Session T33: Focus Session: Organic Electronics and Photonics - Transport in Polymers

Top contact approach to the nanoscale organic electronic systems using novel stencil lithography technique

In this presentation, we proposed a widely adaptable fabrication method to form a nanoscale organic electronic system with top contact electrodes using a Poly(methyl methacrylate) (PMMA) shadow mask which has a transparency, flexibility and high resolution electrode pattern. The stencil lithography technique with the PMMA mask was developed by the combination of the standard electron beam lithography and the micro transfer printing technique. Firstly, this technique was applied to fabricate nanoscale pentacene field effect transistor (FET) which has top contact source and drain electrodes. The configurations of pentacene layer such as position, width and length were controlled by a PMMA shadow mask which was pre-transferred onto a target substrate. Another PMMA shadow mask with electrode pattern was precisely aligned on the pre-deposited pentacene layer and the pairs of gold electrode were defined after the thermal evaporation followed by mechanical detachment of the mask. The channel length of the transistor was varied from 5um to 500nm and placed at regular intervals along the pentacene layer. The electrical performance of the pentacene FET was statistically analyzed according to the channel length variation.   

Infrared spectroscopy of narrow gap donor-acceptor polymer-based ambipolar transistors

Donor-acceptor (D-A) copolymers have recently emerged as versatile materials for use in a large variety of device applications. Specifically, these systems possess extremely narrow band gaps, enabling ambipolar charge transport when integrated in solution-processed organic field-effect transistors (OFETs). However, the fundamentals of electronic transport in this class of materials remain unexplored. We present a systematic investigation of ambipolar charge injection in a family of narrow-gap D-A conjugated polymers based on benzobisthiadiazole (BBT) using infrared (IR) spectroscopy. We observe a significant modification of the absorption edge in polymer-based OFETs under the applied electric field. The absorption edge reveals hardening under electron injection and softening under hole injection. Additionally, we register localized vibrational resonances associated with injected charges. Our findings indicate a significant self-doping of holes that is modified by charge injection. Observations of both electron and hole transport with relatively high carrier mobility strongly suggest an inhomogeneous, phase-separated conducting polymer.   

Improving Ambipolar Charge Injection in Polymer FETs with Carbon Nanotubes

Efficient charge injection is a key issue for organic field-effect transistors (FET). Various methods can be used to optimize injection of either holes or electrons, for example, by modifying the workfunction of metallic electrodes with self-assembled monolayers. For ambipolar FETs this is much more difficult because injection of both charge carriers has to be improved at the same time. Here we demonstrate a simple process to significantly improve ambipolar charge injection in bottom contact/top gate polymer field-effect transistors by adding single-walled carbon nanotubes (SWNT) to the semiconducting polymer at concentrations well below the percolation limit. Such polymer/carbon nanotube hybrid systems are easily produced by ultrasonication and dispersion of SWNT in a conjugated polymer solution. Even at very low nanotube concentrations the charge injection of both holes and electrons, for example, into poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) and poly(9,9-dioctylfluorene) (PFO) is significantly enhanced leading to lower contact resistances and threshold voltages than in FETs with pristine polymer films. This method can be extended to other semiconductors like n-type naphthalene-bis(dicarboximide)-based polymers (e.g. P(NDI2OD-T2)) for which hole injection was greatly enhanced. The proposed mechanism for this effect of carbon nanotubes on injection is independent of the polarity of the charge carriers. It can be maximized by patterning layers of pure carbon nanotubes onto the injecting electrodes before spincoating the pristine polymers leading to almost ohmic contacts for polymers, which usually show only strongly Schottky-barrier-limited injection. This improved injection of holes and electrons allows for a wider range of accessible polymers for ambipolar and thus also light-emitting transistors.   

Elucidating Bias Stress in Vertical and Lateral Charge Transport in Organic Electronics

Bias stress, during which a reduction in source-drain current is observed under continuous application of gate voltage in organic thin-film transistors, originates from trapped mobile charges. Organic semiconductors often exhibit tail states that extend into their band gap; these tail states can act as traps to immobilize charge. Alternatively, defects at the organic semiconductor-dielectric interface can also trap charge. Whether bias stress originates from impurities or defects in the bulk of the organic semiconductor or at the organic semiconductor-dielectric interface, however, remains unclear. By building and testing organic single-carrier diodes having different active layer thicknesses, we can infer the trapping contributions in the bulk of the organic semiconductor relative to those at the organic semiconductor-electrode interface. In conjunction with device characteristics of organic thin-film transistors having different dielectrics, we found that the broad distribution of tail states that is present in poly(3-hexyl thiophene), P3HT, is responsible for bias stress in P3HT-comprising devices. On the other hand, traps at the [6,6]-phenyl-C61-butyric acid methyl ester, PCBM,-dielectric interface are more dominant than those in the bulk in PCBM-containing devices.   

Low-temperature transport in metallic polyaniline

Since the first observation of true metallic transport in polyaniline (PANI) [Lee et al. Nature, 441, 65 (2006)], one of the outstanding properties of metallic state in PANI, the positive temperature dependence of resistance has not been systematically investigated. We studied the underlying mechanism of the intriguing low-temperature transport in PANI synthesized with self-stabilized dispersion polymerization. [Lee et al. Adv. Funct. Mater. 15, 1495 (2005)] The positive temperature dependence was successfully reproduced at all range of low temperatures. More disordered samples showed negative temperature dependence, indicating disorder-induced metal-insulator transition. In addition, the charge-density-dependent transport in PANI will be presented for profound understanding of this metallic state.   

Role of Morphology on Carrier Transport in Conjugated Polymer Thin Films

The effects of morphology on the out-of-plane hole mobility in poly(3-hexylthiophene) (P3HT) films were examined using impedance spectroscopy (IS), time-of-flight (ToF) and charge extraction by a linearly increasing voltage (CELIV). IS was used for the first time to measure the hole mobilities, $\mu $, of P3HT films; $\mu $ was found to be film thickness dependent, increasing over an order of magnitude with increasing film thickness from 100 to 700 nm. These results are in excellent agreement with those measured using ToF and CELIV. IS has an added advantage over ToF and CELIV, as it also provides dc conductivity $\sigma_{\mathrm{dc}}$ and charge carrier density $n$. Both $\sigma_{\mathrm{dc}}$ and $n$ are shown to decrease appreciably with increasing $h$, over the same thickness range. The thickness dependent trends in $\mu $, $\sigma_{\mathrm{dc}}$ and $n$ are consistent with changes in the morphology of these films.   

Charge Transport in Trehalose-Derived Sugar Glasses

Trehalose is a naturally occurring disaccharide with a well-known ability to preserve the biological function of proteins and cell membranes during periods of stress, including dehydration, by stabilizing the conformations of the macromolecules within a glassy matrix. This phenomenon makes use of the propensity of trehalose to interact strongly with protein functional groups and solvating water molecules via hydrogen bonding. Recently, it has been shown that trehalose sugar glasses also support long range charge transport in the form of oxidation-reduction reactions occurring between spatially separated donors and acceptors. Based on an Arrhenius conductivity analysis, along with IR-absorption and dielectric spectroscopy data, we propose that a Grotthuss-like proton hopping mechanism is responsible for the high charge carrier mobility and observed bias-dependent apparent activation energy. The possibility is raised for novel redox reactions to be performed on proteins constrained to specific 3D conformations. This could lead to a deeper understanding of biological processes, such as anhydrobiosis, as well as the development of new biomimetic photovoltaic devices.   

Aliphatic Polymers Bearing Pendant Radical Groups as Charge Carrying Moieties in Organic Electronic Applications

The implementation of highly conjugated polymers has led to an explosion of high-performance organic electronic devices; however, many important synthetic, physical, and mechanical properties of these macromolecules still lag behind polymers with non-conjugated backbones. In order to implement the positive aspects of both macromolecular classes, we have synthesized radical polymers ($i.e.$, where a pendant stable radical group is present on each repeat unit of the polymer) using controlled polymerization mechanisms. We demonstrate that these next-generation conducting polymers have thermal and physical properties similar to that of aliphatic polymers while still retaining charge transport properties akin to those of well-studied conjugated polymer systems. Specifically, we characterize the charge transport ability of radical polymers using a model radical polymer, poly(2,2,6,6-tetramethylpiperidinyloxymethacrylate), and propose a mechanism for charge transport in these molecules. Furthermore, because of the low optical absorption in the visible spectrum associated with non-conjugated polymers, radical polymers are utilized as anodic modifiers in organic photovoltaic devices and show promise in being more stable to environmental conditions than traditional anode-modifying materials.   

Violation of the Wiedemann-Franz law in Conducting Polymers

The free-electron gas model proposed by Drude and Sommerfeld has been enormously successful at describing the electronic and thermal properties of highly electrically conducting materials. A prediction of the free-electron gas model is that the ratio of the electronic component of the thermal conductivity to the electrical conductivity is proportional to a constant multiplied by the absolute temperature. This prediction is known as the Wiedemann-Franz law, and has been widely validated across various classes of materials. The validity of this law however has not been extensively studied in conducting polymer systems, primarily due to the challenges associated with fabricating highly electrical conductivity polymer devices for which both the electrical and thermal conductivity could be measured. Here, we investigate the relationship between thermal and electronic transport in conjugated polymers across a wide range conductivities, and find that the Wiedemann-Franz law is strongly violated. These results demonstrate that the link between charge transport and heat transport is fundamentally different in conjugated polymer systems than in the vast majority of high-conductivity materials.   

Ab initio modeling of electronic properties of DNA: Comparison to experiments

In this work, we model the zero-bias conductance for four DNA strands that were used in Ref. [1]. Our approach consists of three elements: (i) experimental data, (ii) ab initio calculations of DNA and (iii) two parameters to determine the decoherence rates. We find that the coherent conductance is much smaller than the experiments [2]. To understand the reason, we look at the effect of decoherence. By including decoherence, we show that our model can rationalize the measured conductance of the four strands. We find that decoherence on $G:C$ base pairs is crucial in getting agreement with the experiments. However, the decoherence on $G:C$ base pairs alone does not explain the experimentally determined dependence of conductance in strands containing a number of $A:T $base pairs. Including decoherence on $A:T$ base pairs is also essential. By fitting the experimental magnitudes of the conductance for the four DNA molecules, we estimate for the first time that the deocherence rate is 6 \textit{meV} for $G:C$ and 1.5 \textit{meV} for $A:T$ base pairs. [1] Ajit K Mahapatro, et al., Nanotechnology, \textbf{18}, 195202 (2007) [2] Jianqing Qi, et al. http://www.ee.washington.edu/faculty/anant/publications/JianqingQiPaper.pdf.   

Various Magnetoresistance of a New Copolymer, FeCl$_3$ doped Poly(Phenylenevinylene-EDOT-Vinylene)

We synthesized a new alternating copolymer in which ethylenedioxythiophene (EDOT) and phenylene are alternatively linked by vinylene unit (PPVEDOTV). Temperature dependence of conductivity of the FeCl$_3$ doped PPVEDOTV films followed Coulomb gap variable range hopping (VRH). However magnetoresistance (MR) showed different behaviors despite their similar temperature dependence of conductivity. Among the 4 samples, the MR of the most conducting sample was such that initially positive and decreased as the magnetic field increased and upturned as the field increased further (Type A). 2 other samples showed initially negative MR and it crossed over to positive MR (Type B). Lastly the most insulating sample showed monotonic positive MR (Type C). The MR of type B and C were analyzed as the sum of forward quantum interference (FQI) and wavefunction shrinkage (WS) effects and WS effect only, respectively. For the MR of type A, we propose that the initial positive MR is attributed to FQI in less disordered systems.