  Perovskite solar cells with record power conversion efficiency are fabricated
by alloying both hybrid and fully inorganic compounds. While the basic
electronic properties of the hybrid perovskites are now well understood, key
electronic parameters for solar cell performance, such as the exciton binding
energy of fully inorganic perovskites, are still unknown. By performing magneto
transmission measurements, we determine with high accuracy the exciton binding
energy and reduced mass of fully inorganic CsPbX$_3$ perovskites (X=I, Br, and
an alloy of these). The well behaved (continuous) evolution of the band gap
with temperature in the range $4-270$\,K suggests that fully inorganic
perovskites do not undergo structural phase transitions like their hybrid
counterparts. The experimentally determined dielectric constants indicate that
at low temperature, when the motion of the organic cation is frozen, the
dielectric screening mechanism is essentially the same both for hybrid and
inorganic perovskites, and is dominated by the relative motion of atoms within
the lead-halide cage.
