With the increasing development of organic opto-electronic devices

interest in the mechanisms of charge carrier photogeneration

separation

transport and recombination continues to grow. Electromodulation of photoluminescence has been used as an efficient probe to investigate evolution of primary excitation in an electric field. This method can provide useful information on carrier photogeneration

the formation and dissociation of excitons

energy transfer

and exciton recombination in the presence of an electric field. Operation of organic light emitting diodes(OLED) brings electrons and holes from opposite electrodes and generates singlet and triplet excitons. However

triplet excitons are wasted because a radiative transition from triplets is spin-forbidden. Spin statistics predicts that singlet-to-triplet ratio is 1:3 in organic semiconductors. One way to harvest light from triplet excitons is to use phosphorescent materials. These materials incorporate a heavy metal atom to mix singlet and triplet states by the strong spin-orbit coupling. As a result

a spin forbidden transition may occur allowing an enhanced triplet emission. Among phosphorescent materials

factris(2-phenylpyridine) iridium [Ir(PPY)

3

] has attracted attention because of its short triplet lifetime to minimize the triplet-triplet annihilation. High quantum efficiencies were obtained by doping Ir(PPY)

3

in organic molecules and in polymers. The mechanisms of electrophosphorescence has been a interesting topic since high efficient electrophosphorescence was reported. In this paper

we investigated the PL spectrum under the operation condition of OLED based on phosphorescent materials doped polymer. The photoluminescence under the modulation of an electric field and electroluminescence from thin films of poly(N-vinylcarbazol) (PVK) doped by Ir(PPY)

3

are measured. The PL spectra were gotten by reducing EL from whole emission spectra when the bias is above the threshold of EL. For different biases

no very obvious difference in the profiles of photoluminescent spectra from Ir(PPY)

3

doped PVK. The relative intensity of PVK emission peak does not decrease while increasing the bias

which indicates the energy transfer rate from PVK to Ir(PPY)

3

does not change with increasing bias. Both PL spectra in the prescence and absence of electric field are nearly the same but different from their EL spectra. For EL spectra

the emission from PVK host is hardly found. We can deduce that the main EL emissions dont come from PVK chains to Ir(PPY)

3

molecules which obey Fster energy transfer rule

but come from the recombination of injected carriers at Ir(PPY)

3

molecules acting as both the carrier-trap and recombination centers.