Lowering the dielectric mismatch for efficient inverted perovskite solar cells through incorporating cyano-functionalized fullerene additive

Fullerene derivatives especially [1,1]-phenyl-C 61 -butyric-acid-methyl-ester (PC 61 BM) have been commonly used as electron transport layers and additives of inverted (p-i-n) perovskite solar cells (PVKSCs); however the dielectric constant (the ε r ) of PC 61 BM (3.8) is much smaller than those of organolead halide hybrid perovskites (6.5 for CH 3 NH 3 PbI 3 [MAPbI 3 ]), resulting in unfavorable exciton dissociation and impeded interfacial charge transport and consequently undesirable nonradiative recombination. Herein, a novel cyanofunctionalized fullerene in which two cyano groups are incorporated via a rhodanine moiety (abbreviated C 60 -Rhd-CN) was synthesized and introduced into the MAPbI 3 active layer to construct inverted PVKSCs via an anti-solvent method. The involvement of two electron-withdrawing cyano groups within C 60 -Rhd-CN obviously increases the ε r value (5.1) relative to PC 61 BM (3.8), hence lowering the dielectric mismatch between MAPbI 3 and fullerene. Besides, the cyano groups enable strong coordination interactions with the Pb 2+ ions of MAP-bI 3 , rendering efficient defect passivation of perovskite. As a result, C 60 -Rhd-CN incorporation facilitates exciton dissociation and interfacial charge transport and suppresses the nonradiative recombination, affording dramatic power conversion efficiency (PCE) enhancement of inverted PVKSC devices from 18.43% to 20.81%. Furthermore, C 60 -Rhd-CN incorporation helps to improve the ambient and thermal stabilities of PVKSC devices. To elucidate the critical role of cyano groups in increasing the the ε r value and efficiency enhancement, another analogous novel rhodanine-functionalized fullerene with the two cyano groups substituted by a sulfur atom (abbreviated C 60 -Rhd-S) was also synthesized, which delivered a lower PCE of 19.45% than C 60 -Rhd-CN-incorporated devices due mainly to its lower the ε r value (4.0).