Transient response of a bilayer organic light emitting diode: Building-up of external and recombination currents

Abstract

Using a theoretical model of a bilayer organic light emitting diode, we calculate numerically the evolution of carrier densities and electric fields inside the device. The obtained results allow us to detail injection and accumulation of carriers during transient excitation. Charge densities as a function of applied voltage present two distinct thresholds which determine three operating ranges: no injection, unipolar injection, and bipolar injection. Dynamically these thresholds depend on the rise time of the applied voltage and have a clear signature in the current densities. We show that the electroluminescence threshold has static and dynamic values which may be different. Calculated external current in response to a voltage ramp presents two steps related to the evolution of the capacity of the device. This capacitive behavior is observed experimentally.

Dynamic turn-on behavior of organic light-emitting devices with different work function cathode metals under fast pulse excitation

“…͑7͔͒, one of the reasons for the quantitative mismatch is probably the difference in charge injection behavior between dynamic excitation at very low duty by a single fast electric pulse and continuous dc excitation involving high-duty pulsed excitation, for example, the dynamic building up of space charges in organic layers, the orientational polarization of molecules, and so on. 11,19,20 This is because the measured specific capacitance is close to the capacitance per unit area of the condenser with a relative permittivity of 3 ͑representative value of most organic solids͒ and insulator thickness of 110 nm.…”

“…Therefore, the dynamic behavior of EL of OLEDs under electrical fast pulse excitation provides important insights into the internal operation mechanisms ͓injection, transport ͑migration͒, and recombination of charges͔ of OLEDs and the device physics of organic semiconductors. [1][2][3][4][5][6][7][8][9][10][11] For example, it is easy to imagine that the EL dynamics under fast pulse excitation conditions give valuable information about charge carrier injections into organic layers of OLEDs, charge transport in the layers of OLEDs, and so on. The transient response is also important in the context of possible applications for OLEDs in optical communications where their utility will ultimately be limited by the response of OLED light sources to high frequency modulation.…”

Centimeter-scale hole diffusion and its application in organic light-emitting diodes for reducing efficiency roll-off and enhancing operation lifetime

“…Using the "Marburg model'' ( Figure S5a) [13,37] , the reason of carrier accumulation in device C1 is explored in Supplementary Note 2. In the operating hole-dominant device C1 ( Figure S5b), the recombination current IR can be expressed as:…”

Storage of charge carriers on emitter molecules in organic light-emitting diodes

“…In fluorescent OLEDs, such transient overshoots are widely observed and investigated. [8][9][10][11][12][13][14][15][16] They are caused by a number of processes including delayed fluorescence created via triplettriplet annihilation, [13][14][15] recombination of trapped charge carriers, 13 electric field-induced quenching, 16 or a change in the ratio of drift and diffusion currents during and after application of an electric field, which leads to enhanced recombination after turn-off. [8][9][10] In phosphorescent OLEDs, a transient overshoot is typically attributed to a delayed recombination of trapped charge carriers.…”