Molecular Doping in Multi-Molecule Polymer-Dopant Complexes Shows Reduced Coulomb Binding

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.