For bulk material it is almost constant. When u make a solution u have diluted the material in a way that new bonds have been formed between the related atoms and the solvent. This can change the nature of the material. Even in solid state the bandgap is a function of sevaral factors like growth conditions and post-growth ones. Thin film naturally follows the same way and some factors like the degree of amorphousness, thickness, strain/stress etc. can change the bandgap let alone the liquid phase.
Defects (as a function of structural & morphological evolution) usually give rise new states within the bandgap (Urbach tail is one of them for instance) and so changing the bandgap width. However it may be trace but very significant. Grain size can affect it too. Suppose downsizing of particles in a way they reach quantum confinement region. Then, HUMO-LUMO transitions lead to widening of BG.