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For improving the energy conversion efficiency of solar cells, it is essential to reduce the surface recombination velocity of CuInSe2 absorber layer. The use of quaternary alloys with an increasing band gap gradient was also demonstrated to be effectively increased the open-circuit voltage of the cells. The experiments using different concentration ammonium sulfur solutions to proceed surface passivation and sulfurization of CuInSe2 and CuInSe2¡GSb films have been conducted to evaluate their influences on the band gap and other related properties. The band gaps of Cu-rich and In-rich CuInSe2 films did not change after ammonium sulfur treatment. For CuInSe2¡GSb films after immersing (NH4)2Sx solution, the PL spectra gave an evidence of the formation of the quaternary CuInSxSe2-x alloys. The metal contacts to CuInSe2 films with the structures of Mo/P-type CuInSe2/Al and Mo/N-type CuInSe2/Au had been fabricated. Their I-V characteristics indicate that the Schottky Contacts had been successfully formed.

Deposition of thin material films by sputtering is an increasingly common process in the field of silicon (Si)-based photovoltaics. One of the recently developed sputter-deposited materials applicable to Si photovoltaics comprises Si nanocrystals (NCs) embedded in a Si-based dielectric. The particular case of Si nanocrystals in a Silicon Dioxide (SiO2) matrix was studied by fabricating metal-insulator-semiconductor (MIS) devices, in which the insulating layer consists of a single layer of Si NCs in SiO2 deposited by sputtering (Si:NC-MIS devices). These test structures were subjected to impedance measurements. The presence of Si NCs was found to result in two distinct capacitance peaks. The first of these peaks is attributable to the small signal response of states at the insulator/substrate interface, enhanced by the presence of fixed charge associated with the NC layer. The second peak, which occurs without precedent, is due to external inversion layer coupling, in conjunction with a transition between tunnel-limited and semiconductor-limited electron current. Si:NC-MIS devices are also potential test structures for energy-selective contacts, based on SiO2/Si NC/SiO2 double barrier structures fabricated by sputtering. Using a one-dimensional model, current-voltage (I-V) curve simulations were performed for similar structures, in which the Si NCs are replaced by a Si quantum well (QW). The simulations showed that for non-degenerately doped Si substrates, the density of defects in the SiO2 layers can strongly influence the position of I-V curve structure induced by QW quasi-bound states. Passivation of crystalline Si (c-Si) surfaces by sputter-deposited dielectric films is another major application of sputtering for Si photovoltaics. This application was explored for the cases of sputtered SiO2 and hydrogenated Silicon Oxy-Carbide (SiOC:H). For the case of sputtered SiO2, an effective surface recombination velocity of 146 cm/s was achieved for an injection level of 1E15 cm???3. The investigated SiOC:H films were found to be unsuitable for surface passivation of Si, however their passivation performance could be slightly improved by first coating the Si surface with a chemically-grown or sputtered SiO2 layer. The investigations performed into specific aspects of sputter-deposited SiO2, Si NCs, and SiOC:H have highlighted important properties of these films, and confirmed the effectiveness of sputtering as a deposition technology for Si photovoltaics.

The probability of recombination of photogenerated electron hole pairs in crystalline silicon is governed by the density of surface defect states and the density of charge carriers. Depositions of intrinsic hydrogenated amorphous silicon (a-Si:H) in dc saddle field (DCSF) PECVD system and hydrogenated amorphous silicon nitride (SiNx) in rf PECVD system forms a dual layer stack on c-Si, which results in an excellent passivation of the surface and an anti-reflection coating. Response Surface Methodology is used in this work to optimize the deposition conditions of SiNx. Optimization of the response surface function yielded deposition conditions that materialized in a surface recombination velocity of less than 4cm/s. The BACH (Back Amorphous Crystalline silicon Heterojunction) cell concept makes use of this dual layer a-Si:H/SiNx stack to form a high efficiency photovoltaic device. The high quality passivating structure can result in the BACH solar cell device with more than 20% conversion efficiency.

The probability of recombination of photogenerated electron hole pairs in crystalline silicon is governed by the density of surface defect states and the density of charge carriers. Depositions of intrinsic hydrogenated amorphous silicon (a-Si:H) in dc saddle field (DCSF) PECVD system and hydrogenated amorphous silicon nitride (SiNx) in rf PECVD system forms a dual layer stack on c-Si, which results in an excellent passivation of the surface and an anti-reflection coating. Response Surface Methodology is used in this work to optimize the deposition conditions of SiNx. Optimization of the response surface function yielded deposition conditions that materialized in a surface recombination velocity of less than 4cm/s. The BACH (Back Amorphous Crystalline silicon Heterojunction) cell concept makes use of this dual layer a-Si:H/SiNx stack to form a high efficiency photovoltaic device. The high quality passivating structure can result in the BACH solar cell device with more than 20% conversion efficiency.

abstract: Silicon solar cells with heterojunction carrier collectors based on a-Si/c-Si heterojunction (SHJ) have a potential to overcome the limitations of the conventional diffused junction solar cells and become the next industry standard manufacturing technology of solar cells. A brand feature of SHJ technology is ultrapassivated surfaces with already demonstrated 750 mV open circuit voltages (Voc) and 24.7% efficiency on large area solar cell. Despite very good results achieved in research and development, large volume manufacturing of high efficiency SHJ cells remains a fundamental challenge. The main objectives of this work were to develop a SHJ solar cell fabrication flow using industry compatible tools and processes in a pilot production environment, study the interactions between the used fabrication steps, identify the minimum set of optimization parameters and characterization techniques needed to achieve 20% baseline efficiency, and analyze the losses of power in fabricated SHJ cells by numerical and analytical modeling. This manuscript presents a detailed description of a SHJ solar cell fabrication flow developed at ASU Solar Power Laboratory (SPL) which allows large area solar cells with >750 mV Voc. SHJ cells on 135 um thick 153 cm2 area wafers with 19.5% efficiency were fabricated. Passivation quality of (i)a-Si:H film, bulk conductivity of doped a-Si films, bulk conductivity of ITO, transmission of ITO and the thickness of all films were identified as the minimum set of optimization parameters necessary to set up a baseline high efficiency SHJ fabrication flow. The preparation of randomly textured wafers to minimize the concentration of surface impurities and to avoid epitaxial growth of a-Si films was found to be a key challenge in achieving a repeatable and uniform passivation. This work resolved this issue by using a multi-step cleaning process based on sequential oxidation in nitric/acetic acids, Piranha and RCA-b solutions. The developed process allowed state of the art surface passivation with perfect repeatability and negligible reflectance losses. Two additional studies demonstrated 750 mV local Voc on 50 micron thick SHJ solar cell and < 1 cm/s effective surface recombination velocity on n-type wafers passivated by a-Si/SiO2/SiNx stack. / Dissertation/Thesis / Ph.D. Electrical Engineering 2013

abstract: Recent technology advancements in photovoltaics have enabled crystalline silicon (c-Si) solar cells to establish outstanding photoconversion efficiency records. Remarkable progresses in research and development have been made both on the silicon feedstock quality as well as the technology required for surface passivation, the two dominant sources of performance loss via recombination of photo-generated charge carriers within advanced solar cell architectures. As these two aspects of the solar cell framework improve, the need for a thorough analysis of their respective contribution under varying operation conditions has emerged along with challenges related to the lack of sensitivity of available characterization techniques. The main objective of my thesis work has been to establish a deep understanding of both “intrinsic” and “extrinsic” recombination processes that govern performance in high-quality silicon absorbers. By studying each recombination mechanism as a function of illumination and temperature, I strive to identify the lifetime limiting defects and propose a path to engineer the ultimate silicon solar cell. This dissertation presents a detailed description of the experimental procedure required to deconvolute surface recombination contributions from bulk recombination contributions when performing lifetime spectroscopy analysis. This work proves that temperature- and injection-dependent lifetime spectroscopy (TIDLS) sensitivity can be extended to impurities concentrations down to 109 cm-3, orders of magnitude below any other characterization technique available today. A new method for the analysis of TIDLS data denominated Defect Parameters Contour Mapping (DPCM) is presented with the aim of providing a visual and intuitive tool to identify the lifetime limiting impurities in silicon material. Surface recombination velocity results are modelled by applying appropriate approaches from literature to our experimentally evaluated data, demonstrating for the first time their capability to interpret temperature-dependent data. In this way, several new results are obtained which solve long disputed aspects of surface passivation mechanisms. Finally, we experimentally evaluate the temperature-dependence of Auger lifetime and its impact on a theoretical intrinsically limited solar cell. These results decisively point to the need for a new Auger lifetime parameterization accounting for its temperature-dependence, which would in turn help understand the ultimate theoretical efficiency limit for a solar cell under real operation conditions. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2018

The cost of crystalline silicon solar cells must be reduced in order for photovoltaics to be widely accepted as an economically viable means of electricity generation and be used on a larger scale across the world. There are several ways to achieve cost reduction, such as using thinner silicon substrates, lowering the thermal budget of the processes, and improving the efficiency of solar cells. This thesis examines the use of plasma enhanced chemical vapour deposited silicon nitride to address the criteria of cost reduction for n-type crystalline silicon solar cells. It focuses on the surface passivation quality of silicon nitride on n-type silicon, and injection-level dependent lifetime data is used extensively in this thesis to evaluate the surface passivation quality of the silicon nitride films. The thesis covers several aspects, spanning from characterisation and modelling, to process development, to device integration. The thesis begins with a review on the advantages of using n-type silicon for solar cells applications, with some recent efficiency results on n-type silicon solar cells and a review on various interdigitated backside contact structures, and key results of surface passivation for n-type silicon solar cells. It then presents an analysis of the influence of various parasitic effects on lifetime data, highlighting how these parasitic effects could affect the results of experiments that use lifetime data extensively. A plasma enhanced chemical vapour deposition process for depositing silicon nitride films is developed to passivate both diffused and non-diffused surfaces for n-type silicon solar cells application. Photoluminescence imaging, lifetime measurements, and optical microscopy are used to assess the quality of the silicon nitride films. An open circuit voltage of 719 mV is measured on an n-type, 1 Ω.cm, FZ, voltage test structure that has direct passivation by silicon nitride. Dark saturation current densities of 5 to 15 fA/cm2 are achieved on SiN-passivated boron emitters that have sheet resistances ranging from 60 to 240 Ω/□ after thermal annealing. Using the process developed, a more profound study on surface passivation by silicon nitride is conducted, where the relationship between the surface passivation quality and the film composition is investigated. It is demonstrated that the silicon-nitrogen bond density is an important parameter to achieve good surface pas-sivation and thermal stability. With the developed process and deeper understanding on the surface passivation of silicon nitride, attempts of integrating the process into the fab-rication of all-SiN passivated n-type IBC solar cells and laser doped n-type IBC solar cells are presented. Some of the limitations, inter-relationships, requirements, and challenges of novel integration of SiN into these solar cell devices are identified. Finally, a novel metallisation scheme that takes advantages of the different etching and electroless plating properties of different PECVD SiN films is described, and a preliminary evalua-tion is presented. This metallisation scheme increases the metal finger width without increasing the metal contact area with the underlying silicon, and also enables optimal distance between point contacts for point contact solar cells. It is concluded in this thesis that plasma enhanced chemical vapour deposited silicon nitride is well-suited for n-type silicon solar cells.

This thesis presents a work which has been devoted to the growth of silicon thin films on crystalline silicon for photovoltaic applications by means of RF PECVD. The primary goal of this work was to obtain an amorphous growth on any c-Si surface in order to provide an efficient passivation, as required in heterojunction solar cells. Indeed, we demonstrated that epitaxial or mixed phase growths, easy to obtain on (100) Si, would lead to poor surface passivation. We proved that growing a few nm thin a-Si1-xCx:H alloy film was an efficient, stable and reproducible way to hinder epitaxy while keeping an excellent surface passivation by the subsequent deposition of a-Si:H films. Process optimization mainly based on Spectroscopic Ellipsometry, Effective lifetime measurements (Sinton lifetime tester) and current-voltage characterization led us to demonstrate that it was possible to obtain a-Si:H/c-Si heterojunction solar cells with stable VOC of 710 mV and FF of 76 % on flat (n) c-Si wafers, with solar cells of 25 cm2 whose metallization was realized by screen-printing technology. This work has also demonstrated the viability of a completely dry process where the native oxide is removed by SiF4 plasma etching instead of the wet HF removal. Last but not least, the epitaxial growth of silicon thin films, undoped and n or p-type doped, on (100)-oriented surfaces has been studied by Spectroscopic Ellipsometry and Hall effect measurements. We have been able to fabricate homojunction solar cells with a p-type emitter as well as p-i-n structures with an undoped epitaxial absorber on a heavily-doped (p) c-Si wafers.

Electronic components, based on current semiconductor technologies and operating in radiation rich environments, suffer degradation of their performance as a result of radiation exposure. Silicon carbide (SiC) provides an alternate solution as a radiation hard material, because of its wide bandgap and higher atomic displacement energies, for devices intended for radiation environment applications. However, the radiation tolerance and reliability of SiC-based devices needs to be understood by testing devices  under controlled radiation environments. These kinds of studies have been previously performed on diodes and MESFETs, but multilayer devices such as bipolar junction transistors (BJT) have not yet been studied. In this thesis, SiC material, BJTs fabricated from SiC, and various dielectrics for SiC passivation are studied by exposure to high energy ion beams with selected energies and fluences. The studies reveal that the implantation induced crystal damage in SiC material can be partly recovered at relatively low temperatures, for damag elevels much lower than needed for amorphization. The implantation experiments performed on BJTs in the bulk of devices show that the degradation in deviceperformance produced by low dose ion implantations can be recovered at 420 oC, however, higher doses produce more resistant damage. Ion induced damage at the interface of passivation layer and SiC in BJT has also been examined in this thesis. It is found that damaging of the interface by ionizing radiation reduces the current gain as well. However, for this type of damage, annealing at low temperatures further reduces the gain. Silicon dioxide (SiO2) is today the dielectric material most often used for gate dielectric or passivation layers, also for SiC. However, in this thesis several alternate passivation materials are investigated, such as, AlN, Al2O3 and Ta2O5. These materials are deposited by atomic layer deposition (ALD) both as single layers and in stacks, combining several different layers. Al2O3 is further investigated with respect to thermalstability and radiation hardness. It is observed that high temperature treatment of Al2O3 can substantially improve the performance of the dielectric film. A radiation hardness study furthermore reveals that Al2O3 is more resistant to ionizing radiation than currently used SiO2 and it is a suitable candidate for devices in radiation rich applications. / QC 20120117