Solar Cell Stability – May12-09

ABSTRACTSolar cell research is a major emphasis of Iowa State University. Improving the efficiency of solar cells can have a dramatic impact throughout the world. What this project aims to do is improve the stability of amorphous silicon solar cells. The hope is that this will lead to further advancements in thin-film solar cell research. By experimenting with different processing techniques, the goal is to zero in on a strategy to manufacture the most stable amorphous silicon solar cells. An automated setup was also implemented which improves the way inorganic solar cells are tested at the university.

PROBLEM STATEMENTAmorphous silicon solar cells are inherently unstable. The instability arises from defect energy states in the material which are created when high-energy light breaks Si-H bonds leaving dangling Si bonds. Improvements on the number of defects and the stability of a-Si solar cells will be made by adjusting fabrication conditions.

OPERATING ENVIRONMENTThe end product will be used outdoors in every environment. These solar cells should work better then standard a-Si cells even in the following conditions.

•Extreme temperatures•Rain and moisture

FUNCTIONAL REQUIREMENTS•Fill Factor > 60%•Efficiency > 5%•Tauc Band Gap < 1.8eV•Drop in Efficiency after light soaking of no more than 20%

DESIGN CONSTRAINTS•Solar cell design must be reproducible•Software must be intuitive and easy to update•Hardware must reproduce the AM1.5G standard spectrum for solar simulation

ResultsThis project consisted of two major dimensions, (1) Producing an effective way to measure a solar cell’s response to light, and (2) finding a fabrication method that improved on the stability of standard amorphous samples. The automatic I-V setup was built and tested as shown above. The graph above shows the results of the degradation experiments run under 2x sun intensity for 3 samples. For 425°C deposition, cells showed 17% degradation in efficiency and an 11% decline in fill factor. Grading Boron in PPM amounts in the intrinsic layer of the device improved the fill factor above that of the standard device.

Discussion and Future WorkInitial samples aimed to use high temperature anneal of the intrinsic region as a means to reduce defects in the device and improve light-induced instability. Results indicate this method of anneal is not effective at improving stability in a-Si devices, pointing to device properties that are more important than bulk defects when considering instability. Deposition at high temperatures shows improved stability as indicated, but past research has also shown similar improvements. Future work by Dr. Dalal’s PV group hopes to find the device properties most responsible for instability in order to create a-Si cells which degrade by only 5% over time.

Introductory Materials

Automated Light Degradation Setup

Team websitehttp://seniord.ece.iastate.edu/may1209/

Client/AdvisorDr. Vikram Dalal

Team MembersAnthony Arrett EEWei Chen EEWilliam Elliot EEBrian Modtland EEDavid Rincon EE

Contactafarrett@iastate.eduweic@iastate.eduwelliott@iastate.edumodtland@iastate.edurincon@iastate.edu

Project Description

Results

HardwareHardware was chosen to reproduce the solar spectrum and to accurately read power produced. The main components are as follows:

•ABET 10500 Solar Simulator with AM1.5G Filters•Keithley 286 Source-Measure Unit•Keithley 197A Digital Multimeter•Computer running Windows 7 and NI LabVIEW

SoftwareThe program was created using National Instruments’ LabVIEW. It was made to automate a 100 hour measurement cycle for light degradation experiments, export the necessary data, and provide an easy to understand graph. The user interface is easy to understand and the measurements are easily modifiable if necessary.

Solar CellsSolar Cells are fairly simple devices. Light is used to generate energy by creating an electron-hole pair in the intrinsic layer that gets swept out to the P and N terminals by a built-in electric field. If a cell is connected to an external load, these collected carriers flow through the circuit as current.

Advantages and Disadvantages of Amorphous Silicon Solar CellsAmorphous silicon solar cells have an advantage over crystalline silicon cells in that they can absorb more light with less material, meaning they can be manufactured very thin and also at low temperatures. This allows them to be used in innovative ways. However, amorphous silicon solar cells are not stable. The amount of energy produced decreases with light exposure. This degradation of the device can decrease efficiencies up to 40% in the first few months of use.

Development of Testing Procedures

TestingIn order to determine the long term efficiency of the solar cells, they must be tested before, during, and after exposure to light. Tests give important device properties such as fill factor, VOC, ISC, defect density, and quantum efficiency at different wavelengths.

ApproachPrior to this project, testing amorphous silicon solar cells at Iowa State University was very labor intensive and only a few persons were trained to use the devices. As part of our project, we developed a setup to automate testing to make it easier to take measurements on solar cells. Hardware had to be chosen to simulate light, take readings of the power produced, and detect any changes in the light source. Software had to interface with the hardware, take accurate readings over hundreds of hours, and be easy to use.