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24 Sep 2001

Volume 79, Issue 13, pp. 1933-2115

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Endothermic energy transfer: A mechanism for generating very efficient high-energy phosphorescent emission in organic materials

Chihaya Adachi, Raymond C. Kwong, Peter Djurovich, Vadim Adamovich, Marc A. Baldo, Mark E. Thompson, and Stephen R. Forrest

Appl. Phys. Lett. 79, 2082 (2001); http://dx.doi.org/10.1063/1.1400076 (3 pages) | Cited 412 times

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Intermolecular energy transfer processes typically involve an exothermic transfer of energy from a donor site to a molecule with a substantially lower-energy excited state (trap). Here, we demonstrate that an endothermic energy transfer from a molecular organic host (donor) to an organometallic phosphor (trap) can lead to highly efficient blue electroluminescence. This demonstration of endothermic transfer employs iridium(III)bis(4,6-di-fluorophenyl)-pyridinato-N,C2′)picolinate as the phosphor. Due to the comparable energy of the phosphor triplet state relative to that of the 4,4′-N,N′-dicarbazole-biphenyl conductive host molecule into which it is doped, the rapid exothermic transfer of energy from phosphor to host, and subsequent slow endothermic transfer from host back to phosphor, is clearly observed. Using this unique triplet energy transfer process, we force emission from the higher-energy, blue triplet state of the phosphor (peak wavelength of 470 nm), obtaining a very high maximum organic light-emitting device external quantum efficiency of (5.7±0.3)% and a luminous power efficiency of (6.3±0.3)lm/W. © 2001 American Institute of Physics.
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78.60.Fi Electroluminescence
85.60.Jb Light-emitting devices

High efficiency solid-state photovoltaic device due to inhibition of interface charge recombination

Jessica Krüger, Robert Plass, Le Cevey, Marco Piccirelli, Michael Grätzel, and Udo Bach

Appl. Phys. Lett. 79, 2085 (2001); http://dx.doi.org/10.1063/1.1406148 (3 pages) | Cited 169 times

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The performance of solid-state dye-sensitized solar cells based on spiro-MeOTAD was considerably improved by controlling charge recombination across the interface of the heterojunction. This was achieved by blending the hole conductor matrix with a combination of 4-tert-butylpyridine (tBP) and Li[CF3SO2]2N. Open circuit voltages Uoc over 900 mV and short circuit currents Isc up to 5.1 mA were obtained, yielding an overall efficiency of 2.56% at AM1.5 illumination. These values have been fully confirmed at the National Renewable Energy Laboratories for a device with an active area of 1.07 cm2, signifying a dramatic improvement compared to previously reported values for a similar device. © 2001 American Institute of Physics.
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84.60.Jt Photoelectric conversion

Degradation mechanism of phosphorescent-dye-doped polymer light-emitting diodes

Shun-Chi Chang, Gufeng He, Fang-Chung Chen, Tzung-Fang Guo, and Yang Yang

Appl. Phys. Lett. 79, 2088 (2001); http://dx.doi.org/10.1063/1.1404995 (3 pages) | Cited 46 times

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The degradation mechanism of phosphorescent-dye-doped polymer light-emitting diodes (PLEDs) is investigated. The active medium of our PLED is a polymer blend comprising poly(vinylcarbazole) (PVK), [2-(4-biphenylyl)-5-(4-tert-butyl-phenyl)-1,3,4-oxadiazole] (t-PBD), and platinum(II)-2,8,12,17-tetraethyl-3,7,13,18-tetramethylporphyrin (PtOX). The cyclic voltammetry result shows that the reductive reversibility of PtOX is poor. This result suggests that PLED doped with PtOX is not stable if PtOXs trap electrons and turn into anionic PtOX species. This was indeed verified by fabricating single-layer PLEDs with various amounts of electron-transporting material, t-PBD. A slower degradation rate was observed from the devices with higher concentration of t-PBD, because of the reduction of the electron accumulation at the PtOX sites. The half decay lifetime of our phosphorescent polymer LED has been improved by a factor of ∼ 40, from 1.2 to 45 h. © 2001 American Institute of Physics.
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42.70.Jk Polymers and organics
82.80.Fk Electrochemical methods
78.66.Qn Polymers; organic compounds
85.60.Jb Light-emitting devices

Infrared photodetector based on intersubband transitions to minigap-confined states in doped quantum wells

R. P. Leavitt and J. W. Little

Appl. Phys. Lett. 79, 2091 (2001); http://dx.doi.org/10.1063/1.1374482 (3 pages) | Cited 3 times

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We have demonstrated an infrared photodetector based on intersubband transitions in doped (In, Ga)As quantum wells in which the excited state is confined by the minigap between two minibands in a surrounding superlattice. Sharp peaks were observed at 78 K in the infrared photocurrent spectra in the 2.7–3.6 μm wavelength region corresponding to transitions in which the excited state energy is substantially above the (In, Al)As conduction-band edge. Broader, longer-wavelength photocurrent features corresponding to transitions into the lower-lying miniband states were also observed. The spectral features in the photocurrent spectra are well described by calculations of the optical absorption cross section.© 2001 American Institute of Physics.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
07.57.Kp Bolometers; infrared, submillimeter wave, microwave, and radiowave receivers and detectors
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
73.21.Fg Quantum wells

Transient photocurrent overshoot in quantum-well infrared photodetectors

V. Letov, M. Ershov, S. G. Matsik, A. G. U. Perera, H. C. Liu, Z. R. Wasilewski, and M. Buchanan

Appl. Phys. Lett. 79, 2094 (2001); http://dx.doi.org/10.1063/1.1400772 (3 pages) | Cited 4 times

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We report a strongly nonexponential behavior of the transient photocurrent in quantum-well infrared photodetectors (QWIPs) in response to a step-like infrared illumination. The transient photocurrent displays an overshoot on the time scale 0.1–1 ms at low temperatures (T<70 K), exceeding the steady-state photocurrent by as much as ≈ 50%. The overshoot behavior is attributed to a nonlinearity of responsivity caused by the modulation of the electric field in QWIP under relatively high illumination power, when the photocurrent exceeds the dark current. This explanation is confirmed by the experimental data and numerical simulation. These effects can play an important role when QWIPs operate in nonlinear regimes, such as in a heterodyne mode or in low-temperature and low-background applications. © 2001 American Institute of Physics.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
07.57.Kp Bolometers; infrared, submillimeter wave, microwave, and radiowave receivers and detectors
85.30.De Semiconductor-device characterization, design, and modeling

Importance of quadrupolar ordering in antiferroelectric liquid crystal devices

L. A. Parry-Jones and S. J. Elston

Appl. Phys. Lett. 79, 2097 (2001); http://dx.doi.org/10.1063/1.1405423 (3 pages) | Cited 15 times

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A model of switching in antiferroelectric liquid crystals is presented which predicts both helix unwinding in the pretransitional regime and a field induced transition to the ferroelectric state. When the ratio of the coefficients of the quadrupolar and dipolar ordering terms in the bulk free energy expression exceeds a critical value, the latter transition becomes first order, in agreement with experimental observation. We have determined these coefficients in the commercial mixture CS4001. The ratios of the coefficients imply that all three states (antiferroelectric and two ferroelectric) are stable in the ground state, i.e., antiferroelectric liquid crystals are truly tristable. © 2001 American Institute of Physics.
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77.84.Nh Liquids, emulsions, and suspensions; liquid crystals
61.30.Gd Orientational order of liquid crystals; electric and magnetic field effects on order
64.70.M- Transitions in liquid crystals
77.80.Fm Switching phenomena
42.79.Kr Display devices, liquid-crystal devices
42.70.Df Liquid crystals
77.80.B- Phase transitions and Curie point
61.30.Dk Continuum models and theories of liquid crystal structure

Independently addressable subarrays of silicon microdischarge devices: Electrical characteristics of large (30×30) arrays and excitation of a phosphor

S.-J. Park, J. G. Eden, J. Chen, and C. Liu

Appl. Phys. Lett. 79, 2100 (2001); http://dx.doi.org/10.1063/1.1401791 (3 pages) | Cited 13 times

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Large arrays (up to 30×30) of microdischarge devices having separately addressable subarrays have been fabricated in Si and operated continuously in Ne, Ne/Ar, and Ne/Xe gas mixtures at pressures up to 800 Torr. Eight 3×3 arrays fabricated on the same substrate operate simultaneously at voltages as low as 210 V in 400 Torr of Ne and exhibit lifetimes beyond 19 h, or approximately 1 order of magnitude larger than those for earlier arrays in which all devices have a common anode. Four 15×15 arrays have also been tested and, when operated in Ne/Ar or Ne/10% Xe gas mixtures, generate intense fluorescence in the green from a phosphor over an area of 16 mm2. © 2001 American Institute of Physics.
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52.80.Yr Discharges for spectral sources (including inductively coupled plasma)
85.60.Pg Display systems
42.72.Bj Visible and ultraviolet sources

Detection of single x-ray photons by an annular superconducting tunnel junction

L. Frunzio, L. Li, D. E. Prober, I. V. Vernik, M. P. Lisitskii, C. Nappi, and R. Cristiano

Appl. Phys. Lett. 79, 2103 (2001); http://dx.doi.org/10.1063/1.1405425 (3 pages) | Cited 5 times

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We present an experiment that detects photons by use of an annular Nb-based superconducting tunnel junction (STJ). In one magnetic field configuration, we stably trapped a single magnetic fluxon in the STJ barrier during a transition to the superconducting state. This is an innovative configuration which avoids the use of an externally applied field during detector operation. It offers potential benefits for STJs used in imaging arrays. In this configuration, and also in the conventional one with an externally applied parallel magnetic field, we observed current pulses produced by single 6 keV x-ray photons. The pulses were the same for the two configurations. © 2001 American Institute of Physics.
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85.25.Oj Superconducting optical, X-ray, and γ-ray detectors (SIS, NIS, transition edge)
07.85.Fv X- and γ-ray sources, mirrors, gratings, and detectors
74.45.+c Proximity effects; Andreev reflection; SN and SNS junctions
74.25.Uv Vortex phases (includes vortex lattices, vortex liquids, and vortex glasses)
74.25.Sv Critical currents
85.25.Cp Josephson devices
29.40.Wk Solid-state detectors

Gold nanoparticle single-electron transistor with carbon nanotube leads

Claes Thelander, Martin H. Magnusson, Knut Deppert, Lars Samuelson, Per Rugaard Poulsen, Jesper Nygård, and Jørn Borggreen

Appl. Phys. Lett. 79, 2106 (2001); http://dx.doi.org/10.1063/1.1405154 (3 pages) | Cited 42 times

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We have used carbon nanotubes to electrically contact a 7 nm gold particle by scanning-probe manipulation. The result was a single-electron transistor showing a periodic modulation of the current as a function of gate voltage for temperatures up to ∼ 200 K, with the particle responsible for the main features of the electron transport. This interpretation could be verified when the particle was removed and the two nanotubes were moved into electrical contact. © 2001 American Institute of Physics.
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85.35.Gv Single electron devices
85.35.Kt Nanotube devices
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