Micromorph
Encyclopedia
The artificial word micromorph is a combination of the words MICROcrystalline and aMORPHou. It is used for a type of thin-film silicon mulitjunction solar cell
.
the top cell, consisting of a thin amorphous silicon
layer, absorbing the blue light and a thicker microcrystalline silicon bottom cell absorbing the red and near-infrared light of the solar spectrum.
Since the bandgaps of amorphous Silicon (1.7eV) and microcrystalline Silicon (1.1eV) are well suited for tandem solar cells, the Shockley-Queisser limit
of this cell allows conversion efficiencies of over 30%.
In reality this limit can not be reached and typical stable efficiencies are about 9% (world record 11.7%). That is well over the stable efficiencies of single junction thin film silicon solar cells which are around 6%. One reason of the low costs of silicon thin film solar cells is its very low thickness (2µm) compared to silicon Wafer (electronics)
(200µm). In the red and infrared wavelength range 2µm of silicon are not enough to absorb all light and therefore Light trapping is needed.
The advantage of the micromorph aproach is that it keeps the thickness of the amorphous top cell low. This reduces the effect of degradation induced by light (Staebler-Wronski effect
).
In mulitjunction cells top and bottom cell have to produce the same current. But the top cell is limited by the Staebler-Wronski effect
and therefore Light trapping and an intermediate reflector are needed to keep the thickness of the top cell low while increasing its current.
(ZnO intermediate reflector: ZIR) or Silicon oxide
(SiOx intermediate reflector: SOIR) between the top and the bottom cell. Due its lower refractive index of around 2 comparing to the surrounding silicon (4) light is reflected back into the top cell. This increases the top cell current from around 10mA/cm^2 to 12mA/cm^2, but reduces the bottom cell current by an equal amount.
research group of Prof. Arvind Shah by the author J. Meier in the year 1995 [1], but is based on long pioneering research of several authors and of several years. For a detailed list of publications, see the two main research group's publications website under http://pvlab.epfl.ch and under http://www.fz-juelich.de/ief/ief-5/publicger/
Years later, other European, Japanese and American research groups started research activities in the field of improving the conversion efficiency of thin-film silicon solar cells by utilizing the stacked solar cell concept, naming the micromorph device 'hybrid' solar cell or naming the microcrystalline
silicon absorber 'nanocrystalline' or even 'policrystalline' silicon.
The word 'micromorph' has been lately claimed to belong to a equipment manufacturer of silicon coating tools, but in a patent judgement, this claim has not been accepted by European patent office
s [2].
Solar cell
A solar cell is a solid state electrical device that converts the energy of light directly into electricity by the photovoltaic effect....
.
The micromorph cell
In this Multijunction photovoltaic cellMultijunction photovoltaic cell
Multi-junction solar cells or tandem cells are solar cells containing several p-n junctions. Each junction is tuned to a different wavelength of light, reducing one of the largest inherent sources of losses, and thereby increasing efficiency...
the top cell, consisting of a thin 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:...
layer, absorbing the blue light and a thicker microcrystalline silicon bottom cell absorbing the red and near-infrared light of the solar spectrum.
Since the bandgaps of amorphous Silicon (1.7eV) and microcrystalline Silicon (1.1eV) are well suited for tandem solar cells, the Shockley-Queisser limit
Shockley-Queisser limit
In physics, the Shockley–Queisser limit or detailed balance limit refers to the maximum theoretical efficiency of a solar cell using a p-n junction to collect power from the cell. It was first calculated by William Shockley and Hans Queisser at Shockley Semiconductor in 1961...
of this cell allows conversion efficiencies of over 30%.
In reality this limit can not be reached and typical stable efficiencies are about 9% (world record 11.7%). That is well over the stable efficiencies of single junction thin film silicon solar cells which are around 6%. One reason of the low costs of silicon thin film solar cells is its very low thickness (2µm) compared to silicon Wafer (electronics)
Wafer (electronics)
A wafer is a thin slice of semiconductor material, such as a silicon crystal, used in the fabrication of integrated circuits and other microdevices...
(200µm). In the red and infrared wavelength range 2µm of silicon are not enough to absorb all light and therefore Light trapping is needed.
The advantage of the micromorph aproach is that it keeps the thickness of the amorphous top cell low. This reduces the effect of degradation induced by light (Staebler-Wronski effect
Staebler-Wronski effect
The Staebler–Wronski Effect refers to light-induced metastable changes in the properties of hydrogenated amorphous silicon.The defect density of hydrogenated amorphous silicon increases with light exposure, to cause an increase in the recombination current and lead to the reduction in the sunlight...
).
In mulitjunction cells top and bottom cell have to produce the same current. But the top cell is limited by the Staebler-Wronski effect
Staebler-Wronski effect
The Staebler–Wronski Effect refers to light-induced metastable changes in the properties of hydrogenated amorphous silicon.The defect density of hydrogenated amorphous silicon increases with light exposure, to cause an increase in the recombination current and lead to the reduction in the sunlight...
and therefore Light trapping and an intermediate reflector are needed to keep the thickness of the top cell low while increasing its current.
The intermediate reflector
The intermediate reflector is a layer (IRL) of Zinc oxideZinc oxide
Zinc oxide is an inorganic compound with the formula ZnO. It is a white powder that is insoluble in water. The powder is widely used as an additive into numerous materials and products including plastics, ceramics, glass, cement, rubber , lubricants, paints, ointments, adhesives, sealants,...
(ZnO intermediate reflector: ZIR) or Silicon oxide
Silicon oxide
Silicon oxide may refer to either of the following:*Silicon dioxide, SiO2, very well characterized*Silicon monoxide, SiO, not very well characterized...
(SiOx intermediate reflector: SOIR) between the top and the bottom cell. Due its lower refractive index of around 2 comparing to the surrounding silicon (4) light is reflected back into the top cell. This increases the top cell current from around 10mA/cm^2 to 12mA/cm^2, but reduces the bottom cell current by an equal amount.
The word micromorph
This artificial word has first been mentioned in a scientific publication of the University of NeuchâtelUniversity of Neuchâtel
The University of Neuchâtel is a French-speaking university in Neuchâtel, Switzerland. The University has five faculties and more than a dozen institutes, including arts and human sciences, natural sciences, law, economics and theology. The Faculty of Arts and Human Sciences is the largest...
research group of Prof. Arvind Shah by the author J. Meier in the year 1995 [1], but is based on long pioneering research of several authors and of several years. For a detailed list of publications, see the two main research group's publications website under http://pvlab.epfl.ch and under http://www.fz-juelich.de/ief/ief-5/publicger/
Years later, other European, Japanese and American research groups started research activities in the field of improving the conversion efficiency of thin-film silicon solar cells by utilizing the stacked solar cell concept, naming the micromorph device 'hybrid' solar cell or naming the microcrystalline
Microcrystalline
A microcrystalline material is a crystallized substance or rock that contains small crystals visible only through microscopic examination.-See also:* Macrocrystalline* Microcrystalline silicon* Protocrystalline* Rock microstructure...
silicon absorber 'nanocrystalline' or even 'policrystalline' silicon.
The word 'micromorph' has been lately claimed to belong to a equipment manufacturer of silicon coating tools, but in a patent judgement, this claim has not been accepted by European patent office
Patent office
A patent office is a governmental or intergovernmental organization which controls the issue of patents. In other words, "patent offices are government bodies that may grant a patent or reject the patent application based on whether or not the application fulfils the requirements for...
s [2].