In Situ Monitoring the Drying Process of the Bulk Heterojunctions (BHJs) in Organic Solar Cells with a Multi-Spectroscopy

The performance of solution processed organic solar cells (OSCs) is primarily governed by the morphologies of bulk heterojunctions (BHJs) consisting of electron donors and acceptors formed during the drying process. The morphologies of the BHJs depends on many factors, such as solubility of donors and acceptors, solvent, solution concentration, coating methods etc. Stability of OSCs, a key parameter for commercialization, also strongly depends on the morphologies. Therefore, it is essential to understand morphology evolution from solutions to solid films to manipulate the morphology of BHJs for further development of OSCs.<br/>Combining multiple optical techniques in an in situ mode, including laser scattering, absorption, and steady-state photoluminescence (PL), we comprehensively study the morphology evolution and donor/acceptor interactions in different BHJs.<br/>First, to understand how the drying process impacts the blend morphology in OSCs, we studied the film formation processes of three representative BHJs composed of donor PBDB-T with acceptors PC71BM, IT-M, and N2200 by monitoring the drying process from liquids to films with the in situ ultraviolet-visible (UV-vis) absorption spectra and photoluminescent (PL) spectroscopy. The drying and PL quenching dynamics are analyzed during the film formation of both pristine and BHJ films, which indicate that the component with higher molecular weight dominates the blend film formation and the final morphology. This work contributes to a deeper understanding of microstructure formation determined by interplay between donor, acceptor, and solvent during the film drying. (Liu et al., <i>Small Methods</i>, 2021, 2100585, https://doi.org/10.1002/smtd.202100585)<br/>Furthermore, we investigated solvent impacts on the morphology of blend films and performance of the OSCs by monitoring the drying process of PBDBT:PF5–Y5 blends in chlorobenzene (CB), and ortho-xylene (o-XY) with the in situ multifunctional spectroscopy. Finer-mixed donor/acceptor nanostructures obtained in CB-cast layer corresponding to more charge generation in corresponding solar cells was observed. (Yao et al., Sol. RRL 2023, 2201134, https://doi.org/10.1002/solr.202201134 )<br/>In addition, we also revealed the function of a commonly used solvent additive 1-Chloronaphthalene (CN) in enhancing the performance of all-PSCs based on PBDB-T:PF5-Y5 by studying the drying process, which suggests that improved performance of PBDB-T:PF5-Y5 solar cells originated from enhanced crystallinity and hole mobility since CN promotes self-aggregation of PBDB-T during the drying process. (Liu et al., J. Phys. Chem. Lett. 2022, 13, 11696, https://doi.org/10.1021/acs.jpclett.2c03397)<br/>Overall, the versatile in situ spectroscopies can be an important tool for optimizing performance of OSCs via manipulating drying process. Furthermore, the results can be extended to future develop other blend inks for solution-cast organic or hybrid electronics.