Bandgap and exciton binding energies in lead-iodide-based natural quantum-well crystals

We have performed optical absorption and electroabsorption studies on the lead-iodide-based natural quantum-well perovskite-type crystals with different well width (C6H13NH3)2(CH3NH3)m21PbmI3mþ1. With decreasing well thickness, m; the resonance energies of the lowest-energy excitons shift to higher energy due to the increase of the bandgap. The binding energies and oscillator strengths of the excitons drastically increase due to the spatial confinement and image charge effect with decreasing m: The bandgap of (C6H13NH3)2(CH3NH3)m21- PbmI3mþ1 (m ≥ 2) can be reproduced by the effective-mass approximations, while the effective-mass approach is not valid for (C6H13NH3)2PbI4 (m = 1).