Improving the Stability of Hydrogenated Amorphous Silicon Solar Cells

1. Improving the Stability of Hydrogenated Amorphous Silicon Solar Cells SD May 2012-09 ECpE Dept., Iowa State University Advisor/Client – Dr. VikramDalal 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*10-5Ω-1cm-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%

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 film’s ability to conduct a current with exposure to light

14. 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

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 SIDE VIEW TOP VIEW

17. Cost Estimate

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?