P3HT Solution Research Articles

Entanglement of Conjugated Polymer Chains Influences Molecular Self‐Assembly and Carrier Transport

AbstractThe influence of polymer entanglement on the self‐assembly, molecular packing structure, and microstructure of low‐Mw (lightly entangled) and high‐Mw (highly entangled) poly (3‐hexylthiophene) (P3HT), and the carrier transport in thin‐film transistors, are investigated. The polymer chains are gradually disentangled in a marginal solvent via ultrasonication of the polymer solution, and demonstrate improved diffusivity of precursor species (coils, aggregates, and microcrystallites), enhanced nucleation and crystallization of P3HT in solution, and self‐assembly of well‐ordered and highly textured fibrils at the solid–liquid interface. In low‐Mw P3HT, reducing chain entanglement enhances interchain and intrachain ordering, but reduces the interconnectivity of ordered domains (tie molecules) due to the presence of short chains, thus deteriorating carrier transport even in the face of improving crystallinity. Reducing chain entanglement in high‐Mw P3HT solutions increases carrier mobility up to ≈20‐fold, by enhancing interchain and intrachain ordering while maintaining a sufficiently large number of tie molecules between ordered domains. These results indicate that charge carrier mobility is strongly governed by the balancing of intrachain and interchain ordering, on the one hand, and interconnectivity of ordered domains, on the other hand. In high‐Mw P3HT, intrachain and interchain ordering appear to be the key bottlenecks to charge transport, whereas in low‐Mw P3HT, the limited interconnectivity of the ordered domains acts as the primary bottleneck to charge transport.

Tetrafluorene-9,9′-bifluorenylidene as a non-fullerene type electron acceptor for P3HT-based bulk-heterojunction polymer solar cells

2,2′,7,7′-Tetra(9,9-diethyl-9H-fluorene)-9,9′-bifluorenylidene (TFBF) was synthesized as a non-fullerene type electron acceptor for bulk-heterojunction polymer solar cells. The TFBF was synthesized from 2,7-di(9,9-diethyl-9H-fluorene)-9H-fluoren-9-one by a reductive dimerization using Lawesson's reagent. The TFBF was soluble in common organic solvents such as dichloromethane, chloroform, and 1,2-dichlorobenzene. The synthesis of TFBF was identified by 1H NMR, 13C NMR, FAB-Mass, and FT-IR, and the properties of the molecule were characterized by UV–visible absorption and fluorescence spectroscopy. The ionization potential of the TFBF solid thin film was determined by photo-electron spectroscopy, and morphology of the active layer was investigated by atomic force microscope. The measured optical band gap, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of TFBF were 2.32, −5.81, and −3.49eV, respectively. Photoluminescence (PL) spectra of P3HT solution in chloroform were measured at different TFBF concentration. The PL intensity of P3HT solution was decreased by increasing the TFBF concentration. The electron mobility of P3HT:TFBF blend film was measured to be 6.82×10−5cm2/Vs at the electric field of 500 (V/cm)1/2. Photovoltaic devices were fabricated with the structure of ITO/PEDOT:PSS/P3HT:TFBF/LiF/Al. One of the fabricated device showed the highest Voc of 0.70V with Jsc=1.20mA/cm2, FF=0.30, and PCE=0.26% at donor to acceptor composition ratio of 1:0.4.

Single-step fabrication of all-solid dye-sensitized solar cells using solution-processable precursor

A one-pot, solution-processable fabrication method of dye-sensitized bulk heterojunction solar cell is employed to provide an alternate and analogous fabrication perspective for all-solid dye-sensitized solar cells (DSSCs) in a single step. All-solid DSSCs of ZnO/dye/polythiophene derivative (P3HT) are fabricated from a soluble precursor solution of diethylzinc (DEZ), dye and P3HT. Owing to its air and moisture sensitivity, DEZ is decomposed on P3HT layer to form bicontinuous structures of ZnO and P3HT between which dyes are sandwiched. The photoaction spectrum of all-solid DSSC shows that it covered wavelengths up to 750 nm, leading to the conclusion that panchromatic photon harvesting and electron collection are possible due to the synergistic photon harvesting by both the P3HT and the dye sensitizer. ZnO/dye/P3HT interfaces of all-solid DSSC solar cells prepared by a single-step fabrication process.

Uniaxial alignment of poly(3-hexylthiophene) nanofibers by zone-casting approach

The preparation of large area coverage of films with uniaxially aligned poly(3-hexylthiophene) (P3HT) nanofibers by using zone-casting approach is reported. The length and the orientation of the nanofibers are defined by the solubility of the solvent, the P3HT molecular weight and the substrate temperature. The length of the oriented nanofibers could be increased from 1 μm to more than 10 μm by adding poor solvent into the P3HT solution. It is found that for P3HT of relatively low molecular weight, a solvent with relatively low solubility has to be chosen to get the oriented film. While for the high molecular weight P3HT, the solvent with a relatively high solubility has to be used. The well-aligned film could be obtained because of the solute concentration gradient in the region where the critical concentration is reached during the zone-casting process. Particularly, the solvent evaporation rate and crystallization rate must be chosen properly to satisfy the stationary conditions above, which were controlled by an appropriate choice of solvent and substrate temperature. The film prepared by zone-casting approach had microcrystalline P3HT domains with more inter-chain order than spin-coating film. Meanwhile, the P3HT π-π stacking direction was parallel to the alignment direction of the nanofibers.

Enhanced crystallinity and film retention of P3HT thin-films for efficient organic solar cells by use of preformed nanofibers in solution

We report the preparation of films of poly(3-hexylthiophene) nanofibers suitable for fabrication of efficient multilayer solar cells by successive deposition of donor and acceptor layers from the same solvent. The nanofibers are obtained by addition of di-tert-butyl peroxide (DTBP) to a solution of P3HT in chlorobenzene. Interestingly, by varying the concentration of DTBP we are able to control both crystallinity and film retention of the spin-cast films. We also investigate the influence of the DTBP-induced crystallization on charge transport by thin-film transistor measurements, and find a more than five-fold increase in the hole mobility of nanofiber films compared to pure P3HT. We attribute this effect to the synergistic effects of increased crystallinity of the fibers and the formation of micrometer-sized fiber networks. We further demonstrate how it is possible to make use of the high film retention to fabricate photovoltaic devices by subsequent deposition of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) from a chlorobenzene solution on top of the nanofiber film. The presence of a relatively large crystalline phase strongly affects the diffusion behavior of PCBM into the P3HT film, resulting in a morphology which is different from that of common bulk heterojunction solar cells and resembles a bilayer structure, as can be inferred from comparison of the external quantum efficiency spectra. However, a high power conversion efficiency of 2.3% suggests that there is still a significant intermixing of the two materials taking place.

Solvent quality-induced nucleation and growth of parallelepiped nanorods in dilute poly(3-hexylthiophene) (P3HT) solution and the impact on the crystalline morphology of solution-cast thin film

Understanding the chain conformation of conjugated polymers in casting solutions and its impact on the crystalline morphology of solution-cast thin films is crucial for many electronic applications. Using small-angle neutron scattering, we show that well-dissolved poly(3-hexyl thiophene) (P3HT) chains in good solvent (chloroform) form long rectangular parallelepipeds (RPs) via nucleation and growth processes upon increasing the volume fraction of poor solvent (hexane) above a certain critical point. The growth of the RPs is due to the π–π stacking of the P3HT main backbone occurring along the long axis of the RPs. P3HT solutions prepared with different poor solvent volume fractions were drop-cast onto Si-wafers to prepare thin films, which were examined using 2D grazing-incidence X-ray scattering and 1D X-ray diffraction. The results indicate that the RPs grown in solution preferentially orient on the substrate with their two longer axes parallel to the surface after solvent evaporation, and give rise to much improved crystallinity and crystal orientation compared to the disordered chains.

Mass production of a 3D non-woven nanofabric with crystalline P3HT nanofibrils for organic solar cells

A strategy for the in line mass production of a three-dimensional (3D) non-woven nanofabric consisting of crystalline P3HT nanofibrils, created by in situ cooling of the transportation line to feed a P3HT solution for a coating tool, was introduced. The required cooling-temperature with respect to the feeding rate for the overall nanofibril creating process and the yield of the nanofibrils in solution with various organic solvents were determined. Considering the influence of a change in the temperature on the status of the precipitated nanofibrils until feeding it into the spray nozzle, the margin of the surviving nanofibrils at a certain temperature was also investigated. To verify the superiority of our strategy and present directions regarding its application to industry, arrays of organic solar cells based on a 3D non-woven nanofabric structure consisting of P3HT nanofibrils were designed and fabricated using our in situ process combined with a spray-coating system. As a result, through the in situ cooling process, a considerable solar energy harvesting efficiency near 4%, which is a state-of-the-art value in a bi-layer-based solar cell, was obtained.

Effects of aggregation of poly(3-hexylthiophene) in solution on uniaxial alignment of nanofibers during zone casting

We report on the effects of aggregation of P3HT with ordered conformation in solution on improving the uniaxial alignment of the P3HT nanofibers by zone casting. Two approaches were employed to change the aggregation of P3HT: P3HT blending with coil insulating polymer and ultrasonic oscillating. The insulator polymer (i.e. PS) which has good solubility in the solution would disturb the aggregation of P3HT to prevent the chains entanglement. The ultrasonic oscillation can further improve the P3HT aggregation with ordered conformation in the solution. As a result, the P3HT nanofibers in the film grew much orientedly by zone casting the ultrasonic oscillating P3HT/PS polymer blends solution than the same solvent P3HT solution without ultrasonic oscillating and blending. The P3HT pi-pi stacking direction is parallel to the alignment direction of the nanofibers. Meanwhile, the P3HT/PS blend ratio and PS molecular weight have influence on the uniaxial alignment of P3HT nanofibers. Only P3HT/PS is 1:1, the P3HT nanofibers oriented well. The low molecular weight PS can make the P3HT nanofibers orient better than that of the high molecular weight. (C) 2013 Yan-Chun Han. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.

Bulk-heterojunction organic photovoltaic cells fabricated using a high-viscosity solution of poly(3-hexylthiophene) with extremely high molecular weight

In the present work, we demonstrated that the amount of both donor and acceptor materials in the fabrication of BHJ OPVs could be strongly reduced using a high viscosity P3HT solution with the Mw of 680 900. In addition, only slight fluctuations were observed in PCEs of around 3.8% for all OPVs based on P3HT for Mw ranging from 46 600 to 680 900.

Improving the performance of polymer solar cells by adjusting the crystallinity and nanoscale phase separation

In this paper, we report a high-performance P3HT/PCBM bulk-heterojunction solar cell with a power conversion efficiency of 4.85% fabricated by adjusting the polymer crystallinity and nanoscale phase separation using an ultrasonic irradiation mixing approach for the polymer. The grazing incidence X-ray diffraction, UV/Vis spectroscopic, and atomic force microscopic measurement results for the P3HT/PCBM blend films reveal that the P3HT/PCBM film fabricated by ultrasonic irradiation mixing of the P3HT and PCBM solutions for 10 min has a higher degree of crystallinity, a higher absorption efficiency, and better phase separation, which together account for the higher charge transport properties and photovoltaic cell performance.

A solution state diode using semiconductor polymer nanorods with nanogap electrodes

A solution state polymer diode, which uses regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT):dichlorobenzene solution as the semiconductor between highly doped p-type silicon and aluminum electrodes has been built. Electrodes separated by a 40 nm gap enable intra-chain charge carrier transfer through the lengths of single polymer chains. This prevents chain to chain hopping and chain entanglements, increasing carrier mobility. The degradation with time and hysteresis effects of the diodes are measured. An optimal P3HT solution concentration of 6 mg ml−1 is found. A current density of at least 300 mA cm−2 is achieved, indicating at least a six-fold improvement in carrier mobility compared to previously fabricated solid state P3HT diodes.

Mimicking Conjugated Polymer Thin-Film Photophysics with a Well-Defined Triblock Copolymer in Solution

Conjugated polymers (CPs) are promising materials for use in electronic applications, such as low-cost, easily processed organic photovoltaic (OPV) devices. Improving OPV efficiencies is hindered by a lack of a fundamental understanding of the photophysics in CP-based thin films that is complicated by their heterogeneous nanoscale morphologies. Here, we report on a poly(3-hexylthiophene)-block-poly(tert-butyl acrylate)-block-poly(3-hexylthiophene) rod-coil-rod triblock copolymer. In good solvents, this polymer resembles solutions of P3HT; however, upon the addition of a poor solvent, the two P3HT chains within the triblock copolymer collapse, affording a material with electronic spectra identical to those of a thin film of P3HT. Using this new system as a model for thin films of P3HT, we can attribute the low fluorescence quantum yield of films to the presence of a charge-transfer state, providing fundamental insights into the condensed phase photophysics that will help to guide the development of the next generation of materials for OPVs.

Sol–gel transition of poly(3-hexylthiophene) revealed by capillary measurements: phase behaviors, gelation kinetics and the formation mechanism

π-Conjugated organogels of poly(3-hexylthiophene) (P3HT) are prepared via the addition of a marginal solvent, anisole, into solutions of P3HT. We initiate a novel and facile route to determine the gelation threshold of P3HT nanowire dispersions by capillary measurements. The effects of the P3HT concentration (c), anisole volume fraction (φ) and temperature (T) on the phase behaviors are discussed. A thermodynamic c–φ phase diagram is constructed, in which the P3HT dispersion is divided into four regions: solution, sol, sol–gel blend and gel. The concentration and solvent dependent sol–gel transition kinetics shows that the gelation process could be accelerated via increasing c and φ. The gelation temperature gives an exponential relationship with the concentration of P3HT and the anisole volume fraction. Morphological studies reveal the formation process and the topological structure of the P3HT microgel clusters. Based on the above results, a three-stage physical scenario for the sol–gel transition of P3HT dispersions is proposed.

Preparation of Poly(3-hexylthiophene)/[6,6]-Phenyl-C61-Butyric Acid Methyl Ester Langmuir-Blodgett Bulk Hetero Junctions on Indium Tin Oxide

Mono-layers of aggregated Poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) molecules were obtained by using solutions of P3HT, PCBM and P3HT-PCBM mixture without stabilizers such as stearates in chloroform at an air-water interface. 1 to 10 cycle-lifted LB films of P3HT and PCBM were successfully transferred to cleaned bare indium-tin-oxide coated glass substrate by vertical lifting method excluding the first 1 to 2 cycle layer. The dependence of P3HT and PCBM film thickness on the transfer cycles has been explained by the molecular sizes, where four edge-on P3HT molecular and six PCBM molecular stacking which result in thickness was taken into account. Work functions of deposited LB-layers were consistent with those of the ordinary casted films. P3HT and PCBM LB-layers showed optical activity in both infra-red (IR) and visible absorption regions of the spectrum. P-polarized IR absorption owing to C=C and C=O stretching vibrations observed in LB-layered films clearly indicate the enhancement of the orientation of these bonds perpendicular to the substrate surface in contrast to the spin-coated one. Visible optical absorption intensity was increased well in proportion with the lift cycle-numbers of both P3HT and PCBM LB films. The photovoltaic characteristics have been observed in the devices fabricated with P3HT (5 cycles-layer)/PCBM (5 cycles-layer) LB hetero structure as an active layer of the solar cells. The surface pressure of LB compression for the mixture of P3HT and PCBM, that is, bulk hetero mixtures, has also been well built up to 30 mN/m.

Silicon Nanowire/P3HT Hybrid Solar Cells: Effect of the Electron Localization at Wire Nanodiameters

Photoactive hybrid films based on n type silicon nanowires [SiNWs] dispersed in poly(3-hexylthiophene) [P3HT], a p type conjugated polymer known for its good ordering properties, have a main interest for the production of photovoltaic films at a limited cost. Silicon nanowires synthesized at high yield by the oxide assisted growth technique have been dispersed in tetrahydrofuran: THF, and mixed with a P3HT solution in THF to form a blend of the inorganic-organic components in the appropriate proportions. The blend of SiNWs and P3HT have been deposited by spin coating on PEDOT-PSS/ITO substrates leading to the production of 100 nm thick SiNWs/P3HT thin layers of controlled compositions. The quenching of the P3HT fluorescence has shown the effective dissociation of the photogenerated pairs for an optimum composition of 6 SiNWs vol. % in the blend, which is in accordance with the low percolation threshold expected from the high aspect ratio of the nanowires. Current/voltage experiments under illumination have however led to collected photocurrents remaining limited to some 10 μA/cm2 whereas an interesting open circuit voltage of 0.65 V was obtained. It has been possible from surface potential decay experiments to assign the main limiting process to the low electron transport along nanowires of diameter smaller than 10 nm, whereas easy hole transport in the P3HT thickness was obtained. The high densities of silicon surface states acting as electron traps can simultaneously account for efficient charge pair dissociation and low photocurrents in nanosized structures.

Controlled evaporative self-assembly of poly(3-hexylthiophene) monitored with confocal polarized Raman spectroscopy

Highly-ordered, parallel gradient P3HT stripes are fabricated through a facile deposition method based on controlled evaporative self-assembly (CESA). Confocal polarized Raman spectroscopy is employed to determine the orientation of P3HT chains in individual stripes and spacing. This is the first report on orientation at the molecular level, beyond sole morphology, in the patterns assembled by CESA. P3HT chains tend to aggregate into one-dimensional nanowhiskers in solution upon aging, leading to a time-evolved dispersion composed of isolated chains and nanowhiskers. A corresponding evolution of morphology and molecular orientation in the obtained patterns is observed when assembling P3HT solutions with different aging times. The stripes evolve gradually from slim stripes with fingering instabilities for fresh solution into highly regular, perfect stripes for sufficiently aged solution. In the stripe region, P3HT backbone chains align parallel to the contact line for fresh solution whereas perpendicular for aged solution. The thermodynamic multicomponent system gives greater variety to the CESA study.

Exciton migration and charge transfer in chemically linked P3HT–TiO2nanorod composite

Exciton migration and charge transfer in the chemically linked P3HT–TiO2 nanorod composite (P3HT–Si–nr–TiO2) solution were investigated in comparison with pristine P3HT and physically mixed P3HT/LA–nr–TiO2 solutions. The chemically linked P3HT–Si–nr–TiO2 was made by covalently linking in situ polymerized P3HT onto nr–TiO2 using triethoxy-2-thienylsilane as a linker to replace the initial linoleic acid (LA) capping agent on nr–TiO2. The physically mixed P3HT/LA–nr–TiO2 was prepared by adding ex situ synthesized P3HT into the LA–capped nr–TiO2 solution. In the chemically linked sample, charge transfer from P3HT to TiO2 nanorods was found to occur evidenced by photoluminescence (PL) quenching and ultrafast decay dynamics with a timescale of 0.75 ps. However, both the emission spectra and femtosecond dynamics in physically mixed sample overlapped very well with those from pristine P3HT solution, indicating no PL quenching or charge transfer from P3HT to nr–TiO2. In addition, blue shift in absorbance and PL spectra, larger Stokes shift, and structureless PL spectra found in the chemically linked sample indicated that P3HT formed a more coil-like conformation with more twisted torsion disorders than those in pristine P3HT and physically mixed samples. This is consistent with the femtosecond measurement result that torsional relaxation occurred with a longer decay time and higher amplitude. Moreover, intersystem crossings (ISC) from singlet state (S1) to triplet state (T1) in P3HT of the three samples were all found to occur in a comparable timescale of ∼1 ns and showed no dependence on conformational disorders such as torsional defects.

Moving through the Phase Diagram: Morphology Formation in Solution Cast Polymer–Fullerene Blend Films for Organic Solar Cells

The efficiency of organic bulk heterojunction solar cells strongly depends on the multiscale morphology of the interpenetrating polymer-fullerene network. Understanding the molecular assembly and the identification of influencing parameters is essential for a systematic optimization of such devices. Here, we investigate the molecular ordering during the drying of doctor-bladed polymer-fullerene blends on PEDOT:PSS-coated substrates simultaneously using in situ grazing incidence X-ray diffraction (GIXD) and laser reflectometry. In the process of blend crystallization, we observe the nucleation of well-aligned P3HT crystallites in edge-on orientation at the interface at the instant when P3HT solubility is crossed. A comparison of the real-time GIXD study at ternary blends with the binary phase diagrams of the drying blend film gives evidence of strong polymer-fullerene interactions that impede the crystal growth of PCBM, resulting in the aggregation of PCBM in the final drying stage. A systematic dependence of the film roughness on the drying time after crossing P3HT solubility has been shown. The highest efficiencies have been observed for slow drying at low temperatures which showed the strongest P3HT interchain π-π-ordering along the substrate surface. By adding the "unfriendly" solvent cyclohexanone to a chlorobenzene solution of P3HT:PCBM, the solubility can be crossed prior to the drying process. Such solutions exhibit randomly orientated crystalline structures in the freshly cast film which results in a large crystalline orientation distribution in the dry film that has been shown to be beneficial for solar cell performance.

Photoelectrochemical Reactions of Electrodes Modified with Poly(3-hexylthiophene) Nanowires

Nanowires consisting of regioregular poly(3-hexylthiophene) (P3HT) were prepared by whisker precipitation from a solution of P3HT in anisole. Photoirradiation of two electrodes modified with P3HT nanowires or P3HT with visible light afforded cathodic photocurrents in the presence of O2. The photocurrent of the electrode modified with P3HT nanowires was approximately twice as large as that from the electrode modified with P3HT. This is most likely because hole hopping between adjacent thiophenes is facilitated in the P3HT nanowires due to π-π stacking.

Coating on a Cold Substrate Largely Enhances Power Conversion Efficiency of the Bulk Heterojunction Solar Cell

Spin-coating a mixture solution of P3HT and PCBM on a cold substrate largely enhanced the power conversion efficiency (PCE) of the bulk heterojunction (BHJ) solar cells. This concept was based on the abrupt decrease in the solubility of P3HT as solution temperature decreased. The selective precipitation of P3HT on the PEDOT:PSS-coated cold substrate facilitated a desirable rich composition of P3HT at the interface with the PEDOT:PSS layer. The high crystallinity of P3HT suppressed the movement of PCBM during thermal annealing, preventing aggregation of PCBM. The morphological excellence of the pristine film gave a comparable PCE to that made by the conventional fabrication process. After thermal annealing, the device made via coating on a cold substrate showed above 30% increase in PCE from the BHJ solar cells made by the conventional method.