Wei-Che Yen

Near-ultraviolet photodetector based on hybrid polymer/zinc oxide nanorods by low-temperature solution processes

In this article, we have proposed a nanostructured near-ultraviolet photodetector (<400nm) based on the ZnO nanorod/polyfluorene hybrid by solution processes at low temperature. The current-voltage characteristic of the hybrid device demonstrates the typical pn-heterojunction diode behavior, consisting of p-type polymer and n-type ZnO nanorods, respectively. The relative quantum efficiencies of the hybrid device exhibit a nearly three order difference while illuminated under UV and visible light, respectively. The responsivity for the device can reach to 0.18A∕W at 300nm by applying a bias of −2V, which provides a route to fabricate a low-cost near-UV photodetector.

Employing an amphiphilic interfacial modifier to enhance the performance of a poly(3-hexyl thiophene)/TiO2 hybrid solar cell

We have studied two amphiphilic interfacial modifiers: low cost Cu phthalocyanine dye containing ether side chains (Cu–ph–ether dye) and a carboxylic acid- and bromine-terminated 3-hexyl thiophene oligomer (oligo-3HT-(Br)COOH, Mw ∼ 5K) to enhance the interfacial interaction between poly(3-hexyl thiophene) (P3HT) and TiO2 nanorods. A large improvement in the performance of fabricated solar cells was observed using these relatively large molecular modifiers when compared to pyridine-modified TiO2 nanorods. UV-vis spectroscopy and X-ray photoelectron spectroscopy analyses reveal that the modifiers are adsorbed and chemically bonded to TiO2 through unshared electrons associated with the modifiers. Furthermore, the new modifiers increased the hydrophobicity of TiO2 with the order of oligo-3HT-(Br)COOH > Cu–ph–ether dye > pyridine. Synchrotron X-ray spectroscopy studies of the modified hybrid films indicate the crystallinity of P3HT is increased, following the same trend as the hydrophobicity, because the new modifiers function as plasticizers, increasing the flow characteristics of the film. Moreover, the same trend is also observed for the reduced recombination rate and increased lifetime of charge carriers in the device by transient photo-voltage measurement. Thus, the oligo-3HT-(Br)COOH outperforms the Cu–ph–ether dye and pyridine in enhancing the power conversion efficiency (PCE, η) of the solar cell. More than a two-fold improvement is shown compared to pyridine. The results are due to the large size, conductivity, and polar characteristics of the oligo-3HT-(Br)COOH unit, which facilitates both the crystallization of P3HT and the electron transport of the TiO2 nanorods. This study provides a useful route for increasing the efficiency of hybrid solar cellsvia the enhancement of interfacial interactions between organic donors and inorganic acceptor materials.

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.

Nanoscale morphology and performance of molecular-weight-dependent poly(3-hexylthiophene)/TiO2 nanorod hybrid solar cells

We have investigated the effect of polymer molecular weight on the morphology and performance of poly(3-hexylthiophene)/TiO2nanorod hybrid photovoltaic devices by using scanning near field optical microscopy (SNOM), atomic force microscopy (AFM) and confocal Raman microscopy. From the topography and absorption mapping images, it is found that the rod-like structure of the low molecular weight polymer hybrid film consists of a large amount of grain boundaries and has a less continuous absorption mapping image. In contrast, the larger domain structure of the high molecular weight polymer hybrid film exhibits a continuous absorption mapping image, as a result of enhanced polymer stacking and electronic delocalization. The nanoscale morphology of the hybrid samples with different molecular weights also reveals the nature of photovoltaic performance and carrier transport behavior investigated by the time-of-flight technique.

Band gap aligned conducting interface modifier enhances the performance of thermal stable polymer-TiO2 nanorod solar cell

In this paper, we show that the poly(3-hexyl-thiophene)/TiO2 nanorod hybrid material is more thermally stable than the poly(3-hexyl-thiophene)/[6,6]-phenyl C61-bntyric acid methyl ester (P3HT/PCBM) hybrid material. A metal free conducting interface modifier of oligo-3-hexyl thiophene carboxylic acid (oligo-3HT-COOH) has been synthesized that exhibits aligned band gap for the P3HT/TiO2 hybrid. The conducting modifier shows an increase in power conversion efficiency of 4.8 times over an insulating modifier of oleic acid and 2.2 folds improvement over small molecule modifier of pyridine. These increases are due to a reduced recombination rate (42 μs carrier life time) and fast electron injection time of 0.24 ps. This interface modifier makes thermally stable organic-inorganic hybrid materials useful for fabrication of all solution processable solar cells.

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

Colloid TiO(2) nanorods are used for solution-processable poly(3-hexyl thiophene): TiO(2) 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 TiO(2) is higher than that of TiO(2)-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 TiO(2) nanorods because W4 has aligned bandgap with P3HT and TiO(2) to facilitate charge separation and transport. The electronic mobility of two-stage ligand exchanged TiO(2) is improved furthermore except Lut, because it adheres well and difficult to be replaced by W4. The amount of W4 on TiO(2)-tBP is 3 times more than that of TiO(2)-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 TiO(2)-tBP-W4 device has the best performance with 1.4 and 2.6 times more than TiO(2)-pyridine-W4 and TiO(2)-Lut-W4 devices, respectively. The pKa of the pyridine derivatives plays the major role to determine the ease of ligand exchange on TiO(2) which is the key factor mandating the PCE of P3HT:TiO(2) hybrid solar cell. The results of this study provide new insights of the significance of acid-base reaction on the TiO(2) surface for TiO(2)-based solar cells. The obtained knowledge can be extended to other hybrid solar cell systems.

Correlation between nanoscale surface potential and power conversion efficiency of P3HT/TiO2 nanorod bulk heterojunction photovoltaic devices

This is an in depth study on the surface potential changes of P3HT/TiO(2) nanorod bulk heterojunction thin films. They are affected by interlayer structures, the molecular weight of P3HT, the processing solvents and the surface ligands on the TiO(2). The addition of an electron blocking layer and/or the hole blocking layer to the P3HT/TiO(2) thin film can facilitate charge carrier transport and result in a high surface potential shift. The changes in surface potential of multilayered bulk heterojunction films are closely correlated to their power conversion efficiency of photovoltaic devices. Changing ligand leads to the largest change in surface potential yielding the greatest effect on the power conversion efficiency. Merely changing the P3HT molecular weight is less effective and varying the processing solvents is least effective in increasing power conversion efficiency. The steric effect of the ligand has a large influence on the reduction of charge carrier recombination resulting in a great effect on the power conversion efficiency. By monitoring the changes in the surface potential of bulk heterojunction film of multilayer structures, we have obtained a useful guide for the fabrication of high performance photovoltaic devices.

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 lambda(max) of the film absorption is blue-shifted from 520 to 470 nm, and its lambda(max) 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.

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.