MultiJunctionSolarCells (1)

Multi Junction Solar Cells
Nitin Maurya
EUREC (2015-16)
Northumbria University
April 2016

Introduction
Research in renewable technology has been triggered by the energy demand which is
increasing globally at rapid pace and the environmental concerns caused by the use of fossil
sources.Solar energy is by far the most clean and abundant source. Making it cost effective and
at par with prices of fossil fuels is driving research and investment in development of new
cheaper materials and technology..A solar cell or a photovoltaic cell is a device which converts
the sunlight incident on it to useful electrical energy. The key to understand the functioning of it
is to understand first the functioning of p n Junction diode, secondly how the light is absorbed by
a semiconductor material and how the two are put together. A semiconductor solar cell under
the effect of sunlight absorbs the photons from sunlight creating an electron hole pair. The
Photon transfer its energy in excitation of the electron to a higher energy level creating an
electron hole pair in the n type silicon. These excited electrons before been able to recombine
with the hole in the p type material are moved to an external circuit creating an electric current.
Silicone wafer based solar cells are the first generation of solar cells and one of the most
prominent technology in application today at both residential and commercial level.Silicon is the
second most abundant material on earth and is mostly utilised in developing solar cells.It was
first used in 1939 by Russell Ohl. Due to its low efficiency, theoretical maximum of 33%,
scientists have researched different materials for finding photoelectric properties with high
efficiency level.Losses that account for low efficiency in silicon are black body radiation,
radiative recombination and spectral losses. The black body radiation is a phenomena where an
object start emitting radiation beyond absolute zero temperatures. In silicon it accounts normally
to 7% loss in efficiency.Losses due to radiative recombination occurs due to an electron hole
combination before the electron reaches the external circuit to produce electricity. Spectral
losses occurs due to a definite band gap of semiconductor materials that allows a certain
wavelength of light to be absorbed and the rest is either reflected back or converted to heat
which further decreases the efficiency.The second generation of photovoltaics is based on thin
film technology. Substances like copper indium gallium diselenide,amorphous silicon, cadmium
telluride etc are used for this technology. Even though the use of thin film is more practical due
to less mass the low efficiency still holds it back.
To overcome the losses a third generation of solar cells are created combining the advantages
of first and second generation cells.Materials with different band gaps are combined together to
be more efficient as they allow light with different or wider wavelength range to be
absorbed.These kind of solar cells are called multijunction cells.Efficiency as high as 87%
theoretical has been reached by the scientist so far.Since the manufacturing of these kind of
cells was very costly the first application was seen in Space exploration where higher efficiency
is crucial. However for terrestrial applications this technology is still costly and the only
economic application is seen in High concentration Photovoltaics application (HCPV) systems.
Northumbria University,UK
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Design and Fabrication Technology
The most important factors which are considered while designing of multijunction solar cells are
bandgap of each cell, lattice matching and bandgap matching. The bandgap of each cell is very
important since the amount of sunlight absorbed depends on it.The frequency of the light times
the Planck’s constant should be greater than the bandgap for the light to be absorbed.A multi
junction cell with multiple band gaps thus absorbs more photon energy.The diagram below
illustrates a multijunction cell with different bandgaps.
Image 1-Multijunction Layers (Rahim 2012)
The multi junction cell is designed such that the material placed on the top of the layer has the
highest bandgap and hence absorbs highest amount of energy.Some of the light is passed to
the next layer since it has a smaller bandgap.For obtaining same current level in the whole
multijunction cell, the junctions are placed from highest to lowest band gap to allow energy
transfer to happen. The compounds of group III and V with little adjustments in their composition
can be used for providing a big range of bandgap.Lattice constant of a compound is its spatial
distribution at atomic level in a crystal pattern.The matching of crystal lattice constants is very
important to increase efficiency.A mismatch leads to less photocurrents and decrease in open
circuit voltage which causes losses in power.An important characteristic to note is that as
compared to single junction cells, the amount of current generated in multijunction cells is less.
An advantage of this is that there are less resistive losses.However the amount of power
generated is more because the voltage is more as the cells are arranged in series.
There are many techniques for fabrication of multijunction cells.One of them is Metalorganic
Vapour Phase Epitaxy (MOVPE) and the other is molecular beam epitaxy(MBE). In MOVPE
technology, a semiconductor substrate is placed on a reaction chamber and a secondary
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material in the form of pure vapour is deposited over it forming an epitaxial layer.The substrate
is heated to make the substrate and the protection layer of nearly same lattice constant.The
lattice constant in a crystal structure is a measure of the amount of spacing in the atoms. Some
technical issues however arise due to the property of thermal expansion in semiconductor
materials.(Yastrebova 2007) Since the semiconductor materials have different expansion
coefficient, a strain is induced in the cell which causes the wafers to bow and create region of
unsatisfactory contacts and discontinuities.This lead to the formation of non-uniform epitaxial
layer which alters the optical properties of multijunction cell and also are sites for
recombination.This significantly lowers down the efficiency of the cells.Hence effort are made to
make the substrate and epitaxial layer of the same lattice constant. Lattice mismatching develop
dislocation in the structure of crystal which results in a decrease in efficiency. (Yastrebova 2007)
In MBE method molecules are beamed on the substrate to be heated creating a thin layer. The
process is carried out in a high vacuum environment
Image 2- Molecular Beam Epitaxy (Anon 2010)
The scientist therefore are up with the challenge of finding materials which have favourable
band gaps in order to give maximum absorption level and also have a matching lattice constant
so that the material could be grown epitaxially on each other. In triple Junction solar cells the
band gap combination of solar cells with matching lattice is not optimal because the bottom cell
has more incidented levels of photons than in the upper cell. Scientist are searching for new
methods like creating new materials that could replace the bottom cell, forming low-cost solar
cells with improved efficiency New Technologies such as molecular Beam epitaxy has also
been used quite often up in the fabrication of multijunction cells however there are some
problems with it like low crystal quality and issues with scaling for large production.
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Cell Performance and Efficiency
The connection between the cells in a multijunction device is very crucial. The junction generally
is designed in a way that a p doped side is connected to n doped side which creates problems
of opposite polarity.This problem is however solved by introduction of a heavily doped wide
band gap junction connected between the two.The electrons in the n doped side are at same
energy level with the holes in conduction band in the p doped side and cross the barrier and go
to a lower energy state.(Nelson 2011)
In a multijunction solar cell the current produced is dependant on the number of photons from
sunlight that have energy level greater than the bandgap of that material.So while designing the
cell it is important to make the layer thinner if large number of photons have higher bandgap
and made thicker if it has a low absorption constant. Hence the thickness of the material is
decided based on its absorption constant.There are two types of configuration while designing
the solar cells one in series and another in parallel. The advantage of parallel configuration is
that each solar cell can be optimized independently,but due to high level of complications series
configuration is prefered. In series different materials are placed over one another. The cell
which is on the top is generally thick and has a wider range of wavelength absorption.The cells
placed at bottom have less bandgap.
Currently designing of solar cells beyond 6 layers is not economical even in CPV technology
and scientist are working on ways to decrease the cost.Designing the multijunction cell focus on
areas like bandgap of each individual cell and lattice matching. Combining these factors
perfectly results in high efficiency.
Image 3- Quantum Efficiency vs Wavelength (NSSC LAB)
The figure above is an illustration of the bandgap of a three junction cell GaInP2/GaAs/Ge
showing the wavelength covered by the cell.The concept of multijunction cell can be clearly
seen from the spectral response curve seen in this figure. GaAs based cells are in high state of
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research and development due to their good response during Soviet space program and in
some commercial electronics.
Past and Current Application of Multijunction cells
It is projected that the energy consumption is going to be double from the present rate by the
middle of 21st century.Application of photovoltaics will play a crucial role in it and multijunction
technology is proposed by some scientist to be a major contributor.
Till date the major application of multi junction or tandem cells is seen in space programs.The
first research on multijunction cell devices began in the early 1980’s, however research
continued and by beginning of the 90’s they were used in space applications like satellites and
space probes.Due to the high radiation resistance and capable of being operated at high
voltage and low current the multijunction cells like GaAs replaced Silicon which was not fit for
space application because it can not withstand the harsh conditions which predominantly
initiated the use of III- V multijunction cell. As in space there is no atmosphere, the photon flux
has higher concentration above 1.87 eV.As a result the current flowing in the GaAs junction is
less.To overcome this issue the GaAs junction layer is made thinner to allow more photons to
pass through to InGaP layer.In 1997 multijunction cells were first launched in a commercial
sattelite using solar arrays which comprised of dual junction cells by Spectrolab.(Law et al.
n.d.)The efficiency of the cell recorded that time was 25.1 %. Multijunction cells are used in
Mars Rover currently which is an automated motor vehicle that is landed on the surface of
planet Mars for exploration.
At ground level the atmospheric scattering of light reduces the photon flux, so the GaAs layer is
kept thick for terrestrial applications. However with rapid progress in developing new materials
that are cheap, and with use in concentrator systems could make it’s use in terrestrial
applications more economical. A lot of research institutes are working on increasing the
efficiency of these cells.Even companies like Amonix and Spectrolab have their research going
in order to improve efficiency.
Advantages
One of the Major advantage of multijunction solar cells is the increased solar energy
conversation efficiency that they provide for both terrestrial and space applications. The
multijunction solar cells currently manufactured have high efficiency and with use of
concentrated technology and access tracking facilities it is becoming exam economical as
compared to the single junction silicon solar cells. One of the prominent advantage of this
technology is that more amount of energy can be produced from a smaller area.
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Future Design Improvements
1- Use of new materials and inclusion of semiconductor quantum dots
Scientist have discovered some nanomaterials like quantum wells, quantum wire, nanotubes etc
which when added to the multijunction cells have improved its efficiency.Even in single junction
ordinary solar cell has been proven with an efficiency increases up to 63.2% with inclusion of
quantum dots.(Bailey et al. 2008)
One of the major advantages with quantum dots is their optoelectronic properties which can be
altered easily. Quantum dots inclusion in semiconductor materials allows to modify the bandgap
of the material based on their sizes.Hence a same material can have different bandgap for
different layers and can absorb a wider spectrum of sunlight, increasing the efficiency.
An example of the above description is seen in InAs which when quantum dotted with sizes 5nm
,10nm and 12nm have shown a band gap of 1.071eV, 0.553eV and 0.045eV
correspondingly.(Sobolev et al. 2001)The MBE manufacturing technology is used to build up
these cells. By controlling the temperature and rate of growth the size and density of dots can
be controlled. Through the process of impact ionization photons with energy higher than the
bandgap of material can be utilised to make photocurrent.In this method more than one electron
hole pair is created from a single photon.Also the quantum dots are formed in a three
dimensional array so that the electron hole pair can hold for a longer period of time by creating a
strong electronic coupling between them.This phenomena leads to higher electricity generation
at higher voltage.Also for space application quantum dots can help in making favourable
temperature coefficients cells.
2- Design optimization of existing layers
By improving the design of the subcells in multijunction cells its efficiency can be further
increased.An example of this was observed when the top ordered GaInP layer in a triple
junction cell was replaced by a disordered one. The reason of the efficiency increase was more
bandgap in the disordered GaInP ie 1.88eV compared to 1.78 eV in the ordered one. (Law et al.
n.d.)
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Figure - Efficiency increase projections(Dragoman)
In the figure above it can be seen that the efficiency increases from 39% to 42 %, by replacing
GaAs cell with a new material with band gap of 1.25 eV. This layer reduces the number of
photons passing by to Germanium layer thereby increasing the efficiency.(Dragoman)
Another way of improving the efficiency is by increasing the number of cells. As seen in figure
the efficiency increases to 42% from triple junction to four junction cell. Currently scientist were
able to develop six junction cell economically. Five and six junction cell partition the solar
spectrum into narrower wavelength range and all the subcells are matched effectively with the
one producing low current.(Luque & Hegedus 2011)Dragoman)
Some of the advantages seen in five and 6 junction cells are(Arai & Kohei 2013)
1- Reduction in thermalisation losses(Larson et al. 2016)
2-Lower Resistive losses
The low resistive losses are result of less current density and low thermalisation losses are a
result of very fine spectrum division. (King et al. n.d.)
3- Advances in CPV Technology
The use of multi junction cells have been possible for terrestrial applications by using them with
concentrated photovoltaic technology.The CPV technology uses lenses to focus sunlight on a
small area of solar cell.Single axis or dual axis tracking devices are also used to increase the
amount of light incident. By increasing the concentration of the light the performance of the multi
junction cells increases which justifies with the cost of the cell as the output is higher for a small
area. However concentration level of 2000 suns have been practically used experiments are
going on to increase it further.(Marion & Bill 2015)
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Image 1 - Concentrated sphere (Guerin n.d.)
The image above is a concentrator technology developed by Rawlemon which can concentrate
sunlight up to 10000 times for energy purposes and capable of harnessing energy from sunlight.
Conclusion
It is been estimated that the green energy requirement will raise up to 30 TW/year in 2050.This
is the energy requirement from the renewables in order to keep the CO2 level at a minimum
level to alter any climate change.A large portion of this energy is assumed to be supplied by
solar energy and multi junction technology will play a crucial role. Since they have high
efficiency and a potential to go even more higher. In the past progress has been made in
decreasing its cost by changes in the fabrication methods and research is going on with better
results coming up. Compared to other sources of energy solar technology is quite better
because of no fuel requirement and no moving part. Multijunction cells combined with improved
concentrator technology is the future of solar energy.
References
Anon, 2010. Molecular Beam Epitaxy — Electronic & Electrical Engineering. Available at:
https://www.ee.ucl.ac.uk/about/MBE [Accessed April 25, 2016].
Arai, K. & Kohei, A., 2013. Nonlinear Mixing Model of Mixed Pixels in Remote Sensing Satellite
Images Taking Into Account Landscape. International Journal of Advanced Computer
Science and Applications, 4(1). Available at: http://dx.doi.org/10.14569/ijacsa.2013.040115.
Bailey, S.G., Seth, H. & Raffaelle, R.P., 2008. Nanostructured Solar Cells. In Handbook of Self
Assembled Semiconductor Nanostructures for Novel Devices in Photonics and Electronics.
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pp. 552–564.
Guerin, A., Rawlemon’s Spherical Solar Energy-Generating Globes Can Even Harvest Energy
from Moonlight. Available at:
http://inhabitat.com/rawlemon%e2%80%99s-spherical-solar-energy-generating-globes-can-
even-harvest-energy-from-moonlight/ [Accessed April 25, 2016].
King, R.R. et al., High-efficiency space and terrestrial multijunction solar cells through bandgap
control in cell structures. In Conference Record of the Twenty-Ninth IEEE Photovoltaic
Specialists Conference, 2002. Available at: http://dx.doi.org/10.1109/pvsc.2002.1190685.
Larson, D.P., Lukas, N. & Coimbra, C.F.M., 2016. Day-ahead forecasting of solar power output
from photovoltaic plants in the American Southwest. Renewable Energy, 91, pp.11–20.
Law, D.C. et al., Multijunction solar cells with subcell materials highly lattice-mismatched to
germanium. In Conference Record of the Thirty-first IEEE Photovoltaic Specialists
Conference, 2005. Available at: http://dx.doi.org/10.1109/pvsc.2005.1488196.
Luque, A. & Hegedus, S., 2011. Handbook of Photovoltaic Science and Engineering, John
Wiley & Sons.
Marion, B. & Bill, M., 2015. A model for deriving the direct normal and diffuse horizontal
irradiance from the global tilted irradiance. Solar Energy, 122, pp.1037–1046.
Sobolev, N.A. et al., 2001. Enhanced Radiation Hardness of InAs/GaAs Quantum Dot
Structures. Physica Status Solidi , 224(1), pp.93–96.
Yastrebova, N.V., 2007. High-efficiency multi-junction solar cells: Current status and future
potential. SunLab. Available at:
http://sunlab.eecs.uottawa.ca/wp-content/uploads/2014/pdf/HiEfficMjSc-CurrStatusFutureP
otential.pdf.
Brian Nelson , 2011. Multijunction Photovoltaic Devices . Available at :
http://briannelsononlineportfolio.weebly.com/uploads/9/9/8/6/9986689/multijunction_pv_solar_ce
lls.pdf
NSSC Lab IOFFE , Characterization of Solar cells Gallery . Available at :
http://nanopv.ioffe.ru/CharacterisationSCGallery
Rahim Esfandyarpour, 2012. Multi-Junction Solar Cells Available at :
http://large.stanford.edu/courses/2012/ph240/esfandyarpour-r2/
Dragoman Daniela. Multi junction solar cells. Available at :
http://www.unescochair-hhf.ro/uploads/uploads/Daniela_Dragoman_Photovoltaics_4.pdf
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