Facile Photo-Crosslinking of Azide-Containing Hole-Transporting Polymers for Highly Efficient, Solution-Processed, Multilayer Organic Light Emitting Devices

Abstract

A novel framework of azide containing photo-crosslinkable, conducting copolymer, that is, poly(azido-styrene)-random-poly(triphenylamine) (X-PTPA), is reported as a hole-transporting material for efficient solution-processed, multi-layer, organic light emitting diodes (OLEDs). A facile and energy-efficient crosslinking process is demonstrated with UV irradiation (254 nm, 2 mW/cm(2)) at a short exposure time (5 min). By careful design of X-PTPA, in which 5 mol% of the photo-crosslinkable poly(azido-styrene) is copolymerized with hole-transporting poly(triphenylamine) (X-PTPA-5), the adverse effect of the crosslinking of azide moieties is prevented to maximize the performances of X-PTPA-5. Since the photo-crosslinking chemistry of azide molecules does not involve any photo-initiators, superior hole-transporting ability is achieved, producing efficient devices. To evaluate the performances of X-PTPA-5 as a hole-transporting/electron-blocking layer, Ir(ppy)(3)-based, solution-processable OLEDs are fabricated. The results show high EQE (11.8%), luminous efficiency (43.7 cd/A), and power efficiency (10.4 lm/W), which represent about twofold enhancement over the control device without X-PTPA-5 film. Furthermore, micro-patterned OLEDs with the photo-crosslinkable X-PTPA-5 can be fabricated through standard photolithography. The versatility of this approach is also demonstrated by introducing the same azide moiety into other hole-transporting materials such as poly(carbazole) (X-PBC).