A Phenomenological Model for Predicting Thermochromism of Regioregular and Nonregioregular Poly(3-alkylthiophenes)

Stochastic Resonance in Organic Electronic Devices

Stochastic Resonance (SR) is a phenomenon in which noise improves the performance of a system. With the addition of noise, a weak input signal to a nonlinear system, which may exceed its threshold, is transformed into an output signal. In the other words, noise-driven signal transfer is achieved. SR has been observed in nonlinear response systems, such as biological and artificial systems, and this review will focus mainly on examples of previous studies of mathematical models and experimental realization of SR using poly(hexylthiophene)-based organic field-effect transistors (OFETs). This phenomenon may contribute to signal processing with low energy consumption. However, the generation of SR requires a noise source. Therefore, the focus is on OFETs using materials such as organic materials with unstable electrical properties and critical elements due to unidirectional signal transmission, such as neural synapses. It has been reported that SR can be observed in OFETs by application of external noise. However, SR does not occur under conditions where the input signal exceeds the OFET threshold without external noise. Here, we present an example of a study that analyzes the behavior of SR in OFET systems and explain how SR can be made observable. At the same time, the role of internal noise in OFETs will be explained.

Physical Supercritical Fluid Deposition: Patterning Solution Processable Materials on Curved and Flexible Surfaces

We provide the initial demonstration of a general thin film deposition technique that leverages the unique solubility properties of supercritical fluids. The technique is the solution-phase analogue of physical vapor deposition and allows thin films of a semiconducting polymer to be grown without the need for in situ chemical reactions. Film growth is approximately linear with time, indicating that film thickness can be controlled in a straightforward manner by varying the time of deposition. To further demonstrate the flexibility of the technique, we demonstrate precise control over the location of material deposition using a combination of photolithography and resistive heating. The potential for scalable manufacturing is demonstrated by use of a master to control deposition onto a flexible polymer film. Finally, we demonstrate a unique deposition capability of this technique by depositing patterns onto the curved interior of a hemisphere made from a silicone elastomer. This capability is not possible with any printing or line-of-sight deposition technique. More generally, the ability to control the deposition of solution processed materials with high accuracy provides the long sought after bridge between top-down and bottom-up self-assembly.

Effects of linear and branched side chains on the redox and optoelectronic properties of 3,4-dialkoxythiophene polymers

Identification of relevant structure–property relationships on solution-processable conjugated polymers have been shown to improve the performance of various redox properties.

Cyano-Substituted Head-to-Head Polythiophenes: Enabling High-Performance n-Type Organic Thin-Film Transistors

Polythiophenes, built on the electron-rich thiophene unit, typically possess high-lying energy levels of the lowest unoccupied molecular orbitals (LUMOs) and show hole-transporting properties. In this study, we develop a series of n-type polythiophenes, P1-P3, based on head-to-head-linked 3,3'-dialkoxy-4,4'-dicyano-2,2'-bithiophene (BTCNOR) with distinct side chains. The BTCNOR unit shows not only highly planar backbone conformation enabled by the intramolecular noncovalent sulfur-oxygen interaction but also significantly suppressed LUMO level attributed to the cyano-substitution. Hence, all BTCNOR-based polymer semiconductors exhibit low-lying LUMO levels, which are ∼1.0 eV lower than that of regioregular poly(3-hexylthiophene) (rr-P3HT), a benchmark p-type polymer semiconductor. Consequently, all of the three polymers can enable unipolar n-type transport characteristics in organic thin-film transistors (OTFTs) with low off-currents ( I_offs) of 10^-10-10^-11 A and large current on/off ratios ( I_on/ I_offs) at the level of 10⁶. Among them, polymer P2 with a 2-ethylhexyl side chain offers the highest film ordering, leading to the best device performance with an excellent electron mobility (μ_e) of 0.31 cm² V^-1 s^-1 in off-center spin-cast OTFTs. To the best of our knowledge, this is the first report of n-type polythiophenes with electron mobility comparable to the hole mobility of the benchmark p-type rr-P3HT and approaching the electron mobility of the most-studied n-type polymer, poly(naphthalene diimide- alt-bithiophene) (i.e., N2200). The change of charge carrier polarity from p-type (rr-P3HT) to n-type (P2) with comparable mobility demonstrates the obvious effectiveness of our structural modification. Adoption of n-hexadecyl (P1) and 2-butyloctyl (P3) side chains leads to reduced film ordering and results in 1-2 orders of magnitude lower μ_es, showing the critical role of side chains in optimizing device performance. This study demonstrates the unique structural features of head-to-head linkage containing BTCNOR for constructing high-performance n-type polymers, i.e., the alkoxy chain for backbone conformation locking and providing polymer solubility as well as the strong electron-withdrawing cyano group for lowering LUMO levels and enabling n-type performance. The design strategy of BTCNOR-based polymers provides useful guidelines for developing n-type polythiophenes.

Π-Stacking Signature in NMR Solution Spectra of Thiophene-Based Conjugated Polymers

Studies on conjugated polymers seldom report on their NMR characterization in solution. This paper shows how NMR experiments, both ¹H NMR and routine 2D NMR spectra, can help in gaining a further insight into the aggregation behavior of conjugated polymers and could be used to flank the more employed solid-state NMR and other spectroscopy and microscopy techniques in the understanding of the aggregation processes. NMR spectroscopy allows distinguishing, within the class of poorly solvatochromic conjugated polymers, those highly prone to form π-stacked aggregates from the ones that have a low tendency toward π-stacking.

Charge Transport in Intermixed Regions of All-Polymer Solar Cells Studied by Conductive Atomic Force Microscopy

Charge transport in intermixed regions of all-polymer solar cells based on a blend of poly(3-hexylthiophene) (P3HT; electron donor) with poly[2,7-(9,9-didodecylfluorene)-alt-5,5-(4',7'-bis(2-thienyl)-2',1',3'-benzothiadiazole)] (PF12TBT; electron acceptor) was studied by conductive atomic force microscopy (C-AFM). For a blend film fabricated from a chlorobenzene solution, intermixed regions were detected between the P3HT-rich and PF12TBT-rich domains. The overall hole current in the intermixed regions remained almost constant, both before and after thermal annealing at 80 °C, but it increased in the P3HT-rich domains. For a blend film fabricated from a chloroform solution, the entire observed area constituted an intermixed region, both before and after thermal annealing. The overall hole current in this film was significantly improved following thermal annealing at 120 °C. These finely mixed structures with efficient charge transport yielded a maximum power conversion efficiency of 3.5%. The local charge-transport properties in the intermixed region, as observed via C-AFM, was found to be closely related to the photovoltaic properties, rather than the bulk-averaged properties or topological features.

Architectural Effects on Solution Self-Assembly of Poly(3-hexylthiophene)-Based Graft Copolymers

While solution assembly of conjugated block copolymers has been widely used to produce long 1-D nanowires (NWs), it remains a great challenge to provide a higher level of control over structure and function of the NWs. Herein, for the first time, we report the solution assembly of graft copolymers containing a conjugated polymer backbone in a selective solvent and demonstrate that their self-assembly behaviors can be manipulated by the molecular structures of the graft copolymers. A series of poly(3-hexylthiophene)-graft-poly(2-vinylpyridine) (P3HT-g-P2VP) copolymers was designed with two different architectural parameters: grafting fraction (f_g) and molecular weight of P2VP chains (M_n,P2VP) on the P3HT backbone. Interestingly, crystallization of the P3HT-g-P2VP copolymers was systematically modulated by changes in f_g and M_n,P2VP, thus allowing for control of the growth kinetics and curvatures of solution-assembled NWs. When M_n,P2VP (4.4 to 15.1 kg/mol) or f_g (2.8 to 9.2%) of the P3HT-g-P2VP polymers was increased, the crystallinity of the copolymers was reduced significantly. Steric hindrance from the grafted P2VP chains apparently modified the growth of NWs, leading to shorter NWs with a greater degree of curvature for graft copolymers with more hindrance. Therefore, we envision that such conjugated chain-based graft copolymers can be versatile building blocks for producing NWs with controlled length and shape, which can be important for tailoring the optical and electrical properties of NW-based devices.

A Thermochromic Superhydrophobic Surface

Highly enhanced solid-state thermochromism is observed in regioregular poly(3-hexylthiophene), P3HT, when deposited on a superhydrophobic polymer-SiO2 nanocomposite coating. The conformal P3HT coating on the nanocomposite surface does not alter or reduce superhydrophicity while maintaining its reversible enhanced thermochromism. The polymeric matrix of the superhydrophobic surface is comprised of a blend of poly(vinylidene fluoride-co-hexafluoropropylene) copolymer and an acrylic adhesive. Based on detailed X-ray diffraction measurements, this long-lasting, repeatable and hysteresis-free thermochromic effect is attributed to the enhancement of the Bragg peak associated with the d-spacing of interchain directional packing (100) which remains unaltered during several heating-cooling cycles. We propose that the superhydrophobic surface confines π-π interchain stacking in P3HT with uniform d-spacing into its nanostructured texture resulting in better packing and reduction in face-on orientation. The rapid response of the system to sudden temperature changes is also demonstrated by water droplet impact and bounce back on heated surfaces. This effect can be exploited for embedded thin film temperature sensors for metal coatings.

The backbone rigidity and its influence on the morphology and charge mobility of FBT based conjugated polymers

Modulation of the backbone rigidity via proper side chain placement and vinylene group incorporation allowed optimization of solid-state order and OFET performances.

Theoretical study on the torsional potential of alkyl, donor, and acceptor substituted bithiophene: the hidden role of noncovalent interaction and backbone conjugation

The torsional potential of substituted bithiophene is influenced not only by steric repulsion, but also by backbone conjugation and noncovalent interactions.

Synthesis of PCDTBT-based fluorinated polymers for high open-circuit voltage in organic photovoltaics: towards an understanding of relationships between polymer energy levels engineering and ideal morphology control

The introduction of fluorine (F) atoms onto conjugated polymer backbone has verified to be an effective way to enhance the overall performance of polymer-based bulk-heterojunction (BHJ) solar cells, but the underlying working principles are not yet fully uncovered. As our attempt to further understand the impact of F, herein we have reported two novel fluorinated analogues of PCDTBT, namely, PCDTFBT (1F) and PCDT2FBT (2F), through inclusion of either one or two F atoms into the benzothiadiazole (BT) unit of the polymer backbone and the characterization of their physical properties, especially their performance in solar cells. Together with a profound effect of fluorination on the optical property, nature of charge transport, and molecular organization, F atoms are effective in lowering both the HOMO and LUMO levels of the polymers without a large change in the energy bandgaps. PCDTFBT-based BHJ solar cell shows a power conversion efficiency (PCE) of 3.96 % with high open-circuit voltage (VOC) of 0.95 V, mainly due to the deep HOMO level (-5.54 eV). To the best of our knowledge, the resulting VOC is comparable to the record VOC values in single junction devices. Furthermore, to our delight, the best PCDTFBT-based device, prepared using 2 % v/v diphenyl ether (DPE) additive, reaches the PCE of 4.29 %. On the other hand, doubly-fluorinated polymer PCDT2FBT shows the only moderate PCE of 2.07 % with a decrease in VOC (0.88 V), in spite of the further lowering of the HOMO level (-5.67 eV) with raising the number of F atoms. Thus, our results highlight that an improvement in efficiency by tuning the energy levels of the polymers by means of molecular design can be expected only if their truly optimized morphologies with fullerene in BHJ systems are materialized.

Highly coplanar very long oligo(alkylfuran)s: a conjugated system with specific head-to-head defect

Well-defined monodisperse conjugated oligomers, which have planar backbones and are free from the disturbance of substituents, attract broad interest. Herein, we report a series of symmetrical, isomerically pure oligofurans, namely, the 16-mer 16F-6C6 together with the related nF-2C6 (n = 4, 6, 8). Through computational studies and detailed spectroscopic and X-ray characterization, for the first time, we show that the planarity of the furan backbone is almost unaffected by the head-to-head defect which is known to cause considerable twists in its oligo- or polythiophene analogues. We present that the properties of these rigid oligo(alkylfuran)s are strongly influenced by the conjugation length. As the longest monodisperse α-oligofuran synthesized to date, 16F-6C6 was observed to be stable and highly fluorescent. Experimental and computational studies of the redox states of these oligo(alkylfuran)s reveal that 16F-6C6 has singlet biradical (polaron-pair) character in the doubly oxidized ground state: the open-shell singlet (⟨S2⟩ = 0.989) is 3.8 kcal/mol more stable than the closed-shell dication.

Synthesis and morphological studies of a poly(5,6-difluorobenzo-2,1,3-thiadiazole-4,7-diyl-alt-quaterchalcogenophene) copolymer with 7.3% polymer solar cell efficiency

A superior PCE of 7.34% was delivered in the inverted P(FBT-alt-Se2Th2) : PC₇₁BM polymer solar cells.

Influences of the non-covalent interaction strength on reaching high solid-state order and device performance of a low bandgap polymer with axisymmetrical structural units

A high organic field-effect transistor mobility (0.29 cm(2) V(-1) s(-1) ) and bulk-heterojunction polymer solar cell performance (PCE of 6.82%) have been achieved in a low bandgap alternating copolymer consisting of axisymmetrical structural units, 5,6-difluorobenzo-2,1,3-thiadiazole. Introducing the fluorine substituents enhanced intermolecular interaction and improved the solid-state order, which consequently resulted in the highest device performances among the 2,1,3-thiadiazole-quarterthiophene based alternating copolymers.

Hybrid phototransistors based on bulk heterojunction films of poly(3-hexylthiophene) and zinc oxide nanoparticle

Hybrid phototransistors (HPTRs) were fabricated on glass substrates using organic/inorganic hybrid bulk heterojunction films of p-type poly(3-hexylthiophene) (P3HT) and n-type zinc oxide nanoparticles (ZnO(NP)). The content of ZnO(NP) was varied up to 50 wt % in order to understand the composition effect of ZnO(NP) on the performance of HPTRs. The morphology and nanostructure of the P3HT:ZnO(NP) films was examined by employing high resolution electron microscopes and synchrotron radiation grazing angle X-ray diffraction system. The incident light intensity (P(IN)) was varied up to 43.6 μW/cm², whereas three major wavelengths (525 nm, 555 nm, 605 nm) corresponded to the optical absorption of P3HT were applied. Results showed that the present HPTRs showed typical p-type transistor performance even though the n-type ZnO(NP) content increased up to 50 wt %. The highest transistor performance was obtained at 50 wt %, whereas the lowest performance was measured at 23 wt % because of the immature bulk heterojunction morphology. The drain current (I(D)) was proportionally increased with P(IN) due to the photocurrent generation in addition to the field-effect current. The highest apparent and corrected responsivities (R(A) = 4.7 A/W and R(C) = 2.07 A/W) were achieved for the HPTR with the P3HT:ZnO(NP) film (50 wt % ZnO(NP)) at P(IN) = 0.27 μW/cm² (555 nm).

Dithienocoronenediimide-based copolymers as novel ambipolar semiconductors for organic thin-film transistors

A new class of ambipolar donor-acceptor π-conjugated polymers based on a dithienocoronenediimide core is presented. Solution-processed top-gate/bottom-contact thin film transistors (TFTs) exhibit electron and hole mobilities of up to 0.30 cm(2)/V·s and 0.04 cm(2)/V·s, respectively, which are the highest reported to date for an ambipolar polymer in ambient conditions. The polymers presented here are the first examples of coronenediimide-based semiconductors showing high organic TFT performances.

Near-infrared thermochromic diazapentalene dyes

A series of 2,5-diazapentalene containing dyes with tunable energy gaps are visible and near-infrared halochromic towards various acids and their protonated counterparts represent a new class of thermochromic materials with the near-infrared absorption being switched on at room temperature and off above 50 °C.

High Performance Polymer Thin Film Transistors Array Printed on a Flexible Polycarbonate Substrate

High performance poly (3-hexylthiophene) (P3HT) thin film transistors (TFTs) array was fabricated on a polycarbonate substrate by micro-contact printing method. A thin polyimide layer (40 nm) was applied before silicon oxide deposition to improve the electrical properties of the TFT device. Also, the effects of O2 plasma treatment on the field effect mobility and output current behaviors of the devices were investigated. By plasma treatment, the surface roughness of gate dielectric was improved which accounts for the increased field effect mobility and the hole Schottky barrier height in electrode/semiconductor interface was lowered resulting in large drain current in the device. Based on the experiments, we fabricated P3HT TFTs array with 0.025 cm2/V·s in saturation field effect mobility and on/off current ratio of 103 ∼ 104 on a polycarbonate substrate.