Theory of biexcitons in conjugated polymers

A tight binding, two-band Hamiltonian is used to investigate the formation and nonlinear optical properties of biexcitons in ID conjugated polymers. Calculations are performed using the full single and double electron-hole pair basis sets. The biexciton phase region in (α,β) space is numerically calculated for long chains in the polymer limit α and β are the nearest neighbor Coulomb interaction energy and charge transfer energies respectively, made dimensionless through division by the onsite Coulomb energy. Both truncated and extended potentials are studied. The former supports a single biexciton state while the latter supports multiple biexciton states. Calculated two-photon absorption (TPA) spectra contain two types of peaks: one at lower energy due to a charge-transfer, single-exciton and a higher energy peak arising from a biexciton state. Good agreement for the TPA spectrum of poly (di-n-hexylsilane) is obtained.