Spontaneous orientation polarization (SOP) has been frequently observed in the evaporated films of organic light‐emitting diode materials. Because SOP modifies the charge injection and the accumulation properties of the device, understanding and controlling SOP is crucial in optimizing the performance of the device. In this study, we investigated the dominant factors for SOP formation by focusing on intermolecular interactions. We examined the giant surface potential characteristics of coevaporated films incorporating 1,3,5‐tris(1‐phenyl‐1H‐benzimidazol‐2‐yl)benzene (TPBi) that is a typical polar molecule exhibiting SOP. In the coevaporated films of TPBi and nonpolar molecules such as 4,4′‐bis(N‐carbazolyl)‐1,1′‐biphenyl and 4,4′,4″‐tris (carbazol‐9‐yl)triphenylamine, the orientation degree of the permanent dipole moment (PDM) of TPBi is significantly enhanced with diluted TPBi density, though the enhancement is weak on the film with N,N′‐bis(1‐naphthyl)‐N,N′‐diphenyl‐1,1′‐biphenyl‐4,4′‐diamine. The results indicate that the PDM interaction between polar molecules results as a negative factor for SOP formation. Furthermore, we found that SOP formation is suppressed by the surface treatment of the self‐assembled monolayer on the gold substrate, indicating a positive effect of the van der Waals interaction between the molecule and the substrate surface.
Herein, we propose a simple but powerful technique for
investigating
the correlations between the dynamics of charge carriers and excitons.
This technique (DCM-PL) is based on displacement current measurement
(DCM) with simultaneous observation of the photoluminescence (PL)
intensity. By applying this technique to metal–insulator–semiconductor
(MIS) devices incorporating a partial stack of a tris(2-phenylpyridine)iridium(III)
[Ir(ppy)3]-based organic light-emitting diode (OLED), we
are able to investigate the hole accumulation behavior and the corresponding
PL losses due to exciton-polaron quenching (EPQ). Remarkably, the
DCM-PL characteristics revealed that the polarity of the host material
in the emission layer modifies the charge-carrier dynamics and EPQ
properties. Our results contribute to the optimization of OLED device
performance, since EPQ is a key process involved in efficiency roll-off
and device degradation.
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