MXene-based films have been intensively explored for construction of piezoresistive flexible pressure sensors owing to their excellent mechanical and electrical properties. High pressure sensitivity relies on pre-molding a flexible substrate, or regulating the micromorphology of MXene sheets, to obtain a micro-structured surface. However, the two avenues usually require complicated and time-consuming microfabrication or wet chemical processing, and are limited to non-adjustable topographic-electrical (topo-electro) properties. Herein, we propose a lithographic printing inspired in-situ transfer (LIPIT) strategy to fabricate MXene-ink films (MIFs). In LIPIT, MIFs not only inherit ridge-and-valley microstructure from paper substrate, but also achieve localized topo-electro tunability by programming ink-writing patterns and cycles. The MIF-based flexible pressure sensor with periodical topo-electro gradient exhibits remarkably boosted sensitivity in a wide sensing range (low detection limit of 0.29 Pa and working range of 100 kPa). The MIF sensor demonstrates versatile applicability in both subtle and vigorous pressure-sensing fields, ranging from pulse wave extraction and machine learning-assisted surface texture recognition to piano-training glove (PT-glove) for piano learning. The LIPIT is quick, low-cost, and compatible with free ink/substrate combinations, which promises a versatile toolbox for designing functional MXene films with tailored morphological-mechanical-electrical properties for extended application scenarios.

Transparent electrodes based on copper nanowires (Cu NWs) have attracted significant attention owing to their advantages including high optical transmittance, good conductivity, and excellent mechanical flexibility. However, low-cost, high-performance, and environmental friendly solar cells with all-Cu NW electrodes have not been realized until now. Herein, top and bottom transparent electrodes based on Cu NWs with low surface roughness and homogeneous conductivity are fabricated. Then, semi-transparent polymer solar cells (PSCs) with the inverted structure of polyacrylate/Cu NWs/poly(3, 4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) (PH1000)/Y-TiO₂/poly(3-hexylthiophene): [6, 6]-phenyl-C₆₁-butyric acid 3, 4, 5-tris(octyloxy)benzyl/PEDOT: PSS (4083)/Cu NWs/polyimide/polydimethylsiloxane are constructed; these could absorb light from both sides with a power conversion efficiency reaching 1.97% and 1.85%. Furthermore, the PSCs show an average transmittance of 42% in the visible region, which renders them suitable for some specialized applications such as power-generating windows and building-integrated photovoltaics. The indium tin oxide (ITO)- and noble metal-free PSCs could pave new pathways for fabricating cost-effective semi-transparent PSCs.

Copper nanowire (Cu NW) transparent electrodes have attracted considerable attention due to their outstanding electrical properties, flexibility and low cost. However, complicated post-treatment techniques are needed to obtain good electrical conductivity, because of the organic residues and oxide layers on the surface of the Cu NWs. In addition, commonly used methods such as thermal annealing and acid treatment often lead to nanowire damage. Herein, a TiO₂ sol treatment was introduced to obtain Cu NW transparent electrodes with superb performance (13 Ω/sq @ 82% T) at room temperature within one minute. Polymer solar cells with excellent flexibility were then fabricated on the copper nanowire-TiO₂-polyacrylate composite electrode. The power conversion efficiency (PCE) of the cells based on a blend of poly(3-hexylthiophene) (P3HT) and phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) reached 3.11%, which was better than the control devices that used indium tin oxide (ITO)-PET electrodes, and outperforms other Cu NW based organic solar cells previously reported. The PCE of the solar cells based on Cu NW electrodes remained at 90% after 500 cycles of bending, while the PET/ITO solar cells failed after 20 and 200 cycles, with sheet resistance of 35 and 15 Ω/sq, respectively.

In spite of the recent successful demonstrations of flexible and transparent film heaters, most heaters with high optical transmittance and low applied direct current (DC) voltage are silver nanowire (Ag NW)-based or silver grid-based. In this study, flexible and stretchable copper nanowire (Cu NW)-based transparent film heaters were fabricated through a solution-based process, in which a thin layer of hydrophobic polymers was encapsulated on the Cu NW films. The thin polymer layer protected the films from oxidation under harsh testing conditions, i.e., high temperature, high humidity, and acidic and alkaline environments. The films exhibited remarkable performance, a wide operating temperature range (up to 150 ℃), and a high heating rate (14 ℃/s). Defrosting and wearable thermotherapy demonstrations of the Cu NW film heaters were carried out to investigate their practicality. The Cu NW-based film heaters have potential as reliable and low-cost film heaters.

A Cu nanowire (NW)/cuprous oxide (Cu₂O)-based semiconductor-liquid junction solar cell with a greatly enhanced efficiency and reduced cost was assembled. The Cu NWs function as a transparent electrode as well as part of the Cu NWs/Cu₂O coaxial structures, which remarkably benefit the charge separation. The best solar cell reached a conversion efficiency as high as 1.92% under a simulated AM1.5G illumination, which is 106 times higher than that of cells based on fluorine-doped tin oxide and Cu₂O.