Phthalimide-based π-conjugated small molecules with tailored electronic energy levels for use as acceptors in organic solar cells

Abstract

The design, synthesis, and characterization of seven phthalimide-based organic π-conjugated small molecules are reported. The new materials are based on a phthalimide-thiophene-CORE-thiophene-phthalimide architecture. The CORE units utilized were phthalimide (M2), diketopyrrolopyrrole (M3), isoindigo (M4), naphthalene diimide (M5), perylene diimide (M6), and difluorobenzothiadiazole (M7); they were specifically selected to progressively increase the electron affinity of the resulting compound. A small molecule with no core (M1) was synthesized for comparison. Each material was synthesized through optimized direct heteroarylation cross-coupling procedures using bench top solvents in air. Combinations of UV-visible spectroscopy (UV-vis), cyclic voltammetry (CV), differential scanning calorimetry (DSC), ultraviolet photoelectron spectroscopy (UPS) and density functional theory (DFT) were used to characterize each material. The use of various core acceptor building blocks with differing electron affinities resulted in the series M1-M7 having a range of energetically deep LUMO levels and a range of HOMO-LUMO gap energies. Meanwhile, the melting and crystallization temperatures of the molecules M1-M7 were also found to vary according to the change in central acceptor unit. Compounds M1-M7 were employed as acceptors in combination with either the polymeric donor P3HT or small molecule donor DTS(FBTTh2)2 to understand how the LUMO levels of each acceptor influences the open circuit voltage (Voc). It was found that, in general, Voc was only weakly related to the offset between the HOMO energy level of the donor and LUMO level of the acceptor used, with a Voc of up to 1.2 V being achieved for M1. IntroductionDonor-acceptor π-conjugated materials have been extensively used as the active components in sensing devices, thin film transistors, photoswitches, photovoltaics and a variety of other useful applications. 1-3 With respect to solution processed photovoltaics, the development of both donor-acceptor (D-A) conjugated polymers and small molecules has led to the realization of fullerene-based heterojunction (BHJ) organic solar cells (OSCs) with power conversion efficiencies (PCE) reaching beyond 9%. [4][5][6][7] More recently, donor-acceptor (D-A) organic π-conjugated compounds have found utility as electron-accepting components (vide infra) in solution processed BHJ OSCs, a position that is dominated by fullerene derivatives. Key advantages of D-A compounds compared to fullerene include the fact that electronic energy levels, absorption profiles, solubility parameters, and selfassembly tendencies can be precisely controlled through both selection and functionalization of the D and A building blocks. [8][9][10] In addition, the majority of D-A compounds are easily synthesized though standard organic chemistry techniques using widely available starting materials. These advantages of D-A compounds can potentially allow for specifically matched electron-donor electron-acceptor pairings within the BHJ architecture where photon ...

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