Investigation of the effect of donor-acceptor substitution on band gap, band width, and conductivity

Polymers of two donor-acceptor systems, 3-cyano,3'-hydroxybithiophene 1 and 4-dicyanomethylene-4H-cyclopenta[2, 1-b:3,4-b'],3,4-ethylenedioxythiophene (CDM-EDOT) 2, were analyzed with density functional theory. As predicted by perturbation theory, interactions between donors and accepters with very different energy levels are greatly reduced compared to those between fragments with similar energy levels. This leads to localized states and bands with little dispersion. For poly-1 these localized states lie below the valence band and above the conduction band. For 2 localized unoccupied levels lie within the band gap. These acceptor levels account for the high electron affinity of poly-2 and allow for self-doping. Self doping explains the increased intrinsic conductivity of poly-2, the localized nature of the low lying MOs rationalizes the low mobility of n-type carriers in poly-2.