Substituent Effect on the Optoelectronic Properties of Alternating Fluorene-Thiophene Copolymers

A novel series of soluble alternating conjugated copolymers, comprised of 9,9-dihexylfluorene and thiophene or substituted thiophene moieties (P1−P5), were synthesized via the palladium-catalyzed Suzuki coupling reaction. Substitutents on the thiophene include the electron-donating groups of hexyl and hexyloxymethyl group and the electron-withdrawing groups of hexyl carboxylate and cyano. The steric effects of the bulkier substituents outweigh the electronic effects of the substituents on the observed absorption and photoluminescence properties of the copolymers. Therefore, only the cyano substituted copolymer (P4) exhibits a red shift of the electronic spectra with 1.6 times enhancement in the fluorescence quantum yield as compared with the unsubstituted copolymer (P5). The substituents slightly reduce the values of Tg and Td of P5, but all of the reported copolymers have a Td larger than 300 °C

Facile Synthesis of Well-Defined Block Copolymers Containing Regioregular Poly(3-hexyl thiophene) via Anionic Macroinitiation Method and Their Self-Assembly Behavior

P3HT-P2VP block copolymers were synthesized using anionic macroinitiation of a vinyl end-functionalized P3HT. With different block ratio of P2VP to P3HT, the block copolymers exhibit sphere, cylinder, lamellae, and nanofiber nanostructures

Effect of TiO₂ Nanoparticles on Self-Assembly Behaviors and Optical and Photovoltaic Properties of the P3HT-<i>b</i>-P2VP Block Copolymer

An ordered nanostructure can be created from the hybrid materials of self-assembly poly(3-hexyl thiophene-b-2-vinyl pyridine) and nicotinic acid-modified titanium dioxide nanoparticles (P3HT-b-P2VP/TiO2). TEM and XRD analyses reveal that the TiO2 nanoparticles (NPs) are preferentially confined in the P2VP domain of P3HT-b-P2VP whereas TiO2 NPs interact with either pure P3HT or a blend of P3HT and P2VP to produce microsized phase segregation. The morphologies of lamellar and cylindrical structures are disturbed when the loading of TiO2 NPs is 40 wt % or higher. Cylindrical P3HT-b-P2VP/TiO2 exhibits a small blue shift in absorption and photoluminescence spectra with increasing TiO2 loading as compared to P3HT/TiO2. The NPs cause a slightly misaligned P3HT domain in the copolymer. Furthermore, the PL quenching of P3HT-b-P2VP/TiO2 becomes very large as a result of efficient charge separation in the ordered nanodomain at 16 nm. Solar cells fabricated from self-assembly P3HT-b-P2VP/TiO2 hybrid materials exhibit a >30 fold improvement in power conversion efficiency as compared to the corresponding 0.3P3HT-0.7P2VP/TiO2 polymer blend hybrid. This study paves the way for the further development of high-efficiency polymer−inorganic nanoparticle hybrid solar cells using a self-assembled block copolymer

Manipulation of Nanoscale Phase Separation and Optical Properties of P3HT/PMMA Polymer Blends for Photoluminescent Electron Beam Resist

A novel photoluminescence electron beam resist made from the blend of poly(3-hexylthiophene) (P3HT) and poly(methyl methacrylate) (PMMA) has been successfully developed in this study. In order to optimize the resolution of the electron beam resist, the variations of nanophase separated morphology produced by differing blending ratios were examined carefully. Concave P3HT-rich island-like domains were observed in the thin film of the resist. The size of concave island-like domains decreased from 350 to 100 nm when decreasing the blending ratio of P3HT/PMMA from 1:5 to 1:50 or lower, concurrently accompanied by significant changes in optical properties and morphological behaviors. The λmax of the film absorption is blue-shifted from 520 to 470 nm, and its λmax of photoluminescence (PL) is also shifted from 660 to 550 nm. The radiative lifetime is shorter while the luminescence efficiency is higher when the P3HT/PMMA ratio decreases. These results are attributed to the quantum confinement effect of single P3HT chain isolated in PMMA matrix, which effectively suppresses the energy transfer between the well-separated polymer chains of P3HT. The factors affecting the resolution of the P3HT/PMMA electron beam resists were systematically investigated, including blending ratios and molecular weight. The photoluminescence resist with the best resolution was fabricated by using a molecular weight of 13 500 Da of P3HT and a blending ratio of 1:1000. Furthermore, high-resolution patterns can be obtained on both flat silicon wafers and rough substrates made from 20 nm Au nanoparticles self-assembled on APTMS (3-aminopropyltrimethoxysilane)-coated silicon wafers. Our newly developed electron beam resist provides a simple and convenient approach for the fabrication of nanoscale photoluminescent periodic arrays, which can underpin many optoelectronic applications awaiting future exploration

Molecular Structure Effect of Pyridine-Based Surface Ligand on the Performance of P3HT:TiO₂ Hybrid Solar Cell

Colloid TiO2 nanorods are used for solution-processable poly­(3-hexyl thiophene): TiO2 hybrid solar cell. The nanorods were covered by insulating ligand of oleic acid (OA) after sol-gel synthesis. Three more conducting pyridine type ligands: pyridine, 2,6-lutidine (Lut) and 4-tert-butylpyridine (tBP) were investigated respectively to replace OA. The power conversion efficiency (PCE) of the solar cell was increased because the electronic mobility of pyridine-type ligand-modified TiO2 is higher than that of TiO2–OA. The enhancement of PCE is in the descending order of Lut > pyridine > tBP because of the effective replacement of OA by Lut. The PCE of solar cell can be further enhanced by ligand exchange of pyridine type ligand with conjugating molecule of 2-cyano-3-(5-(7-(thiophen-2-yl)-benzothiadiazol-4-yl) thiophen-2-yl) acrylic acid (W4) on TiO2 nanorods because W4 has aligned bandgap with P3HT and TiO2 to facilitate charge separation and transport. The electronic mobility of two-stage ligand exchanged TiO2 is improved furthermore except Lut, because it adheres well and difficult to be replaced by W4. The amount of W4 on TiO2-tBP is 3 times more than that of TiO2–Lut (0.20 mol % vs. 0.06 mol %). Thus, the increased extent of PCE of solar cell is in the decreasing order of tBP > pyridine > Lut. The TiO2-tBP-W4 device has the best performance with 1.4 and 2.6 times more than TiO2-pyridine-W4 and TiO2-Lut-W4 devices, respectively. The pKa of the pyridine derivatives plays the major role to determine the ease of ligand exchange on TiO2 which is the key factor mandating the PCE of P3HT:TiO2 hybrid solar cell. The results of this study provide new insights of the significance of acid-base reaction on the TiO2 surface for TiO2-based solar cells. The obtained knowledge can be extended to other hybrid solar cell systems

Substituent Effect on the Optoelectronic Properties of Alternating Fluorene−Cyclopentadithiophene Copolymers

A novel series of soluble alternating conjugated copolymers, comprised of 9,9-dihexylfluorene and cyclopentadithiophenes (P1−P5), were synthesized via Pd-catalyzed Suzuki coupling reaction in good yields. The UV−vis absorption spectra, fluorescence spectra, fluorescence quantum yields, and cyclic voltammograms of P1−P5 are also reported. The P2 and P3 with electron-donating non-π-substituents (ethylenedioxy and propylenedioxy bridges the 3,3-positions of the thiophene groups) display high fluorescence quantum yields and red-shifted absorption as compared with nonsubstituted P1. However, the P4 and P5 are weakly fluorescent and exhibit blue-shifted absorption which are due to the presence of electron-withdrawing π-substituents (carbonyl and dicyanoethenyl). The behavior of P4 and P5 can be attributed to the significant electronic coupling between the π-substituents and the conjugated polymer backbone that leads to a less allowed optical transition between the ground and the lowest excited state, resembling the meta conjugation effect in phenylene-derived conjugated π-systems. This argument is supported by molecular orbital distribution calculations