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Preparation of ITO free transparent conductive electrode via layer-by-layer deposition of carbon nanotubes and its application for solar cells

Final Report Summary - LBL OF CNTS FOR SCS (Preparation of ITO free transparent conductive electrode via layer-by-layer deposition of carbon nanotubes and its application for solar cells)

This proposal calls for the fabrication of transparent conducting electrode via layer-by-layer assembly of carbon nanotubes for organic photovoltaic application (OPV). The key element of the optoelectronic devices such as light emitting diodes and solar cells is the transparent conductive electrodes. Traditionally, indium tin oxide (ITO) has been utilized as transparent conductive electrode in the optoelectronic devices. However, ITO is not the optimum one for aforementioned devices. Because, limited supply of indium and increasing demand has increased the ITO price drastically since the past decades. In addition, high vacuum and temperature is needed for sputtering ITO on the substrate, which also increases the cost. Besides, the brittle nature of ITO causes device failure upon bending when it is used on flexible substrates. Therefore, there has been great research effort on development of ITO free transparent conductive electrodes. Up to date, researchers have utilized various materials such as conducting polymers, carbon nanotubes (CNTs), graphene and metallic nanowires. However, among these materials, CNTs have been widely utilized, owing to their promising and mature technology. Thus, various methods have been developed to prepare transparent conductive electrode with CNTs; such as spray coating, vacuum filtration, rod/wire coating and layer-by-layer (LBL) deposition. Major advantage of LBL deposition is that it allows one to control the structure of the coatings with actual nanometer scale precision, which includes both normal and lateral packing of the nanoscale building blocks. In other words, it is very easy to control film thickness and morphology, and thus easy to fabricate 3-D interconnected nanomaterial blocks. Generally, the adhesion of the multilayer film to the substrate is very good since the multilayer is formed via covalent bonding or ionic interaction

The main objective of this proposal is to fabricate transparent conductive electrode on glass and PET substrate via LBL deposition of multi-walled CNTs without using any surfactant or polymer and to utilize the mentioned electrode in OPV devices. In order to achieve the goal, we had proposed seven tasks;

WP1: Experiment design and purchasing
WP2: Surface modification of CNTs
WP3: Surface functionalization of glass and PET substrate
WP4: Conventional LBL assembly of CNTs and characterization
WP5: Spin-assisted LBL deposition of CNTs and characterization
WP6: Spray-assisted LBL deposition of CNTs and characterization
WP7: Fabrication of OPV device

Work packages proposed for 3 years have been successfully achieved since the beginning of the project. All of the work mentioned above has been completed. Surface modification of CNTs, which is mandatory for LBL deposition, has been successfully achieved. Next, transparent conducting electrodes have been fabricated by 3 different LBL deposition techniques for comparison. The electrodes were characterized in terms of sheet resistance, optical transmission and morphology. Best results were obtained with conventional LBL assembly both with glass and PET substrate. In addition, treatment of the CNT multilayer films with various acids and heat were carried out in order to increase the electrical conductivity. Finally, P3HT:PCBM based organic photovoltaic devices (OPV) were fabricated and tested. Comparable device efficiencies (2-3%) were obtained with CNT multilayer electrodes both on glass and PET substrate. It was evaluated that it is feasible to use the CNT multilayer electrodes in OPV devices. As continuation of this work, writing new research and small business proposals has been initiated.

There is a high possibility to use final results for development specific optoelectronic devices with the links and resources of TUBITAK Marmara Research Center and Dr. Basarir’s knowledge and experience. The final results are expected to have an impact on the development of nanostructured transparent conducting electrodes for flexible optoelectronic applications in near future. This project plays the key role for writing new project proposals.