Role of Guest Materials in the Lower Stability of Solution-Coated versus Vacuum-Deposited Phosphorescent OLEDs

Abstract

Utilizing different phosphorescent materials as emitter guests, this work investigates the root causes of the lower electroluminescence (EL) stability of solution-coated (SOL) organic light-emitting devices (OLEDs) relative to their vacuum-deposited (VAC) counterparts. The results show that emitter guest molecules aggregate under electrical stress, leading to the emergence of new longer-wavelength bands in the EL spectra of the devices over time. However, the intensity of these aggregation emission bands is much stronger in the case of SOL host:guest systems than that of their VAC counterparts, indicating that guest aggregation occurs much faster in the former. The results reveal that the phenomenon arises from differences in the initial morphologies and are likely associated with the use of solvents in the solution-coating process. Moreover, although excitons can drive this aggregation in the case of SOL emissive layer (EML) devices, the coexistence of excitons and polarons accelerates this phenomenon significantly. The results uncover one of the main causes of the lower stability of OLEDs made by solution coating and reveal the importance of adopting new molecular designs that make them less susceptible to aggregation for the development of SOL OLEDs with high performance.

The Root Causes of the Limited Electroluminescence Stability of Solution-Coated Versus Vacuum-Deposited Small-Molecule OLEDs: A Mini-Review

“…Since the detection of host emission points to incomplete H → G energy transfer, the observations suggest that energy transfer was initially somewhat less efficient in the case of SOL EML. This phenomenon was found to play a direct role in the lower stability of phosphorescent OLEDs based on H:G systems made by solution-coating (Samaeifar and Aziz, 2022). The observations again pointed to differences in molecular distribution or morphology in the case of SOL layers, with more H:G phase separation compared with their VAC counterparts.…”

“…Solution-coating produces film morphologies with some initial phase separation into guest-rich and guest-deficient domains, and the host/guest aggregation accelerates the formation of guest-deficient domains. As a result, it is more difficult for excited host molecules in these domains to lose their excitation energy to guest molecules as quickly, in turn, making them more susceptible to exciton-induced degradation and aggregation (Samaeifar and Aziz, 2022). Photoluminescence images showing the increased H: G phase separation and aggregation in SOL versus VAC 4,4′-bis-(carbazol-9-yl) biphenyl (CBP) films doped with various phosphorescent guests are reproduced in Figure 2A (Yu and Aziz, 2020).…”

Advances in Solution‐Processed OLEDs and their Prospects for Use in Displays

“…, where L 0 is an ini tial luminance and n = 1.7 is an acceleration factor. [54,59] ) Most reported lifetimes of SOLEDs in the literature have been LT 50 of a few tens of hours at L 0 = 1000 cd m −2 , although some reported LT 50 of a few hundred hours (Figure 3 and Table 2). The life times of SOLEDs fabricated in academia are rarely reported, and the reported values are far lower than industrial require ments for display use (Tables 1 and 2 and Figure 3).…”

P‐122: Low Cross‐Linking Temperature Hole Transport Copolymer with Exemplary Hole Transport Ability for Efficient Solution‐Processed OLED

“…Solution-processed organic light emitting diode (s-OLED) technology has increasingly become attractive and is considered as a potential candidate for replacing the current mass production technology, so called, a vacuum evaporation. However, s-OLED still has the demerits of lower performance and stability due to many factors such as interface mixing, [1,2] imbalance charge injection, [3] solvent effect, [4] etc. Among them, interface mixing which is caused by unclear interface between two successively spin-coating layers, typically hole transport layer (HTL) and emitting layer (EML), is probably one of the most serious factors affecting device performance.…”