M. K. Fung

Efficient organic photovoltaic devices using a combination of exciton blocking layer and anodic buffer layer

By using bathophenanthroline (BPhen) as an exciton blocking layer (EBL) at the organic/cathode contact of a standard copper phthalocyanine/C60 organic photovoltaic (OPV) device, power conversion efficiency was substantially increased from 0.86% to 2.64%. The BPhen-based devices showed a 45% increase in power conversion efficiency over that of an equivalent device with an EBL of bathocuproine. The performance improvement was analyzed in terms of the electron energy levels, optical transparencies and electron mobilities of the two EBLs. Based on these results, the roles of and requirements for an effective EBL were discussed. Combining the use of BPhen and a WO3 anodic buffer layer further increased the power conversion efficiency of the OPV device to 3.33%.

Doping-induced efficiency enhancement in organic photovoltaic devices

Performance of organic photovoltaic (OPV) devices is dramatically enhanced by doping suitable fluorescent dyes into the donor and/or acceptor layers. By doping rubrene into standard CuPc∕C60 OPV cell, a high JSC of 30.1mA∕cm2, VOC of 0.58V, and an exceptionally high power conversion efficiency of 5.58% are achieved. The performance improvement is mainly attributed to efficient light absorption by rubrene in the range of 460–530nm where two hosts have low absorbance, leading to more effective exciton formation. Their findings motivate the use of fluorescent dyes for maximizing absorption spectral coverage as well as increasing photon harvesting.

Limits of open circuit voltage in organic photovoltaic devices

Open circuit voltage (Voc) of organic photovoltaic devices has been interpreted with either the metal-insulator-metal (MIM) model or the energy offset between highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor (HOMOD-LUMOA). To elucidate the relation between Voc and the two models, we have used electrodes of a wide range of work functions to connect the CuPc/C60 organic photovoltaic devices. We found that when the work function difference (Δϕelectrodes) between ITO and Al electrode is in the range −3 and 0 eV, Voc increases linearly with Δϕelectrodes as prescribed by the MIM model. Outside this range, Voc saturates with values close to that given by the HOMOD-LUMOA less the exciton binding energy.

Copper hexadecafluorophthalocyanine and copper phthalocyanine as a pure organic connecting unit in blue tandem organic light-emitting devices

A nondoped organic system of copper hexadecafluorophthalocyanine (F16CuPc)∕copper phthalocyanine (CuPc) has been investigated as a connecting unit for deep-blue electrofluorescent tandem organic light-emitting devices (OLEDs) based on 2-methyl-9,10-di(2-naphthyl) anthracene emission. Such devices exhibited a doubling in current efficiency from 0.63to1.47cd∕A at J=100mA∕cm2 as compared to the single-unit device. The pure organic connecting unit showed superior optical transparency (∼100%), resulting in minimal microcavity effect in the devices. Interface dipole and band bending on both sides of the F16CuPc∕CuPc interface suggested the formation of an intrinsic p-n junction, which is a prerequisite of an effective connecting unit leading to a dramatic performance improvement in the tandem OLEDs.

Efficient CsF/Yb/Ag cathodes for organic light-emitting devices

A high-performance cathode consisting of an ultrathin CsF layer and a rare-earth ytterbium (Yb) metal is reported for application in organic electroluminescent devices. Standard tris-(8-hydroxyquinoline) aluminum/α-napthylphenylbiphenyl diamine devices with this bilayer cathode showed dramatically reduced operating voltage and a low turn-on voltage of 2.42 V as compared to 3.75 and 2.95 V in devices using, respectively, the Mg:Ag and single-layer Yb cathodes. At a current density of 200 mA/cm2, devices with the CsF/Yb cathode exhibited high luminance efficiency of 3.45 cd/A and power efficiency of 1.27 lm/W. Analysis by x-ray photoemission spectroscopy suggested that the performance improvement is related to the substantial reduction of electron injection barrier at the cathode/organic interface. It was found that upon Yb deposition, CsF dissociates to liberate low work function Cs metal atoms resulting in a cathode with a lower electron injection barrier and thus a better balance of carriers in the devic...

Applications of Ytterbium in organic light-emitting devices as high performance and transparent electrodes

Potential applications of Ytterbium (Yb) in cathode system for organic optoelectronic/electronic devices were explored in NPB/Alq3 based bi-layer organic light-emitting devices (OLEDs). When a thin (14.5 nm) Yb layer capped with a thicker (200 nm) Ag layer was used as the cathode, the OLEDs show enhanced electron injection over those using the standard Mg:Ag cathode. Performance of the OLEDs with the Yb/Ag cathode is comparable to that using LiF/Al cathode. Interestingly, we also found that Yb can also be used to prepare a highly transparent cathode by coevaporating Yb and Ag to form a Yb:Ag alloy electrode. Surface-emitting (or top emission) and transparent (emission from both surfaces) OLEDs with low turn-on voltage (3.75 V) and high efficiency were prepared with the Yb:Ag electrode. 2002 Elsevier Science B.V. All rights reserved.

Electronic structure and energy band gap of poly (9,9-dioctylfluorene) investigated by photoelectron spectroscopy

The electronic structure of poly (9,9-dioctylfluorene) (PFO) film on a Au-coated Si substrate was investigated by ultraviolet photoelectron spectroscopy (UPS) and x-ray photoelectron spectroscopy (XPS). From the UPS measurement, we obtained the ionization potential (Ip) of the PFO film, Ip=5.60±0.05 eV. From the XPS shake-up peaks of the C1s core level, we estimated the electron energy band gap (Eg) of the film, Eg=3.10±0.10 eV. By comparing the Eg with the optical absorption gap, we found that the value of Eg is closer to the optical absorption maximum than to the optical absorption edge. Therefore, we suggest that the optical absorption maximum may be a better approximation than the optical absorption edge in estimating Eg.

Impact of the metal cathode and CsF buffer layer on the performance of organic light-emitting devices

The influences of different metal cathodes on the performance of organic light-emitting devices were systematically studied. In addition to the well-known effects of metal work function, the effects of reflectivity and reactivity of the metal cathode on the device efficiency and operational stability were explored. The interplays of different metal cathodes and a CsF buffer layer were also studied in standard α-napthylphenylbiphenyl diamine/tris-(8-hydroxyquinoline) aluminum (NPB/Alq3) devices. It was found that when the metal cathode is directly deposited on the organic layer, the device performance improves as the metal work function decreases. This effect is modulated by the metal reflectivity such that rare-earth metal cathodes, which typically have a lower reflectance, have a lower efficiency than alkaline-earth metal cathodes. Device operational stability is found to be related to the reactivity between Alq3 and the metal cathode. Devices with metal cathodes that react detrimentally with Alq3, such ...

Long-lifetime thin-film encapsulated organic light-emitting diodes

Multiple fluorocarbon (CFx) and silicon nitride (Si3N4) bilayers were applied as encapsulation cap on glass-based organic light-emitting diodes (OLEDs). When CFx/Si3N4 bilayers were deposited onto the OLED structure, the devices showed performance worse than one without any encapsulation. The adverse effects were attributed to the damage caused by reaction species during the thin-film deposition processes. To solve this problem, a CuPc interlayer was found to provide effective protection to the OLED structure. With a structure of CuPc/(CFx/Si3N4)×5, the encapsulated device showed an operation lifetime over 8000 h (higher than 80% of that achieved with a conventional metal encapsulation).

Efficiency enhancement and voltage reduction in white organic light-emitting devices

High-efficiency and low operating voltage fluorescent white organic light-emitting devices (WOLEDs) have been realized by doping either 4,7-diphenyl-1,10-phenanthroline (BPhen) or N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) into the blue light-emissive layer. Devices doped with BPhen (or NPB) exhibited a maximum power efficiency of 8.7lm∕W (7.6lm∕W), about 74% higher than that of the reference device (5.0lm∕W). Such performance improvement is ascribed to the incorporation of a better electron-transporting layer and an improved carrier transport through the emissive layer by mixing with the higher drift mobility materials. It provides a simple and general means to improve the power efficiency of WOLED.