Enhancing the performance of organic light-emitting devices by selective thermal treatment

Joule heat generated during device operation has been considered the important factor causing device degradation. We hence fabricated two devices composing indium-tin-oxide (ITO)/N,N′-bis-(1-naphthy)-N,N′diphenyl-1,1′-biphenyl-4-4′-diamine (NPB)/tris-(8-hydroxyquinoline) aluminum (Alq3)/aluminum (Al) and ITO/NPB/Alq3/lithium-fluoride/Al to investigate the effect of heat on device performance by annealing the whole devices at different temperatures or by selectively annealing the desired layer(s) at a certain temperature. The devices annealed at an appropriate temperature, such as 120 °C, showed marked improvement in luminescent efficiency, brightness, and operating stability. Thermal annealing the NPB and Al electrode layers, especially the latter one, greatly improved the performance. The enhancements may be attributed to better electron injection and higher electron/hole recombination efficiency upon appropriate annealing.