Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session B30: Organic Field Effect Transistors I |
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Sponsoring Units: DPOLY FIAP Chair: George Malliaras, Cornell University Room: LACC 505 |
Monday, March 21, 2005 11:15AM - 11:27AM |
B30.00001: Nonlinear charge injection in organic thin-film field effect transistors Behrang Hamadani, Douglas Natelson Transport properties of a series of poly(3-hexylthiophene) organic field effect transistors with Cr, Cu and Au source/drain electrodes were examined over a broad temperature range. A procedure based on dependence of conductance on channel length was used to extract the current-voltage characteristics of the injecting contacts. Distinguishing between the contact and channel contributions in bottom contact organic transistors is an important step toward improved understanding and modeling of the nature of charge injection in these devices. A model of hopping injection into a disordered density of localized states, with emphasis on the primary injection event, is consistent with the field and the temperature dependence of the data over a broad range of temperatures and gate voltages. [Preview Abstract] |
Monday, March 21, 2005 11:27AM - 11:39AM |
B30.00002: Photocurrent Spectroscopy of Pentacene Thin Film Transistors Mihaela Breban, Danilo Romero, Sergey Mezhenny, Vincet Ballorotto, Ellen Williams We investigate charge transport in pentacene thin-film transistors by means of photocurrent spectroscopy. The photocurrent spectrum in these devices reveals contributions from the Davydov-split singlet excitons at 1.85 eV and 1.96 eV, inter-pentacene molecular charge-transfer at 2.1 eV and 2.3 eV, and a broad continuum above 2.25 eV associated with the HOMO- LUMO transitions. By using photon excitations at these characteristic energies, we obtained the optical current- voltage characteristics of the pentacene TFTs by measurements of the source-drain photocurrent as a function of the source- drain and gate voltages, allowing a spectroscopic probe of the various transport mechanisms in these devices. The implications of these results to models of the current-voltage characteristics of pentacene TFTs will be discussed. [Preview Abstract] |
Monday, March 21, 2005 11:39AM - 11:51AM |
B30.00003: Photo-induced effects in Organic Field-Effect Transistors Vitaly Podzorov, Michael Gershenson Most of the organic semiconductors are optically active in visible range. The talk will show that optical effects may play an important role in Organic Field-Effect Transistors (OFETs). As a model system we study the single-crystal OFETs that demonstrate intrinsic transport with high mobility of charge carriers, mu = 5 to 20 cm2/Vs [1,2]. Particularly, we report on an observation of light-induced switching of conductance in the back-gated OFETs with a built-in conduction channel [3]. Other results, including the demonstration of control of the OFET's characteristics with light, will be discussed. [1] V. Podzorov et al., Appl. Phys. Lett. 82, 1739 (2003); ibid. 83, 3504 (2003); [2] V. Podzorov et al., Phys. Rev. Lett. 93, 086602 (2004); [3] V. Podzorov et al., cond-mat/0406738. [Preview Abstract] |
Monday, March 21, 2005 11:51AM - 12:03PM |
B30.00004: 2D Continuum Percolation in Single-Monolayer Pentacene Transistors Byoung-Nam Park, Soonjoo Seo, Paul Evans Geometrical effects arising from the percolation of monolayer islands have a crucial role in the electrical properties of single-layer-scale pentacene thin films. We fabricated and characterized bottom-contact pentacene monolayer transistors on silicon dioxide substrates. In-situ measurements allowed the electrical properties of the pentacene thin films to be determined as a function of pentacene coverage during the deposition of the first monolayer of pentacene. Current begins to flow between the source and drain electrodes of a pentacene transistor at a sharp threshold where the fraction of the surface covered by molecular islands exceeds approximately 0.7. Van der Pauw sheet resistance measurements with slightly higher coverages eliminate the artifacts associated with contact resistance and reveal a much higher mobility than two-contact FET measurements. [Preview Abstract] |
Monday, March 21, 2005 12:03PM - 12:15PM |
B30.00005: Photoinduced Memory Effects in Polymer Field Effect transistors K.S. Narayan, Soumya Dutta Polymer field effect transistors (FETs) exhibit several interesting features upon photoexcitation.$^1$ The large change in conductance upon exposing to light is accompanied by a slow relaxation upon terminating the photoexcitation and this feature was explained on the basis of serial relaxation process due to a hierarchy of the systems with spatial separation of the photogenerated electrons and holes.$^2$ The inherent slow dynamics of the photogenerated carriers in such configuration were exploited to observe the memory effect with repeated write, read, store and erase functions by using the appropriate combination of light and gate voltage.$^3$ We report and discuss these effects based on spectroscopic signatures of the FET in the different states. \\ 1. K. S. Narayan and N. Kumar, Appl. Phys. Lett. 79, 1891 (2001). \\ 2. S. Dutta and K. S. Narayan, Phys. Rev. B 68, 125208 (2003). \\ 3. S. Dutta and K. S. Narayan (to appear in Advanced Materials 2004). [Preview Abstract] |
Monday, March 21, 2005 12:15PM - 12:27PM |
B30.00006: Correlating structure development to performance enhancement in organic semiconductor films Eric Lin, Dean DeLongchamp, Sharadha Sambasivan, Daniel Fischer Measuring the structural development of organic semiconductor films and correlating it to the electrical characteristics of organic field effect transistors (OFETs) is a critical steppingstone to commercialization. Synchrotron-based Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy is a powerful tool that can non-destructively reveal the structure and chemistry of thin organic films. The density of bonds involving carbon, nitrogen, oxygen, and fluorine can be quantified, a composition depth profile can be developed for the top (2-10) nm of the film, and bond orientation can be determined. We employ NEXAFS to investigate chemistry, molecular orientation, and defects in thin organic semiconductor films. These quantities are then correlated to OFET performance. Examples will include classical model systems such as pentacene and regioregular poly(3-hexylthiophene), as well as new molecules. Thermal processing-induced increases in molecular orientation will be described. Structure and performance enhancement by hydrophobic substrate modification will be discussed. Finally, the role of the semiconductor chemical structure itself will be considered. NEXAFS provides excellent correlation of processing to structure to performance, illuminating fruitful development vectors for next-generation materials. [Preview Abstract] |
Monday, March 21, 2005 12:27PM - 1:03PM |
B30.00007: New Strategies for Thin Film Organic Transistors Invited Speaker: This presentation will explore new strategies for creating thin film organic transistors. One strategy embeds a small fraction of dispersed carbon nanotubes in standard organic semiconductors below the threshold for percolation. This method provides high transconductance organic transistors in spite of relatively large source to drain distances. The improvement of the electronic characteristic of such a scheme is equivalent to a 60-fold increase in mobility of the underlying organic semiconductor. The relatively high mobility can be easily achieved without sacrificing the on/off ratio of the device allowing a path to printable electronic materials. A second strategy involves using new molecular semiconductors that attach and assemble on the surface of high-k dielectrics. The molecules are from a new type of linear acene that has its short ends functionalized with endgroups that react with surface oxides. Incorporating this assembly motif into electronic devices allows field effect transistors to be constructed that have about ten thousand molecules in the channel. [Preview Abstract] |
Monday, March 21, 2005 1:03PM - 1:15PM |
B30.00008: High Quality Organic Semiconductor Thin-film Transistors Printed on Plastic Substrates using Transfer Printing Y. Shao, S.A. Solin, D.R. Hines, V.W. Ballarotto, E.D. Williams Transfer Printing has been used to fabricate organic thin-film transistors onto plastic substrates. This method uses pressure and temperature to transfer a patterned layer from one substrate (on which the thin-film material was originally fabricated) to a second substrate. This thermal treatment is similar to an annealing process and is therefore expected to improve the quality of organic films from the small molecule and polymeric classes. X-ray diffraction patterns of a pentacene thin-film transfer printed onto a plastic substrate show sharp $(00l)$ reflections corresponding to a c-axis correlation length of 432$\AA$. X-ray diffraction patterns of the original pentacene thin-film thermally evaporated onto an SiO$_2$ surface also show sharp $(00l)$ reflections but exhibit a c-axis correlation length of 392$\AA$. The transistor characteristics of these high quality, transfer printed organic materials will be correlated with thin-film quality. [Preview Abstract] |
Monday, March 21, 2005 1:15PM - 1:27PM |
B30.00009: N-Type and Ambipolar Charge Transport in Polymer Field-Effect Transistors Jana Zaumseil, Lay-Lay Chua, Peter K.H. Ho, Richard H. Friend, Henning Sirringhaus Most organic field-effect transistors (FETs) readily show p- but not n-type conduction. So far electron conduction has only been found in a few high electron-affinity organic semiconductors. The origin of this difference in electron and hole transport is currently not well understood but the nature of the gate dielectric seems to play a major role. We have recently shown that with the appropriate hydroxyl-free gate dielectrics, such as a benzocyclobutene derivative (BCB) n-type field-effect conduction can be readily observed in a range of polymer semiconductors such as polyfluorenes (e.g. F8BT, F8) and polyphenylenevinylenes (e.g. MEH-PPV, OC$_{1}$C$_{10}$-PPV) which were previously believed to exhibit only p-type field-effect conduction. Here we show that even poly(3-hexylthiophene) (P3HT) which has been widely investigated as a p-channel semiconductor with high hole mobilities is equally able to conduct electrons and shows efficient n-type behaviour in field-effect transistors. Moreover we demonstrate ambipolar charge transport in transistors based on high purity P3HT with balanced electron and hole mobilities. We use these ambipolar transistors to investigate the origin of traps in the bulk and at the semiconductor-dielectric interface represented by hysteretic current-voltage characteristics. This observation of balanced ambipolar charge transport in a conjugated polymer opens the way to organic low power complementary circuits and even light-emitting field-effect transistors. [Preview Abstract] |
Monday, March 21, 2005 1:27PM - 1:39PM |
B30.00010: Water-Dispersible Polyaniline Electrodes for Thin-Film Transistors Kwangseok Lee, Yueh-Lin Loo, Graciela Blanchet, Feng Gao We report a technique for directly patterning water-dispersible conductive polyaniline by a ``stamp-and-spin-cast'' method. This patterning scheme relies on the creation of hydrophilic and hydrophobic regions on an insulating substrate, and then spin-casting an aqueous dispersion of conductive polyaniline on the treated substrate. Since polyaniline selectively adsorbs in the hydrophilic regions, conductive features as small as 5 microns result immediately after spin-casting. We have fabricated polyaniline source and drain electrodes for organic thin-film transistors using this technique. Characterization reveals that polyaniline electrodes and contacts are functionally as effective as gold electrodes; saturation mobilities upwards of 0.1 cm$^{2}$/V.sec and on/off rations of order 10$^{4}$ -- typical of pentacene transistors with gold electrodes -- can be routinely achieved with polyaniline electrodes. Electrical characterization in the low source-drain voltage regime, however, reveals some subtleties. Specifically, the ``hooking behavior'' -- attributed to large contact resistance at the organic semiconductor-metal interface -- typically observed in organic transistors with gold electrodes is absent in polyaniline devices. This observation suggests a smaller contact resistance in transistors with polyaniline electrodes. [Preview Abstract] |
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B30.00011: One-Dimensional Transport in Polymer Nanofibers Y.W. Park, A.N. Aleshin, H.J. Lee, K. Akagi We report our transport studies in quasi-one-dimensional (1D) conductors---helical polyacetylene fibers doped with iodine---and the data analysis for other polymer single fibers and tubes. We found that at 30 K$<$T$<$300 K, the conductance and the current-voltage characteristics follow the power law $G(T) \quad \propto T^{\alpha }$ with \textit{$\alpha $ }$\sim $ 2$:$2--7$:$2 and $I(V)\propto V^{\beta }$with \textit{$\beta \sim $ } 2--5$:$7. Both $G(T)$and$I(V)$ show the features characteristic of 1D systems such as Luttinger liquid or Wigner crystal. The relationship between our results and theories for tunneling in 1D systems is discussed. [Preview Abstract] |
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