Photocurrent enhancement of Ⅲ-Ⅴ solar cells using antireflection coatings with ZnS/MgF2 nanostructures

Abstract

As interest in eco-friendly renewable energy increases, tangible changes have appeared, such as investments in the development of electrification technology for automobiles by leading companies and the continuous growth of demand for solar panels for homes. Globally, commercial solar cells are fabricated by polycrystalline silicon due to the ease of supply of the material and superior cost, but as of 2019, the power conversion efficiency (PCE) of a commercial solar module is around 20%, and the reported high efficiency silicon solar cells do not exceed 30% of PCE. A solar cell is a photoelectric conversion device that uses kinetic energy of excited electrons and holes from a depletion layer by the solar illumination to generate electromotive force. In the case of a single bandgap solar cell, the absorbed light energy is not completely converted into electromotive force, and thermal loss is very significant. In order to solve the problem of energy loss, researches have been conducted to achieve high power conversion efficiency using the concept of intermediate band, multiple junction, and quantum dots. Among them, the multi-junction solar cells have difficulty in fabrication due to the complexity of the process, but have shown the most successful PCE among the high efficiency solar cells studied in the various fields. In order to optimize the external quantum efficiency, optical aid is essential for the solar cells. However, the results of in-depth research aimed at optimizing the external quantum efficiency for complex solar cells such as multi-junction solar cells are still insufficient. In this study, as a part of the research to improve the quantum efficiency of multi-junction solar cell, the growth technique and mechanism of nanostructures, and optimization for optical design were discussed. In the case of multijunction solar cells, the energy absorbed in each sub-cell is different so that various current densities are generated in each sub-cell even in one solar cell. As a result, the current density of the entire solar cell is limited by the low current sub-cell, which indicates the importance of current matching in each sub-cell. It is possible to use a designed thin film to help selective absorption in the active region. In addition, by controlling the nanostructure of the optical material, it is possible to form a thin film that is less dependent on the properties of the material, which make it easy to bring out the quantum efficiency of the optical device to the limit. The first chapter of the thesis deals with background theories and discussions of solid-light interactions, which must be discussed for the ellipsometry analysis, the basis of optical analysis of this study. In Chapter 2, the growth method of nanostructures with optimized optical properties was introduced and the growth mechanism of nanostructures are investigated. In particular, the growth mechanism of nanostructures following the generalized tangent rule, which has not been proved previously, was mathematically derived, and through physical analysis, the fundamental differences between cosine and tangent rules were identified. In Chapter 3, extensive optical calculations on multi-junction solar cells constructed using Group III-V compounds of InGaP/GaAs/InGaAsP/InGaAs were performed using the simulation procedures and stabilized condition of nanostructures required to design an optimized multilayer optical structure. Through this, the anti-reflection film structures optimized for various materials and nanostructures were proposed. Chapter 4 summarizes the procedure and semantics of the computer simulation technique needed to conduct this study, and provides the implemented script. The last chapter summarizs researches that have been conducted to improve the quantum efficiency of solar cells, which are work function control by material synthesis, p-type effective Schottky barrier control through hole transport layer and defect analysis through ellipsometry, effect of localized surface plasmon resonance of silver nanoparticles on solar cell performance, conductive antireflection coating based on ITO.| 친환경 재생에너지에 대한 관심이 증대되면서, 선도 기업들의 자동차의 전동화 기술 개발 투자와 가정용 태양광 패널 수요의 지속적 성장 등 가시적인 변화로 나타나고 있다. 세계적으로 상업용 태양전지는 재료 공급의 용이성과 가격적 우수성으로 인해 다결정 실리콘이 사용되고 있으나, 2019년 현재 상업용 태양전지 모듈의 효율은 20% 전후이고, 보고된 고효율의 실리콘 태양전지도 30%의 효율을 넘지 못한다. 태양전지는 p-n접합을 기반으로 하여, 공핍층에서 여기된 전자 및 전공을 추출하므로써, 빛 에너지를 이용해 여기된 전자의 운동 에너지를 기전력으로 삼는 광전변환소자이다. 이 과정에서 단일 밴드갭의 태양전지의 경우 흡수된 빛에너지가 완전하게 기전력으로 변환되지 못하고 열적으로 손실되는 양이 커진다. 에너지 손실에 대한 문제점을 보완하기 위해 중간밴드, 다중접합, 그리고 양자점 등의 개념을 이용해 광전변환의 고효율화를 달성하려는 연구가 진행되어 왔다. 그 중 다중접합 태양전지는 공정의 복잡도로 인해 제작에 어려움이 있으나, 다양하게 연구되는 초고효율 태양전지 중 가장 성공적으로 높은 광전변환효율을 보여주고 있다. 태양전지는 외부양자효율의 최적화를 위해서 광적 설계를 통해 광학적 보조가 필수적인데, 현재까지 다중접합 등 복잡구조의 태양전지에 대한 외부양자효율 최적화를 목표로 이루어진 심도 있는 연구 결과가 아직 부족한 시점이다. 본 연구에서는 다중접합 태양전지의 양자효율 향상을 위해 수행된 연구의 일환으로, 광학 설계를 위한 나노 구조의 성장 기법과 성장 매커니즘, 그리고 구조의 최적화 방법을 논한다. 다중접합 태양전지의 경우, 태양전지의 각 층에서 흡수되는 에너지가 달라 하나의 태양전지 안에서도 층별로 다양한 전류밀도를 생산한다. 전류가 낮은 층에 의해 태양전지의 전체의 전류밀도가 제한되게 되며, 이로 인해 각 층에서의 전류 매칭의 중요성이 나타난다. 설계된 박막을 이용해서 활성영역에서의 선택적 흡수를 돕는 것이 가능하다. 또한, 광학 재료의 나노구조를 제어하면, 재료의 특성에 더 적게 의존하는 박막을 구성하는 것이 가능해 광소자가 가지고 있는 양자효율을 한계까지 이끌어 내기 위한 최적화가 용이해진다. 논문의 1장에서는 본 연구에서 수행된 광학 분석의 기반인 타원편광 분석기법에서 필히 논의되어야 할, 고체와 빛의 상호작용에 대한 배경이론과 이에 대한 고찰을 다루고 있다. 2장에서는 최적화된 광특성을 가진 나노구조의 성장 방법을 소개하고 구조적 특성에 대한 분석을 통해 성장된 나노 구조의 성장 매커니즘을 분석하였다. 특히나 기존에 증명되지 못했던 generalized tangent rule에 의한 나노구조의 성장 매커니즘을 수학적으로 유도하였고 물리적인 분석을 통해, cosine rule과 tangent rule의 근원적인 차이점을 규명하였다. 3장에서는 최적화된 다층 광학 구조의 설계에 필요한 시뮬레이션의 절차와 안정화된 박막 성장 기법을 이용하여 InGaP / GaAs / InGaAsP / InGaAs의 Ⅲ-Ⅴ족 화합물 반도체를 이용해 구성된 다중접합 태양전지에 대한 광범위한 광학 연산의 결과를 보여주고 다양한 물질과 나노구조에 대해서 최적화된 반사방지막 구조를 제안하였다. 4장에서는 본 연구를 수행하는데 필요했던 전산모사 기법에 대한 절차와 의미에 대해서 정리하였고, 구현된 스크립트를 제공하였다. 마지막 장에서는 학위 기간 중 태양전지의 양자효율 향상을 위해 연구하였던 물질의 합성에 의한 일함수 조절, hole 전송층 설계를 통한 p형 유효 쇼트키 장벽 제어와 타원편광을 통한 결함분석, 은 나노입자의 국소 표면 플라즈몬 공명이 태양전지에 미치는 영향, ITO기반의 전도성 반사방지막 공정 연구를 요약적으로 논하였다.; As interest in eco-friendly renewable energy increases, tangible changes have appeared, such as investments in the development of electrification technology for automobiles by leading companies and the continuous growth of demand for solar panels for homes. Globally, commercial solar cells are fabricated by polycrystalline silicon due to the ease of supply of the material and superior cost, but as of 2019, the power conversion efficiency (PCE) of a commercial solar module is around 20%, and the reported high efficiency silicon solar cells do not exceed 30% of PCE. A solar cell is a photoelectric conversion device that uses kinetic energy of excited electrons and holes from a depletion layer by the solar illumination to generate electromotive force. In the case of a single bandgap solar cell, the absorbed light energy is not completely converted into electromotive force, and thermal loss is very significant. In order to solve the problem of energy loss, researches have been conducted to achieve high power conversion efficiency using the concept of intermediate band, multiple junction, and quantum dots. Among them, the multi-junction solar cells have difficulty in fabrication due to the complexity of the process, but have shown the most successful PCE among the high efficiency solar cells studied in the various fields. In order to optimize the external quantum efficiency, optical aid is essential for the solar cells. However, the results of in-depth research aimed at optimizing the external quantum efficiency for complex solar cells such as multi-junction solar cells are still insufficient. In this study, as a part of the research to improve the quantum efficiency of multi-junction solar cell, the growth technique and mechanism of nanostructures, and optimization for optical design were discussed. In the case of multijunction solar cells, the energy absorbed in each sub-cell is different so that various current densities are generated in each sub-cell even in one solar cell. As a result, the current density of the entire solar cell is limited by the low current sub-cell, which indicates the importance of current matching in each sub-cell. It is possible to use a designed thin film to help selective absorption in the active region. In addition, by controlling the nanostructure of the optical material, it is possible to form a thin film that is less dependent on the properties of the material, which make it easy to bring out the quantum efficiency of the optical device to the limit. The first chapter of the thesis deals with background theories and discussions of solid-light interactions, which must be discussed for the ellipsometry analysis, the basis of optical analysis of this study. In Chapter 2, the growth method of nanostructures with optimized optical properties was introduced and the growth mechanism of nanostructures are investigated. In particular, the growth mechanism of nanostructures following the generalized tangent rule, which has not been proved previously, was mathematically derived, and through physical analysis, the fundamental differences between cosine and tangent rules were identified. In Chapter 3, extensive optical calculations on multi-junction solar cells constructed using Group III-V compounds of InGaP/GaAs/InGaAsP/InGaAs were performed using the simulation procedures and stabilized condition of nanostructures required to design an optimized multilayer optical structure. Through this, the anti-reflection film structures optimized for various materials and nanostructures were proposed. Chapter 4 summarizes the procedure and semantics of the computer simulation technique needed to conduct this study, and provides the implemented script. The last chapter summarizs researches that have been conducted to improve the quantum efficiency of solar cells, which are work function control by material synthesis, p-type effective Schottky barrier control through hole transport layer and defect analysis through ellipsometry, effect of localized surface plasmon resonance of silver nanoparticles on solar cell performance, conductive antireflection coating based on ITO.