Session E11: Organic Electronics - Applied Transport

Mechanisms of transport and electron transfer at conductive polymer/liquid interfaces

Organic semiconductors (OSCs) have incredible prospects for next-generation, flexible electronic devices including bioelectronics, thermoelectrics, opto-electronics, and energy storage and conversion devices. Yet many fundamental challenges still exist. First, solution processing prohibits definitive control over microstructure, which is fundamental for controlling electrical, ionic, and thermal transport properties. Second, OSCs generally suffer from poor electrical conductivities due to a combination of low carriers and low mobility. Third, polymeric semiconductors have potential-dependent, dynamically evolving electronic and chemical states, leading to complex interfacial charge transfer properties in contact with liquids. This talk will focus on the use of alternative synthetic strategies of oxidative chemical vapor deposition and electrochemical deposition to control physical, electronic, and chemical structure. We couple our synthetic efforts with energy-, time-, and spatially resolved spectroelectrochemical and microscopy techniques to understand the critical interfacial chemistry-microstructure-property relationships: first at the macroscale, and then moving towards the nanoscale. In particular, approaches to better understand electron transfer events at polymer/liquid interfaces as a function of: 1.) chemical composition; 2.) electronic density of states (DOS); and 3.) crystallinity and microstructure will be discussed.   

Electrochemical Activation of Radical Polymers for Transparent, Ambipolar Organic Transistors

Closed-shell macromolecules, especially $\pi $-conjugated polymers, have been widely investigated as charge transporting materials for organic electronic devices. Despite their tremendous progress, there are still a number of demands for more functionality in these systems. To meet these critical needs, we firstly introduce a non-conjugated oxidation-reduction (redox) active polymer transistor with open-shelled, charge transporting macromolecules, radical polymers. A large population of the stable redox sites in the radical polymer allow for the rapid electron transfer in solid state. The charge transport mechanism of a model radical polymer, poly(2,2,6,6-tetramethylpiperidine-1-oxyl methacrylate) (PTMA), is established through the application of an ion gel gate. In this system, the ion gel gate acts as a solid electrolyte for ionic penetration into the PTMA layer by low gate potentials. In addition, the balanced redox nature leads to a transparent transistor with ambipolar characteristics. As a result, the activated charge mobility and conductivity values of PTMA are comparable to common semiconducting polymers. Furthermore, the electrochemical doping of PTMA produces ON/OFF current ratios of \sim 10$^{4}$ indicative of high performance as an organic transistor.   

A 2D nanowire network of conjugated polymers and small molecules for sensitive and discriminative biochemical detection

Field effect transistor (FET) biochemical sensors offer high sensitivity and selectivity, along with portable, real-time and low-cost analyte detection, overcoming many drawbacks associated with traditional techniques such as gas chromatography - mass spectrometry. However, most FET sensors rely on receptors that provide specific binding with the target analyte, which present fundamental limitations for at least two reasons: first, sensors with high specificity to many compounds are difficult to produce due to limited availability of receptors and unavoidable non-specific absorption effects, and second, thousands of discrete sensors are needed to discriminate between common analytes. In contrast, the mammalian olfactory system employs a combinatorial receptor coding scheme to identify odors, rather than distinguishing each odorant by a specific receptor. Here we show that orthogonal ambipolar semiconducting nanowire networks formed by aligned growth of crystals in mixtures of pi-conjugated polymers and small molecules offer a highly sensitive and inherently multi-dimensional platform to discriminate multiple analytes based on multiple different electronic read-outs from a single material layer. Such a platform offers new opportunities in artificial nose design.   

Multilayer polymer electrets and their effects in organic electronics1

Thin film polymer electrets are of considerable interest for applications in organo-electronic systems, such as piezoelectrics, field effect transistors, and information storage. By copolymerizing combinations of substituted styrenes we have fabricated multifunctional polymers with tunable electronic and structural properties.\footnote{O.J. Alley {\em et al.}, Macromolecules, {\bf 49}, 3478 (2016)} These polymers can be used to create structured electrets to control static charge and enhance its effect in organo-electronic systems. Copolymerization with vinylbenzo(4-cyclobutene) enables creation of thermally cross-linkable thin films resilient to subsequent exposure to solvent. This permits sequential deposition via spincoating of multilayer dielectric stacks with well-defined interface widths in the range 1.5 – 4 nm, as determined by neutron reflectometry. Via inclusion of fluorinated moieties and/or chargeable groups, such as triphenylamines, at controlled positions in the stacks, we have demonstrated multilayer polymer devices with tunable electronic properties such as reduced gate bias effects or large nontransient threshhold voltage shifts \((\Delta V_{th}=O(30V)\)) as measured in pentacene based thin film transistors.   

Strain-stabilized organic semiconductor thin films leads to mobility enhancement

Optical reflection spectroscopy and grazing incidence wide angle X-ray diffraction are used to study the structure evolution of 6,13-bis(trisopropylsilylethynyl)-pentacene thin films versus deposition temperature. Strain-free solid films exhibit a temperature-dependent blue shift of absorption peaks due to a continuous thermally driven change of the crystalline packing. The strain-stabilization of the high-mobility polymorph known as Form II has been studied in detail. As crystalline films are cooled to room temperature they become strained although cracking of thicker films is observed, which allows the strain to partially relax. Below a critical thickness, cracking is not observed and the films are constrained to the lattice constants corresponding to the temperature at which they were deposited. To study the strain effect on the mobility, aligned thin films are obtained in a temperature range from 25ûC to 135ûC. Blending polar solvents with different vapor pressures leads to well aligned, extremely thin films (\textless 30 nm). We observed that the mobility is greatly improved when the film is more strained. The effect of strain and gate voltage on contact resistance will be studied and reported.   

Understanding polymorph accessibility in molecular semiconductor thin films

We investigated the polymorphic stability and reversibility of thin films of a series of core-halogenated naphthalene tetracarboxylic diimides (NTCDIs) with varying alkyl substitutions. Each derivative adopts its $\beta $-phase upon thermal evaporation; post-deposition processing via solvent-vapor annealing converts the $\beta $-phase to the $\alpha $-phase in two of the four derivatives. NTCDIs adopt layered structures driven by strong intralayer $\pi $-$\pi $ interactions and weaker interlayer van der Waals interactions. The presence of interlayer short contacts effectively locks the structure in place, preventing any molecular reorganization upon post-deposition processing. The absence of such short interlayer contacts is instead correlated with reversible access of both polymorphs; judicious selection of post-deposition processing conditions tunes which polymorph is accessed. This finding is generalizable across a wide range of molecular semiconductors that adopt layered structures, including derivatives of benzothiophene and functionalized acenes. Beyond organic electronics, this finding has implications on pharmaceutics and the food industry as polymorphic stability governs bioavailability and solubility.   

Bias stress in PDI-CN2 and P3HT studied with Kelvin Probe Force Microscopy

We have developed a technique that uses scanning probe microscopy (SPM) to study the real-time injection and extraction of charge carriers in organic semiconductor devices. We investigate PDI-CN2 and P3HT in a back gate field effect transistor geometry with gold electrodes. By positioning the SPM tip at an individual location and using Kelvin probe microscopy to record the potential over time, we can record how the charge carriers respond to changing the gate voltage while the source and drain electrodes are grounded. We see relatively fast screening when carriers are injected into the film. The screening is slower when carriers must escape from traps to exit the film. By incrementally stepping the gate voltage, we can probe different trap depths. By repeating the measurement, we observe the development of longer lived trap states, shown by the longer time recorded to fully screen the gate voltage.   

Elucidating the Impact of the Full Molecular Weight Distribution on Charge Transport in Semiconducting Polymers

Previous reports on the molecular-weight (MW) dependence of charge transport in polymeric semiconductors have focused on individual ensemble-average parameters, such as the weight- or number-average MWs or the polydispersity index. These quantities do not fully describe the molecular weight distributions (MWDs), especially when the distribution is multimodal. Field-effect mobilities have been reported to vary substantially in transistors comprising polymer semiconductors having characteristically different MWDs that are quantified by nominally the same ensemble-average properties. Starting with batches of PCDTPT and P3HT having narrow and distinct MWDs -- accessed through Soxhlet fractionation and controlled polymerization, respectively -- we created homopolymer blends whose MWDs are fully quantified by diffusion-ordered NMR spectroscopy (DOSY). While the mobility of transistors comprising these blends generally increases with increasing loadings of the highest-MW fraction, subtle but quantifiable differences in mobility exist and they reflect differences in the shape of the MWDs of the polymer semiconductors, which we attribute to differences in interdomain connectivity by tie-chains in the active channels.   

Carrier injection of electrons and holes by nanostructure surface control in field effect transistors

Carrier injection from metal electrodes to organic charge transfer layers can greatly be improved for both electrons and holes by nano-structural surface control in organic field effect transistors. We demonstrate a stark contrast for a 2,5-bis(4-biphenylyl)bithiophene (BP2T) and rubrene (RU) active semiconducting layer grown on a modified SiO$_{\mathrm{2}}$ dielectric gate insulator between two different modifications of tetratetracontane and poly(methyl methacrylate) thin films. Important evidence that the field effect transistor (FET) characteristics are independent of electrode metals with different work functions is given by the observation of a conversion of the metal-semiconductor contact from the Schottky limit to the Bardeen limit. Equivalent high efficient carrier injection of both holes and electrons is exemplifed and air-stable light emitting FETs are demonstrated.   

Impact of the air gap in nanowire array transistors

Organic and inorganic semiconducting nanowires are promising for flexible electronic, energy harvesting, and sensing applications. Nanowire arrays processed from solution are particularly attractive for their ease of processing coupled with their potential for high performance. Random stacking has been observed, however, to hinder the collective electrical performance of such nanowire arrays. Here, we employ solution-processed organic semiconducting nanowires as a model system to assess the impact of the air gap that exists under a large portion of the active material in nanowire array transistors. Confocal Raman spectroscopy is used to non-invasively quantify the average air gap thickness which is found to be unexpectedly large - two to three times the nanowire diameter. This substantial air gap acts as an additional dielectric layer that diminishes the buildup of charge carriers, and can affect the measured charge carrier mobility and current on/off ratio by more than one order of magnitude. These results establish the importance of taking the air gap into account when fabricating and analyzing the performance of transistors based on one-dimensional nanostructures, such as organic and inorganic nanowires, or carbon nanotubes.