With the continuous development of science and technology, there is an increasing demand for lightweight, low-power, portable wearable devices, resulting in flexible photodetectors that have great potential and broad market prospects for future practical applications. These detectors are usually highly flexible and can maintain good performance on different curved surfaces and shapes, which is of great significance in meeting the ever-changing market demands of modern technological products. While future flexible photodetectors may focus more on integration and intelligence, to realize the popularity of electronic devices used for smart wearables, many sensors must be integrated into various forms of wearable systems, including clothing, watches, contact lenses, and electronic skin. Therefore, it is crucial to integrate multiple detectors into a miniature system. With advances in materials science and micro-nano technology, the size and weight of flexible photodetectors are expected to be further reduced, making them more suitable for miniaturized and portable devices. The ZnO microwires (MWs) are promising candidates for building highly sensitive ultraviolet (UV) detectors with flexibility, low power consumption, high efficiency and integrability, which could create new possibilities for human wearable systems.

Although ZnO micro-/nanostructures have been widely used in a variety of applications such as constructing photosensitive, communication, wearable devices, and foldable screens. However, researchers still face some challenges in ZnO-based p-n homojunction UV photodetectors. For example, the preparation of stable p-type ZnO materials is still a worldwide challenge, and the device structures of UV photodetectors reported so far have been realized by constructing heterojunctions using n-type ZnO materials with other p-type materials. Limited by the large brittleness and inability to bend of rigid substrates, it is difficult to meet the demands of complex and demanding practical environments. In contrast, ZnO MWs not only have excellent UV absorption properties and high elastic modulus, but also have low cost, high yield, good crystallinity, and controllable synthesis method. Therefore, it is considered as one of the most promising candidates for building miniaturized, integrated, and flexible UV photodetectors. To promote the development of ZnO flexible integrated optoelectronic devices, the team proposed and demonstrated a lightweight, flexible, and integratable crossover ZnO MWs p-n homojunction UV photodetector. The device exhibits excellent UV photodetection capabilities, including a maximum responsivity ~ 2.6 A/W, a specific detectivity ~ 6.3 × 10¹³ Jones, a noise equivalent power of 4.8 × 10^-15 W∙Hz^-1/2 and a good photovoltaic conversion efficiency of ~ 7.8 % upon 360 nm illumination at 0.1 V bias. Also, the obtained rise/decay time (~ 0.48 ms/79.41 ms) demonstrates the excellent optical response speed of the device. The related research results were published in Photonics Research, Volume 12, No. 4, 2024. [ Shulin Sha, Kai Tang, Maosheng Liu, Peng Wan, Chenyang Zhu, Daning Shi, Caixia Kan, Mingming Jiang. High-performance, low-power, and flexible ultraviolet photodetector based on crossed ZnO microwires p-n homojunction[J]. Photonics Research, 2024, 12(4): 648 ]

ZnO-doped Sb (ZnO:Sb) and ZnO-doped Ga (ZnO:Ga) MWs were successfully synthesized by chemical vapor deposition (CVD). The ZnO:Sb and ZnO:Ga MWs were demonstrated to exhibit p-type and n-type carrier transport properties, respectively, by means of field effect transistors with a single wire. And it was successfully used to construct a crossover ZnO:Sb⨂ZnO:Ga MWs homojunction device, the structure of which is shown in Fig. (a). By performing basic electrical characterization tests on the device, its excellent rectification characteristics indicate the formation of a high-quality homojunction between ZnO:Sb and ZnO:Ga MWs (Fig. (b)). The results of different light wavelength testing under constant power conditions show that the device has a significant photoresponse behavior to UV light at -0.1 V bias and exhibits the maximum photoresponse current at 360 nm light irradiation. Subsequently, the unencapsulated ZnO:Sb⨂ZnO:Ga MWs homojunction device was exposed to continuous irradiation of 360 nm UV light, and the peak photocurrent of the device fluctuated only within 10%. And after 60 days of storage (room temperature of 25 °C and humidity of 45%), the device still maintains more than 90% of the photocurrent response, indicating that the device structure can work stably for a long period of time (Fig. (c)).

Fig. (a) Schematic of the constructed ZnO:Sb⨂ZnO:Ga MWs structure; (b) Current-voltage characteristic curves of the ZnO:Sb⨂ZnO:Ga MWs device under dark field; (c) Normalized photocurrent intensity of the device under continuous irradiation of 360 nm light at -0.1 V bias; the inset is the variation of the device photocurrent after different storage times; (d) Schematic of the single-pixel scanning imaging measurement platform; the inset is an optical micrograph of the p-n homojunction array, scale bar: 100 μm; (e) Schematic of the flexible ZnO:Sb⨂ZnO:Ga MWs device array; (f) "MW" imaging results measured by the flexible ZnO:Sb⨂ZnO:Ga MWs device array unit. (g) Photocurrent waveform of the imaged twelfth pixel; (h) Average photocurrent of the imaged full pixel and the corresponding signal-to-noise ratio.

Based on the prepared ZnO:Sb⨂ZnO:Ga MWs homojunction detector, we successfully constructed a flexible array cell on a PET substrate (Fig. (e)). The photoresponse performance

of this flexible array cell can still be maintained above 80% by testing it with different bending angles and bending times. Finally, we integrated the constructed flexible array cell into a real photoelectric imaging system (Fig. (d)), and the obtained image clarity, photoresponse intensity, and photocurrent stability all indicated that the flexible array cell has excellent high-resolution single-pixel imaging capability (Figs. (f-h)).

The development of flexible UV detectors is important for the advancement of new optoelectronic detection technologies. Such devices can provide greater flexibility and convenience in a variety of application scenarios, especially in areas where portable, wearable, or foldable devices are required. Due to their softness and bendability, flexible UV detector devices can better adapt to the shape and movement of the human body, providing a more comfortable and practical user experience. As concepts such as the internet of things (IoT), smart homes and smart cities continue to evolve, flexible UV detector devices will meet the future market demand for multifunctional, integrated, and high-performance photodetectors. This has constantly motivated researchers to make breakthroughs and optimize the related materials and structures to improve the photoelectric response performance and mechanical stability of the devices, thus ensuring the reliability and durability of the devices in practical applications. The development of ZnO MWs-based flexible UV detector devices play an important role in improving the performance of UV detectors, expanding the application areas, and promoting the development of new flexible UV detection technologies. With the deepening of the research and the maturity of the technology, ZnO MWs-based flexible UV detectors are expected to play a more critical role in the future.

The preparation of ZnO MWs homojunction detectors is challenging mainly due to several reasons, including material preparation, interface engineering, carrier transport, and integration and scale-up production. Solving these challenges requires the integration of knowledge and technology from several fields, such as materials science, solid-state physics, and surface science. For example, during material preparation, the p-type doping, quality, and uniformity of ZnO MWs are critical to device performance. Precise control of the growth rate, temperature, atmosphere, etc. is required to ensure the doping concentration, uniformity, and high crystallinity of the ZnO MWs, to obtain the most ideal microstructure and ZnO MWs with different carrier transport properties. Similarly, it remains a major challenge to integrate these constructed high-performance UV detector devices into practical applications and to scale up their production. This involves various considerations such as optimization of process flow, stability of material sources, and control of production cost. Therefore, through our performance study and application exploration of crossed ZnO MWs homojunction devices, this advancement has been instrumental in the development of high-performance, low-power, flexible, and integratable ZnO MWs-based UV photodetector devices.

In the future, the team will further research and explore the material modification, interface optimization, structural design, environmental adaptability, fabrication process, post-processing technology, integration, and packaging, as well as multifunctional applications, with the aim of further optimizing the flexible UV detector of ZnO MWs homojunction to have a higher performance and a wider range of applications.