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Spotlight Archives

Tuning Out-of Plane Molecular Orientation of Contorted Hexabenzocoronene

Spherulites of contorted hexabenzocoronene (HBC) featured on the cover of DPOLY's program booklet. Through post-deposition processing of amorphous HBC films, we can tune the out-of-plane molecular orientation, yielding transistors having variable performance.

 

Guiding Crystallization Around Sharp Corners and Bends

Control over the molecular orientation in organic thin film is demonstrated with precise in-plane spatial resolution over large areas. By exploiting the differential crystallization rates on substrates having different surface energies, the radial symmetry of spherulitic growth can be disrupted by preferentially selecting the molecular orientations that promote growth along the paths of the underlying patterns.

 

Orientation-Independent Charge Transport in Single Spherulites from Solution-Processed Organic Semiconductors

By testing an array of transistors fabricated on a single solution-processed, organic semiconductor spherulite, we found device mobilities to be independent of the general π-stacking direction of molecules in the active channels. This finding is surprising in light of reports on single-crystal transistors, in which charge transport is reported to be fastest along the π-stacking direction. We conclude that charge transport within spherulites is dominated by shallow traps at low-angle intraspherulite grain boundaries, smearing out any charge transport anisotropies that may exist along different crystallographic directions of the organic semiconductor.

   

Mesostructures of Polyaniline Films Affect Polyelectrochromic Switching

The mesoscopic structure of polymer acid-doped polyaniline films heavily influences the kinetics of polyelectrochromic transitions. We demonstrate an inverse correlation between internal surface area and electrochromic switching times.

   

Small-Molecule Thiophene-C60 Dyads As Compatibilizers in Inverted Polymer Solar Cells

The extent to which thiophene-C60 compatibilizers can reduce interfacial energy and prevent domain coarsening and macrophase separation in P3HT/PCBM photovoltaic active layers depends strongly on their molecular architecture. By tuning the length of the thiophene segment, macrophase separation can be effectively suppressed, even upon extended thermal annealing.


 

 

 

Device Characteristics of Bulk-Heterojunction Polymer Solar Cells are Independent of Interfacial Segregation of Active Layers

The ability to reverse the composition profile of active layers by delaminating and transfer-ring P3HT:PCBM thin films has allowed us to probe directly the sole influence of interfacial segregation on solar cell device characteristics on the same device platform.

   

Polyaniline Exhibiting Stable and Reversible Switching in the Visible Extending into the Near-IR in Aqueous Media

Polyaniline template polymerized with poly(2-acyrlamido- 2-methyl-1-propanesulfonic acid) can be polyelectrochromically switched between transparent, green, and purple oxidation states within a (1 V window in aqueous media. Solvent annealing in dichloroacetic acid stabilizes the polyelectrochromic response and imparts near-infrared contrast between the colored and transparent forms.

 
 

Directly Patternable, Highly Conducting Polymers for Broad Applications in Organic Electronics

Post-processing solvent annealing of water-dispersible conducting polymers greatly enhances electrical conductivity by inducing dramatic structural rearrangement. Once treated, these materials can serve as promising alternatives to metal electrodes in organic thin-film transistors, light-emitting diodes, and solar cells.

   

Altering the Thermodynamics of Phase Separation in Inverted Bulk-Heterojunction Organic Solar Cells

By incorporating nonvolatile additives into blends of P3HT:PCBM, the efficiencies of inverted bulk-heterojunction solar cells are improved almost twofold (see figure). These additives selectively partition into the P3HT phase, effectively increasing phase separation between the P3HT and PCBM.

 

 

Controlling Nucleation and Crystallization in Solution-Processed Organic Semiconductors for Thin-Film Transistors

Incorporating small amounts of FTES-ADT in solution-processed thin films of TES-ADT enables fine-tuned control of grain size across 3 orders of magnitude. Using  this process, we were able to correlate OTFT device mobility to grain size in the active layer.  Effective regulation of seeding also eliminates variations in the performance of OTFT active layers, thereby enhancing reliability and reproducibility.

   

Progress and Challenges in Commercialization of Organic Electronics

The field of organic electronics is entering its commercial phase. The recent market introduction of the first prototypes based on organic transistors fabricated from solution is set to augment the existing market presence of organic light-emitting diode applications. Organic photovoltaic products are not far behind. This review provides a brief overview of these devices, examining performance requirements and addressessing the remaining technical challenges in the optimization of next-generation devices.

 

 

Polymer Acid Doped Polyaniline Is Electrochemically Stable Beyond pH 9

Polyaniline (PANI) is a conducting polymer that is protonically doped to its conductive state. Unlike small molecule proton sources, polymer acid dopants can provide long-term stability and water dispersibility to the conductive form of PANI. Our previous research has shown that using poly(2-acrylamido-2-methyl-1-propanesulfonic acid), or PAAMPSA, as a dopant confers the highest reported conductivity for polymer acid-doped PANI systems; we now report stable and reversible PANI-PAAMPSA redox transitions in excess of pH 9 -- another record for polymer acid-doped PANI.

   

Establishing Efficient Electrical Contact to the Weak Cyrstals of Triethylsilylethynyl Anthradithiophene

Triethylsilylethynyl anthradithiophene (TES ADT) forms weak van der Waals crystals in the solid state because its bulky TES side groups limit intermolecular interactions. TES ADT thus melts easily and locally when it experiences heat conduction from the metal evaporation process to form electrical contacts. As such, bottom-contact TES ADT thin-film transistors always outperforms their top-contact counterparts, a contrast to transistors made with other common organic semiconductors.

 

 

Ultra-Low-k Materials Derived from Poly(D,L-lactide-b-pentafluorostyrene) Diblock Copolymers

Microphase separation of poly(D,L-lactide-b- pentafluorostyrene) results in discrete poly(D,L-lactide) nanostructures uniformly dispersed in a poly(pentafluorostyrene) matrix. Acid-catalyzed hydrolysis of the poly(D,L- lactide) block yields a nanoporous fluorinated matrix with well-defined lattice dimensions and an ultra-low-k dielectric constant. The size and shape of the resulting nanopores in the fluorinated matrix can be tuned by varying the initial poly(D,L-lactide) volume fraction, permitting precise control of the dielectric constant.

   

Improving the Electrical Conductivity of Polymer-Acid-Doped Polyaniline by Controlling the Template Molecular Weight

Polymerization of aniline in the presence of poly(2-acrylamido-2-methyl-1-propanesulfonic acid), or PAAMPSA, yields water-dispersible polyaniline whose conductivity depends on PAAMPSA molecular weight, and is amongst the highest recorded for polymer acid-templated polyaniline systems.

 

 
 

Solution-Processsable Organic Semiconductors for Thin-Film Transistors: Opportunities for Chemical Engineers

Solution-processable organic semiconductors enable low-cost and straightforward fabrication of thin-film transistors. The introduction of organic solvents during processing also allows the manipulation of the organic semiconductor morphology and structure, thereby imparting control over device characteristics. This review discusses recent developments of three classes of solution-processable organic semiconductors, including polymer semiconductors, soluble organic semiconductor precursors, and organic semiconductors with solubilizing side groups.

   

Improved Organic TFTs through Solvent-Vapor Annealing

Exposing triethylsilylethynyl antradithiophene (TES ADT) thin film transistors to dichloroethane vapor induces structural rearrangement and crystallization of TES ADT, as shown on the cover (lower section, before annealing, upper section, after). The annealing results in drastic increases in on currents; the saturation mobility increases by two orders of magnitude and the current-voltage hysteresis is largely eliminated.

 

 

Heterogeneous ATRP of Block Copolymer Hydrogels

By polymerizing at the compositional azeotrope, we are able to make statistical copolymers of poly(hydroxyethyl methacrylate-co-dimethylaminoethyl methacrylate) of uniform compositions and narrow molecular weight distributions. When polymerized from a polystyrene macroinitiator, the resulting diblock copolymers spontaneously self assemble to form periodic nanostructures.