Fluorine Substituents Reduce Charge Recombination and Drive Structure and Morphology Development in Polymer Solar Cells

Abstract

Three structurally identical polymers, except for the number of fluorine substitutions (0, 1, or 2) on the repeat unit (BnDT-DTBT), are investigated in detail, to further understand the impact of these fluorine atoms on open circuit voltage (V(oc)), short circuit current (J(sc)), and fill factor (FF) of related solar cells. While the enhanced V(oc) can be ascribed to a lower HOMO level of the polymer by adding more fluorine substituents, the improvement in J(sc) and FF are likely due to suppressed charge recombination. While the reduced bimolecular recombination with raising fluorine concentration is confirmed by variable light intensity studies, a plausibly suppressed geminate recombination is implied by the significantly increased change of dipole moment between the ground and excited states (Δμ(ge)) for these polymers as the number of fluorine substituents increases. Moreover, the 2F polymer (PBnDT-DTffBT) exhibits significantly more scattering in the in-plane lamellar stacking and out-of-plane π-π stacking directions, observed with GIWAXS. This indicates that the addition of fluorine leads to a more face-on polymer crystallite orientation with respect to the substrate, which could contribute to the suppressed charge recombination. R-SoXS also reveals that PBnDT-DTffBT has larger and purer polymer/fullerene domains. The higher domain purity is correlated with an observed decrease in PCBM miscibility in polymer, which drops from 21% (PBnDT-DTBT) to 12% (PBnDT-DTffBT). The disclosed "fluorine" impact not only explains the efficiency increase from 4% of PBnDT-DTBT (0F) to 7% with PBnDT-DTffBT (2F) but also suggests fluorine substitution should be generally considered in the future design of new polymers.

Impact of Backbone Fluorination on π-Conjugated Polymers in Organic Photovoltaic Devices: A Review

“…An auxochromic effect of the fluorine atom has also been observed by these authors, with a significantly enhanced extinction coefficient in solution and in solid state when the fluorine atom density along the conjugated backbone is increased. A similar effect has been previously observed by You and coworkers [59].…”

“…A number of recent reports have shown that fluorination of conjugated polymer backbones can have a strong impact on one or several of these properties, often leading to a more favorable morphology and to improved photovoltaic performances. Adding fluorine heteroatoms has, for instance, been found to enhance the domain purity (lowering the charge carrier recombination rate) [25,59,67], reduce the domain size [28,32,59,67], enhance the structural order [24,25,28,29,59,61,[69][70][71] and promote polymer face-on orientation at the interface with the bottom electrode [28,32,59,61,67,69,72] (increasing the out-of-plane hole mobility) or induce a face-on orientation at the D/A interface [69,73]. However, these effects can be more or less pronounced or even reversed [27,33,67], depending on the molecular structure of the conjugated building blocks as well as on the nature and positioning of the solubilizing side-chains.…”

“…Abe et al, for instance, reported that π-π stacking in PNDT-DTBT-based blends (see Figure 10) remained weak, independent of fluorine substitution [73]. The stronger π-π stacking interactions can counterweigh the steric hindrance of alkyl chains, allowing the introduction of relatively long and branched chains (up to C 24 H 49 ) without jeopardizing the formation of crystalline lamellae [59]. Concomitantly, in some cases the inter-lamellar spacing has been observed to be enlarged as a consequence of more extended side-chain conformation.…”

Noncovalent Interactions in Organic Electronic Materials

“…[84][85][86] Fluorination has been carried out on a wide variety of building blocks used in the synthesis of polymers, including thiophenes, carbazoles, thienothiophenes, benzothiadiazoles, benzotriazoles, benzodithiophenes, indacenodithiophenes, and anthradithiophenes. [87][88][89][90][91][92][93][94][95][96] Recently, we have shown The idea of attaching bulky substituents has also been widely used to control the packing of small molecules. [101][102][103] In particular, attaching bulky triisopropylsilyl (TIPS) groups via ethynyl linkers to the periphery of pentacene triggers the herringbone packing configuration of pentacene to transform into a 2D-brickwork packing arrangement.…”

Conjugated D–A terpolymers for organic field-effect transistors and solar cells

“…[4][5][6][7] This is attributed to the conjugated electronic and polymeric structures of these polymers, resulting in semiconducting properties with good solution processability, mechanical property and thermal stability. 8,9 Among the conjugated polymers, conjugated alternating polymers consisting of both electron donors (D) and electron acceptors (A), referred to as conjugated D-A polymers, [10][11][12][13][14][15][16][17] have been extensively investigated in recent years. The studies indicate that electronic absorptions, highest occupied molecular orbital/lowest unoccupied molecular orbital (HOMO/LUMO) energies (and bandgaps), interchain interactions and thin-film morphologies of conjugated D-A polymers can be effectively tuned [10][11][12][13][14][15][16][17] by properly varying the electron donors and acceptors as well as the linkers.…”