Electroluminescence in Ion-Gel Gated Conjugated Polymer Field-Effect Transistors

We report electroluminescence from ion-gel gated, field-effect transistors based on the conjugated polymer, poly­(9,9′-dioctylfluorene-co-benzothiadiazole) gated by an 1-ethyl-3-methylimidazoliumbis (trifluoromethylsulfonyl) imide/poly (styrene-block-ethylene oxide-block-styrene) ion gel, and investigate the mechanism for light emission. The devices emit light from near the electron-injecting drain electrode when the drain source voltage exceeds approximately the energy gap of the polymer (<i>V</i>_ds <i>> E</i>_g<i>/e</i>). Charge accumulation spectroscopy is used to demonstrate the significant penetration of the negative TFSI^– ions into the F8BT assisted by the application of negative gate voltages, where they lead to significant p-type doping of the bulk of the F8BT film. In contrast, no evidence for diffusion of positive ions with positive gate voltages is observed, and this is consistent with the location of the recombination zone in the proximity of electron injecting electrode and the absence of a comparable electron current at positive gate voltages. We conclude that in the light-emission regime the devices operate more akin to a hole-current dominated light-emitting electrochemical cell than a transistor

Enhanced Ambipolar Charge Injection with Semiconducting Polymer/Carbon Nanotube Thin Films for Light-Emitting Transistors

We investigate the influence of small amounts of semiconducting single-walled carbon nanotubes (SWNTs) dispersed in polyfluorenes such as poly(9,9-di-<i>n</i>-octylfluorene-<i>alt</i>-benzothiadiazole (F8BT) and poly(9,9-dioctylfluorene) (F8) on device characteristics of bottom contact/top gate ambipolar light-emitting field-effect transistors (LEFETs) based on these conjugated polymers. We find that the presence of SWNTs within the semiconducting layer at concentrations below the percolation limit significantly increases both hole and electron injection, even for a large band gap semiconductor like F8, without leading to significant luminescence quenching of the conjugated polymer. As a result of the reduced contact resistance and lower threshold voltages, larger ambipolar currents and thus brighter light emission are observed. We examine possible mechanisms of this effect such as energy level alignment, reduced bulk resistance above the contacts, and field-enhanced injection at the nanotube tips. The observed ambipolar injection improvement is applicable to most conjugated polymers in staggered transistor configurations or similar organic electronic devices where injection barriers are an issue

Electrooptical Spectroscopy of Uniaxially Aligned Polythiophene Films in Field-Effect Transistors

Charge carriers induced in field-effect transistors based on a uniaxially aligned polythiophene polymer, poly­(2,5-bis­(3-alkylthiophen-2-yl)­thieno­[3,2-b]­thiophene) (PBTTT), are investigated by electrooptical charge modulation spectroscopy (CMS). We used a zone-casting deposition method for uniaxial alignment of the nanoribbon crystalline domains of the polymer and examined the optical anisotropy of neutral and charge induced absorptions in FETs. We find that the charge-induced optical absorptions of the polaronic carriers at the interface exhibit a similar degree of uniaxial anisotropy as the absorption spectrum of the neutral polymer bulk suggesting similar degree of polymer alignment at the interface compared with the bulk. We observe subtle differences in the spectral shape of the polaron absorption polarized along and perpendicular to the polymer chain direction. We also detect an additional charge-induced absorption peak appearing at high charge carrier concentrations, which is similar to the charge-induced absorption that is typical for chemically doped films. These observations provide important insight into the interplay between polaron transport and polymer microstructure

Fast Holes, Slow Electrons, and Medium Control of Polaron Size and Mobility in the DA Polymer F8BT

The nature of electron and hole polarons on poly­(9,9-di-<i>n</i>-hexylfluorenyl-2,7-diyl) (pF) and a copolymer poly­[(9,9-di-<i>n</i>-octylfluorenyl-2,7-diyl)-alt-(benzo­[2,1,3]­thiadiazol-4,8-diyl)] (F8BT) has been studied by chemical doping, pulse radiolysis, charge modulation spectroscopy, quantum chemical calculations, and microwave conductivity. While pF exhibits very similar behavior in all respects for the electron and the hole, this paper explores the hypothesis that the donor acceptor (push–pull) nature of F8BT will tend to localize charges. Optical spectra and quantum chemical calculations point to an electron localized on the thiadiazole unit in polar liquids but becoming more delocalized as the solvent polarity decreases. Indeed, in the nonpolar solvent benzene, the electron mobility is only 2.7 times lower than that of the hole, which conversely is shown to be delocalized in all environments and has a similar mobility to polarons on the homopolymer polyfluorene. Advantageous modifications to the optoelectronic properties of conjugated polymers that come about by using alternating donor acceptor repeat units have thus been shown to not significantly hinder charge transport despite the corrugated energy landscape along the backbone

Microstructure of Polycrystalline PBTTT Films: Domain Mapping and Structure Formation

We utilize near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and scanning transmission X-ray microscopy (STXM) to study the microstructure and domain structure of polycrystalline films of the semiconducting polymer poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-<i>b</i>]thiophene) (PBTTT). Total electron yield NEXAFS spectroscopy is used to examine the surface structure of the first 1–2 molecular layers, while bulk-sensitive STXM is used to produce maps of domain orientation and order sampled through the entire film thickness. We study different phases of PBTTT including as-cast, terraced and nanoribbon morphologies produced <i>via</i> spin-coating as well as aligned films of as-cast and nanoribbon morphologies produced by zone-casting. For the terraced morphology, domains are observed that are larger than the size of the terraced surface features, and the calculated degree of order is reduced compared to the nanoribbon morphology. For zone-cast films, we find that, although little optical anisotropy is observed in the bulk of as-cast films, a high degree of surface structural anisotropy is observed with NEXAFS spectroscopy, similar to what is observed in annealed nanoribbon films. This observation indicates that the aligned surface structure in unannealed zone-cast films templates the bulk ordering of the aligned nanoribbon phase. STXM domain mapping of aligned nanoribbon films reveals elongated, micrometer-wide domains with each domain misoriented with respect to its neighbor by up to 45°, but with broad domain boundaries. Within each nanoribbon domain, a high degree of X-ray dichroism is observed, indicating correlated ordering throughout the bulk of the film

Improved Performance of ZnO/Polymer Hybrid Photovoltaic Devices by Combining Metal Oxide Doping and Interfacial Modification

Photoinduced charge separation at hybrid organic–inorganic interfaces is poorly understood and challenging to control. We investigate charge separation at a model system of ZnO/poly­(3-hexylthiophene) (P3HT) and employ Sr doping of ZnO and phenyl-C61-butyric acid (PCBA) self-assembled modification to study and enhance the charge separation efficiency. We find that doping alone lowers the efficiency of charge separation due to the introduction of defect states at the oxide surface. However, with the combination of doping and molecular modification, charge separation efficiency is significantly enhanced due to the passivation of interfacial traps and improved modifier coverage. This demonstrates a complex noncumulative effect of doping and surface modification and shows that with the correct choice of metal oxide dopant and organic modifier, a poorly performing hybrid interface can be turned into an efficient one

High Performance, Low Temperature Solution-Processed Barium and Strontium Doped Oxide Thin Film Transistors

Amorphous mixed metal oxides are emerging as high performance semiconductors for thin film transistor (TFT) applications, with indium gallium zinc oxide, InGaZnO (IGZO), being one of the most widely studied and best performing systems. Here, we investigate alkaline earth (barium or strontium) doped InBa­(Sr)­ZnO as alternative, semiconducting channel layers and compare their performance of the electrical stress stability with IGZO. In films fabricated by solution-processing from metal alkoxide precursors and annealed to 450 °C we achieve high field-effect electron mobility up to 26 cm² V^–1 s^–1. We show that it is possible to solution-process these materials at low process temperature (225–200 °C yielding mobilities up to 4.4 cm² V^–1 s^–1) and demonstrate a facile “ink-on-demand” process for these materials which utilizes the alcoholysis reaction of alkyl metal precursors to negate the need for complex synthesis and purification protocols. Electrical bias stress measurements which can serve as a figure of merit for performance stability for a TFT device reveal Sr- and Ba-doped semiconductors to exhibit enhanced electrical stability and reduced threshold voltage shift compared to IGZO irrespective of the process temperature and preparation method. This enhancement in stability can be attributed to the higher Gibbs energy of oxidation of barium and strontium compared to gallium

Effect of Ozone on the Stability of Solution-Processed Anthradithiophene-Based Organic Field-Effect Transistors

We have investigated the degradation effects of ozone exposure on organic field-effect transistors based on 2,8-difluoro-5,11-bis­(triethylsilylethynyl)­anthradithiophene as the organic semiconducting channel layer, as well as on thin films of this widely used, high-mobility, small molecule semiconductor. Electrical <i>I</i>–<i>V</i> measurements showed a loss of transistor characteristic behavior. We present ¹H Nuclear Magnetic Resonance (NMR) spectroscopy results as well as X-ray Photoemission Spectroscopy (XPS) and Fourier Transform Infrared (FTIR) spectroscopy measurements showing the oxidation of the parent molecule, from which we suggest various possible reaction paths

Structure Influence on Charge Transport in Naphthalenediimide–Thiophene Copolymers

Reported here is a characterization of a series of NDI–thiophene copolymers with one, two, three, and four thiophene units synthesized using Stille polycondensation of dibromo-naphthalene diimide and the trimethylstannylthiophene monomers. The effect of extension of the thiophene donor group is studied in terms of structure-charge transport correlation. The influence of side chains located on the thiophene units of copolymers with two and four thiophene units per monomer is also investigated. Charge transport of both signs is studied experimentally in field-effect transistors. Microstructural data obtained by near-edge X-ray absorption fine structure (NEXAFS) and grazing incidence wide-angle X-ray scattering (GIWAXS) is supported by AFM topography scans. Ultraviolet photoelectron spectroscopy (UPS) and UV–vis spectroscopy data are employed in the measurement of energy levels, and changes with annealing temperature are also discussed. Most of the polymers reach excellent electron and hole mobility with one copolymer (NDI-T4) exhibiting an especially balanced ambipolar charge transport of 0.03 cm² V^–1 s^–1. An odd–even effect in hole mobility is observed with higher values for polymers with an even number of thiophene units. The reported findings indicate that the final charge transport properties are a result of the interplay of many factors, including crystallinity, planarity and linearity of chain, spacing between acceptor units and packing of solubilizing branched side chains

Charge Mobility Enhancement for Conjugated DPP-Selenophene Polymer by Simply Replacing One Bulky Branching Alkyl Chain with Linear One at Each DPP Unit

We demonstrate a simple, but efficient, approach for improving the semiconducting performances of DPP-based conjugated D-A polymers. This approach involves the replacement of one bulky branching alkyl chain with the linear one at each DPP unit in regular polymer PDPPSe-10 and PDPPSe-12. The UV–vis absorption, Raman spectra, PDS data, and theoretical calculations support that the replacement of bulky branching chains with linear ones can weaken the steric hindrance, and accordingly conjugated backbones become more planar and rigid. GIWAXS data show that the incorporation of linear alkyl chains as in PDPPSe-10 and PDPPSe-12 is beneficial for side-chain interdigitation and interchain dense packing, leading to improvement of interchain packing order and thin film crystallinity by comparing with PDPPSe, which contains branching alkyl chains. On the basis of field-effect transistor (FET) studies, charge mobilities of PDPPSe-10 and PDPPSe-12 are remarkably enhanced. Hole mobilities of PDPPSe-10 and PDPPSe-12 in air are boosted to 8.1 and 9.4 cm² V^–1 s^–1, which are about 6 and 7 times, respectively, than that of PDPPSe (1.35 cm² V^–1 s^–1). Furthermore, both PDPPSe-10 and PDPPSe-12 behave as ambipolar semiconductors under a nitrogen atmosphere with increased hole/electron mobilities up to 6.5/0.48 cm² V^–1 s^–1 and 7.9/0.79 cm² V^–1 s^–1, respectively