A Study of Electrical, Structural and Optical Characteristics of Doped ZnO Electrode for Chalcopyrite Solar Cells
- 주제(키워드) ZnO , Solar Cell , Chalcopyrite , TCO
- 발행기관 고려대학교 대학원
- 지도교수 성태연
- 발행년도 2014
- 학위수여년월 2014. 2
- 학위구분 박사
- 학과 일반대학원 신소재공학과
- 원문페이지 135 p
- 실제URI http://www.dcollection.net/handler/korea/000000048630
- 본문언어 영어
- 제출원본 000045793247
초록/요약
Compound Cu-In-Ga-Se (CIGS) thin film solar cell is one of most promising sources for the demands on renewable energies, because of its outstanding energy conversion efficiency. The front electrode of CIGS solar cells are transparent conductive material. Transparent Conducting Oxide (TCO) thin films are generally used for transparent electrode. 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 commercial fabrication. But ITO thin films show the best electrical and optical properties when they are deposited at elevated temperatures (250 ~ 300℃), and CIGS absorption layer damaged at high temperature over 250℃. From this reason, CIGS solar cells are using ZnO based transparent electrode such as Al, Ga, B and F doped ZnO as their window electrode. First, the optical properties of the impurity doped ZnO thin films were analyzed by taking into account for the non-parabolicity in the conduction band. The Drude parameters and the optical band gap shift were analyzed by using the carrier density dependent effective mass determined by the first-order non-parabolicity approximation. The Drude parameters were obtained by applying simple Drude model combined with Lorentz oscillator model for the optical transmittance and reflectance spectrum. The correlation between the optical and Hall analyses were examined by taking the ratios of carrier density, mobility and the resistivity of optical to Hall measurements and by introducing a parameter which represent the ratio of the resistances to electron transport from the inside of the lattice and from the crystallographic defects. Also, it was shown that the measured shifts in optical band gaps could be well represented by theoretical estimation in which the band- gap widening due to band-filling effect and the band gap renormalization due to many-body effect derived for a weakly interacting electron-gas model were combined and the carrier density dependent effective mass was incorporated. ZTO films are known to be feasible to form amorphous phase, which provides a smooth surface morphology as well as etched side wall, when deposited by conventional sputtering technique, and therefore have a potential to be applied for the transparent thin film transistors. ZTO thin films were prepared by combinatorial sputtering of ZnO and SnO2 targets, and the dependence of their electrical and optical properties on the composition and deposition parameters was examined. The Sn content in the films varied in the range of 35 ~ 85 at.%. Deposition was carried out at room temperature, 150 and 300 oC, and the oxygen content in sputtering gas varied from 0 to 1 vol.%. Sn-rich films had better electrical properties, but showed large oxygen deficiency when deposited at low oxygen partial pressure. ZTO films with Sn content lower than 55 at.% had good optical transmission, but the electrical properties were poor due to very low carrier concentration. High Hall mobility of larger than 10 cm2/Vs could be obtained in carrier density range 1017~ 1020 cm-3, and etching rate was measurable for film with Sn content up to 70 at.% by suing a diluted HCl solution, indicating that there is a good possibility of utilizing ZTO films for device application. Ga-doped ZnO (GZO) thin films for CIGS solar cell application were fabricated on the buffer window layer made of intrinsic ZnO (i-ZnO) or Zn-Sn-O (ZTO) thin films which were pre-formed on glass substrates, and the effect of buffer layer on the electrical and structural properties of GZO films was investigated by making direct comparison with the GZO films grown on the bare glass substrate in the same batch. GZO films, which showed good crystallinity and electrical properties when deposited on the bare glass substrate, exhibited deteriorated crystallinity when deposited on the buffer i-ZnO layer, especially at deposition temperature of 150oC. The deteriorated electrical properties of the GZO films grown on the buffer i-ZnO layer at 150oC were found to be closely related with the poor crystallinity which was affected by the poor crystallinity of i-ZnO buffer layer. GZO films formed on amorphous ZTO layers gave comparable crystallinity and electrical properties with those grown on the bare glass substrates, indicating ZTO film as a possible alternative buffer layer for CIGS solar cells.
more목차
CONTENTS
ABSTRACT ----------------------------------------------------------------------------- I
TABLE OF CONTENTS ------------------------------------------------------------- IV
LIST OF FIGURES ------------------------------------------------------------------- VIII
LIST OF TABLES --------------------------------------------------------------------- XIII
1. Introduction -------------------------------------------------------------------------- 1
1.1. Motivation ---------------------------------------------------------------------- 1
1.2. Reference ----------------------------------------------------------------------- 2
2. Theoretical Background ----------------------------------------------------------- 5
2.1. ZnO thin films ----------------------------------------------------------------- 5
2.1.1. Introduction --------------------------------------------------------------- 5
2.1.2. Doped ZnO----------------- ----------------------------------------------- 6
2.1.3. CIGS solar cell Applications of ZnO thin films---------------------- 7
2.2. ZnO thin films ----------------------------------------------------------------- 7
2.2.1.Principle of solar cell ----------------------------------------------------- 7
2.2.2. Solar cell operation------------------------------------------------------- 9
2.2.3. Solar cell parameters----------------------------------------------------- 12
2.2.4. Illuminated current-voltage (I-V) characteristics--------------------- 14
2.2.5. Dark current-voltage (J-V) characteristics---------------------------- 14
2.3. References ---------------------------------------------------------------------- 14
3. Experimental details---------------------------------------------------------------- 26
3.1. Fabrication methods and characterization--------------------------------- 26
3.1.1. Fabrication----------------------------------------------------------------- 26
3.1.2. UV-vis spectroscopy ----------------------------------------------------- 28
3.1.3. Hall measurement--------------------------------------------------------- 30
3.2. References---------------------------------------------------------------------- 33
4. Optical analysis of doped ZnO thin films using non-parabolic conduction band parameters-------------------------------------------------------- 40
4.1. Introduction -------------------------------------------------------------------- 40
4.2. Preparation---------------------------------------------------------------------- 41
4.3. Dielectric modeling------------------------------------------------------------ 42
4.4. Correlation between Hall and optical analyses----------------------------- 46
4.5. Optical band gap analysis----------------------------------------------------- 52
4.6. Conclusion---------------------------------------------------------------------- 58
4.7. References ---------------------------------------------------------------------- 59
5. Electrical, optical and etching properties of Zn-Sn-O thin films deposited by combinatorial sputtering--------------------------------------------- 82
5.1. Introduction -------------------------------------------------------------------- 82
5.2. Preparation---------------------------------------------------------------------- 83
5.3. Properties of ZTO thin films-------------------------------------------------- 84
5.4. Summary------------------------------------------------------------------------ 86
5.5. References----------------------------------------------------------------------- 87
6. The effect of buffer window layer on the properties of Ga-doped ZnO window electrodes used for CIGS solar cells-------------------------------------- 96
6.1. Introduction -------------------------------------------------------------------- 96
6.2. Preparaion----------------------------------------------------------------------- 97
6.3. Effect of different buffer layers---------------------------------------------- 98
6.4. Summary------------------------------------------------------------------------ 102
6.5. References----------------------------------------------------------------------- 103
7. Summary------------------------------------------------------------------------------ 113
ACKNOWLEDGEMENTS 119
