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.