S. Berleb

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.

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.

Space-charge limited conduction with a field and temperature dependent mobility in Alq light-emitting devices

Abstract Electrical transport in organic light-emitting devices (OLEDs) based on tris(8-hydroxyquinolato)aluminium (Alq) is investigated as a function of temperature and organic layer thickness. It is shown that the thickness dependence of the current provides a unique criterion to discriminate between (1) injection limited behavior, (2) trap-charge limited conduction with an exponential trap distribution and a field independent mobility, and (3) trap-free space charge limited conduction with a field and temperature dependent mobility. The observed thickness and temperature dependent current–voltage characteristics are found to be in excellent agreement with trap-free SCLC with a hopping type charge carrier mobility.

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.

The influence of deep traps on transient current–voltage characteristics of organic light-emitting diodes

Abstract Transient current–voltage (I–V) characteristics of organic light-emitting diodes made from both conjugated polymers and low molecular-weight materials show hysteresis effects in the reverse bias regime depending on the direction and speed of the bias sweep. This behaviour is quantitatively investigated here for the example of devices based on N,N′-diphenyl-N,N′-bis(1-naphtyl)-1,1′-biphenyl-4,4′-diamine with Ca and indium–tin oxide as electrodes. To clarify the origin of this peculiarity numerical simulations have been carried out supposing the existence of deep acceptor-like trap states. Typical trends are shown by systematically varying parameters such as measuring conditions, trap characteristics, basic doping level, mobility and injection conditions. Based on the simulated potential and concentration profiles it is shown that the hysteresis of the I–V characteristics is caused by recharging of deep traps for holes. It occurs only if the reverse steady-state current is lower than the trap recharging current and if both currents have different bias dependencies. The origin for the large time needed for the traps to relax into the equilibrium state is clarified. In accordance with the high barrier for the holes at the cathode the calculated reverse current is much smaller than the measured one. Using a new analytical expression for the Schottky diode I–V characteristics for a low-doped thin film device, it is shown qualitatively that in real devices a leakage current should dominate for reverse bias.

Transient electroluminescence measurements on organic heterolayer light emitting diodes

Abstract We have investigated the field dependence of the electron and hole drift mobility in tris-(8-hydroxyquinoline) aluminium (Alq3) in organic multi layer light emitting diodes (LEDs) using transient electroluminescence measurements. The electron transport in thin films of Alq3 can be understood in terms of hopping processes. We obtain a full set of parameters for a modified Poole–Frenkel equation describing the temperature and field dependence of the electron drift mobility. For the measurement of the hole drift mobility, a special device structure was designed. We were able to determine the hole drift mobility in Alq3 and found good agreement with the data obtained from previous time-of-flight (TOF) measurements.

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.

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.

EFFECT OF MAJORITY CARRIER SPACE CHARGES ON MINORITY CARRIER INJECTION IN DYE DOPED POLYMER LIGHT-EMITTING DEVICES

By blending suitable dyes into poly(N-vinylcarbazole) we have fabricated devices which emit light in the whole visible spectrum. Their current–voltage characteristics can be described by space-charge limited currents with effective trapping of the charge carriers by the dye molecules, while the light intensity shows a Fowler–Nordheim-like behavior as a function of the external electric field. However, the anodic space charge changes the field distribution inside the device and leads to a cathodic field enhancement which has to be considered in the Fowler–Nordheim equation. We were able to model the electroluminescence characteristics by assuming tunneling of the minority charge carriers through a triangular barrier. The obtained barrier heights showed a strong dependence on the dye molecules, suggesting that the injection of minority charge carriers takes place directly into the lowest unoccupied molecular orbital of the chromophors. Using poly(p-phenylenevinylene) and an oxadiazole starburst molecule as ...

Full colour electroluminescence using dye-dispersed polymer blends

Abstract By blending suitable dyes into poly( N -vinylcarbazole) (PVK) we have fabricated devices which emit light in the whole visible spectrum. Their current-voltage ( I–V ) characteristics can be described by space-charge limited currents (SCLC) with effective trapping of the charge carriers by the dye molecules, while the light intensity shows a Fowler-Nordheim-like behaviour as a function of the mean electric field. We were able to model the electroluminescence characteristics by assuming tunnelling of the minority charge carriers through a triangular barrier. The obtained barrier heights showed a strong dependence on the dye molecules, suggesting that the injection of minority charge carriers takes place directly into the LUMO levels of the chromophors. The recombination along with the space charge leads to a steeper increase of the current and a Fowler-Nordheim-like I–V characteristic in the double injection regime though the majority carrier current is not injection limited.