In this contribution, we discuss several research results which have contributed
to the vision of a broad application of organic light emitting diodes in displays and
lighting. We discuss the factors which determine the efficiency of OLEDs. First, we
briefly discuss work on the controlled molecular doping of organic semiconductors,
having the same advantageous influence on the device properties than in inorganic
semiconductor devices, in particular in reducing the operating voltage. By comparison
with a simple theory, we show that the voltages achieved with doped OLEDs are
already very close to the thermodynamic limit. We then show, with the example of
red phosphorescent OLEDs, how many properties of the OLED are improved by
Standard organic light emitting diodes (OLEDs) are usually bottom-emitting, i.e. they emit light through a transparent
and electrically conductive substrate. Usually, indium tin oxide (ITO) is used for this purpose. However, as indium is a
very expensive metal, replacing it is of vital interest for cheap OLED mass production, especially when it comes to
lighting applications. We suggest the use of a polymer instead of ITO, carrying out both hole transport and injection. In
contrast to conventional approaches, which use a conductive polymer on top of ITO as smoothening and hole injection
layer, we employ solely a highly conductive polymer in combination with an OLED comprising doped charge transport
layers. This allows us to renounce the ITO layer underneath.
We use a new, highly conductive formulation of PEDOT:PSS, called Baytron® PH 500, with a conductivity of typically
500 S/cm, providing a smooth and electrically well-conductive substrate for the OLED stack. The use of such a
polymeric injection layer and of a doped small-molecule OLED stack results in a low operating voltage of the devices.
The charge transport layers of the OLED consist of MeO-TPD (N,N,N',N'-tetrakis(4-methoxyphenyl)-benzidine) doped
with a low percentage of F4-TCNQ (2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-quinodimethane) for the hole transport layer
and of Bphen (4,7-diphenyl-1,10-phenanthroline) co-evaporated with Caesium for the electron transport layer. We
demonstrate both fluorescent and phosphorescent monochromic OLEDs based on Baytron® PH 500 which achieve good
efficiencies. The OLEDs made on Baytron® PH 500 are compared with devices made on an ITO anode. Although the
polymer possesses a somewhat lower conductivity than ITO, efficient devices can be fabricated. For example, using the
blue emitter Spiro-DPVBi (2,2',7,7'-tetrakis(2,2-diphenylvinyl)spiro-9,9'-bifluorene), we achieve an efficiency of up to
5.1 cd/A. As another example, we discuss green OLEDs based on the triplet emitter Ir(ppy)3 (fac tris(2-phenylpyridine)
iridium) doped in a wide gap material. In this case, even a higher efficiency than on ITO is reached: 62 cd/A at a
luminance of 100 cd/m2, corresponding to an increase in external quantum efficiency by 15% as compared to ITO.
We present highly efficient, low voltage top emitting organic light-emitting diodes (OLEDs) employing the same material (Ag) for both anode and cathode. Benefiting from doped charge carrier injection and transport layers (p-i-n structure) the diodes show comparable or even better electric characteristics than similar bottom emission OLEDs although the work function of the electrodes in the top emission OLEDs and the HOMO/LUMO location of the transport materials do not coincide. A green top emitting OLED with an Ir(ppy)3 doped double emission layer (D-EML) is demonstrated showing an efficiency of 50 cd/A at a brightness of 1000 cd/m2 and at the same time needing a very low driving voltage of only 2.85 V for 1000 cd/m2. By putting an additional organic capping layer on top of the cathode, the optical structure of the device can be tuned and the efficiency of the diodes can be further improved to a maximum efficiency of 78 cd/A at a brightness of 1000 cd/m2. Using the same capping strategy, efficient phosphorescent red and fluorescent blue top emitting OLEDs are demonstrated with efficiencies at 1000 cd/m2 as high as 13.6 cd/A and 8.6 cd/A, respectively.
The use of organic light-emitting diodes (OLEDs) for large area general lighting purposes is gaining increasing interest during the recent years. Especially small molecule based OLEDs have already shown their potential for future applications. For white light emission OLEDs, power efficiencies exceeding that of incandescent bulbs could already be demonstrated, however additional improvements are needed to further mature the technology allowing for commercial applications as general purpose illuminating sources. Ultimately the efficiencies of fluorescent tubes should be reached or even excelled, a goal which could already be achieved in the past for green OLEDs.1 In this publication the authors will present highly efficient white OLEDs based on an intentional doping of the charge carrier transport layers and the usage of different state of the art emission principles. This presentation will compare white PIN-OLEDs based on phosphorescent emitters, fluorescent emitters and stacked OLEDs. It will be demonstrated that the reduction of the operating voltage by the use of intentionally doped transport layers leads to very high power efficiencies for white OLEDs, demonstrating power efficiencies of well above 20 lm/W @ 1000 cd/m2. The color rendering properties of the emitted light is very high and CRIs between 85 and 95 are achieved, therefore the requirements for standard applications in the field of lighting applications could be clearly fulfilled. The color coordinates of the light emission can be tuned within a wide range through the implementation of minor structural changes.
We present highly efficient, low-voltage multilayer organic light emitting diodes based on the phosphorescent emitter tris(2-phenylpyridine) iridium (Ir(ppy)3). The phosphor is doped into various wide gap electron-transport or hole-transport host materials embedded in between doped transport layers. We use 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) doped N,N,N',N'-tetrakis(4-methoxyphenyl)-benzidine (MeO-TPD) as p-type hole-injection and transport layer, while cesium (Cs) and 4,7-diphenyl-1,10-phenanthroline (Bphen) are co-evaporated for the n-type doped electron transport layer. Sandwiched between these two transport layers, we insert one or two emission layers. This p-i-n structure results in efficient carrier injection from both contacts into the doped transport layers and low ohmic losses. Thus, lower operating voltages are obtained compared to conventional undoped OLEDs.
By doping Ir(ppy)3 into a double layer structure of predominantly electron and hole transporting hosts, a power efficiency of 70 lm/W and an external quantum efficiency of 19.5% is achieved at 100 cd/m2 (2.95V). Besides this, the efficiency decays only weakly with increasing current density (or brightness). A quantum efficiency of 13.5% is still obtained at a current density of 100 mA/m2 with a luminance around 50,000 cd/m2. This improvement can be attributed mainly to the fact that we prevent any charge accumulation at hole or electron blocking layers and spread the generation region to both sides of the interface between the two parts of the emission layer. Moreover, losses due to non-radiative decay of triplet excitons diffusing into regions without the phosphorescent dye are avoided.
We demonstrate high-efficiency organic light-emitting diodes (OLEDs) by incorporating a double emission layer (D-EML) into p-i-n-type cell architecture. The D-EML comprises two layers with ambipolar transport characteristics, both doped with the green phosphorescent dye tris(phenylpyridine)iridium [Ir(ppy)3]. The first EML features a bipolar, but predominantly hole transporting host material, 4,4',4''-tris(N-carbazolyl)-triphenylamine (TCTA), while the second EML is made of an exclusively electron transporting host, e.g. 3-phenyl-4-(1'-naphthyl)-5-phenyl-1,2,4-triazole (TAZ); with a weak hole transport capability arising from hopping between dopant sites. The D-EML system of two bipolar layers leads to an expansion of the exciton generation region. Due to its self-balancing character, it avoids accumulation of charge carriers at any interface. Thus, a power efficiency of approximately 77 lm/W and an external quantum efficiency of 19.3% are achieved at 100 cd/m2 at an operating voltage of only 2.65V. More importantly, the efficiency decays only weakly with increasing brightness and a power efficiency of 50 lm/W is still obtained even at 4,000 cd/m2.
The operating voltage of organic light emitting diodes (OLEDs) is important for the power consumption of active or passive matrix displays since it influences both the power consumption of the OLED itself and the power consumption of the driver circuitry. We have shown that very low operating voltages can be achieved in small-molecule OLED by intentional electrical n- and p-type doping. Even more important than the reduction of the voltage is the fact that doping of the charge carrier transport layers improves charge injection, making it basically independent on the actual contact work-functions. Organic light emitting diodes (OLEDs) with electrically doped transport layers show significantly improved properties: For instance, we have achieved a brightness of 100cd/m2 already at a voltage of 2.55V (based on a simple singlet emitter system), well below previous results for undoped small-molecule devices. With phosphorescent emitter dopants, high quantum and power efficiency of OLEDs with doped transport layers can be achieved: operating voltages and current efficiencies of 3.1V and 44cd/A (corresponding to approx. 44lm/Watt at 100cd/m2) are reported here. Inverted and fully transparent devices with parameters comparable to standard bottom-emitting OLED have been demonstrated as well.
By using two-color femtosecond optically heterodyned optical Kerr effect experiment, we studied the real components of third-order nonlinearity of 2,5-dioctyloxy poly(p-phenylene vinylene) (DO-PPV). As pump wavelength was set near the absorption region of the material, the value of x(3) of DO-PPV was improved by one order of the magnitude comparing with that of nonresonant condition. We suggest that this enhancement result from the contribution of the excited state.