Monitoring the Onsets and Kinetics of β Phase Nucleation in Polydiarylfluorene Films Annealed at Different Temperatures

ABSTRACT Polydiarylfluorene stands out as a prime example of a conjugated polymer, renowned for its remarkable light‐emission properties and its capacity to form a distinct ordered crystalline phase—the β phase—with dramatic impact on the optoelectronic properties of this material. Despite extensive research on the morphology of this polymer, the initial stages of nucleation of polymer chains within β phase domains remain elusive. In this study, we investigate the β phase crystallization onset and evolution using a comprehensive set of structural and spectroscopic techniques. In particular, we probe the thermal onset for β phase nucleation by combining differential scanning calorimetry, grazing incidence wide‐angle X‐ray scattering, and absorption, photoluminescence, and Raman spectroscopies. Raman microscopy reveals that β domains are already emerging upon annealing at 210°C. Judicious control of β phase fraction is further shown to be essential for unlocking the optimal light‐emitting properties by amplified spontaneous emission analysis. Remarkably, triplet exciton diffusion probed via continuous‐wave photoinduced absorption spectroscopy uniquely provides definitive evidence for the presence of small, highly isolated β phase domains in films annealed at the lowest temperatures.