Poly (N-vinylcarbazole) (PVK) based polymer light-emitting diode doped with green phosphorescent dye fac-tris (2-phenylpyridinato)iridium(III) [Ir(ppy)(3)] showed two orders of magnitude shorter lifetime than 4,4'-bis (N-carbazolyl)-2,2'-biphenyl : Ir(ppy)(3) based device with the same device structure. Rapid degradation of the polymer based device was primarily originated from the instability of the doubly reduced Ir(ppy)(3)(2-) in PVK, which was found from the reduction of photoluminescence intensity under electrical stress in hole only and electron-hole flowing devices, and the cyclic voltammetry experiments. Large difference of the lowest unoccupied molecular orbital energy level between PVK and Ir(ppy)(3) (similar to 0.5 eV) and low electron mobility of PVK allow electrons transport only through Ir(ppy)(3), so that there is large probability for Ir(ppy)(3) to be doubly reduced upon electron injection. (C) 2013 Elsevier B. V. All rights reserved.