In situ monitoring drying process to disclose the correlation between the molecular weights of a polymer acceptor with a flexible spacer and the performance of all-polymer solar cells

Journal article, 2024

Molecular weight (Mn) and conjugation of polymers can profoundly influence the performance of all-polymer solar cells (all-PSCs) via nanostructures of bulk heterojunctions (BHJs). To study the correlation between Mn and the performance of all-PSCs based on an acceptor with a flexible conjugation-break spacer (FCBS), three batches of acceptors, named PYTS, were synthesized with different number-average Mn from 9, 13 to 19 kDa. Blends with a polymer donor PBDB-T, the all-PSCs based on PYTS with Mn of 9 kDa and 19 kDa, exhibit power conversion efficiencies (PCEs) of 5.99% and 9.43%, respectively, primarily due to the increased short-circuit current density (Jsc) from 13.02 to 18.73 mA cm−2. To disclose the impact of Mn on device performance, dynamics of mixed PBDB-T:PYTS solutions to solid BHJs is studied by monitoring the drying process with home-made in situ multifunctional spectroscopy, which demonstrates that PYTS with Mn of 19 kDa has a longer drying time than the PYTS with Mn of 9 kDa. Prolonged drying of the BHJs with higher Mn PYTS facilitates more tightly packed structures with higher crystallinity. A systematic investigation on the nanostructures of BHJs, charge generation, transport and recombination is carried out with grazing-incidence wide-angle X-ray scattering (GIWAXS), transient absorption spectroscopy (TAS) and characterization of all-PSCs. The results indicate that increased crystallinity in the BHJs benefits exciton dissociation, electron transport, prolonged carrier lifetimes, and decreased non-geminate recombination rate constants in the corresponding devices. Combining the in situ study of drying and the investigation on films and devices provides us a comprehensive understanding of the interplay between Mn, the drying process, the nanostructures of BHJs and device performance. This work not only emphasizes the essential role of Mn in governing the device performance, but also exhibits recorded film formation through the in situ spectroscopy, enabling us to manipulate the nanostructure of BHJs by optimizing Mn of polymers and processing parameters.