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

Abstract

Fullerene derivatives especially [1,1]-phenyl-C61-butyric-acid-methyl-ester (PC61BM) have been commonly used as electron transport layers and additives of inverted (p-i-n) perovskite solar cells (PVKSCs); however the dielectric constant (the εr) of PC61BM (3.8) is much smaller than those of organolead halide hybrid perovskites (6.5 for CH3NH3PbI3 [MAPbI3]), 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 C60-Rhd-CN) was synthesized and introduced into the MAPbI3 active layer to construct inverted PVKSCs via an anti-solvent method. The involvement of two electron-withdrawing cyano groups within C60-Rhd-CN obviously increases the εr value (5.1) relative to PC61BM (3.8), hence lowering the dielectric mismatch between MAPbI3 and fullerene. Besides, the cyano groups enable strong coordination interactions with the Pb2+ ions of MAP-bI3, rendering efficient defect passivation of perovskite. As a result, C60-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, C60-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 εr value and efficiency enhancement, another analogous novel rhodanine-functionalized fullerene with the two cyano groups substituted by a sulfur atom (abbreviated C60-Rhd-S) was also synthesized, which delivered a lower PCE of 19.45% than C60-Rhd-CN-incorporated devices due mainly to its lower the εr value (4.0).