Mei-Yee Chan

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.

High-efficiency green organic light-emitting devices utilizing phosphorescent bis-cyclometalated alkynylgold(III) complexes

A new phosphorescent material of cyclometalated alkynylgold(III) complex, [Au(2,5-F(2)C(6)H(3)-C∧N∧C)(C≡C-C(6)H(4)N(C(6)H(5))(2)-p)] (1) (2,5-F(2)C(6)H(3)-HC∧N∧CH = 2,6-diphenyl-4-(2,5-difluorophenyl)pyridine), has been synthesized, characterized, and its device performance investigated. This luminescent gold(III) complex was found to exhibit rich PL and EL properties and has been utilized as phosphorescent dopants of OLEDs. At an optimized dopant concentration of 4%, a device with a maximum external quantum efficiency (EQE) of 11.5%, corresponding to a current efficiency of 37.4 cd/A and a power efficiency of 26.2 lm/W, has been obtained. Such a high EQE is comparable to that of Ir(ppy)(3)-based devices. The present work suggests that the alkynylgold(III) complex is a promising phosphorescent material in terms of both efficiency and thermal stability, with the additional advantages of its relatively inexpensive cost and low toxicity.

A luminescent cyclometalated platinum(II) complex and its green organic light emitting device with high device performance

A luminescent cyclometalated platinum(II) complex has been demonstrated to show green electrophosphorescence in multilayer organic light-emitting devices (OLEDs) using a dual emissive layer with high current and external quantum efficiencies of 38.9 cd A(-1) and 11.5%, respectively.

Effective organic-based connection unit for stacked organic light-emitting devices

A bilayer connection unit of Mg-doped Alq3 and F4-TCNQ-doped m-MTDATA was investigated for application in stacked organic light-emitting device. This connection unit led to a stacked OLED with a luminous efficiency twice that of a single-unit OLED. Electronic structures, including relevant electron energy levels, of the various interfaces in the stacked OLED were studied by using ultraviolet photoemission spectroscopy and used to discuss the working mechanisms of the stacked OLED. The p-type dopant F4-TCNQ was shown to induce a large band bending of 1.36eV and facilitates efficient carrier injection from the connection units into the carrier-transporting layers.

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.

Application of metal-doped organic layer both as exciton blocker and optical spacer for organic photovoltaic devices

An effective optical spacer based on doping of ytterbium (Yb) metal into bathophenanthroline (BPhen) has been developed for applications in organic photovoltaic (OPV) devices. Utilizing Yb:BPhen as an optical spacer in standard copper phthalocyanine/C60 photovoltaic devices, power efficiency can be increased by four times to 3.42%. Ultraviolet photoemission spectroscopy measurements reveal that the good electron transport between C60 and Yb:BPhen is mainly related to the suitable energy level alignment at the interface. Combining with its high optical transparency and electrical conductivity, the Yb:BPhen film provides a useful means for maximizing the power conversion efficiency of OPV devices.

Functionalized Alkynylplatinum(II) Polypyridyl Complexes for Use as Sensitizers in Dye-Sensitized Solar Cells

A series of platinum(II) alkynyl-based sensitizers has been synthesized and found to show light-to-electricity conversion properties. These dyes were developed as sensitizers for the application in nanocrystalline TiO(2) dye-sensitized solar cells (DSSCs). Their photophysical and electrochemical properties were studied. The excited-state property was probed using nanosecond transient absorption spectroscopy, which showed the formation of a charge-separated state that arises from the intramolecular photoinduced charge transfer from the platinum(II) alkynylbithienylbenzothiadiazole moiety (donor) to the polypyridyl ligand (acceptor). A lifetime of 3.4 μs was observed for the charge-separated state. A dye-sensitized solar cell based on one of the complexes showed a short-circuit photocurrent of 7.12 mA cm(-2), an open circuit voltage of 780 mV, and a fill factor of 0.65, thus giving an overall power conversion efficiency of 3.6%.