M. Meier

Light-emitting diodes based on poly-p-phenylene-vinylene: II. Impedance spectroscopy

Electrical impedance measurements on poly-p-phenylene-vinylene (PPV) light-emitting diodes in the frequency range between 100 Hz and 10 MHz are reported. A significant difference can be revealed between the device characteristics of light-emitting diodes eliminated on indium-tin-oxide (ITO) and those of other high-work-function metals (e.g., Au). Thermal conversion of the precursor polymer on ITO substrates results in a p-type doping of the conjugated polymer PPV. Hence, devices in the configuration ITO/PPV/Al display Schottky behavior, which can be modeled by a simple equivalent circuit of two RC elements in series, representing a bulk and a junction region. The low-frequency device capacitance displays a pronounced voltage dependence and, from a detailed analysis, the ionized acceptor concentration NA, the diffusion potential VD, and the width of the space charge region w can be obtained. Typical values for NA are 1016–1017 cm−3, and for VD within the range 1–1.5 V, resulting in a width w of the space c...

Light-emitting diodes based on poly-p-phenylene-vinylene: I. Charge-carrier injection and transport

Detailed investigations of the device characteristics of poly-p-phenylene-vinylene (PPV) light-emitting diodes are reported. We analyze the influence of various hole- and electron-injecting electrodes on the current–voltage (I–V) characteristics and electroluminescence behavior. Our studies reveal that thermal conversion of the prepolymer on indium–tin–oxide (ITO) substrates leads—in contrast to conversion on Au and other high-work-function metals—to a p-type doping of PPV and, additionally, to the formation of an ohmic hole-injecting contact at the ITO/PPV interface. Hence, devices fabricated with low-work-function metals acting as the electron injecting contact (for example, Al and Ca) display Schottky behavior. These Schottky diodes are distinguished by a high rectification ratio ρr of about 106 and display electroluminescence at bias voltages as low as 1.5 V for ITO/PPV/Ca light-emitting diodes. The I–V characteristics can be quantitatively described within the modified Shockley equation, taking into ...

Transient electroluminescence in poly(p-phenylenevinylene) light-emitting diodes

Abstract Transient behaviour of poly(p-phenylenevinylene) (PPV) thin film devices was investigated via short voltage pulses. Between the application of a rectangular voltage pulse and the first appearance of electroluminescence (EL) a time lag exists, which depends on the pulse height. Response times in the order of 10−3 to 10−6 s in a small voltage range from 2 to 10 V, respectively, were measured. Thus, the estimated mobility of positive charge carriers shows a strong field dependence ranging from 10−7 to 3 × 10−6 cm2 V−1 s−1, which can be described by a Poole-Frenkel-detrapping model. We observed a delayed EL with a lifetime of 80 μs, pointing to a triplet-triplet-annihilation process.

DETERMINATION OF TRAPPING PARAMETERS IN POLY(P-PHENYLENEVINYLENE) LIGHT-EMITTING DEVICES USING THERMALLY STIMULATED CURRENTS

Employing thermally stimulated current (TSC) technique, the existence of distinct trap distributions in poly(p-phenylenevinylene) (PPV) light-emitting devices has been established. In devices with an indium tin-oxide (ITO) anode two TSC peaks in the temperature range between 100 and 150 K are observed. They correspond to trap levels with a depth of 0.03–0.06 eV and 0.13–0.18 eV, respectively. The total density of these trap species is of the order of 1016 cm−3 which is in good agreement with the dopant concentration obtained from capacitance-voltage measurements. The investigated peaks in the TSC spectrum do not occur if a Au electrode is used instead of ITO. Hence, the reaction of ITO with the elimination products (mainly HCl) during the conversion of the PPV precursor leads to the formation of these shallow traps. Deeper trap states with energies between 0.6 and 1 eV have been detected, too. The latter trapping levels appear independently of the anode substrate material and are due to the influence of air.

Control of impurities in PPV light-emitting devices

Abstract The influence of different substrates used for the fabrication of poly( p -phenylene vinylene) (PPV) light-emitting devices on the device characteristics is investigated with different experimental techniques, like current-voltage, brightness-voltage and capacitance-voltage measurements. Using thermally stimulated currents we determine the energetic depth and density of states created by doping of PPV during device fabrication. In devices prepared on indium-tin oxide (ITO) substrates doping with InCl 3 leads to states with a depth of about 0.15 eV and an ionized acceptor concentration in excess of 10 16 cm −3 . These carriers are mobile and form a depletion layer of width 120 nm when a metal with low work function, like Al. is used as cathode. This doping is responsible for the observed Schottky diode behaviour in PPV devices on ITO. With fluorine-doped tin dioxide as transparent hole-injecting contact, trap energies increase slightly to 0.2 eV and the ionized acceptor concentration is lowered by a factor of five. The lower doping concentration leads to an increase of the depletion layer width to about 270 nm and thickness-dependent device characteristics. For PPV convened on gold no doping is detectable with capacitance-voltage measurements and thermally stimulated currents. Photoluminescence measurements show a significant quenching of fluorescence in PPV converted on ITO. We regard this as an important limiting factor for single and heterolayer devices with PPV as emissive material.

Heterolayer light-emitting diodes based on new oxadiazole polymers

Abstract Monolayer light-emitting diodes (LEDs) from poly(1,4-phenylene vinylene) (PPV) usually exhibit relatively low quantum efficiencies. Thus, the external efficiency of an indium-tin oxide (ITO)/PPV/A1 LED is typically 0.001%. In order to increase the quantum yield heterolayer devices with new oxadiazole polymers have been fabricated. These polymers with the electron-withdrawing oxadiazole units facilitate electron injection and transport in bilayer LEDs with PPV as hole transport and emitting layer. Thus, LEDs with external quantum efficiencies up to 0.2% have been achieved. Compared to conventional PPV LEDs no temperature dependence of the quantum efficiency is detectable, indicating improved balanced charge carrier injection at room temperature.

Doping in PPV light-emitting devices fabricated on different substrates

Abstract The influence of different substrates used for the fabrication of poly-(p-phenylene-vinylene) light-emitting devices on the device characteristics is investigated with different experimental techniques, like current-voltage, brightness-voltage and capacitance-voltage measurements. Using thermally stimulated currents we determine the energetic depth and density of states created by doping of PPV during device fabrication. In devices prepared on indium-tin oxide substrates doping with InCl 3 leads to states with a depth of about 0.15 eV and an ionized acceptor concentration in excess of 10 16 cm −3 . These carriers are mobile and form a depletion layer with a width of about 100 nm when a metal with low work function, like Al, is used as cathode. This doping is responsible for the observed Schottky diode behaviour in PPV devices on ITO. With fluorine-doped tin dioxide as transparent hole injecting contact trap energies slightly to 0.2 eV and the ionized acceptor concentration is lowered by a factor of 5. The lower doping concentration leads to an increase of the depletion layer width to about 250–300 nm and thickness dependent device characteristics. For PPV converted on gold no doping is detectable with capacitance-voltage measurements and thermally stimulated currents. Photoluminescence measurements show a significant quenching of fluorescence in PPV converted on ITO. We regard this as an important limiting factor for single and heterolayer devices with PPV as emissive material.

Characterization of light emitting diodes and solar cells based on poly-phenylene-vinylene

Abstract Due to its good processability and thermal stability poly-p-phenylene-vinylene (PPV) is well suited for electronic devices. In this paper we report on electrical properties of PPV-based light-emitting diodes and solar cells. At room temperature I-V-characteristics in the dark reveal a good diode behaviour with a maximum rectification ratio of 10 6 . The threshold voltage for visible electroluminescence (EL) is device dependent and under best conditions we measure values as low as 2 V. Impedance measurements show that our diodes can be described within the Schottky barrier model, e.g. as a serial circuit of resistive and capacitive components, for both the bulk and the junction. At low temperatures the I-V-characteristics become almost symmetrical and EL appears in both current directions, indicating that the Schottky junction is not necessary for the observation of EL in conjugated polymers. Under white light illumination an open-circuit voltage of more than 1 V and a power conversion efficiency of about 0.1 % can be achieved, suggesting the possible application as solar cells. Ageing experiments point to a significant influence of oxygen on the electrical properties and consequently on the lifetime of the devices.

Electroluminescence and photovoltaic effect in PPV Schottky diodes

Abstract Detailed investigations ofI–V characteristics and impedance spectroscopy of light emitting diodes (LEDs) based on poly-p-phenylene-vinylene (PPV) indicate that the devices operate like Schottky diodes.I–V characteristics reveal a good ectification ratio of 104–106 and can be described using the Shockley equation. The emission spectra of the Ls depend on the degree of substitution of the PPV-polymer. The quantum efficiency of LEDs from mixtures of PPV and substituted PPV exceeds the quantum efficiency from pure PPV typically by a factor of 2–5. Under illumination a photovoltaic effect is measured with open circuit voltages up to 1.3 V on Al/PPV/ITO devices. The power conversion efficiency of these solar cells increases with increasing temperature and values between 0.1% and 1% can be achieved.

MetaVInsulatorPolymer - LEDs Based on PPV

Abstract We investigated the current-voltage (I-V) and electroluminescence (EL) characteristic of metal/polymer and metal/insulator/polymer (MIP) LEDs based on poly (1,4-phenylene vinylene) (PPV). The I-V- and EL characteristics of the MIP structures display a pronounced dependence of the insulator thickness and we measure an increase of the quantum efficiency of more than a factor of 40 at an AlOX layer thickness of 3–5nm. The device characteristic is qualitatively understood within inorganic metal/insulator/semiconductor (MIS) theory and can be explained by a voltage dependent barrier for minority carrier injection in connection with a hole blocking barrier at the PPV/insulator interface. With MIP structures we reveal external quantum efficiencies up to 0.01%, comparable to values achieved on monolayer Ca LEDs. The MIP structures, however have the advantage, that a more stable device performance is obtained.