W. Brütting

Device physics of organic light-emitting diodes based on molecular materials

Abstract Electrical transport in single- and hetero-layer organic light-emitting diodes based on aromatic amines like N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD) or N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (NPB) and the aluminium chelate complex Alq (tris(8-hydroxyquinolato)aluminium) has been investigated as a function of temperature and organic layer thickness. It is shown that the thickness dependence of the current–voltage (I–V) characteristics provides a unique criterion to discriminate between (1) injection limited behaviour, (2) trap-charge limited conduction with an exponential trap distribution and a field-independent mobility, and (3) trap-free space-charge limited conduction (SCLC) with a field and temperature dependent mobility. The I–V characteristics of NPB-based hole-only devices with indium–tin oxide anodes are neither purely injection nor purely space-charge limited, although the current shows a square-law dependence on the applied voltage. In Al/Alq/Ca electron-only devices with Alq thickness in the range 100–350 nm the observed thickness and temperature dependent I–V characteristics can be described by SCLC with a hopping-type charge carrier mobility. Additionally, trapping in energetically distributed trap states is involved at low voltages and for thick layers. The electric field and temperature dependence of the charge carrier mobility in Alq has been independently determined from transient electroluminescence. The obtained values of the electron mobility are consistent with temperature dependent I–V characteristics and can be described by both the phenomenological Poole–Frenkel model with a zero-field activation energy ΔE=0.4–0.5 eV and the Gaussian disorder model with a disorder parameter σ=100 meV. Measurements of the bias-dependent capacitance in NPB/Alq hetero-layer devices give clear evidence for the presence of negative charges with a density of about 6.8×10 11 cm −2 at the organic–organic interface under large reverse bias. This leads to a non-uniform electric field distribution in the hetero-layer device, which has to be considered in device description.

Charge carrier mobility in poly(p-phenylenevinylene) studied by the time-of-flight technique

The charge carrier transport in poly(p-phenylenevinylene) (PPV) is investigated by the time-of-flight technique. Mobilities of positive carriers in PPV are determined and the dispersive character of transport is established. The dispersion parameters are analyzed in the frame of a multiple trapping model. The drift mobility of the positive carriers is in the range of 10−5 cm2/Vu2009s at room temperature for an electric field of 105 V/cm and increases with increasing field and temperature. The mobility shows thermally activated behavior with an activation energy of about 0.75 eV at zero field. It is shown that the experimental results can be interpreted by polaron transport.

Very compact tunable solid-state laser utilizing a thin-film organic semiconductor

Optically pumped organic semiconductor lasers are fabricated by evaporation of a thin film of tris(8-hydroxyquinoline) aluminum (Alq(3)) molecularly doped with a laser dye on top of a polyester substrate with an embossed grating structure. We achieve low-threshold, longitudinally monomode distributed-feedback laser operation. By varying the film thickness of the organic semiconductor film, we can tune the wavelength of the surface-emitting laser over 44 nm. The low laser threshold allows the use of a very compact all-solid-state pump laser ( approximately 10 cm long). This concept opens up a way to obtain inexpensive lasers that are tunable over the whole visible range.

Interfacial charges and electric field distribution in organic hetero-layer light-emitting devices

Abstract The electric field distribution in organic hetero-layer light-emitting devices based on N,N′-diphenyl-N,N′-bis(1-naphtyl)-1,1′-biphenyl-4,4′-diamine (NPB) and 8-tris-hydroxyquinoline aluminium (Alq3) has been investigated under different bias conditions using capacitance–voltage measurements. Although this method yields primarily information on the differential capacitance, the data give clear evidence for the presence of negative interfacial charges with a density of 6.8×10 11 e cm −2 at the NPB/Alq3 interface at large reverse bias. This leads to a jump of the electric field at the interface and a non-uniform field distribution in the hetero-layer device.

ELECTRON TRANSPORT IN A STARBURST OXADIAZOLE

In this article we report on the electron mobility of a starburst oxadiazole, 1,3,5 -[(4-tert.-butylphenyl)-2-oxadiazolyl] benzene. For direct evidence of the electron transport capability the charge drift mobility and its field dependence have been measured by the time-of-flight technique. The electron mobility showed a square root dependence of the electric field and was somewhat larger than that for PBD. In addition, the oxadiazole compound has been used as the electron transport layer (ETL) in light-emitting diodes (LEDs) together with poly(1,4-phenylene vinylene). The characteristics of these devices are compared to analogous LEDs with Alq3 as the ETL.

Temperature dependent device characteristics of organic light-emitting devices

Abstract Current–voltage characteristics of single and hetero-layer light-emitting devices with an aromatic diamine (TPD) as hole transport material and tris-8-(hydroxyquinoline) aluminum (Alq 3 ) as electron transport material and emitter have been investigated over a wide temperature range and for various film thickness in order to identify the limiting mechanism: charge carrier transport or injection. From the observed thickness and temperature dependence, pure injection limitation can be ruled out as dominant mechanism. Instead, the voltage dependence of the current density can be well described by power laws j ∝ V m +1 (with V corrected by the built-in potential) with temperature dependent exponents m ranging from 4 to 25. This can be interpreted in terms of space charge limited currents (SCLC) in Alq 3 with an exponential energetic distribution of traps where m is given by m = E t / kT . A reasonable trap energy of 0.15–0.2 eV is obtained by a temperature dependent analysis of the I–V characteristics. However, the thickness dependence cannot be satisfactorily explained by the simple SCLC-model. This indicates that more sophisticated models are required.

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.

Deep level transient spectroscopy (DLTS) of a poly(p-phenylene vinylene) Schottky diode

Abstract Deep level transient spectroscopy measurements have been carried out on ITO/poly( p -phenylenevinylene)/Al organic light emitting diodes that have a depletion region type Schottky barrier at the polymer/metal interface. The very long lived capacitance transients can be successfully described by the de-trapping of p-type majority carriers from a single energy trap level to a Gaussian distribution of transport states. The Gaussian width of 0.10±0.02 eV and trap depth of 0.75±0.05 eV are in excellent agreement with values measured from other unrelated experimental techniques.

Anomalous current-voltage characteristics in organic light-emitting devices

Quasi-reversible current maxima at low voltage, leading to N-type current-voltage characteristics with negative differential resistance, have been observed in different types of organic light-emitting devices including conjugated polymer LEDs, dye-doped polymeric LEDs and LEDs from evaporated small molecules. We have investigated the dependence of this phenomenon on different external parameters, like layer thickness, temperature and time. We found that the usual explanations, e.g. by tunneling, cannot satisfactory explain our observations. Instead, our experiments indicate that spatially local effects are responsible for the anomalous high current flow at low voltage. The implications for device operation and lifetime are discussed.

Diffusion photovoltage in poly(p -phenylenevinylene)

Photovoltage phenomena in poly(p-phenylenevinylene) (PPV) are investigated under pulsed laser illumination. The photovoltage transients are strongly retarded in time depending on sample thickness, laser intensity, and bias illumination. It is shown that the photovoltage in PPV originates from separation of excess electrons and holes due to their concentration gradient and different diffusion coefficients (diffusion photovoltage). The diffusion coefficient of excess holes is found to be on the order of 1×10−6u200acm2/s and it increases with increasing excitation intensity and intensity of bias illumination. The diffusion coefficient of excess electrons is about 1–2 orders of magnitude smaller than for excess holes.Photovoltage phenomena in poly(p-phenylenevinylene) (PPV) are investigated under pulsed laser illumination. The photovoltage transients are strongly retarded in time depending on sample thickness, laser intensity, and bias illumination. It is shown that the photovoltage in PPV originates from separation of excess electrons and holes due to their concentration gradient and different diffusion coefficients (diffusion photovoltage). The diffusion coefficient of excess holes is found to be on the order of 1×10−6u200acm2/s and it increases with increasing excitation intensity and intensity of bias illumination. The diffusion coefficient of excess electrons is about 1–2 orders of magnitude smaller than for excess holes.