Staebler-Wronski effect
Encyclopedia
The Staebler–Wronski Effect (SWE) refers to light-induced metastable changes in the properties of hydrogenated amorphous silicon
.
The defect density of hydrogenated amorphous silicon
(a-Si:H) increases with light exposure, to cause an increase in the recombination current and lead to the reduction in the sunlight
to electricity
conversion efficiency.
It was discovered by David L. Staebler
and Christopher R. Wronski
in 1977. They showed that the dark conductivity and photoconductivity
of hydrogenated amorphous silicon
can be reduced significantly by prolonged illumination with intense light. However, on heating the samples to above 150 °C, they could reverse the effect.
Suggested explanations
Microcrystalline silicon suffers less from the Staebler–Wronski effect than amorphous silicon, suggesting that the disorder in the amorphous silicon Si network plays a major role. Other properties that could play a role are hydrogen concentration and its complex bonding mechanism, as well as the concentration of impurities.
The exact nature and cause of the Staebler–Wronski effect is still not well known. Historically, the most favored model has been the hydrogen bond switching model. It proposes that an electron-hole pair formed by the incident light may recombine near a weak Si–Si bond, releasing energy sufficient to break the bond. A neighbouring H atom then forms a new bond with one of the Si atoms, leaving a dangling bond
. These dangling bonds can trap electron-hole pairs, thus reducing the current that can pass through. However, new experimental evidence is casting doubt on this model. More recently, the H collision model proposed that two spatially-separated recombination events cause emission of mobile hydrogen from Si–H bonds to form two dangling bonds, with a metastable paired H state binding the hydrogen atoms at a distant site.
of the cell. After this initial drop, the effect reaches an equilibrium and causes little further degradation. The equilibrium level shifts with operating temperature
so that performance of modules tend to recover some in the summer months and drop again in the winter months. Most commercially available a-Si modules have SWE degradation in the 10–15% range and suppliers typically specify efficiency based on performance after the SWE degradation has stabilized. In a typical amorphous silicon
solar cell the efficiency is reduced by up to 30% in the first 6 months as a result of the Staebler–Wronski effect, and the fill factor falls from over 0.7 to about 0.6. This light induced degradation is the major disadvantage of amorphous silicon as a photovoltaic material.
Amorphous silicon
Amorphous silicon is the non-crystalline allotropic form of silicon. It can be deposited in thin films at low temperatures onto a variety of substrates, offering some unique capabilities for a variety of electronics.-Description:...
.
The defect density of hydrogenated amorphous silicon
Amorphous silicon
Amorphous silicon is the non-crystalline allotropic form of silicon. It can be deposited in thin films at low temperatures onto a variety of substrates, offering some unique capabilities for a variety of electronics.-Description:...
(a-Si:H) increases with light exposure, to cause an increase in the recombination current and lead to the reduction in the sunlight
Sunlight
Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...
to electricity
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...
conversion efficiency.
It was discovered by David L. Staebler
David L. Staebler
David L. Staebler is an American electrical engineer.He received his BSEE degree from Pennsylvania State University in 1962. The following year he was awarded an MSEE degree from the same institution. He became a member of the technical staff at RCA Laboratories in 1963. In 1970, he gained his...
and Christopher R. Wronski
Christopher R. Wronski
Dr. Christopher R. Wronski is a noted electrical engineer, now Professor Emeritus at Pennsylvania State University, noted for his pioneering research in photovoltaic cells including discovery of the Staebler–Wronski effect.Dr. Wronski received his Ph.D...
in 1977. They showed that the dark conductivity and photoconductivity
Photoconductivity
Photoconductivity is an optical and electrical phenomenon in which a material becomes more electrically conductive due to the absorption of electromagnetic radiation such as visible light, ultraviolet light, infrared light, or gamma radiation....
of hydrogenated amorphous silicon
Amorphous silicon
Amorphous silicon is the non-crystalline allotropic form of silicon. It can be deposited in thin films at low temperatures onto a variety of substrates, offering some unique capabilities for a variety of electronics.-Description:...
can be reduced significantly by prolonged illumination with intense light. However, on heating the samples to above 150 °C, they could reverse the effect.
Explanation
Some experimental results- Photoconductivity and dark conductivity decrease rapidly at first before stabilizing at a lower value.
- Interruptions in the illumination has no effect on the subsequent rate of change. Once the sample is illuminated again, the photoconductivity will drop as though there was no interruption.
- AnnealingAnnealing (metallurgy)Annealing, in metallurgy and materials science, is a heat treatment wherein a material is altered, causing changes in its properties such as strength and hardness. It is a process that produces conditions by heating to above the recrystallization temperature, maintaining a suitable temperature, and...
the amorphous siliconAmorphous siliconAmorphous silicon is the non-crystalline allotropic form of silicon. It can be deposited in thin films at low temperatures onto a variety of substrates, offering some unique capabilities for a variety of electronics.-Description:...
at a few hundred degrees Celsius reverses the effect.
Suggested explanations
Microcrystalline silicon suffers less from the Staebler–Wronski effect than amorphous silicon, suggesting that the disorder in the amorphous silicon Si network plays a major role. Other properties that could play a role are hydrogen concentration and its complex bonding mechanism, as well as the concentration of impurities.
The exact nature and cause of the Staebler–Wronski effect is still not well known. Historically, the most favored model has been the hydrogen bond switching model. It proposes that an electron-hole pair formed by the incident light may recombine near a weak Si–Si bond, releasing energy sufficient to break the bond. A neighbouring H atom then forms a new bond with one of the Si atoms, leaving a dangling bond
Dangling bond
In chemistry, a dangling bond is an unsatisfied valence on an immobilised atom.In order to gain enough electrons to fill their valence shells , many atoms will form covalent bonds with other atoms. In the simplest case, that of a single bond, two atoms each contribute one unpaired electron, and the...
. These dangling bonds can trap electron-hole pairs, thus reducing the current that can pass through. However, new experimental evidence is casting doubt on this model. More recently, the H collision model proposed that two spatially-separated recombination events cause emission of mobile hydrogen from Si–H bonds to form two dangling bonds, with a metastable paired H state binding the hydrogen atoms at a distant site.
Effects
The efficiency of an amorphous silicon solar cell typically drops during the first six months of operation. This drop may be in the range from 10% up to 30% depending on the material quality and device design. Most of this loss comes in the fill factorFill factor
Fill factor may refer to:*Fill factor , the ratio of maximum obtainable power to the product of the open-circuit voltage and short-circuit current*In vision science, the ratio of view areas to the object visible areas....
of the cell. After this initial drop, the effect reaches an equilibrium and causes little further degradation. The equilibrium level shifts with operating temperature
Operating temperature
An operating temperature is the temperature at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the device function and application context, and ranges from the minimum operating temperature to the...
so that performance of modules tend to recover some in the summer months and drop again in the winter months. Most commercially available a-Si modules have SWE degradation in the 10–15% range and suppliers typically specify efficiency based on performance after the SWE degradation has stabilized. In a typical amorphous silicon
Amorphous silicon
Amorphous silicon is the non-crystalline allotropic form of silicon. It can be deposited in thin films at low temperatures onto a variety of substrates, offering some unique capabilities for a variety of electronics.-Description:...
solar cell the efficiency is reduced by up to 30% in the first 6 months as a result of the Staebler–Wronski effect, and the fill factor falls from over 0.7 to about 0.6. This light induced degradation is the major disadvantage of amorphous silicon as a photovoltaic material.
Methods of reducing the SWE
- Using microcrystalline silicon instead of amorphous siliconAmorphous siliconAmorphous silicon is the non-crystalline allotropic form of silicon. It can be deposited in thin films at low temperatures onto a variety of substrates, offering some unique capabilities for a variety of electronics.-Description:...
- Operating at a higher temperature
- Stacking one or more thinner layers of amorphous silicon together with other materials to form a multijunction solar cell. The higher electric field which applies in the thinner layers appears to reduce the SWE.