Reliability of organic light-emitting diodes in low-temperature environment

Organic light-emitting diode (OLED) is an electroluminescent technology that relies on charge-carrier dynamics and is a potential light source for variable environmental conditions. Here, by exploiting a self-developed low-temperature testing system, we investigated the characteristics of hole/electron transport, electro-optic conversion efficiency, and operation lifetime of OLEDs at low-temperature ranging from -40 ^°C to 0 ^°C and room temperature (25 ^°C). Compared to devices operating at room temperature, the carrier transport capability is significantly decreased with reducing temperature, and especially the mobility of the hole-transporting material (HTM) and electron-transporting material (ETM) at -40 ^°C decreases from 1.16× 10^-6 cm²/Vs and 2.60× 10^-4 cm²/Vs to 6.91× 10^-9 cm²/Vs and 1.44× 10^-5 cm²/Vs, respectively. Indeed, the temperature affects differently on the mobilities of HTM and ETM, which favors unbalanced charge-carrier transport and recombination in OLEDs, thereby leading to the maximum current efficiency decreased from 6.46 cdA^-1 at 25 ^°C to 2.74 cdA^-1 at -40 ^°C. In addition, blue fluorescent OLED at -20 ^°C has an above 56% lifetime improvement (time to 80% of the initial luminance) over the reference device at room temperature, which is attributed to efficiently dissipating heat generated inside the device by the low-temperature environment.