Host-to-Guest Energy Transfer and Its Role in the Lower Stability of Solution-Coated versus Vacuum-Deposited Phosphorescent OLEDs

Abstract

Electroluminescence (EL) degradation mechanisms in solution-coated (SOL) host:guest (H:G) systems commonly used in phosphorescent organic light-emitting diodes (OLEDs) are investigated and compared to their vacuum-deposited (VAC) counterparts. Changes in the EL, photoluminescence (PL), and time-resolved PL (TRPL) characteristics of devices comprising SOL or VAC H:G light-emitting layers (EMLs) made of the same materials and in the same device architectures during prolonged electrical driving are compared and analyzed. Hole-only devices are also utilized to study the effects of charges and excitons, separately and combined. Moreover, devices with double EMLs comprising SOL and VAC components are tested to glean additional insights into the role of host excitons in device degradation. The results indicate 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 host-to-guest (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. The findings shed light on one of the root causes of the limited stability of SOL OLEDs.

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

“…Moreover, both devices exhibit an LT50 of about 20 h (for an L 0 of 6000 cd m –2 ). These results prove that the solvent exposure does not affect device performance or lifetime and establish the solvent resistance of ITO/HAT-CN/VB-FNPD layers and interfaces for subsequent experiments …”

“…These results prove that the solvent exposure does not affect device performance or lifetime and establish the solvent resistance of ITO/HAT-CN/VB-FNPD layers and interfaces for subsequent experiments. 19 Previous work showed that increasing the guest concentration can lead to increased stability due to enhanced host-toguest energy transfer and thus reduced exciton stress in wide band gap host materials. 20,24 Therefore, we also tested and compared the EL stability of a similar set of devices with 10 wt % guest concentration.…”

“…One can expect that the guest aggregation leads to less efficient host-to-guest energy transfer, which may, in turn, lead to a shorter device lifetime. 19 To further verify the origin of these new bands, we also tested the effect of varying the guest concentration. Figure S2 shows the normalized EL spectra collected from the fresh devices with Ir(mppy) 3 , Ir(ppy) 2 acac, or TEG with 5 and 10 wt % concentrations.…”

“…The phase separation reduces the ability of excited host molecules to lose their excitation energy promptly, in turn making them more prone to exciton-induced degradation and aggregation. 19,28 To investigate the possibility that guest materials may attain different morphologies in SOL and VAC layers and their influence in the faster guest aggregation in the former, we also measured the surface topography and roughness of neat layers of the guest molecules using AFM. Since in an H:G system, the morphology is dominated by the host, the neat layers of the guest molecules were used here to be able to distinguish and test the effect of the fabrication process on the guest molecules specifically.…”

“…17 Moreover, we observed that SOL host:guest (H:G) systems exhibit lower photoluminescence (PL) and EL stability relative to their VAC counterparts, a behavior associated with a less efficient host-toguest energy transfer in them as a result of host aggregation. 18,19 However, these studies have paid little attention to the role of guest molecules in the lower stability of SOL devices, focusing instead on host materials. The role of guest molecules in the degradation process has been overlooked despite observations that show that SOL H:G systems with the same host material but different guest materials can exhibit differences in their stability.…”