Scott E. Watkins

Tuning the Optoelectronic Properties of Nonfullerene Electron Acceptors

Broad spectral coverage over the solar spectrum is necessary for photovoltaic technologies and is a focus for organic solar cells. We report a series of small-molecule, nonfullerene electron acceptors containing the [(benzo[c][1,2,5]thiadiazol-4-yl)methylene]malononitrile unit as a high electron affinity component. The optoelectronic properties of these molecules were fine-tuned with the objective of attaining strong absorption at longer wavelengths by changing the low-ionization-potential moiety. The electron-accepting function of these materials was investigated with poly(3-n-hexylthiophene) (P3HT) as a standard electron donor. Significant photocurrent generation in the near infrared region, with an external quantum yield reaching as high as 22 % at 700 nm and an onset >800 nm was achieved. The results support efficient hole transfer to P3HT taking place after light absorption by the acceptor molecules. A Channel II-dominated power conversion efficiency of up to 1.5 % was, thus, achieved.

Aggregation of a dibenzo[b,def]chrysene based organic photovoltaic material in solution.

Detailed electrochemical studies have been undertaken on molecular aggregation of the organic semiconductor 7,14-bis((triisopropylsilyl)-ethynyl)dibenzo[b,def]chrysene (TIPS-DBC), which is used as an electron donor material in organic solar cells. Intermolecular association of neutral TIPS-DBC molecules was established by using (1)H NMR spectroscopy as well as by the pronounced dependence of the color of TIPS-DBC solutions on concentration. Diffusion limited current data provided by near steady-state voltammetry also reveal aggregation. Furthermore, variation of concentration produces large changes in shapes of transient DC and Fourier transformed AC (FTAC) voltammograms for oxidation of TIPS-DBC in dichloromethane. Subtle effects of molecular aggregation on the reduction of TIPS-DBC are also revealed by the highly sensitive FTAC voltammetric method. Simulations of FTAC voltammetric data provide estimates of the kinetic and thermodynamic parameters associated with oxidation and reduction of TIPS-DBC. Significantly, aggregation of TIPS-DBC facilitates both one-electron oxidation and reduction by shifting the reversible potentials to less and more positive values, respectively. EPR spectroscopy is used to establish the identity of one-electron oxidized and reduced forms of TIPS-DBC. Implications of molecular aggregation on the HOMO energy level in solution are considered with respect to efficiency of organic photovoltaic devices utilizing TIPS-DBC as an electron donor material.

Reduced roughness for improved mobility in benzodipyrrolidone-based, n-type OFETS

A series of six phenyl-flanked benzodipyrrolidone-based copolymers are designed and synthesised: three exhibit backbone in-plane curvature or torsional twisting, while the other three remain planar and linear. While the former may appear less crystalline by X-ray diffraction, they afford a smoother thin-film surface topology and increased electron mobility in top-gate, bottom-contact organic field-effect transistors.

Synthesis and properties of pyrrolo[3,2-b]pyrrole-1,4-diones (isoDPP) derivatives

The synthesis of three pyrrolo[3,2-b]pyrrole-1,4-dione (isoDPP) derivatives is described, namely 1,3,4,6-tetraphenylpyrrolo[3,2-b]pyrrole-2,5(1H,4H)-dione 2, 1,4-diphenyl-3,6-di(thiophen-2-yl)pyrrolo[3,2-b]pyrrole-2,5(1H,4H)-dione 3, and 1,4-bis(4-(hexyloxy)phenyl)-3,6-di(thiophen-2-yl)pyrrolo[3,2-b]pyrrole-2,5(1H,4H)-dione 7 in which the molecular structures differ in the aromatic ring (phenyl or thiophene) attached to the nitrogen atom. Thin films of 2, 3, and 7 could be formed by evaporation under vacuum. In the case of 2 and 3 GIWAXS measurements showed that the film structural ordering was similar to that measured in single crystals. In contrast GIWAXS showed that 7 had features associated with liquid crystalline materials. Time dependent density functional theory (TDDFT) calculations predicted that the transition between the lowest energy singlet excitation (S1) and the ground state (S0) would be optically forbidden due to the centrosymmetric geometries of compounds. Photophysical measurements showed that the compounds were weakly luminescent, with low radiative rates in solution of order 106 s−1, which are consistent with the TDDFT predictions. Furthermore, photoinduced absorption (PIA) spectroscopy showed that there is a long-lived low energy state, which has been assigned as a triplet and provides a further non-radiative decay pathway for the excited state.

Rhenium(I) phenanthrolines bearing electron withdrawing CF3 substituents: synthesis, characterization and biological evaluation

Rhenium(I) tricarbonyl complexes bearing a 2,4,7,9-tetraphenylphenanthroline ligand, where the 2,9-phenyl groups have either meta- or para-CF3 groups, have been screened against a selection of cell lines. The meta derivative shows anticancer activity against HeLa and A549 cell lines, whereas the para derivative causes proliferation of HL-60 cells, whilst showing toxicity towards A549 cells.

Alkyl side-chain branching point effects in thieno[3,4-c]pyrrole-4,6-dione copolymers

A novel and efficient route to thieno[3,4-c]pyrrole-4,6-dione (TPD) facilitating late-stage alkylation is presented. Four copolymers with alkylated bithiophene (2T) are synthesised, with different TPD N−alkyl side-chain branching points. The effect of steric bulk on solid-state properties is investigated (UV–VIS, differential scanning calorimetry, X-ray diffraction) and C1-branching found to increase crystallinity and solid-state packing in TPD-2T.

Semi-perfluoroalkyl polyfluorene with varying fluorine content: synthesis and photophysical properties

Deposition of multilayer organic electronic devices by solution processing often requires clever material design to avoid intermixing of the various layers. The use of orthogonal solvents for multilayer processing is a simple and effective method to solve this problem. However, the introduction of fluorous substituents to materials normally functionalised with aliphatic side chains has been shown to affect the device performance of these materials. In this study, a series of 9,9-disubstituted polyfluorenes containing varying ratios of semi-perfluoroalkyl to alkyl side chains have been synthesised and characterised. The effect of the semi-perfluoroalkyl chains on photophysical, electrochemical and thermal properties of the polymers was examined. The results indicate that the semi-perfluoroalkyl chains have varying influence on these properties and this is reflected in the previously reported electroluminescent device performances.

Infrared Sensitizers in Titania-Based Dye-Sensitized Solar Cells using a Dimethylferrocene Electrolyte

Vide infra: Two boron-dipyrromethene (BODIPY)-type infrared sensitizers are investigated for use in titania-based dye-sensitized solar cells (DSCs). The DSCs use a solution of dimethylferrocene (Me2 Fc) in nitromethane as electrolyte. The titania conduction band edge is fine-tuned by adding lithium ions from LiTFSI, while the choice of dimethylferrocene as redox shuttle provides the ideal driving force for dye regeneration, allowing the BODIPY-based DSCs to reach unprecedented quantum efficiencies in the near-infrared.

Synthesis of Host Polymers and Guests for Electrophosphorescence

Significant progress has been realized in the design and synthesis of light emitting polymers that emit over the entire visible spectrum. However, up to seventy-five percent of charge recombination events can lead to triplet states that decay non-radiatively. Following the pioneering work in the field of small molecule organic light emitting devices, it has been found that solution processible iridium polymer complexes can be used to harness the wasted triplet energy. In this paper, new results with respect to the electrophosphorescence of solution processible tethered iridium polymer derivatives are presented. Furthermore, our approaches to the design of new high triplet energy conjugated polymer hosts are also reported.

Synthesis of triplet emitters and hosts for electrophosphorescence

Electroluminescence from conjugated polymers can be significantly improved by harnessing the energy of their nonemissive triplet states. Poly(2,7-dibenzosilole) has been prepared and its triplet energy has been measured as 2.14 eV, a figure that is slightly higher than that of polyfluorene (2.09 eV). Two new tris-cyclometalated iridium complexes with blue-to-green emission properties have been prepared and characterized.