With the emergence of fifth-generation (5G) cellular networks, millimeter-wave (mmW) and terahertz (THz) frequencies have attracted ever-growing interest for advanced wireless applications. The traditional printed circuit board materials have become uncompetitive at such high frequencies due to their high dielectric loss and large water absorption rates. As a promising high-frequency alternative, liquid crystal polymers (LCPs) have been widely investigated for use in circuit devices, chip integration, and module packaging over the last decade due to their low loss tangent up to 1.8 THz and good hermeticity. The previous review articles have summarized the chemical properties of LCP films, flexible LCP antennas, and LCP-based antenna-in-package and system-in-package technologies for 5G applications, although these articles did not discuss synthetic LCP technologies. In addition to wireless applications, the attractive mechanical, chemical, and thermal properties of LCP films enable interesting applications in micro-electro-mechanical systems (MEMS), biomedical electronics, and microfluidics, which have not been summarized to date. Here, a comprehensive review of flexible LCP technologies covering electric circuits, antennas, integration and packaging technologies, front-end modules, MEMS, biomedical devices, and microfluidics from microwave to THz frequencies is presented for the first time, which gives a broad introduction for those outside or just entering the field and provides perspective and breadth for those who are well established in the field.

Active metasurfaces provide promising tunabilities to artificial meta−atoms with unnatural optical properties and have found important applications in dynamic cloaking, reconfigurable intelligent surfaces, etc. As the development of semiconductor technologies, electrically controlled metasurfaces with semiconductor materials and devices have become the most promising candidate for the dynamic and programmable applications due to the large modulation range, compact footprint, pixel−control capability, and small switching time. Here, a technical review of active and programmable metasurfaces is given in terms of semiconductors, which consists of metasurfaces with diodes, transistors, and newly rising semiconductor materials. Physical models, equivalent circuits, recent advances, and development trends are discussed collectively and critically. This review represents a broad introduction for readers just entering this interesting field and provides perspective and depth for those well−established.

A pressure-assisted method was used to achieve a remarkable enhancement in the crystallinity and flatness of ultra-thin MAPbI₃ films. This process facilitates an all-round improvement in the photoelectric and optical properties of solar cells.

In this paper, a narrowband bandpass filter comprising substrate integrated waveguide (SIW) and spoof surface plasmon polaritons (SSPPs) is proposed, achieving miniaturization and low insertion loss at Ka-band. SIWs and SSPPs exhibit low and high cut-off responses, respectively, which can be combined for filtering functionalities. By designing I-shaped SSPP structures with large effective length, the cut-off frequencies of SIW and SSPPs can be tailed to converge at certain frequency for narrowband filter, where the SIW mode functions as a quasi-evanescent mode, and the mode impedance ramps fast. A sharp funnel-shaped transition is designed to connect SIW with large impedance and 50-Ω microstrip line for low insertion loss and compact profile. In addition, no transitional SSPP structures are desired in the proposed devices, which further shortens the profile dimension. To verify our design, a hybrid filter at 30 GHz is fabricated and characterized. The measured insertion loss is as low as 1.5 dB, the 3-dB bandwidth is as narrow as 3%, which shows good consistency with the simulation. The proposed SIW-SSPP device has a low profile of 0.70λ × 0.38 λ and may find important applications in frequency division multiplexing.

Thin-film Schottky diodes are one of the key elements in large-area flexible electronics. In such devices, a highly uniform semiconductor film is vital for the device performance. Here, we propose a novel solution-based anodization method to form a conformal oxide semiconductor layer for Schottky diodes. The thickness of the anodized TiO ₂ layer varied from 12 to 22.5 nm. The optimized Pt/TiO ₂ /Ti Schottky diode demonstrated a large barrier height of 1.19 eV, an ON/OFF ratio as high as ${3.5} \times {10}^{{6}}$ at ± 2 V, and an ideality factor of 1.5. The average breakdown electric field was 5.5 MV/cm, which is higher than typical values of conventional solution processed TiO ₂ . The diode with a 15-nm-thick TiO ₂ layer also showed good rectification properties up to 0.7 MHz.