Effects of chalcogen atom substitution on the optoelectronic and charge-transport properties in picene-type π-systems.

A series of π-conjugated 3,10-dialkyl-dinaphtho[1,2-b:2',1'-d]chalcogenophenes incorporating S, Se, and Te as the central chalcogen atom was newly synthesized, and their optoelectronic and charge-transport properties were systematically investigated. High carrier mobilities of up to 4.7 cm2 V-1 s-1 were achieved for solution-processed organic field-effect transistors using these materials as p-channel organic semiconductors.

Solution-processed organic thermoelectric materials exhibiting doping-concentration-dependent polarity.

Recent progress in conducting polymer-based organic thermoelectric generators (OTEGs) has resulted in high performance due to high Seebeck coefficient, high electrical conductivity (σ), and low thermal conductivity obtained by chemically controlling the materials's redox levels. In addition to improving the properties of individual OTEGs to obtain high performance, the development of solution processes for the fabrication of OTEG modules is necessary to realize large thermoelectric voltage and low-cost mass production. However, the scarcity of good candidates for soluble organic n-type materials limits the use of π-leg module structures consisting of complementary elements of p- and n-type materials because of unbalanced transport coefficients that lead to power losses. In particular, the extremely low σ of n-type materials compared with that of p-type materials is a serious challenge. In this study, poly(pyridinium phenylene) (P(PymPh)) was tested as an n-type semiconductor in solution-processed OTEGs, and the carrier density was controlled by a solution-based chemical doping process using the dopant sodium naphthalenide, a well-known reductant. The electronic structures and doping mechanism of P(PymPh) were explored based on the changes in UV-Vis-IR absorption, ultraviolet photoelectron, and X-ray photoelectron spectra. By controlling the dopant concentration, we demonstrate a maximum n-type power factor of 0.81 µW m-1 K-2 with high σ, and at higher doping concentrations, a switch from n-type to p-type TE operation. This is one of the first cases of a switch in polarity just by increasing the concentration of the reductant and may open a new route for simplified fabrication of complementary organic layers.

Seamless growth of a supramolecular carpet.

Organic/metal interfaces play crucial roles in the formation of intermolecular networks on metal surfaces and the performance of organic devices. Although their purity and uniformity have profound effects on the operation of organic devices, the formation of organic thin films with high interfacial uniformity on metal surfaces has suffered from the intrinsic limitation of molecular ordering imposed by irregular surface structures. Here we demonstrate a supramolecular carpet with widely uniform interfacial structure and high adaptability on a metal surface via a one-step process. The high uniformity is achieved with well-balanced interfacial interactions and site-specific molecular rearrangements, even on a pre-annealed amorphous gold surface. Co-existing electronic structures show selective availability corresponding to the energy region and the local position of the system. These findings provide not only a deeper insight into organic thin films with high structural integrity, but also a new way to tailor interfacial geometric and electronic structures.

Luminous butterflies: efficient exciton harvesting by benzophenone derivatives for full-color delayed fluorescence OLEDs.

Butterfly-shaped luminescent benzophenone derivatives with small energy gaps between their singlet and triplet excited states are used to achieve efficient full-color delayed fluorescence. Organic light-emitting diodes (OLEDs) with these benzophenone derivatives doped in the emissive layer can generate electroluminescence ranging from blue to orange-red and white, with maximum external quantum efficiencies of up to 14.3%. Triplet excitons are efficiently harvested through delayed fluorescence channels.

Polymorphism in 9,9-diarylfluorene-based organic semiconductors: influence on optoelectronic functions.

Polymorphism is observed in 9,9-diarylfluorene-based organic semiconductors. The effects of rich phase behavior on the charge transport and photoluminescence properties of these semiconductors are investigated. Polymorphism control is a rational way to tailor the optoelectronic functions of their films.

High performance organic field-effect transistors based on single-crystal microribbons and microsheets of solution-processed dithieno[3,2-b:2',3'-d]thiophene derivatives.

We present π-conjugated dithieno[3,2-b:2',3'-d]thiophene derivatives that act as high-performance p-type organic semiconductors. These molecules self-organize into single-crystal microribbons or microsheets. High carrier mobilities of up to 10.2 cm(2) V(-1) s(-1) and high on/off ratios of ~10(7) have been achieved in organic single-crystal field-effect transistors.

Bifunctional star-burst amorphous molecular materials for OLEDs: achieving highly efficient solid-state luminescence and carrier transport induced by spontaneous molecular orientation.

Bifunctional star-burst amorphous molecular materials displaying both efficient solid-state luminescence and high hole-transport properties are developed in this study. A high external electroluminescence quantum efficiency up to 5.9% is attained in OLEDs employing the developed amorphous materials. It is revealed that the spontaneous horizontal orientation of these light-emitting molecules in their molecular-condensed states leads to a remarkable enhancement of the electroluminescence efficiencies and carrier-transport properties.