Abstract The objective of this proposed study is to develop the new material CuInSe2 for larger area, low cost and high efficiency commercial CuInSe2 based solar cell for the solar resource in Taiwan. Recently, The¢¹-¢»-¢¾2 ternary chalcopyrite semiconductors, CuMX2(M=In, Ga; X=Se, S) have received considerable potential for nonlinear optics and photovoltaic applications such as a promising material for solar cell. For the request of large area homogeneousness, the electrochemical atomic layer epitaxy (ECALE) and molecular beam epitaxy (MBE) are used to deposit and adjust the composition. The combination of the advantages of MBE and ECALE could produce the large area epitaxial layer and get the precise compositions of CuInSe2 films to obtain a high conversion efficiency for commercial solar cell applications. For the study of large area CuInSe2 substrate, first, the electrochemical atomic layer epitaxy (ECALE) was applied to deposit a layer of CuInSe2 thin film on ITO substrate, and then the physical vapor deposition was applied for the annealing process to adjust the composition wanted. At last, a CuInSe2 epitaxial film was grown on the top of substrate under the MBE process. In addition, under the conditions of fixed In molecular beam flux and excess Se molecular beam flux, we can control the Cu/In composition ratio by changing Cu molecular beam flux to get stoichiometric and In-rich or Cu-rich epitaxial films, and predict the type of conductivity. It could be possible to obtain the high reproducibility and stability of the composition and properties of epilayers by controlling the growth parameters carefully. Finally, We hope it can be used in the manufacture of solar cell and get large area high conversion efficiency.

Thesis (Ph.D.)--Murdoch University, 2008. / Thesis submitted to the Faculty of Minerals and Energy. Includes bibliographical references (leaves 238-246)

Thesis (Ph.D.)--Northwestern University, 2008. / Adviser: Hupp, Joseph T. Includes bibliographical references.

Thesis (Ph. D.)--School of Electrical and Computer Engineering, Georgia Institute of Technology, 2004. Directed by Ajeet Rohatgi. / Vita. Includes bibliographical references (leaves 203-211).

Thesis (Ph. D.) -- California Institute of Technology, 2010. / Title from home page (viewed 03/04/2010). Advisor and committee chair names found in the thesis' metadata record in the digital repository. Includes bibliographical references.

Thesis (M. Phil.)--University of Hong Kong, 2002. / Includes bibliographical references (leaves 97-102).

Organic solar cells (OSCs) have shown great promise in becoming the next generation of renewable energy due to its low cost, simple manufacturing process and flexibility. A method of efficiency improvement in OSCs is by incorporating metallic nanoparticles (NPs). While various reports have reported that incorporation of NPs improve OSC efficiencies due to the Localized Surface Plasmon Resonance (LSPR) effect, the investigations have lacked depth and a detailed investigation is necessary to fully understand the device mechanisms of these OSCs. In this thesis, we first investigate OSCs incorporating Au NPs into the hole collection Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) layer. Our theoretical and experimental results show that the very strong near field around Au NPs due to LSPR mainly distributes laterally along the PEDOT:PSS rather than vertically into the adjacent active layer, leading to minimal enhancement of light absorption in the active layer. With optical effects proven to be minor contributors to device performance improvements, we investigate the electrical properties of the OSCs and obtain insights into the detailed device mechanisms. Improvements in power conversion efficiency (PCE) of solar cells are found to originate from the enlarged active layer/PEDOT:PSS interfacial area and improved PEDOT:PSS conductivity. At high NP concentrations, reduced exciton quenching at donor/acceptor junctions is found to cause PCE deterioration. Next, the effects of Au NPs incorporated into the active layer of OSCs with a newly synthesized donor polymer are investigated in detail. Our experimental and theoretical results both show that LSPR introduced by the NPs can enhance the light absorption in the active layer of OSCs because the strong LSPR near field mainly distributes laterally along the active layer. Combined with our previous study, our results strongly suggest that NPs have to be incorporated in the active layer in order to harvest light by the LSPR effect. Meanwhile, our results show that the electrical properties of NPs improve at low concentration of NPs. When NP concentration is increased, the electrical properties deteriorates and counter-diminish the optical enhancement from LSPR and reduces the overall performance improvement. Finally, we demonstrate efficiency improvement in OSCs by ~22% through incorporating Au NPs into all polymer layers. Au NPs are found to have distinct mechanisms in improving device performance when incorporated in different polymer layers. Our results indicate that the efficiency improvement is the accumulated effects of incorporating NPs in the individual layers and that coupling is not observed in this device configuration. On the whole, our findings highlight the importance that both optical and electrical properties need to be studied and optimized simultaneously for achieving enhancement in PCE of OSCs. We have carried out a detailed study on incorporating NP in various layers and our results are highly useful for the design of high efficiency OSCs incorporating metallic NPs. / published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

Sunlight is sustainable, clean and readily available energy source, which is one of the potential alternatives to the traditional energy sources. Recently, the organic photovoltaics (OPVs), in particular polymer solar cells (PSCs), have attracted increasing attention owing to their outstanding properties such as low cost, lightweight, flexible, allowing vacuum-free fabrication process and thin-film architecture. These advantageous material and manufacturing features of PSCs provide the opportunities for many novel applications. However, the lower power conversion efficiencies (PCEs) of PSCs compared to inorganic solar cells hinder their competition in the marketplace. This thesis covers the basic principles of the PSC, strategies for enhancing PCEs as well as the recent development of PSCs. The importance of the source materials has been also demonstrated and discussed. Due to a large number of possibilities, limited resources and time, it is not feasible to do all the work experimentally. Therefore, for continuing advance development of PSCs, the device performance should be modeled as a function of material parameters, which requires the knowledge of material properties, in particular the complex index of refraction N= n - ik. Accurate determination of the optical functions of the active layers and light trapping layers commonly used in PSCs by using the spectroscopic ellipsometry (SE) has been demonstrated. In order to acquire reliable solutions, the methodology including multiple sample analysis, combinations of different measurement techniques, selection of models, the rigorous fitting procedures and the independent verification have been proposed. The obtained information can be used in the simulation to optimize device architectures, model device performance as well as characterize novel materials. / published_or_final_version / Physics / Doctoral / Doctor of Philosophy

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

Organic solar cell (OSC) is a highly promising research field with a strong potential to realize low cost solar cells with flexibility and light weight. Although OSC power conversion efficiency (PCE) exceeding 9% has been achieved recently, great efforts are still needed to strive a PCE over 10% making OSC ready for commercialization. Besides the demand of high PCE, other considerations, such as easy solution process, stability and large area processing, are also required for mass production in future. With the understanding of key technical issues that still challenge OSC towards widely spread applications, our worksarefocusingon1) applying the solution processed inorganic materials to ameliorate the intrinsic drawback in OSCs; and 2)proposing novel and simple solution process to improve electrical properties of OSCs by controlling the film quality thus the electrical properties during the film formation process. Detailed work is listed below: 1. Incorporating of metal nanoparticles (NPs) for improving OSC efficiency Metal NPs are selected as the candidate for improving OSC efficiency through their unique optical and electrical properties. Our results show that (1a) When meal NPs are incorporated in the hole transport layer (HTL) poly-(3, 4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), the PCE of OSCs are improved due to enhanced conductivity and rough surface. (1b) When metal NPs are embedded in the active layer, OSC performance can be further enhanced due to improvement in light absorption and electrical properties. When we incorporate Au NPs in all organic layers of OSCs, accumulation improvements in OSC performances can be achieved. (1c) When metal NPs are incorporated in electron transport layer of TiO2, the experimental results show that the enhanced charge extraction under solar illumination can be attributing to the UV-excited electrons transfer from TiO2electron transport layer and storage by Au NPs. 2. Solution processed metal oxide thin film for high efficient hole transporting layer (HTL) The solution-processed transition metal oxides (TMOs) have attracted great attention due to their superior air-stability properties and universal energy level alignment with organic materials. In this thesis, we propose a one-step method to synthesize low-temperature solution-processed TMOs such as molybdenum oxide and vanadium oxide, with good film quality, desirable electrical properties, and improved device stability, for HTLs applications. 3. Self-assemble metal oxide for high efficient electron transporting layer (ETL) We propose a self-assemble and solution-processed method in fabricating ETLs composed of TiO2 NPs that can simultaneously achieve good film uniformity and homogeneity, and electron transport properties. We believe this new method will be capable for large-area applications in future. 4. Vertical morphology control for active layer. Besides carrier transport layers, the morphology of the active layer will significantly affect its electrical and optical properties and thus device performance. We propose up-side-down method to modify the nano-morphology blend along vertical direction, which is beneficial to vertical charge transport and thus producing higher OSC performances. The film-growth dynamics of polymer blends is studied, which has been neglected in most study of OSC morphology by others. / published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy