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Structural, Optical and Electrical Properties of Zn-In-Sn-O Films

  • Lee, Seunghun (대학원 신소재공학과 신소재공학전공)

초록/요약

Transparent Conducting Oxide (TCO) thin films are strongly degenerated n-type semiconductors with a band-gap over 3.0 eV and, therefore, transparent in the visible wavelength range. Also TCO thin films have a carrier concentration over 1019~1020 cm-3. Currently, TCO thin films have found a wide range of applications such as low-emissivity windows, electro-chromic mirrors and windows, defrosting windows, electromagnetic shielding, flat panel displays and Photovoltaic (PV). Sn-doped In2O3 (ITO) thin films are regarded as the representative of TCO due to its low resistivity (~1x10-4 Ωcm) and transparency (~85%) obtained easily in production scale. But ITO thin films show the best electrical and optical properties when they are deposited at elevated temperatures (250 ~ 300℃). And, it has been reported that the indium is scarce element in earth crust and its price is high and unstable. For silicon heterojunction solar cells, TCO thin films with antireflection layer and current spreading layer have been employed, and mostly used ITO thin films. The TCO films in silicon heterojunction solar cells application must have a low sheet resistance and high transmittance. Especially, it is desirable for TCO films to have work function of over 5.2 eV to prevent the effect of emitter band bending caused by the difference in the work function between the emitter and TCO film. Recently, Zn-In-Sn-O (ZITO) thin film has been reported as a candidate to apply for Thin-film transistor (TFT) Organic photovoltaic, and Organic Light Emitting Diodes (OLED). ZITO thin films fabricated low temperature has high optical and electrical properties comparable to ITO thin films. And ZITO thin film reported to have a high work function over 6.1 eV. They also demonstrated that indium content in ITO could be reduced by introduction of ZnO. In this study, we investigated the structural, electrical, and optical measurements ZITO thin films with the change of Sn and Zn contents for Silicon heterojunction solar cells. And we investigated the detailed silicon heterojunction solar cells characterization applies ZITO thin films. ZITO thin films were deposited on glass substrates (Corning Eagle XG) by radio frequency (rf) magnetron sputtering of four 4-in. diameter targets of ITO (In2O3:SnO2 10 wt.%), AZO (ZnO:Al2O3 2 wt.%), and pure SnO2 (99.99% purity) in combinatorial method. The structural analysis was carried out by X-ray diffraction (XRD) and transmission electron microscopy (TEM). All XRD patterns of ZITO thin films deposited at RT has a smooth peak. All XRD patterns of ZITO thin films deposited at RT has a smooth peak. Nevertheless, the peak at around 31° corresponding to In2O3 shifted slightly toward high angle side as the content of Zn increased. The cross-section view and TEM image of 4.83 at.% Zn (2 target) exhibited a typical fine structure or nanostructure with amorphous phase. On the other hand, the cross-section view and TEM image of 36.9 at.% Zn (2 target) exhibited a purely amorphous phase. All ZITO thin films grown in this study showed outstanding transparency in the Vis and NIR wavelength spectra of over 80%. The optical constants of ZITO thin films have been obtained using the Drude free electron model combined with the Lorentz oscillator model. The optical effective mass of 2 target and 3 target series thin films was in the range of 0.67-0.51 me and 0.51-0.31 me, respectively. We further characterized the conduction band minimum to fermi level from the standard three-dimensional model. The conduction band minimum to fermi energy of 2 target and 3 target series thin films decreased slightly due to reduction in the carrier concentration. The lowest electrical resistivity of 5.5 x 10-4 Ωcm was observed for ZITO thin film with 4.83 at.% Zn by 2 target. The carrier concentration decreased with increasing Zn content. The ZITO thin film with 11.3 at.% Zn by 2 target had a highest work function measured by kelvin probe and UPS of 5.3 eV and 5.2 eV, respectively. We find that ionization potential showed very similar behavior to the work function, not conduction band to fermi energy. We fabricated the silicon heterojunction solar cells using 2 target and 3 target series thin films. The solar cell efficiency increased when the Zn content was increased to 11.3 and 9.44 at.%, and then decreased gradually when the Zn content exceeded 11.3 and 9.44 at.%. Voc and FF showed very similar behavior to the work function. The ZITO thin film with 11.3 at.% Zn by 2 target marked the highest conversion efficiency of 18.3%. After the low-temperature annealing at 150 oC in vacuum, the structural, optical, electrical, and work function properties of all ZITO thin films did not show considerable change. The highest conversion efficiency of 18.74% was obtained annealed ZITO thin film with 11.3 at.% Zn by 2 target. It was shown that low Zn contents (under 15 at.%) of ZITO thin films with high electrical and optical properties as well as with high work function, which is suitable for silicon heterojunction solar cell applications.

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ABSTRACT
TABLE OF CONTENTS
LIST OF FIGURES
LIST OF TABLES

1. Introduction
1.1. Motivation
1.2. Research Objectives
1.3. Reference
2. Theoretical Background
2.1. Transparent Conducting Oxides (TCO) Materials
2.1.1. Introduction
2.1.2. History of TCO Materials
2.1.3. Electrical and Optical Properties
2.1.4. Properties of ITO and ZnO Films
2.1.5. Multicomponent TCO Materials
2.1.6. Thin Film Zn-In-Sn-O Materials
2.2. Solar Cell Fundamentals
2.2.1. Introduction
2.2.2. Solar Cell Parameters
2.2.3. Illuminated Current-Voltage (I-V) characteristics
2.2.4. Dark Current-Voltage (I-V) Characteristics
2.2.5. Quantum Efficiency Characteristics
2.2.6. Silicon Heterojunction Solar Cells
2.3. Reference
3. Film Growth and Characterization
3.1. Introduction
3.2. Film Growth and Measuring Techniques
3.3. Film Composition
3.4. Structural Properties
3.5. Summary
3.6. Reference
4. Optical Properties
4.1. Introduction
4.2. Optical Properties
4.2.1. Transmittance, Reflectance, and Absorptance
4.2.2. Optical Band-gap
4.2.3. Figure of Merit
4.3. Optical Modeling
4.3.1. Optical Analyses
4.3.2. Analysis of Drude Parameters
4.4. Summary
4.5. Reference
5. Electrical Properties and Work Function Analysis
5.1. Introduction
5.2. Electrical Properties: Resistivity, Mobility, and Carrier Concentration
5.3. Work Function Analysis
5.4. Summary
5.5. Reference
6. Application to a-Si:H/c-Si Heterojunction Solar Cells
6.1. Introduction
6.2. Model of the a-Si:H/c-Si Heterojunction Solar Cells
6.3. Device Characterization
6.4.1. Analysis of Illuminated Current-Voltage Characteristics
6.4.2. Analysis of Dark Current-Voltage Characteristics
6.4.3. Analysis of Quantum Efficiency Characteristics
6.4. Summary
6.5. Reference
7. Effect of low-temperature annealing on ZITO thin film
7.1. Introduction
7.2. Effects of low-temperature annealing on ZITO thin films
7.3. Annealing Effects of Silicon Heterojunction Solar Cells
7.4. Summary
7.5. Reference
8. Summary

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