Electrical doping of organic layers is now a well established method for building highly efficient and long living OLEDs.
A unique class of OLED devices called PIN-OLEDs based on redox doping technology is emerging as one key
technology for OLED applications. These devices exhibit high power efficiency and long life time, which are critical
parameters for commercial success. Moreover, PIN OLEDs offer high degree of freedom in choosing layer structures for
optimizing the device performance for specific lighting and display applications. For example, optimizing color and
power efficiency of OLEDs can be easily achieved without compromising the device operating voltage. It is worth to
mention that PIN OLEDS, especially the red emitting PIN OLEDs, exhibit record breaking half life time of more than
one million hours with the starting device brightness of 1000 cd/m<sup>2</sup>. The doping technology also offers benefits to other
organic electronic devices such as OTFTs and photovoltaic devices. This paper briefly discusses the improvements made
on the OLED device performance such as power efficiency and lifetime using doped transport layers. Few examples of
device optimization using doped layers are presented in detail. In addition, a brief discussion on performance of doped
transport layers in photovoltaics is also presented.
Organic light-emitting devices (OLEDs) containing highly efficient phosphorescent emitters and highly conductive
doped organic transport layers were studied. Saturated red devices with luminous efficiency of 15 cd/A operate at <4 V;
hence, they have a record power efficiency of 12 lm/W at 1,000 cd/m<sup>2</sup>.
Additionally, two high-efficiency red OLEDs were serially connected and vertically stacked to create a stacked OLED
having a luminous and power efficiency (at 1,000 cd/m<sup>2</sup>) of 28 cd/A and 12 lm/W, respectively. The electrical
connection between the two OLEDs is enabled by molecular p- and n-type doped organic transport layers.
The single emissive layer red OLED has a projected lifetime (time to half initial luminance) of ~150,000 hrs from an
initial brightness of 500 cd/m<sup>2</sup>. The stacked device shows very similar lifetime characteristics when driven at similar
currents, which results in significantly prolonged lifetime of ~260,000 hrs at an initial luminance of 500 cd/m<sup>2</sup>.
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/m<sup>2</sup> 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/m<sup>2</sup>) are reported here. Inverted and fully transparent devices with parameters comparable to standard bottom-emitting OLED have been demonstrated as well.
We demonstrate an efficient organic electroluminescent devices with p-i-n structure. Anamorphous starburst, 4,4',4'-tris(3-methylphenylphenylamino)triphenylamine doped with a strong organic acceptor, tetrafluoro-tetracyano- quinodimethane serves as the p-type hole transport layer, and 4,7-diphenyl-1, 10-phenanthroline doped with Li as the n-type electron transport layer. A breakthrough is achieved in the performances of device based on pure 8-tris- hydroxyquinoline as an emitter: 100cd/m<SUP>2</SUP> at 2.52V, 1,000cd/m<SUP>2</SUP> at 2.9V and the maximum luminance and efficiency reach 66,000cd/m<SUP>2</SUP> and 5.25 cd/A, respectively. The efficiency can be kept above 3cd/A in a very large luminance region from 100 to 55,000cd/m<SUP>2</SUP>.
Organic light emitting diodes generally suffer from higher operating voltages compared to inorganic ones. This limits their application in passive or active driven displays based on OLED-technology. As was previously shown by our group, p-type doping of the hole injection and transport layer of an organic light emitting diode (OLED) by co-evaporation of a matrix and an acceptor molecule leads to lower operating voltages of the device. In OLEDs using doped transport layers, the use of a proper buffer layer between the doped layer and the light emission layer is essential to yield a high current efficiency and a low operating voltage at the same time. In order to further enhance the device efficiency, we apply here the doping concept to OLEDs with a light emission layer which is doped with a fluorescent dye. This approach proves that doping of the transport layer is able to improve the optoelectronic properties of already highly efficient OLEDs. The doped hole injection and hole transport layer is a Starburst layer p-type-doped with tetrafluoro-tetracyano-quinodimethane (F<SUB>4</SUB>-TCNQ). As blocking layer, a diamine (TPD) is used. The emitter layer consists of quinacridone (QAD) doped aluminum-tris-(8-hydroxy-quinolate) (Alq<SUB>3</SUB>). Holes are injected from untreated ITO, electrons via a lithium-fluoride (LiF)/aluminum cathode combination. For this OLED layer sequence, we achieved a luminance of 100cd/m<SUP>2</SUP> in forward direction at the lowest operating voltage reported for completely non-polymeric OLEDs (3.2-3.4V) with a current efficiency of around 10cd/A.