Popovych K. Electroluminescent properties of EL lamps made of encapsulated ZnS: Cu phosphor.

Electroluminescent properties of flexible EL lamps prepared from encapsulated zinc sulphide powder phosphors doped with copper have been studied. The following parameters: band gap width, photoemission threshold, dielectric permittivity, relaxation energy, total bond energy and elastic constants, along with the energy positions of the electronic states in the band gap and energy band diagram for ZnS/Cu2-xS (x = 0.2) heterostructure were calculated by the method based on linear combination of atomic orbitals and pseudopotential. The decrease in brightness of EL lamps luminescence during 2000 hours operation can be described by an empiric formula which includes at least two constituents of relaxation time: t1 = 90 - 220 hours, t2 = 3100 - 4100 hours. Effect of frequency, temperature and pulse shape on t1 relaxation time was investigated. Activation energy of t1 relaxation process was found to be about 0.24 eV. Structural changes in the ZnS: Cu phosphors are attributed to the moving of Cu impurity atoms from the lattice sites into the interstitials, diffusion of Cu over interstitials, trapping of the inter-stitial atoms by vacancies, diffusion of vacancies, and also, reactions on the boundaries of decay and coagulation centres.