Photovoltaics: Present Status and Future Prospects
Vasilis Fthenakis
Center for Life Cycle Analysis, Columbia University, New York, NY 10027
Summary
This report provides an overview of recent developments in photovoltaics (PV) efficiencies,
production costs, and deployment and discusses how these advances created the potential for
photovoltaics to become a major contributor in energy markets throughout the world.
Specifically the report deals with the following topics: a) PV market evolution; b) photovoltaic
materials and devices; c) PV manufacturing present status and prospects; d) efficiency
advancements for current and evolving technologies; e) sustainability and life cycle
assessments, and f) the intermittency challenge.
The technologies described in this report are crystalline silicon (c-Si), amorphous silicon (a-Si),
cadmium telluride (CdTe), copper indium gallium diselenide (CIGS), copper zinc tin sulfide
(CZTS), dye-sensitized solar cells, and organic photovoltaics (OPV).
1
Table of Contents
1. Introduction
2. Photovoltaic Materials and Devices
3. PV Materials, Cells and Modules: Manufacturing Status
a. Solar Grade Silicon Production
i. Silane and chlorosilane production
ii. Siemens process
iii. Fluidized bed reactor processes
iv. Vapor to liquid deposition
v. Upgraded metallurgical silicon
b. Alternative Si-cell technologies
i. Kerfless wafers
ii. Thin-film silicon
c. Amorphous and Nanocrystalline Silicon (a-Si)
d. Cadmium Telluride (CdTe)
e. Copper Indium Gallium Selenide (CIGS)
i. Co-evaporation
ii. Reactive Sputtering/selenization
iii. Solution-based processes
f. Copper Zinc Tin Sulfide (CZTS)
g. Dye-sensitized solar cells
h. Organic photovoltaics (OPV)\
4. Future Outlook
5. Sustainability-Life Cycle Analysis
6. The Intermittency Challenge
7. Conclusions
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1. Introduction
Global power consumption currently stands at approximately 15 TW (1 TW = 1012 W), the vast
majority of which is generated by the combustion of fossil fuels. The associated release of CO2
from these anthropogenic sources has dramatically altered the composition of the atmosphere
and may detrimentally impact global temperature, sea levels, and weather patterns.
Furthermore, the realization that fossil fuels are not inexhaustible and that enhancing recovery
of coal, oil and natural gas presents additional risks, drive energy policy scenarios that are
based on renewable forms of energy. In most of the proposed scenarios, solar energy is the
primary constituent as it is the major energy source over large regions. Solar energy in addition
to maintaining life on the planet, is used on demand in three basic forms based on
anthropogenic processes: Electricity from the direct conversion of solar energy using
semiconductor materials (solar photovoltaics, PV); electricity from captured thermal energy
(concentrated solar power, CSP), and heat from the sun (solar thermal). The later, in the form
of water solar collectors is cost-competitive with electric heaters in high solar irradiation
regions all over the world. Among the solar electric technologies, traditionally CSP were
considered as the most cost effective for central power generation in high insolation areas,
while PV were considered primarily for small dispersed (mainly residential) applications.
However, since about 2005, a large utility market has been created for PV and the costs of large
PV “solar
farms” has
been reduced
to below that of
comparable
size CSP plants.
Fig. 1. Recent growth of the PV Global Market; annual sales in GW modules(source:
http://solarcellcentral.com/markets_page.html)
3
Overall, the market for photovoltaics has been growing at an average rate of 45% per year over
the past decade. The five year growth rate from 2007 to 2011 was approximately 70% per year,
but this was slowed down to 15% in 2012 as the incentives in several European countries were
reduced (Fig. 1). While the growth numbers are very impressive, the 27 giga-watts installed in
2011 is just a fraction of one percent of the total amount of electricity that was being generated
by all sources, indicating that there is plenty of room for further growth. 1,2
A synopsis of the market historical evolution is pertinent. Japan was the first country to enact
economic incentives for PV and led the global market in the 90s till the early 2000s in both
production and installations. Then in 2003, Germany enacted the Renewable Energy Standard
(RPS) and created an uncapped PV market that became a magnet for PV producers from all
over the world. The large market created in Germany is credited with enabling the economics
of scale in PV production and the manufacturing efficiencies that reduced the cost of producing
PV modules by a factor of three. In the period 2005-2009 cadmium telluride (CdTe) thin film
technology emerged rapidly as the low-cost PV production leader. A single company First Solar
become the world’s higher volume PV manufacturer in 2009 capturing 13% of the global
supply, from a negligible position in 2003. However, the rapid growth of CdTe PV was eclipsed
by an increase of crystalline PV production from China, which in 2010 became the world’s
leading manufacturer. This growth was spurred by strategic government investment, access to
cheap capital, low Chinese labor costs, and probable distortion of economics by
unconventional accounting and a low exchange rate. Protests by the European and US
manufacturers, resulted to the US Commerce Chamber issuing a preliminary determination for
a 31% tariff to be imposed on Chinese imports into the US market to counter-balance the
subsidies and cost distortions of the Chinese imports, whereas proposed legislation in the US
Senate calls for a “US-made” requirement to projects eligible for the business Investment Tax
Credit (ITC) eligibility. Thus, projects using a large fraction (e.g., >30%) of PV modules from
China would be ineligible for the 30% ITC in the United States. Although the market growth and
distribution of market share is fluid, we believe that the market is poised for further expansion
in the next decade as PV has reached cost parity with peak rates in south California, and South
4
Italy and is expected to reach cost parity throughout the U.S. and many other regions by 2030
(Fig. 2).
Fig. 2 Wholesale Levelized Cost of Electricity in the U.S.: Central (utility) status and
projections (Source: Lushetsky J., Solar Technologies Program, US-DOE, 25th EUPV, Valencia,
Spain, Sept. 2010.
This report provides an overview of PV technologies and innovations which have the potential
to significantly contribute to an expansion of the PV markets in developed and developing
countries alike. The descriptions of the status and prospects of PV manufacturing are mainly
based on a National Science Foundation workshop held at the Colorado School of Mines. 3
2. Photovoltaic Materials and Devices
Photovoltaics use semiconductor materials to generate electricity from solar energy. A
semiconductor is a solid, mostly crystalline, material such as silicon, selenium or germanium.
Semiconductors have electrical conductivities greater than insulators but lower than metals
5
which are good conductors. At low temperatures they are insulators but at high temperatures
and/or when excited by sunlight, they conduct electrons. The most commonly used
semiconductor element is silicon belonging to column IV of the periodic table which has four
valence electrons and bond to four other atoms of the same element to have a fully satisfied
valence of eight electrons. A number of compounds can be semiconductors having a valence of
eight electrons. Galium arsenide (GaAs), and cadmium telluride (CdTe), are examples of III–V,
and II–VI compounds, respectively.
Semiconductors do not have ‘free’ electrons’ to exhibit electrical conduction at low
temperatures; however at room temperature, they show a modest electrical conductivity,
which increases as temperature increases. The conductivity behavior of semiconductors may
be explained with a simple model illustrating silicon’s atomic structure. Each silicon atom has
four nearest neighbors. It has 14 electrons in its atomic structure and four of them are in the
outermost orbit which is weekly-bound to the nucleus. These four electrons are called ‘valence
electrons’ which form covalent bonds with the four nearest-neighbor atoms (i.e., each valence
electron of a silicon atom is shared by one of its four nearest neighbors).
Fig.3 (a) Three-dimensional Silicon Lattice
Fig 3(b) Silicon lattice with one Si atom displaced by a pentavalent atom (e.g., As, P);
(c) Silicon Lattice with one Si atom displaced by a trivalent atom (e.g., Ga, B)
Source: Markvart T., Solar Electricity, UN Press4
6
The valence electrons thus help to bind one atom to the nearest neighbor, resulting in these
electrons being tightly bound to the nucleus. In ‘chemically pure’ or ‘intrinsic’ silicon which
contains no foreign atoms, this ideal situation exists at 0 degrees Kelvin, and the crystal
behaves as an electrical insulator since no free electrons are available. When light is absorbed
by the material, the ‘bond’ gets broken and the valence electron becomes a free electron
leaving behind a ‘hole’. The hole is an incomplete
covalent bond. It takes energy equal to ‘band energy’
Eg, a characteristic parameter of a semiconductor, to
separate an electron from the bonding and make it a
‘free’ electron, also referred to as ‘conduction’ electron
(Fig. 4).
Figure 4. Schematic of energy gap in semiconductor materials
For silicon, Eg is equal to 1.1 eV at room temperature. When a hole exists in a bond, it is
relatively easy for a valence electron in a neighboring atom to leave its covalent bond to fill the
hole, leaving a hole in its original position. The ‘hole’ therefore effectively moves opposite to
that of the electron. The movement of holes, behaving like positive charges, constitute an
electrical current. Small amounts of dopants, few parts per million, introduced into the
semiconductor structure can significantly increase the number of electrons available for
breaking away from their atoms.
When, a silicon atom is replaced by a trivalent atom like boron (B) or gallium (Ga) which has
three valence electrons, then bonding with only three nearest neighbors is complete. This atom
can capture an electron from a nearby bond, establishing the fourth bond. The captured
electron is not available for conduction, but a hole created in the original position of the
electron enables conduction of electrons. This is called p-type doping or ‘acceptor’ type doping.
The valence electrons are confined to the valence band; but when light energy equal to or
7
greater than Eg is incident, they move into the conduction band, creating an equal number of
‘holes’ in the valence band. In Fig. 4, Ec represents the lowest energy that conduction electrons
may have and Ev represents the lowest energy that ‘holes’ may have. Eg is the separation of the
conduction band edge from the valence band edge.
If a semiconductor of band gap energy Eg absorbs light energy (E=hν), greater than Eg, each
absorbed photon raises one electron from valence band to conduction band, creating one
electron and one hole. Here ‘h’ is Planck’s constant and ν is the frequency of the light wave. The
carrier generation rate per unit area of the semiconducting surface is therefore a function of Eg;
however, the reflection of incident light at the surface and incomplete collection of charge
carriers affect the generation rate.
The p-n junction
P-n junctions are formed by joining n-type and p-type semiconductor materials, as shown
below. Since the n-type region has excess electrons and the p-type has excess holes, electrons
diffuse from the n-type side to the p-type side. Similarly, holes flow by diffusion from the p-type
side to the n-type side. If the electrons and holes were not charged, this diffusion process
would continue until the concentration of electrons and holes on the two sides were the same,
as happens if two gasses come into contact with each other. However, in a p-n junction, when
the electrons and holes move to the other side of the junction, they leave behind exposed
charges on dopant atom sites, which are
fixed in the crystal lattice and are unable
to move. On the n-type side, positive ion
cores are exposed. On the p-type side,
negative ion cores are exposed. In
summary, electrons move from the ntype to the p-type and holes move from
p-type to n-type and this movement creates is a region where the negative electrons have
become positive and positive holes have become negative. This changing of charges creates a
8
barrier to further movement. An electric field Ê forms between the positive ion cores in the ntype material and negative ion cores in the p-type material; this electric field pulls electrons and
holes in opposite directions. This region is called the "depletion region" since the electric field
quickly sweeps free carriers out, hence the region is depleted of free carriers (Fig. 5). The
width, W, of the depletion zone in p-n silicon devices is in the order of 100 nm. Due to Ê, a
"built in" potential is formed at the junction.
Principle of Operation of a P-N Junction Solar Cell
Solar cell is an electronic device which converts solar energy directly into electrical energy
through the photovoltaic effect. It is a typical semiconductor p-n junction device. The principle
of operation of a p-n- junction solar cell are illustrated in Figures 6 and 7. When the light falls on
the device, the light photons of certain wavelengths are absorbed by the semiconducting
material (excites electrons which move from the valence to the conduction band) and electrical
charge carriers, electrons and holes, are generated. These carriers diffuse to the junction where
a strong electric field exists, the electrons and holes are separated by this field and produce an
electric current in the external circuit, this current, called photo-current, depends on the
incident photon intensity and the nature of semiconductors that constitute the junction device.
Since the extra energy of photons is lost into heat, only part of solar spectrum can be converted
(Fig. 6).
Fig. 6 and 7. Photovoltaic Effect in a Solar Cell
9
Under illumination, a photo-generated current, Iph, flows as a reverse diode current, which is
linearly dependent on the intensity of incident light. Fig. 8 shows the current-voltage (I-V)
curves, without and with incident light.
Fig. 8. I-V characteristics of a solar cell with external voltage and with illumination
The resulting current is: I = -Iph + Io [ exp (qV/γkT) -1 ] and is represented by the lower curve in
Fig. 8. Typically the I-V curves of solar cells are presented only by the fourth quadrant (flipped
up to show positive current) showing the effect of both illumination and the diode, as shown in
Fig. 9.
Fig. 9. Solar Cell I-V Curve
10
Two important parameters are the short-circuit current (Isc), and the open-circuit voltage (Voc).
If the terminals of the junction are shorted, no current flows in the junction. When the junction
is illuminated, a current called ‘short-circuit current’ flows in the junction from p- to n- side or
otherwise depending on which side of the junction, the light is incident. This photocurrent is
due to three contributions: holes generated in the n-region, electrons in the p-region and
electrons and holes in the depletion region before they recombine.
Isc is a function of cell design and is proportional to photon flux. If the load is an open circuit,
the corresponding Vout is called ‘open-circuit voltage’, Voc. Isc and Voc are related to the output
power at the ‘optimum operating point’, Pmax as
Pmax = Voc x Isc x FF
where FF, the fill factor.
FF= Vm x Im/Voc x Isc
Vm and Im are the voltage and current at the optimum operating point. Typically Vm is 75% to
90% of Voc; and Im is 85 % to 99 % of Isc for silicon.
The efficiency of the solar cell is typically measured as:
η = Voc x Isc x FF/ Incident Solar Power
The efficiency will be high is Isc and Voc are high and so also FF. it means the dark current must
be low. A sharp corner in the I-V characteristic is an indicator of high FF.
In practice, the conversion efficiency of a solar cell is measured under Standard Test Conditions
(STC), which include (i) solar irradiance intensity of 1000 watts/m2, (ii) AM 1.5 solar reference
spectrum, and (iii) cell temperature during measurement, of 250C.
The Voc is directly proportional to the band gap energy whereas the generation rate of the
current decreases with increasing energy gap, thus that the cell efficiency reaches a maximum
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at certain range of values of Eg that corresponds to a maximum voltage-current product. More
specifically, a change in energy gap influences the energy conversion of a solar cell in two ways.
First, a larger Eg reduces the reverse saturation current and increases Voc which in turn tends to
increase the efficiency. Secondly, a larger Eg means that fewer photons can be absorbed
because only those photons greater than or equal to Eg are absorbed, which in turn decreases
the efficiency. The net result of these two opposing effects is shown in Fig.10. For singlejunction devices, the calculated efficiency has a maximum of about 30% at Eg equal to about 1.5
eV and falls off on either side of this value.
The semiconducting compounds, cadmium telluride(CdTe), gallium arsenide (GaAs) and indium
phosphide(InP) have Eg around this value. In practice, several semiconductors with energy band
gaps in the range 1.0 to 1.7 eV are
used for maximum efficiency
devices/solar cells. These also
include monocrystalline and
multicrystalline silicon (Eg = 1.12
eV), copper indium diselenide(Eg =
1.05 eV), amorphous silicon (Eg =
1.7 eV), cadmium telluride (Eg =
1.45 eV), gallium arsenide (Eg =
1.25 eV).
Fig. 10. Estimated Efficiency versus Energy Gap of the Cell Material
Figure 11 displays the champion overall power conversion efficiencies for laboratory solar
cells.13, 14, 15 It is shown that the PV technologies are at different points in their respective
learning curves when compared to the Shockley–Queisser (S-Q) limit of ~31% solar energy
conversion efficiency for single junction devices.17 As shown in Figure 10, the band gap energy
dependence of the Shockley–Queisser limit is flat around the maximum, thus the theoretical
12
efficiency limits for these technologies are only marginally different and are not a deciding
factor in their competitiveness with each other. Nevertheless, crystalline silicon, which is the
most mature technology, having been benefitted by investments in IC applications, has reached
record efficiencies of ~24%, whereas the less researched CdTe and CIGS thin-film technologies
have reached record cell efficiencies of 17.3% and 20.4% correspondingly. The module
efficiencies lag those of the laboratory cells; the highest commercial production module
efficiencies are 20.1%, 12.5% and 11.5% correspondingly for mono-crystalline Si, CdTe and CIGS
modules.
Fig. 11. Record efficiencies for different types of solar cells in the laboratory (Source: NREL)
In most cases, the record cells have seen little or no improvement over the past decade while
economies of scale and advances in manufacturing science and technology have fueled the
growth of the PV market through cost reduction and module performance improvements16.
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Solar Cell Design Principles
Solar cell design involves specifying the parameters of a solar cell structure to maximize
efficiency. The theoretical efficiency for photovoltaic conversion is in excess of 86.8%.6
However, the 86.8% figure uses detailed balance calculations and does not describe device
implementation. For silicon solar cells, a more realistic efficiency under one sun operation is
about 29%.7 The maximum efficiency measured for a silicon solar cell is currently 24.7% under
AM1.5G. The difference between the high theoretical efficiencies and the efficiencies measured
from terrestrial solar cells is due mainly to two factors. The first is that the theoretical
maximum efficiency predictions assume that energy from each photon is optimally used, that
there are no unabsorbed photons and that each photon is absorbed in a material which has a
band gap equal to the photon energy. This is achieved in theory by modeling an infinite stack of
solar cells of different band gap materials, each absorbing only the photons which correspond
exactly to its band gap. The second factor is that the high theoretical efficiency predictions
assume a high concentration ratio. Assuming that temperature and resistive effects do not
dominate in a concentrator solar cell, increasing the light intensity proportionally increases the
short-circuit current. Since the open-circuit voltage (Voc) also depends on the short-circuit
current, Voc increases logarithmically with the level of the incident light. Furthermore, since the
maximum fill factor (FF) increases with Voc, the maximum possible FF also increases with light
concentration, allowing concentrators to achieve higher efficiencies.
In designing single junction solar cells, the principles for maximizing cell efficiency involve the
following: a) increasing the amount of light collected by the cell that is turned into carriers; b)
increasing the collection of light-generated carriers by the p-n junction; c) minimizing the
forward bias dark current; d) extracting the current from the cell without resistive losses.
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3. PV Materials and Modules: Manufacturing Status
A. Crystalline Silicon
Silicon is the most developed and best understood semiconductor, benefiting from decades of
development by the integrated circuit (IC) industry. Multi-crystalline silicon (mc-Si)
photovoltaics have the greatest market share followed by mono-crystalline silicon, followed by
CdTe thin-film photovoltaics. Although CdTe thin-film has the lowest module production cost,
mc-Si has higher efficiency, reducing, therefore, the cost requirement for the mounting
structure and installation, since those are proportional to the area required for the installation.
The mc-Si industry is looking forward to lower polysilicon prices, and improvements in wire
cutting technology that reduced wafer thicknesses that could keep c-Si silicon competitive..
Several companies have started producing solar grade silicon at less energy and at a lower cost
than the current production via the“modified Siemens” process, and R&D is taking place for the
production of kerfless wafers, and the development of solar cells from depositing a thin-film
silicon on substrates as amorphous silicon is deposited today.
A.1 Production of Solar-grade Silicon
The solar grade silicon production is known to be the most energy intensive stage in the life
cycle of silicon PVs when the typical Siemens process is used. This process was originally
designed to produce the semiconductor grade silicon, which is purer than the solar grade
silicon. In order to reduce the high electric energy requirement, modified version of Siemens
process which operates under a relaxed condition, have widely been used. In addition, novel
process schemes and equipment have begun emerging such as fluidized bed reactor (FBR) and
upgraded metal-grade (UMG) processes. Figure 12 presents the currently available routes to
produce solar grade silicon; these are reviewed below.
15
Most SGS production is involving purification through gasification of MG silicon to silane gases
and deposition of Si by reducing the silane feedstock gases. A new process that upgrades the
purity of MG silicon (UMG-Si) has also become available to commercial production.
A1.1 Silane and Chlorosilane Production
Most solar grade silicon (SGS) currently is obtained through silicon deposition in reactors from
mono- or trichloro- silane gas; the vast majority of the SGS is produced in Siemens reactors with
a small fraction of the production based on the fluidized bed reactors (FBR). Silane gases are
produced by gasifying metal-grade silicon (MG-Si, >99%) that is acquired by reacting silica
(SiO2) with coke (C) in electric arc furnace. Trichloro-silane (SiHCl3, TCS) gas is synthesized
reacting MG-Si with HCl in a Fluidized Bed Reactor (FBR) at 200-500 ° C: Si + 3HCl → SiHCl3 +
H2. In this process, a side reaction becomes dominant as the temperature increases, i.e., Si +
HCl  SiCl4+ 2H2, producing SiCl4, the byproduct of this process. Therefore, removing the
reaction heat is critical to improve the efficiency of TCS synthesis.7 In mono-silane (MS, SiH4)
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gas production process, first, SiHCl3 is synthesized from MG-Si, SiCl4, and H2 (Si + 3SiCl4 + 2H2
→ 4SiHCl3), then converted to SiH4. Mono-silane is also commercially produced from SiF4
through the following process: SiF4 + NaAlH4 → SiH4 + NaAlF4.7
(a) Siemens Reactor
(b) Fluidized Bed Reactor
Figure 13. CVD Reactors to Produce Polysilicon8
A1.2 Siemens Process
The typical Siemens process based on TCS deposits silicon on rods at 850 - 1100 °C.8 The bell
jar type inner wall of the Siemens reactor should be cooled by water to prevent Si precipitation
on the wall of the reactor, causing significant heat losses (Figure 13 (a)). The electricity
consumption ranges 60-150 kWh per kg of purified silicon.7 This process generates byproducts
of H2, HCl, SiH2Cl2 and SiCl4. The latter is recycled to SiHCl3 by reacting with H2 in a FBR from
the following reactions: SiCl4 + H2 = SiHCl3 + HCl, 3SiCl4+2H2+Si = 4SiHCl3. MS-based Siemens
can lower the deposition temperature to 650-800 °C. 7 But, formation of unwanted fine silicon
powder causes product quality and process control issues. 7,9
Once the silicon rods reach 10-15 cm and no further room to grow inside the Siemens reactor,
the deposition operation should stop and the completed rods be removed. This type of batch
17
process, therefore, requires frequent assembly/disassembly of the reactor, which considered
labor intensive and time consuming.7
A1.3 Fluidized Bed Reactor (FBR) Processes
Fluidized Bed Reactor method produces particle silicon compared with rod shape silicon ingot
produced in the Siemens process. Silane or trichlorosilane gas is continuously supplied released
into a superheated FBR chamber that contains seed grains of silicon for continuous production
(Figure 13 (b)). The electric energy demand of producing SGS through FBR is reported 10-30%
of what is required for the Siemens process.7,9,10 The FBR route is more energy efficient than
the Siemens process for several reasons: It does not waste energy as in the Siemens process,
and produces more silicon per cubic meter of reactor space. 11
This process is easily contaminated due to the large surface area of Si particles than the bar Si
obtained in the Siemens process, and to the fact that those particles frequently contact with
the reactor wall. The particle size of Si should be constant for continuous production in FBR
that seed particles may be supplied to the reactor. The inner wall of the reactor is coated with
SiC as a barrier against contamination. But, in a high temperature beyond 1000°C, carbon can
be released from the coating to contaminate silicon particles.8 To avoid precipitation of silicon
on the wall and contamination, radiant heating such as high frequency, micro waves, and
infrared rays and so on is often used in an attempt to heat only Si particles. 9 The large surface
area of the Si particles also causes trapping of hydrogen gases inside and their burst triggers
generation of unwanted fine silicon powders. An additional dehydrogenation process may be
required at 1150 °C. 9
The MEMC Inc. has been commercially producing SGS in a FBR process since 1987 from MS
(SiH4) synthesized through reduction of SiF4. In addition, a new FBR SGS process in REC uses
SiH4 produced from disproportionation of SiHCl3 (4SiHCl3 → 3SiCl4 + SiH4). It is known that
this method reduces electricity consumption to 20% of the Siemens process and the cost to less
than 70%. 9 MS-FBR has a lower deposition temperature compared with TCS-FBR as MS
deposits at 750°C compared with TCS at 1050°C.12 However, MS-FBR tends to produce a large
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amount of unwanted fine silicon dust, which largely forms at the empty space above the
fluidized bed or inside gas bubbles.7
Hydrogen reduction in FBR using TCS as a raw material also produces purified particle silicon,
which is used by Wacker. However, the details of Wacker FBR process are not well known.12 In
TCS-FBR, residual Cl could be the major impurity, formed from dissociation ofSiCl2 [2, 3]. The
density of the latter is known to be lesser with a lower temperature and high pressure reactor
condition. 7
A1.4 Vapor to Liquid Deposition (VLD)
This is the process used by Tokuyama, Japan. It is similar to the Siemens process but produces
liquid silicon instead of solid silicon bar. The precipitation rate is 10 times faster than the
Siemens process but it is hard to remove impurities. Thus, this process is suitable for SGS
rather than electronic grade silicon which requires a higher purity. 9
A1.5 Upgraded Metallurgical Grade Silicon
Solar-grade silicon based on metallurgical refining processes, often called upgraded
metallurgical-grade silicon (UMG-Si), is expected to play an important role in reducing the
energy and cost of silicon production for photovoltaics. The leading company in this area is
Norway’s Elkem which reports the use of UMG-Si in solar cells with comparable efficiencies to
polysilicon (poly-Si) from the traditional Siemens process. Elkem’s process starts from the
production of metallurgical silicon in a reduction furnace with advanced process controls,
followed by purification stages involving slag treatment, leaching and solidification, and
subsequent post-treatment. The purification stages are designed to remove impurities like
boron and phosphorous which can adversely affect the operation of the p-n junction in solar
cells. Elkem reports having reduced the concentration of B and P to 0.22 ppm and 0.62 ppm
correspondingly, making their product comparable to solar grade silicon from the modified
Siemens process.
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B. Alternative Si-cell Technologies
B.1 Kerfless Wafers
Improvements in wire-saw technology have enabled the reduction of wafer thicknesses from
~400 µm a decade ago to 140 µm at present. However, over than 50% of the silicon is lost as
silicon kerf and recycling of this kerf is difficult as it is contaminated with solvents and
abrasives. Kerfless wafer could potentially reduce Si use by 50% or more . Techniques for the
direct production of wafers from the melt were invented in the 1970s, 20, 21 and after decades of
development they have now reached the market. The two closely related techniques are edgedefined, film-fed growth (EFG) and string ribbon silicon technologies. In the EFG process the Si
wafers are pulled out from the melt through a graphite die using capillary action. This process
was developed by ASE Americas22 and is now employed by Schott Solar. Ribbon silicon is
produced by pulling a pair of high-temperature strings through a crucible of molten Si, and this
technology has been promoted and implemented by Evergreen Solar. These two growth
techniques produce vertical sheets of mc-Si
hick and up to a 100 mm
wide.
The quality of the material produced by these techniques is somewhat inferior to standard
block-cast mc-Si, but is continuously improving with champion laboratory cells now reaching
power conversion efficiencies over 18% and 14.5% efficient modules already on the market.14, 15
Passivation of surface and bulk defects is critical to achieving high efficiency, and this is usually
achieved through the deposition of a hydrogen-rich silicon nitride layer, which also serves as an
anti-reflection (AR) coating.23 These technologies are expected to become more costcompetitive as energy costs continue to rise, but improvements in manufacturing are needed to
compete with conventional c-Si technology (Fig. 3). In particular, further reductions in wafer
thickness coupled to improvements in throughput are viewed as the most important tasks for
scaling these technologies to TW levels. Increasing the grain size and crystal quality will also be
important for further improvements in efficiency.3
20
B2. Thin-film- Silicon
Silicon has an indirect band gap, and, therefore, relatively thick layers are needed to allow for
momentum extraction and energy absorption. It is assumed that silicon must be thicker than
100 μm to effectively absorb light,24 however theoretical modeling has shown that ~40 μm may
be ideal for obtaining maximum performance,25, 26 and that efficient cells could be obtained
with only 1 μm of single crystal silicon with state of the art surface passivation.27 If such thin
layers could be produced using a kerfless process, and therefore avoid the large silicon loss due
to cutting, this would result in an order of magnitude reduction in materials cost and energy
with respect to today’s state-of-the-art wafers. Thin silicon is also amenable to use of bifacial
architectures,28, 29 which harvest light from both directions.
There are two general approaches to the fabrication of thin-film-Si; these are a) heteroepitaxial
growth followed by lift off or removal of a sacrificial substrate30, and b) “peeling” ultrathin
silicon layers off of silicon ingots using techniques such as stress-induced liftoff.31 The startup
company Silicon Genesis has recently introduced a process where this is achieved through a
combination of ion implantation and thermal treatment, producing kerf-free wafers as thin as
25 microns.32 Substantial challenges remain once thin-film-Si is produced. Achieving high
efficiency will require the use of the most advanced technologies with respect to both surface
passivation33, 34 and light trapping.35, 36 However, results to date are encouraging. The University
of Stuttgart has fabricated a 16.7% solar cell from 45 μm thick Si produced by lift off, while 8.2%
modules have been fabricated using 2 μm polycrystalline Si.13 Commercialization of such efforts
will need to address the nontrivial challenge of mechanically handling these very thin wafers
while maintaining high throughput and low cost.
C.
Amorphous and Nanocrystalline Silicon (a/nc-Si)
Solar cells based on hydrogenated amorphous silicon (a-Si:H or a-Si) were invented by Carlson
and Wronski at RCA Labs in 1976.61 Manufacturing is achieved by plasma-enhanced chemical
vapor deposition (PECVD) using mixtures of H2 and SiH4. Hot-wire chemical vapor deposition
has been offered as an alternative,62 but has yet to be implemented in large scale
21
manufacturing. The capability to dope a-Si in a controllable fashion is relatively poor and
attempts to do so leads to the creation of additional recombination centers. Because of these
effects p-i-n device structures are almost always used.63 Benefiting from synergies with the IC
industry a-Si was rapidly commercialized and the first PV products appeared in the early 1980s.
Early devices rapidly surpassed the 10% conversion plateau, but it was quickly recognized that
these devices suffered from light-induced degradation through the now well-known StaeblerWronski effect64; light exposure leads to a reduction of the solar cell efficiency over months
which eventually stabilizes at efficiencies around 6-7%. Nevertheless, for decades a-Si was by
far the most successful thin film technology, achieving market shares in excess of 10% before
the phenomenal growth of CdTe PV production.
Leading manufacturing companies include Sharp and Oerlikon. One of the most attractive
features of a-Si is that devices can be deposited at low temperature (< 200 °C), enabling the
fabrication of lightweight, flexible laminates on temperature sensitive substrates. This is a
unique trait that provides a competitive advantage in markets such as consumer products and
building integrated photovoltaics (BIPV). Though discovered much earlier, 65 another major
change that has occurred over the past decade is the integration of micro (μc-Si) or
nanocrystalline (nc-Si) into device structures. The quality of PECVD deposited material is
strongly influenced by the level of silane dilution in hydrogen, and high H2 dilution levels (>90%)
lead to the formation of crystalline domains within the material. The primary advantage of nc-Si
is that it is much less susceptible to Staebler-Wronski degradation.64 Another important feature
is that a/nc-Si is amenable to the formation of multi-junction devices; most commercial devices
are based on either tandem cells or even triple junction cells. A common configuration is the
“micromorph” tandem, which pairs an a-Si top cell with a nc-Si bottom cell.66 Solar cells with
record efficiencies are based on triple junctions that employ germanium alloys to further
improve absorption in the red region of the solar spectrum.67 A related success story has been
the introduction of the a-Si/c-Si heterojunction with intrinsic thin layer or HIT cell, which boasts
21% conversion efficiency. 68 In addition to boosting current the intrinsic a-Si:H layers appear to
be important for passivation of the underlying c-Si material.
22
D. Cadmium Telluride (CdTe)
The first reports of CdTe-related PV devices appeared in the 1960s40 but efficiencies were very
low till the early 90s.41 CdTe has a number of intrinsic advantages as a light absorber. First, its
band gap of 1.45 eV is ideally positioned to harness solar radiation. Its high optical absorption
coefficient allows light to be fully captured using only two microns of material. CdTe sublimes
homogeneously and the compound’s thermodynamic stability makes it is nearly impossible to
produce anything other than stoichiometric CdTe.42 Thus, simple evaporation processes may be
used for film deposition. Close-space sublimation employs diffusion as the transport
mechanism,41 while very high rates (>
may be obtained using convective vapor
transport deposition.43
Standard CdTe-based devices employ a superstrate configuration: production begins with a
glass substrate followed by the successive deposition of the transparent conducting oxide (TCO,
SnO2:F), the n-type window layer (CdS), the p-type CdTe absorber, and finally the back contact
(ZnTe/Cu/C). CdTe PV manufacturing is uniquely equipped to be integrated with the production
of float line glass44. Glass exits a float line at ~600 °C, which happens to be an optimal
temperature for vapor-phase deposition of the SnO2:F (FTO), CdS, and CdTe. Part of First Solar’s
success has been due to their ability to integrate these various process steps into an in-line
manufacturing process reducing the processing time from glass to a finished module down to
2.5 hours.
With low manufacturing costs established, the biggest opportunities for CdTe lie in the
improvement of device efficiency. Champion cells (Fig. 4) convert just over 50% of their S-Q
potential, while commercial modules are at ~11% power conversion efficiency. Improving
efficiency will require enhancements in both current and voltage. The former is perhaps the
most straightforward route, as much of the blue region of the solar spectrum is absorbed in the
TCO and CdS layers that make up the front contact. Top laboratory cells have replaced the FTO
with advanced TCOs such as cadmium stannate45 and ITO.46 Likewise, the CdS window layer
(2.6 eV) absorbs a significant fraction of the blue light. Integration of advanced front contacts
into manufacturing appears to be the near term strategy. This will not be trivial because ITO is
23
expensive and cadmium stannate is a complex material.47 Furthermore it is not clear what
might be used to substitute for CdS though sulfides of zinc and indium have attracted significant
interest.48
The more daunting challenge is improving the voltage. The open-circuit voltage (Voc) of
champion CdTe cells is well below that of similar band-gap PV materials. For example, the best
Voc obtained in CdTe is 230 mV short of GaAs devices, which has a similar band gap. Short
carrier lifetimes are at the root of this limitation. The combined effect of defects and grain
boundaries limits minority carrier lifetimes in polycrystalline CdTe to a few nS, even the best
devices. These lifetimes are very short compared to almost 1 s for epitaxial CdTe49 or
hundreds of ns for CIGS.50 Sites and Pan51 showed through simulation that increasing the
carrier lifetime or the use of a p-i-n device structures may be two viable routes to increase the
efficiency to above 20%. The short term goal of commercial manufacturers is to raise module
efficiencies from current levels to >15% by 2014 through a combination of process integration,
research, and development. In the longer term they are targeting 18% as an achievable goal for
module efficiency.52
A number of fundamental questions remain unanswered for CdTe PV. At present, the issue of
extending carrier lifetime is partially addressed by chemical passivation. Examples include the
introduction of O2 during CdTe growth,53 post-deposition CdCl2 treatments,54 and controlled
diffusion of Cu from the back contact.55 The empirical recipes associated with these processes
constitute the “black art” of CdTe manufacturing. Clearly a preferable route would be to
understand the nature of the defects states so one could prevent their formation in the first
place or develop alternative and perhaps better passivation strategies. Fundamental research
in understanding these defects and how to passivate them would be transformative leading to
improvements in one of the most promising solar cell technologies. Another fundamental
question concerns the role of grain boundaries in these devices.56 CdTe is an interesting and
unusual material in that solar cells based on polycrystalline CdTe outperform devices made
using single crystal CdTe. It is thought that grain boundaries can have both positive and
detrimental impacts on charge transport, but the current level of understanding is not sufficient
to suggest how one might engineer a desired morphology. The use of p-i-n structures to create
24
high-efficiency devices requires deliberate control of the sample free carrier density, which is
not yet fully understood or achieved. A final area that deserves attention is the back contact. It
is difficult to contact CdTe because it has low conductivity. Moreover, the back contact has
been implicated as a potential contributor to degradation.57, 58 The issues discussed above are
non-trivial and will require substantial investment and fundamental research to resolve.
Another issue to be mentioned with respect to large-scale CdTe manufacturing is perceptions
with respect to both cadmium toxicity and tellurium availability. The toxicity issue is one of
public perception which has been amplified by competitive business interests. Cadmium is
indeed a toxic element, but the risk of exposure once incorporated into PV modules is minimal.
Extensive testing of broken and heated panels has shown that emissions due to fire59 or
leaching60 are negligible. To their credit, all CdTe manufacturers are committed to 100%
ownership of recycling, which in part is related to the issue of Te availability discussed later in
this paper. One also notes that Cd will continue to be produced as a natural byproduct of Zn
mining. Perhaps the best argument for CdTe PV is that it serves as a means to sequester this
element in an environmentally beneficial manner. While scientific arguments can be made that
the toxicity of Cd is not a significant issue, governmental policy in individual countries may
dictate whether this issue hampers the deployment of CdTe solar cell technology.
E. Copper Indium Gallium Selenide (CIGS)
Copper indium diselendide (CuInSe2 or CIS) CIS has a band gap of 1.05 eV slightly off the
maximum efficiency energy gap (fig 10). However the band gap may be continuously
engineered over a very broad range (1.05 – 2.5 eV) by substituting either Ga for In or S for Se. 74
CIGS is currently the efficiency leader among thin film technologies cells with 20.3% efficiencies
currently reported in the laboratory75, 76. Commercial production of CIGS began in 2007, with a
few companies operating facilities with 10-30 MW/year capacities but the foretold large
production has not happened yet as reproducibility and production yield have proven to be
challenging. However, Solar Frontier in Japan recently expanded production to 900 MW.
25
Substrates include soda lime glass, metal foils, or high temperature polyimide (PI). The latter
has garnered substantial interest for applications such as BIPV and small portable applications.
In the case of deposition on flexible substrates it is critical to match the coefficient of thermal
expansion, with highest efficiencies obtained on titanium and stainless steel foils.
Fabrication of the CIGS device begins with the deposition of a Mo back contact followed by
the p-type CIGSS absorber (1-3 μm), a thin buffer layer (50-100 nm), with doped ZnO serving as
the transparent front contact. Many different approach exist for the formation of the CIGSS
absorber.77 At present, the performance of commercial modules is only about 11% although
the efficiency record of lab cells is 20.3% and much of this difference attributed to the quality
of the absorber layer.78 The approaches to CIGS fabrication may be classified into three basic
categories: co-evaporation, reactive sputtering/selenization of metal films, and non-vacuum
and solution printing techniques. Below is a description of the major advantages and issues
associated with each approach (Wolden et al, 2011).
Co-evaporation
Co-evaporation is practiced either in a single or three-stages. The former is employed by Q-cells
and Wurth Solar and has produced 12.7% efficient modules(Q-cells). The three-stage coevaporation is the process that has produced the world-record cells79 but so far it has produced
12.2% efficient modules (Global Solar, Yohkon). Co-evaporation alternates between copper-rich
and copper-poor conditions to produce the large grains and graded Ga/In profiles characteristic
of high efficiency material.80 There are a number of important practical challenges involved in
the manufacturing of CIGS solar cells. Evaporation sources typically have a cosine flux
distribution, and it is difficult to introduce sharp changes in composition or maintain uniformity
over large areas under the diffuse conditions of high vacuum. In addition, sources must be
mounted in a top-down configuration in order for large glass substrates to be supported and
heated to 600 °C. In situ diagnostics such as thermometry and laser light scattering, which are
critical for process control in the batch process, are being adapted for use in the manufacturing
environment.81 Likewise, atomic absorption spectroscopy and X-ray fluorescence are employed
for controlling element flux and in-line detection of film composition, respectively. Another
26
challenge with co-evaporation is that the relatively unreactive Se must always be supplied in
great excess, leading to practical concerns related to condensation and materials management.
Despite these challenges Q-Cells has announced 12-13% mass-produced modules and a 14.2%
champion module with this process. Through systematic optimization and accompanying
improvements in yield this may turn out to be a viable large-scale manufacturing strategy.
Reactive Sputtering/Selenization
Another method for synthesizing CIGS films is selenization of a stack or alloy of the constituent
metal films predeposited on a substrate in a predetermined stoichiometry. There are many
variations of this approach but the most common is a two-step process where the metals are
sputtered onto the substrate and then converted to CIGS through annealing in a chalcogencontaining environment. Practitioners include Solar Frontier who have recently announced a
scaling up of their production to 900 Mw/yr, Avancis, Stion and TSMC Solar. The highest module
efficiency obtained by the two-step process is 12.6% (Solar Frontier). There are many pathways
and intermediates involved in transforming the metal into the chalcopyrite, requiring careful
optimization of the time-temperature-reactant profiles employed.78 This approach creates a
copper-indium-gallium-selenide-sulfide (CIGSS) film, with shifts the band gap towards the
maximum efficiency and also improves the interface with the window layer, which are also
sulfides (CdS, ZnS, In2S3).78
Non-vacuum and Solution-printing Processes
The third general approach to CIGS manufacturing has been to eliminate vacuum processing. In
general these are also two-step processes, application of a coating followed by a high
temperature step for annealing or sintering. Ostensible advantages include reduced capital
requirements, improved materials utilization, potentially lower energy requirements, and
compatibility with roll to roll (R2R) processing.82 A general challenge with the non-vacuum
based approaches is the potential of contamination introduced by either the compounds
themselves or the solvents employed. As such, it has been much harder to produce dense,
homogenous absorber layers. It is also more challenging to produce chemically-graded
27
structures with this technique. Record cell efficiencies trail co-evaporation and metal
selenization, but values up to 14% have been obtained by a number of techniques.
The non-vacuum strategies may be further divided into electrodeposition, particulate
deposition, and solution processes. Electrodeposition has been around for decades83 and
achieved cell efficiencies as high as 13.8%,84 but concerns about up scaling appear to have
limited commercial interest. The particulate route is currently the most actively pursued, with
variations employing particles composed of CIGS, metal, metal oxides, and/or metal selenides.
In all of these methods a coating of particles is first formed on the substrate surface and
reacted and/or sintered at high temperature to form the final film. It was found that CIGS
particles required excessive temperature for sintering.85 Likewise, problems with handling and
premature oxidation have limited the utility of metal particles. The best results have come
using slurries containing mixtures of metal oxide or selenide powders.86 This approach was
pioneered by Kapur and co-workers at ISET,87, 88 and more recently championed by Nanosolar.
The latter has reported 14% efficient cells,89 and has announced that 10-11% modules will be
available in the near future. Solution approaches have employed the use of soluble metal salts,
organometallics, and hydrazine-based compounds. Best results have been obtained with the
latter,90, 91 however the highly reactive and toxic nature of hydrazine poses additional
complications for manufacturing.
With five elements and numerous binary and ternary phases, the CIGSS system presents much
greater complexity than the PV technologies described previously3. Extensive theoretical work
has made great advances in understanding the electronic structure and role of defects in this
system.92 These studies have been aided by improvements in advanced characterization
techniques. Raman and time resolved photoluminescence are becoming useful for identifying
the presence of secondary phases and certain defects.81 It is well-known that sodium plays a
critical role in the morphology and electronic properties of CIGS. When soda lime glass
substrates are used, sodium diffuses into the CIGS layer from the glass. Once the importance of
Na was realized, more controlled and systematic approaches that employ sputtered layers of
Na-containing material have been developed to gain control over Na introduction into the CIGS.
28
There is also significant attention being paid to the window layers deposited on the CIGS
absorber. While CdS remains the leading choice, both indium and zinc sulfides are being
pursued and in some cases commercialized. Part of the interest is due to the desire to remove
Cd, but a second motivation is improving the blue response of these devices. There are strong
interactions between the buffer and the underlying absorber, and simultaneous optimization of
these layers is required for best performance. Some concerns remain about the use of ZnO as
the front TCO, and its potential impact on long-term device stability. Moisture exposure is
particularly detrimental, both to the TCO and the heterojunction itself. Encapsulation in glass
partially alleviates this effect, but further development of transparent ultra-barriers is required
to improve the long term stability of flexible CIGS solar cells.93
A longer-term concern is the availability and price of In. Recycling of indium will alleviate
constraints on CIGS long-term production, but research is needed to develop technologies for
efficient and low cost recycling of all the elements from the CIGS modules. The possibility of
substituting indium/gallium with earth abundant alternatives such as zinc/tin is discussed
below.
F.
Copper Zinc tin Sulfide Selenide (CZTSS) Thin Films
There is interest in using “abundant” materials in thin-film photovoltaics, driven by concerns
about the availability of In, Te, Ga and Ge in the current 2nd generation thin-film PV. Such
materials include oxides and sulfides of base metals (e.g., Fe, Cu) that have band gaps in the
range of 1 – 2 eV. The most successful system to date has been copper-zinc-tin-sulfide
(selenide), or CZTS.125 Pioneered by Katagiri,125 in the past decade champion CZTS devices have
gone from less than 3% to approaching 10% efficiency (Fig. 5). These results have come with
only a handful of publications. CZTS shares great similarities with CIGS, including similar device
structures and fabrication techniques for the formation of the absorber layer. Initial studies
focused on sulfidization of metal layers,126 but more recently co-evaporation127 and nonvacuum techniques128-130 have garnered significant attention. The current efficiency champion
includes Se and was derived from hydrazine precursors.130 The similarities to CIGS may have
29
accelerated CZTS solar cells’ initial success, but these same similarities may become limitations
in the long run.3
G. Dye-sensitized Solar Cells (DSC)
Dye-sensitized solar cells are based on the photo electrochemical effect discovered by Bequerel
in 1839. A relatively new concept, DSC was introduced by Grätzel and co-workers in 1991.99 A
comprehensive review on the complex chemistry and processes involved in this system was
recently published by Hagfelt and co-workers.100 This hybrid material is typically composed of
organometallic dye molecules adsorbed to a mesoporous titania nanoparticle film, with the
pore space filled by an electrolyte. In this structure light is absorbed by the dyes, which then
inject an electron into the conduction band of a wide band gap semiconductor like TiO2. The
electron is transported by hopping through the nanoparticle network to the front contact
where it exits and performs useful work before returning to the platinized back contact. Here
the electron reduces a redox couple, which in turn diffuses through the electrolyte and
regenerates a dye molecule to complete the cycle. Dye sensitization of oxides was well-known
at the time, and Grätzel’s key innovations were in creating a nanoparticle film with high surface
area to improve light harvesting and in choosing components with appropriate kinetics for fast
charge transfer and slow recombination. Grätzel’s group rapidly optimized the device to over
10% within a few years of its introduction.101 This brought the attention of industry and today a
number of small companies including G24i, Solarprint and Dyesol are engaged. Most current
products are directed at the consumer market; for example, DSC on flexible substrates that
replace rechargeable batteries for portable electronics. A unique feature of DSC is that their
performance improves under diffuse and low light conditions,102 enabling their use indoors and
without direct solar exposure. Devices can be fabricated in a number of colors and levels of
transparency, which is an attractive feature for architectural and BIPV applications.
Manufacturing can also be done at low temperature using flexible substrates.
30
It is noted that champion cell efficiency has been stagnant at ~11% for the past 15 years (Figure
5). The three main components in a DSC, the Ru-based dye, the photoanode, and the iodinebased redox couple, have also remained unchanged. Further optimization of any one of these
components individually is not likely to yield significant improvements in efficiency. The recent
review by Hamann and co-workers103 provides an excellent overview of the complexity of the
issues involved. First, the leading dye does not capture much light past 750 nm, and harvesting
the red and near-infrared portion of the spectrum is needed to increase current densities.
Second, the I3-/I- redox couple is positioned with a 550 mV overpotential relative to dye
regeneration. An alternative redox couple could potentially allow the Voc to be improved by up
to 300 mV, but recombination rates are typically much faster with non-iodine redox couples. A
combination of these two changes could elevate device performance to > 16%. However as
cautioned by Hamann and colleagues,103 this will most likely require simultaneous optimization
of both dye and electrolyte and perhaps the development of new photoanodes with faster
charge transport as well. While there are photoanode designs based on wide band gap
semiconductor nanowires that attempt to improve efficiencies, six years after their first
introduction, the efficiencies remain low.
With respect to manufacturing numerous module fabrication strategies are being pursued,
which general can be divided into monolithic or sandwich constructions. The former offers
advantages with respect to materials cost, while the latter may be more amenable to R2R
processing. Substrates include glass, metal, and polymer foils, with best performance being
obtained on glass. Critical issues include stability and the production of large area modules. At
present mini-modules with areas < 100 cm2 are used, with resistance losses being a one of the
major challenges. The stability of a DSC module is strongly related to the device encapsulation.
Accelerated lifetime testing has indicated that carefully encapsulated glass-based DSC can last
for over 20 years, although current products using DSC on plastic substrates have lifetimes of
only a few years. For outdoor applications, the sealing material must, for example, be
mechanically and thermally stable, stable under UV exposure, and chemically inert to the
electrolyte. Moreover, it should prevent mass transport between adjacent cells. The issue so
important that Hagfelt et al. 100 suggested that the leading manufacturing approach for DSC
31
may be the one that provides the most functional encapsulation method. Replacing the liquid
electrolyte with a gel or solid would greatly improve encapsulation issues, but these changes
have resulted in decreased efficiency. Elimination of glass, implementation of R2R
manufacturing methods, and increased lifetimes will be critical to economics, particularly if
device efficiency remains below 12%.
H. Organic PV (OPV)
Spurred by the elemental abundance of carbon, extensive efforts to develop solar cells using
organic semiconductors are underway. Brabec and colleagues104 recently provided a
comprehensive review of the developments in OPV over the past decade and the challenges
that lie ahead. Figure 5 charts the progress of champion cell efficiencies for the past 15 years.
While most technologies have been relatively stagnant in their champion efficiency, organic PV
has made great strides in the past decade, with Heliatek and Konarka being the current
champions, each with devices certified at 8.3%.105, 106 Unfortunately Konarka run out of cash
and went out of business in June of 2012. Others leading the development of OPV for small
devices include Solamer and Plextronics who held the efficiency record in recent years. OPV
devices are comprised of a heterojunction between an electron donor molecule (e.g., P3HT,
poly(3-hexylthiophene) or CuPC, copper phthalocyanine) and an electron acceptor molecule
(e.g., C60 or its derivatives such as PCBM, phenyl-C61-butyric acid methyl ester).107 The
essentially limitless varieties of candidate organic semiconductor materials may be categorized
as either solution-processable (polymers, dendrimers, oligomers, or small molecules) or
vacuum deposited (small molecules or oligomers). Although superficially similar to inorganic p-n
junctions, the OPV junction is fundamentally different. Instead of directly creating an e-/h+ pair,
photon absorption produces an exciton, an uncharged excited state that must diffuse to a
donor/acceptor interface in order to dissociate into a free e-/h+ pair. In organic materials the
exciton can typically only diffuse 5-10 nm before decaying to the ground state, a problem that
limits performance and is typically referred to as the exciton bottleneck. There are two ways to
deal with this. One can make a multilayer device that uses very thin donor/acceptor layers such
32
that a majority of excitons can diffuse to a heterojunction interface.107 This approach is
commonly used in vacuum deposited devices. Or one can reduce the distance the exciton has
to diffuse before reaching the heterojunction by mixing the donor and acceptor materials on a
nanometer length scales to form a single-layer interpenetrating bicontinuous network called a
bulk heterojunction. This approach is commonly used in solution-processable materials.
In the OPV device structure, the heterojunction active layer(s) is(are) sandwiched between a set
of contact electrodes, with buffer layers likely to be present. In the bulk heterojunction
approach an asymmetry in the device must be imposed by either using electrodes of different
work function (typically a front TCO contact modified with a conducting polymer PEDOT-PSS,
poly(3,4-ethylenedioxythiophene) poly (styrenesulfonate) and a back contact metal of Ca or Al)
or by inserting a buffer layer that blocks carriers from leaving one side of the device. An oxide
buffer layer is commonly inserted to block holes from leaving the device through the front TCO
contact, which inverts the direction of operation of the device and allows the use of a high
workfunction Ag back contact.108-110. In the vacuum deposited multilayer approach, co-doping
of buffer layers has been used to great effect to produce a true p-i-n structure that obviates the
need for a mismatch in the contact work-functions.111
There are several challenges to improve the efficiency of organic solar cells.104, 112 These are
being addressed through the development of novel donor and acceptor materials, new buffer
layer and electrode geometries, innovative processing, and through the use of tandem
architectures. A key issue is to significantly raise the short circuit currents (Jsc) to above 20
mA/cm2. Present values are typically 10-12 mA/cm2 with champion values approaching 17
mA/cm2.104 The main problem is that leading photoactive layers do not efficiently harness
photons in the red and infrared region of the solar spectrum. Significant efforts have been
directed at developing improved low band gap polymers.113, 114 Advanced photon management
strategies are also being pursued to increase optical density.115 A second challenge is to
increase the open circuit voltage. Key to this is achieving optimal band alignment of the device
structure and minimizing the band offset between donor and acceptor molecules while
33
retaining efficiency charge transfer. 113,116 It is predicted that the maximal Voc in a standard
donor-acceptor device is 0.6 V less than the bandgap energy/e. Thus the goal is for a Voc of 0.8
– 0.9 V for low band gap absorbers with band gaps ~1.4 – 1.5 eV. Voc’s above 1 V have been
achieved, but only with high band gap materials. Third, the fill factors (FF) have to be increased
beyond 0.7, which has been achieve in only a few champion devices.104 Organic solar cells
typically have poor FF relative to conventional p-n junctions. This is due to high series resistance
and/or carrier recombination as the carrier mobilities in organic thin films are lower than their
inorganic counterparts.
Simultaneous achievement of Jsc = 20 mA/cm2, Voc = 0.8 V, and FF = 0.7, leading to an efficiency
of 11% has not been achieved yet. Doing this in a single junction device will require
simultaneous optimization of all the materials and interfaces. A possibly faster route to this goal
will involve the use of tandem configurations.117 These have been demonstrated using the bulk
heterojunction approach and are being used to effectively boost efficiency in the evaporated
small molecule approach as implemented for instance by Heliatek. Passing the psychological
milestone of 10% efficiency could bring organic solar cells within striking distance of the existing
thin film technologies, particularly because manufacturing costs are expected to be low. 104 Even
with existing materials and devices, the energy payback time for OPV has been estimated at 0.3
years.118 However, an efficiency of closer to 15% may be needed to achieve a true grid parity
LCOE of ~$0.07/kWh.119 Much work still needs to be done to demonstrate acceptable
performance in large area modules. 120 At present OPV submodule (200 cm2) efficiencies from
leading companies are approaching 4%.121 This value lags substantially behind the 9.2%
efficiency in comparable sized DSC modules.13 Also, published champion OPV devices are
fabricated on glass. To be economical, the substrate will likely need to be a low cost flexible
material that is suitable for R2R processing.
Another important issue that has to be resolved is the stability of organic solar cells. The
chemical, physical and mechanical degradation that are predominant in OPV materials and
devices have been well discussed.122 The list of failure mechanisms of OPV cells is long and
certainly as extensive as for any other photovoltaic technology. Major issues include
photodegradation and the sensitivity of OPV components to oxygen, requiring the use of
34
ultrabarriers for encapsulation. The current goal is to increase lifetimes from 3 to 10 years,
which is expected to be sufficient for consumer applications.104 Due to the flexibility of organic
synthesis, it can be estimated that there are on the order of 1013 different material
combinations that could be employed. Whether the right combination of properties (e.g., band
gap, charge mobility, exciton diffusion length, etc.) exists and how to identify them remain
open questions. Optimizing the photoactive organic layer may be best addressed using a
combinatorial approach. On the other hand, candidate structures and trends may be identified
using a rational method that combines computational methods with targeted synthesis.
4. Future Outlook
As discussed earlier, there is a large gap between the module and the cell efficiency for all
currently commercial PV technologies which can likely be narrowed with manufacturing R&D.
For crystalline silicon modules, efficiencies can be improved by maximizing the collection of
current, since little can be done to further improve the voltage. Related efforts include
improvements in front side texturing, integration of back side reflectors, and the use of
advanced AR coatings.37 Another strategy is to reduce the level of shadowing associated with
front contacts by either reducing the line-widths or by completely eliminating them using back
side contacting schemes such as those employed in Sunpower’s high efficiency modules. 38 Also
the limitation of quantum efficiency in the blue region of the spectrum can be addressed with
selective emitter designs and improved passivation strategies.39 Through a combination of
these strategies it is expected that micro-crystalline-Si modules will exceed 20% within the
decade, and mono-crystalline -Si modules would approach 25%.3 Currently the best efficiencies
in nono-crystalline Si modules are slightly above 20% for modules manufactured by SunPower
and Sanyo.
The next few years will prove critical to amorphous silicon technology’s long-term viability.
While necessary to improve stability, the transition from a-Si to nc-Si has come at an expense.
35
Due to its relatively low absorption coefficient,69 nc-Si based devices need to be up to five times
thicker than a-Si to collect sufficient light. This issue is exacerbated by the fact that the
deposition rates for nc-Si are much lower than those for a-Si.70 Combined with the relatively
low efficiencies, this has made manufacturing of a-Si/nc-Si based solar cells prohibitively
expensive when compared to alternative technologies such as CdTe and recently with cheap
imports of c-Si modules from China. Efforts to improve deposition rates include: a) use of very
high frequency (VHF: 25 – 100 MHz) plasma sources; b) operation at higher pressures and c)
development of linear plasma sources to maintain large area uniformity with VHF
modulation.71-73 Barring a significant breakthrough, a/nc-Si may need to focus on market
sectors that benefit from its low temperature, low weight capability. Another strategy might be
to examine if a-Si could be used as a route to form ut-Si, perhaps by coupling with rapid thermal
processing.3
The improvement of the CdTe PV technology is expected to continue as first solar continues to
invest in R&D and others are following their lead (e.g., PrimeStar, Abound). Through
improvements in efficiency and further integration of manufacturing CdTe manufacturing costs
could drop to $0.5/Wp within this decade.
However, the c-Si industry has shown that they can also rapidly reduce costs, and it is important
to note that silicon PV module prices does not have to be reduced as much as CdTe, if their
efficiency is high. As module costs drop below $1/Wp they become increasingly less important,
as balance of system issues including inverters, racks, installation, and space become important
cost drivers. These costs drop with module efficiency, and thus silicon can afford to remain
competitive despite higher manufacturing costs than CdTe. It is too early to project what may
happen to CIGS, though it is plausible to expect that it might end up in a similar region, most
likely somewhere between CdTe and c-Si.3
There have been numerous scientific breakthroughs in the area of 3 rd generation PV,132-135 but
practical devices are far from the point of commercialization.136, 137 In general 3rd generation PV
rely on the use of nanostructures such as quantum dots and nanowires to generate the desired
effects. A critical yet unresolved problem with devices that employ such structures is that they
36
will be dominated by interfaces, the anathema of PV technology.3 Interfaces typically serve as
either recombination centers or barriers to charge transport, and the demonstrated pathway to
high efficiency has been through their elimination. Record heteroepitaxial multi-junction cells
are produced by molecular beam epitaxy.138 The detrimental impact of interfaces is quite
plainly seen by comparing the performance within the silicon system (in terms of performance
c-Si > mc-Si > nc-Si > a-Si). Likewise, record CdTe and CIGS thin film devices are characterized by
their large grain size.139 Development of optimized manufacturing techniques requires
sophisticated modeling to understand how to maintain uniformity with respect to both area
and time. Accompanying this goal is the development of in-line diagnostics for real time process
control. Developments in intelligent and potentially self-correcting control of process flow
would help enable and accelerate throughput volume.
5.
Sustainability –Life Cycle Analysis
Photovoltaics do not need any fuel to produce electricity but energy is needed for generating
their materials, cells, modules, and systems. As in all types of products and systems, a complete
evaluation of their environmental profile must be done under the framework of a life-cycle
analysis.140 The life-cycle of photovoltaics starts from the extraction of raw materials (“cradle”)
and ends with the disposal (“grave”) or the recycling and recovery (“cradle”) of the PV’s
components (Figure 14).
M, Q
M, Q
M, Q
M, Q
M, Q
Raw
Material
Material
Processing
Manufacturing
Operation
M, Q
Decommissioning
Treatment
/Disposal
M, Q
E
E
E
E
E
E
Recycling
M, Q: Material and energy inputs
E: effluents (air, water, solids)
E
Figure 14: Flow of the life-cycle stages, energy, materials, and effluents for PV systems
37
The mining of the raw materials, for example, quartz sand for silicon PVs, copper-, zinc-, and
aluminum-ores for mounting structures and thin-film semiconductors, is followed by the
multiple stages of separation and purification. The silica in the quartz sand is reduced in an arc
furnace to metallurgical-grade silicon that must be purified further into solar-grade silicon (i.e.,
99.999% purity), requiring significant amounts of energy. Metallurgical-grade cadmium,
tellurium, indium, gallium, and selenium for CdTe and CIGS PV primarily are obtained as
byproducts of zinc-, copper-, and lead-smelting, and then further purified to solar grades. The
raw materials include those for encapsulations and balance-of-system components, for
example, silica for glass, copper ore for cables, and iron- and zinc-ores for mounting structures.
The production of all these materials requires large amounts of energy, as does the
manufacture of the solar cells, modules, electronics, and structures, their installation,
operation, and eventually their dismantling and recycling141 or disposal.
Thus, the energy payback time (EPBT) is defined as the period required for a renewable energy
system to generate the same amount of energy (in terms of primary-energy equivalent) that
was used to produce the system itself.
Energy Payback Time (EPBT) = (Emat+Emanuf+Etrans+Einst+EEOL) / (Eagen – Eaoper)
where,
Emat
: Primary energy demand to produce materials comprising PV system
Emanuf : Primary energy demand to manufacture PV system
Etrans
: Primary energy demand to transport materials used during the life cycle
Einst
: Primary energy demand to install the system
EEOL
: Primary energy demand for end-of-life management
Eagen
: Annual electricity generation in primary energy terms
Eaoper : Annual energy demand for operation and maintenance in primary energy terms
38
An indicator more commonly used for comparing different types of energy-production
technologies is the Energy Return on Energy Investment (EROI) that quantifies the benefit that
the user gets out of exploiting an energy source. Usually, it is expressed as the dimensionless
ratio of the energy generated from a system over that energy, or its equivalent from some
other source, that is “invested” in extracting, growing, or processing a new unit of the energy in
question. Thus, EROI even can be used for fuel-based power production that never pays back
its energy requirement as it continuously needs energy in the form of depletable fuel to
operate. The traditional way of calculating the EROI of PV is EROI = lifetime/ EPBT; thus, an
EPBT of 1 year and a life expectancy of 30 years corresponds to EROI of 30:1. The EROI of
conventional thermal generation from fossil fuels has been viewed as much higher than those
of photovoltaics; this recently was shown to be a misconception fostered by using outdated
data and a lack of consistency among calculation methods. 142
Several published PV LCA studies give differing estimates of the EROI. Such divergence reflects
the varied assumptions about key parameters, like product design, solar irradiation,
performance ratio, and lifetime. These assessments also deviate because of the different types
of installation used, such as ground mounts, rooftops, and façades. Also, assessments still are
calculated from outdated information in the literature collected from antiquated PV systems.
An example of such misrepresentation of the current reality was apparent in a recent PE
Magazine article wherein the author stated that “…photovoltaic electricity generation cannot
be an energy source for the future because photovoltaics require more energy than they
produce during their lifetime”. Statements to this effect were not uncommon in the 1970s
based on some early PV prototypes. However, today’s PVs return far more energy than that
embodied in the life cycle of a solar system; Fig.15 illustrates this historical trend to betterment.
To resolve these inconsistencies in the estimates, the International Energy Agency (IEA) PVPS
Task 12 published the “Methodology Guidelines on Life Cycle Assessment of Photovoltaic
Electricity” for conducting balanced, transparent, and accurate LCAs . 143 The results, presented
in Fig. 16, were obtained following these Guidelines and the associated Life Cycle Inventory
39
(LCI) data144; they represent consensus between LCA experts from the ten Task 12 member
countries.
50
mono-c-Si
multi-c-Si
CdTe
5
mono-c-Si
multi-c-Si
CdTe
0.5
1960
1970
1980
1990
2000
2010
2020
Fig. 15. EPBTs of various PV systems were reduced from about 40 yrs to 0.5 yrs from 1970 to
2010
Figure 16 plots the Energy payback times (EPBTs) of three major types of commercial PV
modules , i.e., mono-Si, multi-Si, and cadmium telluride (CdTe).
1.8
1.6
1.4
1.2
BOS
1
0.8
Frame
0.6
Module
0.4
0.2
0
mono-Si PV 14%
multi-Si 13.2%
CdTe PV 10.9%
Figure 16: Energy payback times (EPBTs) of rooftop mounted PV systems for U.S- and Europeanproduction and installation under average U.S. irradiation of 1800 kWh/m2/yr (4.9
kWh/m2/day), a performance ratio of 0.75, and the module efficiencies shown above. Data
40
adapted from de Wild Scholten (2009) and Fthenakis et al. (2009); note that module efficiencies
have increased since 2009, and, correspondingly, EPBTs have decreased.
These results are based on detailed process data obtained through collaborations with thirteen
European- and US-PV manufacturers. 145,146 The EPBT for the same type of systems installed
in the US-SW were decreased in proportion to the solar irradiation ratio (1800/2380) between
the US average- and SW-solar conditions. Thus, for SW irradiation, the EPBTs for the three PV
technologies shown in Fig. 16 are 1.3-, 1.3- and 0.5-years and their corresponding EROIs are
23:1, 23:1 and 60:1.
Fig.17. Energy PaybackTimes for different insolation levels in the United States. This Solar
Resource map was produced by B. Roberts, National Renewable Energy Laboratory for the U.S.
Department of Energy; the colors show annual averages of daily insolation for the south facing
latitude-tilt plane. The EPBTs were estimated by V. Fthenakis, Brookhaven National Laboratory.
41
6. The Intermittency Challenge
The short-term variability of solar power recently garnered much attention because the
installed capacity is increasing very rapidly, and the technology is on its way to become a
significant part of the generator portfolio (power capacity) of several countries, notably
Germany (12%, 2010), and Spain (4.3%, 2010) (German Energy Ministry, 2011; Red Electrica De
Espana, 2010). German Energy Ministry. (2011). Erneuerbare Energien 2010. Vierteljahrshefte
zur Wirtschaftsforschung (Vol. 76). doi:10.3790/vjh.76.1.35
Red Electrica De Espana. (2010). Red Electrica De Espana. Retrieved from http://www.ree.es/
Variability can be smoothened by aggregating solar generation over large geographical regions
and by combining solar and wind generation, since in many places one compliments the other
(e.g., more wind at night than during the day and more in the winter than in the summer). This
has been illustrated for New York State 147 and the ERCOT grid system.148
7. Conclusions
Although recent market uncertainties have slowed down PV production in 2012, the long-term
potential of the technology is clearly upward, and it would accelerate when electricity
production by PV reaches parity with conventional electric power generation technologies in
large regions of the world. It is expected that crystalline silicon will continue dominating the
market for the foreseeable future. However, significant opportunities remain for thin-film
photovoltaics, mainly through continued increase in module efficiencies. If investments in CdTe
and CIGS thin-film PV technologies continue, it is not unreasonable to expect that the module
efficiencies in commercial production will reach those of the high efficiency crystalline-silicon
PV modules. Pending further improvements in efficiency, CdTe’s market share could evolve to
anywhere between 10 – 25%. After decades of being the leading thin film technology, the
prospects of amorphous silicon making a major contribution to the utility sector appear
severely constrained by high manufacturing costs and low efficiency, with the exit of UniSolar
from the market being the evidence of the technology lacking competitive merit. However, the
42
a-Si/nc-Si (micromorph) architecture is still holding up. CIGS has gained a foothold in PV
manufacturing at ~100 MW/year and higher (Solar Frontier) and the demonstrated potential in
efficiency provides reason for guarded optimism. The next five years will be critical for CIGS to
demonstrate robust manufacturing to allow it to break out and join CdTe as a major player in
the global PV market. Although encouraging industrial progress has been made, DSC and OPV
are still limited at present by low efficiency and stability. Konarka, the US front-runner on OPV
recently exited the market as their production costs were relatively high.
Concerning the future, the future of solar PV still looks bright. The industry has consistently
been able to lower the cost of solar panels. This author believes that this trend can be
maintained, and if subsidies are continued for the next 10 years, there is a real prospect for
solar to become cost competitive without any subsidies. What is needed more than anything
else at this critical conjuncture is the maintenance of federal tax incentives and state
“renewable portfolio standard” incentives or the enactment of feed-in-tariffs following
Germany’s example. With solar replacing the current fossil-fuel and nuclear based power
infrastructure, the environmental benefits, which are not accounted in the current cost
structures, will be enormous.
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