Organic semiconductors are promising materials for a variety of applications. However,
different from their crystalline counterparts, they are inherently disordered, and therefore
have low charge carrier mobilities and large nonradiative losses. One strategy to reduce
energetic disorder and boost device efficiency is to introduce a transparent host matrix
into the system. Recent studies on polymer light-emitting diodes have shown that high-
bandgap third components such as poly[9-vinylcarbazole] (PVK) or polystyrene (PS)
reduce the number and depth of trap states and improve charge carrier mobility.[1,2]
However, high polarity of transparent host materials may lead to an even increased
energetic disorder. Other studies found that host polymer polarity critically affects
charge transport: polar matrices increase energetic disorder and significantly reduce
hole mobility, whereas apolar polymers largely suppress this effect.[3]
These findings are particularly relevant for the field of organic solar cells (OSCs)
for two reasons. First, high band-gap hosts can help achieve thicker layers without
compromising transparency, which is beneficial for upscaling. Second, potentially lower
energetic disorder will lead to reduced non-radiative losses and higher charge carrier
mobilities. So far, a detailed study on transparent third components in OSCs has not
yet been reported. We chose materials with different polarity and miscibility with the
donor and acceptor materials to probe the changes in energetic disorder and morphology,
respectively. We studied their impact on the photophysics using PM6:L8-BO as a model
system, and PS and poly[methyl 2-methylpropenoate] (PMMA) as transparent host
materials.
We performed synchrotron grazing incidence wide angle X-ray scattering measure-
ments to probe crystallinity and π-π stacking changes upon different loadings of the wide-
bandgap host. Complementarily, steady-state PL was used to quantify exciton harvest-
ing and to infer information about domain size changes indirectly. These measurements
can help link the host matrix miscibility to morphological changes in the photoactive
layer. We probed charge carrier mobilities using intensity-modulated photocurrent spec-
troscopy. We employed light-intensity-dependent open-circuit voltage measurements to
obtain the recombination ideality factor and relate it to energetic disorder. This allows
to link the morphology and the polarity of the host polymer to device photophysics
changes. Overall, this work aims to clarify how transparent high-bandgap third compo-
nents can enable thicker layers without sacrificing transparency, and how they impact
energetic disorder in organic solar cells.