improving the stability of hydrogenated amorphous silicon solar cells
DESCRIPTION
Improving the Stability of Hydrogenated Amorphous Silicon Solar Cells. SD May 2012-09 ECpE Dept., Iowa State University Advisor/Client – Dr. Vikram Dalal Anthony Arrett, Wei Chen, William Elliott, Brian Modtland, and David Rincon. Problem Statement. - PowerPoint PPT PresentationTRANSCRIPT
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Improving the Stability of Hydrogenated Amorphous Silicon Solar CellsSD May 2012-09ECpE Dept., Iowa State UniversityAdvisor/Client – Dr. Vikram Dalal
Anthony Arrett, Wei Chen, William Elliott, Brian Modtland, and David Rincon
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Problem Statement• Amorphous Silicon Solar Cells are inherently unstable - we want to improve that
• Investigate the instability of a-Si Solar Cells
• Use Stradins’ research to design a baseline a-Si solar cell with less defects over time
• Determine new fabrication recipes that produce more stable a-Si with the best efficiency
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Background of PV Cells
-EHP are created in the depleted intrinsic layer-PIN junction allows us have a bigger depletion layer over PN Junction-Electric field within junction allows faster transport of carriers, and reduces likelihood of recombination
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Background to Amorphous Si• Same tetrahedral bonding as crystalline Si, but does not
have long range crystalline structure
• Random structure leads to dangling bonds in the material, these are considered defects
• Dangling bonds lead to midband gap states
• Hydrogen is used to fill those dangling bonds
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Light-Induced Instability• Discovered that defect density increases with exposure to
light, not necessarily time
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Staebler-Wronski Effect
-Dramatic drop in efficiency after just a few hours of exposure to light-Stable efficiency is the most important attribute-Theorized that light breaks the H-Si bonds, creating dangling bonds in material-Self annealing
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Overview of Plan• Build a device with a higher stable efficiency than is
currently available
• Working off Stradins’ breakthrough to reducing defect density of intrinsic layer
• Experiment with anneal temperatures
• Add graded Boron doping to improve internal field
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High Temperature Anneal
• High temp annealing shows promise in reducing Li-DB
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Boron Doping• Graded Boron doping will create an electric field in the
intrinsic layer
• 10ppm-100ppm
• Electric field will speed up collection process and lower recombination
• Lower recombination leads to higher efficiency
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Functional Requirements
• Photoconductivity > 1*10-5 Ω-1 cm-1
• Dark Conductivity < 1*10-10 Ω-1 cm-1
• Tauc Band Gap < 1.8eV
• Defect density after light soaking < 1*1016 cm-3
• Fill Factor > 60%
• Efficiency > 5%
• Drop in Efficiency after light soaking of no more than 10%
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Non-functional Requirements
• Ability to be reproduced time after time of similar quality
• Ability to convert recipe to mass-production with little
changes
• Samples that are easily measured and tested with devices
at the MRC
• Size of the cell
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Market Overview• The firm projects $1.3 billion in revenues from a-Si based
photovoltaic in the year 2009• Will grow to $4.1 billion in the year 2014
the market share of different PV technology
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Testing of the Solar Cells• Quantum Efficiency
• Indicates a solar cells capability to convert energy• Current vs. Voltage
• Power Efficiency, Fill Factor• Capacitance vs. Voltage
• Used to measure defect density and intrinsic layer thickness• Capacitance vs. Frequency
• Defect Density vs. Energy• Thickness
• Serves a prerequisite to calculating properties of the device• Photoconductivity
• Used to determine the film’s ability to conduct a current with exposure to light
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Automated I-V Setup• Automated I-V measurement of a-Si solar cells
• Find ISC, VOC, Fill Factor, Efficiency, RSHUNT, and RSERIES
• Extended Light Soaking up to 100 hours• Simulated solar exposure to study Staebler-Wronski
instability
• AM1.5 Solar spectrum standard
• Capability for 1x, 2x, 3x, and 4x Solar Irradiance
• LabView software programming
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Specifications of Auto I-V Setup
• Easy-to-use software interface
• NI LabView
• AM1.5 Spectrum for solar simulation
• 100 hour measurements w/ adjustable intervals
• I-V taken every 1 to 5 minutes
• 1x, 2x, 3x, and 4x Suns with the use of lenses
• Reference cell for tracking intensity of the light source
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Detailed Design• Concept Diagram
TOP VIEWSIDE VIEW
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Cost EstimateItem Cost StatusKeithley 236 $3000 (already
purchased)Shipped and Done
Keithley 485 $1000 Ships Early JanuaryABET 10500 $4300
$325 for beam turnerShips Mid-January
USB GPIB Adapter $0 (In Stock) DoneDell Desktop Optiplex 790 w/ 20” Monitor
$784 Ships in ~2 Weeks
Reference Solar Cell $0 (In Stock) Done
NI LabView Software $0 (CSG Install)
TOTAL $9084 w/ Software (no beam turner)
Completed by mid-Feb
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Status Report• Design has been completed for automated IV
measurements
• A proposal has been written up, submitted to our client, and accepted• Now ordering parts and materials for the setup
• Beginning measurements have been taken for different recipes.• QE, I-V, and C-V
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Task Responsibilities• We all did our own separate research and reading to
become acquainted with amorphous silicon.• Measurement Research & Auto I-V: - Tony - QE & LabView setup for auto I-V - William - Light soaking & LabView setup - David - Conductivity & Hardware Research - Chen - Tauc Band gap & Hardware - Brian - Defect Density & Team Leader
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Plan for the Upcoming Semester
• I-V hardware ordered by end of December
• Software implemented by end of January
• Have everything up and running and tested by middle of
February
• Once this this done, continue with device measurements
• Finalize device recipe based on results
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Summary• Our goal is to determine new fabrication recipes that
produce more stable a-Si solar cells• Dangling Bonds cause defects in the structure
• Leads to loss of efficiency• Can combat this with high temp annealing and graded
Boron doping• Automated I-V measurements will save time (added
feature)• Automated I-V tool should be up and running by the end
of February• Finalized device recipe by next April
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QUESTIONS?Comments, Concerns, or Donations?