Session P33: Organic Electronics and Photonics - Structure-Property Relationships

Molecular dynamics simulations for the study of optical properties in conjugated semiconducting molecules

Conformational disorder of conjugated polymers strongly influences their optical and electronic properties. Molecular Dynamics (MD) simulations can provide a quantitative understanding of these effects. Given the ever-expanding range of molecules with potential for device applications, it is critical to systematically establish accurate MD parameters for such simulations. We present an experimentally verified, general and optimised procedure, based on a computationally inexpensive methodology for generating the required MD parameters for conjugated molecules. By combining a large sample ($\sim$1000) of MD generated conformations with DFT calculations for the resulting electronic states we can explore the influence of conformational disorder on the optical properties. Using this scheme, we determine the effect of conformational variation on both linear and two-photon absorption spectra in a number of different conjugated semiconducting oligomers. Our results indicate that, while there exists significant inhomogeneous broadening in the linear absorption, there is only a weak conformational influence on the two-photon absorption spectrum.   

Directed self-assembly of $\pi $-conjugated oligopeptides for supramolecular electronics.

The directed mesoscale engineering of nanoscale building blocks holds enormous promise to catalyze a revolution in new functional materials for advanced electronics. Bio-inspired systems can play a key role in this effort due to their inherent ``programmable'' function. In this work, oligopeptide with defined flanking sequences was appended to $\pi $-conjugated units, thereby directing their assembly processes in a designed manner. By utilizing custom-designed microfluidic devices and controlled acid vapor diffusion, the self-assembly rate was directed and precisely tuned. Notably, the kinetics was found to play a key role in the morphology of self-assembled $\pi $-conjugated oligopeptides. The influence of flanking peptide sequences and $\pi $-conjugated core-core interactions on the self-assembly nanostructure was systematically investigated. Importantly, the electronic properties of the synthetic peptide assembly was explored by integration as the active layer of a field effect transistor. The presented study offers insights to the design and fabrication of supramolecular electronics.   

In-situ observation of dynamic processes during organic semiconductor thin film deposition and strain-stabilization of metastable states

In-situ optical spectromicroscopy in reflection mode is used to study the growth mechanisms and thermal stability of 6,13- bis(trisopropylsilylethynyl)-pentacene (TIPS-pentacene) thin films. The results show that the films form in a supersaturated state before transforming to a solid film. Molecular aggregates are observed by optical spectroscopy in this supersaturated region corresponding to subcritical nuclei in the crystallization process. During deposition on a heated substrate, a progressive blue shift of optical absorption peaks of the solid film is observed at higher deposition temperatures due to a continuous thermally driven change of the crystalline packing. As crystalline films are cooled to ambient temperature they becomes strained although cracking of thicker films is observed, which allows the strain to partially relax. Below a critical thickness of 30 nm, cracking is not observed and the films are constrained to the lattice constants corresponding to the temperature at which they were deposited. An high averaged hole mobility about 2 cm$^{\mathrm{2}}$v$^{\mathrm{-1}}$s$^{\mathrm{-1}}$ is obtained for strained TIPS-pentacene thin films deposited at 135ûC.   

Transient phases during crystallization of solution-processed organic thin films

We report an in-situ study of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C$_{\mathrm{8}}$-BTBT) organic semiconductor thin film deposition from solution via hollow pen writing, which exhibits multiple transient phases during crystallization. Under high writing speed (25 mm/s) the films have an isotropic morphology, although the mobilities range up to 3.0 cm$^{\mathrm{2}}$/V$^{\mathrm{.}}$s. To understand the crystallization in this highly non-equilibrium regime, we employ in-situ microbeam grazing incidence wide-angle X-ray scattering combined with optical video microscopy at different deposition temperatures. A sequence of crystallization was observed in which a layered liquid-crystalline (LC) phase of C$_{\mathrm{8}}$-BTBT precedes inter-layer ordering. For films deposited above 80\textordmasculine C, a transition from LC phase to a transient crystalline state that we denote as Cr1 occurs after a temperature-dependent incubation time, which is consistent with classical nucleation theory. After an additional $\approx $ 0.5s, Cr1 transforms to the final stable structure Cr2. Based on these results, we demonstrate a method to produce large crystalline grain size and high carrier mobility during high-speed processing by controlling the nucleation rate during the transformation from the LC phase.   

Critical Role of Processing on the Thermoelectric Performance of Doped Semiconducting Polymers

The ability to convert excess waste heat into useable energy can significantly help meet the global energy demands. One may capture this waste heat through thermoelectrics devices. In a thermoelectric material, the charge carriers transport both electrical current and heat. Consequently, under a temperature difference ($\Delta $T), a carrier concentration gradient results in a voltage ($\Delta $V), which is related to the Seebeck coefficient, $\alpha \quad =-\Delta $V/$\Delta $T. One of the challenges lies in finding materials that simultaneously have low thermal conductivity ($\kappa )$, high electrical conductivity ($\sigma )$, and high Seebeck coefficient ($\alpha )$. Conjugated semiconducting polymers can potentially meet this demand due to their inherent low thermal conductivity and high electrical conductivity through sufficient doping. Here, we report on the critical role of thermal processing on the enhancement of thermoelectric properties of conjugated polymer thin films. These films were doping using three different mechanisms: acid (toluene sulfonic acid), charge transfer (F$_{4}$TCNQ), and vapor (fluorinated-alkyl trichlorosilane). These thermoelectrics properties will be correlated to the structural and morphological properties of the doped thin-films through various synchrotron X-ray scattering techniques. Lastly, to further elucidate the charge transport mechanism driving the thermoelectric performance, we report on the temperature-dependent measurements of both the Seebeck coefficient and electrical conductivity.   

Influence of Molecular Shape on Molecular Orientation and Stability of Vapor-Deposited Organic Semiconductors

Physical vapor deposition is commonly used to prepare active layers in organic electronics. Recently, it has been shown that molecular orientation and packing can be tuned by changing the substrate temperature during deposition, while still producing macroscopically homogeneous films. These amorphous materials can be highly anisotropic when prepared with low substrate temperatures, and they can exhibit exceptional kinetic stability; films retain their favorable packing when heated to high temperatures. Here, we study the influence of molecular shape on molecular orientation and stability. We investigate disc-shaped molecules, such as TCTA and m-MTDATA, nearly spherical molecules, such as Alq$_{3}$, and linear molecules covering a broad range of aspect ratios, such as p-TTP and BSB-Cz. Disc-shaped molecules have preferential horizontal orientation when deposited at low substrate temperatures, and their orientation can be tuned by changing the substrate temperature. Alq$_{3}$ forms stable, amorphous films that are optically isotropic when vapor deposited over a broad range of substrate temperatures. This work may guide the choice of material and deposition conditions for vapor-deposited films used in organic electronics and allow for more efficient devices to be fabricated.   

Selective crystallization of conjugated polymers into nanowires from graphene coated surfaces.

Solution-based crystallization of conjugated polymers offers a scalable and attractive route to develop hierarchical structures for organic electronic devices, especially solar cells. The introduction of well-defined nucleation sites into metastable supersaturated solutions provides a way to regulate the crystallization behavior, and therefore the morphology of the material. We focus on metastable solutions of poly(3-hexylthiophene) (P3HT) dissolved in mixtures of m-xylene, a marginal solvent, and chlorobenzene, a good solvent. Appropriate levels of supersaturation are identified to suppress homogenous nucleation of crystals at room temperature, while allowing for crystallization on heterogeneous nucleation sites. We show that in these metastable solutions, P3HT selectively crystallizes on graphene-coated surfaces. Through in situ atomic force microscopy, we confirm that nanowires grow vertically in a face-on orientation from highly oriented pyrolytic graphite and graphene. Moreover, this method can be successfully extended to other conjugated polymers with superior electronic properties, such as poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b] thiophene]. Therefore, this method is a promising route to improve the performance of organic electronics.   

Surface induced alignment for semiflexible polymers

The interfacial structure of semiflexible polymers can largely affect the overall performance of applications, such as organic electronics. Due to backbone stiffness, semiflexible polymers, including conjugated polymers, tend to align parallel to an impenetrable surface. The segmental alignment near the wall creates a quadrupolar aligning field, which can interact with the polymer backbones and enhance the chain alignment. In the present work, we combine molecular dynamic (MD) simulations and a lattice version of self-consistent field theory (SCFT) to investigate the surface induced alignment for semiflexible polymers. Using MD simulations of bead-spring chains, we demonstrate that the thickness of the aligned layer is about a persistence length $L_p$ for semiflexible polymers in the isotropic phase. Using the SCFT lattice model, we predict that the amplitude and range of the alignment increase with increasing nematic coupling, quantified by the nematic coupling constant $\alpha$. The impenetrable surface acts as a perturbation on the chain alignment, and the nematic coupling $\alpha$ amplifies the perturbation. By comparing the SCFT results for chains near an impenetrable surface to MD simulations, we can also qualitatively estimate $\alpha$ for semiflexible polymers.   

Revealing molecular order inside and between PBTTT nanoribbons through the polarized X-ray scattering

Electronic and optical properties of conjugated polymers emerge not only through molecular architecture, but also through hierarchical ordering from the molecular to the mesoscale. Characterizing aspects of that ordering critical to properties can be challenging, however. For example, local molecular orientation within and connectivity between ordered structures is considered paramount for charge transport in conjugated polymer films. While some of these aspects can be imaged with state-of-the-art microscopy measurements, true statistical measurements of molecular order and connectivity remain elusive due to the low levels of crystalline packing that limit diffractive and other techniques capable of statistical analysis. Recently, we demonstrated how resonant scattering with polarized soft X-rays (PSoXS) is sensitive to molecular orientation and that such measurements could be used to locate sources and types of ordering within larger nanostructures on a statistical basis. Here we combine forward simulation and measurements of PSoXS on PBTTT nanoribbon films to extract critical information such as the average level of molecular alignment within nanoribbon structures and the level of connectivity between ribbons that promote the dominant charge transport mechanisms in these films. Further development of PSoXS will enable crucial insight into internal molecular order within organic materials tied to optical and electronic properties and how to control these properties for use in novel devices.   

Charge Transport in Conjugated Block Copolymers

Interest in conjugated block copolymers for high performance organic photovoltaic applications has increased considerably in recent years. Polymer/fullerene mixtures for conventional bulk heterojunction devices, such as P3HT:PCBM, are severely limited in control over interfaces and domain length scales. In contrast, microphase separated block copolymers self-assemble to form lamellar morphologies with alternating electron donor and acceptor domains, thereby maximizing electronic coupling and local order at interfaces. Efficiencies as high as 3{\%} have been reported in solar cells for one block copolymer, P3HT-PFTBT, but the details concerning charge transport within copolymers have not been explored. To fill this gap, we probed the transport characteristics with thin-film transistors. Excellent charge mobility values for electron transport have been observed on aluminum source and drain contacts in a bottom gate, bottom contact transistor configuration. Evidence of high mobility in ordered PFTBT phases has also been obtained following thermal annealing. The insights gleaned from our investigation serve as useful guideposts, revealing the significance of the interplay between charge mobility, interfacial order, and optimal domain size in organic block copolymer semiconductors.   

Selective crystallization of regioregularity controlled polythiophene for enhancing mechanical stability and electronic performance.

Considering the many potential applications of organic electronics in portable electronic devices, it is of great importance to develop an electro-active material that possesses mechanical stability and high electronic performance. Coexistence of both properties, however, is very difficult to achieve because good electronic performance is associated with long conjugation length, and high crystallinity often results in stiffness and brittleness. Herein, we utilize P3HT with two different regioregularities: high RR (98{\%}) P3HT has high electronic properties but poor mechanical resilience, and low RR P3HT (68{\%}) exhibits high elasticity and ductility but poor electronic performance. Selective crystallization of high RR P3HT induced by solution assembly allows construction of percolated networks of high RR P3HT nanowires (NWs) embedded in low RR P3HT matrix. Only 5 wt{\%} high RR P3HT is required to reach a hole mobility comparable to that of high RR P3HT, and high RR NWs embedded in film exhibits 20 times higher elongation at break. Selective self-assembly allows us to overcome the fragile nature of highly crystalline conjugated polymers without losing their electronic properties.   

Regio regularity effects on chain mobility and entanglement for poly(3-hexylthiophene)

Poly(3-hexylthiophene-2,5-diyl) (P3HT) is a conjugated polymer that can serve as the active layer in a variety of electronic devices. However, its glass transition temperature (T$_{\alpha })$ and entanglement molecular weight (M$_{e})$ are still in dispute. These parameters are essential for estimating the density of tie chains, which are hypothesized to limit the bulk charge transport. A wide range of molecular weights of both regiorandom (RRa) and regioregular (RRe) P3HT were studied by oscillatory shear rheology. Coupled with the molecular weight distribution from GPC, M$_{e}$ was extracted by fitting the linear viscoelastic data of multiple MW samples using BoB software. Furthermore, two T$_{\alpha }$s were identified for both RRe and RRa P3HT. T$_{\alpha \, }$corresponds to the segmental motion and follows Flory-Fox equation well for various MWs with 2 C \textless T$_{\alpha }$ \textless 14 C, yielding high MW limit of T$_{\alpha \infty }$ $=$ 21 C. RRe has a larger M$_{e}$ than RRa, which might originate from their different T$_{\alpha PE}$ corresponding to the side chain packing. So, further investigation on their packing lengths via dilute solution light scattering will be crucial to understand entanglement in these semiflexible polymers.   

Controlling the out-of-plane orientation of solution-processed organic semiconductor crystals

We demonstrate the ability to control out-of-plane orientation of small-molecule bis(triisopropylsilylethynyl) pyranthrene (TIPS-PY) crystals drop cast from the solution phase onto SiO$_{\mathrm{2}}$ substrates. By tuning solvent-molecule interactions through the incorporation of varying amounts of an anti-solvent during drop casting, we observed a systematic change in the crystal morphology from cross-shaped crystals to needle like crystals using optical microscopy and scanning electron microscopy. 2-D x-ray diffraction experiments revealed that this change in crystal morphology corresponded to a change in the crystallographic orientation of the crystals, from one in which the (l00) plane is parallel to the substrate surface to one in which the (00l) plane is parallel to the substrate surface. Tuning molecule-substrate interactions by modifying the surface energy of the underlying substrate was also found to affect the observed crystal orientation. Because organic semiconductor crystals display large charge transport anisotropies along different crystallographic directions, it is expected that the out-of-plane charge mobility will depend on the TIPS-PY crystal orientation.   

Birefringence and Enhanced Stability in Stable Organic Glasses

Stable glasses can be prepared by physical vapor depositing organic molecules onto a cold substrate at slow rates. These glasses have many exceptional properties such as high thermal stability, high density, and birefringence. Regardless of the molecular shape or intermolecular interactions, birefringence has been observed in various stable glasses produced at low temperatures (below 80{\%} of the molecule's glass transition temperature, T$_{g})$. Here we prepare stable glasses of an organic molecule, 9-(3,5-di(naphthalen-1-yl)phenyl)anthracene, that possesses a nearly isotropic shape and intrinsic fluorescence. Ellipsometry is used to show that all stable glasses prepared in the temperature range from 73{\%} T$_{g}$ to 97{\%} T$_{g\, }$show positive birefringence. Angle- and polarization- dependent photoluminescence measurements show isotropic molecular orientation in these optically birefringent glasses. Furthermore, the values of birefringence are strongly correlated with the enhanced density, implying a general origin of the observed anisotropy in stable glasses. This correlation can elucidate the role of packing in the formation of such high-density glasses.   

Traversing the polymorphic landscape through tuning molecule-molecule, molecule-substrate and molecule-solvent interactions

As subtle changes in the crystalline packing motif of molecular semiconductors can have a large impact on charge transport, a thorough understanding of the accessibility of polymorphs in thin films is needed. Using a series of core-chlorinated naphthalene tetracarboxylic diimides, we demonstrate that the choice of the alkyl substituents at the imide functionalities, as well as the choice of substrate and post-deposition processing conditions, tune the relative strengths of molecule-molecule, molecule-substrate and molecule-solvent interactions, providing a handle over polymorphic selection. We access the triclinic polymorph of NTCDI-CH$_{\mathrm{2}}$C$_{\mathrm{3}}$F$_{\mathrm{7}}$ in thermally evaporated thin films; solvent-vapor annealing induces a reversible transformation to its monoclinic polymorph. The addition of a fluoromethylene group in the alkyl substituent increases molecule-molecule interactions and, accordingly, improves the stability of its triclinic polymorph; this derivative does not undergo a polymorphic transformation with any of the post-deposition conditions we have explored.