  Lead halide perovskites (LHPs) are promising materials for opto-electronics
and photovoltaics, thanks to favorable characteristics and low manufacturing
costs enabled by solution processing. In light of this, it is crucial to assess
the impact of solvent-solute interactions on the electronic and optical
properties of LHPs and of their solution precursors. In a first-principles work
based on time-dependent density-functional theory coupled with the polarizable
continuum model, we investigate the electronic and optical properties of a set
of charge-neutral compounds with chemical formula, PbX$_2$(Sol)$_4$, where
X=Cl, Br, and I, and Sol are the six common solvents. We find that
single-particle energies and optical gaps depend on the halogen species as well
as on the solvent molecules, which also affect the energy and the spatial
distribution of the molecular orbitals, thereby impacting on the excitations.
We clarify that dark states at the absorption onset are promoted by
electron-withdrawing solvents, and we show the correlation between oscillator
strength and HOMO$\rightarrow$LUMO contribution to the excitations. Our results
provide microscopic insight into the electronic and optical properties of LHP
solution precursors, complementing ongoing experimental research on these
systems and on their evolution to photovoltaic thin films.
