Optimization of solvent swelling for efficient organic solar cells via sequential deposition

Compared to bulk heterojunction (BHJ) organic solar cells (OSCs) prepared by the blend casting in “one step process”, sequential deposition (SD) processed OSCs can realize an ideal profile of vertical component distribution due to the swelling of polymer films. Herein, we did trials on several kinds of second solvents for swelling the polymer layer, and investigated the packing structure and morphology of the swollen films and the performance of the resulting devices. We found that an optimized morphology can be achieved by solvent swelling while using orthodichlorobenzene (o-DCB) as the second layer processing-solvent, with polymer donor PffBT-3 as bottom layer, PC71BM as top layer and bicontinuous networks in the middle. Such solvent swelling process also makes the SD method exempt from thermal annealing treatment. The device based on SD yields a power conversion efficiency (PCE) up to 8.7% without any post-treatment, outperforming those from the devices based on SD using other solvents and that (7.06%) from BHJ device, respectively. We also extended the use of this approach to all-polymer blend system, and successfully improved the efficiency from 4.72% (chloroform) to 9.35% (o-DCB), which is among the highest PCEs in all-polymer-based OSCs fabricated with SD method. The results demonstrate that the swelling of the polymer by the second layer solvent is a necessity for SD, paving the way towards additive-free high-performance OSCs.