The precursor solution mixture of P3HT/F 4 TCNQ in chlorobenzene has been prepared by following the known protocol, which is also briefly described in the materials and methods section. The solution mixture so obtained has 10 wt % of dopant to polymer ratio corresponding to ∼5 P3HT repeat units per F 4 TCNQ. The interaction of polymer:dopant pairs might have more than one possible mode modulating the electronic coupling, which also should correspond to different possible interaction strengths. This scenario has been depicted in Fig. . Optical spectroscopy has been used widely to characterize the nature The IR bands at 2169 and 2194 cm -1 correspond to F 4 TCNQ anion (blue shaded), while a weak band at 2202 cm -1 signifies a minor population of partial charge transfer complex. of species in solution mixtures of P3HT/F 4 TCNQ. As mentioned earlier, P3HT/F 4 TCNQ forms an ion-pair, which can also be seen to exist even in the precursor solution. The absorption spectrum of the solution mixture has been shown in Fig. The C≡N stretching frequencies of neutral F 4 TCNQ (2227 and 2214 cm -1 ) do not exist in the solution mixture spectrum, which further validates our hypothesis that all the added F 4 TCNQ are interacting with the polymer backbone to form ion-pairs in solution. In addition, the 2194 (b 1u mode) cm -1 mode has also been shown to be the marker of single ionization signifying that one principal mode of interaction possibly dominates in the solution mixture. But, interestingly, a very weak feature on the blue edge at 2202 cm -1 (shaded red) can also been seen, which reflects a small population corresponding to partial charge transfer complex. This feature has been reported to become more dominant in films over time, suggesting that it represents thermodynamically more stable interactions. Thus, optical absorption measurements in the visible and IR establishes that the precursor solution mixture of P3HT/F 4 TCNQ in chlorobenzene contains F 4 TCNQ predominantly in the form of anions. The optical transitions of F 4 TCNQ anion carry the information of the interacting surrounding, which is in this case are polymer and solvent molecules. Thus, the excited state dynamics of F 4 TCNQ anion can reveal the information regarding the electronic interaction strength between the ion-pair and its heterogeneity.