Advantages and disadvantages of vacuum-deposited and spin-coated amorphous organic semiconductor films for organic light-emitting diodes

Abstract

To realize low-cost fabrication processes for high-performance organic light-emitting diode (OLED) displays and lighting, it has recently become important to understand the properties and structure of solution-processed amorphous films. In particular, to choose an appropriate process to produce OLEDs to meet the demands of a realistic situation, it is necessary to know the general advantages and disadvantages of vacuum-and solution-processed films quantitatively. However, the differences between films formed by these processes is not yet sufficiently clear. Here we systematically compare vacuumdeposited and spin-coated amorphous organic semiconductor films used for OLEDs mainly by ellipsometry, and demonstrate the critical differences in film density, transition temperature, and molecular orientation. We found that the film density, transition temperature, and degree of horizontal molecular orientation of small-molecule spin-coated films are inherently lower than those of the corresponding vacuum-deposited ones. In addition, we show that the transition temperature and molecular orientation of small-molecule spin-coated films of glassy materials are identical to those of "deteriorated" vacuum-deposited films that have experienced a transition induced by heating. Our comprehensive comparisons lead to guidelines for selecting suitable processes and materials for production and clarify the future challenges to be addressed to facilitate development of high-performance solution-processed OLEDs.

Simple model-free estimation of orientation order parameters of vacuum-deposited and spin-coated amorphous films used in organic light-emitting diodes

“…Because of the interference effect caused by multiple reflections between the air=organic and organic= substrate interfaces, spectral shapes and peak wavelengths of absorption seem to depend on film thickness to some degree. 16) For example, the fluctuation of the baselines and their dependence on film thickness can be seen in the spectral region of 400-500 nm in Fig. 3.…”

“…This random orientation of spin-coated films is generally seen for most smallmolecule OLED materials. 16) In the spin-coated films, only the TSBF films have S values that slightly deviate from zero, indicating the possibility that oligomers can be horizontally oriented even in spin-coated films as with polymers. [20][21][22] In Table I, we also show the S values estimated by VASE using the 100-nm-thick as-deposited vacuum-deposited films on Si(100) substrates.…”

“…This result confirms that the molecular orientation in these amorphous organic films becomes random via a transition upon heating to a temperature sufficiently higher than T g , as reported previously. [13][14][15][16] The absorption spectra of all the sample films on fused silica substrates before and after annealing are shown in Fig. 3.…”

Phthalimide‐Based Transparent Electron‐Transport Materials with Oriented‐Amorphous Structures: Preparation from Solution‐Processed Precursor Films

“…Yokoyama and co‐workers showed by means of ellipsometry that small molecule organic semiconductors with anisotropic shapes orient face‐on to the substrate in vacuum‐deposited amorphous films . The same molecules, however, oriented randomly in films prepared by spin‐coating . The inability of spin‐coated films to retain an oriented structure can be explained by the rapid, bulk condensation as the solvent evaporates, which leaves insufficient time for the molecules to achieve a preferential orientation.…”

“…Since solution‐based film growth is preferred over vacuum deposition in terms of low fabrication costs and applicability to large‐area devices, solution process methods for fabricating oriented‐amorphous films are valuable. The fabrication of oriented‐amorphous films by solution process, however, has so far been limited to polymers and oligomers . Though small molecules have advantages of high material purity and easy synthesis, solution‐processed oriented‐amorphous films have not, to the best of our knowledge, been realized for small molecule organic semiconductors.…”