Direct visualization of electric potential distribution in organic light emitting diode by phase-shifting electron holography

Abstract

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 (α-NPD) and tris-(8-hydroxyquinoline)aluminum (Alq3) layers to visualize their built-in potential distribution. The bilayer showed three different electric fields, namely, the fields in the α-NPD layer, near the α-NPD/Alq3 interface, and in the Alq3 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 α-NPD layer, the charge carrier accumulation junction around the α-NPD/Alq3 interface, and the giant surface potential spontaneously polarized in the Alq3 layer.