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.

Identification of a Bridge-specific Intramolecular Exciton Dissociation Pathway in Donor-π-acceptor Alternating Conjugated Polymers 

“…As previously reported, it can also achieve long-distance charge separation between the donor and acceptor, thus ensuring a long-lived charge transfer state. [8] Fig. 1b exhibits the steady-state absorption spectra of the three polymers, the absorption spectra of the three polymers with different π bridges present similar shapes, featured by two distinct absorption bands.…”

“…[5] In consequence, OPV devices that consist of Donor-Acceptor alternating conjugated polymers could become the economically viable alternatives to Si-based solar cells. [6][7][8] The latest researches from various groups have shown that the e ciency of OPV can be improved by using low band gap conjugated alternating copolymers (as seen in PCDTBT, PBDTTT, and PTB families). [9][10][11][12] An important difference between these polymers is that the exciton transition at the lowest energy level exhibits partial charge-transfer characteristics.…”

Molecular Engineering Approaches Towards All‐Organic White Light Emitting Materials

“…A dual fluorescence is often achieved through the structural equilibrium between the locally excited (LE) state and the relaxed ICT state (Figure a) . The extent of the LE and the ICT emission can easily be modulated by varying the substituents, steric restrictions, and polarity of the local environment of the probe . Hence, D‐A based fluorophores have become popular in the last few years to achieve white light emission .…”

A single-stranded coordination copolymer affords heterostructure observation and photoluminescence intensification

“…Heterostructures in polymers (or copolymer configurations) enrich their functionalities, and numerous works have studied organic polymers that feature random ( 1 – 4 ), block ( 5 – 14 ), and sequence-controlled ( 15 – 21 ) copolymer compositions. For example, Roy et al .…”

“…For example, Roy et al . ( 14 ) reported a fundamental intramolecular charge-transfer mechanism for donor-π-acceptor copolymers, which led to the fabrication of energy-efficient organic solar cells. Researchers have also developed supramolecular copolymers based on noncovalent bonds such as hydrogen bonds, metal coordination bonds, and π-π interactions ( 22 – 30 ).…”