Role of Intermolecular Coupling in the Photophysics of Disordered Organic Semiconductors: Aggregate Emission in Regioregular Polythiophene

We address the role of excitonic coupling on the nature of photoexcitations in the conjugated polymer regioregular poly(3-hexylthiophene). By means of temperature-dependent absorption and photoluminescence spectroscopy, we show that optical emission is overwhelmingly dominated by weakly coupled H aggregates. The relative absorbance of the 0–0 and 0–1 vibronic peaks provides a powerfully simple means to extract the magnitude of the intermolecular coupling energy, of approximately 5 and 30 meV for films spun from isodurene and chloroform solutions, respectively.

Figure 1

(a) Normalized room-temperature (RT) absorption and PL spectra of rrP3HT in a 0.0001 wt % isodurene solution. Solid line through PL is a Franck-Condon fit. (b) Temperature-dependent absorption spectra of 1 wt % isodurene solution. (c) Normalized absorption spectra of a film spun from 1 wt % isodurene and chloroform solutions. The PL spectrum of a chloroform film is also shown (○) with a Franck-Condon best fit (solid curve). The PL spectrum was measured at 10 K whereas absorption spectra were measured at RT.

Figure 2

(a) Normalized PLE spectra at room temperature (RT) of a film spun from a chloroform solution at different detection photon energies. (b) PL decay at two photon energies measured at RT and 10 K.

Figure 3

(a) PL spectra of a film spun from chloroform solution at various temperatures. The solid lines through the data are fits to Eq. (2). (b) Time-resolved PL spectra measured at 10 K, normalized to the 0–1 peak. Spectra are integrated over 0.25-ns windows and presented here from 0–3 ns. (c) Jablonski diagram showing the exciton energy level structure. (d) Variation of $\alpha$ with temperature [see Eq. (2)].

Figure 4

Experimental and simulated absorption spectra measured at room temperature for films spun from (a) chloroform and (b) isodurene. Calculations were performed on $\pi$ stacks containing 12 polymers, large enough to ensure spectral convergence. Transition frequency offsets were chosen randomly from a Gaussian distribution with ($1/e$) full width of $2\sigma$ and spatial correlation length $l_0=3$. The final absorption spectrum was averaged over 1500 disorder configurations. For further details, see 12.