Molecular Doping of PCPDT-BT Copolymers: Comparison of Molecular Complexes with and Without Integer Charge Transfer

Abstract

<div> <div> <div> <p>Molecular doping in conjugated polymers is a crucial process for their application in organic photovoltaics and optoelectronics. In the present work we theoretically investigate p-type molecu- lar doping in a series of (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) conjugated oligomers with different lengths and three widely-used dopants with different electron affinities, namely F4TCNQ, F6TCNNQ, and CN6-CP. We study in detail the molecular geometry of possible oligomer-dopant complexes and its influence on the doping mechanisms and electronic system properties. We find that the mechanisms of dop- ing and charge transfer observed sensitively depend on the specific geometry of the oligomer-dopant complexes. For a given complex different geometries may exist, some of which show transfer of an entire electron from the oligomer chain onto the dopant molecule resulting in an integer-charge transfer complex, leaving the system in a ground state with broken spin symmetry. In other ge- ometries merely hybridization of oligomer and dopant frontier orbitals occurs with partial charge transfer but spin-symmetric ground state. Considering the resulting electronic density of states both cases may well contribute to an increased electrical conductivity of corresponding film samples while the underlying physical mechanisms are entirely different. </p> </div> </div> </div>

Microscopic Insights into Charge Formation and Energetics in n-Doped Organic Semiconductors

“…Therefore, as the method of choice, we use spin-unrestricted density functional theory (DFT), which has previously successfully been used to describe molecular doping in similar contexts. As successfully applied in our previous work, 14,24,31 the electronic states are described with the range-separated cam-B3LYP functional and 6-31G** basis set, including van der Waals interaction on the level of Grimme's D3 dispersion correction.…”

“…It should be noted that using the ωB97XD functional and/or a reasonably smaller basis set such as 6-31G* usually gives very close results in terms of integer charge transfer, as tested also in our previous work. 24 The treatment of the dielectric environment uses the polarizable continuum model (PCM) with ϵ = 3. 21 The geometries of all complexes were optimized in the neutral state in the gas phase, so that the geometric change of each molecule caused by possible charge transfer is treated.…”

Microscopic Study of Molecular Double Doping

“…Our previous work showed that CN6-CP induces more robust integer charge transfer (ICT) than other dopants such as F4TCNQ, with less influence of the geometric details of the complexes formed. 22,23 This choice facilitated an easier first computational investigation of molecular double doping.…”

“…In our previous work, it was shown that the ωB97X-D functional gives a similar description of charge transfer processes as the CAM-B3LYP functional. 22 To account for the electrostatic effects of an actual OSC environment, a dielectric background with ε = 3 is used within the polarizable continuum model (PCM). To shed some light on the effect of the chosen ε, we also varied its value from 1.5 to 5.0.…”

“…For example, further details of the charge formation such as the formation of individual polarons and/or bipolarons upon double doping are of significant relevance for the transport properties of the resulting doped system. 17 −19,21−23 In this work, we investigate simultaneous charge transfer between two p(g 4 2T-TT) conjugated polymer (oligomer) chains in a close arrangement and a single CN6-CP p-type dopant molecule, as illustrated in Figure 1a possibility for other electronic processes to occur, 22 the results clearly show double p-type doping in the form of double integer charge transfer from the conjugated chains onto the dopant molecule: a CN6-CP dianion is formed along with two individual singly charged polarons, one on each polymer chain.…”