Abstract
AbstractTop‐emitting organic light‐emitting diodes (OLEDs) are of interest for numerous applications, in particular for displays with high fill factors. To maximize efficiency and luminance, molecular p‐doping of the hole transport layer (p‐HTL) and a highly reflective anode contact, for example, made from silver, are used. Atomic layer deposition (ALD) is attractive for thin film encapsulation of OLEDs but generally requires a minimum process temperature of 80 °C. Here it is reported that the interface between the p‐HTL and the silver anode of top‐emitting OLEDs degrades during an 80 °C ALD encapsulation process, causing an over fourfold reduction in OLED current and luminance. To understand the underlying mechanism of device degradation, single charge carrier devices are investigated before and after annealing. A spectroscopic study of p‐HTLs indicates that degradation is due to the interaction between diffusing silver ions and the p‐type molecular dopant. To improve the stability of the interface, either an ultrathin MoO3 buffer layer or a bilayer HTL is inserted at the anode/organic interface. Both approaches effectively suppress degradation. This work shows a route to successful encapsulation of top‐emitting OLEDs using ALD without sacrificing device performance.
Highlights
We investigated the degradation in current density and luminance in top-emitting Organic light-emitting diodes (OLEDs) upon encapsulation with a low temperature Atomic layer deposition (ALD) process
To understand the underlying mechanism, single charge carrier devices were investigated, and it was found that the interface between the bottom silver electrode and p-doping of the hole transport layer (p-HTL) was the main cause for the decrease in current density and luminance
The top-emitting configuration of OLEDs is preferable in terms of fill factor and aperture ratio in various practical applications
Summary
OLEDs with NBPhen and TPBi showed lower current density and luminance but comparable external quantum efficiency (EQE, Figure S2, Supporting Information) We attribute this difference to the high charge mobility and deep lowest unoccupied molecular orbital level of BPhen.[36,37] when instead using ALD encapsulation, which involves heating the devices to a temperature of 80 °C for several hours, the current density in the BPhen device became dramatically lower (0.8 mA cm−2 at 5 V) and no detectable emission was observed up to a voltage of 7 V. With increasing annealing time the device performance reduced significantly; there was an almost fourfold decrease in current density and luminance at 5 V after 16 h, with the values approaching those observed after ALD encapsulation This indicates that the elevated temperature during the ALD process is the main cause of the observed degradation
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