Direct Visualization Of Electric Potential Distribution In Organic Light Emitting Diode By Phase-Shifting Electron Holography

We utilized phase-shifting electron holography on organic light emitting diodes consisting of N,N '-di-[(1-naphthyl)-N,N '-diphenyl]-(1,1 '-biphenyl)-4,4 '-diamine (alpha-NPD) and tris-(8-hydroxyquinoline)aluminum (Alq(3)) layers to visualize their built-in potential distribution. The bilayer showed three different electric fields, namely, the fields in the alpha-NPD layer, near the alpha-NPD/Alq(3) interface, and in the Alq(3) layer measured as -1.8 +/- 0.4 MV m(-1), -10.0 +/- 2 MV m(-1), 3.1 +/- 0.6 MV m(-1), respectively. We show that they are related to hole accumulation in the alpha-NPD layer, the charge carrier accumulation junction around the alpha-NPD/Alq(3) interface, and the giant surface potential spontaneously polarized in the Alq(3) layer.