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Density functional theory (DFT) calculations were performed using a MPWB1K functional to interpret the charge transport (CT) properties of stacked annelated β-trithiophene molecules. The goal was to help understand how the chemical composition, face-to-face stacking, intra- and intermolecular correlations, and the applied electric field affect the CT properties of this organic semiconductor compared with those of the edge-to-face "herringbone" motif. The variations in the frontier molecular orbitals, energy gap, nonadiabatic electron attachment energies (EAE), and vertical detachment energies (VDE) were investigated under an external electric field as a function of the number of stacked layers (n). Two possible CT pathways, monomer-to-monomer (MM) and dimer-to-dimer (DD) transports, were postulated to determine the charge carrier conductivities of these molecules. The results highlight that the intermolecular electronic couplings and electrostatic interactions can significantly affect the stacking geometry, even in more extended structures; displaced stacks with smaller interlayer spacing resulted in more compact stacking and, thus, higher CT efficiency; face-to-face stacking geometries can help to reduce the energy gap and VDE. Despite the fact that common thiophene-based oligomers adopting edge-to-face herringbone motifs exhibited p-type (hole-transporting) characters, the face-to-face stacked models based on annelated β-trithiophenes exhibited remarkably increased n-type (electron-transporting) performances. The electric field in the z-direction produced a small influence on the DD charge transport, whereas both electron and hole mobilities decreased dramatically in the MM case. More importantly, in MM charge transport, the electric field increases the hole mobility so that it is higher than that of the electron. [ABSTRACT FROM AUTHOR]

Two donor–acceptor (D–A)medium band gap polymers, P1and P2, alkoxyphenylthiophene(APTh) linked benzodithiophene (BDT) as an electron-rich unit and1,3-di(2′-bromothien-5′-yl)-5-(2-ethylhexyl)thieno[3,4-c]pyrrole-4,6-dione (TPD) (A1) or [5,6-bis(octyloxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole] (BT) (A2) as an electron-deficientunit, have successfully been synthesized via microwave-assisted Stillepolymerization and utilized for bulk heterojunction (BHJ) polymersolar cells (PSCs). P1shows a well-distinguished absorptionshoulder between 590 and 620 nm attributed to the π–πstacking of a polymer backbone; such kind of absorption shoulder isnot observed in P2, indicating that the P1has more planar structure than that of P2. This isdue to the fact that the sulfur atom of thiophene spacer and the oxygenatom of carbonyl groups in TPD have more pronounced intramolecularnoncovalent interactions (INCI) in P1than that of thesulfur atom of thiophene spacer and the oxygen atom of alkoxy groupsof BT in P2. The bulk heterojunction polymer solar cells(BHJ PSCs) were fabricated with the configuration of ITO/PEDOT:PSS/polymer(P1or P2):PC71BM/LiF/Al. The P1device shows better photovoltaic performance with open-circuitvoltage (Voc) of 0.91 V and the powerconversion efficiency (PCE) of 4.19% than the P2device(Voc: 0.71 V; PCE: 1.88%) in neat blendfilms under the illumination of AM 1.5G (100 mW/cm2). Upontreating the active layers containing P1and P2with methanol, the PCE of the P1device is increasedfrom 4.19 to 7.14%. In contrast, the PCE of the P2deviceis decreased from 1.88 to 1.82%. Space charge limited current mobility,atomic force microscopy, transmission electron microscopy, time-of-flightsecondary ion mass spectrometry, and impedance spectroscopy studiesstrongly support the enhanced PCE for the P1device isattributed to the increased mobility, nanoscale morphology, and reducedresistance upon methanol treatment; these favorable properties forthe P1polymer are highly correlated with the planarityof the backbone. [ABSTRACT FROM AUTHOR]