Connecting optical absorption to doping in conjugated polymers*

 

Conjugated polymers (CP) exhibit distinctive optical properties that reveal important features of each material. Some peaks in the optical absorption spectra account for different phenomena, including electronic transitions, bonding-antibonding transitions, and presence of polarons. First principles modeling is commonly used to calculate the spectra of an undoped polymer, but it is not computationally feasible for doped CPs because of long-range Coulomb interactions. Here we numerically compute the absorption of doped CPs by solely considering transitions within and between their HOMO and LUMO. We model the DOS based on Arkhipov’s model where doping induces a heavy exponential tail in the DOS. The heaviness of this tail is dependent on both dopant and CP features. We assign a contriution to the DOS from each structurally different domain in the CP and consider optical transitions only within that domain. We compute the absorption across a wide range of doping to show that absorption in the near-IR is related to transitions within the heavy tail of HOMO DOS. Doping in creases energetic disorder via Coulomb interactions, making the DOS tail heavier, resulting in a red shift in the spectrum. Comparing intensities of the NIR and UV/Vis peak, we can extract doping concentration in CPs, which is challenging with other methods since carriers in CPs are localized and do not exhibit a fully developed Hall effect. Hence, we anticipate our work to aid understanding of optical absorption mechanisms and to correlate doping, spectra, and electrical transport in CPs.