Enhanced brightness and efficiency in organic light-emitting diodes using SiO₂ as buffer layer and electron-blocking layer

A novel device structure was fabricated with an inorganic SiO₂ layer inserted between hole-transporting and electron-transporting layer. In device indium-tin oxide (ITO)/poly [2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene](MEH-PPV)/tris-(8-hydroxyquinoline) aluminum (Alq₃)/Al, the recombination zone lies in MEH-PPV layer, but in ITO/MEH-PPV/SiO₂/ Alq₃/Al (device A), the recombination zone moves to Alq₃ layers. In another device ITO/N,N′-bis-(1-naphthyl)-N, N′-diphenyl-1,1′-biphenyl-4-4′-diamine (NPB)/Alq₃/LiF/Al, the brightness and efficiency were enhanced by inserting SiO₂ between NPB and Alq₃ layers, and it results from an improved balance of holes and electrons. Our experimental results demonstrate that the SiO₂ layer acts as not only a buffer layer but also an electron-blocking layer in these devices.