Geçirgen ve iletken elektrot olarak karbon nanotüp katkılı indiyum kalay oksit ince filmlerin geliştirilmesi
Development of carbon nanotube doped indium tin oxide transparent conductive electrode
Yazar: GÖKÇEN GÖKÇELİ
Danışman: PROF. DR. NİLGÜN KARATEPE YAVUZ
İstanbul Teknik Üniversitesi / Enerji Enstitüsü / Enerji Bilim ve Teknoloji Ana Bilim Dalı / Enerji Bilim ve Teknoloji Bilim Dalı
Derece: Yüksek Lisans, 2018
Konu: Enerji = Energy

Özet:
In2O3 kristal yapısının içerisine Sn4+ iyonlarının yerleşmesi ile meydana gelen indiyum kalay oksit (ITO) geçirgen iletken filmler, düşük yüzey direnci (<200 Ω/kare) ve görünür bölgede %80 üzeri geçirgenlik sergilemelerinden dolayı en çok tercih edilen elektrot malzemesi olarak karşımıza çıkmaktadır. Ancak, ITO'nun kırılgan yapıda olması film içerisinde mikro boyutta çatlaklar meydana getirerek cihazın performansını etkileyebilmektedir. Ayrıca, indiyum kaynaklarının sınırlı olması uzun vadede ITO kullanımında bir engel olarak görülmekte ve alternatiflere yönelinmesi gerektiğini düşündürmektedir. Geçirgen iletken elektrotların küresel pazar paylarına dair tahminler ise ITO'nun 2016-2020 döneminde %5,96 yıllık büyüme oranı ile hala önemli ölçüde kullanılmaya devam edeceğini öngörmektedir. Bu nedenle, ITO'nun çözelti bazlı, düşük maliyetli ve kolay uygulanabilir yöntemler kullanılarak üretilmesi önem taşımaktadır.
Tez kapsamında, elektriksel iletkenliği ve mekanik dayanımı yüksek karbon nanotüplerin (CNT) ITO çözeltisi içerisine katkılandırılmasıyla ITO filmin yapısındaki mikroçatlakların giderilerek mekanik özelliklerinin iyileştirilmesi ve yüzey direncinin düşürülmesi üzerinde çalışılmıştır. Ayrıca, sağlanan iyileşmelerle birlikte, ITO filmlerin çalışma ömrünün arttırılması ve maliyetinin düşürülmesi hedeflenmiştir. ITO filmlerin oluşturulmasında dönel kaplama yöntemi kullanılmıştır. CNT'lerin ısıl dayanımı termogravimetrik analiz (TGA) ile tespit edilmiştir. İnce filmlerin yüzey özellikleri ve kalınlıkları taramalı elektron mikroskobu (SEM), kristal yapısı ve ortalama partikül boyutu X-ışını kırınımı (XRD), yüzey direnci 4 nokta temaslı iletkenlik ölçüm sistemi ve optik geçirgenlikleri UV-Vis spektrofotometresi kullanılarak belirlenmiştir.
Literatürdeki kısıtlı bilgilerden yola çıkılarak yürütülen bu çalışmada; ticari çok duvarlı karbon nanotüp (MWCNT), tek duvarlı karbon nanotüp (SWCNT) ve laboratuvarda kimyasal buhar biriktirme (CVD) yöntemi kullanılarak üretilen tek duvarlı karbon nanotüp (LSWCNT) olmak üzere üç farklı CNT, 3:1 oranındaki HNO3:H2SO4 asit karışımı ile muamele edilerek fonksiyonelleştirilmiş ve ITO içerisine çözelti bazlı olarak katkılandırılmıştır. Fonksiyonelleştirme sonrasında tüm CNT'lerin ısıl dayanımında artış gözlenmekle birlikte; ısıl dayanımı en yüksek numune 630 °C değeri ile LSWCNT olmuştur. Tüm filmler için uygun kaplama sayısı 7 olarak belirlenmiştir. MWCNT-ITO ve SWCNT-ITO ince filmlerin yüzey direnci üzerinde katkılandırılan CNT miktarı, ısıl işlem sıcaklık ve süresi gibi parametrelerden hangisinin daha etkili olduğu 23 faktöriyel tasarım yöntemiyle incelenmiştir. Yüzey direncini, sıcaklık ve süre artışının önemli ölçüde azalttığı belirlenmiştir. MWCNT'lerin dispersiyonu ITO çözeltisi içerisinde daha zor olduğundan, 0,1 g/L derişime sahip SWCNT ile katkılandırılmış filmler homojenizasyon ve yüzey direnci açısından daha iyi sonuç vermiştir. En düşük yüzey direnci, 625 °C'de 1 saat süre ile ısıl işlem gören LSWCNT-ITO hibrit elektrot ile elde edilmiştir. Sonrasında yüzey direnci düşük olan filmler H2/Ar ortamında muamele edilerek yapıdaki oksijen boşluklarının ve dolayısıyla elektriksel iletkenliğin arttırılması sağlanmıştır. İndirgen ortamda uygulanan işlem sonrasında LSWCNT-ITO filmlerin yüzey direnci 590 Ω/kare olarak ölçülmüştür. ITO içerisine katkılandırılan tüm CNT'ler kristal yapıyı iyileştirmiştir. Ortalama partikül boyutu MWCNT içeren numunelerde artarken SWCNT ve LSWCNT numunelerinde azalmıştır. ITO filmler görünür bölgede 550 nm'de %83,58 optik geçirgenlik sergilemişlerdir. Ayrıca, optik geçirgenliğin nanotüplerin çeşidinden çok katkılandırma miktarına bağlı olarak değişim gösterdiği belirlenmiştir.

Abstract:
Transparent conductive oxides (TCOs) are essential components of optoelectronic devices such as electrochromics, touchscreen panels, liquid crystal displays and solar cells. TCOs come into prominence owing to their outstanding properties of more than 80% optical transmittance in the visible region of the solar spectrum and sheet resistance lower than 200 Ω/sq. Various compounds including SnO2, In2O3, ZnO, TiO2, CdO and their combinations; indium tin oxide (ITO), aluminum-doped zinc oxide (AZO), fluorine-doped tin oxide (FTO) and gallium-doped zinc oxide (GZO) are used as transparent conductive films in industrial and laboratory scale applications. Among them, ITO films are the most commonly used thin films due to their high transmittance (~90 %) at 550 nm and low sheet resistance (20-100 Ω/sq). Using vacuum based physical vapor deposition (PVD) techniques can be very efficient to obtain low sheet resistance, however, this leads to a dramatic increase in the production cost. Since ITO is the most extensively used transparent conductive electrode, it is very obvious that ITO films should be produced by simple and cost-effective solution-based techniques with enhanced optical and electrical properties. On the other hand, ITO films when prepared with solution-based techniques require several depositions which can cause micro-cracks on the ITO surface. Also, the scarcity of indium sources limits the applications of ITO electrodes in recent years. For this reason, alternative transparent conductive electrodes such as polymers, metal-polymer composites, metal oxide thin films and carbon based materials (CNT or graphene) have been widely studied. CNT is the featured alternative to ITO electrode since it is used for both energy storage and conversion in various devices such as supercapacitors, fuel cells, solar cells, and lithium-ion batteries because of its high chemical stability, electrical conductivity, mechanical strength and high optical transmittance in the visible region and near IR. Unfortunately, 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. The new generation of optoelectronic devices require that the transparent conductive electrodes should be suitable for light and flexible device construction, compatible with the cheap and large-scale production methods. 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.
In this thesis, it was expected that when CNTs doped into the ITO structure they will act as a nano-bridge which increases conductivity and stability. Because of that it was aimed to improve the mechanical properties of ITO by doping with single-walled and multi-walled carbon nanotubes (SWCNT and MWCNT) into ITO structure to reduce the micro-crack formation and increasing the electrical conductivity due to the high mobility of CNTs. Therefore, the service life of ITO electrode will be increased and correspondingly the cost will be reduced. In addition, the single-walled carbon nanotube produced in our laboratory (LSWCNT) by chemical vapor deposition (CVD) technique was doped into the ITO structure to examine the differences between laboratory and commercial type SWCNTs. The thermal resistance of CNTs was investigated by thermogravimetric analysis (TGA). The prepared thin films were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), four-point probe measurement system and UV-Vis spectrophotometer.
ITO solution was prepared by using anhydrous indium (III) chloride (InCl3) and tin (II) chloride dihydrate (SnCl2.2H2O) precursors with acetylacetone and ethanol solvents, respectively. The In:Sn ratio was selected as 9:1 in the solution. CNTs were doped in ITO solution in two different concentration including 0.05 and 0.1 g/L by mixing in the ultrasonic homogenizer. Cleaned bare glasses (1.25x1.25 cm) were coated with the solutions by spin coating method. The coating process was realized in 3 steps; firstly the solution was spread out throughout the glass at 500 rpm, secondly the solution form thin film at 3000 rpm spinning for 30 seconds, and finally the film was dried at 4000 rpm. Different number of coatings such as 1, 3, 5 and 7 were investigated to find out the optimum film thickness. Number of coating was fixed to 7 due to its low sheet resistance compared to other samples. Thickness was measured as 165 nm for this sample by SEM. Before using as the dopant, all types of CNTs were functionalized with acid treatment in the mixture of HNO3:H2SO4 (3:1) for 1 hour at 120°C and they were characterized by TGA. As a consequence of functionalization, the thermal resistance of MWCNTs increased with the increase in temperature from 550 to 600 °C. SWCNTs showed lower thermal resistance (450 °C) compared to MWCNTs. After acid treatment, thermal resistance enhanced to 560 °C. Among CNTs, LSWCNT was the preferable one in terms of its high thermal resistance before (620 °C) and after (630 °C) functionalization.
Thereafter, effects of process parameters were investigated for ITO, MWCNT-ITO, and SWCNT-ITO films by utilizing 23 factorial design. Three factors including CNT concentration (high:0.1 g/L, low:0.05 g/L), annealing time (high:60 min, low:30 min) and annealing temperature (high:550 °C, low: 500 °C) were considered. Sheet resistance measurements of the films were performed by 4-point probe system. For MWCNT-ITO films, although concentration effect was less important, lower concentration value was better in terms of sheet resistance. On the other hand, higher CNT concentration decreased the sheet resistance when SWCNT-ITO films considered. For both SWCNT-ITO and MWCNT-ITO samples, the most effective factor was temperature to decrease sheet resistance of ITO. Also, increasing annealing time had a positive effect on electrical conductivity. Since the LWCNTs can thermally resist up to 630 °C, LSWCNT-ITO films annealed for 60 min at 500, 550, 600 and 625 °C. Lower sheet resistance obtained at 625 °C with the value of 2.19 kΩ/sq among all samples. Before using transparent conductive films as electrode it is important to reduce the sheet resistance. For this reason, the best sample was post-treated at 300 °C for 3 hours in H2/Ar medium with 10% H2. After post-treatment sheet resistance decreased to 590 Ω/sq from 2.19 kΩ/sq. Surface morphology of the thin films was analyzed by SEM. It was observed that the MWCNTs could not be dispersed properly in ITO solution and caused clustering of ITO particles at the different parts of the film. Besides, SWCNT and LSWCNTs dispersed in the surface homogeneously with a reticular appearance differently from ITO films.
Crystalline structure and mean crystalline size were determined by XRD. All CNT dopants provided an enhancement in the crystalline structure. It was observed from the increased peak intensity. Also, when commercial type nanotubes doped into the ITO structure (211) and (431) planes became distinct. Mean crystalline size of ITO thin film was found as 30.86 nm using the Debye-Scherrer equation. While MWCNT dopants increased mean crystalline size probably because of clustering, SWCNT and LSWCNT decreased this value to nearly 21-23 nm due to their small diameter.
UV-Vis measurements demonstrated that the transmittance of ITO films were 83.58% at 550 nm. Transmittance was decreased depending on the concentration of CNT dopant (0.05 or 0.1 g/L).
The study carried out within the scope of this thesis is unique in the point that high temperature resistant nanotubes produced in our laboratory are added to ITO by solution-based methods. In this study, which was conducted by means of limited information in the literature, the surface properties, crystal structure and sheet resistance of ITO with LSWCNTs have been successfully improved. Implementation of the work with easy and low cost solution-based methods is important in terms of applicability to large scale. When produced films are used especially in organic solar cells, it is expected that indium loss caused by corrosion of acidic PEDOT:PSS towards ITO can be prevented.