Organic thin film structures for high-sensitivity imaging of contact stress distributions

We studied the responses of the electrical and optical characteristics of organic light-emitting diodes (OLEDs) with green fluorescent and phosphorescent dyes doped in a polymer matrix to compressive stresses. The phosphorescent OLED converted stresses into measurable and reversible changes in both current density and electroluminescence (EL) intensity. The current showed a nearly linear characteristic response with sensitivity up to 205 μA kPa−1, whereas EL decreased by over three orders of magnitude at 107 kPa. In contrast, the fluorescent OLED exhibited much smaller stress-induced modulations in current and light intensity, and the responses were saturated at stresses above 26 kPa. The discrepancy has been attributed to stress-enhanced back exciton energy transfer between guest and host molecules, which quenches the EL of the phosphorescent OLED, but has a much smaller impact on the performance of the fluorescent OLED.

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► Phosphorescent OLEDs converted stresses as low as 6 kPa into remarkable changes in both current density and EL intensity.
► The phosphorescent OLEDs exhibited good repeatability and stability required for stress sensing applications.
► Stress-induced modulations in current and light intensity in fluorescent OLEDs were much smaller.
► Hybrid OLEDs built on large curved surfaces may directly image stress distributions and sense touch on a par with a human finger.