Hung Phan

High‐Mobility Field‐Effect Transistors Fabricated with Macroscopic Aligned Semiconducting Polymers

A record high OFET hole mobility, as high as 23.7 cm(2) /Vs, is achieved in macroscopic aligned semiconducting polymers. The high mobility is insensitive to the polymer molecular weight. Polymer chains are aligned along the fiber to facilitate intrachain charge transport.

Effects of Stereoisomerism on the Crystallization Behavior and Optoelectrical Properties of Conjugated Molecules

Three stereoisomers of DPP(TBFu)2 are separated and identified to investigate the effects of stereoisomerism on crystal structures and the optoelectrical properties. The crystal structures and FET mobility are sensitive to stereoisomers, in which the mesomer possesses the highest carrier mobility and the greatest crystallization tendency to dominate the crystallization in spin-cast films of the as-synthesized stereoisomeric mixture.

Direct Observation of Doping Sites in Temperature-Controlled, p-Doped P3HT Thin Films by Conducting Atomic Force Microscopy

The distribution of dopant sites in doped poly(3-hexylthiophene) (P3HT) thin films is characterized using optical absorption, grazing-incidence X-ray diffraction, and conducting atomic force microscopy (c-AFM). It is shown that dopant sites can be directly observed using c-AFM and that the solution temperature dramatically impacts phase separation and conductivity in spin-cast films.

Understanding TiO2 size-dependent electron transport properties of a graphene-TiO2 photoanode in dye-sensitized solar cells using conducting atomic force microscopy

Conducting AFM reveals a continuous conduction network of a TiO2 -graphene composite in DSSC due to a more intimate contact between the smaller sized TiO2 -graphene composite nanosheets,which reduces the internal resistance at TiO2 /TiO2 and TiO2 /FTO interfaces and ultimately leads to a faster and more efficient electron transport in the photoanode.

Electrical Instability Induced by Electron Trapping in Low‐Bandgap Donor–Acceptor Polymer Field‐Effect Transistors

The mechanism of electrical instability and the double slope of p-type organic field-effect transistors (OFETs) fabricated from low-bandgap donor-acceptor copolymers are resolved. Those polymers enable electron conduction in the device, which leads to electron trapping and consequent formation of -SiO(-). This causes a turn-on voltage shift, hole-mobility increase, and double-slope occurrence. These findings tremendously impact the molecular design and device engineering of OFETs.

Hole Mobility and Electron Injection Properties of D-A Conjugated Copolymers with Fluorinated Phenylene Acceptor Units

A novel wide-gap conjugated polymer PhF2,5 (Eg = 1.9 eV) is designed to contain alternating cyclopentadithiophene and difluorophenylene unit with the goal of favoring unipolar organic field effect transistor characteristics. The higher lowest unoccupied molecular orbital energy of PhF2,5 increases the barrier to electron injection, leading to unipolar transport and higher on/off ratios, without sacrificing desirable high hole mobilities.

Enhancement of the Photoresponse in Organic Field-Effect Transistors by Incorporating Thin DNA Layers**

A mechanistic study of the DNA interfacial layer that enhances the photoresponse in n-type field-effect transistors (FET) and lateral photoconductors using a solution-processed fullerene derivative embedded with disperse-red dye, namely PCBDR, is reported. Incorporation of the thin DNA layer simultaneously leads to increasing the electron injection from non-Ohmic contacts into the PCBDR active layer in dark and to increasing the photocurrent under irradiation. Such features lead to the observation of the enhancement of the photoresponsivity in PCBDR FETs up to 10(3) . Kelvin probe microscopy displays that in the presence of the DNA layer, the surface potential of PCBDR has a greater change in response to irradiation, which is rationalized by a larger number of photoinduced surface carriers. Transient absorption spectroscopy confirms that the increase in photoinduced carriers in PCBDR under irradiation is primarily ascribed to the increase in exciton dissociation rates through the PCBDR/DNA interface and this process can be assisted by the interfacial dipole interaction.

Electronic properties of conjugated polyelectrolyte/single-walled carbon nanotube composites.

Two narrow-bandgap conjugated polyelectrolytes (CPEs) of identical backbone structure but different pendant charges are used to disperse single-walled carbon nanotubes (SWNTs) in MeOH. Films of the resulting CPE:SWNT composites have electrical conductivity dependent on the SWNT loading, which can be increased with acid vapor treatment. The anionic CPE gives higher electrical conductivity for the composite immediately after deposition, whereas a more-significant increase is observed for the cationic counterpart after acid treatment.

Unraveling the cooperative synergy of zero-dimensional graphene quantum dots and metal nanocrystals enabled by layer-by-layer assembly

Recent years have witnessed a cornucopia of synthetic methods for fabricating carbon–metal nanocomposites. Nonetheless, achieving a cooperative synergy of zero-dimensional carbon nanomaterials and metal nanocrystals is still uncommon. To this end, we performed a controllable structural design comprising customizable alternating layers of active and functional zero-dimensional nanomaterials, which were intimately assembled together. This unique highly-ordered multilayer configuration was afforded by a judicious layer-by-layer (LbL) assembly strategy, enabling the rational and tunable construction of a series of well-defined metal/graphene quantum dots (M/GQDs)n (M = Au, Ag, Pt) multilayers. This strategy allows the direct assembly of customized units of positively-charged graphene quantum dots (GQDs) and negatively-charged metal nanocrystals (NCs), which were integrated in an alternating stacked fashion under a pronounced electrostatic attractive interaction. Moreover, these multilayer thin films demonstrate remarkably efficient and versatile catalytic performance toward the selective organic transformation of aromatic nitro compounds, electrocatalytic methanol oxidation and photoelectrochemical water splitting under simulated solar light irradiation under ambient conditions, attributed to the cooperative synergy of the metal NC and GQD building blocks. More significantly, the catalytic performances of the (M/GQDs)n (M = Au, Ag, Pt) multilayer thin films are tunable via the assembly cycle and sequence, as well as by selecting different metal NC types. This work highlights the significance of the customizable design of GQDs–metal-based systems for various advanced chemical-to-energy conversion applications.

Understanding the Device Physics in Polymer-Based Ionic–Organic Ratchets

High-performance solution-processed ionic-organic ratchets are fabricated using polymer semiconductors. The devices can provide both high short-circuit current and open-circuit voltage at room temperature, and be driven by AC signals with frequencies up to 13.56 MHz. The effects of trap density, mobility, and rectification ratio in the device on short-circuit current are investigated and clarified.