Magnetic field test predicts blue OLED lifetime in the lab

Judging how long a blue OLED will stay bright has meant waiting. Conventionally, engineers run a device for tens to hundreds of hours while watching its brightness fall, and the device wears down during the test itself. A team at the Korea Advanced Institute of Science and Technology (KAIST) has found a shortcut: a magnetic field that nudges the light output by an amount too small to see with the naked eye. The team, led by professor Seunghyup Yoo of the School of Electrical Engineering, published the method in Nature Communications.

OLEDs (organic light-emitting diodes) produce light on their own when current flows through them, and they make up smartphone and TV screens. A screen creates its full range of colors by combining red, green and blue light. The blue phosphorescent devices studied here use electrical energy efficiently to emit blue light, but keeping a long lifetime while maintaining that efficiency is a key challenge in their development.

The researchers operated OLEDs at constant current and varied the magnetic field strength with an electromagnet, then measured the brightness change. Under the field, the devices became slightly brighter. That response combines several effects, and the team isolated the component most closely tied to lifetime degradation, turning it into a single evaluation metric. Applied to four blue devices made from the same light-emitting material but with different internal structures, the ones with a smaller initial response maintained their starting luminance longer in long-term operating tests. The metric showed a strong linear correlation with the logarithm of lifetime, both at 70% and at 50% of initial brightness.

The team also explained why. Over long periods of use, bonds in the organic molecules can break and form defects that interfere with light emission. Molecular simulations indicated that these defects affect the light-emitting process, and that this effect shows up in the brightness response to a magnetic field. The magnetic field is therefore a probe, not a way to extend lifetime.

Display companies and research institutes are the first to gain. When several prototypes share a light-emitting material but differ in structure, a few early measurements can single out the designs most likely to last, so that development effort goes to them. Because not every prototype has to run for long periods before results arrive, the team expects the method to shorten development time and reduce cost. The team gives no timeline for adoption in production.

The result is not yet a general rule. Further validation is needed before the relationship applies to a wider range of OLEDs, and when materials or internal structure change, the main factors responding to the magnetic field can change too. Each device type therefore needs its own analysis. Yoo said the result "can be used to quickly select materials and device structures with long lifetimes in the early stages of OLED development", and that with validation under various conditions the method could serve as a nondestructive evaluation technique.