Organic solar cells (OSCs) have attracted intense attention in recent years due to their advantages of low cost, easy fabrication, and high flexibility compared to its inorganic counterparts. However, due to the conflicts between the short diffusion length of excitons and long absorption length of incident photons, the thickness of OSCs is typically thin, and thus power conversion efficiency (PCE) is generally lower than traditional silicon solar cells. Therefore, an exquisite design of light trapping schemes is essential to the PCE improvement. Generally, physical guideline of light trapping involves two main approaches: geometric optics methods and wave optics methods. The former aims at elongating optical path inside the photoactive layer and thus enhancing photon absorption. For organic thin film solar cells with typical active layer thickness of 100 nm-200 nm, which is in subwavelength scale, we cannot investigate light harvesting mechanism simply by the geometric optics methods and instead wave optics properties should be considered. In this thesis, two different light trapping enhancement designs are proposed. In order to simulate these structures, we built up programs for absorption power calculation based on scattering matrix method (SMM) by rigorously solving Maxwell’s equations. It is worth to point out that, different from the widely-used calculation method by Absorption = 1-Transmission-Reflection, our algorithm can extract the net optical absorption of the active layer rather than the whole OSCs. This improvement is very important because metal absorption, which does not contribute to exciton generation, can be excluded from the result. In Chapter 3, design of organic solar cell incorporating periodically arranged gradient type active layer is presented. This design can enhance light harvesting with patterned organic materials themselves (i.e. self-enhanced active layer design) to avoid degrading electrical performance in contrast to introducing inorganic concentrators into the active layers such as silicon and metallic nanostructures. Our numerical results show that the OSC with a self-enhanced active layer, compared with the conventional planar active layer configuration, has broadband and wide-angle range absorption enhancement due to better geometric impedance matching and prolonged optical path. In Chapter 4, OSC with interstitial lattice patterned metal nanoparticles (NPs) is proposed, which can improve the light blocking of traditional square lattice patterned NPs structure and achieve broadband absorption enhancement. Compared to square lattice design, the plasmonic mode couplings between individual NPs in the interstitial lattice are more versatile and much stronger. Moreover, plasmonic modes can couple to the guided modes, resulting in large enhancement factor at some wavelengths. These works provide a theoretical foundation and engineering reference for high performance OSC designs. / published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

No description available.

In this thesis, fabrication of polymer solar cells through different device designs is presented and the resulted solar cell performance is discussed. Poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) are chosen as the photoactive layer materials as this material combination has been widely used and well investigated. The known properties of P3HT and PCBM make systematical studies and modeling for the effect of device designs on the performance of polymer solar cells possible although this is beyond the scope of this thesis. First, ITO electrodes were fabricated by sputtering and used as the transparent electrode for polymer solar cells. Properties of ITO film fabricated by different sputtering conditions were compared. Radio frequency (RF) sputtered ITO was found to exhibit the best transparency overall. This condition was further applied to the fabrication of ITO electrode for polymer solar cells with light trapping structures. Low temperature processed silicon oxide (SiOx) / titanium oxide (TiOx) periodic structures were fabricated by sol-gel method. Optical transmittance of the bottom electrode was altered by the presence of the reflective coating and thus the absorption in the photoactive layer was affected. By varying the number of layer pairs and thickness of each layer in the reflective coating, improvement of polymer solar cell performance was found by inserting reflective coating for optimized conditions. Finally, semi-transparent polymer solar cells with inverted structure were demonstrated using conductive polymer as the anode. The process in device preparation was vacuum-free and thus could be potentially useful in large-scale roll-to-roll fabrication. / published_or_final_version / Physics / Master / Master of Philosophy

銅銦鎵硒薄膜太陽能電池因為其高效率及相對低廉的成本，商業應用已經開始陸續出現。我們自主研發的集成式銅銦鎵硒薄膜電池生產系統可以全程製作襯底大小為10cm x 10cm 的電池及剃型組件。本研究工作主要分為兩個方向:第一個方向是研究及測試生長高效率太陽能電池及組件的具體條件。通過儀器改進及電池每層鍍膜的條件優化，能夠重複的生長高效率電池及組件; 第二個方向是通過減少銅銦鎵硒吸光習的厚度從而達到降低電池生產成本的目的。 / 銅銦鎵硒採用三步共蒸法製備吸收層。第一步先蒸發銦、鎵、硒三種元素形成n型硒化銦(鎵)薄膜;第二步蒸發銅、硒形成銦鎵硒半導體薄膜; 第三步蒸發一層額外的型硒化銦(鎵)薄膜保證整體電池是p型半導體。三步期間的襯底溫度經過小心調試，以使得合適的鎵梯度能夠在吸收層裹形成。通過每一層的條件優化我們能夠生長出高光電轉換效率的太陽能電池(17%)及組件(12%)。 / 太陽能電池的變溫測試及弱光測試對瞭解其應用潛能存在非常重要的作用。通過多組對比實驗發現銅銦鎵硒電池的溫度係數可以通過增加鎵在吸收層的組分而得到改善。同時，電池的弱光表現可以通過減少銅的量得到很大的提高。STM 的研究發現弱光表現得到改善是因為吸收層顆粒介面電阻的增加而導致的。 / 減少吸收層的厚度有利於進一步減少太陽能電池的材料成本。當電池的吸收層厚度小於一微米時，開路電壓跟短路電流都明顯有所減少，從而導致太陽能電池效率降低。更薄電池效率的提高可以從兩個方面來實現:氧化鋅表面的陷光結構及更加合適的鎵含量的使用。通過這兩艇改進方法，電池效率被提高到14%以上，使得超薄電池有更好的應用前景。 / Cu(In,Ga)Se₂ (CIGS)-based thin film solar cell has been commercialized recently due to its high energy conversion efficiency. We have designed an integrated satellite deposition system for producing CIGS solar cell with substrate size of 10cm x 10cm. This work mainly contains two parts with first part focusing on growing and characterizing high quality baseline solar cells and solar modules and second part concentrating on further reducing the material costs by growing thinner absorber layer with high efficiency. / The most difficult part in growing high quality CIGS solar cells originate from the absorber layers which contain p-type chalcopyrite structures with four different elements: Cu, In, Ga and Se. The widely used three-stage process is employed to co-evaporate In, Ga and Se first, then Cu and Se are evaporated to form the chalcopyrite CIGS structure and additional In, Ga and Se are deposited in the end to ensure an overall Cu deficiency, which is important for getting p-type semiconductors. The substrate temperatures during these three stages are carefully adjusted to introduce proper gallium gradients which is important for collecting electrons efficiently. Together with optimizing other layers we are able to get cell efficiency (area around 0.5 cm²) over 17%. To produce CIGS mini-modules, laser scribing as well as mechanical scribing are employed for series interconnection of individual cells using monolithic integration. The power and speed of laser together with the condition of mechanical scriber are carefully adjusted to ensure a minimum dead area in the module. Module (area around 80 cm²) with efficiency over 12% is produced. / Solar cells were fabricated and tested under varied temperature and weak light conditions. Temperature coefficient is compared between CIGS solar cells and other types of solar cells. Temperature coefficient is improved a lot with higher gallium content in the absorber layer. Weak light performance is shown to be increased a lot when copper percentage is lowered down. In order to examine the origin of beneficial effects from Cu-poor absorber, solar cells are grown with comparable grain sizes using our technique and I-V performances are examined under STM in grain/atomic scale. Leakage current is found to be mainly originates from boundary area. CIGS solar cells with Cu-poor absorber benefit from the reduced leakage from boundary area. / CIGS solar cells with thinner absorber thickness are studied and compared with conventional CIGS solar cells. We have found that high conversion efficiency solar cells can be grown for absorber thickness as thin as 1.5μm. Further reduction in absorber thickness deteriorates solar cell performances in both V∝ and Jsc resulting in conversion efficiency as low as 11%. / Two major approaches are performed to improve solar cell performances. Light trapping by etching AZO top contact for creating pyramid-structures to enhance light scattering. Efficiency is increased by more than 1.5% for solar cells with etched AZO surfaces. Solar cells with efficiency larger than 13% can be grown by using AZO etching. Another approach is by using suitable Ga content in absorber layer. Solar cells with efficiency as high as 14.17% are grown which makes thinner CIGS solar cells very competitive. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Yang, Shihang = 高效率、低成本銅銦鎵硒薄膜太陽能電池的製造 / 楊世航. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2012. / Includes bibliographical references (leaves 101-109). / Abstract also in Chinese. / Yang, Shihang = Gao xiao lu, di cheng ben tong yin jia xi bo mo tai yang neng dian chi de zhi zao / Yang Shihang. / Chapter 1 --- Introduction to Photovoltaics --- p.1 / Chapter 1.1 --- Energy crisis --- p.1 / Chapter 1.2 --- Physics of solar cells --- p.4 / Chapter 1.2.1 --- Light Absorption --- p.4 / Chapter 1.2.2 --- Charge Carrier Separation --- p.8 / Chapter 1.2.3 --- Solar Cell I-V Characteristics --- p.9 / Chapter 1.3 --- Classifications of Solar Cells --- p.11 / Chapter 1.3.1 --- Crystalline silicon solar cell --- p.11 / Chapter 1.3.2 --- Thin film solar cells --- p.12 / Chapter 1.3.3 --- Organic and polymer solar cells --- p.13 / Chapter 1.4 --- Cu(In,Ga)Se₂ (CIGS) based Solar Cells --- p.13 / Chapter 1.4.1 --- State of the art --- p.13 / Chapter 1.4.2 --- Material properties and structures --- p.14 / Chapter 1.4.3 --- CIGS advantages --- p.17 / Chapter 2 --- Integrated CIGS deposition system and fabrication process optimization --- p.21 / Chapter 2.1 --- Introduction to vacuum deposition system --- p.21 / Chapter 2.1.1 --- Integrated CIGS solar cell deposition system --- p.21 / Chapter 2.1.2 --- Ni-Al top grid evaporation system --- p.23 / Chapter 2.2 --- Fabrication processes --- p.23 / Chapter 2.2.1 --- Substrate treatment --- p.23 / Chapter 2.2.2 --- Molybdenum back contact deposition --- p.24 / Chapter 2.2.3 --- CIGS absorber layer formation --- p.26 / Chapter 2.2.4 --- Hetero-junction formation --- p.31 / Chapter 2.2.5 --- Window layer optimization --- p.32 / Chapter 2.2.6 --- Laser and mechanical scribing for mini-modules fabrication --- p.37 / Chapter 2.3 --- Equipment improvements --- p.42 / Chapter 2.3.1 --- Heating uniformity of substrate --- p.42 / Chapter 2.3.2 --- Use of pyrometer for improved control of absorber thickness/composition --- p.43 / Chapter 2.3.3 --- Se cracking unit --- p.45 / Chapter 2.4 --- Characterization of CIGS solar cells --- p.47 / Chapter 2.4.1 --- Morphology, composition and crystallinity --- p.47 / Chapter 2.4.2 --- Depth profile of CIGS --- p.49 / Chapter 2.4.3 --- Electrical property measurements --- p.51 / Chapter 2.5 --- Conclusion --- p.54 / Chapter 3 --- Performance of CIGS solar cells under non-standard test conditions --- p.56 / Chapter 3.1 --- Temperature coefficient measurement of CIGS --- p.57 / Chapter 3.1.1 --- Equipment set-up --- p.57 / Chapter 3.1.2 --- Temperature coefficients for different types of solar cells . --- p.60 / Chapter 3.1.3 --- CIGS solar cells with varied Ga/III composition --- p.65 / Chapter 3.2 --- Weak Light Performance of CIGS --- p.69 / Chapter 3.2.1 --- Introduction --- p.69 / Chapter 3.2.2 --- Experiment --- p.72 / Chapter 3.2.3 --- Results and discussion --- p.73 / Chapter 3.3 --- Conclusion --- p.81 / Chapter 4 --- CIGS solar cells with lower fabrication cost --- p.83 / Chapter 4.1 --- Fabrication cost analysis for commercial CIGS solar cells --- p.83 / Chapter 4.2 --- Thinner CIGS absorber layer --- p.84 / Chapter 4.2.1 --- Solar cell performances with different absorber thicknesses --- p.84 / Chapter 4.2.2 --- Performance improvement for thinner solar cell --- p.87 / Chapter 4.3 --- Conclusion --- p.96 / Chapter 5 --- Conclusion --- p.98 / Chapter 5.1 --- Summary of previous researches --- p.98 / Chapter 5.2 --- Future work --- p.99 / Bibliography --- p.101

Graduation date: 1998

No description available.

Heterojunctions of n-ZnO/p-Si solar cells were fabricated by RF sputtering ZnO:Al onto boron-doped (100) silicon (Si) substrates. Zinc Oxide (ZnO) films were also deposited onto soda lime glass for electrical measurements. Sheet resistance measurements were performed with a four-point-probe on the glass samples. Values for samples evacuated for 14 hours prior to deposition increased from 7.9 to 10.17 and 11.5 O/□ for 40 W, 120 and 160 W in RF power respectively. In contrast, those evacuated for 2 hours started with a higher value of 22.5 O/□, and decreased down to 7.6 and 5.8 O/□. Vacuum annealing was performed for both the glass and the Si samples. Current-voltage measurements were performed on the ZnO/Si junctions in the dark and under illumination. Parameters such as open-circuit voltage, Voc; short-circuit current, Isc; fill factor, FF; and efficiency, eta were determined. A maximum efficiency of 0.25% among all samples was produced, with an I sc of 2.16 mA, Voc of 0.31V and a FF of 0.37. This was a sample fabricated at an RF power of 80 W. Efficiency was found to decline with vacuum annealing. Furthermore, interfacial state density calculated based on capacitance-voltage measurements showed an increase in the value with vacuum annealing. The results found suggest that the interface states may be due to an interdiffusion of atoms, possibly those of Zn into the Si surface. The Electron Beam Induced Current (EBIC) method was used to determine diffusion length to be at a value ∼40--80 mum and therefore a minority carrier lifetime calculated of 3 musec. It was also used to determine the surface recombination velocity (SRV) of the fractured surface of the Si bulk from the fabricated solar cells. An SRV of ∼500 cm/sec was determined from the fractured Si surface, at a point located at 30 and 20 mum away from the junction interface.

No description available.

This thesis focuses on improving the adhesion of electroless metal layers plated to pit contacts in interdigitated, backside buried contact (IBBC) solar cells. In an electrolessly plated, pit contact IBBC cell, the contact grooves are replaced with lines of pits which are interconnected by the plated metal. It is shown, however, that electroless metal layers, plated by the standard IBBC plating sequence, are not adherent on pit contact IBBC solar cells. The cause of this adhesion problem is investigated by examining the adhesive properties of each of the metal layers in the electroless metallization sequence on planar test structures. This investigation reveals that Pd activation of heavily P diffused Si impedes Ni silicide growth and that, in the absence of a silicide at the Ni/Si interface, an electrolessly plated Cu layer will cause the underlying Ni layer to peel away from the substrate. It is also found that the Ni silicidation process itself intermittently causes the unreacted Ni to spontaneously peel away from the substrate. An electroless metallization sequence that results in thick, adhesive Cu deposits on planar &lt 100&gt surfaces is developed in this thesis. It is shown that this process leads to the formation of a Ni silicide on both n- and p- type, heavily diffused surfaces. Fully plated, pit contact IBBC solar cells were not able to be fabricated during the course of this work but it is reasonable to expect that the modified plating sequence developed in this work will result in the metal layers being adhesive on these cells.

Solar energy has the potential to meet all of society’s energy demands, but challenges remain in storing it for times when the sun is not shining. Electrolysis is a promising means of energy storage which applies solar-derived electricity to drive the production of chemical fuels. These so-called solar fuels, such as hydrogen gas produced from water electrolysis, can be fed back to the grid for electricity generation or used directly as a fuel in the transportation sector. Solar fuels can be generated by coupling a photovoltaic (PV) cell to an electrolyzer, or by directly converting light to chemical energy using a photoelectrochemical cell (PEC). Presently, both PV-electrolyzers and PECs have prohibitively high capital costs which prevent them from generating hydrogen at competitive prices. This dissertation explores the design of membraneless electrolyzers and PECs in order to simplify their design and decrease their overall capital costs. A membraneless water electrolyzer can operate with as few as three components: A cathode for the hydrogen evolution reaction, an anode for the oxygen evolution reaction, and a chassis for managing the flows of a liquid electrolyte and the product gas streams. Absent from this device is an ionically conducting membrane, a key component in a conventional polymer electrolyte membrane (PEM) electrolyzer that typically serves as a physical barrier for separating product gases generated at the anode and cathode. These membranes can allow for compact and efficient electrolyzer designs, but are prone to degradation and failure if exposed to impurities in the electrolyte. A membraneless electrolyzer has the opportunity to reduce capital costs and operate in non-pristine environments, but little is known about the performance limitations and design rules that govern operation of membraneless electrolyzers. These design rules require a thorough understanding of the thermodynamics, kinetics, and transport processes in electrochemical systems. In Chapter 2, these concepts are reviewed and a framework is provided to guide the continuum scale modeling of the performance of membraneless electrochemical cells. Afterwards, three different studies are presented which combine experiment and theory to demonstrate the mechanisms of product transport and efficiency loss. Chapter 3 investigates the dynamics of hydrogen bubbles during operation of a membraneless electrolyzer, which can strongly affect the product purity of the collected hydrogen. High-speed video imaging was implemented to quantify the size and position of hydrogen gas bubbles as they detach from porous mesh electrodes. The total hydrogen detected was compared to the theoretical value predicted by Faraday’s law. This analysis confirmed that not all electrochemically generated hydrogen enters the gas phase at the cathode surface. In fact, significant quantities of hydrogen remain dissolved in solution, and can result in lower product collection efficiencies. Differences in bubble volume fraction evolved along the length of the cathode reflect differences in the local current densities, and were found to be in agreement with the primary current distribution. Overall, this study demonstrates the ability to use in-situ HSV to quantitatively evaluate key performance metrics of membraneless electrolyzers in a non-invasive manner. This technique can be of great value for future experiments, where statistical analysis of bubble sizes and positions can provide information on how to collect hydrogen at maximum purity. Chapter 4 presents an electrode design where selective placement of the electrocatalyst is shown to enhance the purity of hydrogen collected. These “asymmetric electrodes” were prepared by coating only one planar face of a porous titanium mesh electrode with platinum electrocatalyst. For an opposing pair of electrodes, the platinum coated surface faces outwards such that the electrochemically generated bubbles nucleate and grow on the outside while ions conduct through the void spacing in the mesh and across the inter-electrode gap. A key metric used in evaluating the performance of membraneless electrolyzers is the hydrogen cross-over percentage, which is defined as the fraction of electrochemically generated hydrogen that is collected in the headspace over the oxygen-evolving anode. When compared to the performance of symmetric electrodes – electrodes coated on both faces with platinum – the asymmetric electrodes demonstrated significantly lower rates of cross-over. With optimization, asymmetric electrodes were able to achieve hydrogen cross-over values as low as 1%. These electrodes were then incorporated into a floating photovoltaic electrolysis device for a direct demonstration of solar driven electrolysis. The assembled “solar fuels rig” was allowed to float in a reservoir of 0.5 M sulfuric acid under a light source calibrated to simulate sunlight, and a solar to hydrogen efficiency of 5.3% was observed. In Chapter 5, the design principles for membraneless electrolyzers were applied to a photoelectrochemical (PEC) cell. Whereas an electrolyzer is externally powered by electricity, a PEC cell can directly harvest light to drive an electrochemical reaction. The PEC reactor was based on a parallel plate design, where the current was demonstrated to be limited by the intensity of light and the concentration of the electrolyte. By increasing the average flow rate of the electrolyte, mass transport limitations could be alleviated. The limiting current density was compared to theoretical values based off of the solution to a convection-diffusion problem. This modeled solution was used to predict the limitations to PEC performance in scaled up designs, where solar concentration mirrors could increase the total current density. The mass transport limitations of a PEC flow cell are also highly relevant to the study of CO2 reduction, where the solubility limit of CO2 in aqueous electrolyte can also limit performance.