Matching Items (3)
Filtering by
- All Subjects: Heterojunctions
- Creators: Holman, Zachary
- Resource Type: Text
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
Layers of intrinsic hydrogenated amorphous silicon and amorphous silicon carbide
were prepared on a polished, intrinsic crystalline silicon substrate via plasma-enhanced chemical vapor deposition to simulate heterojunction device relevant stacks of various materials. The minority carrier lifetime, optical band gap and FTIR spectra were observed at incremental stages of thermal annealing. By observing the changes in the lifetimes the sample structure responsible for the most thermally robust surface passivation could be determined. These results were correlated to the optical band gap and the position and relative area of peaks in the FTIR spectra related to to silicon-hydrogen bonds in the layers. It was found that due to an increased presence of hydrogen bonded to silicon at voids within the passivating layer, hydrogenated amorphous silicon carbide at the interface of the substrate coupled with a hydrogenated amorphous silicon top layer provides better passivation after high temperature annealing than other device structures.
were prepared on a polished, intrinsic crystalline silicon substrate via plasma-enhanced chemical vapor deposition to simulate heterojunction device relevant stacks of various materials. The minority carrier lifetime, optical band gap and FTIR spectra were observed at incremental stages of thermal annealing. By observing the changes in the lifetimes the sample structure responsible for the most thermally robust surface passivation could be determined. These results were correlated to the optical band gap and the position and relative area of peaks in the FTIR spectra related to to silicon-hydrogen bonds in the layers. It was found that due to an increased presence of hydrogen bonded to silicon at voids within the passivating layer, hydrogenated amorphous silicon carbide at the interface of the substrate coupled with a hydrogenated amorphous silicon top layer provides better passivation after high temperature annealing than other device structures.
ContributorsJackson, Alec James (Author) / Holman, Zachary (Thesis advisor) / Bertoni, Mariana (Committee member) / Kozicki, Michael (Committee member) / Arizona State University (Publisher)
Created2016
Description
The maximum theoretical efficiency of a terrestrial non-concentrated silicon solar cell is 29.4%, as obtained from detailed balance analysis. Over 90% of the current silicon photovoltaics market is based on solar cells with diffused junctions (Al-BSF, PERC, PERL, etc.), which are limited in performance by increased non-radiative recombination in the doped regions. This limitation can be overcome through the use of passivating contacts, which prevent recombination at the absorber interfaces while providing the selectivity to efficiently separate the charge carriers generated in the absorber. This thesis aims at developing an understanding of how the material properties of the contact affect device performance through simulations.The partial specific contact resistance framework developed by Onno et al. aims to link material behavior to device performance specifically at open circuit. In this thesis, the framework is expanded to other operating points of a device, leading to a model for calculating the partial contact resistances at any current flow. The error in calculating these resistances is irrelevant to device performance resulting in an error in calculating fill factor from resistances below 0.1% when the fill factors of the cell are above 70%, i.e., for cells with good passivation and selectivity.
Further, silicon heterojunction (SHJ) and tunnel-oxide based solar cells are simulated in 1D finite-difference modeling package AFORS-HET. The effects of material property changes on device performance are investigated using novel contact materials like Al0.8Ga0.2As (hole contact for SHJ) and ITO (electron contact for tunnel-oxide cells). While changing the bandgap and electron affinity of the contact affect the height of the Schottky barrier and hence contact resistivity, increasing the doping of the contact will increase its selectivity. In the case of ITO, the contact needs to have a work function below 4.2 eV to be electron selective, which suggests that other low work function TCOs (like AZO) will be more applicable as alternative dopant-free electron contacts. The AFORS-HET model also shows that buried doped regions arising from boron diffusion in the absorber can damage passivation and decrease the open circuit voltage of the device.
Further, silicon heterojunction (SHJ) and tunnel-oxide based solar cells are simulated in 1D finite-difference modeling package AFORS-HET. The effects of material property changes on device performance are investigated using novel contact materials like Al0.8Ga0.2As (hole contact for SHJ) and ITO (electron contact for tunnel-oxide cells). While changing the bandgap and electron affinity of the contact affect the height of the Schottky barrier and hence contact resistivity, increasing the doping of the contact will increase its selectivity. In the case of ITO, the contact needs to have a work function below 4.2 eV to be electron selective, which suggests that other low work function TCOs (like AZO) will be more applicable as alternative dopant-free electron contacts. The AFORS-HET model also shows that buried doped regions arising from boron diffusion in the absorber can damage passivation and decrease the open circuit voltage of the device.
ContributorsDasgupta, Sagnik (Author) / Holman, Zachary (Thesis advisor) / Onno, Arthur (Committee member) / Wang, Qing Hua (Committee member) / Arizona State University (Publisher)
Created2020