Photophysics properties of blue-emitting polymers

Abstract

We report on an extensive spectroscopic study of thin films of two blue-emitting π-conjugated polymers (PCP), namely ladder-type poly-para-phenylene (mLPPP) and polyfluorene (PFO). In our work, we employed several continuous-wave (cw) spectroscopic techniques, such as absorption, photoluminescence (PL), photoinduced absorption (PA) and PA detected magnetic resonance (PADMR). We have obtained PA spectra measured over a very broad spectral range from 0.05 to 2.5 eV and identify PA bands related to triplet excitons and charged polarons, respectively. We found that the low energy polaron PA band, P1 peaks at unusually low photon energy, indicative of small relaxation energy for polarons in these materials. Similar results were obtained for high-mobility, highly ordered films of regio-regular poly-hexyl-thiophene (RR-P3HT), where the polarons are delocalized in two dimensions. In fact, the P1 band is so low that it overlaps in photon energy with the photoinduced infra-red active vibrations (IRAVs) in the PA spectrum, consequently, this leads to the formation of anti-resonances between IRAVs and the electronic band, which are sharp lines that appear as dips superimposed on the PA band. The spin 1/2 PADMR technique takes advantage of the dependence of the polaron recombination rate on the spin state of the recombining pairs of polarons. The origin of the spin-dependent recombination rate is a considerable difference in nature between singlet and triplet excitons in polymers, which are the two possible by-products of oppositely charged polaron recombination. The motivation for the PADMR study is the great importance of spin-dependent formation cross-sections to commercial light emitting diodes (LED) application. These cross-sections directly determine the maximum possible electroluminescence quantum efficiency (ELQE) for LEDs that are made from these polymers. We found that the ELQE may approach 60% for the blue-emitting polymers. © 2001 Elsevier Science B.V. All rights reserved.