Observation of intermediate-range order in a nominally amorphous molecular semiconductor film

Abstract

Synchrotron X-ray diffraction from a nominally amorphous molecular semiconductor film reveals both the presence of intermediate-range order (IRO) corresponding to crystalline domains with an average size of a few nanometres, and the growth of these domains upon exposure of the film to moisture.Due to a high demand for better, more efficient, and less expensive displays and lighting, the science and engineering community has concentrated efforts on the development, understanding and optimization of new electroluminescent materials. Consequently, organic light emitting devices (OLEDs) have gained interest as an alternative to inorganic semiconductor technology. 1,2 Not surprisingly, device performance is strongly correlated to the microstructure of the organic thin films. While perfect singlecrystalline films promise the best transport properties and quantum efficiencies, they are very difficult to deposit over large areas. Amorphous films have been found to perform significantly better than most polycrystalline films. The high surface roughness associated with the latter is problematic in sandwich-type structures. Moreover, grain boundaries in polycrystalline materials may provide trapping and recombination centres, which can degrade device performance and lifetime. In contrast, the hoppingtype conductivity in amorphous films is adequate for several applications, including OLEDs and organic photovoltaics. 1 As a result, the development of amorphous molecular materials is of great interest. 3 Changes in the film structure that occur during device operation are also of interest, as the structural integrity of the film affects device performance. Recrystallization of an amorphous film, for example, may lead to cracking or delamination, with catastrophic consequences for the device. 4,5 Understanding the local structure of amorphous films of organic semiconductors, and its evolution with device operation, is of fundamental importance to organic electronics. Unfortunately, very little is known about the local structure in such amorphous films. This is due to the fact that amorphous materials are relatively poor scatterers, so that standard X-ray scattering from thin films is very hard to obtain with lab-based X-ray generators. Furthermore, in device-relevant configurations, where the film thickness is of the order of 100 nm, scattering from the substrate can be pronounced and can obscure the faint scattering signatures from the film itself. Both problems can be overcome using synchrotron-based grazing incidence scattering methods, as we will demonstrate below. In this communication we investigate the microstructure of the ruthenium complex [Ru(bpy) 3 ] 2+ (PF 6

Stable and Efficient Solid‐State Light‐Emitting Electrochemical Cells Based on a Series of Hydrophobic Iridium Complexes

“…In that study, it was established that a spin-coated fi lm did indeed consist of nanoscale crystalline domains. [ 52 ] Therefore, it is possible that complexes 1 -4 crystallize in different nanosized domains when spin-coated fi lms are prepared, and this may affect the movement of the counter-anions in the device and thereby the turn-on-time.…”

Long‐Living Light‐Emitting Electrochemical Cells – Control through Supramolecular Interactions

“…It was reported that nanoscale crystalline domains are formed when spincoating concentrated films of iTMCs. [25] To speed up the occurrence of the electroluminescence, small amounts of ionic liquid (IL) can be added to the active layer [26] or short high-voltage pulses can be applied. [27] A rapid turn-on of the luminescence is achieved when a combination of these techniques is used (Fig.…”

Archetype Cationic Iridium Complexes and Their Use in Solid‐State Light‐Emitting Electrochemical Cells

“…Assuming that as in films of [Ru(bpy) 3 ] 2þ nanoscale crystalline domains are also present in the solid films of these Ir-iTMCs, information about the origin of this low ionic mobility can be obtained from the crystal structure. [32] From the crystal structure ( Fig. 1) it can be observed that PF 6 À is enclosed between six iridium cations, which might be the reason for the low mobility of these anions.…”