Multiple horizontal-dip-coating of small molecular emission layers for solution-processable organic light-emitting devices

We report an investigation of small molecular organic light-emitting diodes (SM-OLEDs) which consist of solution-processable light-emitting layers (EMLs) fabricated using a horizontal-dip- (H-dip-) coating method. The EMLs used were composed of a co-mixed small molecular host matrix of hole-transporting 4,4',4 ''-tris(N-carbazolyl)-triphenylamine and electron-transporting 2,7-bis (diphenylphosphoryl)-9,9'-spirobifluorene, doped with blue-, green-, and red-emitting iridium phosphors. To investigate the filmforming ability, the film quality levels of H-dip-coated small molecular EMLs with multiple coatings were determined when increasing the number of coating cycles. It was found that the thickness of the EML increases as the number of cycles of H-dip-coating increases. Moreover, the formation of film defects in the form of nano-pinholes in the EMLs was found to decrease dramatically with an increase in the number of H-dip-coating cycles. By applying three H-dip-coatings of EML solutions, highly homogeneous small molecular EMLs were successfully deposited, demonstrating that the use of the triple-H-dip-coated EMLs in SM-OLEDs results in good device performance, with maximum luminance levels of 25 000 cd m(-2), 79 000 cd m(-2), and 15 000 cd m(-2), with corresponding peak current efficiencies of 15.8 cd A(-1), 23.2 cd A(-1), and 5.7 cd A(-1), for blue, green, and red SM-OLEDs, respectively. These results clearly indicate that H-dip-coated EMLs with multiple coatings will yield bright and efficient all-solution-processable SM-OLEDs.