ss called doping—the
stage is set for creating an electric current. The bottom
layer of the PV cell is usually doped with boron, which
bonds with the silicon to facilitate a positive charge (P).
The top layer is doped with phosphorus, which bonds
with the silicon to facilitate a negative charge (N). The
Figure 2. Three commonly used reflecting schemes for concentrating solar energy to attain high temperatures.
Copyright © 2012 SciRes. SGRE
The Benefits of Integrated Methods in PV Making to Promote Their Efficiency and Achieve Low-Cost Modules 29
Figure 3. Close up of Photovoltaic cell.
surface between the resulting “p-type” and “n-type” semi-
conductors is called the P-N junction (Figure 3). Elec-
tron movement at this surface produces an electrical field
that only allows electrons to flow from the p-type layer
to the n-type layer.
When sunlight enters the cell, its energy knocks elec-
trons loose in both layers. Because of the opposite charges
of the layers, the electrons want to flow from the n-type
layer to the p-type layer, but the electrical field at the P-
N junction prevents this from happening. The presence of
an external circuit, however, provides the necessary path
for electrons in the n-type layer to travel to the p-type
layer. Extremely thin wires running along the top of the
n-type layer provide this external circuit, and the elec-
trons flowing through this circuit provide the cell’s own-
er with a supply of electricity.
Most PV systems consist of individual square cells
averaging about four inches on a side. Alone, each cell
generates very little power (less than two watts), so they
are often grouped together as modules. Modules can then
be grouped into larger panels encased in glass or plastic
to provide protection from the weather, and these panels,
in turn, are either used as separate units or grouped into
even larger arrays. The three basic types of solar cells
made from silicon are single-crystal, polycrystalline, and
amorphous which are described in the following:
· Single-crystal cells are made in long cylinders and
sliced into round or hexagonal wafers. While this
process is energy-intensive and wasteful of materials,
it produces the highest-efficiency cells—as high as 25
percent in some laboratory tests. Because these high-
efficiency cells are more expensive, they are some-
times used in combination with concentrators such as
mirrors or lenses. Concentrating systems can boost ef-
Copyright © 2012 SciRes. SGRE
The Benefits of Integrated Methods in PV Making to Promote Their Efficiency and Achieve Low-Cost Modules
30
ficiency to almost 40 percent. Single-crystal accounts
for 29 percent of the global market for PV [4].
· Polycrystalline cells are made of molten silicon cast
into ingots or drawn into sheets, then sliced into
squares. While production costs are lower, the effi-
ciency of the cells is lower too—around 15 percent.
Because the cells are square, they can be packed more
closely together. Polycrystalline cells make up 62
percent of the global PV market.
· Amorphous silicon (a-Si) is a radically different ap-
proach. Silicon is essentially sprayed onto a glass or
metal surface in thin films, making the whole module
in one step. This approach is by far the least expen-
sive, but it results in very low efficiencies—only
about five percent. Of course, to day the efficiency of
this method is improved.
A number of exotic materials rather than silicon are
under development, such as gallium arsenide (Ga-As),
copper-indium-diselenide (CuInSe2), and cadmium-tel-
luride (CdTe). These materials offer higher efficiencies
and other interesting properties, including the ability to
manufacture amorphous cells that are sensitive to differ-
ent parts of the light spectrum. By stacking cells into
multiple layers, they can capture more of the available
light. Although a-Si accounts for only five percent of the
global market, it appears to be the most promising for
future cost reductions and growth potential. In the 1970s,
a serious effort began to produce PV panels that could
provide cheaper solar power. Experimenting with new
materials and production techniques, solar manufacturers
cut costs for solar cells rapidly, as the following graph
(Figure 4) shows. On the other hand, the number of di-
rect and indirect jobs has been increased dramatically in
recent years (Figure 5).
One approach to lowering the cost of solar electric
power is to increase the efficiency of cells, producing
more power per dollar. The opposite approach is to de-
crease production costs, using fewer dollars to produce
the same amount of power. A third approach is lowering
the costs of the rest of the system. For example, building-
Figure 4. PV manufacturing cut cost.
Figure 5. PV job creating.
integrated PV (BIPV) integrates solar panels into a build-
ing’s structure and earns the developer a credit for re-
duced construction costs. Innovative processes and de-
signs are continually reaching the market and helping
drive down costs, including string ribbon cell production,
photovoltaic roof tiles, and windows with a translucent
film of a-Si. Economies of scale from a booming global
PV market are also helping to reduce costs. More re-
cently, thanks to lower costs, strong incentives, and net
metering policies, the PV industry has placed more focus
on home, business, and utility-scale systems that are at-
tached to the power grid. In some locations, it is less ex-
pensive for utilities to install solar panels than to upgrade
the transmission and distribution system to meet new
electricity demand. In 2005, for the first time ever, the
installation of PV systems connected to the electric grid
outpaced off-grid PV systems in the United States [5]. As
the PV market continues to expand, the trend toward
grid-connected applications will be continuing. This dis-
tributed-generation approach provides a new model for
the utilities of the future. Small generators, spread through-
out a city and controlled by computers, could replace the
large oil and nuclear plants that dominate the landscape
now. Solar energy technologies are poised for significant
growth in the 21st century. More and more architects and
contractors are recognizing the value of passive and ac-
tive solar and learning how to effectively incorporate it
into building designs. Solar hot water systems can com-
pete economically with conventional systems in some
areas. And as the cost of solar PV continues to decline,
these systems will penetrate increasingly larger markets.
In fact, the solar PV industry aims to provide half of all
new U.S. electricity generation by 2025 [6].
4. Three Methods Mostly Used to Pr o moted
PV
4.1. Advanced Thin Film Technologies for Cost
Effective Photovoltaics
The overall challenge is to provide the scientific and
Copyright © 2012 SciRes. SGRE
The Benefits of Integrated Methods in PV Making to Promote Their Efficiency and Achieve Low-Cost Modules
Copyright © 2012 SciRes. SGRE
31
technological basis for industrial mass production of
cost-effective and highly efficient, environmentally sound
and economically compliant large-area thin film solar
cells and modules. By drawing on a broad basis of ex-
perience, the entire range of module fabrication and sup-
porting will be covered: substrates, semiconductor and
contact deposition, monolithic series interconnection,
encapsulation, performance evaluation and applications
by different factories. Photovoltaics have become an in-
creasingly important industrial sector over the past ten
years [7]. Therefore the main challenges are:
· Significantly reducing the cost/efficiency ratio to-
wards €0.5/WP in the long run.
· Providing the know-how and the scientific basis for
large-area PV modules by identifying and testing new
materials and technologies with maximum cost re-
duction.
· Developing the process know-how and the production
technology, as well as the design and fabrication of
specialized equipment, resulting in low costs and high
yield in the production of large area thin film mod-
ules.
To meet these challenges, existing concepts for mate-
rials and technology will be improved and brought to
maturity in close cooperation with industry, and new
options will be investigated for materials and new types
of solar cells to provide the scientific and technological
basis for the next generation of PV devices. Accordingly,
the research activities range from basic research to in-
dustrial implementation [7].
The state-of-the-art for advanced thin film PV tech-
nology and the enhancement within the proposed project
is summarized in Table 1.
Prototypes of a small junction box especially suited for
thin-film modules were developed. Limiting the by-pass
diodes to only one per box allows a reduction in both size
and cost. It is also possible to use the box for parallel
inter-connection of the modules.
4.2. Towards the Production of Cost-Competitive
Photovoltaic Solar Energy by Absorbing the
Most of the Solar Spectrum
Solar radiation is a significant energy source: only appr-
oximately 1000 Joules of energy per second per square
meter are accessible. It is clear to us that strategies to
reach the ultimate goal of a module cost of €1/Wp will
necessarily have to go through the development of con-
cepts capable of extracting the most of every single pho-
ton available. In this respect, each of the three activities
envisaged in this paper to achieve the general goal has to
confront its own challenges. The multi-junction activity
pursues the development of solar cells that approach 40%
efficiency. To achieve this, it faces the challenge of find-
ing materials with a good compromise between lattice
matching and band-gap energy. The thermo photovoltaic
activity bases part of its success on finding suitable emit-
ters that can operate at high temperatures and/or adapt
their emission spectra to the cell’s gap. The other part
relies on the successful recycling of photons so that those
that cannot be used effectively by the solar cells can re-
turn to the emitter to assist in keeping it hot. The inter-
mediate-band solar cell approach addresses the challenge
of proving a principle of operation which would see a
Table 1. Expected enhancement of the state-of-the-art.
Technology State-of-the-art Substrate, process (efficiencies) Planned enhancement in IP (for Europe)
Lab cells
a-Si/μc-Si 12% (Kaneka)
11% (UniNE, FZJ
On glass, PE-CVD 14%
Poly Si 9% (Sanyo) On metal substrate, SPC 15% on foreign substrates
CIGS low gap 19.2% (NREL)
16% - 17% (NREL)
On glass, co-evaporation
On metal foil, co evaporation
18% on metal foil
9% on polyimide foil
CIGS wide gap 12% - 13% (HMI) On glass, sputtering, PVD 13% - 14%, advanced equipment. 10% @ 60% IR transparency for
tandem applications
CIGS tandem 7% (HMI) On glass, co-evaporation 15%
Prototypes, pilot production
a-Si/μc-Si 10% (Kaneka, FZJ) On glass 30 × 30 cm2 (FZJ)
On glass 3738 cm2 (Kaneka)
Equipment for cost-effective production of 10% modules (1 m2 @
costs towards €0.5/Wp)
CIGS wide gap 10% (Sulfurcell) On glass 5 × 5 cm2, sputtering, PVD10% on 125 × 65 cm2
Commercial product
a-Si 6% - 7% (Unisolar,
SCHOTT, Kaneka, ...)
On glass, PE-CVD
CIGS low gap 10% (Shell, Würth) On glass, co-evaporation 11% - 12%, cost-effectiveness, environmentally sound
The Benefits of Integrated Methods in PV Making to Promote Their Efficiency and Achieve Low-Cost Modules
32
significant improvement in the performance of the cells.
In a Project which coordinated by Prof. Antonio Luque
et al. the activity devoted to the search for new molecules
engenders the challenge of identifying molecules capable
of undergoing two-photon processes: that is molecules
that can absorb two low-energy photons to produced a
high-energy excited state or, for example, dyes that can
absorb one high-energy photon and re-emit its energy in
the form of two photons of lower energy. Among all of
the above concepts, the multi-junction approach appears
to be the most readily available for commercialization.
For that, the activity devoted specifically to speeding up
its path to market is the development of trackers, optics
and manufacturing techniques that can integrate these
cells into commercial concentrator systems1.
The multi-junction solar cell approach pursues the
better use of the solar spectrum by using a stack of sin-
gle-gap solar cells incorporated in a concentrator system,
in order to make the approach cost-effective (Figure 6).
The project, at its outset, aimed at cells with an effi-
ciency of 35%. This result has already been achieved by
FhG-ISE in the second year of their project and their
consortium now aims to achieve efficiencies as close as
possible to 40%. In the thermo-photovoltaic approach the
sun heats up, through a concentrator system, a material
called the ‘emitter’, leading to incandescence (Figure 7).
The radiation from this emitter drives an array of solar
cells, thus producing electricity. The advantage of this
approach is that, by an appropriate system of filters and
back-reflectors, photons with energy above and below
the solar cell band-gap can be directed back to the emit-
ter, helping to keep it hot by recycling the energy of
Figure 6. Schematic illustrating the operation of a multi-
junction solar cell in a concentrator system.
Figure 7. Emitter heated up by the sun through a concen-
trator system.
these photons that otherwise would not be converted op-
timally by the solar cells. By the conclusion, it is ex-
pected that the system, made up basically of the concen-
trator, emitter and solar cell array can be integrated and
evaluated. The “intermediate-band” approach pursues
better exploitation of the solar spectrum by using inter-
mediate-band materials. These materials are character-
ized by the existence of an electronic energy band within
what otherwise would be a conventional semiconductor
band-gap. According to the principles of operation of this
cell, the intermediate band allows the absorption of low
band-gap energy photons and the sub sequent production
of enhanced photocurrent without voltage degradation.
This method also expects to identify as many intermedi-
ate-band material candidates as possible, as well as de-
monstrate experimentally the operating principles of the
intermediate-band solar cell by using quantum dot solar
cells as workbenches.
4.3. High Concentration PV Power System
(HICONPV)
Existing and innovative solar concentrators were evalu-
ated for their properties in high-concentration photovol-
taics. Plant types were identified that fulfill the technical
requirements of homogenous irradiation distribution with
solar concentration factors of 500 to 2000 suns and cost-
effective implementation perspectives. The conclusions
were that Modified Spherical Dish (Tailored Concentra-
tor, Figure 8) configurations look more suitable for
meeting current technology requirements than classical
Parabolic Dish solutions. The results shown with this
design are promising. It has been proposed to build and
test a tailored concentrator for HICONPV technology
with this design.
1The Project is coordinated by Professor Antonio Luque Instituto de
Energía Solar assisted by Projektgesellschaft Solare Energiesysteme
GmbH (PSE).
Copyright © 2012 SciRes. SGRE
The Benefits of Integrated Methods in PV Making to Promote Their Efficiency and Achieve Low-Cost Modules 33
Figure 8. Tailored concentrator.
Figure 9. Layout of a solar concentrator.
An innovative heliostat variant was evaluated for its
properties in high-concentration photovoltaics, demon-
strating that the proposed Torque Tube Heliostat design
concept promises significant cost advantages over exist-
ing heliostat designs. This can be achieved with a much
lower construction height of the TTH, which reduces
drastically the wind loads on the structure and the re-
quired specific drive power. The aim of this tailor con-
centrator is to prove the real possibilities of this innova-
tive conceptual design, and to see the performance of the
concept under real manufacturing constraints. The pro-
posed final configuration was not optimized for 1000x
(X equals approximately the area of the optic collector
divided by area of the semiconductor solar cell. Zenith-
Solar system is designed to a concentration factor of
1000X), but rather close, so it is necessary to take into
account the optimized structural heliostat concept, where
the shape of the concentrator is no longer round but rec-
tangular. Rectangular concentrators allow us to keep the
gravity centre lower for the same aperture area (Figure
9). This has a strong influence on the structural design
and the final cost2. In conventional CPV systems, the
excess heat generated in the solar cell needs to be re-
moved to avoid damaging the cell and to maintain high
efficiency of electricity conversion. ZenithSolar utilizes
the heat generated at the solar cell receiver to provide
usable hot water heating, improving overall solar power
conversion efficiency to 75%.
5. Expected Results
The concept of this research project focuses specially on:
· New monolithic integrated modules with efficiencies
of 20% and above.
· Module design for irradiation up to 1000 suns.
· Adaptation of already proven concentrator’s concepts
that promise high quality and high reliability.
REFERENCES
[1] Concentrating Solar Power Program, “Solar Two Dem-
onstrates Clean Power for the Future,” 2000.
http://www.energylan.sandia.gov/sunlab/files/stfuture.pdf
[2] Solar Energy Technologies Program, “Solar Energy Tech-
nologies Program: Multi-Year Program Plan 2001-2011,”
2006.
http://www1.eere.energy.gov/solar/pdfs/set_myp_2007-2
011_proof 1.pdf
[3] Renewable Energy Access, “World’s Largest Solar Pro-
ject Unveiled,” 2005.
http://www.renewableenergyaccess.com/rea/news/story?i
d=35263
[4] Solar Energy Technologies Program, “Solar Energy Tech-
nologies Program: Multi-Year Program Plan 2001-2011,”
2006.
http://www1.eere.energy.gov/solar/pdfs/set_myp_2007-2
011_proof 1.pdf
[5] The Prometheus Institute, “U.S. Market Analysis,” PV
News, Vol. 25, No. 5, 2006, pp. 4-5.
[6] Solar Energy Industries Association, “Our Solar Power
Future: The US Photovoltaics Industry Roadmap through
2030 and beyond,” 2004. http://www.seia.org
[7] Renewable Energy Technologies, “Long Term Research
in the 6th Framework Programme,” European Commis-
sion, Directorate-General for Research, Directorate En-
ergy, Brussels, 2007.
2RESEARCH TRIANGLE PARK, N. C., DuPont Microcircuit Materi-
als (DuPont) today announced the introduction of DuPont™ Solamet®
PV412 photovoltaic (PV) metallization paste, Raleigh, 2009.
Copyright © 2012 SciRes. SGRE