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Organic light-emitting diodes (OLEDs) have reached a huge market as technology for small displays, e.g. in smartphones, and are entering the larger display and solid-state lighting markets as well. In parallel to these commercial successes, the OLED technology is adapted to a multitude of promising new applications, such as in optogenetics and medical therapy. However, it is still challenging to ensure good stability in applications that require high brightness (or high optical power density), in part due to the resulting resistive heating. Increased temperature can lead to a change in morphology of one or several organic layers, e.g. via crystallization of organic molecules, which then reduces electrical and optical performance and likely results in rapid device failure. Aside from an intrinsic resistive heating, heating can also be due to environmental effects during operation or fabrication and encapsulation of devices. For instance, atomic layer deposition (ALD) is a promising technique to form thin, yet highly protective encapsulation layers. However, state-of-the-art ALD processes require relatively high temperature during deposition (< 80 °C).
4,7-diphenyl-1,10-phenanthroline (BPhen) has been widely used as electron transporting layer (ETL) due its high electron mobility, particularly in an organic matrix-dopant system. However, it is well known that thin films of BPhen tend to recrystallize spontaneously. Annealing accelerates crystallization even further due to the relatively low glass transition temperature (Tg) of BPhen (62 °C). A straightforward way to enhance the device thermal stability is to make use of a high Tg material, yet materials have to be carefully adopted to provide appropriate functionality in OLEDs.
In this contribution, we report the improvement of the thermal stability of OLEDs with BPhen based electron transport layers (ETLs) by cesium (Cs) doping. To verify the role of the Cs dopant in the BPhen matrix, recrystallization features of Cs-doped BPhen films with different doping concentrations were investigated using optical microscopy and atomic force microscopy. We also examined the photophysical properties of the films, i.e. photoluminescence (PL) and absorption. PL spectra exhibit monotonic red-shifts and broadening as the Cs doping concentration increases. This presumably indicates formation of metal complexes via interaction between the 1,10-phenanthroline group of BPhen molecules and the Cs ions. It was found that Cs plays a critical role, not only in inhibiting undesired recrystallization of BPhen molecules in a thin-film, but also in allowing BPhen layers to be thermally stable beyond the Tg of neat BPhen.
Next, the electrical and optical properties of blue and red OLEDs that contain BPhen layers with different Cs-doping concentrations as ETL were characterized after annealing at temperatures between 60 and 100 °C. We find that higher doping concentrations lead to a marked increase in thermal device stability (quantified by current density and luminance at a fixed voltage). Making use of this observation, we successfully encapsulated BPhen based OLEDs with thin-film oxide layers using ALD.
The results shown in this work may be transferable to other material systems and can thus provide a useful guideline to enhance the intrinsic thermal durability of organic devices and to render them compatible with processes involving thermal treatment.
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