Flexible image sensors have attracted increasing attention as new imaging devices owing to their lightness, softness, and bendability. Since light can measure inside information from outside of the body, optical-imaging-based approaches, such as X-rays, are widely used for disease diagnosis in hospitals. Unlike conventional sensors, flexible image sensors are soft and can be directly attached to a curved surface, such as the skin, for continuous measurement of biometric information with high accuracy. Therefore, they are expected to gain wide application to wearable devices, as well as home medical care. Herein, the application of such sensors to the biomedical field is introduced. First, their individual components, photosensors, and switching elements, are explained. Then, the basic parameters used to evaluate the performance of each of these elements and the image sensors are described. Finally, examples of measuring the dynamic and static biometric information using flexible image sensors, together with relevant real-world measurement cases, are presented. Furthermore, recent applications of the flexible image sensors in the biomedical field are introduced.

Recent Progress of Flexible Image Sensors for Biomedical Applications.

Challenges and recent advances in photodiodes-based organic photodetectors

Future lightweight, flexible, and wearable electronics will employ visible-light-communication schemes to interact within indoor environments. Organic photodiodes are particularly well suited for such technologies as they enable chemically tailored optoelectronic performance and fabrication by printing techniques on thin and flexible substrates. However, previous methods have failed to address versatile functionality regarding wavelength selectivity without increasing fabrication complexity. This work introduces a general solution for printing wavelength-selective bulk-heterojunction photodetectors through engineering of the ink formulation. Nonfullerene acceptors are incorporated in a transparent polymer donor matrix to narrow and tune the response in the visible range without optical filters or light-management techniques. This approach effectively decouples the optical response from the viscoelastic ink properties, simplifying process development. A thorough morphological and spectroscopic investigation finds excellent charge-carrier dynamics enabling state-of-the-art responsivities >102 mA W-1 and cutoff frequencies >1.5 MHz. Finally, the color selectivity and high performance are demonstrated in a filterless visible-light-communication system capable of demultiplexing intermixed optical signals.

/pdf/color-selective-printed-organic-photodiodes-for-filterless-32nfsseaz7.pdf

Color-Selective Printed Organic Photodiodes for Filterless Multichannel Visible Light Communication

In the last three decades, the development of new kinds of textiles, so-called smart and interactive textiles, has continued unabated. Smart textile materials and their applications are set to drastically boom as the demand for these textiles has been increasing by the emergence of new fibers, new fabrics, and innovative processing technologies. Moreover, people are eagerly demanding washable, flexible, lightweight, and robust e-textiles. These features depend on the properties of the starting material, the post-treatment, and the integration techniques. In this work, a comprehensive review has been conducted on the integration techniques of conductive materials in and onto a textile structure. The review showed that an e-textile can be developed by applying a conductive component on the surface of a textile substrate via plating, printing, coating, and other surface techniques, or by producing a textile substrate from metals and inherently conductive polymers via the creation of fibers and construction of yarns and fabrics with these. In addition, conductive filament fibers or yarns can be also integrated into conventional textile substrates during the fabrication like braiding, weaving, and knitting or as a post-fabrication of the textile fabric via embroidering. Additionally, layer-by-layer 3D printing of the entire smart textile components is possible, and the concept of 4D could play a significant role in advancing the status of smart textiles to a new level.

Integration of Conductive Materials with Textile Structures, an Overview.

Organic photodiodes (OPDs) are set to enhance traditional optical detection technologies and open new fields of applications, through the addition of functionalities such as wavelength tunability, mechanical flexibility, light-weight or transparency. This, in combination with printing and coating technology will contribute to the development of cost-effective production methods for optical detection systems. In this review, we compile the current progress in the development of OPDs fabricated with the help of industrial relevant coating and printing techniques. We review their working principle and their figures-of-merit (FOM) highlighting the top device performances through a comparison of material systems and processing approaches. We place particular emphasis in discussing methodologies, processing steps and architectural design that lead to improved FOM. Finally, we survey the current applications of OPDs in which printing technology have enabled technological developments while discussing future trends and needs for improvement.

/pdf/organic-photodiodes-printing-coating-benchmarks-and-k958c21mtq.pdf

Organic photodiodes: printing, coating, benchmarks, and applications

This paper investigates with a statistical analysis the issue of performance reproducibility and optimization in fully inkjet-printed organic photodetectors on flexible substrates The most crucial process step to obtain reproducible, well performing devices with a high process yield turns out to be the printing of the thin polyethylenimine interlayer used as a surface modifier for the bottom electrode Controlling solution composition and deposition parameters for this layer, a 57 nA cm–2 mean reverse dark current was achieved, with an outstanding standard deviation as low as 15 nA cm–2, with dramatic improvements in process yield (from less than 20% to over 90%) Device performance in terms of dark currents, EQE (from 50% up to 90% at 525 nm, depending on process), and rectification (ratio between forward current and reverse current over 104 and reaching 105 in the best cases) is among the best for fully printed detectors Furthermore, the importance of relative humidity control in the deposition enviro

Reproducible, High Performance Fully Printed Photodiodes on Flexible Substrates through the Use of a Polyethylenimine Interlayer.

We demonstrate photodetectors sensitive to ultraviolet light entirely developed by means of inkjet printing technique and based on titanium dioxide and PEDOT:PSS. Devices have a lateral architecture and are realized on a plastic substrate, thanks to the low thermal budget production process. Pure titania devices behave as standard photodetectors, increasing their conductivity by more than four orders of magnitude upon UV light exposure. Bilayers of PEDOT:PSS and titania show an inverted behavior, with a high conductivity in the dark which drops by seven orders of magnitude upon light exposure: this is likely due to the fast recombination of PEDOT:PSS holes with photogenerated TiO2 electrons. The series connection of pure TiO2 and of PEDOT:PSS/TiO2 bilayer is suggested as the basis for the development of low-power, complementary-like, photosensitive voltage dividers.

https://iopscience.iop.org/article/10.1088/1361-6641/aaf834/pdf

Inkjet printed hybrid light sensors based on titanium dioxide and PEDOT:PSS

Printable UV detector arrays based on light-induced conductance switching in mesoporous titanium dioxide

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Reproducible, High Performance Fully Printed Photodiodes on Flexible Substrates through the Use of a Polyethylenimine Interlayer.

Inkjet printed hybrid light sensors based on titanium dioxide and PEDOT:PSS

Printable UV detector arrays based on light-induced conductance switching in mesoporous titanium dioxide