SD May 2012-09ECpE Dept., Iowa State UniversityAdvisor/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 CostKeithley 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. Rehydrogenation Boron Grading

VOC 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)

Curr

ent (

mA)

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