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Because of the low exciton utilization of fluorescent emitters and inadequately stable blue phosphorescent emitters, hybrid white organic light emitting diodes (WOLEDs) are considered as the most effective way to achieve good white emission. Nevertheless, the serious efficiency roll-off is still a big challenge to be addressed. Generally, the high density and long lifetime of radiative triplet excitons are the main causes. Thus, broadening the exciton distribution region, decreasing the local exciton density, and accelerating the triplet excitons radiated by low-energy emitters will improve this roll-off problem. In this paper, we proposed a hybrid WOLED by simply introducing multi-ultrathin phosphorescent layers in a blue exciplex emitting layer (EML). Compared to the common multi-EML hybrid WOLEDs, the proposal device demonstrates higher efficiency and lower efficiency roll-off at high luminance. The maximum external quantum efficiency, current efficiency, and power efficiency reach 20.4%, 62.8 cd A−1, and 75.9 lm W−1, respectively, and remain 16.5%, 50.7 cd A−1, and 47.1 lm W−1, respectively, at the luminance of 1000 cd m−2. By the analysis of exciton utilization processes, it can be inferred that the improved performance benefits from the energy transfer and wider exciton distribution, thus reducing the exciton density and energy loss due to more efficient exciton utilization. [ABSTRACT FROM AUTHOR]

Organic bulk heterojunction comprising a pentacene and C60 blend can be used as the charge generation layer (CGL) in tandem organic light-emitting diodes (OLEDs) for significantly improved power efficiency. Herein, we systematically study the effects of the ratio and thickness of the CGL on the performance of tandem OLEDs. We have found that the efficiency of tandem OLEDs not only is related to the thickness of the CGL but also strongly depends on the ratio of pentacene to C60 in the CGL. Investigations regarding the capacitance-voltage and atomic force microscopy measurements clearly indicate that different thicknesses of the CGL lead to different behaviors of the charge distributions, and different ratios of pentacene to C60 in the CGL result in different probabilities of charge transfer from pentacene to C60 and imbalanced charge transport, thereby leading to completely different device performance. [ABSTRACT FROM AUTHOR]

Field-effect transistors with organic-inorganic heterojunctions of molybdenum trioxide (MoO3)/pentacene and MoO3/copper-phthalocyanine (CuPc) as active layers were prepared and analyzed. These transistors showed normally-on operation mode and a shift of threshold voltage comparing to the corresponding single-layer organic device. The interfacial electronic structures of MoO3/pentacene and MoO3/CuPc heterojunctions were investigated by ultraviolet photoemission spectroscopy. Significant electron energy level bending and space charge regions of high conductivity were observed at the heterojunction. Formation of the organic-inorganic heterojunctions and characteristics of the corresponding field-effect transistors were analyzed by considering charge exchange at heterojunction interfaces. [ABSTRACT FROM AUTHOR]

We employed organic heterojunction films as all-organic connecting units to fabricate tandem organic photovoltaic cells by continuous deposition. The all-organic connecting units with a better transparence and a lower sublimation temperature became an effective recombination center for electrons and holes photogenerated in front cell and back cell, respectively. Tunnel mechanism was proposed to explain the combination of photogenerated carrier. In the optimized tandem solar cell in which tin phthalocyanine dichloride/copper hexadecafluorophthalocyanine heterojunction is used as connecting units, the open-circuit voltage of 1.04 V is double of the single unit cells, and the PCE was increased by 60%, reaching to 1.8%. Therefore, it is an effective way to use the organic heterojunction films as connecting units in organic tandem cells to improve the device performance dramatically. [ABSTRACT FROM AUTHOR]

Organic thin-film transistors (OTFTs) with a buffer layer sandwiched between source/drain electrodes and organic semiconductor are demonstrated. An intrinsic molecular semiconductor, Lutetium bisphthalocyanine (LuPc2), is used as the buffer layer due to its high carrier density (1016 cm3). Compared with conventional OTFTs, the introduction of the buffer layer leads to on-state current increases from 700 nA to 2.5 μA, field-effect mobility increases from 0.7×10-2 to 1.58×10-2 cm2/V s, and threshold voltage downshifts from -21 to -11 V for the linear region. The on/off current ratio is improved to a level of 104. Mechanisms of performance improvement are attributed to include the difference of the Fermi level and interface dipolar between LuPc2 and Au. Our results demonstrate that it is an effective method to improve linear region characteristics by using a molecular semiconductor as the buffer layer. [ABSTRACT FROM AUTHOR]

We have investigated the interfacial and electrical properties of vanadyl-phthalocyanine (VOPc) metal-insulator-semiconductor devices by the measurement of capacitance and conductance. The devices have been fabricated on ordered para-sexiphenyl (p-6P) layer with silicon nitride (SiNx) as gate insulator. The VOPc/p-6P/SiNx devices have shown a negligible hysteresis, low series resistance, and high operated frequency. Bulk traps have been distinguished from interface traps by two loss peaks in conductance measurement. Trap densities and distribution of trap energy level have been obtained. The improved properties indicate that VOPc/ p-6P devices with SiNx insulator hold a great promise of application in flexible displays. [ABSTRACT FROM AUTHOR]

Efficient parallel tandem organic solar cells are demonstrated by using the highly crystalline chloroaluminum phthalocyanine (AlClPc) films as the planar layer. Their broad photoresponse from 400-900 nm is contributed to the strong near-infrared absorption of the AlClPc films and the good complementarity between AlClPc and zinc phthalocyanine. Importantly, the high external quantum efficiency is obtained in the entire response range with the peak value 65% due to the high carrier mobility of the AlClPc films, and correspondingly the high power conversion efficiency of 3.5% is attributed to the large short circuit current density of 12.15 mA/cm2. [ABSTRACT FROM AUTHOR]

The charge transport and exciton diffusion have been simultaneously enhanced in the highly ordered and oriented zinc phthalocyanine (ZnPc) films which are fabricated by weak epitaxy growth method compared to the normal ZnPc films (charge mobility from 4 × 10-5 cm2/Vs to 1.1 × 10-4 cm2/Vs and exciton diffusion length from 9 nm to 16 nm). The temperature dependent current-voltage (I-V) characteristics are utilized to investigate the charge transport mechanism, which reveal the clear mechanism and the role of traps in different films. These results reveal the direct relationship between traps properties and transport behavior of both exciton and charge. [ABSTRACT FROM AUTHOR]

We investigated interfacial electronic structures of organic/inorganic heterojunctions formed between copper-hexadecafluoro-phthalocyanine (F16CuPc) and molybdenum trioxide or lead monoxide by ultraviolet and x-ray photoemission spectroscopies. Obvious energy level bending in F16CuPc shows the formation of electron depletion and accumulation regions in its contact with molybdenum and lead oxides, respectively. Effects of these depletion/accumulation zones were studied via electronic characteristics of field-effect transistors constructed with these heterojunctions as active layers. Finally, a summary about the heterojunction including organic semiconductor (n-type and p-type) and metal oxide was given, which demonstrated that organic semiconductors can form various space charge regions like conventional inorganic semiconductors. [ABSTRACT FROM AUTHOR]

The significant improvement of power efficiency and efficiency roll-off in tandem white organic light-emitting diodes (WOLEDs) was achieved using interface-modified C60/pentacene organic heterojunction as charge generation layer (CGL). The resulting tandem WOLED exhibits a maximum power efficiency of 53.8 lm W-1 and a maximum current efficiency of 101.5 cd A-1 without any out-coupling techniques. More importantly, the efficiency roll-off is greatly improved, yet remaining power efficiency of 53 lm W-1 at 100 cd m-2 and 45 lm W-1 at 1000 cd m-2, which outperforms that of the conventional CGL-based tandem WOLED. This clearly demonstrates that the organic heterojunction is a promising CGL for high-performance tandem WOLEDs. [ABSTRACT FROM AUTHOR]

We investigated the interfacial electronic structure of the n-n isotype organic heterojunction formed between copper hexadecafluorophthalocyanine (F16CuPc) and phthalocyanatotin (IV) dichloride (SnCl2Pc) using ultraviolet and x-ray photoemission spectroscopies. Energy level bending was observed at the heterojunction interface due to the formation of an electron accumulation region in the F16CuPc layer and an electron depletion region in the SnCl2Pc layer. The formation of organic heterojunction was explained by charge exchange at the interface due to their different work functions. [ABSTRACT FROM AUTHOR]

Highly crystalline organic superlattice has great potential for providing innovative function in organic devices. With studies of the structure and fundamental electronical properties, we have demonstrated the phathalocynine organic superlattice, which is a structure composed of periodically alternating crystalline layers of H2Pc and F16CuPc. A periodical crystal structure and electronic structure appear in this organic superlattice system. High density of mobile electrons and holes distribute periodically in F16CuPc and H2Pc layers, respectively, leading to a significant change in intrinsic properties of organic semiconductors. [ABSTRACT FROM AUTHOR]

Single-crystal-like organic heterojunction films of copper phthalocyanine (CuPc) and copper-hexadecafluoro-phthalocyanine (F16CuPc) were fabricated by weak-epitaxy-growth method. The intrinsic properties of organic heterojunction were revealed through threshold voltage shift of field-effect transistors and measurement of single-crystal-like diodes. At both sides of the heterojunction interface 40 nm thick charge accumulation layers formed, which showed that the long carriers’ diffusion length is due to the high crystallinity and low density of deep bulk traps of single-crystal-like films. This also indicated the electronic properties of organic heterojunction can be adjusted by controlling the growth condition. [ABSTRACT FROM AUTHOR]

Organic photovoltaic cells with a strong absorption spectrum in the near infrared region were fabricated with the structure of indium tin oxide (ITO)/zinc phthalocynine (ZnPc)/lead phthalocynine (PbPc)/C60/Al. PbPc has a broad and strong absorption, while the organic films of PbPc/C60 showed an additional new absorption peak at 900 nm. The absorption in the near infrared region can harvest more photons to invert into photocurrent. Moreover, the introduction of ZnPc thin layer between ITO and PbPc further improved the new absorption peak and the collection of hole carriers at the electrode ITO, which increased the power conversion efficiencies to 1.95% and short-circuit current density to 9.1 mA/cm2 under AM 1.5 solar spectrum. [ABSTRACT FROM AUTHOR]

We investigated electrical properties of vanadyl phthalocyanine (VOPc) metal-insulator-semiconductor (MIS) devices by the measurement of capacitance and conductance, which were fabricated on ordered para-sexiphenyl (p-6P) layer by weak epitaxy growth method. The VOPc/p-6P MIS diodes showed a negligible hysteresis effect at a gate voltage of ±20 V and small hysteresis effect at a gate voltage of ±40 V due to the low interface trap state density of about 1×1010 eV-1 cm-2. Furthermore, a high transition frequency of about 10 kHz was also observed under their accumulation mode. The results indicated that VOPc was a promising material and was suitable to be applied in active matrix liquid crystal displays and organic logic circuits. [ABSTRACT FROM AUTHOR]

The interfaces formed between copper-hexadecafluoro-phthalocyanine (F16CuPc) and 2,5-bis(4-biphenylyl) bithiophene (BP2T) were examined using photoemission and inverse photoemission spectroscopy. It is observed that in F16CuPc/BP2T the heterojunction is characterized by band bending in both materials, while in BP2T/F16CuPc the band bending is confined in BP2T only. The combination of the band bending and finite Debye lengths provides an explanation to the observed ambipolar behavior of the organic thin film transistors based on such heterojunctions. [ABSTRACT FROM AUTHOR]

The authors investigated the switch-on transient properties of p-type vanadium phthalocyanine (VOPc) transistors, which were fabricated by weak epitaxy growth on ordered para-sexiphenyl (p-6P) layer. The overshoot phenomenon of drain current had been observed in the VOPc/p-6P transistors, which was explained by the filling of carriers in traps of organic films. The small overshoot value of about 35% and transient duration time of 2 ms demonstrated the low trap concentration in organic films, which were comparable to the reported hydrogenated amorphous-silicon thin-film transistors. Therefore, the VOPc/p-6P transistors can be applied in active matrix liquid crystal display as switch elements. [ABSTRACT FROM AUTHOR]

The organic films of vanadyl-phthalocyanine (VOPc) compounds showed weak epitaxy growth (WEG) behavior on thin ordered para-sexiphenyl (p-6P) layer with high substrate temperature. The WEG of VOPc molecules standing up on the p-6P layer leaded to high in-plane orientation and their layer-by-layer growth behavior. In consequence, high quality VOPc films were obtained, which were consisted of lamellar crystals. Organic field-effect transistors with VOPc/p-6P films as active layers realized high mobility of above 1 cm2/V s. This result indicated that nonplanar compounds can obtain a device performance better than planar compounds, therefore, it may provide a rule to find disklike organic semiconductor materials. [ABSTRACT FROM AUTHOR]

Organic thin-film transistors (OTFTs) having source/drain electrodes sandwiched between copper phthalocyanine (CuPc) and cobalt phthalocyanine (CoPc) layers, CuPc/CoPc SC OTFTs, are investigated. Comparing their properties with that of CuPc-based top-contact OTFT, field-effect mobility increases from 0.04 to 0.11 cm[sup 2]/Vs, threshold voltage shifts from -13.8 to -8.9 V, and the current on/off ratio maintains at a level of 10[sup 5]. A top-contact OTFT with a layer of CuPc and a layer of CoPc (10%)–CuPc mixture reveals that the combination of CuPc and CoPc enhances charge injection from the source electrode into the active layer and increases the off-state current. The sandwich configuration increases the field-effect mobility, reduce the threshold voltage, and improve the on/off ratio at the same time. Our results indicate that using a double-layer of active organic materials in sandwich configuration is an effective way to improve OTFT performance. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

An organic thin-film transistor (OTFT) having a low-dielectric polymer layer between gate insulator and source/drain electrodes is investigated. Copper phthalocyanine (CuPc), a well-known organic semiconductor, is used as an active layer to test performance of the device. Compared with bottom-contact devices, leakage current is reduced by roughly one order of magnitude, and on-state current is enhanced by almost one order of magnitude. The performance of the device is almost the same as that of a top-contact device. The low-dielectric polymer may play two roles to improve OTFT performance. One is that this structure influences electric-field distribution between source/drain electrodes and semiconductor and enhances charge injection. The other is that the polymer influences growth behavior of CuPc thin films and enhances physical connection between source/drain electrodes and semiconductor channel. Advantages of the OTFT having bottom-contact structure make it useful for integrated plastic electronic devices. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The p-p heterojunction of 5, 6, 11, 12-tetraphenylnaphthacene/vanadyl phthalocyanine, which has been used as the buffer layer, is demonstrated. The highest field-effect mobility is 5.1 cm2/Vs, which is one of the highest reported for polycrystalline rubrene thin film transistors. Current versus voltage characteristics of heterojunction diodes are utilized to investigate the charge injection mechanism, revealing the factors that bring about the improvement of carrier injection and the reduction of contact resistance. These results suggest that our approach is very promising to fabricate high performance organic thin-film transistors for practical applications in organic electronics. [ABSTRACT FROM AUTHOR]