Introducing a Nonvolatile N‐Type Dopant Drastically Improves Electron Transport in Polymer and Small‐Molecule Organic Transistors

Molecular doping is a powerful yet challenging technique for enhancing charge transport in organic semiconductors (OSCs). While there is a wealth of research on p‐type dopants, work on their n‐type counterparts is comparatively limited. Here, reported is the previously unexplored n‐dopant (12a,18a)‐5,6,12,12a,13,18,18a,19‐octahydro‐5,6‐dimethyl‐ 13,18[1′,2′]‐benzenobisbenzimidazo [1,2‐b:2′,1′‐d]benzo[i][2.5]benzodiazo‐cine potassium triflate adduct (DMBI‐BDZC) and its application in organic thin‐film transistors (OTFTs). Two different high electron mobility OSCs, namely, the polymer poly[[N,N′‐bis(2‐octyldodecyl)‐naphthalene‐1,4,5,8‐ bis(dicarboximide)‐2,6‐diyl]‐alt‐5,5′‐(2′‐bithiophene)] and a small‐molecule naphthalene diimides fused with 2‐(1,3‐dithiol‐2‐ylidene)malononitrile groups (NDI‐DTYM2) are used to study the effectiveness of DMBI‐BDZC as a n‐dopant. N‐doping of both semiconductors results in OTFTs with improved electron mobility (up to 1.1 cm² V^?1 s^?1), reduced threshold voltage and lower contact resistance. The impact of DMBI‐BDZC incorporation is particularly evident in the temperature dependence of the electron transport, where a significant reduction in the activation energy due to trap deactivation is observed. Electron paramagnetic resonance measurements support the n‐doping activity of DMBI‐BDZC in both semiconductors. This finding is corroborated by density functional theory calculations, which highlights ground‐state electron transfer as the main doping mechanism. The work highlights DMBI‐BDZC as a promising n‐type molecular dopant for OSCs and its application in OTFTs, solar cells, photodetectors, and thermoelectrics.     

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.     

Effective Controlling of Film Texture and Carrier Transport of a High‐Performance Polymeric Semiconductor by Magnetic Alignment

The controlling of molecular orientation and structural ordering of organic semiconductors is crucial to achieve high performance electronic devices. In this work, large‐area highly oriented and ordered films of an excellent electron transporter Poly{[N,N′‐bis(2‐octyldodecyl)‐1,4,5,8‐naphthalenedicarboximide)‐2,6‐diyl]‐alt‐5,5′‐(2,2′‐bithiophene)} (P(NDI2OD‐T2)) are achieved by improved solution‐cast in high magnetic field. Microstructural characterizations reveal that the chain backbones of P(NDI2OD‐T2) are highly aligned along the applied magnetic field in the films. Based on the synchrotron‐based X‐ray diffraction analysis of the polymer films cast from different solvents, a mechanism which controls the alignment process is proposed, which emphasizes that molecular aggregates of P(NDI2OD‐T2) preformed in the solution initiate magnetic alignment and finally determine the degree of film texture. Furthermore, the time‐modulated magnetic field technique is utilized to effectively control the orientation of π‐conjugated plane of the backbones, thus the degree of face‐on molecular packing of P(NDI2OD‐T2) is enhanced significantly. Thin film transistors based on the magnetic‐aligned P(NDI2OD‐T2) films exhibit an enhancement of electron mobility by a factor of four compared to the unaligned devices, as well as a large mobility anisotropy of seven.     

Effect of Doping Concentration on Microstructure of Conjugated Polymers and Characteristics in N‐Type Polymer Field‐Effect Transistors

Despite extensive progress in organic field‐effect transistors, there are still far fewer reliable, high‐mobility n‐type polymers than p‐type polymers. It is demonstrated that by using dopants at a critical doping molar ratio (MR), performance of n‐type polymer poly[[N,N9‐bis(2‐octyldodecyl)‐naphthalene‐1,4,5,8‐bis(dicarboximide)‐2,6‐diyl]‐alt‐5,59‐(2,29‐bithiophene)] (P(NDI2DO‐T2)) field‐effect transistors (FETs) can be significantly improved and simultaneously optimized in mobility, on–off ratio, crystallinity, injection, and reliability. In particular, when using the organic dopant bis(cyclopentadienyl)–cobalt(II) (cobaltocene, CoCp₂) at a low concentration (0.05 wt%), the FET mobility is increased from 0.34 to 0.72 cm² V^–1 s^–1, and the threshold voltage was decreased from 32.7 to 8.8 V. The relationship between the MR of dopants and electrical characteristics as well as the evolution in polymer crystallinity revealed by synchrotron X‐ray diffractions are systematically investigated. Deviating from previous discoveries, it is found that mobility increases first and then decreases drastically beyond a critical value of MR. Meanwhile, the intensity and width of the main peak of in‐plane X‐ray diffraction start to decrease at the same critical MR. Thus, the mobility decrease is correlated with the disturbed in‐plane crystallinity of the conjugated polymer, for both organic and inorganic dopants. The method provides a simple and efficient approach to employing dopants to optimize the electrical performance and microstructure of P(NDI2DO‐T2).     

Systematic Study of Widely Applicable N‐Doping Strategy for High‐Performance Solution‐Processed Field‐Effect Transistors

A specific design for solution‐processed doping of active semiconducting materials would be a powerful strategy in order to improve device performance in flexible and/or printed electronics. Tetrabutylammonium fluoride and tetrabutylammonium hydroxide contain Lewis base anions, F^? and OH^?, respectively, which are considered as organic dopants for efficient and cost‐effective n‐doping processes both in n‐type organic and nanocarbon‐based semiconductors, such as 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)) and selectively dispersed semiconducting single‐walled carbon nanotubes by π‐conjugated polymers. The dramatic enhancement of electron transport properties in field‐effect transistors is confirmed by the effective electron transfer from the dopants to the semiconductors as well as controllable onset and threshold voltages, convertible charge‐transport polarity, and simultaneously showing excellent device stabilities under ambient air and bias stress conditions. This simple solution‐processed chemical doping approach could facilitate the understanding of both intrinsic and extrinsic charge transport characteristics in organic semiconductors and nanocarbon‐based materials, and is thus widely applicable for developing high‐performance organic and printed electronics and optoelectronics devices.     

Printed,Flexible, Organic Nano‐Floating‐Gate Memory: Effects of Metal Nanoparticles and Blocking Dielectrics on Memory Characteristics

The effects of using a blocking dielectric layer and metal nanoparticles (NPs) as charge‐trapping sites on the characteristics of organic nano‐floating‐gate memory (NFGM) devices are investigated. High‐performance NFGM devices are fabricated using the n‐type polymer semiconductor, 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)), and various metal NPs. These NPs are embedded within bilayers of various polymer dielectrics (polystyrene (PS)/poly(4‐vinyl phenol) (PVP) and PS/poly(methyl methacrylate) (PMMA)). The P(NDI2OD‐T2) organic field‐effect transistor (OFET)‐based NFGM devices exhibit high electron mobilities (0.4–0.5 cm² V^?1 s^?1) and reliable non‐volatile memory characteristics, which include a wide memory window (≈52 V), a high on/off‐current ratio (I_on/I_off ≈ 10⁵), and a long extrapolated retention time (>10⁷ s), depending on the choice of the blocking dielectric (PVP or PMMA) and the metal (Au, Ag, Cu, or Al) NPs. The best memory characteristics are achieved in the ones fabricated using PMMA and Au or Ag NPs. The NFGM devices with PMMA and spatially well‐distributed Cu NPs show quasi‐permanent retention characteristics. An inkjet‐printed flexible P(NDI2OD‐T2) 256‐bit transistor memory array (16 × 16 transistors) with Au‐NPs on a polyethylene naphthalate substrate is also fabricated. These memory devices in array exhibit a high I_on/I_off (≈10^{4 ± 0.85}), wide memory window (≈43.5 V ± 8.3 V), and a high degree of reliability.     

Competitive Absorption and Inefficient Exciton Harvesting: Lessons Learned from Bulk Heterojunction Organic Photovoltaics Utilizing the Polymer Acceptor P(NDI2OD‐T2)

Organic solar cells utilizing the small molecule donor 7,7′‐(4,4‐bis(2‐ethylhexyl)‐4H‐silolo[3,2‐b:4,5‐b′]dithiophene‐2,6‐diyl)bis(6‐fluoro‐4‐(5′‐hexyl‐[2,2′‐bithiophen]‐5‐yl)benzo[c][1,2,5] thiadiazole) (p‐DTS(FBTTh₂)₂ and the polymer acceptor poly{[N,N′‐bis(2‐octyldodecyl)‐1,4,5,8‐naphthalenedicarboximide‐2,6‐diyl]‐alt‐5,5′‐(2,2′‐bithiophene)}(P(NDI2OD‐T2)) are investigated and a power conversion efficiency of 2.1% is achieved. By systematic study of bulk heterojunction (BHJ) organic photovoltaic (OPV) quantum efficiency, film morphology, charge transport and extraction and exciton diffusion, the loss processes in this blend is revealed compared to the blend of [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC₇₁BM) and the same donor. An exciton diffussion study using Förster resonant energy transfer (FRET) shows the upper limit of the P(NDI2OD‐T2) exciton diffusion length to be only 1.1 nm. The extremely low exciton diffusion length of P(NDI2OD‐T2), in combination with the overlap in donor and acceptor absorption, is then found to significantly limit device performance. These results suggest that BHJ OPV devices utilizing P(NDI2OD‐T2) as an acceptor material will likely be limited by its low exciton diffusion length compared to devices utilizing functionalized fullerene acceptors, especially when P(NDI2OD‐T2) significantly competes with the donor molecule for photon absorption.     

A Versatile Method to Fabricate Highly In‐Plane Aligned Conducting Polymer Films with Anisotropic Charge Transport and Thermoelectric Properties: The Key Role of Alkyl Side Chain Layers on the Doping Mechanism

A general method is proposed to produce oriented and highly crystalline conducting polymer layers. It combines the controlled orientation/crystallization of polymer films by high‐temperature rubbing with a soft‐doping method based on spin‐coating a solution of dopants in an orthogonal solvent. Doping rubbed films of regioregular poly(3‐alkylthiophene)s and poly(2,5‐bis(3‐dodecylthiophen‐2‐yl)thieno[3,2‐b ]thiophene) with 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F₄TCNQ) yields highly oriented conducting polymer films that display polarized UV–visible–near‐infrared (NIR) absorption, anisotropy in charge transport, and thermoelectric properties. Transmission electron microscopy and polarized UV–vis–NIR spectroscopy help understand and clarify the structure of the films and the doping mechanism. F₄TCNQ^? anions are incorporated into the layers of side chains and orient with their long molecular axis perpendicular to the polymer chains. The ordering of dopant molecules depends closely on the length and packing of the alkyl side chains. Increasing the dopant concentration results in a continuous variation of unit cell parameters of the doped phase. The high orientation results in anisotropic charge conductivity (σ) and thermoelectric properties that are both enhanced in the direction of the polymer chains (σ = 22 ± 5 S cm^?1 and S = 60 ± 2 µV K^?1). The method of fabrication of such highly oriented conducting polymer films is versatile and is applicable to a large palette of semiconducting polymers.     

N‐Type Conjugated Polymer‐Enabled Selective Dispersion of Semiconducting Carbon Nanotubes for Flexible CMOS‐Like Circuits

Sorting of semiconducting single‐walled carbon nanotubes (SWNTs) by conjugated polymers has attracted considerable attention recently because of its simplicity, high selectivity, and high yield. However, up to now, all the conjugated polymers used for SWNT sorting are electron‐donating (p‐type). Here, a high‐mobility electron‐accepting (n‐type) polymer 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)) is utilized for the sorting of high‐purity semiconducting SWNTs, as characterized by Raman spectroscopy, dielectric force spectroscopy and transistor measurements. In addition, the SWNTs sorted by P(NDI2OD‐T2) have larger diameters than poly(3‐dodecylthiophene) (P3DDT)‐sorted SWNTs. Molecular dynamics simulations in explicit toluene demonstrate distinct linear or helical wrapping geometry between P(NDI2OD‐T2) and different types of SWNTs, likely as a result of the strong interactions between the large aromatic core of the P(NDI2OD‐T2) backbone and the hexagon path of SWNTs. By using high‐mobility n‐type P(NDI2OD‐T2) as the sorting polymer, ambipolar SWNT transistors with better electron transport than that attained by P3DDT‐sorted SWNTs are achieved. As a result, flexible negated AND and negated OR logic circuits from the same set of ambipolar transistors are fabricated, without the need for doping. The use of n‐type polymers for sorting semiconducting SWNTs and achieving ambipolar SWNT transistor characteristics greatly simplifies the fabrication of flexible complementary metal‐oxide‐semiconductor‐like SWNT logic circuits.     

Synthesis,Electronic Structure,and Charge Transport Characteristics of Naphthalenediimide‐Based Co‐Polymers with Different Oligothiophene Donor Units

Naphthalenediimide (NDI)‐based polymers co‐polymerized with thienyl units are an interesting class of polymer semiconductors because of their good electron mobilities and unique film microstructure. Despite these properties, understanding how the extension of the thienyl co‐monomer affects charge transport properties remains unclear. With this goal in mind, we have synthesized a series of NDI derivatives of the parent 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)), which exhibited excellent electron mobility. The strategy comprises both the extension of the donor o‐conjugation length and the heteroatomic fusion of the thiophene rings. These newly synthesized compounds are characterized experimentally and theoretically vis‐à‐vis with P(NDI2OD‐T2) as the reference. UV‐vis data and cyclic‐voltammetry are adopted to assess the effect of the donor modification on the frontier energy levels and on the bandgap. Intra‐molecular polaronic effects are accounted for by computing the internal reorganization energy with density functional theory (DFT) calculations. Finally electrons and holes transport is experimentally investigated in field‐effect transistors (FETs), by measuring current‐voltage characteristics at variable temperatures. Overall we have identified a regime where inter‐molecular effects, such as the wavefunction overlap and the degree of energetic disorder, induced by the different donor group prevail over polaronic effects and are the leading factors in determining electrons mobility.     

Very Low Degree of Energetic Disorder as the Origin of High Mobility in an n‐channel Polymer Semiconductor

Charge transport is investigated in high‐mobility n‐channel organic field‐effect transistors (OFETs) based on 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), Polyera ActivInk? N2200) with variable‐temperature electrical measurements and charge‐modulation spectroscopy. Results indicate an unusually uniform energetic landscape of sites for charge‐carrier transport along the channel of the transistor as the main reason for the observed high‐electron mobility. Consistent with a lateral field‐independent transport at temperatures down to 10 K, the reorganization energy is proposed to play an important role in determining the activation energy for the mobility. Quantum chemical calculations, which show an efficient electronic coupling between adjacent units and a reorganization energy of a few hundred meV, are consistent with these findings.     

A Synergetic Effect of Molecular Weight and Fluorine in All‐Polymer Solar Cells with Enhanced Performance

A synergetic effect of molecular weight (M_n) and fluorine (F) on the performance of all‐polymer solar cells (all‐PSCs) is comprehensively investigated by tuning the M_n of the acceptor polymer 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)) and the F content of donor polymer poly(2,3‐bis‐(3‐octyloxyphenyl)quinoxaline‐5,8‐dyl‐alt‐thiophene‐2,5‐diyl). Both M_n and F variations strongly influence the charge transport properties and morphology of the blend films, which have a significant impact on the photovoltaic performance of all‐PSCs. In particular, the effectiveness of high M_n in increasing power conversion efficiency (PCE) can be greatly improved by the devices based on optimum F content, reaching a PCE of 7.31% from the best all‐PSC combination. These findings enable us to further understand the working principles of all‐PSCs with a view on achieving even higher power conversion efficiency in the future.     

A High‐k Fluorinated P(VDF‐TrFE)‐g‐PMMA Gate Dielectric for High‐Performance Flexible Field‐Effect Transistors

A newly synthesized high‐k polymeric insulator for use as gate dielectric layer for organic field‐effect transistors (OFETs) obtained by grafting poly(methyl methacrylate) (PMMA) in poly(vinylidene fluoride‐trifluoroethylene) (P(VDF‐TrFE)) via atom transfer radical polymerization transfer is reported. This material design concept intents to tune the electrical properties of the gate insulating layer (capacitance, leakage current, breakdown voltage, and operational stability) of the high‐k fluorinated polymer dielectric without a large increase in operating voltage by incorporating an amorphous PMMA as an insulator. By controlling the grafted PMMA percentage, an optimized P(VDF‐TrFE)‐g‐PMMA with 7 mol% grafted PMMA showing reasonably high capacitance (23–30 nF cm^?2) with low voltage operation and negligible current hysteresis is achieved. High‐performance low‐voltage‐operated top‐gate/bottom‐contact OFETs with widely used high mobility polymer semiconductors, poly[[2,5‐bis(2‐octyldodecyl)‐2,3,5,6‐tetrahydro‐3,6‐dioxopyrrolo [3,4‐c]pyrrole‐1,4‐diyl]‐alt‐[[2,2′‐(2,5‐thiophene)bis‐thieno(3,2‐b)thiophene]‐5,5′‐diyl]] (DPPT‐TT), and poly([N,N′‐bis(2‐octyldodecyl)‐naphthalene‐1,4,5,8‐bis(dicarboximide)‐2,6‐diyl]‐alt‐5,5′‐(2,2′‐bithiophene)) are demonstrated here. DPPT‐TT OFETs with P(VDF‐TrFE)‐g‐PMMA gate dielectrics exhibit a reasonably high field‐effect mobility of over 1 cm² V^?1 s^?1 with excellent operational stability.     

Favorable Molecular Orientation Enhancement in Semiconducting Polymer Assisted by Conjugated Organic Small Molecules

A bimodal texturing effect of semiconducting polymers is investigated by incorporating conjugated small molecules to significantly improve the charge transport characteristics via formation of 3D transport pathways. Solution blending of the electron‐transporting polymer, 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)), with small molecular crystals of tetrathiafulvalene and tetracyanoquinodimethane is used, and the thin film microstructures are studied using a combination of atomic force microscopy, transmission electron microscopy, 2D grazing incidence X‐ray diffraction, and surface‐sensitive near‐edge X‐ray absorption fine structure. Blended thin films show edge‐on and face‐on bimodal texture with long‐range order and microstructure packing orientation preferable for electron transport through the channel in organic field‐effect transistors, which is confirmed by high electron mobility 1.91 cm² V^?1 s^?1, small contact resistance, and low energetic disorder according to temperature dependence of the field‐effect mobility. Structural changes suggest a 3D network charge transport model via lamella packing and bimodal orientation of the semiconducting polymers.     

Room‐Temperature‐Operated Ultrasensitive Broadband Photodetectors by Perovskite Incorporated with Conjugated Polymer and Single‐Wall Carbon Nanotubes

In this work, room‐temperature‐operated ultrasensitive solution‐processed perovskite photodetectors (PDs) with near infrared (NIR) photoresponse are reported. In order to enable perovskite PDs possessing extended NIR photoresponse, novel n‐type low bandgap conjugated polymer, poly[(N,N′‐bis(2‐octyldodecyl)‐1,4,5,8‐naphthalene diimide‐2,6‐diyl) (2,5‐dioctyl‐3,6‐di(thiophen‐2‐yl)pyrrolo[3,4‐c]pyrrole‐1,4‐dione‐5,5′‐diyl)] (NDI‐DPP), which has strong absorption in the NIR region, is developed and then employed in perovskite PDs. By the formation of type II band alignment between NDI‐DPP with single‐wall carbon nanotubes (SWCNTs), the NIR absorption of NDI‐DPP is exploited, which contributes to the NIR photoresponse for the perovskite PDs, where perovskite is incorporated with NDI‐DPP and SWCNTs as well. In addition, SWCNTs incorporated with perovskite active layer can offer the percolation pathways for high charge‐carrier mobility, which tremendously boosts the charge transfer in the photoactive layer, and consequently improves the photocurrent in the visible region. As a result, the perovskite PDs exhibit the responsivities of ≈400 and ≈150 mA W^?1 and the detectivities of over 6 × 10¹² Jones (1 Jones = 1 cm Hz^1/2 W^?1) and over 2 × 10¹² Jones in the visible and NIR regions, respectively. This work reports the development of perovskite PDs with NIR photoresponse, which is terrifically beneficial for the practical applications of perovskite PDs.     

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.     

Extremely Low‐Threshold Amplified Spontaneous Emission of 9,9′‐Spirobifluorene Derivatives and Electroluminescence from Field‐Effect Transistor Structure

By doping 2,7‐bis[4‐(N‐carbazole)phenylvinyl]‐9,9′‐spirobifluorene (spiro‐SBCz) into a wide energy gap 4,4′‐bis(9‐carbazole)‐2,2′‐biphenyl (CBP) host, we demonstrate an extremely low ASE threshold of E_th = (0.11 ± 0.05) μJ cm^–2 (220 W cm^–2) which is the lowest ASE threshold ever reported. In addition, we confirmed that the spiro‐SBCz thin film functions as an active light emitting layer in organic light‐emitting diode (OLED) and a field‐effect transistor (FET). In particular, we succeeded to obtain linear electroluminescence in the FET structure which will be useful for future organic laser diodes.     

High‐Performance n‐Channel Thin‐Film Field‐Effect Transistors Based on a Nanowire‐Forming Polymer

A new electrontransport polymer, poly{[N,N′‐dioctylperylene‐3,4,9,10‐bis(dicarboximide)‐1,7(6)‐diyl]‐alt‐[(2,5‐bis(2‐ethyl‐hexyl)‐1,4‐phenylene)bis(ethyn‐2,1‐diyl]} (PDIC8‐EB), is synthesized. In chloroform, the polymer undergoes self‐assembly, forming a nanowire suspension. The nanowire's optical and electrochemical properties, morphological structure, and field‐effect transistor (FET) characteristics are investigated. Thin films fabricated from a PDIC8‐EB nanowire suspension are composed of ordered nanowires and ordered and amorphous non‐nanowire phases, whereas films prepared from a homogeneous PDIC8‐EB solution consist of only the ordered and amorphous non‐nanowire phases. X‐ray scattering experiments suggest that in both nanowires and ordered phases, the PDIC8 units are laterally stacked in an edge‐on manner with respect to the film plane, with full interdigitation of the octyl chains, and with the polymer backbones preferentially oriented within the film plane. The ordering and orientations are significantly enhanced through thermal annealing at 200 °C under inert conditions. The polymer film with high degree of structural ordering and strong orientation yields a high electron mobility (0.10 ± 0.05 cm² V^?1 s^?1), with a high on/off ratio (3.7 × 10⁶), a low threshold voltage (8 V), and negligible hysteresis (0.5 V). This study demonstrates that the polymer in the nanowire suspension provides a suitable material for fabricating the active layers of high‐performance n‐channel FET devices via a solution coating process.     

Self‐Doping Cathode Interfacial Material Simultaneously Enabling High Electron Mobility and Powerful Work Function Tunability for High‐Efficiency All‐Solution‐Processed Polymer Light‐Emitting Diodes

A variety of N ‐hydrogenated/N ‐methylated pyridinium salts are elaborately designed and synthesized. Thermogravimetric and X‐ray photoelectron spectra analysis indicate the intensities of the N? H covalent bonds are strengthened step‐by‐step from 3,3′‐(5′‐(3‐(pyridin‐3‐yl)phenyl)‐[1,1′:3′,1″‐terphenyl]‐3,3″‐diyl)dipyridine (Tm)‐HCl to Tm‐HBr and then Tm‐TfOH, which results in gradually improved cathode interfacial modification abilities. The larger dipole moments of N⁺? H containing moieties compared to those of the N⁺? CH₃ endow them with more preferable interfacial modification abilities. Electron paramagnetic resonance signals reveal the existence of radical anions in the solid state of Tm‐TfOH, which enables its self‐doping property and high electron mobility up to 1.67 × 10^?3 cm² V^?1 s^?1. Using the Tm‐TfOH as the cathode interfacial layers (CILs), the phenyl‐substituted poly(para ‐phenylene vinylene)‐based all‐solution‐processed polymer light‐emitting diodes (PLEDs) achieve more preferable device performances than the poly[(9,9‐bis(3′‐(N ,N ‐dimethylamino)propyl)‐2,7‐fluorene)‐alt ‐2,7‐(9,9‐dioctylfluorene)]‐based ones, i.e., high current density of nearly 300 mA cm^?2, very high luminance over 15 000 cd m^?2 at a low bias of 5 V. Remarkably, the thickness of the CILs has little impact on the device performance and high efficiencies are maintained even at thicknesses up to 85 nm, which is barely realized in PLEDs with small‐molecule‐based electron transporting layers.     

Electrophosphorescent Devices Based on Cationic Complexes: Control of Switch‐on Voltage and Efficiency Through Modification of Charge Injection and Charge Transport

This paper reports an analysis of the properties of polymer light‐emitting devices (PLEDs) doped with iridium complexes. Devices based on charged and neutral complexes doped into poly(vinylcarbazole) (PVK) are presented, and the role of the ions and the charge‐transport properties of the complexes are discussed. In devices with the charged complexes, the concentration of the complex is found to have a profound effect on both the switch‐on voltage and the efficiency. At higher doping concentrations the efficiency is increased and the switch‐on voltage decreased. The increase in efficiency and decrease in switch‐on voltage at higher dopant concentration are found to be due to an alternative charge transport path via the iridium dopant [Ir(bpy)]⁺ (bis(2‐phenylpyridine‐C²,N′)(2,2′‐bipyridine)iridium hexafluorophosphate). However, at lower concentrations the complex becomes an electron trap and the efficiency is reduced. The devices are found to be significantly less efficient than those with neutral complexes. This difference is attributed to the ionic content and the charge trapping properties of the charged complexes. The low efficiency of the charged‐complex‐based devices could be overcome by utilizing a hole‐blocking layer; devices with efficiencies as high as 23 cd A^–1 were obtained.