Direct Probing of Carrier Behavior in Electroluminescence Indium--Zinc-Oxide/N,N$\aku '$-Di-[(1-naphthyl)-N,N$\aku '$-diphenyl]-(1,1$'$-biphenyl)-4,4$'$-diamine/Tris(8-hydroxy-quinolinato)aluminum(III)/LiF/Al Diode by Time-Resolved Optical Second-Harmonic

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By using electric-field-induced second-harmonic generation (EFISHG) measurement, we probed charging and discharging in an $\alpha$-NPD/Alq3 organic light-emitting diode [$\alpha$-NPD, N,N$\aku '$-di-[(1-naphthyl)-N,N$\aku '$-diphenyl]-(1,1$'$-biphenyl)-4,4$'$-diamine; Alq3, tris(8-hydroxy-quinolinato)aluminum(III)] while electroluminescence response was monitored. The EFISHG measurement showed that excessive positive charges accumulated at the $\alpha$-NPD/Alq3 interface in the charging process, accompanying electroluminescence (EL) radiation, and the accumulated excess positive charges disappeared in the discharging process before the EL decayed. Note that the EL radiation was smooth and strong under the high voltage application, while the EL decayed in a similar way. The Maxwell--Wagner model analysis showed no-dependence of the accumulated excessive positive charge on the applied external voltage, suggesting that electrons and holes injected from the opposite electrodes for EL radiation balanced at the interface. The EFISHG measurement will be useful as a direct way to probe carrier behaviors in organic EL devices.
2011-04-25