Long-lived photo excitations in light emitting conjugated polymers

Abstract

In this work findings related to delayed luminescence after optical excitation in a range of solid-state conjugated polymers are described and analysed. Special interest was focused on conjugated light emitting polymers applicable for polymer light emitting diodes such as poly(9,9-bis(2-ethylhexyl)fluorene- 2,7-diyl) (PF2/6), since it appears to be that the major excitations created are non emitting long-lived triplets. Indeed delayed luminescence could be detected in all investigated polymers consisting of two spectral contributions. The high-energy portion identical to the prompt fluorescence is assigned to be delayed fluorescence (DP). The second contribution - vibronically similar but red-shifted to the prompt fluorescence - results from the decay of the first excited triplet state to the singlet ground state and thus is termed phosphorescence (Ph). Experimental results of kinetics, temperature and intensity dependencies for PF2/6 of both, DP and Ph, can be understood qualitatively in terms of a picture based on mobile triplets created via inter system crossing that perform triplet-triplet annihilation (TTA) at early times and become trapped for long times after excitation. In the last chapter an alternative to the TTA picture based on geminate pairs, again formed after optical excitation, is theoretically developed. Two parameters are of importance: the recombination frequency and the initial geminate pair distribution. Then indeed the entire complexity of experimental results, especially the observed power law dependence in the DP kinetics, becomes understandable, even if the final proof for the theory is lacking due to qualitatively poor experimental data. An important conclusion following the geminate pair picture is that the majority of triplets created via inter-system-crossing decay fast and non- emissive rather than forming geminate pairs and hence the real on-chain phosphorescence, as observed in frozen solution, still eludes detection in solid-state conjugated polymer films.