We measure the current–voltage and electroluminescencecharacteristics of single‐heterojunction, vacuum‐deposited organic light‐emitting devices(OLEDs) over a wide range of materials, temperatures, and currents. We find that the current is limited by a large density of traps with an exponential energy distribution below the lowest unoccupied molecular orbital. The characteristic trap depth is 0.15 eV. Furthermore, in metal–quinolate‐based devices,electroluminescence originates from recombination of Frenkel excitons, and its temperature dependence is consistent with the excitons being formed by Coulombic relaxation of the trapped electrons with holes injected from the counter electrode. By semiempirical molecular orbital modeling, we find that the trap distribution obtained from the current–voltage characteristics is consistent with a distribution in the metal–quinolate molecular conformations which result in a continuous, exponential distribution of allowed states below the lowest unoccupied molecular orbital. We discuss the implications of the intrinsic relationship between electroluminescence and current transport in OLEDs for the optimization of efficiency and operating voltage in these devices.

Relationship between electroluminescence and current transport in organic heterojunction light‐emitting devices

Structure and electrical conduction properties of phthalocyanine thin films

The effects of copper phthalocyanine purity on organic solar cell performance

Thin and ultra-thin films of nickel phthalocyanine grown on highly oriented pyrolitic graphite: an XPS, UHV-AFM and air tapping-mode AFM study

The electrical conduction mechanisms of semicrystalline thermoplastic parylene C (-H(2)C-C(6)H(3)Cl-CH(2)-)(n) thin films were studied in large temperature and frequency regions. The alternative current (AC) electrical conduction in parylene C is governed by two processes which can be ascribed to a hopping transport mechanism: correlated barrier hopping (CBH) model at low [77-155 K] and high [473-533 K] temperature and the small polaron tunneling mechanism (SPTM) from 193 to 413 K within the framework of the universal law of dielectric response. The conduction mechanism is explained with the help of Elliot's theory, and the Elliot's parameters are determined. From frequency- and temperature-conductivity characteristics, the activation energy is found to be 1.27 eV for direct current (DC) conduction interpreted in terms of ionic conduction mechanism. The power law dependence of AC conductivity is interpreted in terms of electron hopping with a density N(E(F)) (~10(18) eV cm(-3)) over a 0.023-0.03 eV high barrier across a distance of 1.46-1.54 A.

Experimental and theoretical study of AC electrical conduction mechanisms of semicrystalline parylene C thin films.

The dielectric constant and the dielectric loss of thin films of metal-free phthalocyanine dispersed in polycarbonate using ohmic gold electrodes are investigated in the frequency range 20–105 Hz and within the temperature range 300–388 K. The frequency dependence of the impedance spectra plotted in the complex plane shows semicircles. The Cole–Cole diagrams have been used to determine the molecular relaxation time, τ, The temperature dependence of τ is expressed by thermally activated process. The AC conductivity σ AC (ω) is found to vary as ωs with the index s⩽1, indicating a dominant hopping process at low temperatures. From the temperature dependence of AC conductivity, free carrier conduction with mean activation energy of 0.33 eV is observed at higher temperatures. Capacitance and loss tangent are found to decrease with increasing frequency and increase with increasing temperature. Such characteristics are found to be in good qualitative agreement with existing equivalent circuit model assuming ohmic contacts.

AC conductivity and dielectric measurements of metal-free phthalocyanine thin films dispersed in polycarbonate

Current density-voltage characteristics and thermally stimulated currents have been obtained from thin films of β-nickel phthalocyanine particles dispersed in a polymer binder, when sandwiched between ohmic gold and blocking aluminium electrodes. At low voltages, the current in the forward direction varies exponentially with voltage. At higher voltages, two separate regions of space-charge limited current conduction under forward bias were observed. Under reverse bias, the conduction process at low voltages is determined by Schottky emission over a potential barrier of approximate height, 0·89 eV and thickness 65 nm. At higher voltages, the Poole-Frenkel effect was observed. The activation energy of the hole trapping level determined from the thermally stimulated current measurements has an electric field dependence. The results are interpreted in terms of the Poole-Frenkel effect.

Direct current conductivity and thermally stimulated current of β-nickel phthalocyanine films using gold and aluminium electrodes

Measurements of the temperature dependence of ohmic and space-charge-limited (SCL) bulk currents flowing in the c' direction in crystals of beta -nickel phthalocyanine in vacuum ambient have been used to locate the statistically important energy levels and their state densities in this molecular solid. Two temperature-induced changes in the activation energy E for ohmic conduction have been observed. From these findings the presence is deduced of a discrete electron level 0.43 eV below the conduction band, with a state density of 5*1025 m-3, and a hole level 0.64 eV above the valence band, having a state density of 3*1018 m-3. The thermal band gap is 1.63 eV. Measurement of the thermoelectric power showed that NiPc is n-type over the temperature range 325-425K, the excess donor concentration being 8*1017 m-3. The room temperature electrical conductivity is 1.0*10-3 Omega -1 m-1.

Charge transport in nickel phthalocyanine crystals. I. Ohmic and space-charge-limited currents in vacuum ambient

Electrical and photoelectrical measurements are made on thin films of β-cobalt phthalocyanine particles in Schottky junction cells. The results of current density-voltage characteristics in dark at constant temperature are interpreted in terms of a space charge limited mechanism dominated by an exponential distribution of trapping levels. An analysis of the results enables the determination of the Fermi temperature of the trap distribution. The dependence of the photovoltaic parameters on pigment loading and polymer binder are investigated. Typical polymeric binders used throughout this study are polyvinylacetate (PVA) and polycarbonate (MK). The results show that the polymer has a strong effect on photocurrent generation efficiency in phthalocyanine composite type photovoltaic cell; polymers having a polar group seem desirable for improving the performance.



An dunnen β-Kobalt-Phthalozyaninpartikelschichten in Schottky-ubergangszellen werden elektrische und photoelektrische Messungen durchgefuhrt. Die Ergebnisse der Stromdichte-Spannungscharakteristik im Dunkeln bei konstanter Temperatur werden mit einem Mechanismus der Raumladungsbegrenzung, der durch eine exponentielle Verteilung der Trapniveaus beherrscht wird, interpretiert. Eine Analyse der Ergebnisse ermoglicht, die Fermitemperatur der Trapverteilung zu bestimmen. Die Abhangigkeit der Photospannungsparameter von der Pigmentbeladung und dem Polymerbinder wird untersucht. Typische polymere Binder, die durchweg fur die Untersuchung benutzt werden, sind Polyvinylazetat (PVA) und Polykarbonat (MK). Die Ergebnisse zeigen, das das Polymer einen grosen Einflus auf den Photostromgenerationswirkungsgrad in Phthalozyanin-Photospannungszellen hat; Polymere mit einer polaren Gruppe scheinen fur eine Verbesserung der Leistungsparameter geeignet.

T.G. Abdel-Malik

Papers

AC conductivity and dielectric measurements of metal-free phthalocyanine thin films dispersed in polycarbonate

Direct current conductivity and thermally stimulated current of β-nickel phthalocyanine films using gold and aluminium electrodes

Charge transport in nickel phthalocyanine crystals. I. Ohmic and space-charge-limited currents in vacuum ambient

Dark and Photoelectric Investigation on Thin Films of β‐Cobalt Phthalocyanine Dispersed in Polymers

Transport properties in nickel phthalocyanine thin films using gold electrodes