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

Abstract

Molecular weight (M_n) 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 M_n 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 M_n from 9, 13 to 19 kDa. Blends with a polymer donor PBDB-T, the all-PSCs based on PYTS with M_n 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 (J_sc) from 13.02 to 18.73 mA cm⁻². To disclose the impact of M_n 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 M_n of 19 kDa has a longer drying time than the PYTS with M_n of 9 kDa. Prolonged drying of the BHJs with higher M_n 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 M_n, the drying process, the nanostructures of BHJs and device performance. This work not only emphasizes the essential role of M_n 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 M_n of polymers and processing parameters.