Excitonic coupling dominates the homogeneous photoluminescence excitation linewidth in semicrystalline polymeric semiconductors

We measure and model the homogeneous excitation linewidth of regioregular poly(3-hexylthiophene), a model semicrystalline polymeric semiconductor, by means of two-dimensional coherent photoluminescence excitation spectroscopy. At a temperature of 8 K, we extract a linewidth of $\sim 90$ meV full width at half maximum, which is a significant fraction of the total linewidth. We interpret this homogeneous broadening as a consequence of interchain exciton coupling and discuss it within the context of a weakly coupled aggregate model.

Figure 1

(a) Absorption spectrum of P3HT measured at 10 K and the spectrum of the femtosecond laser used in this work. (b) Schematic of the pulse sequence for the 2D-PLE measurements [11]. We used transform-limited, 13-fs pulses. The spectrally and temporally integrated PL intensity is measured as a function of the time delays $t_{21}$ and $t_{43}$ at a fixed population time $t_{32}$. The phase differences ${\varphi }_{21}$ and ${\varphi }_{43}$ are modulated to isolate the desired nonlinear signals.

Figure 2

Real part of (a) rephasing, (b) nonrephasing, and (c) total correlation spectra, which correspond to the purely absorptive spectral component, measured at 8 K. These data were acquired with a population waiting time $t_{32}=50$ fs.

Figure 3

Numerical simulations of the P3HT rephasing (left) and nonrephasing response (right). The real part is displayed using $2\gamma =30$ meV (top) and $2\gamma =90$ meV (bottom), both with inhomogeneous broadening $\sigma =130$ meV. Here $\gamma$ is the dephasing rate and $\sigma$ is the inhomogeneous spectral width.

Figure 4

(a) Modulus of the measured 2D-PLE rephasing spectrum displayed in Fig. 2. The elongated feature along the diagonal is characteristic of inhomogeneous broadening, limited by the excitation laser spectrum, while the antidiagonal slice (blue dashed line) reveals the homogeneously broadened spectrum [10]. (b) Antidiagonal slice of the photon-echo signal at 2.10 eV (blue open circles) corresponding to the dashed blue line in part (a). The black continuous curve displays the corresponding slice for the simulated spectra using $2\gamma =90$ meV and $\sigma =130$ meV [shown in Fig. 3].