Characteristics of solution-processed small-molecule organic films and light-emitting diodes compared with their vacuum-deposited counterparts

Although significant progress has been made in the development of vacuumdeposited small-molecule organic light-emitting diodes (OLEDs), one of the most desired research goals is still to produce flexible displays by low-cost solution processing. The development of solution-processed OLEDs based on small molecules could potentially be a good approach but no intensive studies on this topic have been conducted so far. To fabricate high-performance devices based on solution-processed small molecules, the underlying nature of the produced films and devices must be elucidated. Here, the distinctive characteristics of solution-processed small-molecule films and devices compared to their vacuum-deposited counterparts are reported. Solutionprocessed blue OLEDs show a very high luminous efficiency (of about 8.9 cd A-1) despite their simplified structure. A better hole-blocking and electron-transporting layer is essential for achieving high-efficiency solutionprocessed devices because the solution-processed emitting layer gives the devices a better hole-transporting capability and more electron traps than the vacuum-deposited layer. It is found that the lower density of the solutionprocessed films (compared to the vacuum-deposited films) can be a major cause for the short lifetimes observed for the corresponding devices.