Organic light-emitting diodes (OLEDs) are subject to thermal stress from Joule heating and the external environment. In this work, neutron reflectometry (NR) was used to probe the effect of heat on the morphology of thin three-layer organic films comprising materials typically found in OLEDs. It was found that layers within the films began to mix when heated to approximately 20 °C above the glass-transition temperature (T) of the material with the lowest T. Diffusion occurred when the material with the lowest T formed a supercooled liquid, with the rates of interdiffusion of the materials depending on the relative T's. If the supercooled liquid formed at a temperature significantly lower than the T of the higher-T material in the adjacent layer, then pseudo-Fickian diffusion occurred. If the two T's were similar, then the two materials can interdiffuse at similar rates. The type and extent of diffusion observed can provide insight into and a partial explanation for the "burn in" often observed for OLEDs. Photoluminescence measurements performed simultaneously with the NR measurements showed that interdiffusion of the materials from the different layers had a strong effect on the emission of the film, with quenching generally observed. These results emphasize the importance of using thermally stable materials in OLED devices to avoid film morphology changes.

We have used steady-state and time-resolved neutron reflectometry to study the diffusion of fullerene derivatives into the narrow optical gap polymer poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) to explore the sequential processing of the donor and acceptor for the preparation of efficient organic solar cells. It was found that when [6,6]-phenyl-C61-butyric-acid-methyl-ester (60-PCBM) was deposited onto a thin film of PCDTBT from dichloromethane (DCM), a three-layer structure was formed that was stable below the glass-transition temperature of the polymer. When good solvents for the polymer were used in conjunction with DCM, both 60-PCBM and [6,6]-phenyl-C71-butyric-acid-methyl-ester (70-PCBM) were seen to form films that had a thick fullerene layer containing little polymer and a PCDTBT-rich layer near the interface with the substrate. Devices composed of films prepared by sequential deposition of the polymer and fullerene had efficiencies of up to 5.3%, with those based on 60-PCBM close to optimized bulk heterojunction (BHJ) cells processed in the conventional manner. Sequential deposition of pure components to form the active layer is attractive for large-area device fabrication, and the results demonstrate that this processing method can give efficient solar cells.

layers. This stacked structure and the order in which the materials are layered are critical in ensuring optimal device performance. A common arrangement of the organic layers within a device is to have a central emissive layer with adjacent charge transport layers. The charge transport layers serve to transport holes or electrons from the electrodes within the device, ideally trapping them in the emissive layer for exciton formation and radiative decay. As such, it is important that the nature and ordering of these layers do not undergo any significant change during device operation as this will lead to a change in performance and in particular the efficiency of the device. It has been observed that exposure to temperatures above ambient (thermal stress) can lead to a significant drop in device performance. [3] It has been suggested that the drop in device performance caused by thermal stress is due to one or more of the organic layers within a device undergoing a glass transition and subsequent accelerated diffusion. [4,5] It is therefore important that the origin of this change in performance is understood, with a specific focus on temperatures that an OLED might be exposed to in a commercial device, either externally, or by joule heating during operation. [6,7] Atomic force microscopy, [8,9] scanning tunneling microscopy, [10] infrared imaging, and impedance spectroscopy [11] have all been used to study the change in film structure on heating. Reflectometry techniques can also be utilized to nondestructively probe the internal layered structure of a multilayer film under thermal stress. For example, studies using X-ray reflectometry showed that a multilayer device containing a copper phthalocyanine (CuPc) hole injection layer, an N,N′-bis(naphthalen-1-yl)-N,N′-diphenylbenzidine (NPB) hole transport layer, a tris(8-hydroxyquinoline)aluminium (Alq 3 ) emissive layer, and a lithium fluoride (LiF) electron injection layer deposited onto ITO on glass maintained discrete layers up to 100 °C, while annealing at 120 °C caused intermixing of the NPB and Alq 3 and the Alq 3 and LiF layers. [12,13] While the electron density differences of the compounds in that particular layered film were sufficient to provide adequate scattering length density (SLD) contrast for the use of X-ray reflectometry, this is generally not the case, particularly for films that primarily comprise A common feature of organic light-emitting diodes is their stacked multilayer structure, which is critical for efficient charge injection and transport, and light emission. In this study, it is found that a blended layer of the holetransport material tris(4-carbazol-9-ylphenyl)amine with 6 wt% fac-tris(2-phenylpyridyl)iridium(III) [Ir(ppy) 3 ] readily undergoes interdiffusion with adjacent layers of typical charge transport materials: bathocuproine; 1,3,5-tris(Nphenylbenzimidazol-2-yl)benzene; N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine; and N,N′-bis(naphthalen-1-yl)-N,N′-diphenylbenzidine. This process is followed using combined neutron reflectom...

Controlling the orientation of the emissive dipole has led to a renaissance of organic light-emitting diode (OLED) research, with external quantum efficiencies (EQEs) of >30% being reported for phosphorescent emitters. These highly efficient OLEDs are generally manufactured using evaporative methods and are comprised of small-molecule heteroleptic phosphorescent iridium(III) complexes blended with a host and additional layers to balance charge injection and transport. Large area OLEDs for lighting and display applications would benefit from low-cost solution processing, provided that high EQEs could be achieved. Here, we show that poly(dendrimer)s consisting of a non-conjugated polymer backbone with iridium(III) complexes forming the cores of firstgeneration dendrimer side chains can be co-deposited with a host by solution processing to give highly efficient devices. Simple bilayer devices comprising the emissive layer and an electron transport layer gave an EQE of >20% at luminances of up to ≈300 cd/ m 2 , showing that polymer engineering can enable alignment of the emissive dipole of solution-processed phosphorescent materials.

Organic light-emitting devices containing solution-processed emissive dendrimers can be highly efficient. The most efficient devices contain a blend of the light-emitting dendrimer in a host and one or more charge-transporting layers. Using neutron reflectometry measurements with in situ photoluminescence, we have investigated the structure of the as-formed film as well as the changes in film structure and dendrimer emission under thermal stress. It was found that the as-formed film stacks comprising poly(3,4-ethylenedioxythiophene):polystyrene sulfonate/host:dendrimer/1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (where the host was deuterated 4,4'-N,N'-di(carbazolyl)biphenyl or tris(4-carbazol-9-ylphenyl)amine, the host:dendrimer layer was solution-processed, and the 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene evaporated) had well-defined interfaces, indicating good wetting of each of the layers by the subsequently deposited layer. Upon thermal annealing, there was no change in the poly(3,4-ethylenedioxythiophene):polystyrene sulfonate/host:dendrimer interface, but once the temperature reached above the T of the host:dendrimer layer, it became a supercooled liquid into which 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene dissolved. When the film stacks were held at a temperature just above the onset of the diffusion process, they underwent an initial relatively fast diffusion process before reaching a quasi-stable state at that temperature.