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Elucidating the interrelation between the molecular structure and charge transport properties in conjugated polymer thin films is an essential issue in developing the design principle of high‐performance polymer materials for application in organic electronics. In particular, the backbone planarity is suggested to be a key element that governs the transport performance, especially in recently developed donor–acceptor (D–A)‐type copolymers exhibiting high mobility, whereas the direct evaluation of the intrinsic transport performance, usually realized only within the small crystalline domains, is difficult by using conventional macroscopic measurements. Here, it is demonstrated that a D–A type copolymer, PDPPF‐DTT, which consists of furan‐flanked diketopyrrolopyrrole (DPP) and dithienothiophene (DTT) units in the conjugated backbone, exhibits a highly efficient charge transport performance within the crystalline domains with a remarkably low activation energy of less than 8 meV, based on microscopic measurements using field‐induced electron spin resonance spectroscopy. This high transport performance is primarily caused by the high backbone planarity realized by introducing furan‐flanked DPP and fused dithienothiophene units, which is demonstrated from the density functional theory calculations. This result provides a microscopic indication of the effectiveness of the present molecular design to produce a planar backbone and realize highly efficient charge transport performance. [ABSTRACT FROM AUTHOR]

The properties of molecularly doped films of conjugated polymers are explored as the crystallinity of the polymer is systematically varied. Solution sequential processing (SqP) was used to introduce 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) into poly(3-hexylthiophene-2,5-diyl) (P3HT) while preserving the pristine polymer's degree of crystallinity. X-ray data suggest that F4TCNQ anions reside primarily in the amorphous regions of the film as well as in the P3HT lamellae between the side chains, but do not π-stack within the polymer crystallites. Optical spectroscopy shows that the polaron absorption redshifts with increasing polymer crystallinity and increases in cross section. Theoretical modeling suggests that the polaron spectrum is inhomogeneously broadened by the presence of the anions, which reside on average 6-8 Å from the polymer backbone. Electrical measurements show that the conductivity of P3HT films doped by F4TCNQ via SqP can be improved by increasing the polymer crystallinity. AC magnetic field Hall measurements show that the increased conductivity results from improved mobility of the carriers with increasing crystallinity, reaching over 0.1 cm2 V−1 s−1 in the most crystalline P3HT samples. Temperature-dependent conductivity measurements show that polaron mobility in SqP-doped P3HT is still dominated by hopping transport, but that more crystalline samples are on the edge of a transition to diffusive transport at room temperature. [ABSTRACT FROM AUTHOR]

The detailed characterization of a dialkoxyphenylene-difluorobenzothiadiazole based conjugated polymer poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)] (PPDT2FBT) is reported. PPDT2FBT closely tracks theoretical photocurrent production while maintaining a high fill factor in remarkably thick films. In order to understand the properties that enable PPDT2FBT to function with thick active layers, the effect of film thickness on the material properties and device parameters was carefully studied and compared to three benchmark polymers. Optical modeling, grazing incidence wide angle X-ray scattering, cross-sectional transmission electron microscopy, transient photoconductivity, and extensive device work were carried out and have clarified the key structural features and properties that allow such thick active layers to function efficiently. The unique behavior of thick PPDT2FBT films arises from high vertical carrier mobility, an isotropic morphology with strong, vertical π-π stacking, and a suitable energy band structure. These physical characteristics allow efficient photocurrent extraction, internal quantum efficiencies near 100% and power conversion efficiencies over 9% from exceptionally thick active layers in both conventional and inverted architectures. The ability of PPDT2FBT to function efficiently in thick cells allows devices to fully attenuate incident sunlight while providing a pathway to defect-free film processing over large areas, constituting a major advancement toward commercially viable organic solar cells. [ABSTRACT FROM AUTHOR]

The power conversion efficiency of solar cells based on a conjugated polymer (donor) and a fullerene derivative (acceptor) is very sensitive to the morphology of the active layer. One detrimental feature, which is often encountered in non-optimal morphologies, is the occurrence of fullerene blobs in a finely mixed matrix containing both donor and acceptor material. Here, the effects of such fullerene blobs are studied in detail with a three-dimensional drift-diffusion model. It includes the effects of exciton diffusion and quenching; space-charge; recombination, generation, drift and diffusion of charge carriers; and the injection/extraction of carriers at the contacts. The influence of blob size and shape, and matrix composition are quantified. The latter has the strongest effect on the overall efficiency, as most of the current is transported through the mixed phase. The total current flowing out of the solar cell can be split up in a part which comes from the interfacial region between the acceptor phase and the mixed phase, and a part that stems from the mixed phase itself. Depending on the bias voltage and the morphology, one or the other contribution is dominant. Finally, it is shown how both contributions can be computed with a simple one-dimensional drift-diffusion simulator. [ABSTRACT FROM AUTHOR]

Current organic semiconductors for organic photovoltaics (OPV) have relative dielectric constants (relative permittivities, εr) in the range of 2-4. As a consequence, Coulombically bound electron-hole pairs (excitons) are produced upon absorption of light, giving rise to limited power conversion efficiencies. We introduce a strategy to enhance εr of well-known donors and acceptors without breaking conjugation, degrading charge carrier mobility or altering the transport gap. The ability of ethylene glycol (EG) repeating units to rapidly reorient their dipoles with the charge redistributions in the environment was proven via density functional theory (DFT) calculations. Fullerene derivatives functionalized with triethylene glycol side chains were studied for the enhancement of εr together with poly( p-phenylene vinylene) and diketopyrrolopyrrole based polymers functionalized with similar side chains. The polymers showed a doubling of εr with respect to their reference polymers in identical backbone. Fullerene derivatives presented enhancements up to 6 compared with phenyl-C61-butyric acid methyl ester (PCBM) as the reference. Importantly, the applied modifications did not affect the mobility of electrons and holes and provided excellent solubility in common organic solvents. [ABSTRACT FROM AUTHOR]

Naphthalenediimide (NDI)-based polymers co-polymerized with thienyl units are an interesting class of polymer semiconductors because of their good electron mobilities and unique film microstructure. Despite these properties, understanding how the extension of the thienyl co-monomer affects charge transport properties remains unclear. With this goal in mind, we have synthesized a series of NDI derivatives of the parent poly{[ N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene) (P(NDI2OD-T2)), which exhibited excellent electron mobility. The strategy comprises both the extension of the donor o-conjugation length and the heteroatomic fusion of the thiophene rings. These newly synthesized compounds are characterized experimentally and theoretically vis-à-vis with P(NDI2OD-T2) as the reference. UV-vis data and cyclic-voltammetry are adopted to assess the effect of the donor modification on the frontier energy levels and on the bandgap. Intra-molecular polaronic effects are accounted for by computing the internal reorganization energy with density functional theory (DFT) calculations. Finally electrons and holes transport is experimentally investigated in field-effect transistors (FETs), by measuring current-voltage characteristics at variable temperatures. Overall we have identified a regime where inter-molecular effects, such as the wavefunction overlap and the degree of energetic disorder, induced by the different donor group prevail over polaronic effects and are the leading factors in determining electrons mobility. [ABSTRACT FROM AUTHOR]