Spontaneous formation of metastable orientation with well-organized permanent dipole moment in organic glassy films

High-density stable glasses formed on soft substrates

Enabled by surface-mediated equilibration, physical vapour deposition can create high-density stable glasses comparable with liquid-quenched glasses aged for millions of years. Deposition is often performed at various rates and temperatures on rigid substrates to control the glass properties. Here we demonstrate that on soft, rubbery substrates, surface-mediated equilibration is enhanced up to 170 nm away from the interface, forming stable glasses with densities up to 2.5% higher than liquid-quenched glasses within 2.5 h of deposition. Gaining similar properties on rigid substrates would require 10 million times slower deposition, taking ~3,000 years. Controlling the modulus of the rubbery substrate provides control over the glass structure and density at constant deposition conditions. These results underscore the significance of substrate elasticity in manipulating the properties of the mobile surface layer and thus the glass structure and properties, allowing access to deeper states of the energy landscape without prohibitively slow deposition rates.

n-Type boron β-diketone-containing conjugated polymers for high-performance room temperature ammonia sensors

Organic semiconductor (OSC) gas sensors with good mechanical flexibility have received considerable attention as commercial and wearable devices. However, due to poor resistance to moisture and low conductivity, the improvement in the sensing capability of individual OSCs is limited. Reported here is a promising pathway to construct a series of conjugated organic polymers (COPs) with well-defined pyrimidine (Py-COP) or boron β-diketone (BF-COP) units. Unlike traditional metal- or carbon-based hybrid materials, the developed COPs can provide abundant absorption sites for gaseous analytes. As a result, the as-prepared BF-COP results in an excellent sensing response of over 1500 (Ra/Rg) toward 40 ppm of NH3 at room temperature, which is the highest value among those of pristine COPs as n-type sensing materials. Notably, they can maintain their initial sensing responses for two months and 90% relative humidity resistance. Combining the results of in situ Fourier transform infrared spectroscopy and theoretical calculations, the β-diketone skeleton is found to activate the surface electronic environment, verifying that the electron-deficient B ← O groups are adsorption centers. The B/N-heterocyclic decoration effectively modulates the redox properties and electronic interactions, as well as perturbs charge transfer in typical π-conjugated COPs. These results offer insight into developing highly efficient OSC gas sensors, which potentially have broadened sensing applications in the areas of organoboron chemistry.

Impact of Intermittent Deposition on Spontaneous Orientation Polarization of Organic Amorphous Films Revealed by Rotary Kelvin Probe

The control of the molecular orientation and resultant polarization is essential for improving the performance of organic optoelectronic devices. Conventionally, the substrate temperature and deposition rate are tuned to control the molecular orientation of vapor-deposited films. In this study, we proposed a novel method, referred to as "intermittent deposition", in which the polarization direction and magnitude are controlled by introducing intervals during physical vapor deposition. The rotary Kelvin probe measurement of the Alq3 and TPBi films clearly showed a time-dependent decrease in the surface potential owing to the surface relaxation of the molecular orientation immediately after deposition. Through a series of intermittent depositions, in which the deposition shutter is repeatedly opened and closed at certain intervals, a relaxed surface layer was built up, and we could control the polarization magnitude. For the Alq3 film, even the polarization direction was switched. The proposed new deposition method is applicable to general organic molecules, not limited to polar molecules, thereby potentially tuning the conduction properties of organic devices and fabricating novel devices.

Controlling Spontaneous Orientation Polarization in Organic Semiconductors─The Case of Phosphine Oxides

Upon film growth by physical vapor deposition, the preferential orientation of polar organic molecules can result in a nonzero permanent dipole moment (PDM) alignment, causing a macroscopic film polarization. This effect, known as spontaneous orientation polarization (SOP), was studied in the case of different phosphine oxides (POs). We investigate the control of SOP by molecular design and film-growth conditions. Our results show that using less polar POs with just one phosphor-oxygen bond yields an exceptionally high degree of SOP with the so-called giant surface potential (slope), reaching more than 150 mV nm-1 in a neat bis-4-(N-carbazol(yl)phenyl)phenyl phosphine oxide (BCPO) film grown at room temperature. Additionally, by altering the evaporation rate and substrate temperature, we are able to control the SOP magnitude over a broad range from 0 to almost 300 mV nm-1. Diluting BCPO in a nonpolar host enhances the PDM alignment only marginally, but combining temperature control with dipolar doping can result in highly aligned molecules with more than 80% of their PDMs standing upright on the substrate on average.

Spontaneous orientation polarization of flavonoids

Spontaneous orientation polarization (SOP) is macroscopic electric polarization that is attributed to a constant orientational degree of dipole moments of polar molecules on average. The phenomenon has been found in small molecules like H2O at low temperatures and π-conjugated molecules employed in organic light-emitting diodes. In this study, we demonstrate that a thin film of baicalein, a flavonoid compound found in natural products, exhibits SOP and resultant giant surface potential (GSP) exceeding 5500 mV at a film thickness of 100 nm. Vacuum-deposition of baicalein under high vacuum results in smooth and amorphous films, which enables the generation of GSP with a slope of 57 mV/nm in air, a value comparable to the representative of an organic semiconductor showing GSP, tris(8-hydroxyquinoline)aluminum(III) (Alq3). We also found the superior photostability of a baicalein film compared to an Alq3 film. These findings highlight the potential of baicalein in new applications to organic electronics.

Indolocarbazoles with Sterically Unrestricted Electron-Accepting Anchors Showcasing Aggregation-Induced Thermally Activated Delayed Mechanoluminescence for Host-Free Organic Light-Emitting Diodes

We investigated the effects of sterically nonrestricted electron-accepting substituents of three isomeric indolocarbazole derivatives on their aggregation-induced emission enhancement, mechanochromic luminescence and thermally activated delayed fluorescence. The compounds are potentially efficient emitters for host-free organic light-emitting diodes. The films of indolocarbazole derivatives exhibit emissions with wavelengths of fluorescence intensity maxima from 483 to 500 nm and photoluminescence quantum yields from 31 to 58%. The ionization potentials of the solid samples, measured by photoelectron emission spectrometry, are in the narrow range of 5.78-5.99 eV. The electron affinities of the solid samples are in the range of 2.99-3.19 eV. The layers of the derivatives show diverse charge-transporting properties with maximum hole mobility reaching 10-4 cm2/Vs at high electric fields. An organic light-emitting diode with a light-emitting layer of neat compound shows a turn-on voltage of 4.1 V, a maximum brightness of 24,800 cd/m2, a maximum current efficiency of 12.5 cd/A and an external quantum efficiency of ca. 4.8%. When the compounds are used as hosts, green electroluminescent devices with an external quantum efficiency of ca. 11% are obtained. The linking topology of the isomeric derivatives of indolo[2,3-a]carbazole and indolo[3,2-b]carbazole and the electron-accepting anchors influences their properties differently, such as aggregation-induced emission enhancement, mechanochromic luminescence, thermally activated delayed fluorescence, charge-transporting, and electroluminescent properties. The derivative indolo[3,2-b]carbazole displays good light-emitting properties, while the derivatives of indolo[2,3-a]carbazole show good hosting properties, which make them useful for application in electroluminescent devices.

Synthesis of 2,5,8-Tris(1-phenyl-1H-benzo[d]imidazol-2-yl)benzo[1,2-b:3,4-b':5,6-b″] Trithiophenes and Their Spontaneous Orientation Polarization in Thin Films

To investigate the relationship between molecular structures and spontaneous orientation polarization (SOP) in organic thin films, 2,5,8-tris(1-phenyl-1H-benzo[d]imidazol-2-yl)benzo[1,2-b:3,4-b':5,6-b″] trithiophene (TPBTT) and its ethyl derivative (m-ethyl-TPBTT) were synthesized. Variable angle spectroscopic ellipsometry and two-dimensional grazing-incidence wide-angle X-ray scattering showed that the vacuum-deposited films of TPBTT and m-ethyl-TPBTT had a higher degree of molecular orientation parallel to the substrate compared with that of prototypical 2,2',2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) due to the larger π-conjugated benzotrithiophene core. However, TPBTT films showed a lower SOP of +54.4 mV/nm than did the TPBi film (+77.3 mV/nm), indicating that the molecular orientation alone did not determine the SOP. In contrast, m-ethyl-TPBTT showed a larger SOP of +104.0 mV/nm in the film. Quantum chemical calculations based on density functional theory suggested that the differences in the stable molecular conformation and the permanent dipole moments between TPBTT and m-ethyl-TPBTT caused the differences in SOP. These results suggest that the simultaneous control of the orientational order and conformation of the molecules is important to achieving a large SOP in films.

Hidden singularities in spontaneously polarized molecular solids

Films of dipolar molecules formed by physical vapor deposition are, in general, spontaneously polarized, with implications ranging from electron transfer in molecular optoelectronic devices to the properties of astrochemical ices in the interstellar medium. Polarization arises from dipole orientation, which should intuitively decrease with increasing deposition temperature, T. However, it is experimentally found that minimum or maximum values in polarization vs T may be observed for cis-methyl formate, 1-propanol, and ammonia. A continuous analytic form of polarization vs T is developed, which has the property that it is not differentiable at all T. The minima and maxima in polarization vs T are marked by singularities in the differential of this analytic form. This exotic behavior is presently unique to films of dipolar species and has not been reported, for example, in the related magnetic phases of spin glasses.

Process Engineered Spontaneous Orientation Polarization in Organic Light-Emitting Devices

Polar molecules with appreciable permanent dipole moments (PDMs) are widely used as the electron transport layer (ETL) in organic light-emitting devices (OLEDs). When the PDMs spontaneously align, a macroscopic polarization field can be observed, a phenomenon known as spontaneous orientation polarization (SOP). The presence of SOP in the ETL induces considerable surface potential and charge accumulation that is capable of quenching excitons and reducing device efficiency. While prior work has shown that the degree of SOP is sensitive to film processing conditions, this work considers SOP formation by quantitatively treating the vapor-deposited film as a supercooled glass, in analogy to prior work on birefringence in organic thin films. Importantly, the impact of varying thin-film deposition rate and relative temperature is unified into a single framework, providing a useful tool to predict the SOP formation efficiency for a polar material, as well as in blends of polar materials. Finally, in situ photoluminescence characterization and efficiency measurements reveal that SOP-induced exciton-polaron quenching can be reduced through an appropriate choice of processing conditions, leading to enhanced OLED efficiency.

Morphology-Dependent Optoelectronic Properties of Pentacene Nanoribbon and Nanosheet Crystallite

Organic, single crystals have emerged as unique optoelectrical materials due to their highly ordered structure and low defects. In this work, pentacene nanoribbons and nanosheets were selectively fabricated by controlling their growth temperature. The results show that their photoluminescence (PL) activity and electrical properties were strongly dependent on their geometrical morphology and molecular stacking mode such as the degree of π-orbital overlap and intermolecular interaction. The pentacene nanoribbon crystal exhibited a higher PL intensity compared with the nanosheet configuration; conversely, its electrical conductivity was poor. The low-temperature PL measurement indicated that there are stronger π-π stacking interactions in the nanosheet crystal than in the nanoribbon crystal, leading to exciton quenching and higher conductivity. Our study demonstrated that a unique optoelectronic property of organic crystals can be obtained by controlling the crystal's morphology, which offers potential guidance for the future design and development of organic crystal optoelectronics.