Side-Chain Engineering of Conjugated Polymers: Distinguishing Its Impact on Film Morphology and Electronic Structure

Abstract

Solution processing of conjugated polymers is key for low-cost processing of organic electronic devices. To improve solubility, introducing alkyl side chains is a commonly employed approach, known for its impact on film morphology. The morphology of polymer films is a key aspect of the structure− function relationship in organic electronic devices with a strong impact on their overall efficiency. Although planarity of conjugated backbones is advantageous for exciton and charge carrier mobilities and the overall degree of order, it leads to aggregation in solution. This reduces solubility but can result in interesting structures. Side-chain-mediated backbone torsion, in contrast, greatly enhances solubility, facilitating synthesis and control of molecular weight, but often impairs performance. Detailed insight into the impact of side chains on both morphology and electronic structure is therefore of high demand. We demonstrate time-resolved electron paramagnetic resonance spectroscopy to be perfectly suited to probe the orientation as well as the overall degree of order in conjugated polymer films, while simultaneously revealing details of the electronic structure. By systematically studying the impact of additional side chains, we distinguish their impact on the film morphology and electronic structure. Additional side chains decrease exciton delocalization but not the overall degree of order in the film. Delocalization is therefore only connected to backbone planarity. Using magnetophotoselection experiments, we additionally present clear evidence for the preferential face-on orientation of the amorphous polymer backbone on the substrate. This is crucial for the efficiency of both organic photovoltaic devices and organic light-emitting diodes.

Elucidating the Influence of Side-Chain Circular Distribution on the Crack Onset Strain and Hole Mobility of Near-Amorphous Indacenodithiophene Copolymers

“…We ascribe the immediate decline in mobility to the increased twist between monomer units which weakens the electronic coupling between adjacent conjugated rings, thereby decreasing the efficiency of intramolecular charge transport. 38 As the BPD monomer is further incorporated into the backbone, charge transport along the backbone worsens. The distance between π−π interactions in local aggregates also lengthens, as evidenced by the increase in (010) d-spacing beginning at 35% BPD incorporated and beyond, making intermolecular charge transfers less efficient.…”

From Chlorinated Solvents to Branched Polyethylene: Solvent‐Induced Phase Separation for the Greener Processing of Semiconducting Polymers

“…[1][2][3] This intriguing class of materials has been strategically designed and used for the development of various innovative technologies, including organic field-effect transistors (OFETs), organic photovoltaics, and organic light-emitting diodes. [4,5] In contrast to their inorganic counterparts, organic π-conjugated semiconducting polymers possess tunable charge transport properties, [6][7][8] intrinsically low Young's modulus, [9,10] and can be processed at large scale through various solution deposition techniques such as inkjet printing or roll-to-roll patterning, [11,12] making them remarkable candidates to fabricate low-cost large-area flexible, conformable, and stretchable electronics. This notable combination of properties has propelled the fields of organic electronics forward, leading to the fabrication of organic electronic devices with constant improvements in performance and functionality.…”

Impact of Side Chains of Conjugated Polymers on Electronic Structure: A Case Study

“…Based on the assignment of a positive sign to D from DFT calculations and given the information of the orientation of the D tensor within the molecule, we can draw some conclusions from the zero-field triplet sublevel populations obtained by spectral simulations. The vanishing contribution of pz associated with the Dz component oriented perpendicular to the aromatic plane, in line with magnetophotoselection experiments on the (non-hexylated) D-A fragment [55], can be rationalized based on the spin-density distribution obtained from DFT calculations. However, we would not try to deduce, in reverse, a positive sign of D only based on a disk-like (oblate) spin-density distribution of a planar aromatic system.…”

“…Furthermore, an overall face-on orientation with respect to the substrate was deduced, which is advantageous for charge carrier injection or extraction in OLED or OPV devices, respectively [51,52,53,54]. Additionally, we investigated the impact of side chains on the film morphology using a series of PCDTBT polymers with increasing degree of alkyl side chains, allowing us to distinguish between effects on electronic structure and morphology [55].…”