Fundamental optical absorption and photo-conduction in PbI 2 single crystals

Abstract

This paper describes measurements of the optical absorption of gel-grown PbI 2 in the region from 0.5–15 μ and photoconductive measurements in the region of the absorption edge at about 0.525 μ. The edge is dichromatic, with a splitting of 0.048 eV which is explained in terms of a split valence band. The absorption edge of PbI 2 for E ⊥ c is found to obey Urbach's rule for α = 50 cm −1 to 10 4 cm −1 in the temperature range 144–361°K and has a temperature dependence which may be represented by (ΔE/ΔT)α = (9.95-0.60 ln α) × 10 −4 e V °K . A well defined absorption band at 4.2 μ and an activation energy of 0.26 eV evaluated from the measurements of dark current vs. 1 T are attributed to a deviation from stoichiometry. The exciton effective mass in PbI 2 is estimated to be M = 0.094 m 0.