Exciton–Polaron‐Induced Aggregation of Wide‐Bandgap Materials and its Implication on the Electroluminescence Stability of Phosphorescent Organic Light‐Emitting Devices

Abstract

The degradation mechanisms of phosphorescent organic light-emitting devices (PhOLEDs) are studied. The results show that PhOLED degradation is closely linked to interactions between excitons and positive polarons in the host material of the emitter layer (EML), which lead to its aggregation near the EML/electron transport layer (ETL) interface. This exciton-polaroninduced aggregation (EPIA) is associated with the emergence of new emission bands at longer wavelengths in the electroluminescence spectra of these materials, which can be detected after prolonged device operation. Such EPIA processes are found to occur in a variety of wide-bandgap materials commonly used as hosts in PhOLEDs and are correlated with device degradation. Quite notably, the extent of EPIA appears to correlate with the material's bandgap rather than with the glass-transition temperature. The fi ndings uncover a new degradation mechanism, caused by polaron-exciton interactions, that appears to be behind the lower stability of OLEDs utilizing wide-bandgap materials in general. The same degradation mechanism can be expected to be present in other organic optoelectronic devices.

Exciton–Polaron‐Induced Aggregation of Organic Electroluminescent Materials: A Major Degradation Mechanism in Wide‐Bandgap Phosphorescent and Fluorescent Organic Light‐Emitting Devices

“…A close examination of Figure 2 b,c also reveals detectable, yet smaller, spectral changes. The appearance of new longer-wavelength bands suggests that they may be the result of molecular aggregation in these materials, similar to that observed in several other OLED material systems, [ 15,18 ] in this case induced by the electrical driving. The differences clearly correspond to the emergence of new well defi ned bands at longer wavelengths.…”

Degradation Mechanisms in Blue Phosphorescent Organic Light-Emitting Devices by Exciton–Polaron Interactions: Loss in Quantum Yield versus Loss in Charge Balance

“…As can be seen, the main mCP emission peak at 400 nm decreases over time. Accompanying the decrease is the appearance of a new band with peak at ∼500 nm, which we have previously attributed to emission from mCP aggregates Figure c presents the PL spectra of this device before and after the electrical stress conditions.…”

Influence of the Guest on Aggregation of the Host by Exciton–Polaron Interactions and Its Effects on the Stability of Phosphorescent Organic Light-Emitting Devices

“…Interestingly, the spectrum of the neat CBP after heating shows clear vibronic bands, indicating that the CBP molecules become more highly ordered, which is consistent with the aggregation notion and the observed crystallization. The different spectral features in this figure versus those in Figure can be attributed to the different extent of aggregation in the two cases, as was explained in more detail in our previous work . Furthermore, a comparison of spectra from the heated versus unheated 0.25% Ir­(ppy) 3 -doped films in Figure b reveals the following: (1) Significant PL in the 350–450 nm range, typical of that of CBP, appears after heating, indicating that energy transfer from CBP to the guest becomes less efficient.…”