sd may 2012-09 ecpe dept., iowa state university advisor/client – dr. vikram dalal anthony arrett,...
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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
* Team Leader
Improving the Stability of Hydrogenated Amorphous Silicon Solar Cells : Design for Enabling Technology
Problem Statement
• Many solar cells, particularly those based on Amorphous Silicon are inherently unstable - we want to design equipment for measuring changes in performance
• We also want to study processes for improving stability of a-Si solar cells
Design Requirements
• The equipment must be able to measure continuously for 1000 hours
• The equipment must replicate the standard sunlight spectrum (AM1.5)
• The equipment must be able to provide different intensities of light so as to do accelerated testing
• The equipment must be automated and export data for analysis by EXCEL and MATLAB
Project Plan/Progression• Research Based – study and understand the problem
• Study the characteristics of a-Si solar cells
• Design equipment for meeting the needs of the client –
• Identify the various pieces of equipment needed
• Select options
• Do cost analysis of various options
• Select cost-efficient equipment that meets the needs and
is expandable
• Automate the measurements
Background: Solar Cells made from a-Si:H
• EHPs are created in the depleted intrinsic layer
• Carriers separated and collected by internal electric field
• Random structure leads to defect states in the material - these are centers for undesired carrier recombination
• Dangling bonds lead to mid-band gap states
• Hydrogen is used to fill those dangling bonds
Staebler-Wronski Effect
Efficiency drops quickly after exposure to light
To overcome this problem:
• Study various cell configurations
• Various cell processing techniques
• Make cells
• Measure cell performance vs. time
• See which technique works best and why
Example: One technique: Stradins et al.
Note how post-annealedMaterials are more stable
Questions:Would devices be more stableAs well?Can we make good devices usingthis technique?
NREL research by Stradins (et al) shows lower dangling bond densities in films
Hardware Selected and Built
• System required that could expose the cell, as well as source and measure current vs. voltage
• Also needed a reference cell meter – check for stability of light source
• ABET 10500 Solar simulator – meets solar spectrum
• Keithley 236 – Source-Measure Unit – meets automation requirement
• Keithley 197 – Digital current meter- simple but reliable
Budget for Stability SetupItem Cost
Keithley 236 SMU $3000
Keithley 197a $600
ABET 10500 $4300
USB GPIB Adapter $0 (In Stock)
Dell Desktop Optiplex 790 w/ 20” Monitor
$784
Reference Solar Cell $0 (In Stock)
Misc. Hardware $0 (Found @ MRC)
NI LabView Software $0 (CSG Install)
TOTAL $8684 w/ Software
Problems: ABET 10500 Solar Simulator
• Problem: Found to be too rich in UV light compared to solar spectrum • Did not meet the specs even though the vendor claimed it did
• Solution: Fix it with a UV filter
• Problem: Most filters degrade in UV
• Solution: Design and build our own using amorphous Silicon-carbide film
AM1.5G Standard Spectrum
Comparison of ABET to other lamps• Too much UV from the ABET arc lamp• UV light is high energy – causes bonds to break• Need UV filter to better simulate degradation in sunlight
Too much UV!
What we made
Silicon Carbide Filters• Can adjust band gap energy between 1.7eV and 4.0eV
• Change Methane (CH4) to Silane (SiH4) ratio
• Adjustable thickness (nm) – adjusts amt of absorbed light• Can tune filter for our application• Does NOT degrade like plastic filters We designed a series of
films to approximate an ideal filter - getting closer and closer
Ideal
12 3
4
5
6
1. Initialization2. Sweep3. I-V Curve4. Key Calculations5. Export Data6. Loop Iteration
LabVIEW Program
Data exported to EXCEL For analysis
Experiment• High-temperature annealed devices
• Deposition at 400°C• Annealed after i-layer deposited at temps ranging from 350°C -
425°C
• High-temperature growth• Deposition of entire device at temps up to 450°C• No Anneal
• Use Boron grading in both experiments to try and improve devices
• Measure device properties: I-V, QE, Defect Density, etc.• Light degradation done at 2x Sun for 60+hrs
High Temperature Anneal
Anneal Temp. RehydrogenationBoron
GradingVOC ISC FF
400°C No No 0.866V 1.23mA 58.6%
425°C No No 0.861V 1.23mA 60.3%
400°C No Yes 0.871V 1.32mA 61.6%
425°C No Yes 0.842V 1.32mA 59.3%
400°C Yes Yes 0.868V 1.12mA 61.2%
425°C Yes Yes 0.855V 1.09mA 57.8%
Standard - - 0.822V 1.29mA 64.5%
High Temperature Growth
Growth Temperature
Boron Grading VOC ISC FF
400°C No 0.907V 1.00mA 54.7%
425°C No 0.854V 1.08mA 56.7%
400°C Yes 0.884V 1.07mA 63.1%
425°C Yes 0.877V 1.13mA 63.8%
450°C Yes 0.866V 1.15mA 66.9%
Standard - 0.822V 1.29mA 64.5%
I-V Comparison
-0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2
-1.5
-1
-0.5
0
0.5
1
1.5
2
Current vs. Voltage
425C Anneal
425C Dep. w/o Boron Grading
425C Dep w/ Boron Grading
300C Standard
Voltage (V)
Cu
rren
t (m
A)
Degradation of Fill Factor
Best so far!
Summary of Results• Devices via High-temp anneal have the largest currents,
but they degrade more than standard devices
• Devices via High-temp growth degrade less
• Boron grading raises ISC and FF in devices
• Defect densities vs. energy increase with exposure to light
• Mid-gap defects don’t correlate with degradation• Fail our initial hypothesis based on Stradins et al. THEIR METHOD
DOES NOT WORK
Future Work
• Detailed device analysis
• FTIR for chemical analysis of Si-H bonds
• Subgap QE to detect energy states in bandgap region
• Further experiments to detect changes in defect densities
• Study of how fundamental material changing under light exposure
• Study of changes in interfaces
• Degradation under various light intensities
Future Use• Stability setup will be used in the long-term
• System will be used to measure stability on inorganic solar cells
• Software is designed to be adjustable
• Software is easy-to-fix if problems arise
• Hardware requires little maintenance
Conclusions• Original Hypothesis Failed
• High-temp anneal does not produce stable devices
• Used a different process to make more stable
• Insight gained into the structure of a-Si Solar cells
• Hardware setup will help ISU research for years
• Commercial solar simulator failed specs- modified it to approximately meet desired spectrum
• Setup is modifiable for future research needs, e.g. testing at different intensities and temperatures for accelerated testing
Lessons Learnt• Not everything in literature is true – some processes fail
• Use fundamental understanding to invent new processes
• Commercial equipment often does not meet specs• Identify the problem and then solve it
• LabVIEW is very powerful for automating equipment• Great training in actually automating a set up and making it work
• A system is more than a sum of its components • When designed and built right, it provides a very versatile testing
environment