2.2 The in situ PL spectrum
The PL spectrum can reflect the phase separation process of solvent volatilization through changes in fluorescence efficiency and luminescence peak position.[69] With the evaporation of solvent, the degree of aggregation and cluster formation will gradually increase, which will lead to the increase of fluorescence efficiency.[70] Furthermore, the interaction between organic molecules will gradually strengthen due to the formation of aggregates and clusters, which will lead to the redshift of fluorescence peak position.[71] When the phase separation reaches a certain degree, the fluorescence efficiency will reach the maximum value, and the fluorescence peak position will also reach a stable state. While in situ photoluminescence (PL) measurements are relatively straightforward to conduct, it is crucial to exercise caution when analyzing the data. The reason for PL quenching during the drying of BHJ blends is the result of various molecular interactions, such as photochemical oxidation, reactions in the excited state, rearrangement of molecules, transfer of energy, and the creation of complexes in the ground state.[72] To fully utilize the in situ PL spectrum for BHJ drying, it is necessary to elucidate the mechanism of PL quenching during the drying process.[73] The experimental equipment is shown in Figure 1b. Engmann et al. studied the film-forming process of P3HT:PCBM blends using in situ PL.[74] During the early phase, the PL intensity of P3HT diminishes as the mean separation between evenly dispersed fluorophores and quencher molecules decreases. In the second stage, PL intensity of P3HT drops sharply and then recovers. The abrupt decline in PL is ascribed to a prolonged exciton diffusion distance, enabling reach to quenchers situated at greater distances, subsequently leading to domain coarsening that ultimately leads to the restoration of PL. In the third stage, the PL decreases, and the emission ratio continues to increase. The emission intensity of ordered polymer domains primarily determines the overall PL, while fluorescence from the disordered phase is greatly reduced. Huang et al. studied P3HT spectra at different concentrations.[75] When the concentration is low, the PL peak position is 570 nm. When the concentration increases, the peak strength increases, indicating that P3HT is a single-chain state. When the concentration rose to 0.02%, PL intensity decreased sharply with the increase of concentration. This is because the P3HT molecules come into contact, causing the fluorescence to be quenched and the intensity to decrease. When the concentration was 1%, new peaks appeared at 640 nm and 670 nm. This is because the molecules are π-π stacking, forming ordered aggregates, and the energy of the system is reduced.