Molecular dipole engineering-assisted strain release for mechanically robust flexible perovskite solar cells

Abstract

Mechanical endurance and long-term operational stability are critical factors for the commercialization of flexible perovskite solar cells (f-PSCs), attributable to defects in the perovskite layer. In this study, we synthesized a series of –CN additives with different molecular dipoles, including 2′-fluoro-[1,1′-biphenyl]-3,5-dicarbonitrile (1F-2CN), 2′,6′-difluoro-[1,1′-biphenyl]-3,5-dicarbonitrile (2F-2CN), and 2′,3′,4′-trifluoro-[1,1′-biphenyl]-3,5-dicarbonitrile (3F-2CN). Two –CN groups in the additives could coordinate with Pb²⁺ defects; moreover, fluorine (F) atoms could regulate the dipole moment of the additives and form hydrogen bonds with the charged FA⁺ group. Consequently, these –CN additives could successfully sew the defects at the grain boundaries (GBs) of perovskites and release the GB stress, resulting in low Young's modulus and high mechanical flexibility. Simultaneously, additives could weaken the interactions between the charge carriers and the longitudinal optical phonons as well as promote carrier extraction and transport. More importantly, 2F-2CN with a stronger molecular dipole could better enhance the efficiency and stability of f-PSCs. Thus, noteworthy power conversion efficiencies (PCEs) of 21.87%, 23.64%, 24.08%, and 23.30% were obtained for the control, 1F-2CN-, 2F-2CN-, and 3F-2CN-based f-PSCs, respectively. To date, the PCE of 24.08% has been the highest reported PCE for inverted f-PSCs. Benefiting from perfect perovskite films, unpackaged f-PSCs with modified additives exhibited excellent mechanical reliability as well as good light, heat, and air stability. Our work provides new insights into the molecular dipoles of –CN additives for defect passivation, stress relief, and enhancement of mechanical flexibility in perovskite films.