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, P 1 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 P 1 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.