Development and Optimization of Sputtered Cu2ZnSn(S,Se)4-Based Photoelectrodes for High-Performance Photoelectrochemical Water Splitting = 스퍼터링 공정을 이용한 케스터라이트 기반 광전극의 개발 및 최적화: 고성능 광전기화학적 수전해 응용

다국어 초록 (Multilingual Abstract)

Photoelectrochemical (PEC) water splitting presents a sustainable pathway for producing clean hydrogen energy using solar power. The performance of this technology critically depends on the photoelectrode, which must efficiently absorb sunlight and generate charge carriers. This dissertation focuses on developing a high-performance photocathode for hydrogen production using Cu2ZnSn(S,Se)4 (CZTSSe), a kesterite material chosen for its earth-abundant, low-cost, and non-toxic constituents. Despite these advantages, CZTSSe suffers from a high concentration of native point defects, such as CuZn antisites, which act as recombination centers and significantly limit its photo-conversion efficiency. To address this fundamental challenge, a systematic materials engineering approach was employed.
First, the bandgap of the CZTSSe absorber layer was precisely tuned to an optimal value of 1.10 eV for the hydrogen evolution reaction (HER) by controlling the S/(S+Se) compositional ratio. This optimization enhanced light absorption and charge transport properties, achieving an initial photocurrent density of 33.50 mA/cm².
Second, to ensure long-term stability and practical applicability, the system was transitioned from a highly corrosive acidic electrolyte to a neutral pH environment. To overcome the sluggish HER kinetics in neutral media and mitigate the intrinsic defect issues, a cation-doping strategy was implemented. While toxic Cd doping confirmed the effectiveness of suppressing Zn-related defects and improving crystallinity, non-toxic Ag and Ge were subsequently investigated as eco-friendly alternatives to control Cu- and Sn-related defects, respectively.
Finally, a synergistic co-doping strategy using both Ag and Ge was developed to suppress both Cu- and Sn-related defects simultaneously. This complementary defect engineering approach significantly promoted grain growth and dramatically reduced charge recombination at grain boundaries. As a result, the Ag,Ge co-doped CZTSSe photocathode achieved a half-cell solar-to-hydrogen (HC-STH) efficiency of 4.22% in a neutral electrolyte, which stands as one of the highest reported performances for kesterite-based photocathodes.
This work provides a comprehensive strategy to overcome the intrinsic limitations of kesterite materials through systematic cation doping. By demonstrating high-efficiency solar hydrogen production in a practical, neutral pH environment with non-toxic elements, this study holds significant academic and industrial implications for the advancement of scalable PEC technology.

국문 초록 (Abstract)

광전기화학(PEC) 물 분해 기술은 태양광을 이용하여 청정 수소 에너지를 생산하는 지속 가능한 방법으로 주목받고 있다. 이 기술의 핵심은 태양광을 효율적으로 흡수하여 전하를 생성하는 광전극의 성능에 달려있다. 본 연구에서는 풍부한 매장량과 저렴한 비용, 친환경적인 특성을 지닌 Cu2ZnSn(S,Se)4 (CZTSSe) 케스터 라이트 소재를 p-타입 광음극으로 활용하여 고효율 수소 생산을 목표로 하였다. 그러나 CZTSSe는 CuZn 반자리(antisite)와 같은 고유한 점결함(point defect) 농도가 높아 전하 재결합 손실이 크고, 이는 광전변환 효율을 저해하는 주된 원인으로 작용한다. 이러한 문제를 해결하기 위해 본 연구에서는 체계적인 재료 공학적 접근법을 적용하였다. 첫째, S/(S+Se) 조성비 제어를 통해 CZTSSe 흡수층의 밴드갭을 수소 발생 반응에 최적화된 1.10 eV로 정밀하게 조절하였다. 이를 통해 향상된 광 흡수 및 전하 수송 특성을 확보하여 초기 광전류 밀도를 33.50 mA/cm2까지 높였다. 둘째, 장기 구동 안정성과 실용성을 고려하여 기존의 부식성 강한 산성 전해질 대신 중성 전해질 환경을 도입하였다. 중성 환경에서의 느린 수소 발생 반응 속도를 극복하고, 고질적인 결함 문제를 해결하기 위해 양이온 도핑 전략을 적용하였다. 독성이 있는 Cd 도핑을 통해 Zn 관련 결함 억제와 결정성 향상 효과를 확인하였고, 이를 바탕으로 친환경적인 Ag와 Ge를 대체 도펀트로 도입하여 각각 Cu 및 Sn 관련 결함을 효과적으로 제어하였다. 마지막으로, Ag와 Ge를 동시에 도핑하는 시너지 효과를 활용하여 Cu와 Sn 관련 결함을 동시에 억제하는 상호보완적 결함 제어 전략을 구현하였다. Ag와 Ge 동시 도핑은 결정립 성장을 촉진하고, 결정립계(grain boundary)에서의 전하 재결합을 획기적으로 감소시켰다. 그 결과, 중성 전해질 환경에서 4.22%라는 높은 반쪽 전지 태양광-수소 전환(HC-STH) 효율을 달성하였으며, 이는 기존 케스터 라이트 기반 광음극 연구 중 최고 수준의 성능이다. 본 연구는 케스터라이트 소재의 고질적인 결함 문제를 체계적인 양이온 도핑 전략을 통해 해결하고, 이를 통해 친환경적이고 실용적인 중성 환경에서 고효율 수소 생산 가능성을 입증했다는 점에서 큰 학문적, 산업적 의의를 가진다.

목차 (Table of Contents)

• Chapter 1. Introduction 1
• 1.1 Global Warming 1
• 1.2 Renewable Energy Resources 4
• 1.3 Hydrogen Energy 5
• 1.4 Photoelectrochemical Water Splitting 8
• 1.5 Why Cu2ZnSn(S,Se)4? 13
• 1.6 Shortcomings of Kesterite Materials for Application 15
• 1.7 Cation doping strategies for high-performance CZTSSe thin films 17
• 1.8 Thin Film Deposition Techniques and Advantages of Sputtering 22
• 1.9 References 23
• Chapter 2. Bandgap Tuning and Thin-Film Optimization of CZTS-based kesterite materials 27
• 2.1 Motivation and Background 27
• 2.2 Experimental Method 29
• 2.2.1 Preparation of active layer 29
• 2.2.2 Preparation of photocathodes and solar cells 29
• 2.2.3 Materials Characterizations 30
• 2.2.4 PEC performance measurements 31
• 2.2.5 Photovoltaic performance measurements 32
• 2.3 Results and Discussion 33
• 2.3.1 Crystallographic properties of CZTSe, CZTSSe, CZTS thin films 33
• 2.3.2 Chemical properties of CZTSe, CZTSSe, CZTS thin films 39
• 2.3.3 Photoelectrochemical performances 41
• 2.3.4 Photovoltaic performances 49
• 2.4 Conclusion 52
• 2.5 References 53
• Chapter 3. Defect Suppression and Onset Potential Enhancement of CZTSSe photocathode 56
• 3.1 Motivation and Background 56
• 3.2 Experimental Method 59
• 3.2.1 Preparation of the cation doped CZTSSe layer 59
• 3.2.2 Fabrication of photocathodes and photovoltaics 59
• 3.2.3 Materials Characterizations 60
• 3.2.4 PEC measurements 61
• 3.2.5 Photovoltaic performance measurements 63
• 3.3 PEC Performance of Cd Doped CZTSSe Photocathodes 65
• 3.3.1 Crystallographic properties of Cd doped CZTSSe thin films 65
• 3.3.2 Chemical properties of Cd doped CZTSSe thin films 71
• 3.3.3 PEC performance and GC 73
• 3.3.4 EIS and UPS analysis 85
• 3.3.5 KPFM analysis 91
• 3.3.6 AS anlaysis 95
• 3.4 PEC Performance of Ag or Ge Doped CZTSSe Photocathodes 97
• 3.4.1 Crystallographic properties of Cd doped CZTSSe thin films 97
• 3.4.2 Chemical properties of Cd doped CZTSSe thin films 99
• 3.4.3 PEC performances 101
• 3.4.4 KPFM analysis 103
• 3.4.5 EIS and UPS analysis 105
• 3.5 Conclusion 108
• 3.6 References 109
• Chapter 4. Synergistic Defect Mitigation of CZTSSe photocathode 115
• 4.1 Motivation and Background 115
• 4.2 PEC Performances 116
• 4.3 KPFM analysis 118
• 4.4 EIS and UPS analysis 120
• 4.5 CZTSSe-BiVO4 system 122
• 4.6 Degradation mechanism 124
• 4.7 Conclusion 127
• 4.8 References 128
• Chapter 5. Summary and Conclusion 129
• Abstract in Korean 132