Single-walled carbon nanotubes and PEDOT [electronic resource] : PSS for stretchable and novel electronics
Keywords: Chemical Engineering.

Conducting polymers and carbon allotropes hold great promise for next-generation electronics due to their excellent mechanical and electrical properties. These unique properties enable their use in the field of stretchable electronics, particularly in displays, touch screens, circuits, solar cells, implantable medical devices, and robotic systems with skin-like capabilities. In addition, they enable the fabrication of functional devices on curved and mobile surfaces and offer high durability. The unique attribute of stretchable electronics is their ability to be significantly stretched and compressed, which is different from flexible electronics where only flexibility is demonstrated. The main challenge in this field is to fabricate these devices in a low-cost large-scale process without compromise to device performance. The goal of this thesis is to develop processing methods and device architectures to use films of single-walled carbon nanotubes (SWNTs) and poly-(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) to fabricate next-generation devices including thin film transistors, transparent and stretchable pressure and strain sensors, stretchable solar cells, and all-carbon solar cells. A derivative of the polymer polythiophene, PEDOT:PSS films have shown the potential for use in transparent electrode applications. However, suspensions of these films are available in aqueous solutions, which limit their deposition on normally hydrophobic elastomeric substrates. As a result, the fabrication of stretchable and conductive PEDOT:PSS films for transparent electrodes has not been demonstrated. Here, we used a fluorosurfactant to improve the wetting properties of these solutions on the elastomeric substrate polydimethylsiloxane (PDMS). Films with a sheet resistance of less than 50 [Ohms]/□ at ~82% transparency were fabricated and used as anodes in indium tin oxide (ITO) free solar cells. Additionally, by applying a pre-strain on the PDMS substrates prior to deposition, and then releasing the pre-strain, out of plain buckles were formed on the PEDOT:PSS films enabling stretchability. These films were reversibly stretchable with little change in conductivity and were used in the first demonstration of a stretchable solar cell. These findings will enable the use of these highly conductive and stretchable PEDOT:PSS films as electrodes in stretchable optoelectronic devices. SWNTs are seamless tubes with a diameter on the order of 1 nm and a length of several to hundreds of microns with exceptional charge transport and strength. Because they can be deposited from solution, they offer the potential for low-cost and large-area fabrication on a variety of substrates. However, until now efforts to fabricate stretchable films of SWNTs have typically shown only uni-axial stretchability and resistance increases by an order magnitude or more in the stretched state. The impressive capabilities of stretchable SWNT films, combining high conductivity ([sigma] > 100 S cm-1) and transparency (> 80%) at high bi-axial strains ([epsilon] ≥ 150%) has remained a challenge. Here, we spray-coated films of SWNTs on PDMS to produce transparent, conductive, and bi-axially stretchable electrodes. By applying a series of strain-and-release cycles we were able to form "nano-springs" in the SWNTs that allowed them to become reversible stretchable up to 150% of their original length while maintaining high conductivity. These films were used as electrodes in arrays of transparent, stretchable capacitors, which behave as pressure and strain sensors. These sensors can be used in touch screen display applications with pressure sensitive functionalities and in robotic systems with skin-like capabilities. Furthermore, these SWNT films were used as cathodes in the first demonstration of an all-carbon solar cell in which all major components of the solar cell were fabricated from carbon allotropes. All-carbon based devices offer the potential for low-cost, stable (chemical and physical), and unique applications. However, the integration of all-carbon components as electrodes and active layers has posed a challenge in the fabrication of these devices. Here, we fabricate an active layer consisting of a bilayer of polymer-sorted semiconducting SWNTs and the fullerene C60 for solar cells. With standard electrodes these solar cells displayed a power conversion efficiency of 0.46%. For the all-carbon structure, a reduced graphene oxide (rGO) film was used as the anode and an n-type doped SWNT film was as the cathode, demonstrating the feasibility of all-carbon electronics. In summary, the use of SWNT and PEDOT:PSS films in stretchable and next-generation electronics highlights their unlimited potential. By overcoming challenges in fabrication and integration, these films are formed using low-cost and large-area methods on elastomeric substrates to impart stretchability. In addition, due to their unique properties new devices can be realized that can pave a new era of emerging electronics.