Subpicosecond Exciton Dynamics in Polyfluorene Films from Experiment and Microscopic Theory

Abstract

Electronic energy transfer (EET) in organic materials is a key mechanism that controls the efficiency of many processes, including light harvesting antenna in natural and artificial photosynthesis, organic solar cells, and biological systems. In this paper we have examined EET in solid-state thin-films of polyfluorene, a prototypical conjugated polymer with ultrafast photoluminescence experiments and theoretical modelling. We observe EET occurring on a 680 ± 300 fs timescale by looking at the depolarisation of photoluminescence. An * To whom correspondence should be addressed independent, predictive microscopic theoretical model is built by defining 125 000 chromophores containing both spatial and energetic disorder appropriate for a spin-coated thin film. The model predicts time-dependent exciton dynamics, without any fitting parameters, using the incoherent Förster-type hopping model. Electronic coupling between the chromophores is calculated by an improved version of the usual line-dipole model for resonant energy transfer. Without the need for higher level interactions, we yet find that the model is in general agreement with the experimentally observed 680 ± 300 fs depolarisation caused by EET. This leads us to conclude that femtosecond EET in polyfluorene can be well described by conventional resonant energy transfer, as long as the relevant microscopic parameters are well captured. The implications of this finding are that dipole-dipole resonant energy transfer can in some circumstances be fully adequate to describe ultrafast EET without needing to invoke strong or intermediate coupling mechanisms.1

Light Harvesting for Organic Photovoltaics

“… 42 , 43 For elongated chromophores, such as conjugated polymers, a line-dipole approximation gives satisfactory results with modest computational resources. 44 − 46 …”

Enhancing Exciton Diffusion Length Provides New Opportunities for Organic Photovoltaics

“…[35][36][37][38][39] Further hopping after this requires thermal activation or FRET to nonnearest neighbors; hence, the diffusion coefficient initially decreases and then reaches a thermal equilibrium value. 40 The energy distribution of excitons in thermal equilibrium is called the occupied density of states (ODOS). In the case of no structural relaxation in the excited state, the center of the ODOS is shifted by s 2 /kT to lower energy relative to the center of the DOS.…”

Perspective of dye-encapsulated conjugated polymer nanoparticles for potential applications

“…The criterion for effective energy transfer is the large overlap between the emission spectrum of donor and the absorption spectrum of acceptor. Besides dipole-dipole approximation, line-dipole approximation is considered in polymer-based energy transfer [9,47]. In addition, the distance between donor and acceptor also dictates the efficiency of energy transfer.…”