V[subscript OC] enhancement in polymer solar cells with isobenzofulvene–C[subscript 60] adducts

Abstract

We report the use of isobenzofulvene–C[subscript 60] adducts in bulk heterojunction organic solar cells, synthesized via the [4 + 2] cycloaddition of C[subscript 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[subscript 61]-butyric acid methyl ester). This large increase of the LUMO level is attributed to cofacial π-orbital interactions between the fullerene surface and the isobenzofulvene π–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[subscript 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[subscript OC], the short-circuit current densities (J[subscriptSC]) were improved in the devices containing the epoxide analogs of the isobenzofulvene–C[subscript 60]. Notably the epoxide derivative of the monoadduct (IBF–Ep) exhibited ∼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[subscript OC]s.