Nanoclusters and Quantum Dots: Study of Their Recombination Kinetics for Solar Energy Applications

Abstract

The field of nanomaterials have shown us over the years that materials behave differently when we reach the nano-domain. One of the recent examples are noble metal nanoclusters (NCs). We all know gold and silver are metallic in nature but when we decrease the size to about 100 nm, surface plasmon behavior appears due to the resonance of electrons in a confined environment. But further decrease of particle size to less than 2 nm produces quantum confinement effect which give rise to discrete electronic structures like molecules. The optoelectronic properties of these NCs are of particular importance for solar energy applications. Gold nanoclusters (Au NCs) with molecule-like behavior have emerged as a new light harvester in various energy conversion systems. Despite several important strides made recently, efforts toward the utilization of NCs as a light harvester have been primarily restricted to proving their potency and feasibility. In solar cell applications, ground-breaking research with a power conversion efficiency (PCE) of more than 2% has recently been reported. Because of the lack of complete characterization of metal cluster-sensitized solar cells (MCSSCs), however, comprehensive understanding of the interfacial events and limiting factors which dictate their performance remains elusive. In this regard, the first part the research in this dissertation provides deep insights into MCSSCs for the first time by performing in-depth electrochemical impedance spectroscopy (EIS) analysis combined with physical characterization and density functional theory (DFT) calculations of Au NCs. In particular, the effect of the size of the Au NCs and electrolytes on the performance of MCSSCs are elucidated which reveal that they are significantly influential on important solar cell characteristics such as the light absorption capability, charge injection kinetics, interfacial charge recombination, and charge transport. Besides offering comprehensive insights, the highest ever PCE of 3.8% was achieved in this research. Among other metal NCs, Ag NCs are the best contender to be used as sensitizer in solar cells. But Ag NCs suffer from low stability and so far only a couple of reports have provided some evidence that Ag NCs indeed have ability of charge transfer to TiO2. In this dissertation the potency of the Ag NCs as sensitizer is also thoroughly investigated. A facile method based on thermal reduction was developed to synthesize Ag NCs with varying sizes which can be controlled by adjusting the synthesis temperature. Direct evidence of electron transfer to TiO2 was obtained by transient absorption spectrometry. A comprehensive analysis of recombination kinetics of Ag NCs based solar cells was also done by EIS. Along with all the analysis a highest PCE of 1.43% was also achieved which showed Ag NCs indeed have a potential to work as photoactive material in solar cells. The idea of solar paint was put forward few years ago as a transformative single coat method to synthesize photoanode of quantum dots sensitized solar cells (QDSSCs) but due to various technological challenges, not much progress have been made in this field. Chapter 5 provides the research which have been done in an effort to revive this idea. PbS based solar paints were synthesized by pseudo-successive ionic layer and adsorption reaction (p-SILAR) and a highest PCE of 1.4% was achieved. Whole synthesis process was carried out in ambient conditions and detailed XPS analysis was carried out to elucidate the changes which happen on the surface during fabrication process. PbS has a potential to achieve very high photocurrent (JSC) and to exploit this potential a temperature controlled successive ionic layer and adsorption reaction (SILAR) was developed which is the focus of chapter 6. By controlling the temperature a large number of PbS quantum dots (QDs) can be loaded onto TiO2 with controlled size that favors the charge transfer. By this methodology a high JSC of 30 mA/cm2 was achieved and insights into reasons for this high JSC were provided by studying the recombination kinetics by EIS. |나노소재 분야에서 물질이 나노 크기의 영역에 도달하면 기존과 다른 경향성을 나타낸다는 것은 다년간 잘 알려진 사실이다. 이러한 경향성을 잘 보여주는 예시 중 최근에 나타난 것이 귀금속 나노클러스터 (nanocluster, NCs)이다. 금이나 은과 같은 경우 일반적인 환경에서는 금속성을 나타내지만, 약 100 nm의 크기로 줄어들면 제한된 공간에서의 전자 공진으로 인해 표면 플라즈몬 현상이 발생하게 된다. 만약 2 nm 이하로 입자크기가 줄어들면 마치 분자처럼 분리된 전자구조를 갖게 되고 이로 인해 양자구속효과가 발생한다. 이러한 나노클러스터의 광전기적 특성은 태양 에너지 분야에 있어 특히나 중요하게 작용한다. 분자와 같이 작동하는 금 나노클러스터는 다양한 에너지 변환 시스템에서 새로운 감광제로서 떠오르고 있다. 하지만 최근에 발견된 여러가지 중요한 특성들에도 불구하고, 감광제로써 나노클러스터를 활용하기 위한 노력은 주로 효율과 가능성 측면에서 제한되어 있다. 최근 태양전지 분야에서 2%가 넘는 에너지 변환 효율 (power conversion efficiency, PCE)를 나타내는 획기적인 연구가 보고된 바 있다. 하지만 금속클러스터 감응형 태양전지 (metal cluster-sensitized solar cells, MCSSCs)의 전체적인 특성에 대한 분석이 부족하여, 표면에서의 반응과 성능을 결정하는 제한 요소에 대한 포괄적인 견해는 찾기 어려웠다. 이와 관련하여, 본 논문의 첫 번째 연구는 MCSSCs분야에선 처음으로 금 나노클러스터의 물리적 특성과 밀도 범함수 이론 (density functional theory, DFT) 계산을 포함한 전기화학적 인피던스 분광법 (electrochemical impedance spectroscopy, EIS) 분석을 수행하여 심도 있는 분석을 진행하였다. 특히 금 나노클러스터의 크기와 전해질이 빛 흡수 용량, 전자 주사 에너지, 표면 전자 재결합, 전자 수송과 같은 태양전지의 주요 특성에 막대한 영향을 미침으로써 MCSSCs의 성능에 관계한다는 것을 해명하였다. 이러한 포괄적인 분석 외에도, 3.8%라는 이례적인 PCE를 달성하였다. 다른 금속 나노클러스터 중에서 은 나노클러스터는 태양 전지의 감광제로서 사용될 만한 가장 유력한 경쟁후보이다. 그러나 은 나노클러스터는 안정성이 무척 낮은 편인 탓에, 지금까지 오직 두건의 논문에서만 실제로 은 나노클러스터가 TiO2로의 전자 수송 능력을 가지고 있다는 것을 증명하였다. 본 논문에서는 감광제로서의 은 나노클러스터의 효능도 철저하게 검토하였다. 합성 온도를 조절함으로써 은 나노클러스터의 크기를 조절할 수 있는 열 환원 합성법을 개발하였으며, 순간흡광분광기 (transient absorption spectrometry)를 통하여 TiO2로의 전자 수송에 대한 직접적인 분석결과를 얻을 수 있었다. 은 나노클러스터 태양전지의 재결합 에너지에 대한 분석은 금 나노클러스터의 경우와 같이 EIS를 통하여 구하였다. 이러한 분석들을 통하여 은 나노클러스터가 태양전지 광 활성물질로서 활용될 수 있는 가능성을 보여주는 1.43%의 높은 PCE를 달성할 수 있었다. 솔라 페인트 아이디어는 양자점 감응형 태양 전지 (quantum dots sensitized solar cells, QDSSCs)의 광전극을 합성하기 위한 변식 단일 코팅방법 (transformative single coat method)으로써 몇 년 전에 제시되었지만, 여러 가지 기술적인 문제로 인하여 별다른 진전을 보이지 못하고 있다. 제 5 장에서는 이 아이디어를 보다 효율적으로 활용하기 위한 연구 내용이다. PbS 기반 솔라 페인트는 의사 연속 이온 층 흡착 반응 (pseudo-successive ionic layer and adsorption reaction, p-SILAR)을 통하여 합성하였고 1.4%에 달하는 PCE를 달성하였다. 전체 합성과정은 대기조건하에서 진행되었으며, 제조 공정 중 표면에서 일어나는 변화를 규명하기 위하여 보다 자세한 XPS 분석을 실시하였다. 6장에서는 높은 수준의 광전류 (photocurrent, JSC)를 나타낼 수 있는 가능성을 가지고 있는 PbS와, 이 가능성을 발휘 시키기 위해 개발된 온도 조절 연속 이온 층 흡착 반응 (successive ionic layer and adsorption reaction, SILAR) 합성법 개발에 초점을 맞추었다. 온도를 조절을 통한 방법은 전자 수송에 용이하게 사이즈가 조절된 PbS 양자점 (quantum dots, QDs)을 TiO2 위에 다량 증착시키는 것이 가능하게 하였다. 이 방법을 통해 30 mA/cm2의 높은 JSC 를 달성하였고, EIS를 통한 재결합 에너지 분석으로 이처럼 높은 JSC 를 갖는 이유를 해명하였다.; The field of nanomaterials have shown us over the years that materials behave differently when we reach the nano-domain. One of the recent examples are noble metal nanoclusters (NCs). We all know gold and silver are metallic in nature but when we decrease the size to about 100 nm, surface plasmon behavior appears due to the resonance of electrons in a confined environment. But further decrease of particle size to less than 2 nm produces quantum confinement effect which give rise to discrete electronic structures like molecules. The optoelectronic properties of these NCs are of particular importance for solar energy applications. Gold nanoclusters (Au NCs) with molecule-like behavior have emerged as a new light harvester in various energy conversion systems. Despite several important strides made recently, efforts toward the utilization of NCs as a light harvester have been primarily restricted to proving their potency and feasibility. In solar cell applications, ground-breaking research with a power conversion efficiency (PCE) of more than 2% has recently been reported. Because of the lack of complete characterization of metal cluster-sensitized solar cells (MCSSCs), however, comprehensive understanding of the interfacial events and limiting factors which dictate their performance remains elusive. In this regard, the first part the research in this dissertation provides deep insights into MCSSCs for the first time by performing in-depth electrochemical impedance spectroscopy (EIS) analysis combined with physical characterization and density functional theory (DFT) calculations of Au NCs. In particular, the effect of the size of the Au NCs and electrolytes on the performance of MCSSCs are elucidated which reveal that they are significantly influential on important solar cell characteristics such as the light absorption capability, charge injection kinetics, interfacial charge recombination, and charge transport. Besides offering comprehensive insights, the highest ever PCE of 3.8% was achieved in this research. Among other metal NCs, Ag NCs are the best contender to be used as sensitizer in solar cells. But Ag NCs suffer from low stability and so far only a couple of reports have provided some evidence that Ag NCs indeed have ability of charge transfer to TiO2. In this dissertation the potency of the Ag NCs as sensitizer is also thoroughly investigated. A facile method based on thermal reduction was developed to synthesize Ag NCs with varying sizes which can be controlled by adjusting the synthesis temperature. Direct evidence of electron transfer to TiO2 was obtained by transient absorption spectrometry. A comprehensive analysis of recombination kinetics of Ag NCs based solar cells was also done by EIS. Along with all the analysis a highest PCE of 1.43% was also achieved which showed Ag NCs indeed have a potential to work as photoactive material in solar cells. The idea of solar paint was put forward few years ago as a transformative single coat method to synthesize photoanode of quantum dots sensitized solar cells (QDSSCs) but due to various technological challenges, not much progress have been made in this field. Chapter 5 provides the research which have been done in an effort to revive this idea. PbS based solar paints were synthesized by pseudo-successive ionic layer and adsorption reaction (p-SILAR) and a highest PCE of 1.4% was achieved. Whole synthesis process was carried out in ambient conditions and detailed XPS analysis was carried out to elucidate the changes which happen on the surface during fabrication process. PbS has a potential to achieve very high photocurrent (JSC) and to exploit this potential a temperature controlled successive ionic layer and adsorption reaction (SILAR) was developed which is the focus of chapter 6. By controlling the temperature a large number of PbS quantum dots (QDs) can be loaded onto TiO2 with controlled size that favors the charge transfer. By this methodology a high JSC of 30 mA/cm2 was achieved and insights into reasons for this high JSC were provided by studying the recombination kinetics by EIS.