Broadening of Distribution of Trap States in PbS Quantum Dot Field-Effect Transistors with High-k Dielectrics.

We perform a quantitative analysis of the trap density of states (trap DOS) in PbS quantum dot field-effect transistors (QD-FETs), which utilize several polymer gate insulators with a wide range of dielectric constants. With increasing gate dielectric constant, we observe increasing trap DOS close to the lowest unoccupied molecular orbital (LUMO) of the QDs. In addition, this increase is also consistently followed by broadening of the trap DOS. We rationalize that the increase and broadening of the spectral trap distribution originate from dipolar disorder as well as polaronic interactions, which are appearing at strong dielectric polarization. Interestingly, the increased polaron-induced traps do not show any negative effect on the charge carrier mobility in our QD devices at the highest applied gate voltage, giving the possibility to fabricate efficient low-voltage QD devices without suppressing carrier transport.

Suppressing molecular vibrations in organic semiconductors by inducing strain.

Organic molecular semiconductors are solution processable, enabling the growth of large-area single-crystal semiconductors. Improving the performance of organic semiconductor devices by increasing the charge mobility is an ongoing quest, which calls for novel molecular and material design, and improved processing conditions. Here we show a method to increase the charge mobility in organic single-crystal field-effect transistors, by taking advantage of the inherent softness of organic semiconductors. We compress the crystal lattice uniaxially by bending the flexible devices, leading to an improved charge transport. The mobility increases from 9.7 to 16.5 cm(2) V(-1) s(-1) by 70% under 3% strain. In-depth analysis indicates that compressing the crystal structure directly restricts the vibration of the molecules, thus suppresses dynamic disorder, a unique mechanism in organic semiconductors. Since strain can be easily induced during the fabrication process, we expect our method to be exploited to build high-performance organic devices.

Thin Films of Highly Planar Semiconductor Polymers Exhibiting Band-like Transport at Room Temperature.

We report the observation of band-like transport from printed polymer thin films at room temperature. This was achieved from donor-acceptor type thiophene-thiazole copolymer that was carefully designed to enhance the planarity of the backbone and the resulting transfer integral between the macromolecules. Due to the strong molecular interaction, the printed polymer film exhibited extremely low trap density comparable to that of molecular single crystals. Moreover, the energy barrier height for charge transport could be readily reduced with the aid of electric field, which led formation of extended electron states for band-like charge transport at room temperature.

High mobility and low density of trap states in dual-solid-gated PbS nanocrystal field-effect transistors.

Dual-gated PbS nanocrystal field-effect transistors employing SiO2 and Cytop as gate dielectrics are fabricated. The obtained electron mobility (0.2 cm(2) V(-1) s(-1) ) and the high on/off ratio (10(5) -10(6) ), show that the controlled nanocrystal assembly (obtained with self-assembled monolayers), as well as the trap density reduction (using Cytop as dielectric), are crucial steps for the future application of nanocrystals.

Temperature-independent transport in high-mobility dinaphtho-thieno-thiophene (DNTT) single crystal transistors.

The angular and temperature dependence of the field-effect mobility are investigated for p-type DNTT single crystals in a vacuum-gap structure. Temperature-independent transport behavior and weak mobility anisotropy are observed, with the best mobility approaching 10 cm(2) V(-1) s(-1) . Structural characterization and simulation suggest exceptionally high-quality and high-purity crystals.