Effects of the carrier transport layer thickness on the bending durability of flexible OLEDs

In this work, flexible organic light-emitting diode on polyethylene terephthalate (PET) substrate with N, N'-Bis-(1-naphthalenyl)-N, N'-bis-phenyl-(1, 1'-biphenyl)-4, 4'-diamine (NPB) serve as the hole transport layer (HTL), TPBi as electron transport layer (ETL), and 4-(Dicyanomethylene)-2-tert-butyl-6-(1, 1, 7, 7-tetramethyljulolidin-4-yl-vinyl)-4H-pyran (DCJTB) as dopant were fabricated. And the effects of HTL thickness and ETL thickness on the bending durability were explored and optimized. The bending durability was characterized by the ratio of the luminance after 100 bending cycles (L-100) or 1000 cycles (L-1000) to that before bending or initial (L-0). The results show that for a given thickness of Alq(3), the device of structure 'ITO/NPB/Alq(3)/Al' having 80 nm NPB layer exhibited the best bending durability, but low luminance. Overall, the device with 60 nm NPB layer has a good performance both in luminance and bending durability. For a given NPB layer of 60 nm in devices with TPBi as ETL, the optimal TPBi layer thickness for the best bending durability is 25 nm. Experimental measurements of surface of the samples were carried out with an SEM. From the observation results, the main influence of the bending on the device performance comes from the destruction of the organic layer and the aluminum layer.