Abstract—Alternative electrode materials for optoelectronic devices have been widely investigated in recent years. Since indium tin oxide (ITO) is the most preferred transparent conductive electrode, producing ITO films by simple and cost-effective solution- based techniques with enhanced optical and electrical properties has great importance. In this study, single- and multi-walled carbon nanotubes (SWCNT and MWCNT) incorporated into the ITO structure to increase electrical conductivity, mechanical strength, and chemical stability. Carbon nanotubes (CNTs) were firstly functionalized by acid treatment (HNO 3 :H 2 SO 4 ), and the thermal resistance of CNTs after functionalization was determined by thermogravimetric analysis (TGA). Thin films were then prepared by spin coating technique and characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), four-point probe measurement system and UV-Vis spectrophotometer. The effects of process parameters were compared for ITO, MWCNT-ITO, and SWCNT-ITO films. Two factors including CNT concentration and annealing temperature were considered. The UV-Vis measurements demonstrated that the transmittance of ITO films was 83.58% at 550 nm, which was decreased depending on the concentration of CNT dopant. On the other hand, both CNT dopants provided an enhancement in the crystalline structure and electrical conductivity. Due to compatible diameter and better dispersibility of SWCNTs in the ITO solution, the best result in terms of electrical conductivity was obtained by SWCNT-ITO films with the 0.1 g/L SWCNT dopant concentration and heat-treatment at 550 °C for 1 hour. Keywords—CNT incorporation, ITO electrode, spin coating, thin film. INTRODUCTION I. NE of the key components of the solar cells is a transparent electrode layer since it may remarkably influence the cell performance by transmitting incoming photons to the active layer with high efficiency. ITO is a commonly used transparent conductive electrode because it shows low sheet resistance (<200Ω/sq) and high transmittance (>80%) in the visible region [1] owing to its wide bandgap G. Gokceli is with the Istanbul Technical University, Institute of Energy, Renewable Energy Department, 34469 Maslak, Istanbul, Turkey (phone: +90 212 285 7393; e-mail: ggokceli@itu.edu.tr). O. Eksik is with the Istanbul Technical University, Faculty of Chemical and Metallurgical Engineering, Chemical Engineering Department, 34469 Maslak, Istanbul, Turkey (phone: +90 212 285 6234; e-mail: eksikos@itu.edu.tr). E. Ozkan Zayim is with the Istanbul Technical University, Faculty of Science and Letters, Physics Department, 34469 Maslak, Istanbul, Turkey (phone: +90 212 285 3009; e-mail: ozesra@itu.edu.tr). N. Karatepe is with the Istanbul Technical University, Institute of Energy, Renewable Energy Department, 34469 Maslak, Istanbul, Turkey (phone: +90 212 285 3940; e-mail: kmnilgun@itu.edu.tr). more than 3.75 eV [2]. Commercial ITO films are mostly prepared by vacuum-based magnetron sputtering technique which can be very efficient to obtain low sheet resistance; however, this causes a dramatic increase in the production cost [3]. Because of that, there have been several attempts in recent years to produce ITO films by low-cost and simple techniques as well as to optimize the process for large-scale productions by solution-based techniques [4], [5] or replace with novel electrode materials [6]. It is unfortunate that the solution based techniques are applied to produce ITO films, and problems such as the brittle nature of ITO and high sheet resistance can be observed [8]. CNT is one of the featured alternatives to ITO electrode owing to flexibility, chemical stability and relatively low-cost [7]. However, different from the individual CNT, the conductivity of the CNT film is affected by the high contact resistance in the junction points of the tubes which requires post-treatment such as acid treatment [9]. As a consequence, the use of ITO or CNT as the transparent conductive electrode brings about different advantages and disadvantages. For this reason, it is possible to produce highly efficient electrode material for next-generation optoelectronics by CNT doping into ITO structure instead of using separately [10]. In this study, it was expected that, when CNTs are doped into the ITO structure, they will act as a nano-bridge which increases conductivity and stability. Because of that, it was aimed at not only improving the mechanical properties of ITO by doping with single-walled carbon nanotube (SWCNT) and multi-walled carbon nanotube (MWCNT) into ITO structure and reducing the micro-crack formation but also increasing the electrical conductivity due to the high mobility of CNTs. EXPERIMENTAL II. A. Materials For the preparation of ITO solution, anhydrous indium(III) chloride (InCl 3 , Alfa Asesar, 99.99%), tin(II) chloride dihydrate (SnCl 2 .2H 2 O, Merck, 98%), acetylacetone (Merck, 99%) and absolute ethanol (Merck, 99%) were used. MWCNTs (outer diameter:13 nm, >95% purity) were purchased from Baytubes, Bayer and SWCNTs (outer diameter: 1.55 nm, >90% purity) were purchased from carbon solutions. sulfuric acid (H 2 SO 4 , Merck, 98 %) and nitric acid (HNO 3 , Merck, 65 %) were used for the functionalization of CNTs. G. Gokceli, O. Eksik, E. Ozkan Zayim, N. Karatepe A Comparative Study of Single- and Multi-Walled Carbon Nanotube Incorporation to Indium Tin Oxide Electrodes for Solar Cells O World Academy of Science, Engineering and Technology International Journal of Materials and Metallurgical Engineering Vol:13, No:1, 2019 24 International Scholarly and Scientific Research & Innovation 13(1) 2019 ISNI:0000000091950263 Open Science Index, Materials and Metallurgical Engineering Vol:13, No:1, 2019 waset.org/Publication/10009943