Yi-Huan Lee

Effect of TiO2 Nanoparticles on Self-Assembly Behaviors and Optical and Photovoltaic Properties of the P3HT-b-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/TiO(2)). TEM and XRD analyses reveal that the TiO(2) nanoparticles (NPs) are preferentially confined in the P2VP domain of P3HT-b-P2VP whereas TiO(2) 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 TiO(2) NPs is 40 wt % or higher. Cylindrical P3HT-b-P2VP/TiO(2) exhibits a small blue shift in absorption and photoluminescence spectra with increasing TiO(2) loading as compared to P3HT/TiO(2). The NPs cause a slightly misaligned P3HT domain in the copolymer. Furthermore, the PL quenching of P3HT-b-P2VP/TiO(2) 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/TiO(2) hybrid materials exhibit a >30 fold improvement in power conversion efficiency as compared to the corresponding 0.3P3HT-0.7P2VP/TiO(2) 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.

Self-assembly and phase transformations of π-conjugated block copolymers that bend and twist: from rigid-rod nanowires to highly curvaceous gyroids

In this study, a series of π-conjugated block copolymers of regioregular poly(3-hexyl thiophene)-b-poly(2-vinyl pyridine) (P3HT–P2VP) was synthesized and their self-assembly behavior and the detailed thermodynamic phase diagram were explored. By a combination of TEM, SAXS, WAXS, and UV-VIS measurements, it was found that the π-conjugated P3HT in their various self-assembled nanodomains could be a rigid rod, or a semi-rigid chain, or even a fully flexible chain. With the P2VP volume fraction, ϕ, = 0.20, the P3HT–P2VP displays a nanowire structure with a fully extended all-transP3HT rod structure across the width of the nanowires, indicating a prevailing rod–rod interaction between P3HT blocks over the microphase separation interaction between the constituent blocks. With ϕ = ∼0.30 to 0.6, the P3HT–P2VPs show a highly ordered lamellar structure with the P3HT block exhibiting as a semirod-like chain composed of shorter rods connected by twisted 3HT units. With ϕ > 0.68, the π-conjugated block copolymers display self-assembling nanostructures of hexagonal close packed cylinders and spheres, indicating that P3HT adopts a fully coil-like structure that favors interfacial curvatures. In particular, for the P3HT–P2VP with ϕ = 0.68, a gyroid phase, the first of its kind for π-conjugated block copolymers, was observed upon heating. For the nanowire structured P3HT–P2VP, a liquid crystalline phase transition from the smectic-like crystalline state to a nematic structure was observed at ∼200 °C. The observed microstructures and transformations reveal the importance of the semirigid nature of π-conjugated P3HT chains and provide new guidelines for the organization of π-conjugated block copolymers for future optoelectronic applications.

In-situ template synthesis of a polymer/semiconductor nanohybrid using amphiphilic conducting block copolymers.

In this study, we synthesized organic/inorganic hybrid materials containing cadmium sulfide (CdS) nanoparticles using a novel amphiphilic conducting block copolymer as a synergistic structure-directing template and an efficient exciton quencher of the hybrid. The amphiphilic rod-coil block copolymer of polyphenylene-b-poly(2-vinyl pyridine) (PPH-PVP) was first prepared from its coil-coil precursor block copolymer of poly(1,3-cyclohexadiene)-b-poly(2-vinyl pyridine) (PCHD-PVP) by using sequential anionic polymerization followed by the aromatization reaction of converting the PCHD block to form conducting PPH. The synthesized PCHD-PVP block copolymers self-assembled into different bulk nanostructures of lamellae, cylinders, and spheres at a volume fraction similar to that of many coil-coil block copolymer systems. However, an enhanced chain-stiffness-induced morphological transformation was observed after the aromatization reaction. This is evidenced by the TEM observation in which both spherical and cylindrical structured PCHD-PVPs transform into lamellar structured PPH-PVPs after aromatization. In addition to the bulk-phase transformation, the rigid-rod characteristic of the conducting PPH block also affects the self-assembling property of the block copolymers in their solution state. CdS nanoparticles were synthesized in situ in a selective solvent of THF using PCHD-PVP and PPH-PVP micelles as nanoreactors. The PPH-PVP/Cd ion in THF exhibits a new ringlike structure of uniform size (approximately 50 nm) with PPH in the inner rim and complexed PVP/Cd ions in the outer rim as a result of the effects of strong intermolecular forces between PPH segments and the solvophobic interaction. CdS nanoclusters were subsequently synthesized in situ from the PPH-PVP/Cd(2+) ring structure, forming a nanohybrid with intimate contact between the PPH domain and CdS nanoparticles. In particular, we found that there is an efficient energy/electron transfer between the conducting PPH domain and CdS nanoparticles in the hybrid, resulting in an enhanced PL quenching effect. The novel nanohybrid shows the potential to be used for optoelectronic applications.

Solution self-assembly and phase transformations of form II crystals in nanoconfined poly(3-hexyl thiophene) based rod-coil block copolymers

Solution processing of π-conjugated polymers constitutes a major low-cost manufacturing method for the fabrication of many new organic optoelectronic devices. The solution self-assembly kinetics of π-conjugated rod-coil block copolymers of symmetric poly(3-hexyl thiophene)-b-poly(2-vinyl pyridine) (P3HT-P2VP) during drying and the phase transformations of the subsequently dried samples were studied by using a combination of TEM, SAXS, WAXS and DSC measurements. During solution drying in chlorobenzene, a good solvent for the copolymer, P3HT-P2VP first formed nanoseed aggregates followed by the directional growth of nanofibrils driven by the formation of prevailing form II P3HT crystals within its nanofibril core confined by the surrounding domain of P2VP blocks. This result was in sharp contrast when a similar molecular weight P3HT homopolymer was solution self-assembled in chlorobenzene, nearly free from confinement, in which case the resulting nanofibrils consisted of a mixture of majority form I and form II crystals. Solvent-cast films of P3HT-P2VP nanofibrils with form II crystals were heat-/cold-treated and showed solid-state phase transformations from form II crystals to form I crystals, both within nanofibrils with annealing, indicating the metastability of the form II crystals with temperature. A disordered state followed with increasing temperatures which, when cooled, induced the formation of a thermodynamically stable lamellar phase with only form I P3HT crystals. Correspondingly, the study provides new strategies for controlling polymorphs and nanostructures of π-conjugated block copolymers for future applications using solution processing and subsequent heat treatment.

Self-assembled all-conjugated block copolymer as an effective hole conductor for solid-state dye-sensitized solar cells.

An all-conjugated diblock copolymer, poly(2,5-dihexyloxy-p-phenylene)-b-poly(3-hexylthiophene) (PPP-b-P3HT), was synthesized and applied as a hole transport material (HTM) for the fabrication of solid-state dye-sensitized solar cells (ss-DSCs). This copolymer is characterized by an enhanced crystallinity, enabling its P3HT component to self-organize into interpenetrated and long-range-ordered crystalline fibrils upon spin-drying and ultimately endowing itself to have a faster hole mobility than that of the parent P3HT homopolymer. Transient photovoltage measurements indicate that the photovoltaic cell based on PPP-b-P3HT as the HTM has a longer electron lifetime than that of the reference device based on P3HT homopolymer. Moreover, comparing the two ss-DSCs in terms of the electrochemical impedance spectra reveals that the electron density in the TiO2 conduction band is substantially higher in the PPP-b-P3HT device than in the P3HT cell. Above observations suggest that the PPP block facilitates an intimate contact between the copolymer and dye molecules absorbed on the nanoporous TiO2 layer, which significantly enhances the performance of the resulting device. Consequently, the PPP-b-P3HT ss-DSC exhibits a promising power conversion efficiency of 4.65%. This study demonstrates that conjugated block copolymers can function as superior HTMs of highly efficient ss-DSCs.

Effect of rod–rod interaction on self-assembly behavior of ABC π-conjugated rod–coil–coil triblock copolymers

A new class of ABC π-conjugated rod–coil–coil triblock copolymers of poly(diethylhexyloxy-p-phenylene vinylene)-b-poly (2-vinyl pyridine)-b-polystyrene (PPV-PVP-PS) was synthesized and its self-assembly behavior was explored. Three different triblock copolymers of PPV-PVP-PS1, PPV-PVP-PS2, and PPV-PVP-PS3, each with PPV, PS, and PVP, respectively, as the major species in the copolymers, were used to study the effects of copolymer composition and rod–rod interaction between PPV blocks on their morphology. Transmission electron microscopy (TEM), polarizing optical microscopy (POM), and simultaneously measured small-angle (SAXS) and wide-angle (WAXS) X-ray scattering experiments as a function of different annealing conditions revealed the details of the copolymer morphology, molecular packing, and their phase transitions. Despite their large differences in the rod volume fraction, fPPV, from 0.43 to 0.18, all three triblock copolymers adopted a self-assembled lamellar structure, in sharp constrast with the observation of many non-lamellar structures typically exhibited by ABC coil–coil–coil triblock copolymers with similar segregation strength. For PPV-PVP-PS1 with its major species PPV rod coupled with a single-phase symmetric PVP-PS diblock precursor, PPV-PVP-PS1 self-organized to form a triple-lamellar phase with each domain corresponding to the three respective blocks. Investigation of the molecular packing of PPV rods within their domain through the analysis of the 1D electron density profile suggests the PPV rods adopted a smectic C monolayer organization below its order–disorder transition temperature (TODT). For PPV-PVP-PS2 with its PS-rich asymmetric PVP-PS diblock precursor that displayed a disordered micelle structure, PPV-PVP-PS2 with fPPV of only 0.19 still exhibited a triple-lamellar phase with PPV forming a broken lamellar layer, thus preventing the excessive chain stretching of the coil blocks on the otherwise long-range ordered PPV lamellar phase. A similar broken triple-lamellar phase can also be observed for the PVP-rich PPV-PVP-PS3 with a low fPPV of only 0.18. Simultaneous SAXS and WAXS measurements show that all three triblock copolymers undergo the ordered lamella-to-disorder transition and the smectic/isotropic transition at the same temperature, indicating that the rod–rod interaction between PPV rods plays a critical role in forming and stabilizing these lamellar structures. The observation of the phase transformations is in good agreement with a recent mean-field prediction of a rod–coil–coil triblock copolymer system.

A new strategy for co-assembling π-conjugated polymer/cadmium sulfide hybrids into an efficient charge-transporting nanochannel array by using an all-conjugated diblock copolymer motif

In this study, a novel organic/inorganic hybrid material containing cadmium sulfide (CdS) nanoparticles was synthesized in situ by using an all π-conjugated block copolymer as both a synergistic long-range ordered structure-directing template and an efficient exciton quencher for the nanohybrid. The conducting–conducting block copolymer of poly(2,5-dihexyloxy-para-phenylene)-b-poly(3-hexyl thiophene) (PPP-P3HT) was used as a linear nanoreactor for the quantum dot synthesis to yield nanohybrids with highly ordered donor/acceptor (D/A) nanowire morphology that exhibit enhanced optoelectronic properties. The synthesized PPP-P3HT/CdS-OH nanohybrid possessed a one-dimensional D/A coaxial nanowire structure of uniform width (∼10 nm), wherein the synthesized CdS quantum dots were embedded, resulting in a display of superior long-term stability in a solution medium. By a simple drop casting process, the hybrid materials with highly ordered organic/inorganic nanochannel arrays could also be preserved in a thin film state, as evidenced by TEM and X-ray measurements, for device applications. The PPP-P3HT block copolymer exhibited significantly better hole mobility than its P3HT homopolymer counterpart. In addition, the resulting PPP-P3HT/CdS-OH hybrid systems displayed a considerably enhanced PL quench effect and shorter exciton lifetime due to efficient charge transfer between the P3HT block and the confining CdS nanoparticles in the P3HT/CdS coaxial domain of the hybrid nanowire structure. Therefore, the PPP-P3HT/CdS-OH nanohybrids can be used as bicontinuous nanochannels for efficient charge separation and charge transport, which show potential for use in future optoelectronic applications.

One-step in situ hydrothermal fabrication of D/A poly(3-hexylthiophene)/TiO2 hybrid nanowires and its application in photovoltaic devices

In this study, we developed a novel in situ hydrothermal method to fabricate self-assembled P3HT/TiO2 hybrid nanowires, wherein a facile one-step synthetic strategy was utilized to co-organize P3HT molecules and titanium precursors into highly elongated hybrid nanowires, followed by a hydrothermal process in an autoclave to in situ transform the titanium precursors into crystalline TiO2 nanoparticles on the P3HT nanofibrils. P3HT nanofibrils were utilized as a structure-directing motif to achieve a favorable dispersion of electron acceptor (A) TiO2 nanocrystals of 10–15 nm in diameter embossed along the nanofibrils, as well as an efficient electron donor (D) for the nanohybrid. In particular, the crystallization temperature of anatase-phase TiO2 nanoparticles with high crystallinity obtained via the hydrothermal method was significantly reduced to 130 °C in an elevated pressure of ∼7 bars as compared to the conventional calcination temperature of 450 °C at ambient pressure for TiO2 nanocrystal synthesis, therefore, allowing the synergistic one-step fabrication of both highly crystalline TiO2 nanoparticles embossed on highly crystalline long-range ordered P3HT nanofibrils. As a consequence of the structural development, this P3HT/TiO2 embossed nanohybrid could afford significant improvements in its D/A interfacial contact area for effective charge separation without the need for capping ligands typically used in ex situ D/A blend systems, as well as an efficient pathway for charge transport, leading to enhanced optoelectronic properties and device performance. The highest conversion efficiency of 0.14% was presented by the P3HT/TiO2 embossed hybrid device, which was a remarkable improvement as compared to only 0.03% from an ex situ P3HT/TiO2 hybrid device. This novel in situ approach shows a feasible way to fabricate organic/inorganic nanohybrid materials of conjugated copolymers with different inorganic nanoparticles for the applications of future optoelectronic devices.