Relating Chemical Structure to Device Performance via Morphology Control in Diketopyrrolopyrrole-Based Low Band Gap Polymers

Abstract

We investigated the structure-morphology-performance relationship of diketopyrrolopyrrole (DPP)-based low band gap polymers with different donor cores in organic field effect transistors (OFETs) and organic photovoltaics (OPVs). The change in the chemical structure led to strong physical property differences, such as crystalline behavior, blend morphology, and device performance. In addition, the choice of solvents and additives enabled one to fine tune the properties of these materials in the condensed state. For instance, when thin films were processed from solvent mixtures, both in the pure polymer and in a blend, we observed an enhanced edge-on orientation and the formation of thinner and longer polymer fibrils. In the BHJ blends, processing from a solvent mixture reduced the size scale of the phase separation and promoted the formation of a fibrillar network morphology, having a polymer-PCBM mixture filling the interfibrillar regions. The characteristic length scale of the fibrillar network dictated the specific inner surface area, which directly correlated to the performance in the OPV devices. When the BHJ mixture was processed from a single solvent, a large-scale phase separated morphology was observed that was stratified, normal to the film surface. A strong scattering anisotropy was observed in the resonant soft X-ray scattering of the blends that provided insight into the packing of the polymer chains within the fibrils. The morphology and performance trend in OPVs paralleled the performance in an OFET, suggesting that similar processing conditions should be considered in OFET fabrication.

Charge Transport in Organic and Polymeric Semiconductors for Flexible and Stretchable Devices

“…[ 19,20,59,75 ] Actually, the correlation between microstructure and electronic properties in conjugated polymers is still far from being clearly articulated because of their molecular intermediate states between order and disorder, which also signifi cantly prevents the study of physical phenomena from theory. [ 48,76 ] Fortunately, the continuous improvement of structural-measurement techniques and their combined utilization, for example, GIXRD, [ 21 ] scanning transmission electron microscopy (STEM), [ 24 ] nearedge X-ray absorption fi ne structure (NEXAFS), [ 22 ] resonant scattering of polarized soft X-rays (P-SoXS), and 2D-NMR, [ 57 ] etc., have enabled experimental access to probe more-detailed microscopic clues toward deeper understanding of the fundamental charge transport in conjugated polymers.…”

“…[13][14][15][16][17][18] This has stimulated the rapid development of this fi eld and given the chance for scientists to deeply understand the origin of high mobility in conjugated polymers. [19][20][21] For example, some clues suggest the high mobility in conjugated polymers should be attributed to the improvement of conjugated polymer chain orders and planar-conjugated structure. [22][23][24] Better understanding of the charge-transport physics and structure-property relationship in conjugated polymers provides more directions for rational design of organic and polymeric semiconductors [ 25 ] with high device performance, [ 26 ] as well as advancing them in fl exible and stretchable electronics.…”

Controlling the Dominant Length Scale of Liquid–Liquid Phase Separation in Spin‐coated Organic Semiconductor Films

“…Thin fi lms with smaller length scales are commonly made by adding an extra solvent and thus processing from a four-component ink. [ 15 ] Although these droplet-like morphologies are not directly relevant to high-performing solar cells, the relevance of these three-component inks is twofold. Firstly, we are currently far from understanding the complex processes that determine the length scales when processing four-component inks to an optimized solar cell.…”

Combining theory and experiment for X-ray absorption spectroscopy and resonant X-ray scattering characterization of polymers

“…This anisotropic scattering occurs in samples that are globally isotropic in-plane and the direction of scattering intensity anisotropy changes with incident X-ray polarization direction, suggesting this feature arises due to core-level transitions with specific dipole moment directions and local molecular arrangement. Several cases of scattering anisotropy have been observed in blends of conjugated polymers and polymer-fullerene blends [68,71,70,76,72,75], but it can also be observed in a single semicrystalline polymer, as shown in Energy (eV)…”