Exciton–Exciton Annihilation in Mixed‐Phase Polyfluorene Films

Abstract

Singlet–singlet annihilation is studied in polyfluorene (PFO) films containing different fractions of β‐phase chains using time‐resolved fluorescence. On a timescale of >15 ps after excitation, the results are fitted well by a time‐independent annihilation rate, which indicates that annihilation is controlled by 3D exciton diffusion. A time‐dependent annihilation rate is observed during the first 15 ps in the glassy phase and in the β‐phase rich films, which can be explained by the slowdown of exciton diffusion after excitons reach low‐energy sites. The annihilation rate in the mixed‐phase films increases with increasing fraction of β‐phase present, indicating enhanced exciton diffusion. The observed trend agrees well with a model of fully dispersedβ‐phase chromophores in the surrounding glassy phase with the exciton diffusion described using the line‐dipole approximation for an exciton wavefunction extending over 2.5 nm. The results indicate that glassy andβ‐phase chromophores are intimately mixed rather than clustered or phase‐separated.

Subpicosecond Exciton Dynamics in Polyfluorene Films from Experiment and Microscopic Theory

“…The latter assumption is consistent with the large annihilation rates previously reported for PF8 films. 23 The one remaining parameter, the effective ESA absorption dipole d XX magnitude, we fix against the experiment at a given excitation power as detailed below.…”

“…Experimental results for polyfluorene of different types of morphology were reported previously. 23 Here we extend our microscopic model detailed above to include exciton-exciton annihilation events in the dynamics. To this end, we add the term…”

“…Furthermore Shaw et al 23 measured the temporal decay of the photoluminescence for a range of different pump intensities. With all the parameters fixed above, in figure 10 we compare our computed data with these measurements.…”

“…The combined influence of many factors, including sample morphology 22,23 , molecular conformations [24][25][26] , packing and traps 27,28 , exciton hopping and annihilation makes the problem of organic semiconductor device optimization complex, as it involves a wide range of energies, timescales and parameters that are difficult to isolate. Disentangling one from another is challenging and requires a combination of both experimental and theoretical approaches.…”

“…Disentangling one from another is challenging and requires a combination of both experimental and theoretical approaches. Simulations of large-scale systems have been conducted using a Monte-Carlo simulation approach [29][30][31][32] or a Master-equation based approach [33][34][35][36][37] , while experiments have used spectral-and time-resolved photoluminescence decay 23,38,39 or transient absorption spectroscopy 39,40 to monitor the ultrafast photophysics. Furthermore a number of theoretical simulations model charge-separated excitations [41][42][43][44][45][46] , which are particularly relevant to current flow in devices such as photovoltaics and LEDs.…”