Modulating Crystallinity of Poly(3-hexylthiophene) via Microphase Separation of Poly(3-hexylthiophene)–Polyisoprene Block Copolymers

A series of poly­(3-hexylthiophene)-<i>block</i>-polyisoprene (P3HT-<i>b</i>-PI) diblock copolymers (DBCP) and polyisoprene-<i>block</i>-poly­(3-hexylthiophene)-<i>block</i>-polyisoprene (PI-<i>b</i>-P3HT-<i>b</i>-PI) triblock copolymers (TBCP) with accurately controlled molecular architecture were synthesized via highly efficient coupling reaction between aldehyde end-functionalized P3HT and living anionic polyisoprene. The self-assembly behaviors, considering morphology and crystallinity, of the thermal annealed bulk samples of these DBCPs and TBCPs containing various PI content were systematically investigated. The DBCPs behaved very differently from most published P3HT BCP systems, showing elongated fibers with preserved crystallinity regardless of the PI fraction. More noteworthy, with PI fraction less than 40 wt %, the DBCPs exhibited parallel straight fibers longer than several micrometers accompanied by concurrent enhanced crystallinity. The unique microstructure of the DBCPs might originate from moderate microphase separation between P3HT and PI as well as high flexibility of PI to conduct the packing of P3HT. The TBCPs, by contrast, exhibited highly curved interdomain boundaries with significant depressed crystallinity, resembling P3HT diblock copolymers in the strong phase segregation regime, as more pronounced entanglement of the two terminal PI segments would restrict the movement of P3HT

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

Rational Design of Versatile Self-Assembly Morphology of Rod–Coil Block Copolymer

Controlling the nonlamellar and bicontinuous nanostructures through changing volume fraction is a well-developed technique for coil–coil block copolymer, but it is not always effective for rod–coil block copolymer due to strong rod–rod interaction. Versatile self-assembly morphology of rod–coil copolymer can be achieved by simultaneously adjusting the rod–rod interaction, rod–coil interaction, and conformational asymmetry. This approach has been investigated by using poly­(3-alkylthiophene)-<i>b</i>-poly­(methyl methacrylate) as a model. By altering the alkyl side chain of polythiophene from linear hexyl to longer dodecyl and to branch 2-ethylhexyl, both rod–coil and rod–rod interaction are decreased with increasing spatial occupation of alkyl side chain which have been quantitatively determined for this type of rod–coil copolymer. With tunable conformational asymmetry, competition between rod–rod and rod–coil interactions, and crystallization-driven force, the presence of versatile morphology, i.e., lamellar and hexagonal structures, cylinder-to-gyroid phase transition, and disordered phase, can be observed for long-sought composition at approximately <i>f</i>_rod = 0.5. The finding described here can provide new insights into the self-assembly behaviors of rod–coil block copolymer for scientists to manipulate and obtain the desired order morphology in high performance optoelectronic applications

Synthesis, Morphology, and Optical and Electrochemical Properties of Poly(3-hexylthiophene)-<i>b</i>-poly(3-thiophene hexylacetate)

A series of all-conjugated diblock copolythiophenes of poly­(3-hexylthiophene)-<i>b</i>-poly­(3-thiophene hexylacetate) (P3HT-<i>b</i>-P3THA) were synthesized via modified sequential Grignard metathesis polymerization. The living P3HT was formed first, then reacting with the monomer of P3THA. By using 2-bromo-3-hexyloxycarbonylmethylene-5-iodothiophene instead of dibromo monomer in metal exchange reaction and by controlling the polymerization temperature relatively low at 16–20 °C, the reaction between carboxylate group and Grignard reagent can be minimized and the polymerization can be controlled; low PDI (<1.3), high regioregularity (>95%), and well-controlled block ratios of block copolymer were obtained. The introduction of carboxylate group in the side chain of one of the monomers, and controlling the side-chain length difference by only three atoms between two monomers, there are profound effects on the optical and electrochemical properties and morphologies of the block copolymers. The electron-withdrawing carboxylate causes the absorption maximum of copolymer in solution to be blue-shifted from that of pristine P3HT, and the extent of blue shift is increased monotonically with increasing the molar ratio of P3THA. However, in thin film, the intermolecular π–π stacking plays a role in the absorption behavior of copolymer which decreases the extent of blue shift. The HOMO level of the copolymer is lowered by 0.38 eV from that of P3HT due to the presence of P3THA block. The crystalline structure of the copolymer can be controlled according to the molar ratio of each block. Crystalline–amorphous, crystalline–crystalline, and cocrystalline structures are observed in the bulk samples when the block molar percentage of P3THA is increased from 22, 40, to 50 and higher, respectively. Microphase separation is clearly present in the thin film fabricated from the copolymer containing crystalline–amorphous and crystalline–crystalline structures. The observation of various crystalline structures in a single type of all-conjugated diblock copolymer is very significant and provides a new approach to simultaneously manipulate the optical and electronic properties and nanostructures of conducting polymers by simply changing their compositions

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

Synthesis and Self-Assembly of Poly(diethylhexyloxy-<i>p</i>-phenylenevinylene)-<i>b</i>-poly(methyl methacrylate) Rod−Coil Block Copolymers

A series of poly(diethylhexyloxy-p-phenylenevinylene-b-methyl methacrylate) (DEH-PPV-b-PMMA) polymers with narrow polydispersity (PDI 1H nuclear magnetic resonance (1H NMR) and gel permeation chromatography (GPC). Transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) studies reveal the details of copolymer morphology. The DEH-PPV-b-PMMA system presented here has higher block segregation strength than many previously studied rod−coil block copolymers yet still shows experimentally accessible phase transitions with respect to temperature. As a result, this molecule offers new insight into the competition between rod−rod and rod−coil interactions that occurs in the system. The DEH-PPV rods are organized as a monolayer that is inclined with the lamellar normal (smectic C) for the copolymers containing low volume fraction of PMMA coil (<54%). However, as the coil fraction increases, the strips containing DEH-PPV pack into hexagonal lattice. In contrast to previous work which demonstrated similar morphologies, the sequence of reversible liquid crystalline and microphase phase transitions is altered as a result of the increased block segregation. Upon heating, the low coil fraction copolymers exhibit a series of clear transitions of smectic−lamellar to amorphous−lamellar to disordered structures. In high coil fraction copolymers, the transitions between smectic−hexagonal to amorphous−hexagonal and smectic−hexagonal to disorder structures could not be clearly differentiated. The order-to-disorder temperature (ODT) decreases slowly with increasing coil fraction while the smectic-to-isotropic transition (SI) temperature stays relatively unchanged. The steady SI temperature suggests that the strong rod−rod interaction keeps the liquid crystalline rod in the nanodomain structure regardless of the amount of coil segment in the copolymers

Formation Mechanism and Control of Perovskite Films from Solution to Crystalline Phase Studied by in Situ Synchrotron Scattering

Controlling the crystallization and morphology of perovskite films is crucial for the fabrication of high-efficiency perovskite solar cells. For the first time, we investigate the formation mechanism of the drop-cast perovskite film from its precursor solution, PbCl₂ and CH₃NH₃I in <i>N</i>,<i>N</i>-dimethylformamide, to a crystalline CH₃NH₃PbI_3–<i>x</i>Cl_<i>x</i> film at different substrate temperatures from 70 to 180 °C in ambient air and humidity. We employed an in situ grazing-incidence wide-angle X-ray scattering (GIWAXS) technique for this study. When the substrate temperature is at or below 100 °C, the perovskite film is formed in three stages: the initial solution stage, transition-to-solid film stage, and transformation stage from intermediates into a crystalline perovskite film. In each stage, the multiple routes for phase transformations are preceded concurrently. However, when the substrate temperature is increased from 100 to 180 °C, the formation mechanism of the perovskite film is changed from the “multistage formation mechanism” to the “direct formation mechanism”. The proposed mechanism has been applied to understand the formation of a perovskite film containing an additive. The result of this study provides a fundamental understanding of the functions of the solvent and additive in the solution and transition states to the crystalline film. It provides useful knowledge to design and fabricate crystalline perovskite films for high-efficiency solar cells

Enhancing P3HT/TiO₂ Hybrid Photovoltaic Performance by Incorporating High Surface Potential Silica Nanodots into Hole Transport Layer

We offer a novel approach to improve the performance of P3HT/TiO2 hybrid photovoltaic devices by incorporating either hydroxyl- or amino-functionalized silica nanodots (SND–OH or SND–NH2) into the hole transport layer of the PEDOT:PSS. The SNDs serve as screens between conducting polymer and ionomer PSS to improve the phase separation and charge transport of the PEDOT:PSS hole transport layer. The power conversion efficiency (PCE) was thus improved by 1.45 and 2.61 fold for devices fabricated with PEDOT:PSS containing 1 wt % of SND–OH (SND–OH device) and 1 wt % of SND–NH2 (SND–NH2 device), respectively, when compared with the devices fabricated by neat PEDOT:PSS. The increase in PCE arises from an increase in short circuit currents, which are affected by the phase separation of PEDOT:PSS with possessing incorporated SNDs. The low surface potential of hydroxyl-functionalized SNDs (SND–OH) is easily aggregated in the PEDOT:PSS solution and forms large-sized phase separation in the PEDOT:PSS film. The aggregation of SND–OH causes slight decreases in the resistance of PEDOT:PSS thin film from (61 ± 1 to 69 ± 4)× 106 Ohm/square and a decrease in the shielding effects of the SNDs. In contrast, the high surface potential of amino-functionalized SNDs are dispersed uniformly in the PEDOT:PSS solution and form morphologies with small-sized domains in the PEDOT:PSS film. As a result, the sheet resistance of PEDOT:PSS thin films is decreased from (61 ± 1 to 46 ± 3) × 106 Ohm/square. Therefore, the SND–NH2 device exhibits greater performance over the SND–OH devices