(Invited) Performance of Poly(3,4-ethylenedioxythiophene) Thin Films on Fabrics for Wearable Device Applications Using Oxidative Chemical Vapor Deposition

Abstract

Wearable electronics is a growing field of electronic devices integrated into wearable applications such as clothing. These electronic devices can be fabricated onto any flexible materials including fabrics and other clothing materials. The potential for the creation of new wearable device applications can range from the most important medical applications to ones for novel enjoyment. While this type of technology is emerging, limits from traditional inorganic conductive materials have not lent themselves to the development of these technologies. These traditional materials lack the overall flexibility and fabric performance to apply themselves to the development of wearable devices. Polymeric materials have been found to have superior mechanical flexibility when integrated into fabrics, and are advantageous to the development of wearable electronic components. Poly(3,4-ethylenedioxythiophene) (PEDOT) is one of a number of promising conjugated polymers due to its high electrical conductivity, visible regime transparency, and tunable work function. It is also extremely stable and has shown promise in organic electronic and optoelectronic devices such as solar cells, thin film transistors and chemical sensors. This stability and PEDOT’s mechanical flexibility make it a promising material for use in various wearable electronic applications.As an emerging technique, oxidative chemical vapor deposition (oCVD) is a deposition process that utilizes mild temperatures and vacuum to generate a chemical reaction between a vaporized source monomer (in this case EDOT) and an oxidizer (in this case FeCl3). Through the use of a temperature controlled vacuum chamber the synthesized resulting material (PEDOT) can be deposited onto specific substrates of almost any kind, in a controllable thin film form. Thin film thickness can be accurately controlled depending on target substrate temperature, deposition time, and monomer introduction rate. In comparison to conventional solution-based deposition processes, oCVD has specific advantages which grants itself to the creation of wearable electronic devices. Films deposited from the use of oCVD better conform to the target substrate. This conformality gives PEDOT the ability to maintain a fabrics breathability by maintaining areas in the fabric where air can pass through. This PEDOT’s excellent stability allows flexible properties to be maintained when depositing on fabric substrates. This allows oCVD to be extremely versatile and promising for wearable electronic devices.What will be presented is the performance of PEDOT thin films on fabric substrates compared to reference samples grown on glass and Si substrates. Performance of electrical, mechanical, surface, and chemical properties will be analyzed through the use of various testing equipment. To mimic the daily use in clothing and confirm the stability and mechanical flexibility of PEDOT thin films, films deposited on fabric will be compared before and after undergoing mechanical bending cycles, where sheet resistance and conductivity will be measured after each cycle to monitor potential film degradation. After undergoing 100 mechanical bending cycles, PEDOT films deposited on glass shown that they can maintain nominal conductivity measurements with an average variation from nominal of 10% depending on substrate temperature. Note that due to the nature of fabric having a rough and curvy surface, precise contact with the film is difficult to obtain measurements. Therefore, the term nominal is used for the values of conductivity. Due to the measured conductivity not degrading after 100 bending cycles, it shows PEDOT’s ability to maintain its performance and its mechanical flexibility. Atomic force microscopy (AFM) will be utilized to obtain topographical microstructure images and to analyze root mean square (RMS) roughness. Scanning electron microscopy (SEM) will be used to determine visual conformality of PEDOT thin films on fabric substrates. Fourier-transform infrared spectroscopy (FTIR) is utilized to confirm specific chemical bonding typical to PEDOT films, and UV-Vis spectroscopy to investigate optical properties such as onset of absorption to accurately determine band gap, which has been found to be between 1.8 and 2.0 eV depending on film thickness. All of these properties will be compared across different target substrate temperatures ranging from 30 °C to 90 °C. These findings on vapor-processed conjugated polymers on fabrics with high conductivity and mechanical flexibility are expected to significantly contribute to the realization of high performance and sustainable wearable electronic devices, in which development requires enhanced uniformity, flexibility, and breathability.The authors gratefully acknowledge the financial supports of the U.S. NSF Award No. ECCS-1931088; the Purdue Research Foundation (Grant No. 60000029); and the Improvement of Measurement Standards and Technology for Mechanical Metrology (Grant No. 20011028) by KRISS. Figure 1