SD May 2012-09 ECpE Dept., Iowa State University Advisor/Client – Dr. Vikram Dalal

1. Improving the Stability of Hydrogenated Amorphous Silicon Solar Cells : Design for Enabling Technology SD May 2012-09 ECpE Dept., Iowa State University Advisor/Client – Dr. VikramDalal Anthony Arrett, Wei Chen, William Elliott, Brian Modtland*, and David Rincon * Team Leader

2. 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 Problem Statement

3. 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 Design Requirements

4. 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 Project Plan/Progression

5. 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

6. Staebler-Wronski Effect Efficiency drops quickly after exposure to light

7. Study various cell configurations Various cell processing techniques Make cells Measure cell performance vs. time See which technique works best and why To overcome this problem:

8. Example: One technique: Stradins et al. Note how post-annealed Materials are more stable Questions: Would devices be more stable As well? Can we make good devices using this technique? NREL research by Stradins (et al) shows lower dangling bond densities in films

9. 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 Hardware Selected and Built

10. Budget for Stability Setup

11. 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 Problems: ABET 10500 Solar Simulator

12. AM1.5G Standard Spectrum

13. 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!

14. 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 What we made

15. LabVIEW Program Initialization Sweep I-V Curve Key Calculations Export Data Loop Iteration 5 3 2 1 4 6 Data exported to EXCEL For analysis

16. 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 Experiment

17. High Temperature Anneal

18. High Temperature Growth

19. I-V Comparison

20. Degradation of Fill Factor Best so far!

21. 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 Summary of Results

22. 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 Work

23. 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 Future Use

24. 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 Conclusions

25. 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 • LabVIEWis 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 Lessons Learnt