Optical studies of Poly[p-(2,5-didodecylphenylene)ethynylene] in Thin Films

Abstract

ABSTRACTUnlike many other conjugated polymers popular for use in LEDs, poly[p-(2,5-didodecylphenylene)ethynylene] (DPPE), shows less interchain interaction between polymer chains after annealing to an ordered film. Evidence of this decreased interchain interaction can be seen in the emission spectra of pristine and annealed films. Thin films of pristine DPPE show a broad featureless green emission, characteristic of an excimer-like state, whereas annealed films blue-shift and become structured, resembling dilute solution emission. DPPE packs into ordered domains after annealing, shown by birefringence, polarization anisotropy NSOM, and x-ray crystallography data. The lack of chromophore interaction in the ordered domains can be explained through a brick-wall-type packing motif, where the polymer backbone is insulated by the side chains of offset neighboring polymers. Electroluminescence spectra of pristine and annealed LEDs are nearly identical, both having green emission, unlike their different photoluminescence spectra. This is evidence that light emission in LEDs comes only from excimer sites within the film, which would indicate a decreased performance for the annealed devices compared to pristine devices. Preliminary data of pristine and annealed LEDs supports this theory, with most pristine devices having a slightly higher electroluminescence intensity compared to annealed devices.