Session X50: Organic Electronics IV: Doping and Electronics

Rapid and non-destructive optical patterning of conjugated polymers for device applications

A significant obstacle for the industrial development of organic electronic devices is the lack of a patterning technology having the disruptive power that photolithography exerted in traditional microelectronics. Here we present a new scalable patterning technology for organic semiconductors that takes advantage of the existing photolithography infrastructure and is compatible with digital direct-write patterning and sequential roll-to-roll (R2R) solution coating. The Moule group works on a series of solubility control techniques including the use of marginal solvents and polymer doping, that reduce the solubility of polymers at room temperature, but allow patterning at elevated temperatures. Using these techniques, we are able to vertically stack and laterally pattern mutually soluble polymer layers, which are vital processing steps needed to expand the use of organic semiconductors in device applications. Optimization of these techniques has yielded diffraction limited film patterning with regular features of 200-300 nm with only solution processing steps and direct write laser patterning. We have also recently shown that vertically patterned layers are stable, even with solvent exposure times of hours. This presentation will cover the fundamentals of optical patterning of organic semiconductors and delve into details of how to create doped microdomains, dopant diffusion, and the relationship between polymer crystallinity, dopant diffusion rate, and pattern fidelity.
  

Branched Side Chains Influence the Efficacy of Doping in Conjugated Polymers

The principles that govern effective charge transfer between dopants and semiconducting polymers are poorly understood. It is currently unclear how the position of the dopant in the thin film can affect carrier mobility. Here, we report the evolution in spectroscopic and electrical properties of a model conjugated polymer upon exposure to two types of dopants: a strong oxidant (F₄TCNQ) and a strong acid (HTFSI). The model polymer was poly(3-(2′-ethyl)hexylthiophene) (P3EHT), a branched side chain analogue of the well-characterized polymer P3HT. We find that F₄TCNQ forms a charge transfer complex (CTC) with P3EHT resulting in a maximum electrical conductivity of 3×10^–5 S cm^–1. We postulate that the branched side chains of P3EHT constrain the position of F₄TCNQ within the P3EHT crystallites, resulting in partial charge transfer between the donor and acceptor. Conversely, protonation of the polymeric backbone from HTFSI increases the electrical conductivity of P3EHT to 4×10^–3 S cm^–1, two orders of magnitude higher than when F₄TCNQ is used. This work shows that a favorable energetic offset between the donor and acceptor is not sufficient to predict the charge transfer mechanism, but also relies on structural constraints of incorporating a dopant molecule into the polymer film.   

Understanding the molecular doping process of semiconducting polymers through In-situ conductivity measurements and structural characterization

Polymers with conjugated semiconducting backbones show promise for use in organic electronics, such as thin film transistors, organic photovoltaics and the emerging technology of organic thermoelectrics. Molecular doping is a vital step in the controlling the charge transport of semiconducting polymers. Here, we report on how molecular doping affects the conductivity of polythiophene-based polymers over the course of vapor doping, and how different dopants (fluorinated TCNQs) with variable HOMO-LUMO overlap with our polymers affect the observed conductivity trends. Through in situ conductivity experiments, we show that for all three dopants tested, the conductivity follows a distinct profile that shows rapid increase of conductivity up until an optimal time, after which it falls and then equilibrates between 90% to 50% of the optimal conductivity. In conjunction with our in situ conductivity experiments, we report on scattering and spectroscopy experiments such as GIWAXS, UV-Vis, and EPR spectroscopy on polymer-dopant films at various time-points throughout the doping process. This study shows the potential of these methodologies for use on various polymer-dopant pairings to further advance our understanding of molecular doping dynamics.   

Electrical and magnetic characterization of doped conjugated polymers with pendent stable radicals

Conjugated polymers such as poly(3-hexylthiophene-2,5-diyl) (P3HT) are being considered as potential backbone materials for organic radical battery electrodes to increase conductivity over unconjugated backbones, without sacrificing the electrochemical activity of radical groups. This is important for efficient current collection. Although conjugated polymers are among the most conductive organic materials at ambient temperatures, covalent attachment of stable radical such as such as 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) to undoped, regioregular P3HT creates steric hindrances that result in an exponential decrease of film conductivity as a function of radical content. To recover the conductivity, we dope the conjugated backbone using either iodine or 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). We characterize electrical properties of the doped conjugated radical polymer by varying both the pendent radical concentration and the doping level. Additionally, we used electron paramagnetic resonance to investigate the interplay between polarons and stable radical electrons. Transport measurements confirm that the conductivity increases by a few to several orders of magnitude, depending on the dopant.   

Electronic and Optical Properties of N-doped BBL Polymer

Since their discovery, conducting polymers have been widely studied over the last few years in many optoelectronic applications. Applying redox chemistry, the intrinsic conductivity of the polymers could be altered by adding/removing an electron to make the polymer as an n-type/p-type material, respectively. The charge transport in electronic devices depends on the performance of both the p-type and n-type materials. P-doped polymers have been extensively studied over n-doped polymers due to lack of stable n-doped polymers. The need of the hour is to study n-type polymer to improve the performance of organic electronic devices. Here we study n-doped poly(benzobisimidazobenzophenanthroline) (BBL) using ground-state and time-dependent (TD) density functional theory (DFT). High electron mobilities (~0.1 cm² V^–1 s⁻¹), high structural and thermo-oxidative stability make BBL as an ideal n-doped polymer. To understand the electronic structure and optical spectroscopy of n-doped BBL, a thorough theoretical study is carried out. Formation of polaron and bipolaron due to the reduction of BBL is analyzed by ωB97XD functionals DFT with the 6-31+G(d) basis set. UV visible absorption spectra from TDDFT calculations show the transition from polaron/bipolaron to the conduction band.   

Non-conjugated Radical Polymers as Transparent Conductors in Organic Electronic Devices

Radical polymers are an emerging class of organic electronic materials that are composed of a non-conjugated macromolecular backbone and with stable open-shell moieties present on their pendant groups. However, their utilization in solid-state organic electronic devices has been limited due to the commonly-held belief that this lack of conjugation impedes their ability to conduct charge effectively in the solid state. Through a combination of experiment and computation we alter this archetype, and we demonstrate that pristine (i.e., not doped) radical polymers are able to achieve relatively high electrical conductivity values (i.e., > 20 S m^-1) near room temperature through appropriate polymer processing. This places our radical polymer electrical conductivity on par with many common commercial grades of doped conjugated polymers. Moreover, the non-conjugated macromolecular backbones of radical polymers allow for high optical transparency values to be achieved across the visible spectrum. As such, we demonstrate how these macromolecules are successfully incorporated as transparent conducting thin films in organic photovoltaic cells, perovskite solar cells, and electrochromic modules in order to improve the performance and lifetime of these next-generation optoelectronic devices.   

Modeling Charge Transport in Insulating Materials for High Voltage Direct Current (HVDC) Application

High Voltage Direct-Current (HVDC) is rapidly growing for long-distance power transmission applications where typical AC solutions are not efficient (generally >80km). However, extruded insulation material systems utilized in AC applications are challenged in DC applications due to the accumulation of injected charge within the insulation, which can significantly alter the electrical field and subject the insulation system to electrical stresses that are many times the “design stress” (Laplacian). Charge accumulation within polymeric insulating systems has been related to the temperature and stress-dependence of conductivity. A steady-state solution for the space charge profile and resulting electric field has been developed. However, the assumption of steady state avoids the treatment of charge injection and the rate of charge transport, and may not capture transient phenomena which are entirely relevant for the DC application. Therefore, a computational model of the transient development of the space charge field can provide valuable insight into the development of materials suitable for extruded DC power cables. We developed a finite element analysis method treating the charge injection process and conduction process based on a trapped-charge hopping mechanism.   

High-k and High-Temperature Polymer Dielectrics for Electric Energy Storage and Organic Electrics

Dipolar glass polymers are attractive for high-k and low-loss dielectric applications because of the enhanced orientational polarization from highly dipolar groups, e.g., amide and sulfone groups. In this work, we explored the opportunity of side-chain dipolar glass polymers as advanced dielectric materials for polymer film capacitor and gate dielectric in organic electronics. We design a new class of high-temperature side-chain dipolar glass polymer, sulfonylated poly(2,6-dimethyl-1,4-phenylene oxide) (SO₂-PPO). No cooperative segmental motion or ferroelectric loss was observed for these side-chain dipolar glass polymers. Due to efficient rotation of highly polar methylsulfonyl side groups below the glass transition temperature (T_g ~ 220 °C), enhanced dipolar polarization and thus high dielectric constant of 6-8 was obtained for these SO₂-PPOs. The discharged energy density reached as high as 22 J/cm³. Due to its high T_g, the SO₂-PPO₂₅ film exhibited a low dielectric loss and low leakage current. These side-chain dipolar glass polymers are promising for high-temperature and low-loss electric energy storage and electronic applications.   

Magneto-electrical signatures of spin dependent processes in organic semiconductors

Room temperature quantum and coherent spin phenomena in organic semiconductors have recently gained interest in the scientific community. Besides the potential for future developments towards quantum-based sensors, computing, and information, established technologies like organic light emitting diodes and organic photovoltaics can benefit near term from harnessing higher order effects such as triplet-triplet annihilation (TTA) and singlet fission (SF). Recently, an observed “half-bandgap” emission in small molecule-based OLEDs has been attributed to a Dexter transfer of triplet charge transfer states into triplet exciton states, followed by triplet-triplet annihilation to produce an emitting singlet. We will present on key requirements to be able to observe TTA in Rubrene and DiFTES-ADT, in heterojunction-based OLED devices test structures. We will show that a sub-bandgap turn-on does not require higher order recombination processes such as TTA and is thus not a reliable measure. However, if competing interfacial recombination pathways are suppressed, TTA can be readily observed in the luminescence-current density-voltage characteristics (L-J-V) at mid to high current densities. We will discuss how these processes relate to the observed Magneto-electro-luminescence response.   

Programmable molecular-scale diode based on standard molecules/2D semiconductor hybrid molecular junction

Since Aviram and Ratner initially proposed the possibility of a molecular-scale rectifier in 1974, diverse type of molecular diodes driven by a specific molecular itself or the asymmetric coupling has been extensively demonstrated [1-2]. In this study, we propose a new class of molecular diode based on a hybrid molecular junction system that is composed with the 2D semiconductor (MoS₂ or WSe₂) and the standard self-assembled monolayer (SAM) (alkyl- or conjugated molecules). The 2D semiconductor and the SAM are sandwiched between the Au probe tip and Au bottom electrode using conductive atomic force microscopy (CAFM) technique. In the case of the molecular junction with monolayer MoS₂ and OPT2, the diode feature with rectification ratio > ~10³ was observed. Furthermore, the rectification ratio can be programmed according to the number of MoS₂ layers, the type of 2D semiconductor, molecular length, and molecular group. Our suggested rectifier architecture can provide potential benefits to simply implement the molecular diode function and propose the idea to improve the diode performance.
[1] Avriram, A.; Ratner, M. A. Chem. Phys. Lett. 1974, 29, 277-283
[2] Diez-Perez, I.; Hihath, J.; Lee, Y.; Yu, L.; Adamska, L.; Kozhushner, M. A.; Oleynik, I. I.; Tao, N. Nat. Chem. 2009, 1, 635-641   

The Structural, Electronic and Optical Properties of γ-glycine Under Pressure: A First Principles Study

Non-linear devices and photonics are set to be prevalent in our everyday lives with future applications in quantum optics, plasma physics, and laser manufacturing.The crystallized amino acid γ-glycine is a large band gap insulator that shows promise in the aforementioned fields. In order to better understand its physical properties, the effect of pressure on the structural, electronic and optical properties of γ-glycine were investigated through a first principles calculation approach based on density functional theory. A band gap of 5.026 eV was found, and was shown to decrease with an increase of pressure, due to the widening of the conduction and valence bands which is reaffirmed through the densities of states. Furthermore, the DOS establishes that the conduction bands are mainly dominated by the O 2p and C 2p orbitals and that γ-glycine exhibits electronic stability under 0.0-3.0 GPa. The absorption spectra was calculated using Many-Body Green’s functions GW, which both revealed a slight blueshift in the absorption spectra with an absorption peak in the deep UV region, and verifies the validity of γ-glyince as a prospective candidate for NLOs. The information presented in this paper might be useful in providing insight for the application of γ-glycine in future technology..   

Electro-absorption in Metal Nanoparticles within Glass; Comparison with Quantum Dots in Nonconjugated Conductive Polymers

Quadratic electro-optic effect / Kerr coefficient of metal nanoparticles within glass and comparison with iodine doped nonconjugated conductive polymers have been recently reported. In this report, we will discuss results of the measurement of electro-absorption in gold nanoparticles in glass and compare the results with published data on electro-absorption in quantum dots within doped nonconjugated conductive polymers. The electro-absorption was found to increase quadratically with the applied electric field. The magnitude of electro-absorption appears to increase as d^-3, where d is diameter of metal nanoparticle. More detailed work on this is in progress. This dependence on d is similar to the recently reported results on Kerr coefficient. This is expected since electro-absorption is proportional to the imaginary part of χ⁽³⁾ and Kerr coefficient is proportional to the real part of the same.