The presence of three different interaction modes in precursor solution of P3HT/F 4 TCNQ signifies heterogeneity in the conformational states. The π-stack interaction mode in C2 is the strongest in nature. In comparison to PBTTT, the excitonic coupling strengths for the π-πstack configuration is weaker in P3HT (∼250 cm -1 for PBTTT as compared to ∼150 cm -1 for P3HT). Thus, the π-π-stack interaction mode should be less detrimental in terms of effective doping in films for P3HT in comparison to PBTTT. Due to the smaller interactions energies in slipped stack and side-chain modes, there should be continuous exchange of population amongst these populations. These weaker interaction modes (especially side-chain) have been known to exist in doped polymer films as well. In fact, recent studies propose side-chain interaction modes to be the major population in some of molecular doped polymer films.