Seth M. McAfee

Key components to the recent performance increases of solution processed non-fullerene small molecule acceptors

In recent years, the intensive development of π-conjugated small molecule acceptors has yielded viable alternatives to fullerene acceptors in state-of-the-art organic photovoltaic devices. Small molecule acceptors are designed to replicate the favourable electronic properties of fullerenes and to overcome their inherent optical and stability deficiencies. Concurrently, advances in device engineering through rigorous optimization have seen the development of intricate device architectures and led to impressive performance increases. This review highlights a number of recent high performance non-fullerene acceptors, focusing on the design of π-conjugated structures, device optimization and the ensuing power conversion efficiencies.

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

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.

An Electron‐Deficient Small Molecule Accessible from Sustainable Synthesis and Building Blocks for Use as a Fullerene Alternative in Organic Photovoltaics

An electron-deficient small molecule accessible from sustainable isoindigo and phthalimide building blocks was synthesized via optimized synthetic procedures that incorporate microwave-assisted synthesis and a heterogeneous catalyst for Suzuki coupling, and direct heteroarylation carbon-carbon bond forming reactions. The material was designed as a non-fullerene acceptor with the help of DFT calculations and characterized by optical, electronic, and thermal analysis. Further investigation of the material revealed a differing solid-state morphology with the use of three well-known processing conditions: thermal annealing, solvent vapor annealing and small volume fractions of 1,8-diiodooctane (DIO) additive. These unique morphologies persist in the active layer blends and have demonstrated a distinct influence on device performance. Organic photovoltaic-bulk heterojunction (OPV-BHJ) devices show an inherently high open circuit voltage (Voc ) with the best power conversion efficiency (PCE) cells reaching 1.0 V with 0.4 v/v % DIO as a processing additive.

Utility of a heterogeneous palladium catalyst for the synthesis of a molecular semiconductor via Stille, Suzuki, and direct heteroarylation cross-coupling reactions

The commercially available silica-supported heterogeneous catalyst SiliaCat® DPP-Pd has proven to be highly active, robust, and reusable for the synthesis of a thiophene–phthalimide-based molecular semiconductor under microwave-irradiation reaction conditions. A Stille reaction protocol demonstrated that SiliaCat® DPP-Pd outperformed well-known homogeneous catalysts, Pd(PPh3)4 and Pd(PPh3)2Cl2, in terms of performance and catalyst loading, while also exhibiting tolerance to ambient reaction conditions and two-fold recyclability for the formation of product. The success established for SiliaCat® DPP-Pd catalyzed Stille reactions via microwave irradiation was extended to optimize Suzuki coupling and direct heteroarylation protocols. Notably, direct heteroarylation with SiliaCat® DPP-Pd exhibited excellent selectivity and perturbed the formation of homo-coupled aryl bromides, two side reactions that are known to plague this type of cross-coupling reaction.

The structural evolution of an isoindigo-based non-fullerene acceptor for use in organic photovoltaics

The structural evolution of a functional isoindigo-based non-fullerene acceptor led to the development of three new materials to address the deficiencies of the original framework. Owing to the versatility of the structure and the flexibility of the synthetic procedures, these three new materials were accessible from previously optimized reaction conditions and similar precursor materials. The influence of structural modification on the optical, electrochemical and thermal properties were assessed and correlated with DFT calculations to provide compelling evidence for the effect of each substitution and how they relate to each particular adaptation. The structure–property relationships were investigated for their photovoltaic performance in solution processable BHJ devices fabricated in inverted architectures. Evaluation of device performance demonstrated that a single modification did not improve on the efficiency of the original structure, but the combination of both induced non-planarity, and increased electron affinity of the fourth iteration showed the potential of our framework, with PCE reaching 1.9%.

Applying direct heteroarylation synthesis to evaluate organic dyes as the core component in PDI-based molecular materials for fullerene-free organic solar cells

Direct heteroarylation has emerged as a versatile and powerful tool to access π-conjugated materials through atom-economical Pd-catalyzed carbon–carbon bond forming reactions. Employing this synthetic protocol has enabled the facile evaluation of a series of organic dyes in a PDI-dye-PDI framework. Material properties are largely dictated by the PDI components, but the incorporation of either thienoisoindigo, diketopyrrolopyrrole or isoindigo has been shown to influence the ionization potential and absorption profiles of the final materials. Solution-processable organic solar cell devices were fabricated to investigate the influence of the different dye cores on photovoltaic performance when paired with the donor polymer PTB7-Th. It was found that the diketopyrrolopyrrole-based material out-performed the other organic dyes, demonstrating energy losses of less than 0.6 eV, promising efficiencies when cast from non-halogenated solvents and the ability to dictate self-assembly induced by small volume fractions of the high-boiling solvent additive 1,8-diiodooctane to reach best device efficiencies of 4.1%.

Design and computational characterization of non-fullerene acceptors for use in solution-processable solar cells.

In an effort to seek high-performance small molecule electron acceptor materials for use in heterojunction solar cells, computational chemistry was used to examine a variety of terminal acceptor-conjugated bridge-core acceptor-conjugated bridge-terminal acceptor small molecules. In particular, we have systematically predicted the geometric, electronic, and optical properties of 16 potential small-molecule acceptors based upon a series of electron deficient π-conjugated building blocks that have been incorporated into materials exhibiting good electron transport properties. Results show that the band gap, HOMO/LUMO energy levels, orbital spatial distribution, and intrinsic dipole moments can be systematically altered by varying the electron properties of the terminal or core acceptor units. In addition, the identity of the conjugated bridge can help fine-tune the electronic properties of the molecule, where this study showed that the strongest electron affinity of the conjugated π-bridge increased the stability in the HOMO and LUMO energies and increased the band gap of these small-molecule acceptors. As a result, this work points toward an isoindigo (C5) core combined with C2-thienopyrrole dione (A5) terminal units as the most promising small molecule acceptor material that can be fine-tuned with the choice of conjugated bridge and may be considered as reasonable candidates for synthesis and incorporation into organic solar cells.

A non-fullerene acceptor with a diagnostic morphological handle for streamlined screening of donor materials in organic solar cells

Utilizing the N-annulated PDI acceptor PDI–DPP–PDI, a simple air-processed and air-tested organic photovoltaic device fabrication procedure has been established to streamline the screening of donor materials. Post-deposition solvent vapour annealing of the active layer blend results in a preferential re-organization dictated by PDI–DPP–PDI and is responsible for significant increases in device performance. Employing this method, a series of low-cost and scalable donor polymers were screened to help select promising candidates as alternatives to the expensive high-performance donor polymer PTB7-Th. Universal improvements in performance upon solvent vapour annealing with a range of donor polymers highlighted the advantages of PDI–DPP–PDI and this post-deposition treatment. This facile screening protocol identified PDTT-BOBT as the most suitable PTB7-Th alternative in the surveyed series, with the best device efficiencies reaching 4.5% PCE compared to control devices with PTB7-Th at 4.6% PCE.

Exploiting direct heteroarylation polymerization homocoupling defects for the synthesis of a molecular dimer

We have utilized a C–H homocoupling defect that plagues direct heteroarylation polymerization reactions to access a molecular dimer based on diketopyrrolopyrrole and perylene diimide. This synthetic route, which also incorporates a direct heteroarylation mono-substitution, proved to be an effective method to access this class of compound. The dimer, PDI-DPP-DPP-PDI, presented an overall electron-deficient π-conjugated molecular framework with strong optical absorption from 400–800 nm. The low energy absorption from 600–800 nm is a feature not accessible by the monomeric analogue, highlighting the potential of this dimerization method to be used for spectral engineering of π-conjugated materials.

Optoelectronic engineering with organic dyes: utilizing squaraine and perylene diimide to access an electron-deficient molecule with near-IR absorption

Applying direct heteroarylation cross-coupling reactions to the synthesis of π-conjugated materials has facilitated the creation of molecular libraries in an efficient and more sustainable way than was previously possible with traditional cross-coupling methods. Utilizing this approach, we have been able to realize new compounds with tailored optoelectronic properties. In this work, we disclose the design, synthesis and characterization of an optically and electrochemically engineered molecular compound composed of a bis-indole squaraine dye flanked by N-annulated perylene diimide units to access an electron-deficient π-conjugated structure with strong light absorption ranging from the visible to near-infrared and reversible ambipolar redox behavior.Graphical Abstract