AVS2004 Session OF+EM-ThA: Molecular and Organic Films and Devices - Optoelectronic
Thursday, November 18, 2004 2:40 PM in 304C
OF+EM-ThA-3 Plasma Damage-Free Deposition of Metal and ITO Electrodes on Organic Light Emitting Diodes by Using Mirror Shape Target Sputtering(MSTS)
H.-K. Kim, D.-G. Kim, K.A. Lee, M.-S. Huh, S.H. Jeong, K.I. Kim (Samsung SDI, Korea)
We report a successful fabrication of plasma damage-free organic light-emitting diodes (OLED) by using a mirror shape target sputter (MSTS) technique. Compared to leakage current (1*10@super-1@ ~10@super-2@mA/cm@super2@ at -6V) of the OLED consisted of Al cathode grown by conventional DC magnetron sputtering, that of the OLED with an Al cathode grown by MSTS shows much lower leakage current at reverse bias (1*10@super-5@ mA/cm@super2@ at -6V), indicative of no plasma damages. Therefore, the MSTS technique is expected to be useful in plasma damage-free and low temperature deposition technique for top and bottom-emitting OLEDs and flexible OLED. Possible mechanism is given to explain plasma damage free deposition of the metal and ITO electrodes by using current-voltage characteristics, SEM, AFM, XRD, and TEM examinations.
OF+EM-ThA-4 Magnetic Field Effects in Transient Electroluminescence (EL) from Alq@sub 3@/NPB Bi-layer Organic Light Emitting Diodes
J. Wilkinson, A.H. Davis, K. Bussmann, J.P. Long (Naval Research Laboratory)
The long electron spin lifetimes commonly found in organic materials make organic light emitting diodes (OLEDs) potential candidates for spin-injection controlled light sources. However, direct-current measurements of electroluminescence (EL) in OLEDs with non-magnetic electrodes (i.e. with no spin injection) show a variation in EL with applied magnetic field that must be understood. For example, EL can increase by as much as 6% around 0.1 Tesla (T) before decreasing by up to 20%, at 2 T.@footnote 1@ This shows that magnetic field effects are not due to spin injection, but are an intrinsic property of the light emission process. To probe the causes of these processes, we have performed sensitive transient EL experiments in the low-field regime (H = 80 mT) where the magnetic field enhances EL. The OLED is driven with rectangular voltage pulses producing a temporal response in EL with features down to the system resolution of 70 ns. The time-dependent magnetic field effect, defined as @DELTA@EL/EL = [EL(H) - EL(0)]/EL(0) increases EL by 6% for a 3.6 V drive at room temperature, as in direct-current measurements. But in addition @DELTA@EL/EL has interesting transient behavior when the device is first turned on, and again after the drive pulse is turned off during a long-lived delayed EL signal. As the device turns on, @DELTA@EL/EL doubles relative to its steady state value, which it attains on the microsecond timescale. Immediately after device turn-off, a long-lived increase in @DELTA@EL/EL is detected as well. The measured tendency for the magnetic field enhancement of EL to decrease as the transient EL increases is consistent with drive-dependent measurements. Together, these experiments indicate that high concentrations of non-equilibrium carriers or excitons interfere with the magnetic enhancement process. @FootnoteText@ @footnote 1@ A.H. Davis and K. Bussmann, Organic Light-Emitting Materials and Devices VII, eds. Z.H. Kafafi and P.A. Lane, 5214, 57-63 (2004).
OF+EM-ThA-5 Molecular N-Type Doping of NTCDA by Pyronin B
C.K. Chan, A. Kahn (Princeton University)
Molecular n-type doping of 1,4,5,8-naphthalene tetracarboxylic dianhydride (NTCDA) by pyronin B (PyB) is observed using ultraviolet photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPES). The leuco form of the dopant molecule is prepared in situ by heating the stable PyB chloride salt until sublimation@footnote 1@. UPS of the neat PyB film shows that the highest occupied molecular orbital (HOMO) of the material is 5.69eV below the vacuum level (E@sub vac@), whereas the lowest unoccupied molecular orbital (LUMO) of NTCDA is at 4.08eV below E@sub vac@, as determined by IPES. Despite this relatively large energy difference between donor and electron transport states, the deposition of small amounts (<2Å) of PyB on pristine NTCDA films leads to a shift of the HOMO away from the Fermi-level by nearly 0.20eV, indicative of n-type doping of NTCDA by PyB and in agreement with the results of Werner et al.@footnote 1@. Interface and bulk energy levels of co-evaporated films show similarly efficient doping. Current-voltage measurements on doped NTCDA diode devices will also be presented. @FootnoteText@ @footnote 1@ A. G. Werner, F. Li, K. Harada, M. Pfeiffer, T. Fritz, and K. Leo, Appl. Phys. Lett. 82, 4495 (2003).
OF+EM-ThA-6 Electrical Doping of Poly(9,9-dioctylfluorenyl-2,7-diyl) with Tetrafluoro-Tetracyanoquinodimethane by Solution Method
J.H. Hwang, A. Kahn (Princeton University)
Electrical doping of organic materials has received attention for enhancing carrier injection and lowering drive voltages. We investigate here p-type doping of poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO) with tetrafluorotetracyanoquinodimethane (F4-TCNQ) using a solution method. Doped and undoped films were compared using ultraviolet photoelectron spectroscopy (UPS) and current-voltage (I-V) measurement. Undoped PFO was prepared from 0.1wt% of tetrahydrofuran (THF) and p-xylene. For doped PFO, 5% of F4-TCNQ relative to each repeat unit of PFO was added to each solution. Undoped and doped films (~100Å thick) were spun on ITO or Au substrates in nitrogen, annealed in nitrogen at 50°C to remove residual solvent, and loaded without ambient exposure in an ultra-high vacuum chamber. UPS spectra were recorded for each film. The energy of the highest occupied molecular orbital (HOMO) was measured with respect to the Fermi level (E@sub F@). The ionization energy of PFO, determined as the difference between vacuum level (measured from the onset of photoemission) and HOMO, was found to be 5.75 eV, which is ~0.5 eV larger than the electron affinity of F4-TCNQ (5.24eV)@footnote 1@.In spite of the fact that this difference is significantly larger than for ZnPc (0.04 eV)@footnote 1@ and a-NPD (0.28 eV)@footnote 2@, F4-TCNQ p-dopes the polymer. EF-HOMO is 1.1 eV and 1.4 eV for undoped PFO on Au and ITO, respectively, and drops by 0.2 eV for PFO:F4-TCNQ, showing a shift in the expected direction for p-doping. We also performed I-V measurements on Au/1200Å PFO/ITO, which show an order of magnitude increase in current in doped PFO, consistent with higher conductivity and/or lowering of the hole-injection barrier. These measurements show that electron transfer from host to dopant occurs and produces p-type doping. @FootnoteText@ @footnote 1@ W. Gao, A. Kahn, Appl. Phy. Lett., 79, 4040 (2001)@footnote 2@ W. Gao and A. Kahn, J. Appl. Phys. 94, 359 (2003).
OF+EM-ThA-7 Negative Capacitance in Hetero-Layered Organic Light Emitting Diodes
L.S.C. Pingree, B.J. Scott, T.J. Marks, M.C. Hersam (Northwestern University)
Negative capacitance (NC) has been measured by impedance spectroscopy and correlated with time domain waveforms in ITO/NBP/Alq@sub 3@/Al OLED macroscopic devices. In addition, this behavior has been measured by Nanoscale Impedance Microscopy@footnote 1@ on 8 µm x 8 µm microscopic OLED devices. Beyond providing evidence of the scaling of NC, this AFM based technique provides spatially resolved capacitance variations in these structures. Due to the field dependent mobility of the charge carriers, and their subsequent dependence upon both bias and space charge, a slow rise time (t@sub R@) in the current response is detected when a step voltage is applied to the OLED. Such behavior is typical of a NC impedance response@footnote 2@. The typical t@sub R@ varies from 100 ms at 1 volt to 15 ms at 10 volts for electrons, and 100 ms at 1 volt to 1 ms at 10 volts for holes. The onset of NC in the frequency domain correlates strongly with the hole t@sub R@, and the corresponding frequencies are exponentially dependent upon the applied bias. These results agree with the exponential dependence of mobility upon applied field given by Poole-Frenkel theory. Also, the variation between the carrier rise times suggests that Richardson-Schottky injection dominates electron flow. Correlation of the data with device physics implies that NC behavior is hole dominated since the flow of electrons is modulated solely by holes trapped at the Alq@sub 3@/NPB interface, whereas holes respond to both bias and space charge. Additionally, temporal variations in the behavior of light emission as a function of frequency were acquired though the use of a dual lock-in technique. @FootnoteText@ @footnote 1@R. Shao et. al. Appl. Phys. Lett. 82 1869 (2003).@footnote 2@M. Ershov et. al. IEEE Trans. Elect. Dev. 45 2196 (1998).
OF+EM-ThA-8 Electronic Structure and Molecular Orientation of Conducting Polymer Films Produced via Surface Polymerization by Ion Assisted Deposition
S. Tepavcevic, Y. Choi (University of Illinois at Chicago); M. Bissen, D. Wallace (University of Wisconsin-Madison); L. Hanley (University of Illinois at Chicago)
Conducting polymer films are grown by mass-selected, hyperthermal organic cations coincident on a surface with a thermal beam of organic monomers, in a process termed surface polymerization by ion assisted deposition (SPIAD).@footnote 1,2@ SPIAD is applied here to create polymer films from thiophene ions and either @alpha@-terthiophene neutrals (3T SPIAD) or p-terphenyl neutrals (3P SPIAD). Mass spectrometry and x-ray photoelectron spectroscopy (XPS) verify the polymerization of both 3T and 3P SPIAD films. The electronic structure and molecular orientation of these films are probed by valence band XPS, ultraviolet photoelectron spectroscopy (UPS) and polarized near-edge x-ray absorption fine structure spectroscopy (NEXAFS). Valence band XPS and UPS of the 3T SPIAD films produced with 200 eV ions and an ion/neutral ratio 1/150 display similar spectral features as polythiophene films prepared electrochemically. A new state is observed 1 - 3 eV below the Fermi level in the 3T SPIAD film spectra which is not observed in films prepared by evaporation of 3T. This new state is attributed to an extended @pi@* bonding band along the conjugated aromatic chain of the polymerized 3T. Carbon K-edge NEXAFS probes the unoccupied @pi@* and @sigma@* bands of 3T SPIAD films which appear similar to those of the evaporated 3T film. Polarized NEXAFS show that the 3T SPIAD film is at least partially oriented with their molecular axes close to the normal of the substrate surface. The 3P SPIAD film prepared at 200 eV with an ion/neutral ratio 1/100 display more electron delocalization over the @pi@* bonding band compared with the 3P evaporated film. Polarized NEXAFS shows that as little or no orientation in the 3P SPIAD film, in contrast to the highly oriented 3P film. @FootnoteText@ @footnote 1@S. Tepavcevic, Y. Choi, and L. Hanley, J. Amer. Chem. Soc. 125 (2003) 2396. @footnote 2@Y. Choi, S. Tepavcevic, Z. Xu, and L. Hanley, Chem. Mater. (2004) in press.