Ultrafast bridge planarization in donor-π-acceptor copolymers drives intramolecular charge transfer

Abstract

Donor-π-acceptor conjugated polymers form the material basis for high power conversion efficiencies in organic solar cells. Large dipole moment change upon photoexcitation via intramolecular charge transfer in donor-π-acceptor backbone is conjectured to facilitate efficient charge-carrier generation. However, the primary structural changes that drive ultrafast charge transfer step have remained elusive thereby limiting a rational structure-function correlation for such copolymers. Here we use structure-sensitive femtosecond stimulated Raman spectroscopy to demonstrate that π-bridge torsion forms the primary reaction coordinate for intramolecular charge transfer in donor-π-acceptor copolymers. Resonance-selective Raman snapshots of exciton relaxation reveal rich vibrational dynamics of the bridge modes associated with backbone planarization within 400 fs, leading to hot intramolecular charge transfer state formation while subsequent cooling dynamics of backbone-centric modes probe the charge transfer relaxation. Our work establishes a phenomenological gating role of bridge torsions in determining the fundamental timescale and energy of photogenerated carriers, and therefore opens up dynamics-based guidelines for fabricating energy-efficient organic photovoltaics.

Fig. 1

Energetics of photoinduced processes in neat low-bandgap donor-π-acceptor copolymers. Photogenerated excitons can lead to ultrafast intramolecular charge transfer (ICT) and polaron pair states

Fig. 2

Spectroscopic characterization of TDPP-BBT copolymer. a Structural representation of TDPP-BBT copolymer. b Steady-state absorption of the 90 μm copolymer solution in chlorobenzene is represented by black curve while the emission spectra is represented by orange dotted curve where 570 nm is the excitation wavelength; 650, 700, and 800 nm are the pump wavelength used for transient absorption measurements. c Ground state stimulated Raman spectra of TDPP-BBT polymer in chlorobenzene where 816 nm is the excitation pulse. Asterisk mark represents instrumental artifact

Fig. 3

Transient absorption of TDPP-BBT in chlorobenzene. a Actinic pump wavelength-dependent (650, 700, and 800 nm) excited state absorption spectra at 1 ps; b kinetics fitting at 1050 nm and 1350 nm for 650 nm excitation showing the presence of a short-lived and long-lived species

Fig. 4

Femtosecond stimulated Raman spectra of TDPP-BBT in chlorobenzene. Excited state Raman spectra at different pump-probe delay times has been shown. The ground state and excited state Raman signals are represented by black and red traces respectively. Asterisk mark represents instrumental artifact

Fig. 5

Transient changes in the Raman peak frequency and area. a Peak shift of 1422 and 1367 cm⁻¹ mode (Inset) in about 160 and 450 fs timescale respectively; both the modes reflect red shifted spectra. b Peak shift of 1315 and 1503 cm⁻¹ mode (inset). The 1315 cm⁻¹ mode is red shifted in 200 fs and then blue shifted in 10 ps while 1503 cm⁻¹ mode shows blue shift in 2 ps. c Relative peak area changes of 1228 cm⁻¹ with respect to 1422 cm⁻¹ mode occur in 430 fs timescale. d Relative Raman intensity change of 1228 cm⁻¹ as a function of thiophene dihedral angle as obtained from DFT calculation

Fig. 6

Schematic of the exciton dynamics and its driving molecular coordinates. FSRS measurements directly probe the exciton relaxation via thiophene bridge planarization leading to hot delocalized ICT character, and its subsequent cooling dynamics