Delayed electroluminescence: a new tool for studies of OLED fundamental properties

Abstract

We measured delayed electroluminescence in abrupt heterojunction undoped and doped small molecule organic light emitting diodes (OLEDs) based on NPB and AlQ 3 hole and electron transport and emitter molecules, after the excitation currents are switched off and reverse bias applied to the sample. The experiments indicate that delayed light emission is a result of two distinct processes: emissive excited singlet state generation by either triplet-triplet annihilation or recombination of trapped positive and negative charges in the device. Under reverse device bias these two mechanisms have distinctly different signatures. Undoped devices show dominant light emission contribution from triplet-triplet annihilation, while in rubrene and coumarine doped devices delayed light emission comes predominantly from recombination of trapped charge. Therefore these molecules act as recombination centers when doped into AlQ 3 .

Degradation Mechanisms in Organic Light-Emitting Diodes with Polyethylenimine as a Solution-Processed Electron Injection Layer

“…As a result of such a reverse bias, charge carriers are detrapped or removed from interfaces and hence produce a sudden increase (a spike) in the delayed EL signal. A detailed description of the experimental setup can be found elsewhere. − …”

Triplet-polaron quenching by charges on guest molecules in phosphorescent organic light emitting devices

“…Details of delayed EL measurement setup were reported in our previous work. [8][9][10] First, to determine the width of the electron-hole (e-h) recombination zone (i.e., where excitons are formed) and how far it extends into the EML layer away from the HTL/EML (i.e., where the majority of TPQ presumably occurs), we study a series of devices of the following structure: ITO/MoO 3 (10 nm)/NPB (40 nm)/CBP:Ir(ppy) 3 (92%/8%) (x nm)/CBP(40-x nm)/BAlq (10 nm)/Alq 3 (30 nm)/ Mg:Ag. Their EL characteristics are summarized in Table I.…”

“…In this technique, the devices are driven with a 0.5 ms width square pulse forward bias, and persistent (delayed) EL is collected $0.3 ms after the end of the forward bias pulse, using an optical chopper system. [8][9][10] This delay time which is significantly longer than the lifetime of Ir(ppy) 3 triplet state lifetime (<1 ls) is selected to ensure the absence of any contributions from prompt EL in the collected signal. As such, any collected signal will arise from radiative decay of excitons that are formed after the bias is turned off.…”