Indolocarbazole-Based Photo-Crosslinkable Hole-Transporting Layer for Efficient Solution-Processed Organic Light-Emitting Diodes.

We designed and synthesized a new indolocarbazole-based polymer, poly(N,N-diphenyl(5,11-dihexylindolo[3,2,1-jk]carbazol-2-yl)amine) (PICA), for solution-processed organic light-emitting diodes (OLEDs). The highest occupied and lowest unoccupied molecular orbital energy levels of this polymer, -5.25 and -2.46 eV, respectively, are suitable for hole transport from the anode to the emissive layer. PICA was photo-crosslinked by UV irradiation with ethane-1,2-diyl bis(4-azido-2,3,5,6-tetrafluorobenzoate) (FPA) as the photoinitiator. Successful crosslinking was confirmed by a decreased intensity in the azide-stretching FT-IR peak and solvent test with toluene (a suitable solvent for PICA). The PICA film photo-crosslinked with 3 wt% FPA showed enhanced solvent resistance (90%) compared to the non-crosslinked neat PICA film (<20%). Moreover, OLED devices using PICA-based hole-transporting layers exhibited better device performance (EQE/LE/PE: 8.88%/12.97/8.12 in red devices, 10.84%/38.47 cd/A/25.06 lm/W in green devices) than those using poly-TPD:FPA. We demonstrated that the photo-crosslinked PICA can be applied as a hole-transporting layer in solution-processed OLEDs.