Session X33: Organic Electronics and Photonics - Organic Electronic Devices

Charge Transport Properties in Polymer Brushes.

Electrical transport properties in poly(3-methyl)thiophene (P3MT) brushes have been studied. The P3MT brushes correspond to a new type of surface-tethered, vertically oriented conjugated molecular wires, sandwiched between two metallic electrodes to form the electrode-molecule-electrode (EME) devices. P3MT is a highly conjugated polymer, a "workhorse" material for organic electronics and photonics. The P3MT brushes were grown on ITO surfaces with controlled length (between 2 and 100 nm). The top electrodes were transfer-printed Au films with lateral dimensions between 200 nm and 50 $\mu$m. I-V and differential conductance measurements were performed using conductive AFM and 4-terminal techniques. Tunneling and field-emission measurements in EME devices with molecular lengths $<$ 5 nm show HOMO mediated direct hole tunneling with energy barriers of 0.3 and 0.5 eV at the respective interfaces with ITO and Au. The transport properties in longer brushes are indicative of the two quasi-Ohmic interfaces with a characteristic offset in the conductance minimum of 0.12 V biased toward the ITO. Temperature dependent parameters have been examined at various molecular lengths. The drift mobility and the interplay between intra- and intermolecular transport have been investigated.   

Hall effect and band-like carrier transport in high mobility polymer transistors

A microscopic understanding of charge carrier transport in polymeric semiconductors is essential to improve the state of the art of flexible or printed electronic devices. In particular, thin-film field-effect transistors based on donor-acceptor polymers are in the focus of current literature reaching high charge-carrier mobility. In this work, we demonstrate the Hall effect and the temperature dependence of the charge carrier mobility based on uniaxially ordered CDT-BTZ donor-acceptor copolymer films. Uniaxially ordered films of CDT-BTZ with hexadecyl (C16) and eicosyl (C20) sidechains showed mobility of 5.6 cm$^{\mathrm{2}}$/Vs and 11.4 cm$^{\mathrm{2}}$/Vs respectively. The activation energy of the mobility decreased with the increasing carrier density, and finally the negative temperature dependence of the mobility was observed. Both polymers showed Hall effect, which also indicates the presence of extended electronic states. The temperature and carrier density dependence will be further discussed in the presentation.   

Growth and characterization of organic ferroelectric croconic acid thin films

Using vapor phase evaporation, we have studied the growth of the croconic acid (CCA) thin films, at various conditions such as temperature, thickness, growth speed, and substrates. The morphology of thin film was measured by atomic force microscopy (AFM); the ferroelectric property was confirmed by piezoresponse force microscopy (PFM). A critical thickness of 40 nm and optimal temperature of -30 celsius were found for continuous films, while the substrate and growth speed are found to play a minimal role. According to the reflection high energy electron diffraction (RHEED), the CCA films are polycrystalline. For a 40 nm continuous film, the roughness is about 3 nm, while the coercive voltage for the ferroelectric domain switching is approximately 7V. This is the first molecule ferroelectric thin film. The successful growth of continuous CCA films enhances the applications potential of CCA, which is a molecular crystal of ferroelectricity.   

\textbf{Gate-controlled energy barrier at a graphene/molecular semiconductor junction}

The formation of an energy barrier at a metal/molecular semiconductor junction is both a ubiquitous phenomenon as well as the subject of intense research in order to improve the performance of molecular semiconductor-based electronic and optoelectronic devices. For these devices, a junction with a large energy barrier provides rectification, leading to a diode behavior, whereas a relatively small energy barrier provides nearly-ohmic behavior, resulting in efficient carrier injection (extraction) into the molecular semiconductor. Typically, a specific metal/molecular semiconductor combination leads to a fixed energy barrier; therefore, the possibility of a gate-controlled energy barrier is very appealing for advanced applications. Here [S. Parui \textit{et al}, Adv. Fun. Mat. \textbf{25}, 2972 (2015)], we present a graphene/C$_{\mathrm{60}}$ junction-based vertical field-effect transistor in which we demonstrate control of the interfacial energy-barrier such that the junction switches from a highly rectifying diode at negative gate voltages to a nearly-ohmic behavior at positive gate voltages and at room temperature. We extract an energy-barrier modulation of up to 660 meV, a transconductance of up to five orders of magnitude and a gate-modulated photocurrent.   

Influence of Morphological Disorder on In- and Out-of-Plane Charge Transport in Conjugated Polymer Films

We report the unequal impacts of morphological disorder on in- and out-of-plane charge transport in thin films of poly(3-hexylthiophene) (P3HT) fabricated by both conventional spin-casting and the novel technique Matrix-Assisted Pulsed Laser Evaporation (MAPLE). MAPLE produces films with inhomogeneous globular subfeatures with dimensions on the order of 100 nm. Optical absorbance spectroscopy corroborates that MAPLE-deposited films are more energetically disordered, but possesses average conjugation lengths comparable to spin-cast P3HT. Both in- and out-of-plane carrier transport measurements of MAPLE-deposited films show characteristics that reflect a higher degree of energetic disorder and broadened density of states. Whereas in-plane carrier mobilities of MAPLE-deposited thin-film transistors are comparable to spin-cast analogues (8.3 x 10$^{-3}$ cm$^{2}$V$^{-1}$s$^{-1}$ versus 5.5 x 10$^{-3}$ cm$^{2}$V$^{-1}$s$^{-1})$, the out-of-plane mobilities of MAPLE-deposited samples are nearly an order of magnitude lower (4.1 x 10$^{-4}$ cm$^{2}$V$^{-1}$s$^{-1}$ versus 2.7 x 10$^{-3}$ cm$^{2}$V$^{-1}$s$^{-1})$. The unusual ensemble of properties and behaviors arising from the unique morphologies produced by MAPLE provide important perspectives on the extent to which disorder impacts different mechanisms of charge transport in conjugated polymers.   

Polarization-induced transport in TIPS-pentacene field-effect transistors

The dielectric constant of polymer ferroelectric dielectrics can be tuned by changing the temperature, offering a platform for monitoring the changes in interfacial transport in organic field-effect transistors (FETs), as the polarization strength is tuned. Temperature dependent transport studies of FETs have been carried out from a solution-processed organic semiconductor, 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene), using both ferroelectric and non-ferroelectric gate insulators. Non-polar dielectric based TIPS-pentacene FETs show a clear activated transport in contrast to the ferroelectric dielectric polymer, poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE), where a negative temperature coefficient of the mobility is observed in the ferroelectric temperature range. We attribute the weak temperature-dependence of the charge carrier mobility to a polarization fluctuation driven transport resulting from a coupling of the charge carriers to the surface phonons of the polar dielectric. The negative coefficient of mobility ($\frac{d\mu}{dT}<0$) observed with ferroelectric dielectrics is not a signature of an extended-state conduction but rather denotes polarization fluctuation driven transport.   

Radical Polymer Utilization for Interfacial Improvement of Organic Field-Effect Transistors

Metal-semiconductor interfacial contact is one of the crucial factors for high-performance organic electronic device applications. In particular, the performance of organic field-effect transistors (OFETs) is critically dependent on the engineering of the interface between the organic semiconductor and the source/drain electrodes. Here, we modulate the performance of pentacene-based OFETs through the inclusion of a specific radical polymer, poly(2,2,6,6-tetramethylpiperidine-1-oxyl methacrylate) (PTMA), at the pentacene-gold electrode interface. Using a simple and fast inkjet printing method, the OFET performance is highly enhanced by the systematic deposition of a thin PTMA layer. The insertion of the radical polymer has an impact on the highly-improved OFET performance due to its redox charge transport ability and the amorphous nature allowing the stable growth of the pentacene. The synergistic effect facilitates the charge injection at the interface of the metal and organic semiconductor, resulting in the highly improved OFET performance. As such, the fundamental insights associated with radical polymers can be widened and their utilization as a highly-improved, low-cost interfacial modifier in myriad organic electronic devices is of great promise.   

Small Molecule Doping of Radical Polymers for Enhanced Electronic Performance

Radical polymers have emerged as a class of conducting polymers that show immense potential for solid state electronic applications. However, very little has been done to explore the small molecule doping of these materials for increased electrical performance. Here, we present the characterization of the charge transport ability of a model radical polymer, poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA), doped with varying levels of the small molecule, 4-Acetamido-2,2,6,6-tetramethyl-1-oxopiperidinium tetrafluoroborate (TEMPOnium). We demonstrate that the addition of the TEMPOnium to PTMA thin films creates a distinct relationship between doping level and electrical conductivity. At optimal doping levels, we find that the electrical conductivity of PTMA thin films increases by over an order of magnitude. Furthermore, we illustrate the competing effects of electrical and ionic conductivity that exists in this system by probing the dependence of current on time in these thin films. Finally, we show that the TEMPOnium doping greatly enhances the film quality of these typically brittle PTMA thin films. We anticipate that these findings will encourage novel methods to enhance the electrical performance of these open shell systems in the solid state.   

Understanding the growth of organic semiconductors on semiconducting surfaces

A selective mechanism for tuning the molecule-substrate interaction has been a long sought after goal towards tailored molecular growth. Using first-principles theory and scanning tunneling microscopy, we show that by controlling the strength of orbital hybridization between phthalocyanine molecules and the deactivated Si surface via the selective p-d orbital coupling, we can tune the molecular ordering and molecular orientation at the hetero-interface. This mechanism offers a novel approach to balance the critical interactions, leading to controlled long-ranged ordered molecular growth [1]. [1] S.R. Wagner, B. Huang, C. Park, J. Feng, M. Yoon, and P. Zhang, Phys. Rev. Lett. 115, 096101 (2015). This work was supported by the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy and partly supported by the Materials Sciences and Engineering Divisions, Office of Basic Energy Sciences, U.S. Department of Energy.   

Stencil Nano Lithography Based on a Nanoscale Polymer Shadow Mask: Towards Organic Nanoelectronics

A stencil lithography technique has been developed to fabricate organic-material-based electronic devices with sub-micron resolution. Suspended polymethylmethacrylate (PMMA) membranes were used as shadow masks for defining organic channels and top electrodes. Arrays of pentacene field effect transistors (FETs) with various channel lengths from 50 $\mu $m down to 500 nm were successfully produced from the same batch using this technique. Electrical transport measurements showed that the electrical contacts of all devices were stable and the normalized contact resistances were much lower than previously studied organic FETs. Scaling effects, originating from the bulk space charge current, were investigated by analyzing the channel-length-dependent mobility and hysteresis behaviors. This novel lithography method provides a reliable means for studying the fundamental transport properties of organic materials at the nanoscale as well as enabling potential applications requiring the fabrication of integrated organic nanoelectronic devices.   

Ab initio studies of the optoelectronic properties of biphenyl derivatives in OLEDs

The influence of molecular conformation on electron relaxation and photophysical properties of a series of biphenyl derivatives have been investigated using density functional theory (DFT) and time-dependent DFT(TDDFT). The calculated absorption and emission properties of the series as well as phosphorescence quantum yield are in good agreement with the available experimental data. The spin orbit coupling values and the S $\to $ T intersystem-crossing matrix elements and crossing rate constants are also explored as a function of the twist angle between the rings. The T $\to $ S$_{\mathrm{0}}$ radiative and non-radiative transition rates are calculated and discussed for each member of the series. In addition, the T $\to $ S$_{\mathrm{0}}$ radiative transition rate constant is calculated for twisted biphenyls and compared to those for planar molecules.   

A Comparison Between Magnetic Field Effects in Excitonic and Exciplex Organic Light-Emitting Diodes

In flat-panel displays and lighting applications, organic light emitting diodes (OLEDs) have been widely used because of their efficient light emission, low-cost manufacturing and flexibility. The electrons and holes injected from the anode and cathode, respectively, form a tightly bound exciton as they meet at a molecule in organic layer. Excitons occur as spin singlets or triplets and the ratio between singlet and triplet excitons formed is 1:3 based on spin degeneracy. The internal quantum efficiency (IQE) of fluorescent-based OLEDs is limited 25{\%} because only singlet excitons contribute the light emission. To overcome this limitation, thermally activated delayed fluorescent (TADF) materials have been introduced in the field of OLEDs. The exchange splitting between the singlet and triplet states of two-component exciplex systems is comparable to the thermal energy in TADF materials, whereas it is usually much larger in excitons. Reverse intersystem crossing occurs from triplet to singlet exciplex state, and this improves the IQE. An applied small magnetic field can change the spin dynamics of recombination in TADF blends. In this study, magnetic field effects on both excitonic and exciplex OLEDs will be presented and comparison similarities and differences will be made.