High electron affinity (EA) molecules p-type dope low ionization energy (IE) polymers, resulting in an equilibrium doping level based on the energetic driving force (IE-EA), reorganization energy, and dopant concentration. Anion exchange doping (AED) is a process whereby the dopant anion is exchanged with a stable ion from an electrolyte. We show that the AED level can be predicted using an isotherm equilibrium model. The exchange of the dopant anion (FeCl₃^-) for a bis(trifluoromethanesulfonamide) (TFSI^-) anion in the polymers poly(3-hexylthiophene-2,5-diyl) (P3HT) and poly[3-(2,2-bithien-5-yl)-2,5-bis(2-hexyldecyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione-6,5-diyl] (PDPP-2T) highlights two cases in which the process is nonspontaneous and spontaneous, respectively. For P3HT, FeCl₃ provides a high doping level but an unstable counterion, so exchange results in an air stable counterion with a marginal increase in doping. For PDPP-2T, FeCl₃ is a weak dopant, but the exchange of FeCl₃^- for TFSI^- is spontaneous, so the doping level increases by >10× with AED.

Merocyanines (MCs) are a versatile class of small-molecule dyes. Their optoelectronic properties are easily tunable by chemically controlling their donor-acceptor strength, and their structural properties can be tuned by simple side-chain substitution. This manuscript demonstrates a novel series of MCs featuring an indoline donor with varying hydrocarbon side-chain length (from 6 to 12 carbons) and a tert-butyl-thiazole acceptor, labeled InTBT. Bulk heterojunction organic photovoltaics are fabricated with a [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) acceptor and characterized. Films composed of I8TBT:PCBM and I9TBT:PCBM produced the highest power conversion efficiency of 4.5%, which suggests that the morphology is optimized by controlling the side-chain length. Hierarchical structure formation in InTBT:PCBM films is studied using grazing incidence X-ray diffraction (GIXRD), small-angle neutron scattering (SANS), and atomic force microscopy (AFM). When mixed with PCBM, InTBTs with ≤8 side-chain carbons form pure crystalline domains, while InTBTs with ≥9 side-chain carbons mix well with PCBM. SANS demonstrates that increasing side-chain length increases the InTBT-rich domain size. In addition, a branched hexyl-dodecyl side-chain IHDTBT:PCBM film was studied and found to exhibit the worst-performance organic photovoltaic (OPV) device. The large-branched side chain inhibited mixing between IHDTBT and PCBM resulting in large segregated phases.

Many organic electronic devices require vertically layered structures to operate. This manuscript demonstrates an additive solution process for depositing multiple layers of semiconducting polymer (SP) films by controlling film solubility with molecular dopants. During multi-layer deposition the bottom layers are exposed to a series of solvent environments that swell the SP films. We use neutron reflectometry (NR) to quantify the film thickness change and solvent content during solvent exposure in a single poly-3-hexylthiophene (P3HT) layer. The film thickness increases by 40-80% with exposure to good solvents. Four layer thin-films composed of alternating protonated and deuterated P3HT layers were additively coated from solution. NR measurements reveal high individual layer purity and that extensive solvent soaking induces no mixing between layers. Finally, two-point conductivity measurements demonstrate that P3HT:P3HT layer interfaces have no effect on vertical conductivity. This facile process enables additive layering of mutually soluble SP films and can be used to design novel electronic device architectures.

Recent theories suggest that low frequency dynamic intramolecular and intermolecular motions in organic semiconductors (OSCs) are critical to determining the hole mobility. So far, however, it has not been possible to probe these motions directly experimentally and therefore no unequivocal and quantitative link exists between molecular-scale thermal disorder and macroscale hole mobility in OSCs. Here we use inelastic neutron scattering to probe thermal disorder directly by measuring the phonon spectrum in six different small molecule OSCs, which we accurately reproduce with first principles simulations. We use the simulated phonons to generate a set of electron-phonon coupling parameters. Using these parameters, the theoretical mobility is in excellent agreement with macroscopic measurements. Comparison of mobility between different materials reveals routes to improve mobility by engineering phonon and electron-phonon coupling.