 19.70% with a remarkable JSC of 29.20 mA cm−2 and a VOC of 0.856 V while the unencapsulated device retains 95.0% of its original efficiency after 1,000 hours of storage at air ambient, indicating excellent long-term stability. Moreover, impressive open-circuit voltage of 1.36 V is observed in wide band-gap (WBG) perovskite solar cells (PSCs) with a bandgap of 1.77 eV with maximum power conversion efficiencies of 29.35% (certified 28.98%) in all-perovskite tandem solar cells.
Serving as hole selective contact for organic solar cells and perovskite solar cells, Br-4PhCz is an alternative to PEDOT:PSS with superior performance and the convenience of solution deposition at low concentration, i.e. 1 mM.