Next-generation wearable electronics will need to be mechanically flexible and stretchable such that they can be conformally attached onto the human body. Photodetectors that are available in today's market are based on rigid inorganic crystalline materials and they have limited mechanical flexibility. In contrast, photodetectors based on organic polymers and molecules have emerged as promising alternatives due to their inherent mechanical softness, ease of processing, tunable optoelectronic properties, good light sensing performance, and biocompatibility. Here, the recent advances of organic photodetectors in terms of both optoelectronic and mechanical properties are outlined and discussed, and their application in wearable electronics including health monitoring sensors, artificial vision, and self-powering integrated devices are highlighted.

Organic Photodetectors for Next-Generation Wearable Electronics.

Conjugated polymer chains have many degrees of conformational freedom and interact weakly with each other, resulting in complex microstructures in the solid state. Understanding charge transport in such systems, which have amorphous and ordered phases exhibiting varying degrees of order, has proved difficult owing to the contribution of electronic processes at various length scales. The growing technological appeal of these semiconductors makes such fundamental knowledge extremely important for materials and process design. We propose a unified model of how charge carriers travel in conjugated polymer films. We show that in high-molecular-weight semiconducting polymers the limiting charge transport step is trapping caused by lattice disorder, and that short-range intermolecular aggregation is sufficient for efficient long-range charge transport. This generalization explains the seemingly contradicting high performance of recently reported, poorly ordered polymers and suggests molecular design strategies to further improve the performance of future generations of organic electronic materials.

http://absimage.aps.org/image/MAR14/MWS_MAR14-2013-020288.pdf

A general relationship between disorder, aggregation and charge transport in conjugated polymers

Organic photodetectors (OPDs) have gained increasing interest as they offer cost-effective fabrication methods using low temperature processes, making them particularly attractive for large area image detectors on lightweight flexible plastic substrates. Moreover, their photophysical and optoelectronic properties can be tuned both at a material and device level. Visible-light OPDs are proposed for use in indirect-conversion X-ray detectors, fingerprint scanners, and intelligent surfaces for gesture recognition. Near-infrared OPDs find applications in biomedical imaging and optical communications. For most applications, minimizing the OPD dark current density (Jd) is crucial to improve important figures of merits such as the signal-to-noise ratio, the linear dynamic range, and the specific detectivity (D*). Here, a quantitative analysis of the intrinsic dark current processes shows that charge injection from the electrodes is the dominant contribution to Jd in OPDs. Jd reduction is typically addressed by fine-tuning the active layer energetics and stratification or by using charge blocking layers. Yet, most experimental Jd values are higher than the calculated intrinsic limit. Possible reasons for this deviation are discussed, including extrinsic defects in the photoactive layer and the presence of trap states. This provides the reader with guidelines to improve the OPD performances in view of imaging applications.

https://onlinelibrary.wiley.com/doi/epdf/10.1002/adfm.201904205

Organic Photodetectors and their Application in Large Area and Flexible Image Sensors: The Role of Dark Current

Optical sensor development has been pursued over past 25 years, which has also seen development of novel chemistries, materials, sensor designs and optical instrumentation. In addition, novel signal and data processing techniques have been adopted in the sensor development. New techniques and materials in chemical and biological sensing have set further stages of advanced research. One of the most recent technological advances in this area has been the development of optical nanosensors, which are sensors with dimensions on the nanometer scale. Nanosensors of various types (e.g. optical, electrochemical) have been published in the literature over the past decade. Optical nanosensors, like the larger and macro scale sensors, can be either chemical or biological, depending on the type of probe used. Both types of sensors have been studied to provide a reliable method of monitoring various chemicals in microscopic environments and have been used to detect molecules within single cells. The field of nanosensing is relatively new and reports in the literature presenting any problems associated with such a scale of analysis are sparse. This paper will briefly review some of the current developments in the field of optical nanosensors.

Optical sensors

Recent Progress on Broadband Organic Photodetectors and their Applications

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

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

Fully Digitally Printed Image Sensor Based on Organic Photodiodes

The fabrication of electronics on the basis of biofriendly materials aims to counterbalance the negative trends conveyed by the short life-cycle of electronics. Furthermore, these materials open the possibility to develop optoelectronic technologies which will be in contact with the human body. In this work, we present an electrochromic display fabricated by resource- and energy-efficient digital printing techniques. The biodegradation of the device is certified under the ISO 14855 standard. The display comprises of a poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) electrochromic layer, a gelatin-based electrolyte and Au electrodes deposited on a cellulose di-acetate substrate. We investigate the impact of various naturally sourced ionic species on the ionic conductivity of the electrolyte and the figures of merit of the display. The printed devices show an electrochromic contrast of 32 ± 4% and switching times of 3.0 ± 1.4 s, comparable to the spincoated reference devices. The utilization of inkjet printing enables the fabrication of different device designs with individually addressable pixels. The display can be worn innocuously on the skin without loss of performance thanks to the self-adhesion properties of the gelatin hydrogel. The present work highlights the use of industrial relevant technology for the fabrication of truly ecofriendly optoelectronic systems.

/pdf/biodegradable-inkjet-printed-electrochromic-display-for-4ksd3b8k9u.pdf

Biodegradable inkjet-printed electrochromic display for sustainable short-lifecycle electronics

Digitally printed organic photodiodes (OPDs) are of great interest for the cost-efficient additive manufacturing of single and multidevice detection systems with full freedom of design. Recently reported high-performance non-fullerene acceptors (NFAs) can address the crucial demands of future applications in terms of high operational speed, tunable spectral response, and device stability. Here, we present the first demonstration of inkjet and aerosol-jet printed OPDs based on the high-performance NFA, IDTBR, in combination with poly(3-hexylthiophene), exhibiting a spectral response up to the near-infrared (NIR) region. These digitally printed devices reach record responsivities up to 300 mA/W in the visible and NIR spectrum, competing with current commercially available technologies based on Si. Furthermore, their fast dynamic response with cutoff frequencies surpassing 2 MHz outperforms most of the state-of-the-art OPDs. The successful process translation from spin-coating to printing is highlighted by t...

Noah Strobel

Papers

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

Organic photodiodes: printing, coating, benchmarks, and applications

Fully Digitally Printed Image Sensor Based on Organic Photodiodes

Biodegradable inkjet-printed electrochromic display for sustainable short-lifecycle electronics

Non-Fullerene-Based Printed Organic Photodiodes with High Responsivity and Megahertz Detection Speed