Abstract
Conjugated polymers composed of semirigid backbone and the flexible side chains for facilitating their solubility in common organic solvents are the key materials of polymer-based optoelectronics, including solar cells, light-emitting diodes and thin-film transistors (TFTs). In the case of bulk heterojunction (BHJ) polymer solar cells, poly(3-hexylthiophene) (P3HT) has been considered as one of the most promising donor materials due to a good combination of charge transport properties, solution processability and the ease of synthesis with controllable regioregularity and molecular weight. The morphology of the active layer, which has a significant impact on the power conversion efficiency of the BHJ polymer solar cell, is strongly influenced by the dissolution condition of the conjugated polymer for film casting due to the existence of the memory effect; therefore, investigating the structure of the solution structure of conjugated polymer and its effect on the morphology of the subsequently cast film is an important fundamental task. This thesis is focused on elucidating the structures and phase transformation processes of P3HT in the solution and bulk state. We systematically studied the gelation process of the semidilute solution of P3HT with xylene induced by prolonged aging. A macrophase separation generating P3HT-enriched phase and solvent-rich phase of μm in length scale occurred during the aging. Phenomenologically, the phase separation took place via the nucleation-and-growth mechanism driven by the formation of nanowhiskers in the P3HT-enriched domain. The P3HT-enriched macrodomains were mechanically stabilized by the networking of the nanowhiskers and their jamming in the system led to the gel property. Therefore, P3HT/xylene gel was characterized by a hierarchical structure with distinct morphological entity at different length scales, namely, the macrodomains with μm length scale composed of the nanowhiskers assembled by the crystallites. The addition of PCBM was found to greatly slow down the gelation, which implied the existence of favorable interaction between P3HT and PCBM in xylene. The effect of the solution structure on the crystallization kinetics and relevant structural development in the subsequently cast films of P3HT has been studied to resolve the memory effect underlying the morphological formation of conjugated polymer films. We compared the crystallization rate of P3HT in the film cast from the gel (called “gel-cast film”) with that in the film cast from the liquid solution (called “solution-cast film”) to understand the effect of solution structure on the structural development in the subsequently cast film of conjugated polymer. The in-situ SAXS measurement and thermal analysis revealed that both the cold and melt crystallization of P3HT in the gel-cast film were much slower than those in the solution-cast counterpart. The retardation of crystallization rate in the gel-cast film was attributed to the abundance of the nanowhiskers, which remained largely undisrupted even when the temperature was higher than the crystal melting point. In this case, the the crystallization of P3HT occurred predominantly within the individual nanowhiskers and the meshes in the networks of the whiskers, where their limited sizes in at least one dimension imposed a strong spatial constraint to the crystal growth and chain motion for crystallization. Finally, we attempt to resolve the role of regioregularity on the self-assembly process of P3HT. The regioregularity of P3HT is normally characterized by the regioregularity (RR) value defined as the mole percentage of head-to-tail (HT) dyad in the backbone. Here we revealed that the by the content of HT-HT triad also plays an important role governing the self-assembly of P3HT in solution and bulk state by investigating the gelation rate and crystallization kinetics of two P3HT samples bearing almost identical RR value and average molecular weight. It was found that the semidilute xylene solution of the sample with slightly smaller RR value exhibited a faster gelation driven by the formation of crystalline nanowhiskers. In the bulk state, this sample also displayed a faster crystallization kinetics and higher melting points. Detailed characterization of the configurational structure by nuclear magnetic resonance (NMR) spectroscopy showed that the sample with smaller RR value contained an obviously higher population of HT-HT triad sequence. Therefore, in addition to the amount of HT dyad represented by the RR value, the population of the consecutive HT sequence along the polymer chain plays a significant role in the self-assembly processes of P3HT. Knowledge on this system parameter, which can be deduced conveniently from 13C NMR spectroscopy, is hence essential for the delicate control of the morphology of P3HT. This thesis has thus provided a systematic understanding of the structures and crystallization behavior of P3HT in both solution and bulk state. The results shall be fundamentally important for understanding and controlling the morphological formation of the P3H film induced by crystallization for opto-electronic applications.