Synthesis, Characterization, Charge Transport, and Photovoltaic Properties of Dithienobenzoquinoxaline- and Dithienobenzopyridopyrazine-Based Conjugated Polymers
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Two donor–acceptor polymers (P1 and P2) based on dithienobenzoquinoxaline (M1) and dithienobenzopyridopyrazine (M2) as acceptor and indacenodithiophene as donor were synthesized via Stille polycondensation. The fused dithienobenzene unit in M1 and M2 units can improve the intermolecular stacking of polymer and also decrease the steric hindrance. P1, with dithienobenzoquinoxaline acceptor, shows a band gap of 1.61 eV. The band gap of P2 was reduced to 1.48 eV after changing to dithienobenzopyridopyrazine as the acceptor unit. The mobilities of P1 and P2 reach 5.6 × 10–2 and 1.5 × 10–2 cm2 V–1 s–1, respectively. The results from photovoltaic measurements showed a very promising PCE of 6.06% for the P1/PC71BM blend system without any thermal or solvent treatments, showing a great offer for the roll-to-roll manufacturing of PSCs.Keywords:
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Steric properties of crystallographically and computationally determined structures of linear palladium(0) and square planar palladium(II) complexes of di(tert-butyl)neopentylphosphine (P(t-Bu)2Np), tert-butyldineopentylphosphine (P(t-Bu)Np2), and trineopentylphosphine (PNp3) have been determined. Structures of linear palladium(0) complexes show that steric demand increases as tert-butyl groups are replaced with neopentyl groups (P(t-Bu)2Np < P(t-Bu)Np2 < PNp3). In square planar palladium(II) complexes, PNp3 gives the smallest steric parameters, whereas P(t-Bu)Np2 has the largest steric demand. The change in the steric demand of PNp3 compared to P(t-Bu)2Np and P(t-Bu)Np2 results from a significant conformational change in PNp3 depending on the coordination number of the metal. The steric properties of these ligands were also probed by measuring the equilibrium constant for coordination of free phosphine to dimeric [(R3P)Pd(μ-Cl)Cl]2 complexes. Binding equilibria follow the same trend as the steric parameters for square planar complexes with PNp3 having the highest binding constant. In contrast to the normal trend, the neopentylphosphines show increased pyramidalization at phosphorus with increasing steric demand. We hypothesize that this unusual dependence reflects the low back side strain of the neopentyl group, which allows the ligand to be more pyramidalized while still exerting a significant front side steric demand.
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The vertical composition distribution of a bulk heterojunction (BHJ) photoactive layer is known to have dramatic effects on photovoltaic performance in polymer solar cells. However, the vertical composition distribution evolution rules of BHJ films are still elusive. In this contribution, three BHJ film systems, composed of polymer donor PBDB-T, and three different classes of acceptor (fullerene acceptor PCBM, small-molecule acceptor ITIC, and polymer acceptor N2200) are systematically investigated using neutron reflectometry to examine how donor–acceptor interaction and solvent additive impact the vertical composition distribution. Our results show that those three BHJ films possess homogeneous vertical composition distributions across the bulk of the film, while very different composition accumulations near the top and bottom surface were observed, which could be attributed to different repulsion, miscibility, and phase separation between the donor and acceptor components as approved by the measurement of the donor–acceptor Flory–Huggins interaction parameter χ. Moreover, the solvent additive 1,8-diiodooctane (DIO) can induce more distinct vertical composition distribution especially in nonfullerene acceptor-based BHJ films. Thus, higher power conversion efficiencies were achieved in inverted solar cells because of facilitated charge transport in the active layer, improved carrier collection at electrodes, and suppressed charge recombination in BHJ solar cells.
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The stacking of layers forming three-dimensional periodic structures is explored in the general case, where neither the layers nor the stacking need to be close-packed, and the connectivity number for the system may be either two or four. Procedures are described whereby all possible stacking variants can be systematically derived for a given number of layers, and for a given number of possible stacking positions. The latter depends on the structure of the layer and on the stacking vector.
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Steric properties of ligands are an important parameter for tuning the reactivity of the corresponding complexes. For various ligands used in mononuclear complexes, methods have been developed to quantify their steric bulk. In this work, we present an expansion of the buried volume and the G-parameter to quantify the steric properties of 1,8-napthyridine-based dinuclear complexes. Using this methodology, we explored the tunability of the steric properties associated with these ligands and complexes.
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Steric properties of ligands are an important parameter for tuning the reactivity of the corresponding complexes. For various ligands used in mononuclear complexes, methods have been developed to quantify their steric bulk. In this work we present an expansion of the buried volume and G-parameter to quantify the steric properties of 1,8-napthyridine based dinuclear complexes. Using this methodology, we explored the tuneability of the steric properties associated with these ligands and complexes.
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Steric energy and the semi-empirical method of calculating steric energy are discussed in the language of potential surfaces. Equations are given relating ΔH00 for several types of reactions commonly used to exhibit steric effects experimentally to the steric energy of the molecules involved in the reactions. The equations of Westheimer and Mayer for the calculation of the steric energy of a molecule are generalized in several respects. The difference in ΔH00 (formation) between cis and trans−2-butene is calculated as a steric effect. Because of various complications the application of the method in its present form to molecules such as H2O, NH3, PF3, etc., is rather unsatisfactory.
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