An official website of the United States government
Abstract Tuning structures of solution‐state aggregation and aggregation‐mediated assembly pathways of conjugated polymers is crucial for optimizing their solid‐state morphology and charge‐transport property. However, it remains challenging to unravel and control the exact structures of solution aggregates, let alone to modulate assembly pathways in a controlled fashion. Herein, aggregate structures of an isoindigo–bithiophene‐based polymer (PII‐2T) are modulated by tuning selectivity of the solvent toward the side chain versus the backbone, which leads to three distinct assembly pathways: direct crystallization from side‐chain‐associated amorphous aggregates, chiral liquid crystal (LC)‐mediated assembly from semicrystalline aggregates with side‐chain and backbone stacking, and random agglomeration from backbone‐stacked semicrystalline aggregates. Importantly, it is demonstrated that the amorphous solution aggregates, compared with semicrystalline ones, lead to significantly improved alignment and reduced paracrystalline disorder in the solid state due to direct crystallization during the meniscus‐guided coating process. Alignment quantified by the dichroic ratio is enhanced by up to 14‐fold, and the charge‐carrier mobility increases by a maximum of 20‐fold in films printed from amorphous aggregates compared to those from semicrystalline aggregates. This work shows that by tuning the precise structure of solution aggregates, the assembly pathways and the resulting thin‐film morphology and device properties can be drastically tuned.
more »
« less
Tuning conformation, assembly, and charge transport properties of conjugated polymers by printing flow
Intrachain charge transport is unique to conjugated polymers distinct from inorganic and small molecular semiconductors and is key to achieving high-performance organic electronics. Polymer backbone planarity and thin film morphology sensitively modulate intrachain charge transport. However, simple, generic nonsynthetic approaches for tuning backbone planarity and the ensuing multiscale assembly process do not exist. We first demonstrate that printing flow is capable of planarizing the originally twisted polymer backbone to substantially increase the conjugation length. This conformation change leads to a marked morphological transition from chiral, twinned domains to achiral, highly aligned morphology, hence a fourfold increase in charge carrier mobilities. We found a surprising mechanism that flow extinguishes a lyotropic twist-bend mesophase upon backbone planarization, leading to the observed morphology and electronic structure transitions.
more »
« less
- PAR ID:
- 10134468
- Date Published:
- Journal Name:
- Science Advances
- Volume:
- 5
- Issue:
- 8
- ISSN:
- 2375-2548
- Page Range / eLocation ID:
- eaaw7757
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
The molecular morphology and dynamics of conjugated polymers in the bulk solid state play a significant role in determining macroscopic charge transport properties. To understand this relationship, molecular dynamics (MD) simulations and quantum mechanical calculations are used to evaluate local electronic properties. In this work, we investigate the importance of system and simulation parameters, such as force fields and equilibration methods, when simulating amorphous poly(3-hexylthiophene) (P3HT), a model semiconducting polymer. An assessment of MD simulations for five different published P3HT force fields is made by comparing results to experimental wide-angle X-ray scattering (WAXS) and to a broad range of quasi-elastic neutron scattering (QENS) data. Moreover, an in silico analysis of force field parameters reveals that atomic partial charges and torsion potentials along the backbone and side chains have the greatest impact on structure and dynamics related to charge transport mechanisms in P3HT.more » « less
-
null (Ed.)Understanding the influence of polymer molecular weight on the morphology, photophysics, and photovoltaic properties of polymer solar cells is central to further advances in the design, processing, performance and optimization of the materials and devices for large scale applications. We have synthesized six number-average molecular weight ( M n ) values (21–127 kDa) of biselenophene–naphthalenediimide copolymer ( PNDIBS ) via direct heteroarylation polymerization and used them to investigate the effects of the acceptor polymer molecular weight on the charge transport, blend photophysics, blend morphology, and photovoltaic properties of all-polymer solar cells (all-PSCs) based on PNDIBS and the donor polymer PBDB-T . The short-circuit current and power conversion efficiency (PCE) of the PBDB-T : PNDIBS blend devices were found to increase with increasing M n until reaching peaks at an optimal molecular weight of 55 kDa and then decreased with further increases in M n . The maximum PCE of 10.2% observed at the optimal M n value of 55 kDa coincided with optimal blend charge transport properties, blend photophysics, and blend morphology at this critical molecular weight. Compared to the bi-continuous network of ∼5.5–6.5 nm crystalline domains with predominantly face-on molecular orientations observed at 55 kDa, a relatively disordered microstructure with larger scale phase separation was evident at higher M n while more finely packed crystalline domains were seen at 21 kDa. The sensitivity of the device efficiency to the active layer thickness was found to also depend on the PNDIBS M n value. These results highlight the importance of tuning the molecular weight of the polymer components to optimize the morphology, charge transport, photophysics and efficiency of all-polymer solar cells. The results also provide new insights on structure–property relationships for a promising n-type semiconducting copolymer.more » « less
-
Solid polymer electrolytes offer potential improvements to lithium ion batteries that include extending their operating temperature range and improving the safe use of the batteries by inhibiting lithium dendrite formation. Because solid polymer electrolytes replace traditional liquid electrolytes as the lithium ion transport medium and also act as the electrode separator, these materials must offer good ionic conductivity along with providing good interfacial contact with the electrode material. This work presents the synthesis and characterization of polymer blends comprised poly(ethylene oxide) and phosphonium ionenes. Ionenes are a class of polycation that includes positive charges within the polymer backbone. Because the positive charge is a part of the polymer chain, the spacing and distribution of these charges have a significant impact on the properties of ionenes. This research focuses on determining the role of charge spacing and distribution of charges along the backbone of phosphonium ionenes on their ability to transport lithium ions. To accomplish this, phosphonium ionenes are blended with low molecular weight poly(ethylene oxide) (e.g. less than 3,000 g/mol) at mass ratios of 20:1, 10:1, and 5:1. The resulting blended solid polymer electrolyte membranes are evaluated for their thermal, mechanical and electrochemical properties along with their charge/discharge performance in coin cell batteries. The dependence of phosphonium ionene structure as well as the composition of SPE blends will be presented.more » « less
-
Abstract The physics of charge transport across the interface in an inorganic Si/organic conducting polymer junction diode has received little attention compared to the inorganicp–nsilicon diode. One reason is the amorphous nature of the organic polymer and the polymer chain orientation which introduces disorder and barriers to charge flow. Herein we first present an easy technique to fabricate an inorganic/organic,p-Si/n-poly(benzimidazobenzophenanthroline-BBL) junction diode. The physics of charge transport across the heterojunction, and in the BBL film is then analyzed from the device current-voltage characteristics as a function of temperature in the range 150 K <T< 370 K. The temperature dependence of the diode ideality parameter and of the saturation current density demonstrate that tunneling enhanced charge recombination via exponential trap distributions in the depletion region was responsible for charge transport across the junction. Furthermore, the temperature dependence of the diode conductance revealed that thermal activation and hopping both contributed to charge transport in the BBL film away from the junction. BBL is a ladder polymer with a discrete layered crystal structure that is oriented perpendicular to the substrate. Such polymer chain orientation, combined with a distribution of bond lengths and numerous conjugation paths available for charge delocalization result in the multiple charge transport mechanisms as observed in the diode.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1126/sciadv.aaw7757
