Molecular Weight‐Induced Structural Transition of Liquid‐Crystalline Polymer Semiconductor for High‐Stability Organic Transistor

In order to fabricate polymer field‐effect transistors (PFETs) with high electrical stability under bias‐stress, it is crucial to minimize the density of charge trapping sites caused by the disordered regions. Here we report PFETs with excellent electrical stability comparable to that of single‐crystalline organic semiconductors by specifically controlling the molecular weight (MW) of the donor‐acceptor type copolymer semiconductors, poly (didodecylquaterthiophene‐alt‐didodecylbithiazole). We found that MW‐induced thermally structural transition from liquid‐crystalline to semi‐crystalline phases strongly affects the device performance (charge‐carrier mobility and electrical bias‐stability) as well as the nanostructures such as the molecular ordering and the morphological feature. In particular, for the polymer with a MW of 22 kDa, the transfer curves varied little (ΔV_th = 3～4 V) during a period of prolonged bias stress (about 50 000 s) under ambient conditions. This enhancement of the electrical bias‐stability can be attributed to highly ordered liquid‐crystalline nanostructure of copolymer semiconductors on dielectric surface via the optimization of molecular weights.     

Tailoring Morphology and Structure of Inkjet‐Printed Liquid‐Crystalline Semiconductor/Insulating Polymer Blends for High‐Stability Organic Transistors

Inkjet printing of semiconducting polymers is desirable for realizing low‐cost, large‐area printed electronics. However, sequential inkjet printing methods often suffer from nozzle clogging because the solubility of semiconducting polymers in organic solvents is limited. Here, it is demonstrated that the addition of an insulating polymer to a semiconducting polymer ink greatly enhances the solubility and stability of the ink, leading to the stable ejection of ink droplets. This bicomponent blend comprising a liquid‐crystalline semiconducting copolymer, poly(didodecylquaterthiophene‐alt‐didodecylbithiazole) (PQTBTz‐C12), and an insulating commodity polymer, polystyrene, is extremely useful as a semiconducting layer in organic field‐effect transistors (OFETs), providing fine control over the phase‐separated morphology and structure of the inkjet‐printed film. Tailoring the solubility‐induced phase separation of the two components leads to a bilayer structure consisting of a polystyrene layer on the top and a highly crystalline PQTBTz‐C12 layer on the bottom. The blend film is used as the semiconducting layer in OFETs, reducing the semiconductor content to several tens of pictograms in a single device without degrading the device performance. Furthermore, OFETs based on the PQTBTz‐C12/polystyrene film exhibit much greater environmental and electrical stabilities compared to the films prepared from homo PQTBTz‐C12, mainly due to the self‐encapsulated structure of the blend film.     

Decoupling the Bias‐Stress‐Induced Charge Trapping in Semiconductors and Gate‐Dielectrics of Organic Transistors Using a Double Stretched‐Exponential Formula

A novel strategy for analyzing bias‐stress effects in organic field‐effect transistors (OFETs) based on a four‐parameter double stretched‐exponential formula is reported. The formula is obtained by modifying a traditional single stretched‐exponential expression comprising two parameters (a characteristic time and a stretched‐exponential factor) that describe the bias‐stress effects. The expression yields two characteristic times and two stretched‐exponential factors, thereby separating out the contributions due to charge trapping events in the semiconductor layer‐side of the interface and the gate‐dielectric layer‐side of the interface. The validity of this method was tested by designing two model systems in which the physical properties of the semiconductor layer and the gate‐dielectric layer were varied systematically. It was found that the gate‐dielectric layer, in general, plays a more critical role than the semiconductor layer in the bias‐stress effects, possibly due to the wider distribution of the activation energy for charge trapping. Furthermore, the presence of a self‐assembled monolayer further widens the distribution of the activation energy for charge trapping in gate‐dielectric layer‐side of the interface and causes the channel current to decay rapidly in the early stages. The novel analysis method presented here enhances our understanding of charge trapping and provides rational guidelines for developing efficient OFETs with high performance.     

Air‐ and Active Hydrogen‐Induced Electron Trapping and Operational Instability in n‐Type Polymer Field‐Effect Transistors

Organic field‐effect transistors (OFETs) have attracted much attention for the next‐generation electronics. Despite of the rapid developments of OFETs, operational stability is a big challenge for their commercial applications. Moreover, the actual mechanism behind the degradation of electron transport is still poorly understood. Here, the electrical characteristics of poly{[N,N‐9‐bis(2‐octyldodecyl)‐naphthalene‐1,4,5,8‐bis(dicarboximide)‐2,6‐diyl]‐alt‐5,59‐(2,29‐bithiophene)} (P(NDI2OD‐T2)) thin‐film transistors (TFTs) as a function of semiconductor/dielectric interfacial property and environment are systematically investigated, in particular, how the copresence of water, oxygen, and active hydrogen on the surface of dielectric leads to a sharp drop‐off in threshold voltage. Evidence is found that an acid–base neutralization reaction occurring at the interface, as a combined effect of the chemical instability of dielectrics and the electrochemical instability of organic semiconductors, contributes to the significant electron trapping on the interface of P(NDI2OD‐T2) TFTs. Two strategies, increasing the intrinsic electrochemical stability of semiconductor and decreasing the chemical reactivity of gate dielectric, are demonstrated to effectively suppress the reaction and thus improve the operational stability of n‐type OFETs. The results provide an alternative degradation pathway to better understand the charge transport instability in n‐type OFETs, which is advantageous to construct high‐performance OFETs with long‐term stability.     

Comparison of the Operation of Polymer/Fullerene,Polymer/Polymer,and Polymer/Nanocrystal Solar Cells: A Transient Photocurrent and Photovoltage Study

We utilize transient techniques to directly compare the operation of polymer/fullerene, polymer/nanocrystal, and polymer/polymer bulk heterojunction solar cells. For all devices, poly(3‐hexylthiophene) (P3HT) is used as the electron donating polymer, in combination with either the fullerene derivative phenyl‐C₆₁‐butyric acid methyl ester (PCBM) in polymer/fullerene cells, CdSe nanoparticles in polymer/nanocrystal cells, or the polyfluorene copolymer poly((9,9‐dioctylfluorene)‐2,7‐diyl‐alt‐[4,7‐bis(3‐hexylthien‐5‐yl)‐2,1,3‐benzothiadiazole]‐2,2‐diyl) (F8TBT) in polymer/polymer cells. Transient photocurrent and photovoltage measurements are used to probe the dynamics of charge‐separated carriers, with vastly different dynamic behavior observed for polymer/fullerene, polymer/polymer, and polymer/nanocrystal devices on the microsecond to millisecond timescale. Furthermore, by employing transient photocurrent analysis with different applied voltages we are also able to probe the dynamics behavior of these cells from short circuit to open circuit. P3HT/F8TBT and P3HT/CdSe devices are characterized by poor charge extraction of the long‐lived carriers attributed to charge trapping. P3HT/PCBM devices, in contrast, show relatively trap‐free operation with the variation in the photocurrent decay kinetics with applied bias at low intensity, consistent with the drift of free charges under a uniform electric field. Under solar conditions at the maximum power point, we see direct evidence of bimolecular recombination in the P3HT/PCBM device competing with charge extraction. Transient photovoltage measurements reveal that, at open circuit, photogenerated charges have similar lifetimes in all device types, and hence, the extraction of these long‐lived charges is a limiting process in polymer/nanocrystal and polymer/polymer devices.     

Inkjet‐Printed Organic Electrodes for Bottom‐Contact Organic Field‐Effect Transistors

A graphite thin film was investigated as the drain and source electrodes for bottom‐contact organic field‐effect transistors (BC OFETs). Highly conducting electrodes (10² S cm^?1) at room temperature were obtained from pyrolyzed poly(l,3,4‐oxadiazole) (PPOD) thin films that were prepatterned with a low‐cost inkjet printing method. Compared to the devices with traditional Au electrodes, the BC OFETs showed rather high performances when using these source/drain electrodes without any further modification. Being based on a graphite‐like material these electrodes possess excellent compatibility and proper energy matching with both p‐ and n‐type organic semiconductors, which results in an improved electrode/organic‐layer contact and homogeneous morphology of the organic semiconductors in the conducting channel, and finally a significant reduction of the contact resistance and enhancement of the charge‐carrier mobility of the devices is displayed. This work demonstrates that with the advantages of low‐cost, high‐performance, and printability, PPOD could serve as an excellent electrode material for BC OFETs.     

Realization of electrically stable organic field-effect transistors using simple polymer blended dielectrics

Organic field-effect transistors (OFETs) were fabricated using polymer blended gate dielectrics in an effort to enhance the electrical stability against a gate bias stress. A poly(melamine-co-formaldehyde) acrylated (PMFA) gate dielectric layer with great insulating properties was blended with polypentafluorostyrene (PFS), a type of hydrophobic fluorinated polymer. Although the overall electrical performance dropped slightly due to the rough and hydrophobic surfaces of the blend films, at the blend ratio (10%), the OFET’s threshold voltage shift under a sustained gate bias stress applied over 3 h decreased remarkably compared with an OFET based on a PMFA dielectric alone. This behavior was attributed to the presence of the hydrophobic and electrically stable PFS polymer, which provided a low interfacial trap density between the gate dielectric and the semiconductor. A stretched exponential function model suggested that the energetic barrier to create trap states was high, and the distribution of energetic barrier heights was narrow in devices prepared with PFS.     

Ambient Processable and Stable All‐Polymer Organic Solar Cells

In this work, the way in which ambient moisture impacts the photovoltaic performance of conventional PCBM and emerging polymer acceptor–based organic solar cells is examined. The device performance of two representative p‐type polymers, PBDB‐T and PTzBI, blended with either PCBM or polymeric acceptor N2200, is systemically investigated. In both cases, all‐polymer photovoltaic devices processed from high‐humidity ambient conditions exhibit significantly enhanced moisture‐tolerance compared to their polymer–PCBM counterparts. The impact of moisture on the blend film morphology and electronic properties of the electron acceptor (N2200 vs PCBM), which results in different recombination kinetics and electron transporting properties, are further compared. The impact of more comprehensive ambient conditions (moisture, oxygen, and thermal stress) on the long‐term stability of the unencapsulated devices is also investigated. All‐polymer solar cells show stable performance for long periods of storage time under ambient conditions. The authors believe that these findings demonstrate that all‐polymer solar cells can achieve high device performance with ambient processing and show excellent long‐term stability against oxygen and moisture, which situate them in an advantageous position for practical large‐scale production of organic solar cells.     

Hydrocarbons‐Driven Crystallization of Polymer Semiconductors for Low‐Temperature Fabrication of High‐Performance Organic Field‐Effect Transistors

While many high‐performance polymer semiconductors are reported for organic field‐effect transistors (OFETs), most require a high‐temperature postdeposition annealing of channel semiconductors to achieve high performance. This negates the fundamental attribute of OFETs being a low‐cost alternative to conventional high‐cost silicon technologies. A facile solution process is developed through which high‐performance OFETs can be fabricated without thermal annealing. The process involves incorporation of an incompatible hydrocarbon binder or wax into the channel semiconductor composition to drive rapid phase separation and instantaneous crystallization of polymer semiconductor at room temperature. The resulting composite channel semiconductor film manifests a nano/microporous surface morphology with a continuous semiconductor nanowire network. OFET mobility of up to about 5 cm² V^?1 s^?1 and on/off ratio ≥ 10⁶ are attained. These are hitherto benchmark performance characteristics for room‐temperature, solution‐processed polymer OFETs, which are functionally useful for many impactful applications.     

Self‐Assembled,Millimeter‐Sized TIPS‐Pentacene Spherulites Grown on Partially Crosslinked Polymer Gate Dielectric

Here, a highly crystalline and self‐assembled 6,13‐bis(triisopropylsilylethynyl) pentacene (TIPS‐Pentacene) thin films formed by simple spin‐coating for the fabrication of high‐performance solution‐processed organic field‐effect transistors (OFETs) are reported. Rather than using semiconducting organic small‐molecule–insulating polymer blends for an active layer of an organic transistor, TIPS‐Pentacene organic semiconductor is separately self‐assembled on partially crosslinked poly‐4‐vinylphenol:poly(melamine‐co‐formaldehyde) (PVP:PMF) gate dielectric, which results in a vertically segregated semiconductor‐dielectric film with millimeter‐sized spherulite‐crystalline morphology of TIPS‐Pentacene. The structural and electrical properties of TIPS‐Pentacene/PVP:PMF films have been studied using a combination of polarized optical microscopy, atomic force microscopy, 2D‐grazing incidence wide‐angle X‐ray scattering, and secondary ion mass spectrometry. It is finally demonstrated a high‐performance OFETs with a maximum hole mobility of 3.40 cm² V^?1 s^?1 which is, to the best of our knowledge, one of the highest mobility values for TIPS‐Pentacene OFETs fabricated using a conventional solution process. It is expected that this new deposition method would be applicable to other small molecular semiconductor–curable polymer gate dielectric systems for high‐performance organic electronic applications.     

Improved Operational Stability of Polymer Light‐Emitting Diodes Based on Silver Nanowire Electrode Through Pre‐Bias Conditioning Treatment

For the first time, highly efficient and flexible polymer light emitting diodes (PLEDs) based on silver nanowire (AgNW) electrode, with improved operational stability by simply applying pre‐bias conditioning treatment, are demonstrated. Reverse bias conditioning performed before J–V–L measurement of the PLEDs enables the rough AgNW networks to function properly as a bottom electrode by stabilizing current characteristics, and the devices continue to show consistent operational performances. Conditions of applied bias and thicknesses of active layer are controlled for optimization and it is found that high reverse voltage is required to obtain current stabilization. Adequate thickness of polymer is also necessary to avoid breakdown induced by reverse bias. The essential effect of pre‐bias conditioning on the improved performances of PLEDs is investigated, and it is found that morphological change of AgNW networks contribute to the improvement in device performance. Some of the AgNWs that appear to be pathway of leakage current are deformed, and surface roughness (RMS) of the AgNW film is decreased while the sheet resistance of the film is maintained when the reverse bias conditioning is applied. It is also revealed that pre‐bias conditioning is independent from directionality of the applied bias when utilizing insulating polymer sandwiched between two electrodes.     

Device Physics of White Polymer Light‐Emitting Diodes

The charge transport and recombination in white‐emitting polymer light‐ emitting diodes (PLEDs) are studied. The PLED investigated has a single emissive layer consisting of a copolymer in which a green and red dye are incorporated in a blue backbone. From single‐carrier devices the effect of the green‐ and red‐emitting dyes on the hole and electron transport is determined. The red dye acts as a deep electron trap thereby strongly reducing the electron transport. By incorporating trap‐assisted recombination for the red emission and bimolecular Langevin recombination for the blue emission, the current and light output of the white PLED can be consistently described. The color shift of single‐layer white‐emitting PLEDs can be explained by the different voltage dependencies of trap‐assisted and bimolecular recombination.     

Hierarchical Multi‐Step Folding of Polymer Bilayers

A highly complex multi‐step folding of isotropic stimuli‐responsive polymer bilayers resulting in a variety of 2D and 3D structures is reported. Experimental observations allow determination of empirical rules, which can be used to direct the folding of polymer films in a predictable manner. In particular, it is demonstrated that these rules can be used for the design of a 3D pyramid. The understanding and know‐how attained in this study allow the very simple design of highly complex, self‐folding 3D objects and open new horizons for 3D patterning, important for the design of microfluidic devices, biomaterials, and soft electronics.     

Manipulating the Hysteresis in Poly(vinyl alcohol)‐Dielectric Organic Field‐Effect Transistors Toward Memory Elements

The origins of hysteresis in organic field‐effect transistors (OFETs) and its applications in organic memory devices is investigated. It is found that the orientations of the hydroxyl groups in poly(vinyl alcohol) (PVA) gate dielectrics are correlated with the hysteresis of transfer characteristics in pentacene‐based OFETs under the forward and backward scan. The applied gate bias partially aligns the orientations of the hydroxyl groups perpendicular to the substrate as characterized by reflective absorption Fourier transform infrared spectroscopy (RA‐FTIR), in which the field‐induced surface dipoles at the pentacene/PVA interface trap charges and cause the hysteresis. Treating PVA with an anhydrous solvent eliminates the residual moisture in the dielectrics layer, allowing for more effective control of the induced dipoles by the applied gate bias. OFETs of dehydrated‐PVA dielectrics present a pronounced shift of the threshold voltage (ΔV_Th) of 35.7 V in transfer characteristics, higher than that of 18.5 V for untreated devices and results in sufficient dynamic response for applications in memory elements. This work highlights the usage of non‐ferroelectric gate dielectrics to fabricate OFET memory elements by manipulating the molecular orientations in the dielectrics layer.     

A Cross‐Linkable Donor Polymer as the Underlying Layer to Tune the Active Layer Morphology of Polymer Solar Cells

For polymer solar cells (PSCs) with conventional configuration, the vertical composition profile of donor:acceptor in active layer is detrimental for charge carrier transporting/collection and leads to decreased device performance. A cross‐linkable donor polymer as the underlying morphology‐inducing layer (MIL) to tune the vertical composition distribution of donor:acceptor in the active layer for improved PSC device performance is reported. With poly(thieno[3,4‐b]‐thiophene/benzodithiophene):[6,6]‐phenyl C₇₁‐butyric acid methyl ester (PTB7:PC₇₁BM) as the active layer, the MIL material, PTB7‐TV , is developed by attaching cross‐linkable vinyl groups to the side chain of PTB7. PSC device with PTB7‐TV layer exhibits a power conversion efficiency (PCE) of 8.55% and short‐circuit current density (J_SC) of 15.75 mA cm^?2, in comparison to PCE of 7.41% and J_SC of 13.73 mA cm^?2 of the controlled device. The enhanced device performance is ascribed to the much improved vertical composition profile and reduced phase separation domain size in the active layer. These results demonstrate that cross‐linked MIL is an effective strategy to improve photovoltaic performance of conventional PSC devices.     

Polymer Dielectrics and Orthogonal Solvent Effects for High‐Performance Inkjet‐Printed Top‐Gated P‐Channel Polymer Field‐Effect Transistors

We investigated the effects of a gate dielectric and its solvent on the characteristics of top‐gated organic field‐effect transistors (OFETs). Despite the rough top surface of the inkjet‐printed active features, the charge transport in an OFET is still favorable, with no significant degradation in performance. Moreover, the characteristics of the OFETs showed a strong dependency on the gate dielectrics used and its orthogonal solvents. Poly(3‐hexylthiophene) OFETs with a poly(methyl methacrylate) dielectric showed typical p‐type OFET characteristics. The selection of gate dielectric and solvent is very important to achieve high‐performance organic electronic circuits.     

Organozinc Compounds as Effective Dielectric Modification Layers for Polymer Field‐Effect Transistors

The interface between the organic semiconductor and dielectric plays an important role in determining the device performance of organic field‐effect transistors (OFETs). Although self‐assembled monolayers (SAMs) made from organosilanes have been widely used for dielectric modification to improve the device performance of OFETs, they suffer from incontinuous and lack uniform coverage of the dielectric layer. Here, it is reported that by introduction of a solution‐processed organozinc compound as a dielectric modification layer between the dielectric and the silane SAM, improved surface morphology and reduced surface polarity can be achieved. The organozinc compound originates from the reaction between diethylzinc and the cyclohexanone solvent, which leads to formation of zinc carboxylates. Being annealed at different temperatures, organozinc compound exists in various forms in the solid films. With organozinc modification, p‐type polymer FETs show a high charge carrier mobility that is about two‐fold larger than a control device that does not contain the organozinc compound, both for devices with a positive threshold voltage and for those with a negative one. After organozinc compound modification, the threshold voltage of polymer FETs can either be altered to approach zero or remain unchanged depending on positive or negative threshold voltage they have.     

Polymer light‐emitting diode prepared with an ionomer and polyaniline

Light‐emitting diodes of poly[2‐methoxy‐5‐(2′‐ethyl‐hexyloxy)‐1,4‐phenylenevinylene] (MEH‐PPV) have been fabricated from sodium sulphonated polystyrene (SSPS) ionomer as an electron‐injecting and hole‐blocking material and polyaniline doped with camphor sulphonic acid (PANI‐CSA) as a polymer anode. The presence of PANI in an ITO/PANI/MEH‐PPV/AI device makes little difference to its optical output and quantum efficiency. SSPS shows good electron injection in an ITO/MEH‐PPV/SSPS/AI device. When SSPS ionomer is introduced in an ITO/PANI/MEH‐PPV/SSPS/A1 device, a larger enhancement in optical output is obtained and its quantum efficiency is also enhanced because of its balanced charge injection. The electroluminescent device associated with SSPS ionomer and PANI is a well‐designed structure to improve stability and efficiency. Copyright © 2000 John Wiley & Sons, Ltd.     

A Highly Crystalline Fused‐Ring n‐Type Small Molecule for Non‐Fullerene Acceptor Based Organic Solar Cells and Field‐Effect Transistors

N‐type organic small molecules (SMs) are attracting attention in the organic electronics field, due to their easy purification procedures with high yield. However, only a few reports show SMs that perform well in both organic field‐effect transistors (OFETs) and organic solar cells (OSCs). Here, the synthesis and characterization of an n‐type small molecule with an indacenodithieno[3,2‐b]thiophene (IDTT) core unit and linear alkylated side chain (C₁₆) (IDTTIC) are reported. Compared to the state‐of‐the‐art n‐type molecule IDTIC, IDTTIC exhibits smaller optical bandgap and higher absorption coefficient, which is due to the enhanced intramolecular effect. After mixing with the polymer donor PBDB‐T, IDTIC‐based solar cells deliver a power conversion efficiency of only 5.67%. In stark contrast, the OSC performance of IDTTIC improves significantly to 11.2%. It is found that the superior photovoltaic properties of PBDB‐T:IDTTIC blends are mainly due to reduced trap‐assisted recombination and enhanced molecular packing coherence length and higher domain purity when compared to IDTIC. Moreover, a significantly higher electron mobility of 0.50 cm² V⁻¹ s⁻¹ for IDTTIC in OFET devices than for IDTIC (0.15 cm² V⁻¹ s⁻¹) is obtained. These superior performances in OSCs and OFETs demonstrate that SMs with extended π‐conjugation of the backbone possess a great potential for application in organic electronic devices.     

Kinetically Controlled Crystallization in Conjugated Polymer Films for High‐Performance Organic Field‐Effect Transistors

Ordering of semiconducting polymers in thin films from the nano to microscale is strongly correlated with charge transport properties as well as organic field‐effect transistor performance. This paper reports a method to control nano to microscale ordering of poly{[N,N′‐bis(2‐octyldodecyl)‐naphthalene‐1,4,5,8‐bis(dicarboximide)‐2,6‐diyl]‐alt‐5,5′‐(2,2′‐bithiophene)} (P(NDI2OD‐T2)) thin films by precisely regulating the solidification rate from the metastable state just before crystallization. The proposed simple but effective approach, kinetically controlled crystallization, achieves optimized P(NDI2OD‐T2) films with large polymer domains, long range ordered fibrillar structures, and molecular orientation preferable for electron transport leading to dramatic morphological changes in both polymer domain sizes at the micrometer scale and molecular packing structures at nanoscales. Structural changes significantly increase electron mobilities up to 3.43 ± 0.39 cm² V^?1 s^?1 with high reliability, almost two orders of enhancement compared with devices from naturally dried films. Small contact resistance is also obtained for electron injection (0.13 MΩ cm), low activation energy (62.51 meV), and narrow density of states distribution for electron transport in optimized thin films. It is believed that this study offers important insight into the crystallization of conjugated polymers that can be broadly applied to optimize the morphology of semiconducting polymer films for solution processed organic electronic devices.