Theses and Dissertations
This collection includes both ASU Theses and Dissertations, submitted by graduate students, and the Barrett, Honors College theses submitted by undergraduate students.
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- Creators: Honsberg, C. (Christiana B.)
Description
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.
ContributorsHerasimenka, Stanislau Yur'yevich (Author) / Honsberg, C. (Christiana B.) (Thesis advisor) / Bowden, Stuart G (Thesis advisor) / Tracy, Clarence (Committee member) / Vasileska, Dragica (Committee member) / Holman, Zachary (Committee member) / Sinton, Ron (Committee member) / Arizona State University (Publisher)
Created2013
Description
Silicon photovoltaics is the dominant contribution to the global solar energy production. As increasing conversion efficiency has become one of the most important factors to lower the cost of photovoltaic systems, the idea of making a multijunction solar cell based on a silicon bottom cell has attracted broad interest. Here the potential of using dilute nitride GaNPAs alloys for a lattice-matched 3-terminal 2-junction Si-based tandem solar cell through multiscale modeling is investigated. To calculate the electronic band structure of dilute nitride alloys with relatively low computational cost, the sp^3 d^5 s^* s_N tight-binding model is chosen, as it has been demonstrated to obtain quantitatively correct trends for the lowest conduction band near Γ, L, and X for dilute-N GaNAs. A genetic algorithm is used to optimize the sp^3 d^5 s^* tight-binding model for pure GaP and GaAs for their optical properties. Then the optimized sp^3 d^5 s^* s_N parametrizations are obtained for GaNP and GaNAs by fitting to experimental bandgap values. After that, a virtual crystal approach gives the Hamiltonian for GaNPAs alloys. From their tight-binding Hamiltonian, the first-order optical response functions of dilute nitride GaNAs, GaNP, and GaNPAs are calculated. As the N mole fraction varies, the calculated critical optical features vary with the correct trends, and agree well with experiment. The calculated optical properties are then used as input for the solar device simulations based on Silvaco ATLAS. For device simulation, a bottom cell model is first constructed to generate performance results that agree well with a demonstrated high-efficiency Si heterojunction interdigitated back contact (IBC) solar cell reported by Kaneka. The front a-Si/c-Si interface is then replaced by a GaP/Si interface for the investigation of the sensitivity of the GaP/Si interface to interface defects in terms of degradation of the IBC cell performance, where we find that an electric field that induces strong band bending can significantly mitigate the impact of the interfacial traps. Finally, a lattice-matched 3-terminal 2-junction tandem model is built for performance simulation by stacking a dilute nitride GaNP(As) cell on the Si IBC cell connected through a GaP/Si interface. The two subcells operate quasi-independently. In this 3-terminal tandem model, traps at the GaP/Si interface still significantly impact the performance of the Si subcell, but their effects on the GaNP subcell are relatively small. Assuming the interfacial traps are well passivated, the tandem efficiency surpasses that of a single-junction Si cell, with values close to 33% based on realistic parameters.
ContributorsZou, Yongjie (Author) / Goodnick, Stephen M. (Thesis advisor) / Honsberg, C. (Christiana B.) (Committee member) / King, Richard R. (Committee member) / Vasileska, Dragica (Committee member) / Arizona State University (Publisher)
Created2019