LUMINESCENCE OF PBI2 CU+

Under band-to-band irradiation, the PbI2:Cu+ crystals are strongly luminescent at 77 K. In the emission spectrum one can notice, besides the characteristic excitonic band E at 2.5 eV, three additional bands denoted D, C and M, with maxima located at 2.42, 2.06 and 1.76 eV respectively. Correlated studies of luminescence decay time, photoconductivity, excitation spectra and thermostimulated currents have revealed the presence of different luminescence centres with and without appearance of Cui ions. In the case of C emission (2.06 eV) which increases with increasing concentration of impurities, a dependence of the decay time versus excitation wavelength has been observed. This can be interpreted as the result of the coexistence of two types of recombination mechanisms: one fast, bimolecular, of the Schon-Klasens type, which is dominant at excitation in the fundamental absorption band and the other slow, dominant at excitation in the excitonic band which seems to be due to tunnelling donor-acceptor recombination. The M emission (1.76 eV) is generated through the prior band-to-band irradiation of the sample and is related to the formation of a complex luminescent centre consisting of Cu+ and Pb+ ions, the latter resulting from products of the photodecomposition of PbI2 under band-to-band irradiation. We also consider that the recombination of free holes wth trapped electrons around the defects in the sample is the main process for the origin of the D emission that is observed in both pure and doped crystals.