The excitonic coupling values obtained are: ∼75 cm -1 for side-chain, ∼150 cm -1 for π-π-stack, and ∼69 cm -1 for slipped stack. Interestingly, the slipped stack configuration gives weak excitonic coupling. This observation can be understood by closely examining the slipped stack configuration ( see Fig. in Supplementary Information). The quinoidal ring of F 4 TCNQ anions in C3 configuration is placed in between the two thiophene rings resulting into poorer electronic overlap of the dopant with either of the rings in P3HT. To further compare our simulation results with experimental IR spectra of the precursor solution, we calculated IR-vibrational frequencies for F 4 TCNQ in different interaction modes, as shown in Fig. . Since there are two interacting F 4 TCNQ anions per P3HT chain, the calculated IR spectrum is extremely crowded. To compare it with the experimentally observed C≡N stretching frequencies of F 4 TCNQ anion in solution, the calculated IR-transitions are plotted as Gaussian with a linewidth of 5 cm -1 and a scaling factor of 0.95, which is added to the frequencies due to the limited accuracy of the dispersion corrected density functional. As expected, the π-π-stack configuration, C2 (red) along with the slipped-stack configuration, C3 (blue) contributes most to the high frequency region, presumably due to formation of a fractional CTCs resulting from π-orbital overlap. Side chain interaction mode C1 (in black) has insignificant contribution towards the higher frequency region. Thus, our results suggest that the experimentally observed IR absorption feature at 2202 cm -1 may be dominated by the contributions of the thermodynamically stable π-π-stack configuration.