A solution processible semiconducting polymer interlayer for blue light-emitting diodes

Abstract

Polyimide containing triphenylamine derivative (TPD-PI) was synthesized to prepare a polymer interlayer having insolubility in common nonpolar solvents for light-emitting polymers. The TPD-PI was prepared from the synthesized diamine monomer and 4, 4'-(hexafluoropropylidene)-diphthalic anhydride by the standard two-step polymerization method, which involved ring-opening polymerization and subsequent cyclodehydration. We incorporated this novel semiconducting polymer (TPD-PI) between the hole conducting layer (PEDOT:PSS) and the blue-emitting layer (PFO, poly(dioctylfluorene)) of a light-emitting diode. The molecular architecture of the TPD-PI interlayer means that it is soluble in most aprotic polar solvents, and completely insoluble in acetone, toluene, and xylene, which are used as solvents for most emitting layers; these solubility properties make the direct spin-coating of the interlayer possible. The surfaces of interlayer have stability against the general organic solvents for a polymeric layer with a nanometre scale. The efficiency increases from 0.25 to 0.351 cd A(-1) and the lifetime rises by 50% as a result of the insertion of the interlayer, which also has advantages in the following areas: interface structure, solubility characteristics, energetic position, hole transport of the interlayer, and thermal stability of the polyimide main chain.