Liquid–Solid Transfer Process of Ordered Structures in Efficient Polymer Photovoltaic Materials

The single-chain conformations and aggregated structures of conjugated polymers in precursor solutions affect the morphology of films and their photoelectric properties. Dynamic/static light scattering, small-angle neutron scattering, high-resolution transmission electron microscopy, and selected area electron diffraction were employed to elucidate the single-chain conformations and aggregated structures of efficient donor polymers (PBDT-TTz and D18-Cl) in chloroform (CF, nonaromatic solvent) and chlorobenzene (CB, aromatic solvent) for a range of concentrations (0.03-5.0 mg/mL). D18-Cl presented a more rigid and extended chain conformation in dilute CF solution. However, the relatively flexible PBDT-TTz easily formed pi-pi interaction with the aromatic CB molecules, thereby extending the main chain's conjugation length. Regulation of the extended single-chain conformation in dilute solution resulted in the formation of a compact, ordered aggregated structure at increased concentrations. In the liquid-solid cross-phase transfer process, the ordered structures in the precursor solution were effectively inherited by the subsequent films. From dilute to concentrated solutions, the multistage self-assembled structures of conjugated polymers directly affected their solid-state packing and photoelectric properties of the resulting films. Through the regulation of the ordered aggregation in their precursor solution, the hole mobilities of PBDT-TTz and D18-Cl were increased by nearly an order of magnitude, and the power conversion efficiency of the solar cells was enhanced by 28.8%.