Investigating the Causes of the Lower Electroluminescence Stability of OLEDs with Solution-Coated versus Vacuum-Deposited Host:Guest Systems

Organic light-emitting devices (OLEDs) are increasingly being used in commercial flat display products from mobile phones and smart watches to televisions. Although OLEDs have become a recognizable product to consumers only recently, their exceptional potential over competing display technologies – liquid crystal displays (LCDs) primarily – has been demonstrated for decades. While LCDs use backlighting, OLEDs are self-emissive, making it possible for each pixel to be turned on and off individually, resulting in lower power draw and deeper black levels. Perhaps one of the most unique properties of OLEDs arises from the low-temperature fabrication process, as this allows for the use of flexible plastic substrates and thus inexpensive large-scale processing. Further down the line, the possibility of fabrication of OLEDs via a solution-coating process presents an opportunity for lower-cost applications, especially solid-state lighting products.
\nFrom a fabrication standpoint, OLEDs can be made via one of two approaches: vacuum-deposition or solution-coating. Vacuum-deposition is currently the main one used in the manufacturing of commercial OLED products since it allows for complicated multiple-layer devices and gives excellent device performance. However, this method has major drawbacks, such as inefficient utilization of materials, high equipment cost, high vacuum requirements, and complicated color patterning processes. Solution-coating, in contrast, provides significant advantages in terms of material utilization and fabrication costs, especially for large-area products. It also allows using inkjet printing for color patterning, offering additional advantages in reducing fabrication costs. However, the EL stability of solution-coated (SOL) OLEDs continues to be significantly lower in comparison to their vacuum-deposited (VAC) counterparts. The short lifetime is currently the main obstacle preventing the commercialization of low-cost OLEDs via solution-coating.
\nWhile several studies have investigated degradation mechanisms in SOL OLEDs and identified excitons and polarons to be leading culprits, the root causes underlying the relatively faster degradation in these systems are still not clearly understood. Moreover, most of those investigations have focused on neat SOL layer systems comprised of only a single material, and host:guest (H:G) systems, typically used in the light-emitting layer (EML) of phosphorescent OLEDs, have not been adequately investigated. In addition, the studies have paid little attention to the role of guest molecules in the lower stability of SOL devices, focusing instead on the host materials. Moreover, it is necessary to find new approaches to improve the stability of SOL OLEDs and surmount this long-standing challenge for SOL OLED technology. Therefore, the main focus of this work is to (i) understand the role of host-to-guest (H → G) energy transfer and guest materials in the lower stability of SOL versus VAC phosphorescent OLEDs, and (ii) explore approaches to increase SOL device stability. This study led to a number of new findings.
\nFirst, our studies indicated that the faster degradation of SOL EML devices relative to their VAC EML counterparts under electrical stress is due – at least in part – to the less efficient H → G energy transfer in these systems, which accelerates molecular aggregation in the EML. Interactions between excitons and polarons in the EMLs induce this aggregation phenomenon which occurs more strongly in the case of SOL EMLs compared to their VAC counterparts because of the higher host exciton concentration in the former as a result of the less efficient H → G energy transfer. 
\nIn addition, our results demonstrated that emitter guests aggregate as a result of electrical stress, giving rise to the emergence of new longer-wavelength bands in the EL spectra of devices after prolonged operation. However, the intensity of these aggregation emission bands is much stronger in the case of SOL H:G systems than their VAC counterparts, indicating that guest aggregation occurs much faster in the former. Results also showed that the differences in behavior arise from differences in the initial film morphologies, and are likely associated with the solvent used in the solution-coating process. Moreover, although excitons can drive this aggregation in the case of SOL EML devices, the coexistence of excitons and polarons accelerates this phenomenon significantly in these devices, possibly through exciton-polaron-induced aggregation (EPIA).
\nNext, a co-doped system was introduced as a novel approach for enhancing the lifetime of SOL phosphorescent OLEDs. The findings revealed that the intensity of guest aggregation emission bands is much stronger in devices with a single dopant compared to their co-doped counterparts, indicating a faster occurrence of guest aggregation in the former. Moreover, devices utilizing the co-doped system exhibit a 3× longer half-life (LT50) than devices with a single dopant. 
\nFinally, the improvement of SOL OLED lifetime was presented using increasing the guest concentrations. To achieve this, thermally-activated delayed fluorescence (TADF) emitters, capable of being incorporated into the EML at a relatively high concentration, were doped into the host material at varying concentrations. The results showed that increasing guest concentration from 10 wt. % to 30 wt. % in H:G systems leads to a more efficient H → G energy transfer, resulting in a longer LT50.

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