Solvent Vapor Conditioning of Polymer–Polymer Organic Solar Cells

Despite rapid advances in all-polymer organic solar cells (OSCs), challenges remain in process optimization of their donor-acceptor heterostructure. Here, using all-polymer OSCs based on the state-of-the-art PM6:PYIT donor-acceptor system, we show that hot mixing of the donor and the acceptor polymer solutions can retain the desired disaggregated state of the acceptor polymer, significantly extending the pot life from 10 min to over 15 h, without harming the power conversion efficiency (PCE) of the fabricated solar cells. This enhanced processing stability enables systematic solvent vapor conditioning (SVC) to be conducted on the polymer films. We show that treatment by certain solvent vapors at activity ratios of 0.01-0.1, corresponding to subsaturation vapor pressures of 1-10%, respectively, can improve not only the open-circuit voltage (V_OC), short-circuit current density (J_SC), and fill factor (FF) of these cells, and thus their PCE, but also, surprisingly, improve their shunt resistance by suppressing the dark leakage current density. For example, SVC with chloroform at an activity of 0.1 for 1 min improves PCE from 13.2% to 14.7% and shunt resistivity from 4 to 24 kΩ cm². In contrast, exposure to saturated chloroform vapor degrades all cell parameters. The SVC leads to a small swelling of the films, estimated to be less than a few percent. But Vis-NIR spectroscopy reveals changes to the state of J-aggregation of the PYIT, suggesting a refinement of packing and/or conformational order of the polymer chain segments in the absence of long-range diffusion. Our results open a scalable and inexpensive route to improving polymer-polymer OSCs, while highlighting their extreme sensitivity to residual solvent vapors in the manufacturing atmosphere. Finally, the suppression of leakage current is also advantageous to improving the performance of indoor PV cells under low-light conditions.