Effect of hole mobilities through the emissive layer on space charge limited currents of phosphorescent organic light-emitting diodes

2014 ◽  
Vol 47 (2) ◽  
pp. 375-385 ◽  
Author(s):  
Yang Luo ◽  
Yu Duan ◽  
Ping Chen ◽  
Yi Zhao
Author(s):  
Sarah E. Feicht ◽  
Ory Schnitzer ◽  
Aditya S. Khair

We analyse electron and hole transport in organic light-emitting diodes (OLEDs) via the drift–diffusion equations. We focus on space-charge-limited transport, in which rapid variations in charge carrier density occur near the injecting electrodes, and in which the electric field is highly non-uniform. This motivates our application of singular asymptotic analysis to the drift–diffusion equations. In the absence of electron–hole recombination, our analysis reveals three regions within the OLED: (i) ‘space-charge layers’ near each electrode whose width is much smaller than the device width , wherein carrier densities decay rapidly and the electric field is intense; (ii) a ‘bulk’ region whose width is on the scale of , where carrier densities are small; and (iii) intermediate regions bridging (i) and (ii). Our analysis shows that the current scales as , where is the applied voltage, is the permittivity and is the electric mobility, in contrast to the familiar diffusion-free scaling . Thus, diffusion is seen to lead to a large increase in current. Finally, we derive an asymptotic recombination–voltage relation for a kinetically limited OLED, in which charge recombination occurs on a much longer time scale than diffusion and drift.


2021 ◽  
Author(s):  
◽  
Rebecca Jane Sutton

<p>Organic light emitting diodes (OLEDs) are an emerging technology based on electrically conducting polymer films, with great promise for large area lighting and flexible ultra-thin displays. However, despite the rapid technological development, there is still a poor understanding of the degradation and spindependent recombination processes that take place inside an OLED. In this thesis, Electron Paramagnetic Resonance (EPR) was used to investigate these processes in blue-emitting OLEDs.  A successful procedure was developed and refined for fabricating OLEDs with the structure ITO/PEDOT:PSS/emissive layer/Al/Ag, with and without the PEDOT:PSS hole-transporting layer. The organic emissive layer was either F8BT, PFO, or PVK:OXD-7:FIrpic (PB). These OLEDs were fabricated in air and with a geometry optimised for EPR experiments. Critical features for satisfactory devices were found to be a sufficiently thick organic layer and minimal exposure to the air.  A compact apparatus was developed for simultaneous light output, current, and voltage measurements on the OLEDs while in an inert glove box environment. Electroluminescence and current-voltage parameters measured for these devices showed predominantly trap-controlled space-charge-limited conduction.   OLEDs with PFO as the emissive layer and with a PEDOT:PSS layer were investigated with conventional, electrically-detected (ED) and optically-detected (OD) EPR techniques. EDEPR and ODEPR signals were observed at ~9.2 GHz and in the low (<50 mT) and high (~330 mT) magnetic field regimes and were found to change markedly with time during operation as the device degraded. The low field signals initially showed a composite broad quenching and superimposed narrow enhancing response centred around zero field strength. These signals were attributed to magneto-resistance (MR) and magneto-electroluminescence (MEL). Following operational ageing, a third, narrow quenching line was observed in the MR and the ratio of the initial two MR responses changed substantially. These effects are tentatively attributed to a hyperfine interaction.  For both EDEPR and ODEPR, quenching high field resonances with a g-value (gyromagnetic ratio) of 2.003±0.001 were observed. The current-quenching resonance gradually diminished during operation and after 4–5 hours was replaced by a current-enhancing resonance. The appearance of this latter resonance could be explained by chemical changes in the OLED due to the diffusion of oxygen through the device from the oxygen-plasma-treated ITO. A working model is proposed which can explain this observed change as spindependent trapping and recombination at free radicals, although the model requires further experimentation to test its validity.</p>


Sign in / Sign up

Export Citation Format

Share Document