The development of high performance organic light emitting diodes (OLEDs) for display and lighting
applications has attracted considerable research interest from both academia and industry. In this work, the
designs of simplified phosphorescent OLEDs with exceptionally high efficiency are discussed. It is found
that discrete blocking layers and a double emission zone are unnecessary to achieve high efficiency in
optimized phosphorescent OLEDs. Due to the elimination of these redundant layers, the device structure can
be highly simplified. It is also shown that single-layer, two organic component devices are feasible with
Significant development has been made on phosphorescent organic light emitting diodes (PHOLEDs) over
the past decade, which eventually resulted in the commercialization of widely distributed active-matrix organic light
emitting diode displays for mobile phones. However, higher efficiency PHOLEDs are still needed to further reduce
the cost and lower the power consumption for general lighting and LED backlight applications. In particular, red
PHOLEDs currently have in general the lowest efficiencies among the three primary colors, due most likely to the
energy-gap law. Therefore, a number of groups have of made use of various device configurations, including
insertion of a carrier blocking or exciton confining layer, doping the transport layers, as well as employing multiple
emissive zone structures to improve the device efficiency. However, these approaches are rather inconvenient for
commercial applications. In this work, we have developed a simpler way to boost the performance of red PHOLEDs
by incorporating an exciton harvesting green emitter, which transfers a large portion of the energy to the co-deposited
red emitter. A high external quantum efficiency (EQE) of 20.6% was achieved, which is among the best
performances for red PHOLEDs.
Non-blocking Phosphorescent Organic Light Emitting Diode (NB-PHOLED) is a highly simplified device structure that
has achieved record high device performance on chlorinated ITO, flexible substrates, also with Pt based
phosphorescent dopants and NB-PHOLED has significantly reduced efficiency roll-off. The principle novel
features of NB-PHOLED is the absence of blocking layer in the OLED stack, as well as the absence of organic hole
injection layer, this allows for reduction of carrier accumulation in between organic layers and result in higher
The band alignment at metal-organic interfaces has been extensively studied; however the electrodes in real devices
often consist of metals modified with dielectric buffer layers. We demonstrate that interface dipole theory, originally
developed to describe Schottky contacts at metal-semiconductor interfaces, can also accurately describe the injection
barriers in real organic electronic devices (i.e, at
electrode-organic interface). It is found that theoretically predicted
hole injection barriers for various archetype metal-organic and
metal-dielectric-organic structures are in excellent
agreement with values extracted from experimental measurements.
Fullerene (C<sub>60</sub>) has been found to form a universal hole injection interface in the Metal/C<sub>60</sub> anode structure. This bilayer structure opens up possibilities to select various highly conductive metals as anodes to replace indium tin oxide (ITO).
Organic light emitting diodes (OLEDs) utilizing Au/C<sub>60</sub> and Mg/C<sub>60</sub> bilayer anodes were fabricated.
Electroluminescence (EL) efficiencies of devices with Au/C<sub>60</sub> anodes surpassed the ITO baseline device by ~ 25%. It was found that the hole injection current for the Au/C<sub>60</sub> anode can be tuned via the C<sub>60</sub> interlayer thickness in the range of 1 - 5 nm. Single carrier
hole-only (HO) devices with different metal and metal/C<sub>60</sub> bilayer anodes were studied. With
the insertion of a 3 nm thick C<sub>60</sub> buffer layer between the anode metal and hole transport layer (HTL), an increase in hole
injection current of more than two orders was realized. Based on device modeling we extracted C<sub>60</sub>-induced dipoles on
the metal surfaces. These dipole values agree well with values obtained by Kelvin probe and photoemission
measurements. What is more, the dipole values effectively pin the work function of all metals to a common value of ~ 4.6 eV, creating a universal hole injection barrier regardless of the pristine anode metal work function.