Xin SUN (Fudan Univ. and IMS) and Hua ZHAO (Fudan Univ.)
[Phys. Rev. B 56, 16 (1997)]
The exciton plays an essential role in the electroluminescence of polymer. But, how big is the binding energy of excitons in polymer is a controversial issue. The binding energy depends on the attractive interaction between electron and hole, and the existing theories use the extended Hubbard model to calculate the binding energy of excitons. However, the polymer is a broad-band system, the off-diagonal part of the electron interaction is effective. Actually, the extended Hubbard model only contains the diagonal part of electron interaction, which is density correlation, while the off-diagonal part reflects the bond correlation, which is a characteristic for the polymer. Therefore it is needed to study the effect of the off-diagonal part of electron interaction on the binding energy of excitons. We particularly studied the effects from the bond-bond interaction W and the bond-site interaction X, which are the only off-diagonal terms in the nearest neighbor. Our calculation shows that W and X affect the excitons oppositely: the former suppresses the excitation energy of excitons whereas the latter increases it except for the 2Ag state. We find that the screening, which controls the bond-bond correlation, is a main reason to reduce the binding energy of the excitons. Our studies also shows that the off-diagonal interactions affect the singlet exciton with small binding energy even in normal screening, but for large exciton energy, such effects are negligible.