Influence of fluorine substituents on the film dielectric constant and open-circuit voltage in organic photovoltaics

The Influence of Backbone Fluorination on the Dielectric Constant of Conjugated Polythiophenes

“…[50] The increase in ε r upon fluorination reflects the trend observed in quinoxalinebased conjugated polymers and further suggests the dielectric constant of conjugated poly mers respond differently to fluorination when compared with nonconjugated poly mers. [4,11] At present, it is unclear from our results whether this difference can be assigned solely to the increased dipole moment, as predicted by DFT calculations, or if nanoscale ordering, and other microstructure effects (e.g., crystallite dipoles) are at play, as found for P(VDFTrFECFE) and related high dielectric polymers. [19] In the case of these polyethylenebased fluorinated polymers, the anisotropy in crystalline domains resulting from the introduction of different comonomers is exploited.…”

“…[27] Similarly, two separate studies have shown that partial backbone fluorination of quinoxalinebased conjugated polymers led to substantial increases in permittivity, and in both cases, fluorination drove increased OPV performance. [4,11] In addition to these efforts, backbone fluorination of conjugated polymers is also moti vated by factors other than dielectric constant improvements. Stabilization of both the highestoccupied molecular orbital and the lowestunoccupied molecular orbital through the addition of electron withdrawing fluorine atoms is a common approach to increase the V oc .…”

Origin of Open-Circuit Voltage Loss in Polymer Solar Cells and Perovskite Solar Cells

“…The dielectric constant ε r has been reported to be ≈3 for most of conjugated polymer films and to be 3.9 for PCBM films. 29,[45][46][47] In this calculation, ε r = 3.5 was used as an average value for all the polymer solar cells studied and R was employed as a fitting parameter. On the basis of Equation (2) with ε r = 3.5 and R = 1.2-1.6 nm, the ∆E C is roughly estimated to be ≈0.3 eV, which is consistent with the difference between E g eff and E DA .…”

Enhancement of photovoltaic performance by increasing conjugation of the acceptor unit in benzodithiophene and quinoxaline copolymers

“…The E opt g of PTOBDTTQ2 is lower than reported previously. 33,35,36,38 These phenomena indicate that the more planar TQ2 unit not only effectively broadens the absorption spectra, but also exhibits good p-p stacking in the solid state, consistent with the design of the polymer.…”

“…The quinoxaline derivative 2,3-diphenyl-5,8-di(thiophen-2-yl)-quinoxaline (TQ1), which has two separated phenyl rings, has the advantages of easy synthesis and versatility. However, most of the polymers based on TQ1 have a large E g and low charge carrier mobility 33,35 as a result of the less planar structure caused by the two separated phenyl rings. 36 We report here a more planar dibenzo[a,c]phenazine that facilitates intermolecular packing and charge transport of the acceptor unit.…”