PROGRESS IN PHOTOVOLTAICS: RESEARCH AND APPLICATIONS
Prog. Photovolt: Res. Appl. 2007; 15:51–68
Published online 19 June 2006 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/pip.715
Broader
Perspectives
The Potential of III-V
Semiconductors as
Terrestrial Photovoltaic
Devices
Matteo Bosi and Claudio Pelosi*,y
IMEM-CNR Institute, Parco area delle Scienze 37/A, 43010 Fontanini (Parma), Italy
III-V semiconductors, GaAs and in particular InGaP, are used in many different
electronic applications, such as high power and high frequency devices, laser diodes
and high brightness LED. Their direct bandgap and high reliability make them ideal
candidates for the realisation of high efficiency solar cells: in the past years they have
been successfully used as power sources for satellites in space, where they are able to
produce electricity from sunlight with an overall efficiency of around 30%. Nowadays, the use of arsenides and phosphides as photovoltaic (PV) devices is confined
only to space applications since their price is much higher than conventional Si flat
panel modules, the leading PV market technology. But with the introduction of multijunction solar cells capable of operating in high concentration solar light, the area
and, therefore, the cost of these cells can be reduced and will eventually find an application and market also on Earth. This article will review the situation of semiconductor solar cell materials, focusing on Si, GaAs, InGaP and multijunction solar cells
and will discuss future trends and possibilities of bringing III-V technology from
space to Earth.Copyright # 2006 John Wiley & Sons, Ltd.
key words: photovoltaic; solar cells; III-V semiconductors; GaAs; InGaP
Received 7 November 2005; Revised 10 March 2006
INTRODUCTION
S
ince their introduction in the mid 1950s, solar cells have experienced an incredible development. Silicon
PV devices were initially employed as reliable and convenient power sources for space satellites, whilst
in the last two decades they have found an application on Earth, where they can provide a reliable source
of electricity in isolated places or be connected to the grid in order to produce electricity.
* Correspondence to: Claudio Pelosi, IMEM-CNR Institute, Parco area delle Scienze 37/A, 43010 Fontanini (Parma), Italy.
y
E-mail: pelosi@imem.cnr.it
Contract/grant sponsor: SPINNER.
Copyright # 2006 John Wiley & Sons, Ltd.
52
M. BOSI AND C. PELOSI
At the beginning of 1980, III-V semiconducotor devices, based on GaAs deposited by means of epitaxial
techniques on GaAs substrates, began to be used in PV applications, and thanks to their superior physical properties and enhanced efficiency compared to Si, were soon adopted in space satellites.
Several years later, more complex heterostructures based on arsenides and phosphide multijunction solar
cells were developed and realised on Ge substrates, and important improvements achieved in the 1990s permitted to surpass the 20% efficiency rate and obtain a significative boost in satellite power sources. By the
end of the year 2000 the 30% efficiency goal was reached, thanks to a triple junction InGaP/GaAs/Ge device.
Nowadays, III-V semiconductor devices have almost completely replaced Silicon as the main component for
space flat PV modules, thanks to their far greater efficiency, low weight and better radiation resistance.
Nevertheless, Si PV is probably one of the most significant examples of a space technology which has finally
succeeded in finding an application on Earth, and now has great potential for changing the world-wide energy
scenario, thanks to its capability of producing renewable energy. The latter is a particularly attractive characteristic, considered the importance and the necessity to develop a self-sustainable future for mankind and the
problems derived from global warming and climate change.
Even if III-V semiconductors seized the role of Si in space applications, nowadays terrestrial Si PV is one of
the fastest growing industry and has enormous potential for changing the lifestyle of millions of people both in
emerging and in ‘civilised’ countries. It has taken almost 50 years since the birth of the first Si solar cell right
through to the adoption of large arrays of Si PV modules to produce a significant amount of elecricity on Earth
at convenient prices, passing through an extensive use of Si in space.
Recalling the example of Si history, it is desirable that arsenides and phosphide eventually find an application
as PV devices on Earth in the coming decades, and in this article several achievements needed to reach this goal
and possible future scenarios will be analysed.
TOWARDS A NEW WORLD-ENERGY SCENARIO
The 21st century will be characterised by a huge energetic demand: world population is steadily increasing
(65 billion in 2005, an estimated 75 billion in 2020 and 9 billion in 20501), and there are hundreds of millions of people in emerging countries (China and India in particular) which are rapidly reaching the welfare
of Western countries. The world’s total energy consumption for 2002 was estimated at about 12 1014 KWh2,
of which about 1/10 was used as electricity. Fossil fuels, in particular coal, oil and gas, contributed with
about 80% and 65% to the generation of world total energy and electricity, respectively (Figure 1). It is
obvious that these resources on Earth are finite, and it is also clear that their use is causing great social
and economical problems for the world, in relation to geopolitical instabilities in controlling energy sources
and global climate: greenhouse gases, which are thought to be the main reason for global warming and climate change, are mostly produced by burning fossil fuels.
In order to preserve human civilisation and to grant social and economical development to billions of
people in the third world, an answer to the ‘energetic problem’ must be found in the coming decades and
should be considered as a main topic of political discussion in all countries. The data reported by the International Energy Agency2 suggests that transport (terrestrial, aerial, marine) and heating have the biggest
role in energy consumption (about 9/10 of the total energy used in the world is different from electricity);
consequently, any energy policy focused on electricity alone is misleading, and in energy-saving attention
should mainly be directed towards the development of new means of transportation, rationale goods delivery
and optimisation of housing and thermal insulation facilities, since electricity has just a second-order contribution to the use of energy. Nevertheless, electricity is the basis of all civilised countries and industrialisation, and its access is a fundamental step towards achieving people welfare.
As reported in Figure 1, 65% of the World’s total electricity consumption is obtained from fossil
fuels; about 20% comes from nuclear power plants and the rest is produced by means of renewable sources,
such as hydropower, biomasses, PV, eolic and geothermal sources: these sources are seen as the most
promising ways of granting electricity to the whole world, and can help to reach a self-sustaining energy
system.
Copyright # 2006 John Wiley & Sons, Ltd.
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Figure 1. Share of total energy and electricity consumption in the world in 20022
On the other hand, conventional nuclear power poses unanswered questions such as waste handling, potential
hazards of accidents and explosions, similarity with a technology for the realisation of nuclear weapons, plant
site security, nuclear fuel control as well as its availability and therefore should not be considered as a reliable
answer in the long-term.3,4 Moreover, it has been calculated that building new nuclear power plants (even with
the latest fission light-water technology), costs twice as much as building new wind farms, 5 to 10 times as much
as gas-fired cogeneration or trigeneration in buildings and factories (at the net of credit for their recovered heat),
and 3 to 30 times as much as making electrical apparatus more efficient.5
It should also be mentioned that the different renewable sources are not in competition as they fulfil many
different needs and are suitable in extremely different environments. They all have pros and cons (identified for
example, in the location of installation, suitable climatic conditions and presence of primary supply—wind or
sun), can be effectively combined to provide a reliable and continuous power supply, and can be scaled from a
production going from a few milliwatts to giga/megawatt power plants.
Currently, wind power is one of the most promising and economic ways of generating renewable electricity,
since it has a very short money payback period (3–5 years), low ownership cost and high energy production rate.
Its main drawbacks are the noise pollution produced by the turbines and the need to be installed in windy
regions in order to be convenient, so it can’t be used everywhere.6 The Authors consider the visual impact
of wind farms as a minor problem derived mainly from people’s cultural background.
Solar power, instead, is still very expensive (although prices per kWp have rapidly decreased fast in the past
years), has a longer money payback rate (8–10 years even with a feed-in tariff) and poses important technological challenges, but can be directly used by the final client even in remote locations, does not necessarily
require a grid connection (or work in a "net metering’ buyback, if a grid is present), is noiseless, maintenance
free, reliable for more than 20 years7 and can be integrated in consumer electronics for low-power applications.
Moreover, solar irradiation in third world countries is extremely high and is seen as the most obvious way to
electrify isolated communities.
Since the total amount of solar energy reaching the Earth’s surface has been calculated at more than 10 000
times the world’s total energy consumption, it is obvious that this source, if correctly and efficiently used, can
supply the substantial part or even all of our future energetic needs. Nevertheless, markets and clients are still
dubious regarding the adoption of eolic energy and PV modules, as their price is perceived as being too high
Copyright # 2006 John Wiley & Sons, Ltd.
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M. BOSI AND C. PELOSI
Table I. Cost of energy production by different sources. Minimum and maximum values
refer to different countries
Market price
Hydroelectricity
Biomass
Wind energy
Photovoltaic (stand alone)
Photovoltaic (grid connected)
Coal
Oil
Nuclear
2–8
5–6
4–12
25–45
9–20
2–15
2–11
2–4
Externality cost (scent/KWh) (after 8)
0–1
0–3
01–025
06
06
2–15
3–11
02–078/6–4276
Externalities for nuclear energy reported in the reference list8 include costs which should appear in case
of a rector fault and for handling of toxic wastes, but as reported in the reference list76 were probably
understimated. Photovoltaic costs (referring to conventional flat Si panles) depends on the climate and
latitude where the panel is installed, and are for large photovoltaic plants.
compared to fossil fuels, as reported in Table I. But it has to be stressed that current energy rates do not include
externalities, i.e. ‘The major impact and costs originating from the production and consumption of energyrelated activities such as fuel cycles’.8 These costs arise when an activity with a specified price has an impact
on another activity, and imposes on the latter an additional cost that was not accounted for in the former price.
Greenhouse gases, car exhaust and chemical waste are typical examples of externalities of fossil fuel use,
because their social costs (not only monetary, but also related to global warming, cancer, illness, etc) are not
considered in the market price. If the price of electricity generated by fossil fuels included externality costs,
renewable sources could become more competitive, as reported in the reference list.8The adoption of renewables for power generation is slow because of the existence of several development and market penetration barriers, most of which are not scientifically related.9 Some of these impediments are economic (lack of
competitiveness and internalisation of external costs of energy use), institutional (lack of co-ordination in
governments and institutions, long-term planning policy requirements), network (monopoly of generation,
transmission and distribution of electricity), social (lack of interest in future energy development and sustainability, doubts about new technologies), financial (lack of funding for research or pilot-installation). Only a
long-term policy can aid a niche market in developing, expanding and gaining the political and economical
power needed to bring a new technology out of the niche and distribute it to everyone. This road was
followed by Germany, now the second world leader in the installation of PV solar panels,10 also thanks to
the feed-in tariff introduced in 2000.
There are several reports and market studies that analyse the trend in PV installation all over the world.11–17
During the past 4 years Japan, Germany and USA have emerged as leaders in the total kWp installed, sharing
together about 90% of the world market. Globally, in 2003 about 753 MWp PV systems were installed, and the
1 GWp barrier was exceeded in 2004. The trend in the PV market has shown an annual increase of about 30%
from 2000 until now, making PVone of the fastest growing industries. Because of this, PV is attracting more and
more actors, even if the 10 biggest companies alone share about 90% of the global market; according to some
studies, a demand increase is expected thanks also to the introduction of feed-in tariff in more and more countries (e.g. Italy and Spain) and the fulfilment requirements of the Kyoto Protocol: in order to reduce CO2 emission, the European Council has stated that in 2010, up to 20% of electricity should be generated by renewable
energy, while in 2030 up to 4% of the World’s electricity should be generated by PV. Estimates suggest that
world energy consumption in 2050 will be about 25 Gtoe, or 27 1014 kWh and, if the world wants to avoid
social, economic and environmental problems, the largest part of this energy should be produced by renewables.
Moreover, the development of emerging countries as well as their energy policy should not follow the development trend of today’s civilised countries, particularly with regards to electricity generation and transport
management. Concerning PV (and renewables in general), a very long policy planning period is most definitely
needed, in order to support the creation of market demand, the assignment of public funding (with ‘roofprogrammes’ or feed-in tariffs), and in order to continue a heavy research and development process.
Copyright # 2006 John Wiley & Sons, Ltd.
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Nowadays, the leading PV technology is Silicon-based, either monocrystalline or polycrystalline. Since
1950, Silicon has been the most important material in the microelectronic industry, where it is used with a very
high purity degree (electronic grade Si—EGS). Up to now, the PV industry has relied upon Si scraps (known as
Solar Grade Silicon—SGS), from microelectronic Si producers.18,19 At the turn of the century, the microelectronic industry faced a sudden fall in demand due to an end in ‘new economy’ speculations, whilst the PV industry began its expansion. Manufacturers sold electronic Si to PV cell producers, which on the other hand
depended only on these feedstocks, without making efforts to build an independent source of SGS.
Since then, chip makers have increased their production and become once again the main Si consumers, using
about 20 000 t of pure Si per year ( in comparison, the PV market consumes about 10 000 t of Si per GWp
installed). But since selling EGS to the microelectronic industry is more convenient than SGS for PV, at the
moment, the Si offer is experiencing an imbalance and the PV industry is probably going to face a sudden
raw material shortage in the next few years. If alternative sources of SGS are not created, a steep increase in
prices will be unavoidable, with effects on module prices and demand. Some market studies suggest that, due to
these reasons, PV demand will be relatively flat in the next 3–4 years, and then pick up after 2010, when feedstock problems will hopefully have been solved. This could also be an occasion for new technologies (thin
films, III-V semiconductors) to emerge and gain a share of the market.
SILICON SOLAR CELLS
The first Si p/n cells were developed at Bell Labs (USA) in 1954.20 They had an efficiency of about 6%, but their
potential as power supply for space satellites soon became clear. The first extensive and important solar cells
application occurred in 1958, on both the Soviet satellite Sputnik 3 and the US Vanguard, which mounted 8%
efficiency n-type Si with diffused boron as p dopant. The success of Si in space also came from the technical
know-how derived from electronic applications, such as power converters (AC to DC current conversion, HF
generation), and electronic circuits for radio communications, to the availability of several growth techniques
and ohmic contact deposition.
Since these pioneering applications, Si solar cells have become more and more used in space, and a lot of
efforts have been made to increase their efficiency and reliability, in order to obtain better radiation resistant
material, and achieve thinner and lighter modules.17 Antireflection coatings with SiO or TiOx, back surface field
and window layers have been developed in order to maximise photon collection. Advances in crystal growth
technology have permitted to obtain materials with a higher degree of purity and high carrier lifetimes (>1 ms).
In the 1970s cheaper Si modules with lower efficiencies were tested, either amorphous,21,22 polycrystal,23 or
thin film based.24,25
Until 1970 Si was the only available power source for space missions, reaching a conversion efficiency of
14–15%. The stringent requirements that solar cells had to meet made cost optimisation a minor problem: the
high price of solar cells in the ‘70s and 80’s was the main reason for their limited use on Earth, often confined to
small electronic devices such as calculators or swatches. A notable exception was represented by the 35 KWp
PV system installed in the Papago Indian Reservation in Arizona, the world’s first PV human installation in
which solar energy was used for pumping water and distributing electricity to 15 houses until 1983, when grid
electricity reached the zone.
GaAs
GaAs is a III-V compound with a direct bandgap of 142 eV, widely used in infrared LED and LASERs,26 fiber
optic drivers and receivers, high speed microelectronics monolithic microwave frequency integrated circuits
(MMIC)27,28 and high efficiency solar cells.
GaAs has some electronic properties, which are superior to those of Si29: it has a higher electron saturation
velocity and a higher electron mobility, allowing operation at frequencies above 250 GHz. Furthermore, high
frequency GaAs devices are less noisy than Si devices, whilst the higher breakdown voltage permits to work at a
Copyright # 2006 John Wiley & Sons, Ltd.
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M. BOSI AND C. PELOSI
higher power than that of an equivalent Si device. These characteristics permitted the use of GaAs IC in mobile
phones, satellite communication, microwave point-to-point links, and some radar systems.
However, Si still has some major advantages over GaAs, such as its cheaper price, larger substrate diameter
(up to 1200 ), the possibility to use SiO2 as an insulator or as an excellent gate oxide and higher hole mobility.
Despite some drawbacks in the fabrication of microelectronic devices, GaAs has far superior potential than Si
in the fabrication of solar cells: the direct bandgap of GaAs allows the absorption of the entire solar spectrum in
less than 3 mm; theoretical calculations30,31 have shown that the optimum bandgap that a single junction solar
cell should have in order to reach the best AM15—1 sun conversion efficiency is about 134 eV, closer to the
GaAs gap than to the Si one (112 eV).Being much more difficult to grow and to optimise, compared to Si, III-V
semiconductors have experienced a slower development. The first GaAs solar cells (with an efficiency of 3%)
were developed at the Russian Ioffe Physico Technical Institute (PTI) in 1960; it was soon recognised that this
material was more radiation resistant than Si and had a higher temperature stability. As for Si, the first important
GaAs application as a PV device was obtained in space: the Russian spacecrafts Venera-2 and Venera-3 reached
Venus in 1965 and were powered by GaAs solar cells.The difficulties in crystal growth technology (epitaxial
GaAs was mainly grown from the liquid phase) and in obtaining sharp junction (doping was realised by diffusion) prevented III-V development until the 1980s, when epitaxial deposition techniques like MOVPE and
MBE began to be extensively used to deposit thin films on GaAs or Ge substrates: the pioneering work of
Manasevit et al. on GaAs MOVPE is described in the reference list.32 MOVPE growth uses standard precursors
as arsine and trimethyl gallium, while doping is realised by Si (n type) with Silane and by Zn or C (p-type),
carried in a growth chamber as dimethyl zinc or carbon tetrabromide, respectively. Homoepitaxy of GaAs poses
very few technological problems, since good crystal quality substrates, either p or n doped and semi-insulant,
are available with EPD < 500 cm2.
Heteroepitaxy of GaAs on Ge, despite the small lattice and thermal mismatch (respectively 008% and 1%)
requires a particular epitaxial procedure in which a thin (some tenth of nm) GaAs or AlGaAs buffer layer is
inserted between the Ge substrate and the GaAs layer.33 In order to achieve perfect matching with Ge, a small
amount of In (about 1%) can be added to GaAs, to avoid the creation of misfit dislocations generated by lattice
mismatch.34
Ge (001) substrates that can be grown n or p-type doped, are available with 600 diameters (with 800 R&D prototypes) and exhibit a perfect crystalline structure (EPD ¼ 0 cm2); they have a non-polar surface, while the
GaAs surfaces exhibit polarity, with the As face having different properties from Ga face. The ability to control
the GaAs/Ge interface is crucial to achieve good device performance and reliability, since the polar/non polar
nature of this interface is the source of Anti-Phase Domain (APD) formation and cross-doping. APD boundaries
degrade device performances by creating leakage currents, while interdiffusion poses problems in the formation
of the p-n junction. In order to avoid APD, Ge substrates used for GaAs heteroepitaxy are [001] oriented and
miscut by about 6 –9 off towards the [111] direction.
MULTIJUNCTION PV DEVICES
Sunlight is a continuous wave spectra, but on the Earth’s surface some long-wave frequencies are absent due to
the absorption from air molecules. As it is well known, a semiconductor can only absorb photons with an energy
above its bandgap; moreover, in this process, any photon with an energy greater than Eg generates a photocarrier
which is thermalised with a series of phonon emissions to the bottom of the conduction band: in this process a
large part of the energy exceeding the bandgap is wasted as heat, and therefore is useless for photovoltaic conversion.
An ideal device for achieving the best efficient use of the solar spectrum should be constituted by an infinite
number of p–n junction, each with a bandgap Eg which differs by an infinitesimal Eg from the adjacent one,
with the highest gap being at the top of the illuminated device and the lowest at the bottom. In this way,
every photon with energy Eg will be absorbed and converted by an appropriate semiconductor without any
thermalisation loss. Of course, since this is not possible, the parcelling of the solar spectrum should be achieved
Copyright # 2006 John Wiley & Sons, Ltd.
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Figure 2. Making a better use of solar spectrum by using two or more PV device in parallel (A) or in series (B)
in a discrete way, the most simple one being the use of a two or three p–n junction (2J–3J) with different bandgaps.
There are two different ways of building such PV devices Figure 2): one is to use two or more different single
cells, split the incoming light with an appropriate optic (prisms or dichroic mirrors) and direct an appropriate
part of the solar spectrum towards the single cell, in dependence of its bandgap. This approach, in its simplicity,
poses a series of technical problems, such as the alignment and mechanical packaging of the optical parts, which
should be precise and reliable in time, and the control of frequency separation of the incoming light with a sharp
crossover filter made of an appropriate material.
Discussion regarding the possibility of realising a single PV device composed of different materials, each one
tuned to a different spectral region, began in the early 1960s, but was realised only when advanced epitaxial
techniques were developed. The method, which is widely used in today’s space solar cells, is based on the realisation, by MBE or MOVPE, of a single monolithic cell in which the different junctions are grown one on the
top of the other, in order to obtain a two-terminal device. These junctions are connected in series, and can be
seen as an array of diodes in which an emitter is connected to the base of the following diode. As a result of this
kind of electrical connection, each junction has to generate the same number of carriers, and when this happens
all the p/n junctions in the device must be crossed by the same current: this is the ‘current matching’ condition
that a multi junction solar cell must fulfil. In order to achieve this requisite, all the single junctions must be
optimised in both thickness and doping, and a careful study of the single parts must be carried out to obtain
the current dependence versus the growth parameters.
Besides current matching, once the number of cells constituting a multi-junction device has been fixed, the
choice of the individual bandgaps should be carefully chosen in order to maximise the theoretical conversion
efficiency; an extended and well referenced summary of theoretical efficiencies for multi-junction solar cells
can be found in the reference list.35 Considering a 2J device, thanks to a thermodynamical consideration, it is
possible to obtain ‘maps of efficiency’ in which the maximum PV conversion efficiency is plotted against the
top and bottom cell bandgap, as the one reported in the reference list.36–38 For a tandem cell, by optimising the
thickness of the top junction for a particular bandgap combination, a maximum conversion efficiency of 36%
(AM15) would be obtained by choosing semiconductors with a bandgap of 11 eV and 17 eV, respectively.37 Si
has a bandgap of 11 eV, but it’s difficult to grow a Si-lattice matched semiconductor with a 17 eV bandgap.
Since substrate lattice matching is yet another limiting condition, the material of choice for the realisation of
multi-junction solar cells is represented by III-V arsenides and phosphides: the unique possibility of growing not
only GaAs, but also lattice matched AlGaAs, InGaAs and AlInGaP alloys on Ge has contributed to the definitive
success of III-V solar cells. By combining GaAs and InGaP (Eg ¼ 19 eV) on either GaAs or Ge substrates it is
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possible to reach a maximum theoretical efficiency of about 34% (AM15, 1 sun), and today the world record
efficiency for this 2J device is 303%.39
InGaP
In1-xGaxP alloy is lattice matched to GaAs for x ¼ 0516, and for this composition its bandgap value is 19 eV.
With the addition of aluminium (AlInGaP alloy), it is theoretically possible to vary the bandgap between
143 eV and 22 eV, always maintaining lattice matching with GaAs. These materials are widely used in the
fabrication of high brightness LED with colors ranging from green to red40,41 and in the fabrication of
InGaP/GaAs heterojunction bipolar transistors (HBT) or metal semiconductor field effect transistors (MESFET) for high frequency and high power applications.42,43
InGaP has superior properties compared to the AlGaAs alloy,44,45 another semiconductor with the same
bandgap range and lattice matched to GaAs, and a potential candidate for the realisation of 2J top-cell; InGaP
has a lower density of DX recombination centres, a lower surface recombination velocity, a higher etching
selectivity between InGaP/GaAs layers than AlGaAs/GaAs junctions and a lower affinity to oxygen (it is more
simple to grow high crystal quality InGaP/GaAs heterolayers than AlGaAs/GaAs ones, in which Al atoms tend
to oxydise during epitaxial growth).
InGaP can be grown by MOVPE using phospine, trimethyl gallium and trimethyl indium as precursors, and
doping is achieved by adding Si (as silane) or zinc (as dimethyl zinc) for n-type or p-type doping, respectively.
A common problem during InGaP/GaAs growth is the mutual diffusion and atomic exchange of species at the
heterointerface, which smooth out its abruptness and degrade device performance.46,47 Several methods for
overcoming this problem have been proposed, including inserting an intermediate layer between InGaP and
GaAs,48 hydride flow modulation49 or the use of particular gas switching sequences.50 Thermodynamical analysis of InGaP/GaAs interface has been performed and the nature of the phenomenon suggested.51
In0484Ga0516P is a disordered alloy, but it has a spontaneous CuPt–type ordering for a composition of
x ¼ 05, which coexists at different extents with the disordered phase52–55: in the ordered phase, In and Ga
occupy alternating [111] planes of the group-III sublattice, leading to a reduced bulk symmetry and to a lower
band gap; the degree of ordering, correlated with the band gap energy shift, is found to vary in dependence of
MOVPE growth conditions, and the atomic structure of the surface has been shown to play a key role in the
ordering effect. A precise control of growth parameters is essential for the reproducible fabrication of electronic
devices and solar cells, since order degree and band gap values are closely related.
In0484Ga0516P (often referred to as InGaP2 or simply InGaP) is the material of choice for the development of
tandem GaAs/InGaP solar cells, as first proposed by Olson et al. in 1989.56 The possibility to use Ge as a substrate opened the way to the addition of another junction, realising a triple junction (3J) solar cell: an n or p
doped Ge substrate can be doped either p or n by diffusion during the growth process, with a trimethyl gallium
or arsine flow. In this way a p-n Ge junction with a bandgap of 07 eV is obtained and used to absorb infrared
photons.
Recent advances in metal organic synthesis have permitted to obtain metal organic compounds with Ge, in
order to achieve a better epitaxial Ge p-n junction by means of a higher degree of control in thickness, interfaces
and material quality.57
Moreover, Ge is mechanically harder than GaAs, allowing to realize thinner and thus lighter substrates. This
characteristic has made Ge the substrate of choice for space applications: even if the manufacturing of III-V PV
devices is far more expensive than producing Si solar cells, the higher power–weight ratio permits to reduce
weight and consequently launch costs.
The first multiple junction solar cells were developed at the National Renewable Energy Laboratory (NREL)
in the US: Figure 3 shows a cross section of this complex device, including three different cells (InGaP, GaAs,
Ge) connected in series by tunnel junction layers. This kind of device holds a record efficiency for a 1 cm2 PV
cell, reaching 32% at 1 sun.39
Recalling the history of Si solar cells, 3J solar cells were initially used as a power source for space modules:
the most notable employment of InGaP/GaAs/Ge devices was on the Spirit and Opportunity robots that are now
Copyright # 2006 John Wiley & Sons, Ltd.
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Figure 3. Schematic of a triple junction solar cell
exploring Mars’ surface. These cells were designed to be highly reliable in an extremely hazardous atmosphere
(with a solar spectrum different from that on Earth) and maximise efficiency in an application for which the cost
is, as usual, a minor problem.
The different cells in a MJ structure are interconnected by tunnel junctions (TJ),58,59 thin (about 10nm) heavily doped pþ–nþ layers, which offer a low resistance to current flow. When forward biased, and below a certain
current threshold, TJ behaves like a low-value resistor. In order to avoid parasitic light absorption, TJ are usually
realised with high bandgap material, often AlGaAs. They represent one of the most critical steps in the growth
of a multi junction device, since they must be defect-free and require a precise control of thickness, doping and
interface abruptness.
TOWARDS FOUR AND MORE JUNCTIONS
Several research groups are currently trying to increase the number of junctions or find an alternative cell design
in order to obtain a better use of the solar spectrum.
A reduction of the band gap exceeding 01 eV per atomic per cent of N content, along with a lattice parameter
reduction, was observed in GaNxAs1-x for x < 0015.55 This discovery has opened an interesting possibility of
using N containing alloys such as InGaPN and InGaAsN for long wavelength optoelectronic devices: 13 mm
and 155 mm InGaAsN/GaAs quantum well (QW) lasers have been reported as light sources for optical fiber
communications.60,61 Such lasers are expected to have an improved thermal stability compared to common
InGaAsP/InP lasers based on the InP substrate, due to the large conduction band offset of InGaAsN/GaAs heterostructures. It has been shown that N concentration as low as a few per cent lowers the band gap of the Ga162
and the Ga1-xInxAs1-yNy alloy
xInxAs1-yNy and Ga1-xInxP1-yNy alloy systems by a significant fraction of eV,
63
with y035x is lattice–matched to GaAs (or Ge). Kondow et al. proposed and demonstrated InGaNAs 1 eV
laser, while Friedman et al.64 proposed it for 1 eV GaAs lattice matched solar cell, because the addition of
10 eV junction to the InGaP/GaAs/Ge structure has the potential for greatly increasing efficiency in a monolithic multijunction solar cell: the bandgaps of materials in this stacking sequence are very close to the best
theoretical ones for achieving maximum efficiency for 4J solar cell.
Working prototypes of InGaAsN-based solar cells lattice-matched to GaAs with an active region of 1 eV have
been demonstrated, and extensive studies have been aimed at understanding the best precursor for carrying N
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M. BOSI AND C. PELOSI
Figure 4. Schematic of 4,5,6 junction solar cells with maximum 1-sun AM15 theoretical efficiency36,64
into the growth chambers and at studying the change in the lattice, surface structure and alloy composition due
to the addition of N.36,65–68
Unfortunately, diluted nitride crystal quality is so low that the fourth junction limits the overall current of the
device, making the addition of the new junction useless. The high density of defects lowers the minority carrier
diffusion length, and the efficiency of the 4J cell is hindered by the low current imposed by the InGaNAs
junction.
Since adding a fourth cell reduces the current flowing in the device, the problem could in principle be solved
by the addition of one or more junctions and by using a different stack of cells: a 5J AlInGaP/InGaP/AlInGaAs/
InGaAs/Ge, with thinner individual layer thickness, can operate at lower current but at a higher overall voltage,
reaching a theoretical efficiency of over 55% at AM0.36 The use of thinner layers makes the device less minority
carrier lifetime dependent, and lower crystal quality materials can be used. Since total current flowing in 5J
devices is lower than in 4J cells, it is possible to insert a sixth junction by using today’s InGaNAs technology:
this material has all the characteristics necessary to be used in a 6J cell in which AlInGaP/InGaP/AlInGaAs/
InGaAs/InGaNAs/Ge alloys are stacked. The efficiency of a multi junction cell like this one would be even
superior when compared to the 5J one (Figure 4).
MARKET PERSPECTIVES FOR III-V SOLAR CELLS
The manufacturing of high efficiency multi junction III-V solar cells requires a huge amount of money, because
of the large investment needed for MOVPE reactors (well over 1,5 106 s for one 12 400 multiwafer production
reactor), for the research and development of the complex growth processes, and for the high cost of substrates,
precursors and safety related issues.
The Microelectronic compound semiconductor industry, that often deals with a chip size of about or less than
1 mm2, is capable of optimising the yield and the unitary cost thanks to the large number of devices realised on a
single wafer. For example, the cost of a single Ge or GaAs substrate (>100$ for 400 diameter) is divided at least
by the number of working chips obtained from it (which can be as high as 4000–5000).
The main drawback of the PV flat panel industry, on the contrary, is the large area of solar cells needed to
produce a significant amount of energy: a large area means a large impact of the substrate and precursor costs
on the final device price, a long growth period, the need for several MOVPE reactors in order to achieve a sufficiently high number of working devices. Moreover, a large device (even a 1 cm2 cell is a significant area for a
semiconductor) with few defects exhibiting electrical activity would either be defective or work at lower efficiency. This is a very important issue, as when dealing with low area devices a single defect may harm one single
device among thousands, whereas in large area devices, one defect may render 1 out of 10 devices useless.
This is the main reason why III-V solar cells, while being used successfully in space on satellites and modules, have yet to be used on Earth in commercial flat panel applications, where large area cells have to be used.
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As an example, let’s consider Spectrolab (www.spectrolab.com), one of the world leaders in III-V space solar
cell production (its cells were mounted on Spirit and Opportunity robots): they claim to possess a facility with
10 fully automated production epitaxial MOVPE reactors and be able to achieve a wafer throughput capacity of
more than 7000 400 wafers per week. If the total area of these 400 wafers were all converted in III-V solar cells
with an efficiency of 40%, a panel area of 2950 m2 per year would be obtained, with a total power of 1,18 MWp,
which could generate up to 2 GWh in a year, if mounted in a conventional flat panel system in a sunny region.
As a comparison, the average initial investment for a 1 MW wind turbine project is of about s11 106, less than
the cost of a single MOVPE production reactor. The wind turbine could produce, in an appropriate region,
nearly the same amount of energy as the 2950 m2 III-V solar cells, but would require much less technology,
risk, financing, realisation issues and would have a much lower payback time.So, it may seem that high
efficiency multi junction III-V solar cells, that now lead the PV space technology, could find difficulties in being
used on Earth. This, fortunately, is not completely true, since it is possible to lower the used area and to boost
efficiency by using cells under concentrated light.
SOLAR CELLS UNDER CONCENTRATION
Concentrating solar light is a very old idea: Archimedes, in 213 BC, used mirrors to concentrate sunlight to burn
Roman ships. Nowadays, we can use a similar idea to collect solar light and concentrate its energy on a single
small area solar cell: in this way it is possible to save a total cell area equal to the concentration intensity and
realise a potentially cheaper PV system, since the expensive semiconductor material is replaced by a relatively
inexpensive optical concentrator, usually made from molten plastic and steel frames. A concentrator PV system
usually includes a sun-tracking apparatus, which utilises light sensors and one or two axis step-motors, so it is
always pointed towards the sun. The tracking system permits to maximise light collection and obtain a significant boost in the energy produced during the year.
The detailed description of PV concentrator systems is not the purpose of this article: for a complete review
of concentrator solar systems and related issues, the reader is addressed to references69–71 where, as well as
technical descriptions and economical analysis, the authors also report some reasons to why concentrators still
haven’t found a market share in PV technology.
The most important topics related to concentrator systems rely on concentration ratio, on how to concentrate
light (by means of Fresnel lenses or parabolic mirrors), on the reliability of sun tracking and of the whole system
and on its final sale price, which should of course, be substantially lower compared to an analogous flat panel
design, and on the kind of solar cells installed. Moreover, since solar cells have a very long lifetime (up to 20
years), all the concentrator components (frames, lenses, motors, electronics, etc) should have a comparable lifetime, in order to avoid expensive maintenance costs.
A boost in cell efficiency is observed by light concentration, since the open circuit voltage depends on the
current: for example, 3J solar cells with VOC ¼ 26 V at one sun, if operated at 1000 concentration will boost
its VOC to 31 V, with an increase of about 20%: as a practical example, the InGaP/GaAs/Ge 3J cell presented in
the reference list72 has an efficiency of 25% at 1 sun but, if the concentration ratio increases to 1000 , efficiency rises to 34%.
Solar cells adopted in concentrator systems require several modifications compared to the design adopted in
space, in order to support the higher operating temperature and current; in particular, critical issues in concentrator cells are the lowering of the series resistance and current matching in MJ structures, since the spectrum at
AM15 is different from the AM0 spectrum, particularly at the high energy side.
The use of Si solar cells on concentrators limits the concentration ratio to about 300–400 : above this value
too much current would be generated, resulting in device fault. Nevertheless, saving on the semiconductor area
would enable the use of the much expansive and efficient EGS: prototypes of Si cell for 100 concentration
have reached an efficiency of 268%, while the best 1 sun laboratory EGS solar cell reaches the maximum efficiency of 247%.39
III-V compounds, especially with multijunction design (three and more junctions), have the potential to
exceed an efficiency of 40%. C. Algora71 presented an in-depth analysis of cost factors for concentrator solar
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cells, defining a minimum concentration level at which the use of solar cells becomes economically convenient.
Since the final price is much more important in defining the success of a product on the market than the technology content, or even efficiency, it seems that in order to be economically convenient III-V PV devices should
be coupled to a concentration ratio of almost 1000 . At such high values, it is possible to think of solar cells
with dimensions comparable to those of LED, i.e. about 1 mm2: this is the approach currently being followed by
the Madrid Instituto de Energia Solar, where a small Fresnel lens concentrator is being developed for use with
GaAs or InGaP/GaAs cells grown on GaAs, reaching an efficiency of 30% at AM15 and 1000 suns.73 The
dimension adopted for the GaAs devices would permit to use the established LED manufacturing and packaging
technology, to obtain thousands of solar cells devices on a single wafer and to project a cost of 25 s/W at an
annual production of 10 MW.
Semiconductor compounds, and arsenides in particular, have proved to be extremely reliable in applications
such as LASERs, where the devices are more complex than a solar cell (consisting of several heterointerfaces,
thinner layers, quantum wells), have more stringent requirements (being subject to higher power and current),
necessitate low dislocation density and must guarantee high reliability. The evolution and widespread utilisation
of GaAs lasers for high power application has proven its reliability and structural perfection: there should be no
doubt about the possibility of using GaAs solar cells in high concentrator devices (even above 1000 ) without
fault risks. The possibility of dealing with the high power required by a high concentration application is exclusively confined to stable crystalline compounds, and, on the other hand, the use of such a high concentration is
the only viable way of minimising the area necessary to assemble a solar cell and achieve an extremely high
energy output at an economically convenient cost. The ultimate goal should be to lower the PV cost to about 1
s/Wp, and to realise modules with an efficiency of 40%: in this scenario, PV generation would become costcompetitive with other sources for electricity generation.
Recent results from Spectrolab74 and NREL,75 reported at 2005 conferences, have presented significant
advances towards this target. Their approach is to use a lattice mismatched cell (GaInAs, 1 eV) with either
InGaP/GaAs/GaInAs or InGaP/GaInAs/Ge structure, and a compositional graded buffer alloy to relieve the
strain due to the mismatch. Cells with an efficiency of 39% on low concentrations (250 ) were developed
and, based on these results, the milestone of an efficiency of 40% is thought to be near.
There are also some companies (SunPower Corporation, Amonix Inc. EDTEK Inc) which produce concentrator systems and cells to be used in these systems: the presence of some market actors, aimed at producing
components of these apparatus, could eventually drive the PV sector towards the utilisation of this technology.
Besides commercial companies, a lot of research groups all over the world are developing their own original
research concentrator system, and nowadays international PV conferences often include a concentration PV
session.
Several large ‘PV fields’ have been installed in the past years, all using Si cells: notable examples are the
Bavaria Solar park (Germany, 10 MWp), the Springerville Generating Station Solar System (Tucson, Arizona,
USA, 459 MWp), the Serre Power Plant (Italy, 33 MWp), and the Floriade exhibition hall PV system
(Netherland, 23 MWp). There seems to be a renewed interest in very large solar installations, in contrast with
the micro-generation and use of a great number of small roof-top systems.
The use of concentration also provides a more rationale use of the installation terrain and a boost in energy
production for large solar power plants: currently there is one project in Europe, the EUCLIDES (European
Concentrated Light Intensity Development of Energy Sources) photovoltaic concentrator located in Tenerife
(Spain), which is testing a large 480 KWp system that can contribute to a significant reduction, of up to
50%, in PV system costs.
Concentration could also challenge conventional roof-top Si arrangements: thanks to an improved kWp/m2
generation, a concentration system installation could be possible also in large apartment buildings and not only
in uni-familiar homes. Moreover, a better integration of PV elements with architectonic design could be
achieved.
Solar cells with five or six junctions have a very high theoretical efficiency and could eventually find an application in future concentration installations. However, several issues, such as material quality and especially the
effect of solar spectrum variation during the day and year on the current generation of each subcell remain to be
addressed.
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In conclusion, there is growing interest towards concentration, both in R&D and in the market, which allows
us to look to the future with optimism.
FUTURE TRENDS IN PV
Depletion of oil and gas resources with increasing costs for their security and control, changes in world climate
with an increasing number of extreme atmospheric events in a single year, lack of access to energy for 1/3 of the
world’s population and the growth of emerging countries, with their huge thirst for energy, are posing problems
that must be solved by means of new energy sources, with low externality costs. Thanks to this world-wide
scenario, renewable sources are attracting more and more attention: renewed social, political and market interest for PV permits to depict the future of PV technologies as being very bright, also thanks to the great efforts
and research projects that are currently being made, in order to commercialise modules at lower prices and find
new materials and solutions for increasing efficiency and s/Wp ratio.
Si flat panel technology is nowadays the most mature and current market leader, with projections of huge
growth in the coming years; the major problem that today Si faces is the low feedstock of SGS, essentially
dependent on electronic industry scraps, which would probably hinder sales in the next 2–3 years due to a consequent price rise. The Si cell industry is currently working on the creation of an independent material feedstock
supply and on the reduction of the quantity of semiconductors used in solar cell production, with the use of
either thinner and more efficient layers, deposition of amorphous Si on cheap substrates like glass, plastic or
steel, novel growth techniques like ribbon and enhanced sawing methods to save material. Current commercial
PV systems are usually made from multi-crystalline Si and cost about 4–5 $/Wp: they reach an efficiency of
about 15–16% (AM15. 1 sun), while laboratory crystalline Si cells score an efficiency of 247%.
Thanks to its simplicity and development, Si PV is the best understood and studied technology, and several
works has been addressed in order to understand not only the energy payback ratios of PV Si modules76 (which
recover in 3–4 years all the energy used to build them), but also the complete life cycle assessment of the module, i.e. how to describe and evaluate all the possible environmental impact from construction to disposability
versus the environmental impact of the fabrication of crystalline Si modules.77,78
Moreover, with Si being one of the most abundant elements on Earth (25% of the first 15 km of the Earth crust
contains Si), it is expected that there will be plenty of raw material to fabricate solar panels, and a depletion of
this resource will not present a problem.
On the other hand, III-V semiconductors have the potential for breaking the 40% barrier thanks to the
unique possibility of using a multijunction monolithic device. Even if GaAs technology is ascertained and
MOVPE processes are continuously improving, it is difficult to imagine an adoption of III-V solar cells on
Earth other than their integration on high-concentration solar systems, due to the large area required for
manufacturing solar cells and their high cost per cm2, and this could eventually be the ultimate challenge
for the coming years.
III-V PV technology is still confined to space applications; before its can be used on Earth, there are still
several issues that should be considered, such as waste handling, the environmental impact of production facilities, life-cycles of modules etc. These studies have been extensively performed for Si, but only a few articles
deal with III-V material.79,80
Regarding the precursors used for MOVPE deposition of III-V cells (and in general for electronic devices) it
is difficult to find studies about their abundance on Earth. Authors are aware of reports which study the abundance of Gallium,81 Indium, Germanium, Arsenic,82 but in some cases they are restricted only to domestic US
availability.
Some materials, as Indium and Gold (used for electrical contact) can be recycled from wasted electronic
refined and reused devices. Anyhow, precise estimates of the availability of precursors for III-V deposition
are not available. There is a general agreement in considering Germanium to be the most critical element,
because it is used to fabricate thick substrates. Data reported from82 the States that the US Germanium reservoir
is of 400 000 kg, is estimated to last for 25 years at the consumption rate of 2003. Following a simple calculation, if all this Germanium were used to make 150 mm thick substrates for advanced III-V solar cells with an
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efficiency of 40% used at 2000 concentration, a maximum of 04 TWp would be obtained: this would permit
to produce about 400 TWh per year, a small fraction of the 16 000 TWh consumed in 2002.
So, it seems obvious that the priority tasks for III-V PV research for the next decades are 1) to utilise III-V
solar cells under high concentration and 2) to find a new and cheaper substrate for III-V solar cells, made from
an abundant element on Earth.
One of the major constraints of III-V crystal growth is indeed the choice of an appropriate substrate, lattice and
thermally matched to the epilayer. In the mid 90s, the adoption of Ge instead of GaAs was essentially due to weight
difference, and it permitted to save on satellite launch cost; now Ge is also cheaper than GaAs, and can be used as
an active junction, thus achieving an efficiency boost and a cost reduction. Since the 80s, III-V Si heteroepitaxy has
been the ‘Holy Grail’ of the whole semiconductor industry: the growth of GaAs on Si, if successful, will allow a
strong cost reduction and integration of fast electronic and optical devices on Si. There are several research groups
involved in developing methods for growing GaAs over Si: in the past years, several III-V devices have been realised on Si substrates, including single junction GaAs solar cells,83 GaAs LEDs84 and InGaAs laser diodes.85
A lot of efforts are currently being made in developing a reliable heteroepitaxy method for the deposition of
GaAs/Si.85 The most promising techniques include thick ( 10 mm) graded SiGe layers deposited by ultra high
vacuum CVD86,87 or by low-energy plasma enhanced CVD,88 the adoption of the two-step GaAs MOVPE
growth method followed by thermal cycle annealing,89,90 and low temperature atomic hydrogen-assisted
MBE,91 but the main drawback of all these approaches is essentially the high density of threading dislocation
generated by the lattice and thermal mismatch (respectively 4% and 60%); nevertheless Ringel et al.87 stress
that a critical value for the dislocation density exists (about 106 cm2) under which the minority carriers recombine before meeting the defects. They realised a GaAs/SiGe/Si solar cell on a Ge ‘virtual substrate’ grown on Si,
with about 106 cm2 dislocation, with an efficiency comparable to a cell grown directly on Ge.
Another solution for using Si as substrate for III-V PV device would be to epitaxially grow an InGaNP
alloy92,93: GaP has a band gap of 226 eV and is nearly lattice matched to Si, but its indirect band gap is a major
impediment to the realisation of solar cells. By adding In and N it is possible to lower the bandgap, maintain
lattice matching and, thanks to the presence of N, obtain material with direct bandgap. Currently InGaPN cells
reach an efficiency of about 6% but suffer from problems due to the low crystal quality of the dilute nitride alloy
and due to the low current that they can support.
A great potential for this system is however, the possibility of realising a double junction solar cell with bandgaps of 112 and 17 eV, almost matching the maximum theoretical efficiency for a 2J device.
Finally, in order to save on substrate costs of III-V cells, it is possible to remove the substrate from the epilayer by a lift off process94–96 and reuse it for successive growths. The epifilm can be bonded to a foreign substrate, in order to get mechanical support and both electrical and thermal conductivity. Moreover, it is possible
to bond the III-V layer to another Si solar cell to form a multijunction cell according to the mechanical-stacking
procedures. Problems regarding this approach are due to the complexity of the lift off process and to the stacking on the foreign substrate, in particular with regards to the electrical connection.
SUMMARY
The potentiality of III-V semiconductors solar cells for terrestrial applications were analysed and discussed.
GaAs and InGaP solar cells are nowadays used in space applications and recalling the example and the development of Si space PV technology, it is expected that in some decades arsenides and phosphide based devices
will be used on Earth to produce electricity from the Sun.
Some major problems in the development of III-V PV technology have been identified; in order to surpass the
intrinsic cost limit due to large area epitaxial deposition, the use of high concentration (1000 and above) with
small devices is expected to represent the most convenient solution to realise high efficiency III-V solar panels
with low s/Wp cost.
III-V devices are expensive but offer notable advantages, as reproducible aging (they are monocrystalline),
possibility to manage high energy density (e.g. lasers) and are nowadays a well-established technology. On the
other hand, concentrated PV is a complex and mature application, but is developing fast.
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Unfortunately, as data from world energy consumption suggests, the energetic future of mankind cannot only
rely on electricity generated by renewables, since almost 90% of the total energy used in the past years has been
dedicated to moving veichles and heating. In order to develop a sustainable future for humanity, a solution to
this imbalance must be pursued with a convergence of political, economic and social interests. Nevertheless,
since access to electricity is the key to granting people welfare, advances in renewable energy sources and PV in
particular should be accomplished.
For a long-term policy and sustainability of the III-V PV system several other issues remain to be addressed,
as the study of the overall life-cycle of solar cells, their environmental impact from precursors extraction to
module disposal; an alternative to the Ge substrates should be found, with the hope to integrate high-efficiency
GaAs/InGaP solar cells on Si. An opportune policy from governments is asbolutely essential, if we want renewables to gain social appreciation and research groups to continue to receive funding for improving materials and
techonlogy.
Besides the use of more and more efficient single and double junction III-V solar cells in concentrator modules, a future development of III-V PV devices could include the addition of more junction to monolithic cells,
in order to achieve four and more junctions, to reach and surpass efficiencies of 50%, and to find new solution
for the integration of small devices in concentration systems.
Acknowledgements
One of the authors (M.B.) thanks the financial support from SPINNER.
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