Combining complementary emissions of hole- and electron-transport layers for ultra-simple white organic light-emitting diodes achieving high device performance

In this work, we proposed a novel and ultra-simple all fluorescent di-chromatic white organic light-emitting diodes (WOLEDs) containing only three non-doped organic layers, which are yellow emitting hole-transport layer, blue emitting electron-transport layer, and charge adjusting layer inserted between hole- and electron-transport layers. White light emission can be realized by simultaneously combining complementary emissions of hole- and electron-transport layers. Based on this structure, a series of all fluorescent di-chromatic WOLEDs were developed, and it was demonstrated the emission spectra could be easily improved by simply changing the thickness of charge adjusting layer. As a consequence, the optimized di-chromatic WOLED achieves a high color rendering index (CRI) of 86, to the best of our knowledge, which is among the highest CRI for such simple structures reported in the literatures. And the resulting device also exhibits high color stability with a slight change of (0.00, −0.01) for CIE coordinates at a wide luminance range from 1011 cd/m² to 9310 cd/m². Moreover, efficiency roll-off is rarely observed for target device before luminance reaching 10,000 cd/m². Such high device performance can be attributed to i) absolutely complementary emissions for hole- and electron-transport layers; ii) a balanced excitons distribution on two transport layers; and iii) a wide carriers recombination zone extending throughout both transport layers. We believe this work will provide a guide to develop simplified but high performance WOLEDs.