Ge Ziyi and Li Wei from the Chinese Academy of Sciences synthesized a series of -CN additives with different molecular dipole moments, including 2′-fluoro-[1,1′-biphenyl]-3,5-dicarbonitrile (1F-2CN), 2′,6′-difluoro-[1,2′-biphenyl]-3,5-dicarbonitrile (2F-2CN) and 2′,3′,4′-trifluoro-[1,6′-biphenyl]-3,5-tricarbonitrile (3F-2CN). The two -CN groups in the additives can coordinate with Pb2+ defects, while the fluorine atoms can tune the dipole moment of the additives and form hydrogen bonds with charged FA+ groups. As a result, the additives successfully stitched defects at the perovskite grain boundaries and released interfacial stresses, leading to low Young’s modulus and high mechanical flexibility.

Meanwhile, the additives can weaken the interaction between charge carriers and longitudinal optical phonons, and facilitate carrier extraction and transport. In addition, 2F-2CN with a stronger molecular dipole can better enhance the efficiency and stability of flexible perovskite solar cells (f-PSCs). The f-PSCs based on 1F-2CN, 2F-2CN and 3F-2CN achieved power conversion efficiencies (PCE) of 21.87%, 23.64%, 24.08% and 23.30%, respectively. Benefiting from the perfect perovskite films, the unmodified f-PSCs showed excellent mechanical reliability as well as good light, thermal and air stability.