Resonant Raman scattering dispersion in poly(dithieno[3,4-b : 3 ',4-d]-thiophene): 2A(g) spectroscopy

Small band gap semiconducting pi-conjugated polymers may, in principle, be more efficient as the active media in solar cells, due to their better adaptability to the solar spectrum. In luminescent pi-conjugated polymers, interband photon absorption results in the radiative recombination of the photogenerated excitons, which competes with charge separation. The photoluminescence (PL) quantum efficiency is largely determined by the relative energies of the optically allowed lowest odd parity (LOP, e.g. 1B(u))excited state and the optically forbidden lowest even parity (LER e.g. 2A(g)) excited state. If E(LEP)< E(LOP), the quantum efficiency is small and charge separation following photoexcitation is more likely than in efficient PL polymers. The determination of the relative position of LEP and LOP states is therefore essential for the possible utilization of small band gap pi-conjugated polymers in practical devices. It was previously shown that resonant Raman scattering dispersion might serve as a spectroscopic tool for the determination of the LEP states, since the dispersion is solely determined by the dependence of the LEP state on the conjugation length, rather than that of the optical gap, or LOP state. In this work, we use the Raman spectra measured in poly(dithieno-thiophene) (PDTT1) in order to estimate the LEP energy level. We find that the LEP energy level is in the vicinity of the optical gap.