Photodetectors (PDs), as an indispensable component in electronics, are highly desired to be flexible to meet the trend of next‐generation wearable electronics. Unfortunately, no in‐depth reviews on the design strategies, material exploration, and potential applications of wearable photodetectors are found in literature to date. Thus, this progress report first summarizes the fundamental design principles of turning “hard” photodetectors “soft,” including 2D (polymer and paper substrate‐based devices) and 1D PDs (fiber shaped devices). In short, the flexibility of PDs is realized through elaborate substrate modification, material selection, and device layout. More importantly, this report presents the current progress and specific requirements for wearable PDs according to the application: monitoring, imaging, and optical communication. Challenges and future research directions in these fields are proposed at the end. The purpose of this progress report is not only to shed light on the basic design principles of wearable PDs, but also serve as the roadmap for future exploration in wearable PDs in various applications, including health monitoring and Internet of Things.

Solar radiation, especially ultraviolet (UV) light, is a major hazard for most skin-related cancers. The growing needs for wearable health monitoring systems call for a high-performance real-time UV sensor to prevent skin diseases caused by excess UV exposure. To this end, here a novel self-powered p-CuZnS/n-TiO UV photodetector (PD) with high performance is successfully developed (responsivity of 2.54 mA W at 0 V toward 300 nm). Moreover, by effectively replacing the Ti foil with a thin Ti wire for the anodization process, the conventional planar rigid device is artfully turned into a fiber-shaped flexible and wearable one. The fiber-shaped device shows an outstanding responsivity of 640 A W , external quantum efficiency of 2.3 × 10 %, and photocurrent of ≈4 mA at 3 V, exceeding those of most current UV PDs. Its ultrahigh photocurrent enables it to be easily integrated with commercial electronics to function as a real-time monitor system. Thus, the first real-time wearable UV radiation sensor that reads out ambient UV power density and transmits data to smart phones via wifi is demonstrated. This work not only presents a promising wearable health monitor, but also provides a general strategy for designing and fabricating smart wearable electronic devices.

ZnS, as one of the first semiconductors discovered and a rising material star, has embraced exciting breakthroughs in the past few years. To shed light on the design principles and engineering techniques of ZnS for improved/novel optoelectronic properties, the fundamental mechanisms and commonly employed strategies are proposed in this review. Recent progress on modifications of ZnS allows it to be extensively and effectively used in versatile applications, including transparent conductors, UV photodetectors, luminescent devices, and catalysis, which are clearly and comprehensively summarized in this work. Novel functional devices springing up from the newly developed ZnS-based materials are highlighted as well. This review not only provides a scientific insight into the advances of ZnS-based materials, but also touches on the future opportunities in this inspiring field.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201802029. relatively high charge recombination rate and low charge transport process inhibit the optoelectronic properties of ZnS as a high-performance photodetector. As a window layer of optoelectronic devices, a lack of high electrical conductivity typically hinders the practical use of ZnS in solar cells, photodiodes, light-emitting devices, etc. As a photocatalyst, the transparency of ZnS becomes a drawback as it suffers from a poor utilization of sunlight.There is not a perfect material, but the potential of developing a material is beyond limitation. The past few years have witnessed extensive designing and engineering of ZnS to explore its potentials in specific applications. [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] This review summarizes the recent breakthroughs on ZnS-based materials as excellent candidates in optoelectronic devices, photocatalysis, and other novel functional devices. More importantly, it sheds light on the design principles and fundamental mechanisms of the improved performance of ZnS through strategies such as developing novel nanostructures, bandgap engineering, alloying with other materials, etc. To the best of our knowledge, it is the first comprehensive review on the design principles and material engineering of ZnS for novel/improved properties and intended applications. Briefly, current literature on a variety of ZnS-based structures was first summarized, ranging from 0D to 2D nanostructures. Then, the representative applications of ZnS-based materials are described, which are mainly consisted of four parts, as shown in Figure 1: transparent conductors, UV photodetectors, luminescent devices, and catalysis. Each part contains the latest design strategies of ZnS for the intended applications, fabrication techniques, representative examples, and the state-of-the-art devices. Finally, it outlines the challenges and opportunities in each field. In particular, we provide our perspectives on the future research directions of ZnS in energy and environment.

Photodetectors, which convert the light signal into other forms of signal, have been under the spotlight of research for many years because they are widely applied in monitoring, communication, and imaging. Most of the currently available photodetectors can output electrical signals to indicate the transient light intensity, while some display color change to reveal the absorbed light dosage. However, there is no device that can tell the transient light intensity and accumulated light dosage at the same time. Here, a paper‐based wearable photodetector that can simultaneously measure transient light intensity and accumulated light dosage is reported. The phosphomolybdic acid/citric acid system, whose color change can be observed by the naked eye, is designed as the photochromic material to combine with photodetective materials (using 2D Sr2Nb3O10 and ZnO nanoparticle as examples) on paper. Such paper‐based photodetector fully utilizes natural hygroscopicity and softness of paper, showing decent flexibility. Its optoelectronic signal remains stable even after 1000 cycles of bending. To the best of one's knowledge, this is the first photodetector that can tell light intensity and dosage simultaneously. This work introduces a new type of wearable photodetector by structure design and material selection, shedding light on more novel works for convenient and practical photodetection.

sı Supporting InformationThe Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpclett.1c03090.Experimental characterization and detailed performances of devices (PDF)