Arthur D. Hendsbee

Electron deficient diketopyrrolopyrrole dyes for organic electronics: synthesis by direct arylation, optoelectronic characterization, and charge carrier mobility

Four electron deficient small molecules based on the diketopyrrolopyrrole (DPP) chromophore were synthesized using microwave-assisted direct arylation reactivity. These molecules are based upon an acceptor–donor–acceptor–donor–acceptor (A1–D–A2–D–A1) framework, where DPP is utilized as the central acceptor (A2) unit. We compared the effect of naphthalimide vs. phthalimide terminal acceptors (A1), and different DPP (A2) alkyl groups, on the optical, thermal, electrochemical and electronic properties. A combination of absorption and emission spectroscopy, differential scanning calorimetry, thermal gravimetric analysis, cyclic voltammetry, ultraviolet photoelectron spectroscopy, charge carrier mobility, and DFT calculations were used to characterize the four materials. All compounds were found to have narrow band-gaps, deep HOMO/LUMO levels, and were able to effectively act as electron transport materials.

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.

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.

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%.

Facile synthesis of unsymmetrical and π-extended furan-diketopyrrolopyrrole derivatives through C–H direct (hetero)arylation using a heterogeneous catalyst system

A mono functionalization of bis-furandiketopyrrolopyrrole (DPPFu2) via direct (hetero)arylation has been developed. This method allows for rapid access to a new family of narrow band-gap, π-extended DPP derivatives via consecutive C–H bond activation between mono-arylated DPP and aryl bromides with heterogeneous catalysts in a simple fashion.

Phthalimide–thiophene-based conjugated organic small molecules with high electron mobility

A series of low-cost phthalimide end-capped oligothiophene small molecules with variations to the terminal alkyl chain and number of thiophene units in the conjugated core have been synthesized and investigated. All molecules exhibit H-aggregation in the solid-state but different crystal structures and electronic properties, showing that subtle chemical modifications can result in dramatic changes to molecular self-assembly. Field-effect transistors display high electron mobilities of up to 0.2 cm2 V−1 s−1.

N-Annulated perylene diimide dimers: acetylene linkers as a strategy for controlling structural conformation and the impact on physical, electronic, optical and photovoltaic properties

The geometry of organic π-conjugated small molecules can impact the morphology of blended-thin films and subsequent performance in opto-electronic devices. In this report, we investigate the role of molecular conformation of perylene diimide (PDI) dimers designed to act as non-fullerene acceptors in organic solar cells. A series of three PDI dimers is presented in which the PDI chromophores are directly linked via the bay position (PDI2, 3) or separated by one (PDI2Ac, 4) or two (PDI2Ac2, 5) acetylene spacers. In all cases, the exo-position of the PDI dimers is N-annulated. New compounds 4 and 5 were synthesized via an optimized and facile synthetic pathway. Directly linked PDI dimers adopted a highly twisted conformation whereas adding two acetylene spacers rendered the PDI chromophores coplanar. 1H NMR spectroscopic analysis of each dimer revealed a highly sensitive electronic structure that is strongly influenced by the acetylene spacers. It was found that compounds 4 and 5 with less twisted structures exhibited similar electron affinities but lower ionization potentials, lower organic solvent solubility, and red-shifted optical absorption spectra when compared to the highly twisted dimer 3. In addition, 4 and 5 showed a stronger tendency to aggregate in both solution and the solid state. This had a large impact on the performance of organic solar cells using these materials as electron acceptors. Bulk-heterojunction solar cells based upon a PTB7-Th:3 active layer could reach high power conversion efficiencies of 5.23%. In contrast, PTB7-Th:4 and PTB7-Th:5 based devices had ∼5 times lower performance owing to the formation of unfavourable active layer morphologies.

N-annulated perylene diimide dimers: the effect of thiophene bridges on physical, electronic, optical, and photovoltaic properties

In this work, we report on the synthesis, characterization, and photovoltaic properties of two new N-annulated PDI derivatives connected with one (M1) or two (M2) thiophene bridging units and directly compare to a twisted PDI dimer with no thiophene units (tPDI-Hex). Compounds M1 and M2 were synthesized using an optimized direct heteroarylation carbon–carbon bond forming method. A combination of optical absorption and emission spectroscopy, cyclic voltammetry, polarized optical microscopy, X-ray diffraction, and density functional theory were used to characterize all compounds. Introduction of thiophene units into the PDI dimer scaffold served to red-shift the optical absorption and lower electron affinities. A decrease in the fluorescence intensity of M1 and M2 relative to tPDI-Hex was observed and attributed to photoinduced electron transfer. Notably, both M1 and M2 exhibited less structured thin-films as evidenced by optical microscopy and thin film X-ray diffraction. Photovoltaic properties were evaluated by fabricating bulk heterojunction solar cells with the inverted architecture ITO/ZnO/BHJ/MoOx/Ag and using PTB7-Th as the donor polymer. All solar cells were made and tested in air, and only ‘as-cast’ active layers were evaluated to ensure a direct comparison amongst the three acceptors. Solar cells with PTB7-Th:tPDI-Hex gave power conversion efficiencies of ∼5% with Vocs of 0.95 V, akin to our previous results. Comparatively, solar cells with PTB7-Th:M1 and PTB7-Th:M2 active layers exhibited higher Voc values upwards of 1.05 V but only had PCE of 1.1–1.2% when cast from chloroform, which we attributed to the formation of large domain sizes. A change in casting solvent to o-xylene resulted in improved bulk heterojunction morphology by reducing domain size and a doubling of the PCE values to ∼2% was realized for devices using M1 and M2 as electron acceptors. Use of the high boiling solvent, trimethylbenzene, resulted in further improvements in PCE for M1 and M2, increasing the PCE for devices using these acceptors to ∼3%. Importantly, devices made using the new materials exhibit low energy losses in the range of 0.5–0.7 eV.

Environment friendly solvent processed, fullerene-free organic solar cells with high efficiency in air

Fullerene-free organic solar cells (OSCs) have attracted significant interest in the research community over the past few years. Their efficiency has risen rapidly, with multiple reports of record power conversion efficiencies (PCEs) breaching 14%. While encouraging, these performance metrics are often achieved with the utilization of toxic halogenated solvents for the fabrication process, which is less attractive for large-scale manufacturing. Dimeric perylene diimide (PDI) electron transport materials are currently considered amongst the key candidates for the realization of low-cost, highefficiency “green-processed” OSCs. The low-cost synthetic versatility of the PDI skeleton allows for a range of chemical “fine-tuning” and the chromophore has excellent photochemical stability and strong light absorption in the visible region. This report will detail our research into OSCs using PDI dimers as the electron acceptors and active layer fabrication using non-halogenated solvents. PTB7-Th was chosen as the donor material, owing to its good solubility in nonhalogenated solvents, complementary light absorption and suitable energy level alignment for pairing with our PDI acceptors. Two different PDI dimers having linear and branched alkyl chains are studied. We have previously shown that PTB7-Th:PDI based solar cells with active layers processed from 2Me-THF, o-xylene, or 1,2,4-trimethylbenzene could reach PCEs from 5-6%. The processing solvent can be extended to toluene with solar cells exhibiting PCEs of 5%. Thus, this work highlights the many processing options for the PTB7-Th / N-annulated PDI dimer active layer combination.