Synthesis of 2,5,8-Tris(1-phenyl‑1<i>H</i>‑benzo[<i>d</i>]imidazol-2-yl)benzo[1,2‑<i>b</i>:3,4‑<i>b</i>′:5,6‑<i>b</i>″] Trithiophenes and Their Spontaneous Orientation Polarization in Thin Films

To investigate the relationship between molecular structures and spontaneous orientation polarization (SOP) in organic thin films, 2,5,8-tris(1-phenyl-1H-benzo[d]imidazol-2-yl)benzo[1,2-b:3,4-b′:5,6-b″] trithiophene (TPBTT) and its ethyl derivative (m-ethyl-TPBTT) were synthesized. Variable angle spectroscopic ellipsometry and two-dimensional grazing-incidence wide-angle X-ray scattering showed that the vacuum-deposited films of TPBTT and m-ethyl-TPBTT had a higher degree of molecular orientation parallel to the substrate compared with that of prototypical 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) due to the larger π-conjugated benzotrithiophene core. However, TPBTT films showed a lower SOP of +54.4 mV/nm than did the TPBi film (+77.3 mV/nm), indicating that the molecular orientation alone did not determine the SOP. In contrast, m-ethyl-TPBTT showed a larger SOP of +104.0 mV/nm in the film. Quantum chemical calculations based on density functional theory suggested that the differences in the stable molecular conformation and the permanent dipole moments between TPBTT and m-ethyl-TPBTT caused the differences in SOP. These results suggest that the simultaneous control of the orientational order and conformation of the molecules is important to achieving a large SOP in films

Electric Field Effects on Photoluminescence of Polyfluorene Thin Films: Dependence on Excitation Wavelength, Field Strength, and Temperature

Electroabsorption (E-A) and electrophotoluminescence (E-F) spectra of thin films of blue-light-emitting poly(9,9-dioctylfluorene) (PFO) have been measured at temperatures ranging from 25 to 295 K to examine both the optical property and excitation dynamics of these films in the presence of external electric fields of 0−1.2 MV cm−1. Electric field effects on excitation dynamics depend on not only applied field strength but also excitation wavelength and temperature. For photoexcitation at 344 or 402 nm, E-F spectra observed with low applied fields show only the Stark shift, whereas fluorescence quenching is induced only by strong electric fields. For photoexcitation at a shorter wavelength of 298 nm, on the other hand, field-induced quenching of fluorescence is observed even with a weak electric field at any temperature. The presence of a nonradiative process from highly excited states that is effectively affected by electric fields is suggested

Helical Morphologies of Thermotropic Liquid-Crystalline Chiral Schiff-Based Rod−Coil Amphiphiles

A series of Schiff-based rod−coil molecules possessing thermotropic liquid-crystalline (LC) character have been prepared whereas a sugar-based moiety was introduced to the chain end of the molecules so as to create chiral amphiphiles for self-assembly. The self-assembly of the chiral Schiff-based rod−coil amphiphiles gave rise to a variety of specific LC textures. A banded morphology under polarized light microscopy (PLM) can be observed; the appearance of the banded texture is strongly dependent upon the length of hydrophobic tail that determines the twisting power of self-assembled hierarchical superstructures with helical sense. As a result, the banded spherulites are identified as quaternary helical morphology with a collection of the tertiary chiral structures (i.e., helical twists) so as to give regular extinction in PLM attributed to zero-birefringence effect. Consistent with the observation of helical morphologies, the occurrence of chiral smectic C (SmC*) phase can only be found in samples with enough alkoxyl chain length, suggesting the existence of strong correlation for morphological evolution from molecular level to macroscopic object with the formation of SmC*. A hypothetic model about the bilayer structure within the SmC* structure is thus given to elucidate the morphological evolution. Consequently, the self-assembly of the chiral amphiphiles with thermotropic LC character represents the mechanism for the chirality transfer in different length scales

Electric-Field-Induced Enhancement/Quenching of Photoluminescence of π-Conjugated Polymer S3-PPV: Excitation Energy Dependence

The effects of electric field on absorption and photoluminescence (PL) of films of sulfide-substituted PPV derivative S3-PPV, poly[2-(phenyl)-3-(4′-(3,7-dimethyloctyloxy)phenyl)-1,4-phenylenevinylene-co-2-(11′-decyl sulfanylundecanyloxy)-5-methoxy-1,4-phenylene vinylene], were investigated. Electroabsorption (E-A) and electrophotoluminescence (E-PL) responses of S3-PPV show the Stark shifts, indicating a significant alternation in the molecular polarizability (Δα̅) associated with the optical transitions. Field-induced enhancement or quenching is also observed for PL of S3-PPV, depending on the photoexcitation energy, whereas the shape of the PL spectra is independent of the excitation wavelength. The field effects on the decay profiles of PL indicate that the quenching results from a diminished population of the emitting states on excitation at 300 nm, whereas the PL is enhanced on excitation at 471 nm because the emitting state has an increased lifetime. The efficiency of field-assisted generation of electron−hole pairs produced through excitons monotonically increases with increasing excitation energy, and the nonradiative decay rate in the emitting state is diminished by electric fields in S3-PPV. The photoirradiation of S3-PPV in ambient air resulted in rapid degradation of the polymer film

Synthesis of a New Ladder-Type Benzodi(cyclopentadithiophene) Arene with Forced Planarization Leading to an Enhanced Efficiency of Organic Photovoltaics

We have developed a new heptacyclic benzodi­(cyclopentadithiophene) (BDCPDT) unit, where 3,7-positions of the central benzo­[1,2-b:4,5-b′]­dithiophenes (BDT) subunit are covalently rigidified with 3-positons of the two external thiophenes by two carbon bridges, forming two external CPDT rings that share two thiophene rings with the central BDT core. The distannyl-BDCPDT building block was copolymerized with 1,3-dibromo-thieno­[3,4-c]­pyrrole-4,6-dione (TPD) by Stille polymerization to afford a new alternating donor–acceptor copolymer PBDCPDT-TPD. The implementation of forced planarization greatly suppresses the interannular twisting to extend the effective conjugated length and preserve the interactions between the donor and acceptor segments. The device using the PBDCPDT-TPD/PC71BM (1:3 in wt%) blend processed with dimethyl sulfoxide as an additive delivered a marked PCE of 6.6% which represents a significant enhancement compared to the device using the corresponding nonfused polymer analogue with a PCE of 0.2%

Self-Assembled Poly(ethylene glycol) Buffer Layers in Polymer Solar Cells: Toward Superior Stability and Efficiency

In this study, we have systematically investigated the mechanism behind the formation of nanoscale self-assembled polymer buffer layers at the cathode interfaces of polymer solar cells. Poly(ethylene glycol) (PEG) molecules in a polymer blend, comprising poly(3-hexylthiophene) and [6,6]-phenyl-C₆₁-butyric acid methyl ester, spontaneously migrated to the surface where they reacted with the Al cathode to form ohmic contacts. In terms of thermodynamics, the surface energy of the substrates played an important role in triggering the vertical-type morphology. From a kinetics point of view, PEG polymers having lower molecular weights readily underwent vertical phase separation prior to solidification of the polymer films, due to their higher mobilities, whereas PEG polymers of higher molecular weights tended to become trapped in the active layer. Employing this knowledge, we prepared organic photovoltaic cells exhibiting both high efficiency and appreciable improvement in stability

Solution-Processed Nanocomposites Containing Molybdenum Oxide and Gold Nanoparticles as Anode Buffer Layers in Plasmonic-Enhanced Organic Photovoltaic Devices

Solution-processed nanocomposites containing molybdenum oxide (MoO₃) and gold nanoparticles (Au NPs) have been used as anode buffer layers in organic photovoltaic devices (OPVs). The resulting devices exhibit a remarkable enhancement in power conversion efficiency after Au NPs were incorporated into the device. Such enhancements can be attributed to the localized surface plasmon resonance induced by the metallic nanostructures. We have also found that the rate of exciton generation and the probability of exciton dissociation were increased. Furthermore, the devices made of the MoO₃ buffer layer containing Au NPs exhibited superior stability. This work opens up the possibility of fabricating OPVs with both high efficiency and a prolonged lifetime

Variation of Helical Twisting Power in Self-Assembled Sugar-Appended Schiff Base Chiral Rod−Coil Amphiphiles

A series of sugar-appended Schiff base chiral rod−coil amphiphiles with various alkoxy chain lengths have been synthesized in order to elucidate the variation of the helical twisting power (HTP), that is, the inverse of the pitch length, of the helical texture in the self-assembly of the amphiphiles. The HTP induced by chiral sugar in the self-assembled helical morphology was dependent upon the alkoxy chain length. Increasing the alkoxy chain length caused the self-assembled morphology to change from platelet-like texture to helical-twist morphology with varying pitch length and then revert to the platelet-like texture. This result demonstrates that the HTP reaches a maximum as the alkoxy chain length changes. The transformation from platelet-like to helical-twist morphology is induced by significant steric hindrance, when the effective size of adjacent alkoxy chains reaches the threshold of helical twisting and bending, resulting in the formation of a chiral smectic C phase. However, as the alkoxy chain length increases further, the disordering of the alkoxy chain conformation in the smectic-like layered structure may give rise to a structural imperfection that reduces the steric-hindrance effect. Eventually, the steric-hindrance effect may reach a compromise with the structural imperfection to produce a platelet-like morphology, leading to the formation of a low-order smectic phase

Tuning Molecular Conformations to Enhance Spontaneous Orientation Polarization in Organic Thin Films

Three isomeric derivatives of 2,2′,2″-(1,3,5-benzinetriyl)-tris­(1-phenyl-1-H-benzimidazole) (TPBi) bearing ethyl groups on the N-phenyl moieties were synthesized to elucidate the effects of intramolecular interactions on spontaneous orientation polarization (SOP) in thin films. The films of the TPBi derivatives displayed enhanced SOP with a surface potential change of up to 1.8 times that for TPBi, and the p-substituted derivative exhibited the largest potential change reported to date (+141.0 mV/nm). Density functional theory calculations and single-crystal structure analysis suggest that the introduction of the ethyl groups switched the stable molecular conformation from C1 to C3 symmetry. Through analysis of the structural anisotropy in the films by spectral ellipsometry and two-dimensional (2D) grazing-incidence wide-angle X-ray scattering, we conclude that the conformational change of the molecules was the major factor underlying the SOP enhancement

Tuning Molecular Conformations to Enhance Spontaneous Orientation Polarization in Organic Thin Films

Three isomeric derivatives of 2,2′,2″-(1,3,5-benzinetriyl)-tris­(1-phenyl-1-H-benzimidazole) (TPBi) bearing ethyl groups on the N-phenyl moieties were synthesized to elucidate the effects of intramolecular interactions on spontaneous orientation polarization (SOP) in thin films. The films of the TPBi derivatives displayed enhanced SOP with a surface potential change of up to 1.8 times that for TPBi, and the p-substituted derivative exhibited the largest potential change reported to date (+141.0 mV/nm). Density functional theory calculations and single-crystal structure analysis suggest that the introduction of the ethyl groups switched the stable molecular conformation from C1 to C3 symmetry. Through analysis of the structural anisotropy in the films by spectral ellipsometry and two-dimensional (2D) grazing-incidence wide-angle X-ray scattering, we conclude that the conformational change of the molecules was the major factor underlying the SOP enhancement