The efficiency and stability of blue organic light emitting devices (OLEDs) continue to be a primary roadblock to
developing organic solid state white lighting. For OLEDs to meet the high power conversion efficiency goal, they
will require both close to 100% internal quantum efficiency and low operating voltage in a white light emitting
device.1 It is generally accepted that such high quantum efficiency, can only be achieved with the use of
organometallic phosphor doped OLEDs. Blue OLEDs are particularly important for solid state lighting. The simplest
(and therefore likely the lowest cost) method of generating white light is to down convert part of the emission from a
blue light source with a system of external phosphors.2 A second method of generating white light requires the
superposition of the light from red, green and blue OLEDs in the correct ratio. Either of these two methods (and
indeed any method of generating white light with a high color rendering index) critically depends on a high efficiency
blue light component.3
We report studies on blue and white organic light-emitting devices (OLEDs) based on the deep-blue electrophosphorescent
bis(4',6'-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate (FIr6). Using high triplet
energy charge transport layers and a dual-emissive-layer structure as well as the <i>p-i-n</i> device structure, we have achieved
external quantum efficiencies of 20% and maximum power efficiency of 36 lm/W in these deep-blue OLEDs. White
OLEDs with a CRI of 79 and a maximum power efficiency of 40 lm/W were also demonstrated by incorporating red and
green phosphorescent dopants together with FIr6.
Organic light emitting devices (OLEDs) have demonstrated the potential for solid state lighting as well as full color
display applications. Use of triplet harvesting phosphorescent materials has led to very high efficiency OLEDs especially
in green and red phosphorescent OLEDs. However in case of blue OLEDs the efficiency achieved is still room for
improvement. Charge balance is a very important factor for achieving high efficiency organic light emitting diodes. In
most OLED devices, hole mobility of hole transport layer is orders of magnitude higher than the electron mobility of
electron transport layer. We study how this affects the charge balance and hence the device performance in the blue
phosphorescent OLEDs with Iridium (III)bis
[(4,6-di-fluorophenyl)- pyridinato-N,C2´] picolinate (FIrpic) emitter.
Charge balance is studied in these devices and the devices are found to be hole dominant. Additionally, effect of charge
balance on device performance is demonstrated with different electron transport layers. Using this approach, a very high
efficiency of 60 Cd/A (50 lm/W) is achieved with
3,5´-N,N´-dicarbazole-benzene (mCP) host.
We have studied the effects of hole transporting layers and electron transporting layers on efficiencies of Iridium(III)bis
[(4,6-di-fluorophenyl)-pyridinato-N,C2'] picolinate (FIrpic) doped 3,5'-N,N'-dicarbazole-benzene (mCP) host blue
PHOLEDs. We found that the device efficiency is very sensitive to the hole transporting materials used and both the
triplet energy and carrier transport properties affect the device efficiency. On the other hand, there is no apparent
correlation between the device efficiency and the triplet energy of the electron transporting material used. Instead, the
device efficiency appears to be determined by the electron mobility of the electron transporting layer only.
In this paper, we demonstrate that the light extraction efficiency of an OLED is a strong function of the location of the
recombination zone and the ratio of the extracted mode to the substrate guided mode varies from 22% to 55%. The large
variation of the extraction efficiency in most OLEDs is the direct result of optical cavity effect present in the devices. In
addition, we show that the light intensity profile varies from a Lambertian shape to a non-Lambertian shape depending
of the device geometry.
Conference Committee Involvement (1)
Display, Solid-State Lighting, Photovoltaics, and Optoelectronics in Energy