Molecular migration behaviors in organic light-emitting diodes with different host structures

Over the past years, organic light-emitting diodes (OLEDs) have attracted increasing interest because of their great potential for use in high-quality flat-panel displays and solid-state lighting. One of the basic requirements in any emissive device is to provide adequate stability to ensure a sufficiently long lifetime. Recently, it was observed that small molecules migrate toward the ITO anode under a direct driving voltage while retaining their original structures. To prevent this bias-driven migration of small molecules, a chemical structure with a higher steric hindrance could be introduced as a blockade, thus molecular migration could be suppressed and the device half-life increased. In this work, OLED devices with different hosts, including CBP, mCP, SimCP2, and SimCP, with increasing steric hindrances are fabricated. The spatial distribution of the tracking molecules after operation for different lengths of time is examined by using X-ray photoelectron spectroscopy (XPS) with in situ   high-energy C60+ and low-energy Ar+ co-sputtering for depth profiling. It is found that the bias-driven molecular migration is suppressed and the device half-life prolonged as the steric hindrance of the host increases.

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► The extent of bias-driven migration of small molecules was monitored by XPS depth profile.
► Migration was suppressed in the order of CBP, mCP, SimCP2, and SimCP host materials. Device lifetime increased with suppressing molecular migration.
► With increasing dipole moment, molecular weight, or density, active materials also act as blockades.