Organic Optoelectronic Materials (Volume II)

Organic optoelectronic materials are rapidly being commercialized at present. The liquid crystal display (LCD) is one of the most successful examples of commercialization of organic optoelectronic materials. The phase or polarization state of light can be modulated by an external electric field. Modulation efficiency can be enhanced provided that the organic medium has a large optical or electrical anisotropy. On the other hand, nanosized organic materials with physical anisotropy can also be used as sensors to detect external changes of environment. In addition, materials also have potential as energy-converting and -harvesting components. Following the first volume, we expanded the scope of the topic and invited researchers to submit papers on various optoelectronic materials and devices. The issue includes the synthesis of new materials, analysis of physicochemical properties, and fabrication and instrumentation of optoelectronic devices.

Recently, interest in cholesteric liquid crystals (ChLCs) has grown and actively studied in non-display applications because of their helical structure. Sohn et al. demonstrate a simple fabrication method of a uniform-lying-helix (ULH) cholesteric liquid crystal cell for phase grating device applications [1]. The ULH state was stabilized by fabricating the pitches of ChLCs as half of the cell gap to obtain the fingerprint texture with homeotropic anchoring. With the given grating period, the diffraction efficiency of the ULH cell can be maximized by optimizing the cell gap. It was found that the fabricated grating device can provide a large diffraction angle of 10° and a low operating voltage of 3 V with a diffraction efficiency of 30%. The demonstration suggests the potential applications for diffraction optics, optical interconnect, and beam steering devices.

Switchable window (smart window) has been a rapidly developing innovative technology to provide a comfortable environment for occupants while saving energy for heating, cooling, and artificial lighting. Ji et al. proposed a novel approach to reduce the energy consumption of the smart window by using a guest-host liquid crystal (GHLC) cell [2]. The angular-selective absorption of the GHLC cell was carefully investigated and it was found that angular-selective absorption is desired because the light transmitted through windows during the daytime is predominantly obliquely incident from direct sunlight. Owing to the absorption anisotropy of guest dichroic dyes, a GHLC cell can absorb the obliquely incident light, while allowing people to see through windows in normal view. Therefore, the cell can provide a comfortable environment for occupants, and reduce the energy required for cooling by blocking the solar heat incident from the oblique direction. The GHLC cell can be switched between the transparent and opaque states for normal viewing. The rising (falling) time was 6.1 (80.5) ms when the applied voltage was 10 V.

In the LCD application, deriving an optimum resistance level of an LC alignment polyimide (PI) layer is important because of the trade-off between the voltage holding and surface-discharging properties. Ko et al. introduced a photocontrolled variable-resistivity PI layer to systematically investigate the voltage holding and discharging properties of the FFS n-LC modes, according to the PI resistivity (ρ) levels [3]. The resistivity level of the optically modulated PI varied from ρ = 0.95 × 10¹⁵ to ρ = 5.36 × 10¹³ Ω∙cm within a wide tunable range by controlling the Ar laser irradiation intensity. In addition, the frequency-dependent temporal transmittance properties and dynamic residual voltage curves are measured according to the photocontrolled resistivity variation. The proposed experimental scheme is a feasible approach in the delicate material engineering of LC alignment PI layers required to resolve PI-dependent image flickering and sticking issues occurring in low-frequency-driven FFS n-LC modes adopted for power-saving operation in mobile display panels.

Ganonoo et al. developed multilayer thin films composed of titanium nitride (TiN) and aluminum nitride (AlN) for advanced and cheap photovoltaic applications [4]. To achieve a high film growth rate, the TiN and AlN layers were deposited by radio frequency reactive sputtering at room temperature. Aesthetically considerable blue, green, and yellow color glasses were fabricated from the derivates of the glass/ITO/TiN/AlN multilayer structure. A uniform color glass with high transmittance of more than 80% is demonstrated. Moreover, the advancement in color design freedom was verified by a simulation based on wave optics of the TiN/AlN multilayer. This development implies a good potential in commercial BIPV system applications.

Together with the development of fine-pitch 3D wafer-level packaging in the semiconductor industry, concerns have been raised over minimizing the damage to the probe and micro-interconnect structures. Le et al. improved the current-carrying capacity (CCC) and minimizing damage to the probe by fabricating an Au-NiCo probe which has an Au layer inside the NiCo and an Au layer outside the NiCo surface [5]. This design allowed the device to enhance high CCC (150 % higher), have less contact force, stress, and deformation of various interconnect structures compared to the NiCo probe. The maximum allowable current capacity of the 5000-µm-long Au–NiCo probe was 750 mA. These results indicate that the proposed Au–NiCo probe will be a prospective candidate for advanced wafer-level testing, with better probing efficiency and higher test yield and reliability than the conventional vertical probe.

Interestingly, Tan et al. found that combining two crystals—a pure crystal phase 1 with green-yellow emission and a CHCl₃-containing co-crystal phase (1•2CHCl₃) with orange-red emission—can modulate the structure properties of inorganic single crystals [6]. A cyclic chalcone derivative was synthesized and its pure crystal phase 1 was demonstrated to show attractive photoinduced motion behavior, indicating that this type of cyclic chalcone is a promising candidate for the construction of crystal machines. The CHCl₃-containing co-crystal phase 1•2CHCl₃ was prepared by regulating the crystallization environment. Crystal 1•2CHCl₃ shows relatively large-redshifted emission compared with crystal 1, which is ascribed to its special halogen bond network structure. This would blueshift its emission to green as 1 loses CHCl₃, further confirming the key effect of the solvent molecule on crystal emission color. These findings suggest that some solvent molecules could also function as important contributors or participants in fine-tuning the molecular orbitals via noncovalent interactions, such as halogen bonding, despite their non-conjugated structure. This in-depth crystal structure investigation together with the theoretical calculations gives clear evidence that: (1) the special “molecular pair” structure in crystal effectively facilitates the LUMO orbitals’ overlap and thus endows the crystal with (2 + 2) cycloaddition reactivity and then triggers the crystal jumping behavior; (2) the CHCl₃ molecules in crystal 1•2CHCl₃ form clusters and tie fluorescent molecules together via abundant halogen bonds, thus forming a halogen bond network system with lower energy gap compared with the solvent-free crystal 1 system. This work would overturn the previous perception that the structurally simple solvent molecules without conjugation cannot greatly affect the structure and properties of pure organic single crystals and provide a new strategy to construct multi-colored organic fluorescent crystals.

Nguyen et al. applied a frequency-modulated interferometer in a novel, compact, and high-precision axial error measurement [7]. A sinusoidal injection current frequency of 3 MHz was used to modulate the frequency of a laser diode (LD), and then the modulated LD was used as the light source of the Michelson interferometer. The axial error was determined from the interference signal of the interferometer using the synchronous detection method. Measurement resolution in the order of approximately 10 nm can be achieved. Moreover, the primary experimental result proved that modulation frequency up to 20 MHz for the LD was possible. The harmonics can be seen clearly by the FFT analysis. The system can operate accurately even though vibration and environmental disturbances are present.

Recent developments in reducing the pixel size of complementary metal-oxide semiconductor (CMOS) image sensors have led to the increasing of the noise source due to dark current, thus lowering the quality of the image. Chai et al. analyzed the effects of carbon implantation on the fluorine diffusion and the dark current characteristics of the CMOS image sensor [8]. As the carbon was implanted with doses of 5.0 × 10¹⁴ and 1 × 10¹⁵ ions/cm² in the N+ area of the FD region, the retained dose of fluorine was improved by more than 131% and 242%, respectively, than that with no carbon implantation, indicating that the higher concentration of the carbon implantation, the higher the retained dose of fluorine after annealing. As the retained fluorine concentration increased, the minority carriers of electrons or holes decreased by more Si-F bond formation, resulting in increasing the sheet resistance. When carbon was implanted with 1.0 × 10¹⁵ ions/cm², the defective pixel, dark current, transient noise, and flicker were much improved by 25%, 9.4%, 1%, and 28%, respectively, compared to that with no carbon implantation. The investigation proved that the diffusion of fluorine after annealing could be improved by carbon implantation, leading to improvement of the dark current characteristics.

To summarize, the second volume of the Special Issue on “Organic Optoelectronic Materials” includes broader areas compared to the first volume, and various applications of organic materials are expected in the future.