Molecular doping in few-molecule polymer-dopant complexes shows reduced Coulomb binding

Abstract

The mechanistic study of molecular doping of organic semiconductors (OSC) requires an improved understanding of the role and formation of integer charge transfer complexes (ICTC) on a microscopic level. In the present work we go one crucial step beyond the simplest scenario of an isolated bi-molecular ICTC and study ICTCs formed of up to two poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b,3,4-b′′]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDT-BT) oligomers and up to two CN6-CP molecules. We find that depending on geometric arrangement, complexes containing two conjugated oligomers and two dopant molecules can show p-type doping with double integer charge transfer, resulting in either two singly doped oligomers or one doubly doped oligomer. Interestingly, compared to an individual oligomer-dopant complex, the resulting in-gap states on the doped oligomers are significantly lowered in energy. Indicating that, already in the relatively small systems studied here, Coulomb binding of the doping-induced positive charge to the counter-ion is reduced which is an elemental step towards generating mobile charge carriers through molecular doping.