Improving the Stability of Hydrogenated Amorphous Silicon Solar CellsSD May 2012-09ECpE Dept., Iowa State UniversityAdvisor/Client – Dr. Vikram Dalal

Anthony Arrett, Wei Chen, William Elliott, Brian Modtland, and David Rincon

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

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

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

Light-Induced Instability• Discovered that defect density increases with exposure to

light, not necessarily time

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

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

High Temperature Anneal

• High temp annealing shows promise in reducing Li-DB

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

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*1016 cm-3

• Fill Factor > 60%

• Efficiency > 5%

• Drop in Efficiency after light soaking of no more than 10%

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

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

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

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

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

Detailed Design• Concept Diagram

TOP VIEWSIDE VIEW

Cost EstimateItem Cost StatusKeithley 236 $3000 (already

purchased)Shipped and Done

Keithley 485 $1000 Ships Early JanuaryABET 10500 $4300

$325 for beam turnerShips Mid-January

USB GPIB Adapter $0 (In Stock) DoneDell Desktop Optiplex 790 w/ 20” Monitor

$784 Ships 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

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

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

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

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

QUESTIONS?Comments, Concerns, or Donations?