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1. Design rules for dynamic-template-directed crystallization of conjugated polymers

   https://doi.org/10.1039/C9ME00042A  

   Mohammadi, Erfan; Qu, Ge; Kafle, Prapti; Jung, Seok-Heon; Lee, Jin-Kyun; Diao, Ying (January 2020, Molecular Systems Design & Engineering)

   The multiscale morphology and device performance of printed semiconducting polymers are highly sensitive to the substrate/ink interfacial properties during solution coating. There is an urgent need for general design rules correlating the substrate properties and conjugated polymer (CP) morphology, which do not yet exist. Dynamic surfaces are particularly promising for templating highly crystalline and highly aligned conjugated polymer thin films and have been shown in recent studies. Herein, we implement the dynamic-templating method using a series of liquid-infused nanoporous substrates as a tool to study the impact of template reconfigurability and chemistry on the multiscale morphology of conjugated polymer thin films, using a high performing donor–acceptor polymer (DPP-BTz) as a model compound. By quantifying the enthalpy of adsorption, we demonstrate that the strength of template–CP interactions directly measures the effectiveness of dynamic surfaces in promoting conjugated polymer crystallization and alignment. We further show that the enthalpy of interactions increases by enhancing the template dynamics and is sensitively modulated by template chemistry. Specifically, increasing the template–CP interactions leads to a larger domain size and higher degree of crystallinity in templated conjugated polymer thin films prepared by meniscus-guided solution coating. This observation validates our hypothesis that dynamic templates function by promoting the nucleation of conjugated polymers. We also demonstrate that such dynamic-template-dependent morphology is independent of coating speed. Notably, the enhanced morphological properties modulate the charge carrier mobility in field-effect transistors (FETs) over an order of magnitude reaching a hole mobility of 2.8 cm 2 V −1 s −1 . This work is a significant step towards establishing general guidelines on how the substrate–ink interfacial properties influence morphology and performance of solution coated CP thin films. 

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2. Directed assembly of metal nanoparticles in polymer bilayers

   https://doi.org/10.1039/C7ME00104E  

   Hsu, Su-Wen; Long, Yuhan; Subramanian, Aatheya G.; Tao, Andrea R. (January 2018, Molecular Systems Design & Engineering)

   The integration of layer-by-layer (LbL) and self-assembly methods has the potential to achieve precision assembly of nanocomposite materials. Knowledge of how nanoparticles move across and within stacked materials is critical for directing nanoparticle assembly. Here, we investigate nanoparticle self-assembly within two different LbL architectures: (1) a bilayer composed of two immiscible polymer thin-films, and (2) a bilayer composed of polymer and graphene that possesses a “hard-soft” interface. Polymer-grafted silver nanocubes (AgNCs) are employed as a model nanoparticle system for systematic experiments – characterizing both assembly rate and resulting morphologies – that examine how assembly is affected by the presence of an interface. We observe that polymer grafts can serve to anchor AgNCs at the bilayer interface and to decrease particle mobility, or can promote particle transfer between layers. We also find that polymer viscosity and polymer mixing parameters can be used as predictors of assembly rate and behavior. These results provide a pathway for designing more complex multilayered nanocomposites. 

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3. Using Graphs to Quantify Energetic and Structural Order in Semicrystalline Oligothiophene Thin Films

   https://doi.org/10.1039/C8ME00028J  

   Van, Ellen; Jones, Matthew; Jankowski, Eric; Wodo, Olga (January 2018, Molecular Systems Design & Engineering)

   In semicrystalline conjugated polymer thin films, the mobility of charges depends on the arrangement of the individual polymer chains. In particular, the ordering of the polymer backbones affects the charge transport within the film, as electron transfer generally occurs along the backbones with alternating single and double bonds. In this paper, we demonstrate that polymer ordering should be discussed not only in terms of structural but also energetic ordering of polymer chains. We couple data from molecular dynamics simulations and quantum chemical calculations to quantify both structural and energetic ordering of polymer chains. We leverage a graph-based representation of the polymer chains to quantify the transport pathways in a computationally efficient way. Next, we formulate the morphological descriptors that correlate well with hole mobility determined using kinetic Monte Carlo simulations. We show that the shortest and fastest path calculations are predictive of mobility in equilibrated morphologies. In this sense, we leverage graph-based descriptors to provide a basis for the quantitative structure-property relationships. 

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4. Near full light absorption and full charge collection in 1-micron thick quantum dot photodetector using intercalated graphene monolayer electrodes

   https://doi.org/10.1039/c9nr09901h  

   Chen, Wenjun; Ahn, Seungbae; Balingit, Marquez; Wang, Jiaying; Lockett, Malcolm; Vazquez-Mena, Oscar (February 2020, Nanoscale)

   Quantum dots (QDs) offer several advantages in optoelectronics such as easy solution processing, strong light absorption and size tunable direct bandgap. However, their major limitation is their poor film mobility and short diffusion length (<250 nm). This has restricted the thickness of QD film to ∼200–300 nm due to the restriction that the diffusion length imposes on film thickness in order to keep efficient charge collection. Such thin films result in a significant decrease in quantum efficiency for λ > 700 nm in QDs photodetector and photovoltaic devices, causing a reduced photoresponsivity and a poor absorption towards the near-infrared part of the sunlight spectrum. Herein, we demonstrate 1 μm thick QDs photodetectors with intercalated graphene charge collectors that avoid the significant drop of quantum efficiency towards λ > 700 nm observed in most QD optoelectronic devices. The 1 μm thick intercalated QD films ensure strong light absorption while keeping efficient charge extraction with a quantum efficiency of 90%–70% from λ = 600 nm to 950 nm using intercalated graphene layers as charge collectors with interspacing distance of 100 nm. We demonstrate that the effect of graphene on light absorption is minimal. We achieve a time-modulation response of <1 s. We demonstrate that this technology can be implemented on flexible PET substrates, showing 70% of the original performance after 1000 times bending test. This system provides a novel approach towards high-performance photodetection and high conversion photovoltaic efficiency with quantum dots and on flexible substrates. 

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5. Transport of charge carriers and optoelectronic applications of highly ordered metal phthalocyanine heterojunction thin films

   https://doi.org/10.1039/d1cp00889g  

   Qian, Chuan; Sun, Jia; Gao, Yongli (April 2021, Physical Chemistry Chemical Physics)

   null (Ed.)

   Organic semiconductor thin films based on polycrystalline small molecules exhibit many attractive properties that have already led to their applications in optoelectronic devices, which can be produced by less expensive and stringent processes. Conduction of electric charges typically occurs in polycrystalline organic thin films. Unavoidably, the crystalline domain size, orientation, domain boundaries and energy level of the interface affect the transport of the charge carriers in organic thin films. In this comprehensive perspective, we focus on highly ordered organic heterojunction thin films fabricated by the weak epitaxy growth method. Transport of charge carriers in these highly ordered organic heterojunction thin films was systematically studied with various characterization techniques. Recent advances are presented in high-performance optoelectronic applications based on highly ordered organic heterojunction thin films, including organic photodetectors, photovoltaic cells, photomemory devices and artificial optoelectronic synapses. 

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