Bulk heterojunction all-polymer solar cells have attracted much interest due to the possibility of achieving high open-circuit voltage and more efficient light absorption compared to polymer/fullerene blend system. However, molecular engineering of the morphological aspect of all-polymer devices has proven more difficult as compared to polymer:fullerene devices. In addition, the anisotropic charge transport properties of the polymers require careful control of their packing structure and orientation suitable for their use in all-PSCs. Here, we have investigated the effect of solvent–polymer interaction on the morphology, crystallinity, and device performance of poly-(3-hexylthiophene) (P3HT) and poly{2,7-(9,9-didodecyl-fluorene)-alt-5,5-[40,70-bis(2-thienyl)-20,10,30-benzothia-diaole]}(PF12TBT) blend system. 3-Hexylthiophene (3-HT), which had the similar structural units with both donor and acceptor materials, was chosen as the solvent additive to be added into the main solvent chlorobenzene (CB), to adjust the solvent–polymer interaction. With the 3-HT percentage increasing from 5 to 30% in CB solution, the solvent–polymer interaction between polymer and solvent molecules decreased slightly according to the calculated solubility parameters and interaction parameters. As a result, nanoscale phase-separated and interconnected morphology with decreased domain size of both donor and acceptor was formed. Meanwhile, the order of P3HT molecule was enhanced which resulted from the extended film drying time and increased molecular planarity after incorporation of 3-HT. In addition, by changing solution state and film drying time, we obtained the identical molecular orientation for PTB7-th/PNDI blend. When varying the solvents from CN, oDCB to CB, the PNDI chain aggregation increased in solution and film drying time decreased. Consequently, the donor/acceptor molecular orientation changed from face-on/edge-on to face-on/face-on, which would facilitate excitons dissociation at D/A interface. Therefore, the PCE of devices processed by CN, oDCB, and CB improved from 0.53 % to 3.52 % ultimately.

Control of conjugated polymer blends morphology and the molecular orientations at the interface: Insight into the phase separation mechanism

252nd National Meeting (2016)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.