검색 상세

A study of In2O3-based films for transparent electrodes in organic photovoltaics

  • 김준호 (JUN HO KIM, 대학원 신소재공학과 신소재공학전공)

초록/요약

Organic photovoltaics (OPVs) have been widely researched in recent years because of their potential applications which have the advantages of low manufacturing cost, lightweight device and high flexibility. Therefore, OPVs is considered as a core technology of the future green energy industry. In order to obtain high performance OPVs, it is very important to develop high quality transparent electrodes with a low resistance, high transparency, chemical stability, and a high work function. Considering the exaction formation and charge correction efficiencies in the OPVs, the development of a highly transparent conducting oxide (TCO) electrode is important because TCO is the primary contributor to the series resistance and transparency of the OPVs. Compared to metal cathodes such as Ca:Al, LiF/Al, and Ag films in OPVs, the TCO anode has a higher resistivity which acts as a main series resistance in OPVs. As TCO electrodes for OPVs, Sn-doped In2O3 (ITO), Zn-doped In2O3 (IZO), Al-doped ZnO (AZO), Ga-doped ZnO (GZO), In-Zn-Sn-O (IZTO), TiO2-doped ITO (TITO), Nb-doped TiO2 (NTO), and oxide-Ag-oxide multilayers have been reported. Among several TCO materials, ITO films are mainly used as an anode in OPVs due to their low resistance and high transmittance. In particular, OPVs using the near infrared (NIR) wavelength region has attracted interest because they can be applied in high efficient tandem OPVs. However, ITO anodes have a low transmittance in the NIR wavelength region due to a high carrier concentration above ~1021 cm-3 even though they have a low resistivity. Considering OPVs using the NIR wavelength region, ITO anode with a low transmittance in the NIR wavelength region is not desirable as a transparent electrode. To solve the problem of conventional ITO anodes, high transparent and mobility TCOs including Nb-, Zr-, W-, Mo-, Ni-, and Ge-doped In2O3, have attracted great attention as highly transparent TCOs for photovoltaics. In chapter 4.1, the TCO have demonstrated high transmittances in the NIR wavelength region because of their low resistivity resulting from the high carrier mobility unlike for ITO films. Among high mobility TCO electrodes, W-doped In2O3 (IWO) films have been extensively investigated because they demonstrated a low resistivity, higher valence electrons than Sn dopants, and a high transmittance in the NIR wavelength region. Newhouse et al., reported that a IWO film increased the carrier mobility up to ~112 cm2/V-s by increasing the electron relaxation time. In chapter 4.2, we investigated INO films prepared by a NiO and In2O3 co-sputtering process for OPV applications as transparent anodes. By optimizing RTA temperature, we obtain an INO film with a resistivity of 5.6610-4 Ohm-cm and an optical transparency of 91.23 %, which are acceptable values for anodes in OPVs. In addition, we compared the performances of OPVs fabricated on the as-deposited and annealed INO film to correlate sheet resistance of the anode and PCE value of OPVs. In chapter 4.3, we investigated the characteristics of Nb-doped In2O3 (INbO) films prepared by co-sputtering of Nb2O5 and In2O3 for use in transparent anodes for organic photovoltaics (OPVs). To optimize the Nb dopant composition in the In2O3 matrix, the effect of the Nb doping power on the resistivity and transparency of the INbO films were examined. The electronic structure and microstructure of the INbO films were also investigated using synchrotron X-ray absorption spectroscopy and X-ray diffraction examinations in detail. At the optimized Nb co-sputtering power of 30 W, the INbO film exhibited a sheet resistance of 15 Ohm/square, and an optical transmittance of 86.04 % at 550 nm, which are highly acceptable for the use as transparent electrodes in the fabrication of OPVs. More importantly, the comparable power conversion efficiency (3.34 %) of the OPV with an INbO anode with that (3.31 %) of an OPV with a commercial ITO anode indicates that INbO films are promising as a transparent electrode for high performance OPVs. In chapter 4.4, we developed PEDOT:PSS-free OPVs using WO3 and In2O3 mixed electrodes acting as a buffer hole injection layer (HIL) and anode simultaneously. Through the co-sputtering and rapid thermal annealing (RTA) of WO3 and In2O3, we achieved buffer and anode-integrated transparent electrodes with a sheet resistance of 17 Ohm/square, a transmittance of 90.32 %, and a work function of 4.83 eV, all of which are comparable to values obtained with a conventional ITO anode. Due to the existence of WO3 in the In2O3 matrix, OPVs fabricated on an IWO electrode with no acidic PEDOT:PSS buffer layer showed a PCE of 2.87 %. Therefore, a transparent IWO electrode simultaneously acting as an HIL and anode layer can be considered a promising transparent electrode for cost-efficient and reliable OPVs because it could eliminate the use of acidic PEDOT:PSS HIL.

more

목차

ABSTRACT I
TABLE OF CONTENTS IV
LIST OF FIGURES VIII
LIST OF TABLES XV
1. Introduction 1
1.1. Introduction to transparent conducting oxides (TCOs) 1
1.2. The need for renewable energy 4
1.3. Applications of TCOs to solar energy 5
1.4. What is photovoltaic? 6
1.5. Why organic photovoltaics? 9
1.6. Reference 18

2. Literature survey 21
2.1. Transparent Conducting Oxides based on In2O3 oxide 21
2.1.1. Band structures of In2O3 oxide 21
2.2. Properties of Transparent Conducting Oxides 23
2.2.1. Electrical Properties 23
2.2.2. Van der Pauw Hall measurement and 4-point probe 24
2.2.3. Optical Properties 27
2.3. Principle of solar cell operation 30
2.3.1. Solar cell parameters 30
2.3.2. Current density-voltage characteristics of solar cells 31
2.4. References 40

3. Experimental procedures and Characterization 42
3.1. Experimental procedures 42
3.2. Characterization Tools 45
3.2.1. Sputtering system 45
3.2.2. XRD measurements 48
3.2.3. Surface Characterization via Scanning Probe Microscopy 48
3.2.4. Work Function Measurement 50
3.2.5. Bulk Heterojunction Devices 50
3.3. References 56

4. Results and discussion 57
4.1. Rapid thermal annealed WO3-doped In2O3 films for transparent electrodes in organic photovoltaics 57
4.1.1. Introduction 57
4.1.2. Experimental details 59
4.1.3. Results and discussion 61
4.1.4. Conclusion 67
4.1.5. References 77
4.2. Effects of rapid thermal annealing on electrical, optical and structural properties of Ni-doped In2O3 anodes for bulk heterojunction organic solar cells 79
4.2.1. Introduction 79
4.2.2. Experimental details 81
4.2.3. Results and discussion 83
4.2.4. Conclusion 88
4.2.5. References 96
4.3. Highly transparent Nb-doped indium oxide electrodes for organic solar cells 98
4.3.1. Introduction 98
4.3.2. Experimental details 101
4.3.3. Results and discussion 104
4.3.4. Conclusion 112
4.3.5. References 123
4.4. Buffer and anode-integrated WO3-doped In2O3 electrodes for PEDOT:PSS-free organic photovoltaics 127
4.4.1. Introduction 127
4.4.2. Experimental details 129
4.4.3. Results and discussion 131
4.4.4. Conclusion 140
4.4.5. References 149

5. Summary and conclusions 151
ACKNOWLEDGEMENTS XVI

more
반출 Meta View 목록