In this work, we have used SAXS and WAXS techniques, combined with STEM, to elucidate important structural aspects of colloidal suspensions and superlattices of PNPls, both in solid state and in solvents. With the information acquired by SAXS, we could observe stacking of PNPls in organic solvents suspensions and the influence of the viscosity of these liquids in their stacks. The use of hydrocarbon solvents with different number of carbons and different viscosities is a convenient strategy to induce the aggregation of the PNPls into their stacks without the use of antisolvents. According to the results from SAXS, the distance between two adjacent particles is short enough to allow for efficient Förster resonant energy transfer, which is predicted to reach values higher than 50%, in contrast to dilute dispersions, in which FRET is negligible, as previously reported by our group. Our findings deepen the understanding on PNPls self-assembly properties and allow for the rational development of colloidally stable PNPls superlattices. This work opens new possibilities for fundamental studies of their collective optical properties and applications. Further systematic studies on the viscosity and temperature dependence of the stacking threshold are expected to improve the control over the final superlattice morphology.