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.

Molecular orientation in organic light-emitting diodes (OLEDs) is now regarded as an important factor that affects device efficiency. However, methods to quantitatively estimate the degree of molecular orientation in OLEDs are currently limited, and they require constructing a model of an optical structure. Here, we propose a simple model-free method to estimate the orientation order parameters (S) of molecules in amorphous OLED films from their absorption spectra using the randomization of molecular orientation induced by heating. This method is used to quantitatively estimate the S values of vacuum-deposited and spin-coated films and clearly demonstrate the random orientation in the latter.

No abstract