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.