π–π Aggregation Behavior of Conjugated Polymers in Semi‐Dilute Solutions

Abstract This study evaluates the π–π aggregation behavior of conjugated polymers in ink (that is, semi‐dilute polymer solutions) and its relation to thin‐film microstructures using UV–vis absorption spectroscopy and viscosity measurements. Three types of conjugated polymers characterized by different thin‐film microstructures, namely, semicrystalline poly(3‐hexylthiophene‐2,5‐diyl) (P3HT), weakly ordered poly[2,6‐(4,4‐bis‐(2‐ethylhexyl)‐4 H ‐cyclopenta [2,1‐ b ;3,4‐ b ′]dithiophene)‐ alt ‐4,7(2,1,3‐benzothiadiazole)] (PCPDTBT), and ordered aggregate poly{[N,N'‐bis(2‐octyldodecyl)naphthalene‐1,4,5,8‐bis(dicarboximide)‐2,6‐diyl]‐alt‐5,5′‐(2,2′‐bithiophene)} (P(NDI2OD‐T2)), are dissolved in chlorobenzene at concentrations ranging from dilute (0.01 mg mL −1 ) to semi‐dilute (30 mg mL −1 ). The solution‐state UV–vis absorption spectra indicate that the polymer chains adopt disordered conformations (P3HT), intrachain locally‐ordered conformations (PCPDTBT), and intra/interchain ordered aggregates (P(NDI2OD‐T2)) in dilute solutions. Despite polymer chain overlap, the conjugated polymers do not exhibit π–π aggregation in semi‐dilute solutions. Instead, the intrachain and interchain π‐conjugation observed in the dilute solutions are maintained in the semi‐dilute solutions. Consequently, the P(NDI2OD‐T2) and PCPDTBT thin‐film microstructures predominantly and partially inherit the polymer conformational state in ink, respectively. The P3HT ink, where chain planarization and inter‐chain π‐stacking occur during condensation, does not exhibit structural correlation with the film. The insights gained from this study can guide both the design and processing of conjugated polymers films through printing and coating techniques toward low‐cost mass‐produced organic electronics.