published_or_final_version / Physics / Doctoral / Doctor of Philosophy

Ma, Xuhang = 柔性襯底銅銦鎵硒太陽能電池製備工藝 / 馬續航. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2013. / Includes bibliographical references (leaves 88-91). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstracts also in Chinese. / Ma, Xuhang = Rou xing chen di tong yin jia xi tai yang neng dian chi zhi bei gong yi / Ma Xuhang.

在這個對可再生能源需求不斷增加的新時代,聚合物太陽能電池成為其中一個最熱門的研究題目。在過去十年,聚合物太陽能電池的研究大多集中在液體處理的塊材異質接面結構。與傳統的‘雙層’結構比較,塊材異質接面結構提供一個更大的給體-受體界面面積讓電荷分離及減少電荷重組,從而帶來更高的能量轉換效率。目前使用塊材異質接面結構的聚合物太陽能電池最高能量轉換效率約為8 %。最近在文獻中公佈的一篇文章中,聲稱‘雙層’器件也可以達到與塊材異質接面器件媲美的高效率。這類‘雙層’器件採用相連沉積的給體層P3HT 和以二氯甲烷作溶劑的受體層PCBM 製作。然而,也有報告指在熱退火處理下PCBM 會擴散進入P3HT 的非晶區從而堤高能量轉換效率。我的工作是研究這類所謂的‘雙層’系統。藉運用紫外-可見光光譜儀和掠角X 光繞射,對埰用不同處理手法的純P3HT 薄膜的聚集和結晶性能進行研究。結果發現P3HT 的聚集和結晶性能可能沒有直接的相關性。同時也發現通過縮短底層P3HT 溶劑的揮發時間,這類液體鑄型‘雙層’器件的能量轉換效率得以提高。通過加熱P3HT 薄膜來加快變乾的速度,可以造具較低聚集度和較高結晶密度的P3HT 層。比起常規P3HT 薄膜可以提高能量轉換效率大約15 %。同時也對重量比為1:1 的複合P3HT:PCBM 薄膜的聚集和結晶性能進行了研究。實驗結果進一步表明P3HT 的聚集和結晶性能可能沒有直接的相關性。雖然實驗未有充分運用所有其他可能的處理方法和物理参數對器件進行全面優化,但這項研究卻為液體鑄型雙層聚合物太陽能電池開闢一條新的優化路線。 / In this new age with increasing demand of renewable energy sources, organic solar cell (OSC) has become one of the most intense research topics. The major attention of research in OSC was given to the solution-processed bulk heterojunction (BHJ) architecture during the past ten years. Compared to traditional ‘bilayer’ architecture, the BHJ structure provides a greater donor-accepter interface area for charge separation and reduced recombination, which leads to a higher power conversion efficiency (PCE). The highest PCE reported using BHJ was about 8 %. In a recent article published in the literature, it was claimed that ‘bilayer’ devices can also achieve a high efficiency comparable to that for BHJ devices. Such ‘bilayer’ devices were fabricated by sequential deposition of the donor P3HT and acceptor PCBM layers using dichlormethane as the solvent of PCBM. However, it was also reported that the PCBM could diffuse into the amorphous regions of P3HT during thermal annealing to cause the efficiency enhancement. My work is to study such so-called ‘bilayer’ system. By using UV-Visible spectrophotometer and grazing incidence XRD, the aggregation and crystallization properties of the pure P3HT films fabricated using different processing methods were studied. It was found that the P3HT aggregation may not have direct correlations with crystallinity. It is also discovered that the PCE of such solution-cast bilayer devices are increased by shortening the evaporation time of the underlying P3HT layer from a solution. By heating the P3HT film to accelerate its drying, a less aggregated P3HT layer with a higher density of crystallites could be obtained. The PCE could be enhanced by about 15 % over that using a regular P3HT film. The aggregation and crystallization properties of the composite P3HT:PCBM films with a ratio of 1:1 by weight were also studied. The experimental results further indicate that the P3HT aggregation may not have direct correlations with crystallinity. Although the devices have not been fully optimized by exhausting all the other possible processing and physical parameters, this study opens up a new route to optimize solution-cast bilayer polymer solar cells. / Detailed summary in vernacular field only. / Wong, Man Kwong = 聚合物太陽能電池研究 / 王文廣. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2012. / Includes bibliographical references. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstracts also in Chinese. / Wong, Man Kwong = Ju he wu tai yang neng dian chi yan jiu / Wang Wenguang. / Abstract --- p.I / 摘要 --- p.III / Acknowledgement --- p.VI / Chapter Chapter 1 --- Introduction to Organic Solar Cells --- p.1 / Chapter 1-1 --- The history of the development of solar cells --- p.1 / Chapter 1-2 --- Motivation of developing polymer solar cells --- p.2 / Chapter 1-3 --- The materials used in polymer solar cells --- p.4 / Chapter 1-4 --- Basic working principles of the polymer solar cells --- p.6 / Chapter 1-5 --- Definition of some important parameters of solar cells --- p.8 / Chapter 1-5-1 --- Short circuit current density (J[subscript Ssubscript C]) --- p.8 / Chapter 1-5-2 --- Open circuit voltage (V[subscript Osubscript C]) --- p.9 / Chapter 1-5-3 --- Fill factor (FF) --- p.10 / Chapter 1-5-4 --- Power conversion efficiency (PCE) --- p.11 / Chapter 1-6 --- An outline of this thesis --- p.12 / Chapter Chapter 2 --- Experimental Procedures --- p.13 / Chapter 2-1 --- Common preparation procedures in all the experiments --- p.13 / Chapter 2-1-1 --- The cleaning of the glass and indium tin oxide (ITO) substrates --- p.13 / Chapter 2-1-2 --- The preparation of different donor and accepter solution --- p.13 / Chapter 2-1-3 --- The coating of PEDOT:PSS on the glass and ITO substrates --- p.14 / Chapter 2-1-4 --- The thermal vapor deposition of the cathode layer for solar cell devices --- p.14 / Chapter 2-2 --- The detailed experimental procedures of the active layers in different chapters --- p.15 / Chapter 2-2-1 --- Fabrication of the active layers in chapter 3 --- p.15 / Chapter 2-2-2 --- Fabrication of the active layers in chapter 4 --- p.17 / Chapter 2-2-3 --- Fabrication of the active layers in chapter 5 --- p.18 / Chapter 2-3 --- Measurement of the properties of the film samples and the performance of the solar cells --- p.19 / Chapter 2-4 --- Experimental errors --- p.20 / Chapter Chapter 3 --- A Survey on Polymer Solar Cells and Some Preliminary Works --- p.21 / Chapter 3-1 --- Traditional bilayer polymer solar cells --- p.21 / Chapter 3-2 --- Bulk heterojunction polymer solar cells --- p.22 / Chapter 3-3 --- Studies of thermal annealing effects --- p.26 / Chapter 3-4 --- Studies of solvent annealing effects --- p.28 / Chapter 3-5 --- Studies of the relationship between the thickness and the optical density of the films --- p.34 / Chapter 3-6 --- Solution-cast bilayer polymer solar cells --- p.35 / Chapter 3-7 --- Previous studies on solution-cast bilayer polymer solar cells --- p.39 / Chapter Chapter 4 --- Studies on Solution-Cast Bilayer Polymer Solar Cells --- p.42 / Chapter 4-1 --- Motivation and the basic ideas of my research work --- p.42 / Chapter 4-2 --- Higher PCE through the use of a low pressure environment of film formation --- p.43 / Chapter 4-3 --- Higher J[subscript Ssubscript C] through the use of chloroform as the P3HT solvent --- p.46 / Chapter 4-4 --- Higher PCE through the use of a pre-dried heating process --- p.52 / Chapter 4-5 --- Dependence of the device performances on the pre-dried heating temperature --- p.55 / Chapter 4-6 --- Effect of a small amount of PCBM in the underlying P3HT layer --- p.59 / Chapter 4-7 --- Short summaries --- p.62 / Chapter Chapter 5 --- Studies on P3HT:PCBM Blend Layers --- p.64 / Chapter 5-1 --- Motivation --- p.64 / Chapter 5-2 --- Effects on the P3HT aggregation and crystallization of the P3HT:PCBM blend layers due to different treatments --- p.64 / Chapter 5-3 --- Studies of the effects of the removal of PCBM on the P3HT aggregation and crystallization in P3HT:PCBM blend layers --- p.67 / Chapter 5-4 --- Short summaries --- p.70 / Chapter Chapter 6 --- Conclusion --- p.71 / Chapter 6-1 --- Summaries of the research work --- p.71 / Chapter 6-2 --- Ideas for future studies --- p.72 / Bibliography --- p.74

Wang, Mingdong. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2011. / Includes bibliographical references. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract also in Chinese.

在本篇論文中，我們成功地在透明導電襯底上製備了一系列準一維納米材料陣列。我們首先製備了氧化鋅納米線陣列，然後把它們用作氧化鋅/硒化鎘核殼納米線纜陣列中的核以及合成硒化鎘和碲化鎘納米管陣列所需的犧牲模板。最後，金納米管陣列則是利用之前製備的硒化鎘納米管陣列為模板合成的。氧化鋅納米線陣列是通過高溫的熱蒸法和低溫的水熱法製備的。水熱法製備的氧化鋅納米線陣列的電導高於熱蒸法製備的氧化鋅納米線陣列，這使得水熱法製備的氧化鋅納米線更適合採用與電相關的後續處理方法。當氧化鋅納米線陣列被用作犧牲模板來製備納米管時，水熱法製備的氧化鋅納米線能被輕易地完全去除。基於這些認識，我們主要採用電化學沉積法在水熱法製備的氧化鋅納米線陣列表面沉積硒化鎘，得到了氧化鋅/硒化鎘核殼納米線纜陣列。接下來，我們將納米線纜陣列光電極和沉積了鉑催化劑的對電極組裝成三文治結構的太陽能電池。研究發現，採用多硫電解液的電池性能比碘基電解液的電池好，其中成分為1摩爾每升硫化鈉，1摩爾每升硫和1摩爾每升氫氧化鈉的多硫電解液的電池效率最高。當去除電化學沉積法生長的氧化鋅/硒化鎘和氧化鋅/碲化鎘核殼納米線纜陣列中的氧化鋅核以後，便在導電襯底上得到了硒化鎘和碲化鎘的納米管陣列。儘管兩種納米管陣列都對可見光有很強的吸收，但是，硒化鎘納米管陣列相比碲化鎘納米管陣列，表現出較高的光響應和較好的光催化降解亞甲基藍的活性。這是因為該樣品中的光生載流子能有效分離，同時能參與化學反應的表面積也較大。最後，我們選用硒化鎘納米管陣列作為模板，利用化學方法製備了金納米管陣列。金納米管的尺寸可以通過控制硒化鎘納米管模板來加以調節。當我們將具有拉曼活性的4-巰基苯甲酸分子吸附到金納米管的表面時，其拉曼散射相比未吸附時，顯著地增強了約四個數量級，如此大的提高來源於金納米管表面附近的局域電場增強效應。 / In this thesis, we demonstrated the synthesis of a series of pseudo-one-dimensional nanostructure arrays on transparent conducting substrates. We started with ZnO nanowire arrays, which were then served as the core for the ZnO/CdSe core/shell nanocable arrays formation. Further taking the ZnO as sacrificial templates led to the formation of CdSe (and CdTe) nanotube arrays. Finally, Au nanotube arrays were fabricated using the CdSe nanotube arrays as the template. ZnO nanowire arrays were synthesized via high-temperature thermal evaporation method (TE) and low temperature hydrothermal method (HT). The electrical conductivity of HT samples on the substrates was higher than that of the TE counterparts, making it attractive for further electrical-based processing. When serving as the sacrificial templates for nanotube fabrication, HT nanowires can be completely removed with ease. Based on these understanding, ZnO/CdSe core/shell nanocable arrays were obtained mainly via electrochemical deposition of CdSe on HT ZnO nanowire arrays. Nanocable-array-photoelectrode was assembled with a Pt-coated counter electrode into a sandwiched solar cell. Polysulfide electrolytes with various compositions were found to work better than iodine-based ones for such cells, and the cell with the polysulfide electrolyte containing 1 M Na₂S, 1 M S and 1 M NaOH showed highest efficiency. Removal of the ZnO cores in the electrodeposited ZnO/CdSe and ZnO/CdTe nanocable arrays left CdSe and CdTe nanotube arrays on the conducting substrate. Although strong visible-light absorption was observed from both two nanotube arrays, higher photocurrent and better photocatalytic degradation activity of methlyene blue were recorded from CdSe-nanotube-array samples (as compared to the CdTe ones), owing to effective charge separation and large surface area for chemical reactions. Lastly, Au nanotube arrays were synthesized via chemical method using CdSe nanotube arrays as the template. The dimensions of the Au nanotubes, as replicated from CdSe nanotubes, were tunable. When absorbed on the Au nanotube arrays surface, the Raman scattering of 4-mercaptobenzoic acid (a Raman-active molecule) was greatly enhanced for~4 orders of magnitude compared to the signals from the dry powder of the same molecule. Such large increase was due to the strong local electrical field enhancement near the Au nanotubes surface. / Detailed summary in vernacular field only. / Zhu, Haojun = 準一維納米結構在光伏、光催化及等離子體激元方面的應用 / 朱浩君. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2012. / Includes bibliographical references (leaves 141-168). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract also in Chinese. / Abstracts in English and Chinese. / Zhu, Haojun = Zhun yi wei na mi jie gou zai guang fu, guang cui hua ji deng li zi ti ji yuan fang mian de ying yong / Zhu Haojun. / Abstract --- p.i / 摘要 --- p.iii / Acknowledgements --- p.iv / Contents --- p.v / List of Figures --- p.viii / List of Tables --- p.xviii / Chapter Chapter 1 --- Introductions --- p.1 / Chapter Chapter 2 --- Background --- p.4 / Chapter 2.1. --- Nanostructured Photovoltaic (PV) Solar Cells --- p.4 / Chapter 2.1.1. --- Fundamental physics of nanostructures for solar cell applications --- p.5 / Chapter 2.1.2. --- Inorganic nano-architectures for PV cells --- p.9 / Chapter 2.2. --- Nanostructures for Photocatalytic Degradation of Organic Pollutants --- p.18 / Chapter 2.2.1 --- Overview of photocatalytic degradation of organic pollutants --- p.19 / Chapter 2.2.2 --- Photocatalysis under visible light illumination --- p.24 / Chapter 2.3. --- Plamonic Noble Metal Nanostructures --- p.29 / Chapter 2.3.1 --- Surface plasmons of noble metal nanostructures --- p.29 / Chapter 2.3.2 --- Applications of plasmonic noble metal nanostructures in solar energy conversion and sensing --- p.35 / Chapter Chapter 3 --- Methodologies and Instrumentations --- p.45 / Chapter 3.1. --- Materials Growth Methodologies --- p.45 / Chapter 3.1.1. --- Thermal evaporation (TE) methods --- p.45 / Chapter 3.1.2. --- Hydrothermal (HT) methods --- p.47 / Chapter 3.1.3. --- Electrodeposition (ED) methods --- p.49 / Chapter 3.1.4. --- Prototype solar cells assemble --- p.52 / Chapter 3.2. --- Characterization Techniques --- p.53 / Chapter 3.2.1. --- Morphological, structural, and compositional analysis using electron microscopy based techniques --- p.53 / Chapter 3.2.2. --- Photoelectrochemical (PEC) performance test --- p.63 / Chapter 3.2.3. --- Photocatalytic degradation of organic pollutants --- p.65 / Chapter 3.2.4. --- Single-particle scattering imaging and spectroscopy --- p.67 / Chapter Chapter 4 --- ZnO Nanowire Arrays on Conducting Substrates -- A Comparison on the Growth Methodology --- p.71 / Chapter 4.1. --- Introduction --- p.71 / Chapter 4.2. --- Experimental --- p.72 / Chapter 4.3. --- Results and Discussions --- p.75 / Chapter 4.3.1 --- Morphologies, crystal structures and chemical compositions --- p.75 / Chapter 4.3.2 --- ZnO nanowire arrays used as electrodes --- p.80 / Chapter 4.3.3 --- ZnO nanowire arrays used as sacrificial templates in electroplating . --- p.85 / Chapter 4.4. --- Conclusions --- p.88 / Chapter Chapter 5 --- ZnO-core/CdSe-shell Nanocable Arrays for Photovoltaic Solar Cells --- p.89 / Chapter 5.1. --- Introduction --- p.89 / Chapter 5.2. --- Experimental --- p.90 / Chapter 5.3. --- Results and Discussions --- p.93 / Chapter 5.3.1 --- Synthesis of the ZnO-core/CdSe-shell nanocable arrays on ITO/glass --- p.93 / Chapter 5.3.2 --- The photovoltaic (PV) performance --- p.100 / Chapter 5.4. --- Conclusions --- p.107 / Chapter Chapter 6 --- CdSe and CdTe Nanotube Arrays as Visible-light-driven Photocatalyst for Organic Pollutant Degradation --- p.108 / Chapter 6.1. --- Introduction --- p.108 / Chapter 6.2. --- Experimental --- p.109 / Chapter 6.3. --- Results and Discussions --- p.112 / Chapter 6.3.1. --- Morphology, crystal structure, and chemical composition of the nanotube arrays --- p.112 / Chapter 6.3.2. --- Optical properties --- p.116 / Chapter 6.3.3. --- Photoelectrochemical (PEC) performance --- p.117 / Chapter 6.3.4. --- Photocatalytic activities --- p.120 / Chapter 6.4. --- Conclusions --- p.123 / Chapter Chapter 7 --- Fabrication of Au Nanotube Arrays and Their Plasmonic Properties --- p.124 / Chapter 7.1. --- Introduction --- p.124 / Chapter 7.2. --- Experimental --- p.125 / Chapter 7.3. --- Results and Discussions --- p.127 / Chapter 7.3.1. --- Morphology, crystalline structure, and chemical composition of Au nanotube arrays --- p.127 / Chapter 7.3.2. --- Au nanotube formation mechanism --- p.129 / Chapter 7.3.3. --- Plasmonic properties of Au nanotube arrays on ITO/glass substrates --- p.131 / Chapter 7.3.4. --- Plasmonic properties of single Au nanotubes --- p.133 / Chapter 7.3.5. --- Au nanotube arrays on ITO/glass as SERS substrates --- p.134 / Chapter 7.4. --- Conclusions --- p.138 / Chapter Chapter 8 --- Conclusions --- p.139 / Bibliography --- p.141

Xu, Yeming = I-III-VI₂χχχII-VI和I-III-VI₂χχχχχχχχχχχχχχχ : 合成, 表征以及潛在应用 / 徐業明. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2011. / Includes bibliographical references (leaves 130-134). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract also in Chinese. / Xu, Yeming = I-III-VI₂ zu yi ji II-VI he I-III-VI₂ zu fu he na mi cai liao he tuo men de yi zhi jie gou : he cheng, biao zheng yi ji qian zai ying yong / Xu Yeming.

The materials Fe���(Si,Ge)(S,Se)���, Cu���PS[subscript 4-x]Se[subscript x] (0 ��� x ��� 4), and Cu���PxAs[subscript 1-x]S��� (0 ��� x ��� 1) have been synthesized and studied as new earth-abundant absorbers for single and multijunction photovoltaic cells as well as solar fuel generation. The synthesis, single-crystal growth, and optical and electrical properties of these materials are described and discussed in Chapter 2, 3, and 4. Inspired by the photovoltaic absorber Cu���ZnSnS���, the new compound CuZnPS��� has been discovered; its structure and properties are discussed in Chapter 5, including a comparative analysis to Cu���PS���. The compound YZrF��� (Chapter 6) has been synthesized and evaluated for the first time as a new optical host for green up-conversion. / Graduation date: 2013

This thesis presents an investigation into a wide range of potential materials for organic photovoltaic (PV) devices. A variety of optical techniques are used to define physical parameters for each material such as the photoluminescence quantum yield (PLQY), absorption coefficient and exciton diffusion length. Electrical characterisation is used to determine the optimal structure for devices fabricated with these materials. A number of novel materials are presented in this thesis. These include new polymers, both soluble and precursor, and a relatively new class of material, the conjugated dendrimer. These are highly configurable branching molecular structures that enable fine tuning of material properties. Work on polymers presented in this thesis investigates how such materials can be improved by testing the effect of small changes to their molecular structure. One of these changes had significant effects upon the overall material characteristics. The introduction of a dipole across a polymer successfully created a charge separating material without the need for an extra species such as C60 to be present. The introduction of the conjugated dendrimer to PV applications allows significant scope for molecular engineering. Dendrimers enable tight control over certain aspects of the molecular properties. Small changes can be made such as colour tuning or solubility that enable optimisation to be performed on the molecular level, rather than on device structure. Such changes produced significantly higher internal quantum efficiencies (> 90%) than typical polymer devices and offer the prospect of power conversion efficiencies in excess of 10%. Time-resolved luminescence (TRL) spectroscopy was used to characterise the behaviour of photogenerated excitons within organic films. The investigation of exciton diffusion length was performed upon two polymers, each utilising two different time-resolved methods; diffusion to a quencher and exciton-exciton annihilation. It was found that diffusion in polythiophene films is anisotropic and the photoluminescence lifetime is dependent upon film thickness. This is explained by the formation of self-ordered microstructures during the spin coating process. Data modelling was performed which took into account both the thickness variation and the interaction of excitons with a quenching interface producing a much more realistic approach than previously published work.

Organic solar cells offer the possibility for lightweight, flexible, and inexpensive photovoltaic devices. This thesis studies the physics of a wide range of materials designed for use in organic solar cells. The materials investigated include conjugated polymers, conjugated dendrimers, and inorganic nanocrystals. The materials studied in this thesis fall into five categories: conjugated polymers blended with a buckminsterfullerene derivative PCBM, nanocrystals synthesised in a conjugated polymer matrix, conjugated polymers designed for intramolecular charge separation, conjugated dendrimers blended with PCBM, and nanocrystals synthesised in a matrix of conjugated small molecules or dendrimers. Conjugated polymers blended with PCBM have been extensively studied for photovoltaic applications, and hence form an ideal test bed for new experiments. In this thesis this blend was used to achieve the first pulsed electrically detected magnetic resonance experiments on organic solar cells. Nanocrystals are attractive for photovoltaics because it is possible to tune their band gap across the solar spectrum. In this thesis a one-pot synthesis is used to grow PbS and CdS nanocrystals in conjugated polymers, soluble small molecules, and dendrimers, and characterisation is performed on these composites. Previous work on dendrimer: nanocrystal composites has been limited to non-conjugated molecules, and the synthesis developed in this thesis extends this work to a conjugated oligomer and a conjugated dendrimer. This synthesis can potentially be extended to a variety of conjugated soluble small molecule: nanocrystal and dendrimer: nanocrystal systems. Conjugated dendrimers have been successfully employed in organic light emitting diodes, and in this thesis they are applied to organic solar cells. Materials based on fluorene and cyanine dye cores show excellent absorption tunability across the solar spectrum. A set of electronically asymetric polymers designed for intramolecular charge separation were investigated. Quenching of the luminescence was observed, and light induced electron paramagnetic resonance measurements revealed that photoexcitation led to approximately equal numbers of positive polarons and nitro centred radical anions. This indicates that charge separation is occurring in these molecules.

The objective of this thesis is to develop low-cost high-efficiency crystalline silicon solar cells which are at the right intersection of cost and performance to make photovoltaics (PV) affordable. The goal was addressed by improving the optical and electrical performance of silicon solar cells through process optimization, device modeling, clever cell design, fundamental understanding, and minimization of loss mechanisms. To define the right intersection of cost and performance, analytical models to assess the premium or value associated with efficiency, temperature coefficient, balance of system cost, and solar insolation were developed and detailed cost analysis was performed to quantify the impact of key system and financial parameters in the levelized cost of electricity from PV.