Space charge capacitance and the physical mechanism of negative capacitance in organic light-emitting diodes (OLEDs) by transient current response analysis are investigated for the first time.

Detailed transient current response characteristics under pulsed voltage in MoO3-based organic light-emitting diodes (OLEDs) are first investigated. The transient current can be divided into three parts: positive current spike (I P), steady-state current (I S), and negative current spike (I N). It is found that I S is the operating current of OLED devices. I P and I N show a linear relationship with pulsed voltage amplitude. Higher I N than I P is induced by the difference of anode Fermi level and highest occupied molecular orbital (HOMO) of NPB. We also derive an expression for duty ratio with space charges. Results show that I P and I N correspond to the charging and discharging processes of space charge, respectively. The current spikes and threshold duty ratio are found only to be affected by the injection barrier and independent of internal device structure.

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