Dopant-free X-shaped D-A type hole-transporting materials for p-i-n perovskite solar cells

Azomethine compounds are accessible for palladium-free routes, paving a way for developing highly efficient and eco-friendly hole-transporting materials. This study reports three organic dopant-free X-shaped molecules (named D31, D32, and D33) were systematically designed, synthesized and characterized for fabricating p-i-n perovskite solar cells. The X-shaped design is based on a benzene core unit with four arms attached. Two of them are triphenylamines and two are azomethine bridges connected to functionalized phenyl rings (-H, -OCH3, -CN). These materials show suitable energy levels with respect to that of CH3NH3PbI3 perovskite. Based on this design, it is found that the hydrophobic nature of the three new compounds not only favors the formation of large grained and dense perovskite films but also improves stability of the devices. More encouragingly, the cyano-substituted D33 with donor-acceptor (D-A) type structure exhibit the superiority of high hole mobility and good film-forming property. The optimized unencapsulated device based on D33 in ambient environment exhibit 17.85% efficiency and retained 70% of the initial PCE after 400 h.