Tracking the coherent generation of polaron pairs in conjugated polymers

Abstract

The optical excitation of organic semiconductors not only generates charge-neutral electron-hole pairs (excitons), but also charge-separated polaron pairs with high yield. The microscopic mechanisms underlying this charge separation have been debated for many years. Here we use ultrafast two-dimensional electronic spectroscopy to study the dynamics of polaron pair formation in a prototypical polymer thin film on a sub-20-fs time scale. We observe multi-period peak oscillations persisting for up to about 1 ps as distinct signatures of vibronic quantum coherence at room temperature. The measured two-dimensional spectra show pronounced peak splittings revealing that the elementary optical excitations of this polymer are hybridized exciton-polaron-pairs, strongly coupled to a dominant underdamped vibrational mode. Coherent vibronic coupling induces ultrafast polaron pair formation, accelerates the charge separation dynamics and makes it insensitive to disorder. These findings open up new perspectives for tailoring light-to-current conversion in organic materials.

Figure 1. Two-dimensional electronic spectroscopy of annealed rr-P3HT thin films.

(a) Linear absorption spectrum (red) and simulation (black), based on a displaced oscillator model, coupling ground and exciton states to the C=C stretch mode at 1,450 cm⁻¹ (23-fs period). (b) Differential transmission (ΔT/T) spectra at a delay of 350 fs, normalized to the pump energy. Positive (ΔT/T>0) bands are due to the bleaching of excitonic transitions, whereas the negative signal around 1.89 eV is assigned to photoinduced absorption of polaron pairs. (c–f) Absorptive 2DES maps of annealed rr-P3HT thin films at selected waiting times of (c) 2.3 fs, (d) 11.5 fs, (e) 25.4 fs and (f) 69.3 fs. At early waiting times (c–e) Cross sections along both the excitation and detection energy reveal distinct splitting of the bleaching peaks into four vibronic resonances, labelled to. The corresponding peaks are marked with dark blue circles. These splittings are the characteristic signature of strong vibronic coupling resulting in the formation of hybrid exciton-polaron-pair (XP) modes. The cross peaks with negative amplitude at detection energy of 1.89 eV originate from photoinduced absorption of polaron pairs. At longer waiting times (f) the splitting along the detection energy washes out and the resulting cross sections match those of the low-energy vibronic peaks in ΔT/T spectra. At all waiting times, negative amplitudes are observed for detection energy around 1.89 eV, monitoring polaron pair peak dynamics.

Figure 2. Dynamics of relevant peaks in the absorptive 2DES maps.

(a) 2DES map at waiting time of 115 fs. We detect a series of positive (A_2D>0) bleaching peaks and negative peaks for detection around 1.89 eV, reflecting photoinduced absorption of polaron pairs. The corresponding resonances are marked by dashed lines. (b–e) The dynamics of different positive vibronic peaks (open circles) show pronounced, long-lived oscillations, predominantly with 23-fs period (fit: red line). Fourier spectra (insets in b,c) reveal additional oscillatory components at 500⁻¹, 1,000⁻¹ and 2,000 cm⁻¹. (f) Remarkably, the negative polaron pair peak amplitude (open circles) displays strong oscillations with 23-fs period and rises within 100 fs (blue line). All these features are distinct signatures of coherent polaron pair formation.

Figure 3. Theoretical analysis.

(a) Displaced harmonic oscillator model consisting of four electronic levels strongly coupled to the C=C stretch mode. Coherent coupling among exciton and polaron pair and the dominant vibrational mode results in hybridized states. (b,c) Simulated absorptive 2D maps in the absence of inhomogeneous broadening at waiting times of 6 and 120 fs, respectively. Positive peaks reflect bleaching of the transitions. The negative peak around 1.89 eV is due to excited state absorption from the manifold to the state. Cross sections reveal distinct spectral splitting of the bleaching peaks, in good agreement with the experimental observations (Fig. 2a,b). (d) Dynamics of the positive diagonal peak amplitude at 2.06 eV. (e) Dynamics of the polaron pair peak for excitation at 2.26 eV and detection at 1.87 eV. A significant part of the polaron pair population is already formed during the first vibrational period. Fourier transforms (d,e insets) reveal the vibrational mode at 1,450 cm⁻¹ and a weaker component around 500 cm⁻¹ arising from electronic coupling between and . (f) Simulated linear absorption spectrum in the absence (green) and presence (red) of inhomogeneous broadening. For comparison, the black curve depicts the actual experimental data.