Phenanthrodithiophene (PDT)-Difluorobenzothiadiazole (DFBT) Copolymers: Effect on Molecular Orientation and Solar Cell Performance of Alkyl Substitution onto a PDT Core

Synthesis, characterization, and solar cell application of three 4,7-dialkylated phenanthro[1,2-b:8,7-b′]dithiophene (PDT)-difluorobenzothiadiazole (DFBT copolymers (P1-P3) with different linear alkyl side chains to improve solubility, molecular weight, and molecular orientation are described. The utilization of Ir-catalyzed direct borylation and sequential functionalization can selectively afford the target 4,7-dialkylated PDT as the monomers. Migita-Kosugi-Stille coupling in the presence of CuI can accelerate polymerization to afford high-molecular-weight polymers along with their improved solubility. The effect of alkyl substitution at the 4,7-positions on the electronic structure of PDT-DFBT copolymers is negligible. By installation of additional alkyl chains at the 4,7-positions of PDT, the synthesized polymers P1-P3 have lower intermolecular interaction than that of nonalkylated P0, but they still maintained aggregation behavior in solution. In addition, they formed a favorable face-on orientation with a short π-stacking distance of 3.6 Å, which can enhance their carrier transport ability, resulting in high J_sc and FF. As a result, their fabricated solar cells reached a PCE exceeding 6%, which are about 1.7-fold higher than that of P0. Comparison of alkyl side chain length at the 4,7-positions of PDT revealed that all polymers formed a predominantly face-on orientation and have a similar face-on ratio in blended films, but their crystallinity was decreased as the carbon chains at the 4,7-positions of PDT became shorter. On the other hand, the polymers with short alkyl side chains tended to have low surface roughness and small domain size of active layers, which is an ideal phase separation structure for high-performance PSCs. From these results, it could be seen that the polymers have a trade-off relationship between their domain size and crystallinity, but the impact of alkyl side chain length on their photovoltaic properties is rather small. Thus, the construction of face-on orientation is highly important to achieve a high PCE. Among three polymers, the P3/PC₆₁BM-based solar cell with an optimal nanoscale phase separation structure with bicontinuous domain showed the highest PCE of up to 6.6%.