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The aim of this article is to investigate the origin of the open circuit voltage (Voc) in organic heterojunction solar cells. The studied devices consist of buckminsterfullerene C60 as acceptor material and an oligophenyl-derivative 4,4′-bis-(N,N-diphenylamino)quaterphenyl (4P-TPD) as donor material. These photoactive materials are sandwiched between indium tin oxide and p-doped hole transport layers. Using two different p-doped hole transport layers, the built-in voltage of the solar cells is independently changed from the metal contacts. The influence of the built-in voltage on the Voc is investigated in bulk and planar heterojunctions. In bulk heterojunctions, in which doped transport layers border directly on the photoactive blend layer, Voc cannot exceed the built-in voltage significantly. Though, in planar heterojunctions, Voc is identical with the splitting of quasi-Fermi levels at the donor-acceptor interface and is thus primarily determined by the difference of the lowest unoccupied molecular orbital of C60 and the highest occupied molecular orbital of 4P-TPD. In planar heterojunctions, the open circuit voltage can exceed the built-in voltage. Furthermore, the investigations show that the efficiency of organic solar cells can be improved by using p-doped charge transport layers with optimized energy level alignment to the active materials. The optimized planar heterojunction shows a fill factor of up to 65.5% and a Voc of 0.95 V. For solar cells with insufficient energy level alignment between the photoactive layer system and the hole transport layer, a reduced Voc in bulk heterojunction cells and a characteristic S shape of the I-V characteristics in planar heterojunction cells are observed. [ABSTRACT FROM AUTHOR]

The optical outcoupling of top-emitting organic light-emitting diodes (OLEDs) can be improved by a thin dielectric capping layer on top of the transparent cathode. We investigate the emission properties of a set of top-emitting OLEDs with the same device structure, but different organic capping layer thicknesses to understand the capping layer effect. We demonstrate that the distribution pattern of the emitted light from top-emitting OLEDs depends strongly on the capping layer thickness, showing not only a maximum current efficiency enhancement by a factor of 1.38 (up to 78 cd/A in forward direction at 1000 cd/m2), but also a quantum efficiency improvement by 35%. This leads to a device efficiency of up to 17.8% and 69 lm/W at a brightness of 1000 cd/m2. We show that this efficiency enhancement is not due to the redistribution of emitted light, but mainly due to the improvement of outcoupling efficiency by changing the optical structure of the devices with the organic capping layer. The maximum outcoupling efficiency is achieved at a capping layer thickness where the top contact stack (cathode+capping layer) shows low absorption and high reflection. [ABSTRACT FROM AUTHOR]

We present a study on n doping of C60 thin films by acridine orange base [3,6-bis(dimethylamino)acridine(AOB)] combining conductivity, field effect, and Seebeck measurements. An increase of more than six orders of magnitude in conductivity is observed for a doping ratio of 6 mol %, accompanied by a decrease in the activation energy from 0.64 to 0.15 eV compared to the undoped C60. We observe a clear doping effect immediately after sample preparation, but also a further activation by annealing or illumination. The field effect and Seebeck measurements confirm n-type conduction of C60 thin films and show that deep donor states are formed in AOB-doped C60 thin films. A field effect mobility of 0.2 cm2/V s is achieved for a doping level of 1.8 mol %. Near Infrared (NIR) and Fourier transform infrared (FTIR) spectra demonstrate electron transfer from the dopant to the matrix: For C60 doped with AOB, C60- is present in NIR absorption and FTIR spectra. On the other hand, a peak corresponding to acridine orange [3,6-bis(dimethylamino)acridinium chloride (AOBH+)] is also observed in the FTIR spectrum of C60:AOB, where AOBH+ corresponds to AOB with one additional proton attached. Electrochemical data of AOB and AOBH+ in acetontrile suggest that the AOB radical cation is not stable, but is rapidly transformed into a compound with similar properties to AOBH+. Conductivities of C60 thin films doped with bis(ethylenedithio)-tetrathiafulvalene were also investigated to confirm that the doping effect of AOB in C60 does not result from a simple electron transfer from AOB to C60. [ABSTRACT FROM AUTHOR]

Green phosphorescent organic light-emitting devices (OLEDs) employing tris (2-phenylpyridine) iridium doped into a wide energy gap hole transport host have been studied. N, N, N', N'-tetrakis (4-methoxyphenyl)-benzidine doped with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane is used as a hole injection and transport layer, 4,7-diphenyl-1,10-phenanthroline and cesium are coevaporated as a n-doped electron transport layer, and an intrinsic emission layer is sandwiched between these two doped layer. Such a p-i-n device features efficient carrier injection from both contacts into the doped transport layers and low ohmic losses in these highly conductive layers. Thus, low operating voltages are obtained compared to conventional undoped OLEDs. By modifying the device structure, we optimized the carrier balance in the emission layer and at its interfaces. For the optimized device, the maximum power efficiency is 53 lm/W, and a luminance of 1000 cd/m² is reached at 3.1 V with a power efficiency of 45 lm/W. [ABSTRACT FROM AUTHOR]

We have studied the behavior of various intrinsic emission zones on the characteristics of organic light-emitting diodes with a p-doped hole-transport layer and an n-doped electron-transport layer based on our previous work [J. S. Huang, M. Pfeiffer, A. Wemer, J. Blochwitz, K. Leo, and S. Liu, Appl. Phys. Lett. 80, 139 (2002)]. This configuration is referred to as a PiN structure. Because the p- and n-doped regions occupy nearly 80% of the total thickness in our PiN device, the intrinsic region becomes a narrow layer between two doped regions. This intrinsic region includes the region where the radiative recombination occurs. Thus, the nature of this layer plays an important role in determining the actual device performance. Employing 8-tris-hydroxyquinoline aluminum as an emitter, we investigated the influence of the thickness of the emitter layer on the performance of the device. The optimum thickness of the emitter layer is found to be 20 nm. Combining the fluorescence dye doping method, we have optimized the PiN structure device. Two emitter systems have been used: Alq[sub 3] doped with two highly fluorescent laser dyes, Quinacridone or Coumarin 6, respectively. We have demonstrated the influence of the thickness and the doping of the emission zone on the characteristics of a doped emitter device with PiN structure, and obtained higher-efficiency PiN structure devices. The different properties of PiN devices corresponding to two different emitter dopants with different trapping effect are also discussed. [ABSTRACT FROM AUTHOR]

The emission properties of a set of top-emitting organic light emitting diodes (OPLEDs) with the same device structure, but different organic capping layer thickness are investigated to study the capping layer effect. Results reveal that this efficiency enhancement is not due to the redistribution of emitted light, but mainly due to the improvement of outcoupling efficiency by changing the optical structure of the devices with the organic capping layer.

A study on n doping of [C.sub.60] thin films by acridine orange base [3,6-bis(dimethylamino)acridine(AOB)] combining conductivity, field effect, and Seebeck measurements is presented. The collected electrochemical data suggest the AOB radical cation is not stable, but is rapidly transformed into a compound with similar properties to [[3,6-bis(dimethylamino)acridinium chloride (AOBH.sup.+].

The study conducted on green phosphorescent organic light-emitting devices (OLEDS) employing tris (2-phenylpyridine) iridium doped in energy gap hole transport host with different thickness of emission layer is presented. The modifications done on P-I-N device structure to obtain maximum power efficiency and low operating voltages is described.

The behavior of different intrinsic emission zones on the characteristics of organic light-emitting diodes with a p-doped hole-transport layer, and an n-doped electron-transport layer is studied and the configuration is known as PiN structure as p- and n-doped region occupy 80% of total thickness in the PiN device. The influence of the thickness of the emitter layer on the performance of the device is investigated.