Extraordinary Redox Activities in Ladder-Type Conjugated Molecules Enabled by B ← N Coordination-Promoted Delocalization and Hyperconjugation.

The introduction of B ← N coordinate bond-isoelectronic to C-C single bond-into π-systems represents a promising strategy to impart exotic redox and electrochromic properties into conjugated organic molecules and macromolecules. To achieve both reductive and oxidative activities using this strategy, a cruciform ladder-type molecular constitution was designed to accommodate oxidation-active, reduction-active, and B ← N coordination units into a compact structure. Two such compounds (BN-F and BN-Ph) were synthesized via highly efficient N-directed borylation. These molecules demonstrated well-separated, two reductive and two oxidative electron-transfer processes, corresponding to five distinct yet stable oxidation states, including a rarely observed boron-containing radical cation. Spectroelectrochemical measurements revealed unique optical characteristics for each of these reduced/oxidized species, demonstrating multicolor electrochromism with excellent recyclability. Distinct color changes were observed between each redox state with clear isosbestic points on the absorption spectra. The underlying redox mechanism was elucidated by a combination of computational and experimental investigations. Single-crystal X-ray diffraction analysis on the neutral state, the oxidized radical cation, and the reduced dianion of BN-Ph revealed structural transformations into two distinct quinonoid constitutions during the oxidation and reduction processes, respectively. B ← N coordination played an important role in rendering the robust and reversible multistage redox properties, by extending the charge and spin delocalization, by modulating the π-electron density, and by a newly established hyperconjugation mechanism.

Electronic and Morphological Studies of Conjugated Polymers Incorporating a Disk-Shaped Polycyclic Aromatic Hydrocarbon Unit.

As more research findings have shown the correlation between ordering in organic semiconductor thin films and device performance, it is becoming more essential to exercise control of the ordering through structural tuning. Many recent studies have focused on the influence of side chain engineering on polymer packing orientation in thin films. However, the impact of the size and conformation of aromatic surfaces on thin film ordering has not been investigated in great detail. Here we introduce a disk-shaped polycyclic aromatic hydrocarbon building block with a large π surface, namely, thienoazacoronenes (TACs), as a donor monomer for conjugated polymers. A series of medium bandgap conjugated polymers have been synthesized by copolymerizing TAC with electron donating monomers of varying size. The incorporation of the TAC unit in such semiconducting polymers allows a systematic investigation, both experimentally and theoretically, of the relationships between polymer conformation, electronic structure, thin film morphology, and charge transport properties. Field effect transistors based on these polymers have shown good hole mobilities and photoresponses, proving that TAC is a promising building block for high performance optoelectronic materials.

A divergent route to core- and peripherally functionalized diazacoronenes that act as colorimetric and fluorescence proton sensors.

Combining core annulation and peripheral group modification, we have demonstrated a divergent synthesis of a family of highly functionalized coronene derivatives from a readily accessible dichlorodiazaperylene intermediate. Various reactions, such as aromatic nucleophilic substitution, Kumada coupling and Suzuki coupling proceed effectively on α-positions of the pyridine sites, giving rise to alkoxy, thioalkyl, alkyl or aryl substituted polycyclic aromatic hydrocarbons. In addition to peripheral group modulation, the aromatic core structures can be altered by annulation with thiophene or benzene ring systems. Corresponding single crystal X-ray diffraction and optical studies indicate that the heteroatom linkages not only impact the solid state packing, but also significantly influence the optoelectronic properties. Moreover, these azacoronene derivatives display significant acid-induced spectroscopic changes, suggesting their great potential as colorimetric and fluorescence proton sensors.

Reducing exciton binding energy by increasing thin film permittivity: an effective approach to enhance exciton separation efficiency in organic solar cells.

Photocurrent generation in organic solar cells requires that excitons, which are formed upon light absorption, dissociate into free carriers at the interface of electron acceptor and donor materials. The high exciton binding energy, arising from the low permittivity of organic semiconductor films, generally causes low exciton separation efficiency and subsequently low power conversion efficiency. We demonstrate here, for the first time, that the exciton binding energy in B,O-chelated azadipyrromethene (BO-ADPM) donor films is reduced by increasing the film permittivity by blending the BO-ADPM donor with a high dielectric constant small molecule, camphoric anhydride (CA). Various spectroscopic techniques, including impedance spectroscopy, photon absorption and emission spectroscopies, as well as X-ray spectroscopies, are applied to characterize the thin film electronic and photophysical properties. Planar heterojunction solar cells are fabricated with a BO-ADPM:CA film as the electron donor and C60 as the acceptor. With an increase in the dielectric constant of the donor film from ∼4.5 to ∼11, the exciton binding energy is reduced and the internal quantum efficiency of the photovoltaic cells improves across the entire spectrum, with an ∼30% improvement in the BO-ADPM photoactive region.

Solution-processable triindoles as hole selective materials in organic solar cells.

We report the use of two solution-processable triindoles, triazatruxene (TAT), and N-trimethyltriindole (TMTI), as hole selective materials in organic solar cells. The unique optical and electronic properties of these molecules make them suitable as a hole extracting/electron blocking layer, i.e. transparency in the visible region due to a wide bandgap, high LUMO (lowest unoccupied molecular orbital) energy level, modest HOMO (highest occupied molecular orbital) level, and high hole carrier mobility. TAT is shown to have a LUMO at -1.68 eV, a HOMO at -5.03 eV, and a bandgap of 3.35 eV, whereas TMTI has a LUMO at -2.05 eV, a HOMO at -5.1 eV, and a bandgap of 3.05 eV, obtained from cyclic voltammetry measurements and absorption spectroscopy. Planar heterojunction photovoltaic devices, consisting of a solution processed transparent TAT (or TMTI) layer and a vapor-deposited C60 layer, exhibited efficiencies of up to 0.71 % (or 0.87 %). In these bilayer devices, the excitons are primarily generated in the C60 layer and undergo dissociation in the interfaces via hole transfer from the C60 layer to the TAT (or TMTI) layer. Additionally, spin-casting methanol solution of TAT on the top of P3HT:PCBM bulk heterojunction in an inverted device produced a hole selective interfacial layer between the photoactive layer and anode, leading to a 26% efficiency increase as compared to a control device without the TAT layer.

Near-infrared azadipyrromethenes as electron donor for efficient planar heterojunction organic solar cells.

We report the use of three solution processable azadipyrromethene (ADPM) based compounds, i.e., ADPM, B,F(2)-chelated ADPM (BF2-ADPM), and B,O-chelated ADPM (BO-ADPM), as electron donors in planar heterojunction solar cells. These small molecules possess exceptional light harvesting capability with high extinction coefficients (~1 × 10(5) M(-1) cm(-1) in solutions) and broad absorption spectra up into the near-infrared region. Planar heterojunction organic solar cells, consisting of a solution processed electron donor layer and a vapor deposited C(60) acceptor layer, have been constructed to give power conversion efficiencies of 0.56, 0.69, and 2.63% for ADPM, BF2-ADPM, and BO-ADPM, respectively, under AM 1.5G simulated 1 sun solar illumination. A high open circuit voltage (V(oc)) of ~0.8 V was achieved for the BO-ADPM/C(60) device, which is among the highest reported values for organic solar cells with photocurrent generation in the near-infrared region beyond 1.5 eV.

Diketopyrrolopyrrole-containing oligothiophene-fullerene triads and their use in organic solar cells.

We report the characterization of a series of oligothiophene-diketopyrrolopyrrole-fullerene triads and their use as active materials for solution processed organic solar cells (OSCs). By incorporating the diketopyrrolopyrrole (DPP) core with electron rich oligothiophene units and electron withdrawing fullerene units, multifunctional electronic molecules have been prepared; these molecules show high solubility in common organic solvents, excellent photophysical properties with high extinction coefficients (1 × 10(4) to 1 × 10(5) M(-1) cm(-1)) and broad absorption spectra coverage (250-800 nm), as well as suitable molecular orbital energy levels (HOMO of approximately -5.1 eV, LUMO of approximately -3.7 eV). Solution-processed thin-film organic field effect transistors (OFETs) from these triads revealed good n-type characteristics with electron mobilities up to 1.5 × 10(-3) cm(2) V(-1) s(-1). With these multifunctional triads, single-component OSCs have been fabricated, exhibiting power conversion efficiencies (PCEs) of up to 0.5 % under AM 1.5 G simulated 1 sun solar illumination. Blending these molecules with poly(3-hexylthiophene) (P3HT) afforded bulk heterojunction OSCs with PCEs reaching as high as 2.41%.

Quinacridone-based molecular donors for solution processed bulk-heterojunction organic solar cells.

New soluble quinacridone-based molecules have been developed as electron donor materials for solution-processed organic solar cells. By functionalizing the pristine pigment core of quinacridone with solubilizing alkyl chains and light absorbing/charge transporting thiophene units, i.e., bithiophene (BT) and thienylbenzo[c][1,2,5]thiadiazolethienyl (BTD), we prepared a series of multifunctional quinacridone-based molecules. These molecular donors show intense absorption in the visible spectral region, and the absorption range and intensity are well-tuned by the interaction between the quinacridone core and the incorporated thiophene units. The thin film absorption edge extends with the expansion of molecular conjugation, i.e., 552 nm for N,N'-di(2-ethylhexyl)quinacridone (QA), 592 nm for 2,9-Bis(5'-hexyl-2,2'-bithiophene)-N,N'-di(2-ethylhexyl)quinacridone (QA-BT), and 637 nm for 4-(5-hexylthiophen-2-yl)-7-(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (QA-BTD). The change of molecular structure also influences the electrochemical properties. Observed from cyclic voltammetry measurements, the oxidation and reduction potentials (vs ferrocene) are 0.7 and -1.83 V for QA, 0.54 and -1.76 V for QA-BT, and 0.45 and -1.68 V for QA-BTD. Uniform thin films can be generated from both single component molecular solutions and blend solutions of these molecules with [6,6]-phenyl C70-butyric acid methyl ester (PC70BM). The blend films exhibit space-charge limited current (SCLC) hole mobilities on the order of 1×10(-4) cm(2) V(-1) S(-1). Bulk heterojunction (BHJ) solar cells using these soluble molecules as donors and PC70BM as the acceptor were fabricated. Power conversion efficiencies (PCEs) of up to 2.22% under AM 1.5 G simulated 1 sun solar illumination have been achieved and external quantum efficiencies (EQEs) reach as high as ∼45%.

Low band gap dithieno[3,2-b:2',3'-d]silole-containing polymers, synthesis, characterization and photovoltaic application.

A series of low band gap silole-containing polymers were synthesized with different alkyl side chains and a power conversion efficiency (PCE) of 3.43% was obtained.

A new n-type low bandgap conjugated polymer P-co-CDT: synthesis and excellent reversible electrochemical and electrochromic properties.

A n-type low bandgap conjugated polymer based on perylene diimide and cyclopenta[2,1-b:3,4-b']dithiophene, which exhibits excellent reversible electrochemical properties in both n-doping/dedoping and p-doping/dedoping processes, was designed, synthesized and characterized.