π-π Stacking Modulation via Polymer Adsorption for Elongated Exciton Diffusion in High-Efficiency Thick-Film Organic Solar Cells.

Developing efficient organic solar cells (OSCs) with thick active layers is crucial for roll-to-roll printing. However, thicker layers often result in lower efficiency. This study tackles this challenge using a polymer adsorption strategy combined with a layer-by-layer approach. Incorporating insulator polystyrene (PS) into the PM6:L8-BO system creates PM6+PS:L8-BO blends, effectively suppressing trap states and extending exciton diffusion length in the mixed donor domain. Adding insulating polymers with benzene rings to the donor enhances π-π stacking of donors, boosting intermolecular interactions and electron wave function overlap. This results in more orderly molecular stacking, longer exciton lifetimes, and higher diffusion lengths. The promoted long-range exciton diffusion leads to high power conversion efficiencies of 19.05% and 18.15% for PM6+PS:L8-BO blend films with 100 and 300 nm thickness, respectively, as well as a respectable 16.00% for 500 nm. These insights guide material selection for better exciton diffusion, and offer a method for thick-film OSC fabrication, promoting a prosperous future for practical OSC mass production.

Correlating Charge Transfer Dynamics with Interfacial Trap States in High-Efficiency Organic Solar Cells.

The charge transfer between the donor and acceptor determines the photogenerated carrier density in organic solar cells. However, a fundamental understanding regarding the charge transfer at donor/acceptor interfaces with high-density traps has not been fully addressed. Herein, a general correlation between trap densities and charge transfer dynamics is established by adopting a series of high-efficiency organic photovoltaic blends. It is found that the electron transfer rates are reduced with increased trap densities, while the hole transfer rates are independent of trap states. The local charges captured by traps can induce potential barrier formation around recombination centers, leading to the suppression of electron transfer. For the hole transfer process, the thermal energy provides a sufficient driving force, which ensures an efficient transfer rate. As a result, a 17.18% efficiency is obtained for PM6:BTP-eC9-based devices with the lowest interfacial trap densities. This work highlights the importance of interfacial traps in charge transfer processes and proposes an underlying insight into the charge transfer mechanism at nonideal interfaces in organic heterostructures.