2.1 The in situ UV-vis absorption spectrum
In situ UV-vis is an analytical technique used to monitor and study chemical reactions, material transformations, and changes in optical properties, which can be used to characterize the absorption spectrum of organic semiconductor. The change of peak shape of absorption spectrum often reflects the change of its aggregation form.[62-64] For example, the extension of conjugation produces redshifts, and the increase in aggregates homogeneity results in sharper peaks or shoulders.[65] In addition, the change of absorbance of a certain wavelength can also be measured to study the film crystallization kinetics.[66] Figure 1a illustrates the schematic diagram of the in situ UV-vis absorption.[67] Shen et al. used in situ UV-visible spectroscopy to investigate the effect of different temperatures on the film-forming rate of the PM6:Qx-1 blend system. In the first stage, the peak position of Qx-1 at 718 nm showed no significant change during the solvent evaporation process. During the second phase, there was a rapid red-shift (830 nm) in the absorption peak of Qx-1. As the temperature increased, the second stage duration decreased from 6.50 s at 40 °C, to 5.52 s at 60 °C, 3.36 s at 80 °C, and finally 2.74 s at 100 °C. The position of the absorption peak for Qx-1 started to redshift, suggesting gradual acceptor aggregation and crystallization. The absorption peak of PM6 did not change significantly (600 nm). In the third stage, the Qx-1 absorption peak reached a constant position with no significant changes. Yu et al. studied the relationship between PTB7 molecular chain length and its aggregation state by using UV-vis absorption spectroscopy.[68] With a low number of monomer units (1.5), the solution’s absorption peak is primarily focused around 500 nm, but as the number of monomer units increases (3.5), the absorption peak gradually shifts towards 570 nm. As the number of monomer units increases, the level of conjugation within the molecular chain also increases, which reduces the energy difference between energy levels and causing a spectral redshift.