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Since the electroluminescence in polymer comes from the transportation and combination of carriers in the electric field,it is essential to study the polarization property of the excitation in polymer.First we obtain the analytic solutions of exciton and biexciton,then their polarizabilities are calculated using the Green function method.An important result is that we analytically proved that the polarizability of the exciton is positive and that of the biexciton is negative.It follows that the polarization will be reversed when the exciton becomes biexciton.

In external electrical field,the self-trapping biexciton in non-degenerate ground state conjugated polymer has an abnormal characteristic-its static polarizability was found to be negative.We calculated the dynamic response function χ(ω) of biexciton.By putting ω to the zero limit,we got the static polarizability χ which proved to be negative.We compared it with that of self-trapping exciton and ground state and gave some analysis on the origin of negative polarizability.Finally,we investigated the behavior of χ(ω) when ω is small,and deduced that if more biexcitons are excited,the n(ω) of the material will decrease,whereas if more excitons are excited,the n(ω) will increase.

Due to the one-dimensional characteristic, the excitation of electrons in polymers will be self-trapped, and two excitons can be combined to form one biexciton. This combination process is one of the most important channels leading to form the biexciton, the efficiency of is much higher than the two-photon process. This kind of combination is accompanied by lattice distortion. By employing the dynamic equation, the relaxation process of this combination is investigated in this paper, and it is found that the relaxation time is about 160fs. The effect of external electrical field on the combination is also studied, the results show that when the electric field E is greater than 0.5MV/cm, instead of the biexciton, two bipolarons are produced.