Abstract
We report the use of isobenzofulvene-C-60 adducts in bulk heterojunction organic solar cells, synthesized via the [4 + 2] cycloaddition of C-60 with an in situ generated isobenzofulvene intermediate. The LUMO energy levels of these adducts are 20-180 meV higher than that of PCBM ([6,6]-phenyl-C-61-butyric acid methyl ester). This large increase of the LUMO level is attributed to cofacial pi-orbital interactions between the fullerene surface and the isobenzofulvene pi-system (aromatic ring and double bond). Raised LUMO levels of fullerenes, together with their desirably slow recombination dynamics, led to higher open-circuit voltages (V-OC) in bulk heterojunction polymer solar cells (up to 0.75 V for bisadducts) relative to cells tested in parallel using the well-known PCBM as the fullerene acceptor. In addition to enhanced V-OC, the short-circuit current densities (J(SC)) were improved in the devices containing the epoxide analogs of the isobenzofulvene-C-60. Notably the epoxide derivative of the monoadduct (IBF-Ep) exhibited similar to 20% enhancement of power conversion efficiency (PCE) compared to reference P3HT: PCBM solar cells. A combination of optical and electronic methods was used to investigate the origin of the PCE enhancement observed with these new fullerene acceptors with particular attention to the increased V(OC)s. (c) 2016 Elsevier B.V. All rights reserved.
