Exploring Alkyl Chains in Benzobisthiazole-Naphthobisthiadiazole Polymers: Impact on Solar-Cell Performance, Crystalline Structures, and Optoelectronics.

The shapes and lengths of the alkyl chains of conjugated polymers greatly affect the efficiencies of organic photovoltaic devices. This often results in a trade-off between solubility and self-organizing behavior; however, each material has specific optimal chains. Here we report on the effect of alkyl side chains on the film morphologies, crystallinities, and optoelectronic properties of new benzobisthiazole-naphthobisthiadiazole (PBBT-NTz) polymers. The power conversion efficiencies (PCEs) of linear-branched and all-branched polymers range from 2.5% to 6.6%; the variations in these PCEs are investigated by atomic force microscopy, two-dimensional X-ray diffraction (2D-GIXRD), and transient photoconductivity techniques. The best-performing linear-branched polymer, bearing dodecyl and decyltetradecyl chains (C12-DT), exhibits nanometer-scale fibers along with the highest crystallinity, comprising predominant edge-on and partial face-on orientations. This morphology leads to the highest photoconductivity and the longest carrier lifetime. These results highlight the importance of long alkyl chains for inducing intermolecular stacking, which is in contrast to observations made for analogous previously reported polymers.

Following the TRMC Trail: Optimization of Photovoltaic Efficiency and Structure-Property Correlation of Thiophene Oligomers.

Semiconducting conjugated oligomers having same end group (N-ethylrhodanine) but different central core (thiophene: OT-T, bithiophene: OT-BT, thienothiophene: OT-TT) connected through thiophene pi-linker (alkylated terthiophene) were synthesized for solution processable bulk-heterojunction solar cells. The effect of the incorporation of an extra thiophene to the central thiophene unit either through C-C bond linkage to form bithiophene or by fusing two thiophenes together to form thienothiophene on the optoelectronic properties and photovoltaic performances of the oligomers were studied in detail. Flash photolysis time-resolved microwave conductivity (FP-TRMC) technique shows OT-TT has significantly higher photoconductivity than OT-T and OT-BT implying that the former can outperform the latter two derivatives by a wide margin under identical conditions in a bulk-heterojunction solar cell device. However, the initial photovoltaic devices fabricated from all three oligomers (with PC71BM as the acceptor) gave power conversion efficiencies (PCEs) of about 0.7%, which was counterintuitive to the TRMC observation. By using TRMC results as a guiding tool, solution engineering was carried out; no remarkable changes were seen in the PCE of OT-T and OT-BT. On the other hand, 5-fold enhancement in the device efficiency was achieved in OT-TT (PCE: 3.52%, VOC: 0.80 V, JSC: 8.74 mA cm(-2), FF: 0.50), which was in correlation with the TRMC results. The structure-property correlation and the fundamental reasons for the improvement in device performance upon solvent engineering were deduced through UV-vis absorption, atomic force microscopy, bright-field transmission electron microscopy, photoluminescence quenching analysis and two-dimensional grazing incidence X-ray diffraction studies.

p/n-Polarity of thiophene oligomers in photovoltaic cells: role of molecular vs. supramolecular properties.

Molecular and supramolecular properties play key roles in the optoelectronic properties and photovoltaic performances of organic materials. In the present work, we show how small changes in the molecular structure affect such properties, which in turn control the intrinsic and fundamental properties such as the p/n-polarity of organic semiconductors in bulk-heterojunction solar cells. Herein, we designed and synthesized two acceptor-donor-acceptor type semiconducting thiophene oligomers end-functionalized with oxazolone/isoxazolone derivatives (OT1 and OT2 respectively). The HOMO-LUMO energy levels of both derivatives were found to be positioned in such a way that they can act as electron acceptors to P3HT and electron donors to PCBM. However, OT1 functions as a donor (with PCBM) and OT2 as an acceptor (with P3HT) in BHJ photovoltaic cells, and their reverse roles results in either no or poor performance of the cells. Detailed studies using UV-vis absorption and fluorescence spectroscopy, time-correlated single photon counting, UV-photoelectron spectroscopy, density functional theory calculations, X-ray diffraction, and thermal gravimetric analysis proved that both molecular and supramolecular properties contributed equally but in a contrasting manner to the abovementioned observation. The obtained results were further validated by flash-photolysis time-resolved microwave conductivity studies which showed an excellent correlation between the structure, property, and device performances of the materials.

Thermally assisted photonic inversion of supramolecular handedness.

Spiraling into control: A photoresponsive supramolecular assembly demonstrates that light, along with heating (Δ) and cooling (), can cause chiral communication between molecules. This effect leads to bias in the helicity of the complex, causing a reversible switching of macroscopic handedness, as shown by a reversal of sign of the circularly polarized luminescence (CPL) that is emitted.

Oligo(p-phenylene-ethynylene)-derived super-π-gelators with tunable emission and self-assembled polymorphic structures.

Linear π-conjugated oligomers are known to form organogels through noncovalent interactions. Herein, we report the effect of π-repeat units on the gelation and morphological properties of three different oligo(p-phenylene-ethynylene)s: OPE3, OPE5, and OPE7. All of these molecules form fluorescent gels in nonpolar solvents at low critical gel concentrations, thereby resulting in a blue gel for OPE3, a green gel for OPE5, and a greenish yellow gel for OPE7. The molecule-molecule and molecule-substrate interactions in these OPEs are strongly influenced by the conjugation length of the molecules. Silicon wafer suppresses substrate-molecule interactions whereas a mica surface facilitates such interactions. At lower concentrations, OPE3 formed vesicular assemblies and OPE5 gave entangled fibers, whereas OPE7 resulted in spiral assemblies on a mica surface. At higher concentrations, OPE3 and OPE5 resulted in super-bundles of fibers and flowerlike short-fiber agglomerates when different conditions were applied. The number of polymorphic structures increases on increasing the conjugation length, as seen in the case of OPE7 with n=5, which resulted in a variety of exotic structures, the formation of which could be controlled by varying the substrate, concentration, and humidity.

Light-induced Ostwald ripening of organic nanodots to rods.

Ostwald ripening allows the synthesis of 1D nanorods of metal and semiconductor nanoparticles. However, this phenomenon is unsuccessful with organic π-systems due to their spontaneous self-assembly to elongated fibers or tapes. Here we demonstrate the uses of light as a versatile tool to control the ripening of amorphous organic nanodots (ca. 15 nm) of an azobenzene-derived molecular assembly to micrometer-sized supramolecular rods. A surface-confined dipole variation associated with a low-yield (13-14%) trans-cis isomerization of the azobenzene moiety and the consequent dipole-dipole interaction in a nonpolar solvent is believed to be the driving force for the ripening of the nanodots to rods.

Synthesis and properties of amphiphilic photoresponsive gelators for aromatic solvents.

A sugar-based photoresponsive supergelator, N-glycosylazobenzene that shows selective gelation of aromatic solvents is described. The partial trans-cis isomerization of the azobenzene moiety allows photoinduced chopping of the entangled gel fibers to short fibers, resulting in controlled fiber length and gel-sol transition. The gelator is useful for the selective removal of toxic aromatic solvents from water.

Self-assembly of thienylenevinylene molecular wires to semiconducting gels with doped metallic conductivity.

Oligo(thienylenevinylene) (OTV) based gelators with high conductivity are reported. When compared to OTV1, OTV2 having an increased conjugation length forms relatively strong gels with a metallic conductivity of 4.8 S/cm upon doping which is the highest value reported for an organogelator. This new class of conducting gels is expected to be useful for organic electronics and photonics application, particularly for bulk heterojunction devices.