Investigation of Opto-Electrical, Mechanical and Surface Properties of AgNWs-PEDOT: PSS Based Transparent and Conductive Electrodes for Bio-Sensing Applications

Ultra-flexible organic photodetectors (OPDs) have experienced a rapid development in recent years, making them increasingly significant in the field of flexible electronics, wearable technology and medical applications. They have emerged as promising optoelectronic devices for several applications including biosensing. For example, OPDs in the field of biosensing are used to monitor vital signs such as, heart rate, oxygen saturation and blood glucose levels. When fabricated on ultrathin flexible substrates organic photodetectors provide conformal surface contact thus allowing for efficient light absorption and detection, thereby enhancing the sensitivity and the performance.<br/>Transparent conductive electrodes (TCEs) are considered to be one of the most crucial components in OPDs as they play a significant role in improving their efficiency as they effectively take part in light transmission and charge extraction. Typically, TCEs are designed to exhibit high transparency in the visible spectrum thus allowing the incoming light to pass through them with minimum loss. The flexible nature of the TCEs will allow for the conformal contact on to curved or flexible substrates and thus allows the seamless integration of OPDs with biological tissues enabling for the continuous and real-time monitoring of the physiological parameters.<br/>Among the various available materials, one dimensional AgNWs are perceived as a promising candidate for TCEs for flexible OPDs due to some of their remarkable properties such as low percolation threshold, high electrical conductivity, high optical transparency and excellent flexibility. However, their long-term stability is still challenging as AgNWs are susceptible to oxidation thus limiting their reliability in flexible devices. Hence to solve this issue the combination of AgNWs with conducting polymers such as poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) as transperent hybrid electrode has been explored. PEDOT: PSS alone offers excellent chemical stability and can therefore further contribute to the stability of AgNWs by preventing from oxidation, thus maintaining the overall integrity of the film from various environmental degradation conditions. Moreover, the hydrophobic properties of PEDOT: PSS helps to shield the AgNWs from the detrimental effects of moisture such as oxidation and loss in electrical conductivity. Additionally, PEDOT: PSS when incorporated in AgNWs will help to reduce the interfacial resistance between the nanowires thus help in the overall stability and electrical conductivity of the film.<br/>In this work, we have investigated the opto-electrical, mechanical and surface properties of silver nanowires/poly (3,4-ethylenedioxythiophene): poly(styrenesulfonate) (AgNWs-PEDOT: PSS) hybrid electrode fabricated on 5-µm ultrathin parylene C substrate. The fabricated electrode exhibits low sheet resistance of 27 Ω/Sq, and shows an average optical transmission of &gt;85% throughout the visible spectrum which is suitable for optoelectronic applications. Besides the comparable work function of 5 eV with respect to standard ITO, the surface rms roughness of the electrode is effectively reduced from 8 nm to 4 nm by spin coating PEDOT: PSS as planarization layer. With a stable R/R0 ≈1 over continuous 50k bending cycles at a radius of curvature of 0.1 mm, the electrode exhibits excellent mechanical characteristics suitable for ultraflexible and wearable applications. Utilizing this electrode, we have realized an ultraflexible organic photodetector that can be conformally adhere to human skin for biosensing applications. The OPD is fabricated in normal atmospheric conditions that can withstand 5k bending cycles when bent at a radius of curvature of 0.2 mm while maintaining similar values in ON/OFF ratio under white light illumination.