Low energy tail of the exciton luminescence band in 2H-PbI2 and its relation to Urbach rule

We have measured luminescence spectra of 2H-PbI₂ at different temperatures between 77 and 180 K. The lower energy tail of the luminescence band due to free exciton annihilation decreases exponentially with decreasing the photon energy over an order of magnitude, which corresponds to the Urbach tail in the absorption spectrum. This fact suggests that the exciton state corresponding to the exponential tail is in the thermal equilibrium and the steepness coefficient δ₀ = 1.67 in Urbach rule is obtained also from the luminescence spectra.