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 ε r) 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 ε r 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 ε r 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 ε r value (4.0). [Figure not available: see fulltext.].