Molecular structure and conformational design of Donor-Accepter conjugated polymers to enable predictable optoelectronic property

Tuning the optoelectronic property Donor-Accepter conjugated polymers (D-A CPs) is highly important in designing various organic optoelectronic devices ranging from the photovoltaic, photodetector, light editing diodes and photoresistors. Low-bandgap D-A CPs allow polymer solar cells to harvest near-infrared photons to increase their power conversion efficiency further. However, there remains a critical challenge in chemical approaches to control bandgap, which originates from a synergistic effect of chain conformation and molecular orbitals. In this work, we studied the chain conformation and corresponding optical spectra for high-performance D-A CPs in the single-chain state by multimodal variable-temperature scattering, spectroscopy techniques, and molecular dynamics simulations. We found a critical role of side-chain length and branch point in the persistence length and optical absorption, which can be attributed to the steric effects. The increasing bulkiness of side chains by increasing side-chain length or spacer length rigidifies the polymer chain due to decreased population in cis conformation. Systematic blue shifts have been observed with increasing alkyl chain length. The optical shift changes according to an odd-even effect of the branching point. In both cases, the local backbone planarity is responsible for the optical shift. The length of oligothiophene donor units has also been shown to influence the persistence length and optical absorption by a combined effect of chain conformation and band energy alignment of D-A units. Chain rigidity decreases with the introduction of longer oligothiophene units. However, a controversial role of oligothiophene unit length on the optical properties of D-A CPs has been observed. Theoretical calculations on band structures have shed light on the nature of the controversial trend based on the intra-chain super-exchange (SE) mechanism, where the band energy alignment plays a key role. Our findings bridge the fundamental knowledge gaps to design new CPs with desired optoelectronic properties via molecular engineering for next-generation electronic devices.