1. 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

2. 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

3. 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

4. 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

5. Light-Induced Instability Discovered that defect density increases with exposure to light, not necessarily time

6. 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

7. 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

8. High Temperature Anneal High temp annealing shows promise in reducing Li-DB

9. 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

10. 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*10 16 cm -3 Fill Factor > 60% Efficiency > 5% Drop in Efficiency after light soaking of no more than 10%

11. 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

12. 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

13. 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 films ability to conduct a current with exposure to light

14. Automated I-V Setup Automated I-V measurement of a-Si solar cells Find I SC, V OC, Fill Factor, Efficiency, R SHUNT, and R SERIES 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

15. 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

16. Detailed Design Concept Diagram TOP VIEWSIDE VIEW

17. Cost Estimate ItemCostStatus Keithley 236$3000 (already purchased) Shipped and Done Keithley 485$1000Ships Early January ABET 10500$4300 $325 for beam turner Ships Mid-January USB GPIB Adapter$0 (In Stock)Done Dell Desktop Optiplex 790 w/ 20 Monitor $784Ships 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

18. 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

19. 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

20. 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

21. 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

22. QUESTIONS? Comments, Concerns, or Donations?