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Photovoltaic solar energy: Conceptual framework
Priscila Gonçalves Vasconcelos Sampaio
a,
⁎
, Mario Orestes Aguirre González
b
a
Federal University of the Semi-Árido, Brazil
b
Federal University of the Rio Grande do Norte, Brazil
ARTICLE INFO
Keywords:
Photovoltaic solar energy
Systematic literature research
ABSTRACT
The purpose of this article is to understand the state of art of photovoltaic solar energy through a systematic
literature research, in which the following themes are approached: ways of obtaining the energy, its advantages
and disadvantages, applications, current market, costs and technologies according to what has been approached
in the scientific researches published until 2016. For this research, we performed a qualitative and quantitative
approach with a non-probabilistic sample size, obtaining 142 articles published since 1996–2016 with a slitting
cut. The analysis result of this research shows that studies about photovoltaic energy are rising and may perform
an important role in reaching a high-energy demand around the world. To increase the participation of
photovoltaic energy in the renewable energy market requires, first, to raise awareness regarding its benefits; to
increase the research and development of new technologies; to implement public policies a programs that will
encourage photovoltaic energy generation. Although crystal silicon solar cells were predominant, other types of
cells have been developed, which can compete, both in terms of cost reduction of production, or in terms of
greater efficiency. The main applications are dominated by telecommunications, water pumping, public lighting,
BIPV, agriculture, water heating, grain drying, water desalination, space vehicles and satellites. The studies
found on photovoltaic solar energy are all technical, thus creating the need for future research related to the
economic viability, chain supply coordination, analysis of barriers and incentives to photovoltaic solar energy
and deeper studies about the factors that influence the position of such technologies in the market.
1. Introduction
With the increase of population and technologic and economic
development, human beings need more energy to create a better life
environment. However, burning traditional fossil fuels is causing a
series of environmental problems, such as climate change, global
warming, air pollution and acid rain
[1–3].
Therefore, there is an urgent need for the development of renewable
energy Technologies, in order to deal with the political, economic and
environmental challenges that are involved in generate electricity. The
appearance of such energies in the last years has largely propelled the
interest among investigators, politics and industry leaders in under-
standing the economic viability of the new energy source
[2,4].
Capturing solar energy through photovoltaic panels, in order to
produce electricity is considered one of the most promising markets in
the field of renewable energy. Due to its fast growth perspective and
high levels of investment involved, the photovoltaic market is now
being more disputed around the world, especially in Europe, China and
in the United States. In Brazil, the advances are starting to be
significant, especially after the insertion of solar energy in Brazil's
energy matrix, and the beginning of solar energy auctions at a time in
which the energy sector is facing difficulties due to the reduction of
hydroelectric energy, which is currently Brazil's main energy matrix,
and the increase in electricity prices.
Research on photovoltaic solar energy has increased in recent years,
as has the number of publications in journals. Based on what has been
exposed, this study aims to answer the following question: “How does
photovoltaic solar energy has been approached in scientific studies
published between 1996 until 2016?” For such, we performed a
systematic literature research followed by a structured of the contents
published on photovoltaic solar energy.
Besides the ongoing introduction, the article is structured from a
division dedicated to showing the research method adopted in the
study. Posteriorly, we will expose the classification of the articles,
followed by the analysis of the themes (definition, mean of obtaining,
advantages, disadvantages, applications, current state in the market,
costs and technologies) discussed in the analyzed pieces, at last, in the
final section we have the conclusion regarding the theme and a
suggestions for possible future studies.
http://dx.doi.org/10.1016/j.rser.2017.02.081
Received 17 June 2016; Received in revised form 31 December 2016; Accepted 22 February 2017
⁎
Correspondence to: Federal University of the Semi-Arid Bernardino M Veras Street 47 59625360 Mossoró Brazil.
E-mail addresses:
prisamp@yahoo.com.br (P.G.V. Sampaio), mario@ct.ufrn.br (M.O.A. González).
Renewable and Sustainable Energy Reviews 74 (2017) 590–601
Available online 02 March 2017
1364-0321/ © 2017 Elsevier Ltd. All rights reserved.
MARK
2. Research method
This research can be characterized, regarding its object, as a
systematic literature research. In
[5] a systematic literature research
is defined as a trustworthy research approach, due to fact that it is
broad and explicitly presents the utilized means and obtained results.
Corroborating with this idea
[6] register that systematic literature
review has the goal of generating structure knowledge about a research
theme.
The research was conducted in five stages.
Fig. 1.
During the first stage, we defined the problem of our research as:
“How does photovoltaic solar energy has been approached in scientific
studies published between 1996 until 2016?”. From that definition, we
then went to the second stage, which was consisted of finding articles
through the Metasearch of CAPES (Brazil's Higher Education
Coordination of Personnel Perfecting) Periodicals portal through the
utilization of the following keywords: “solar energy”, “sun power”,
“photovoltaic solar energy”“photovoltaic cells”.Nofilters were used to
limit the period of years of the research, which was made among all the
texts published until 2016.
In the third stage was made the capture of the articles that
contained in their title these key words. Then the selection of the
articles was done from the reading of the abstracts. During the
development of this stage, articles that had no relationship with the
objective of this research were excluded. In this way, 142 articles were
selected. In the fourth stage of the research the texts were read and
extracted, as well as their classification in terms of structure and
content through the elaboration of a spreadsheet database in the
Microsoft Excel program. In the fifth stage, the results were drafted
for subsequent publication.
3. Classification of the analyzed articles
The study research included the reading of 142 articles, which were
published in a time interval between 1996 and 2016. The year with the
largest number of publications was 2014, with 26 items. In second
place, with 15 publications, it was the year 2011.
As regards to the Journals of the publications, out of the 142
articles, the most important one was the Renewable and Sustainable
Energy Reviews, with about 22% of publications followed by Solar
Energy, Solar Energy Materials & Solar Cells, Energy Policy and
Renewable Energy which together add up to 35% of the publications.
On the issue of countries with more publications on the key words of
this research, the prevalence was the United States, with 26 publica-
tions, China (15) and Germany (14). Other countries such as Japan,
Italy, Spain, Denmark, South Korea, Belgium, Croatia, Belgium,
Lithuania, Scotland, Greece, UAE, Singapore, Spain, Australia,
Australia, Brazil, India, Poland, Switzerland, Sweden, Thailand,
Poland, Australia, Pakistan, Israel, Morocco, Mexico, Malaysia, Chile,
Turkey, United Kingdom and Taiwan, Norway got together 87 pub-
lished texts.
When classifying the texts, we took into account 5 items, which are:
type of study, approach, goals, object and focus of the research. As to
the type of study that was used in the articles, the literature review was
predominant, with 75 texts, followed by the experimental study with
31, as seen in
Table 1.
The majority of the articles were classified, according to the
approach, as being qualitative, with 59% of the total amount, being
followed by the qualitative and quantitative (31%) and quantitative
(10%). As for the goals, the exploratory and exploratory-descriptive
classifications prevailed with 75 and 58 texts, respectively, followed by
the explanatory classification (5), descriptive (3) and exploratory-
descriptive (1).
When analyzing the focus of the research, we could see that the
theoretical focus obtained a rate of 87%. As for the theoretical-
industrial focus, as well as the industrial focus had a rate of 6% of
the texts, followed by the domestic focus, research center and
theoretical-domestic focus (3%, 3% and 1%, respectively).
When it comes to the object, 84 analyzed articles were characterized
as literature research. Next, we have the texts classified as laboratory
research (34), field research (20), literature and field research (3) and
literature and laboratory research (1).
Among the 142 articles read in this study, 74 served as basis for
item 4. Photovoltaic solar energy.
4. Photovoltaic solar energy
The photovoltaic solar energy (PV) is one of the most growing
industries all over the world, and in order to keep that pace, new
developments has been rising when it comes to material use, energy
consumption to manufacture these materials, device design, produc-
tion technologies, as well as new concepts to enhance the global
efficiency of the cells
[7–9]. The understanding of photovoltaic solar
energy from the point of view of the authors consulted in the first stage
of this research is presented in
Table 2.
It can be observed that the definitions presented by the authors on
photovoltaic solar energy have terms in common, being them: “elec-
tricity”, “solar radiation”, “direct generation”, “conversion”. Thus, we
can adopt as a concept of photovoltaic solar energy the following
definition: electricity obtained directly from the conversion of solar
energy.
The conversion of solar radiation into electricity occurs due to the
photovoltaic effect, which was observed by the first time by Becquerel
in 1839
[8,9,17–23]. This effect occurs in materials known as
semiconductors, which present two energy bands, in one of them the
presence of electrons is allowed (valence bad) and in the other there is
no presence of them, i.e., the band in completely “empty” (conduction
band), see
Fig. 2. The semiconductor material more commonly used is
the silicon, second most abundant element on Earth. Its atoms are
Fig. 1. Stages the research.
Table 1
Classification of the type of research.
Type research Amount Type research Amount
CS 23 S 2
EX 38 LR-EX 2
AR 1 LR-S 1
LR 75
CS: Case Study; EX: Experimental; AR: Action Research; LR: Literature Review; S:
Survey.
P.G.V. Sampaio, M.O.A. González
Renewable and Sustainable Energy Reviews 74 (2017) 590–601
591
characterized by having four electrons that connect to its neighbors,
creating a crystal network.
The function of sunlight on the photovoltaic effect is to supply an
amount of energy to the outermost electron to make it possible for him
to move from the valence band to the conduction band in the material,
thereby generating electricity. As [14] in the case of silicon, specifically,
it is needed 1.12 eV (electro volts) for electrons to exceed the GAP.
Further, according to
[19], the semiconductor material must be able to
absorb a large part of the solar spectrum.
Virtually all photovoltaic devices incorporate a PN junction in a
semiconductor, which through a photo voltage is developed. These
devices are also known as solar cells or photovoltaic cells [19]. A typical
solar cell is shown in
Fig. 3. The PN junction is the main part of the cell
where the light receiving portion is the N-type material in the part
below this the material is P-type.
The main advantages and disadvantages of photovoltaic solar
energy are described in
Table 3.
Compared to conventional power generation sources, such as those
using fossil fuels, photovoltaic technology does not bring the serious
environmental problems that these sources cause during generation,
such as climate change, global warming, air pollution, acid rain and so
on. Another advantage in relation to fossil fuels is that solar energy
does not need to be extracted, refined or transported to the generation
site, which is close to the load. However, during its life cycle, it
consumes a large amount of energy and emits some greenhouse gases
in some stages (manufacturing process of solar cells, assembly of
photovoltaic modules and transport of material, among others)
[5,9,17,27].
Photovoltaic technologies, consume per unit of electricity produced,
64 times more material resources, 7 times more human resources and
10 times more capital than nuclear technology. Although this data is
biased, this is a clear indication of the extreme inefficiency of PV
technologies in regions of moderate sunshine to help achieve the goal
of providing a resource-efficient, efficient electricity supply system. Due
to the intermittent nature of electricity production in these regions,
parallel electricity supply infrastructure needs to be provided
[28].
In relation to other renewable sources, photovoltaic solar energy
presents a lower incidence of damages to the environment where it is
being generated, which does not occur with the energy produced by the
hydroelectric plants, where for the construction of hydroelectric plants
the course of the river is changed and extensive areas of production of
food and forests are flooded. Another important factor is the cost of
operation, which for hydraulic power generation is high compared to
the cost of operating a solar plant. Despite the decrease in generation
during cloudy days, energy from the sun is abundant, while the volume
of water in the dams during periods of drought is limited. If compared
to wind energy, photovoltaic solar energy is silent and can be generated
in urban areas since panels can be installed on the roof.
Despite its limitations, the photovoltaic power generation systems
allow the installation of a short-term power plant, with the possibility
to generate several MW in less than a year. As the environmental
impacts, they are minimal, photovoltaic systems remove the need for
preliminary studies that require long-term assessment, unlike the
highly polluting systems
[15].
Using photovoltaic solar energy is used in both spatial and Earth
applications, as seen in
Table 4.
The large-scale photovoltaic application occurs through photovol-
taic plants installed in both water and land. To conserve valuable land
and water, installing solar photovoltaic systems in water bodies such as
oceans, lakes, reservoirs, irrigation ponds, wastewater treatment
plants, wineries, fish farms, dams and canals may be an option
attractive. Floating type photovoltaic solar panels have numerous
advantages compared to grounded solar panels, including fewer
Table 2
Definition of solar photovoltaics.
Author Definition of solar PV
[10] It is the direct conversion of sunlight into electricity.
[11] Energy based on semiconductor technology that converts sunlight into
electricity.
[12] It is the most elegant method to produce electricity by converting
abundant sunlight.
[13] Energy that converts sunlight into electricity by means of a single
junction LED (or several junctions).
[14] Direct generation of electricity from sunlight.
[15] Renewable source of energy by converting solar light into electricity.
[1] Energy that generates electricity from solar energy.
[3] Direct conversion of radiation into electricity.
[16] Energy source that converts light directly into electricity without gas
emissions or noise.
[17] It is the direct conversion system that converts sunlight into electricity
without the help of machines or mobile devices.
Fig. 2. Band of valence, band gap (GAP) and the conduction band: insulator, conductor
and semiconductor.
Fig. 3. Photovoltaic cell.
Table 3
Advantages and disadvantages of solar photovoltaics.
Authors
Advantages Reliable system
[15,24]
Low cost of operation and maintenance [15–17,23]
Low maintenance [23,24]
Free energy source [15,17]
Clean Energy [1,2,4,15,17,24–26]
High Availability [2,15,24]
The generation can be made closer to
the consumer
[15]
Does not cause environmental impacts
/Environmental friendly
[1,17,23,26]
Potential to mitigate emissions of
greenhouse gases
[1,17]
Noiseless [16,17]
Disadvantages Limitations in the availability of
systems on the market
[15]
High initial cost [15,16,20,23,25]
Needs a relatively large area of
installation
[15]
High dependence on technology
development
[16]
Geographical conditions (solar
irradiation)
[16]
P.G.V. Sampaio, M.O.A. González
Renewable and Sustainable Energy Reviews 74 (2017) 590–601
592
obstacles to block sunlight, convenient energy efficiency, and higher
power generation efficiency due to their lower temperature under
panels. In addition, the solar installation brings benefits to the aquatic
environment because shading of the plant prevents excessive evapora-
tion of water, limits algae growth and potentially improves water
quality
[35].
The installation of photovoltaic plants in the desert may be one of
the most suitable places for the use of photovoltaic solar energy due to
the high levels of solar radiation. In the Atacama desert in Chile, for
example, it is a viable option capable of contributing to the continued
supply of sustainable electricity in the north of the country, contribut-
ing to the stabilization of electricity prices, thus benefiting the Chilean
mining industry [36,37].
4.1. Elements of the photovoltaic solar energy system
A typical photovoltaic solar system consists of four basic elements:
Photovoltaic module, charge controller, the inverter and battery when
necessary (
Fig. 4).
The photovoltaic module consists of photovoltaic cells, i.e., the
surfaces that generate electricity, which convert directly solar energy
into electricity. These surfaces have no moving parts to wear out or
suffer breakdowns and works without the use of fuel without vibrations
without noise and without harming the environment
[15–17,24].
As for the charge controller, it has the function to preserve the
batteries from being overcharged or discharged completely, increasing
its useful life. The inverter, in turn, is responsible for converting the
power generated by photovoltaic panels (electricity generating DC –
DC) to alternating current – AC voltage levels and network frequency.
Batteries are used in photovoltaic systems to store the surplus
produced by the modules to be utilized at night or on days with low
sunshine or overcast
[15,17].
4.2. Photovoltaic technologies
According to
[38,39], there is a wide variety of photovoltaic cell
technologies in the marketplace today, using different types of materi-
als, and an even larger number will be available in the future.
Photovoltaic cell technologies are generally categorized into three
generations, depending on the raw material used and the level of
commercial maturity.
•
First generation Photovoltaic systems (fully commercial) that use
the technology of crystalline silicon (c-Si) both in its simple crystal-
line form (sc-Si) as well as in the multicrystalline form (mc-Si).
•
Second generation photovoltaic systems are based on thin film
photovoltaic technologies and generally include three main families:
(1) Amorphous silicon (a-Si) and micro amorphous silicon (a-Si /μc-
Si); (2) cadmium telluride (CdTe); and (3) copper indium selenide
(CIS) and copper, indium gallium dieseline (CIGS).
•
Third generation photovoltaic systems include organic photovoltaics
technologies that are still in demonstration or have not been widely
marketed and new concepts in development.
There are some requirements for a solar cell material to be
considered ideal: bandgap between 1.1 and 1.7 eV, because the smaller
the gap, the easier it is to promote an electron from one band to the
other and thereby increase the conduction of this material; consisting
of readily available materials, non-toxic; easy fabrication technique,
Table 4
Applications of solar photovoltaics.
Applications Description Authors
Spacecraft Photovoltaic energy is converted into electrical energy to be applied in on-board equipment of the spacecraft.
The main technology used in this application are gallium arsenide cells which, despite having a high cost
compared to silicon cells, shows good efficiency
[8,17,22,29,30].
Water pumping Water pumping of wells and rivers used in farms for irrigation of plantations, for livestock and for domestic
consumption
[8,17,22,29–33].
Lighting street Used to illuminate parking spaces, signage and other outdoor areas. Photovoltaic panels are usually mounted
in the lighting structure or integrated in the pole itself and carry a rechargeable battery, which powers the
lamps. For installation there is no need to open ditches, wiring and similar preparations needed for
traditional lighting systems
[22,29–32].
Building integrated photovoltaic systems
( BIPV )
It is a set of photovoltaic systems and technologies that are integrated into the building, forming part of its
external covering like roofs and facades. Are considered as a functional part of the building structure, being
architecturally integrated into the building design. Simultaneously serving as building envelope material and
power generator
[8,16,29,30,33,34].
Telecommunications It is used in the generation of electricity in isolated telecommunication stations for the operation of
equipment such as communication radios, radio communication devices, telemetry stations, public
telephones, PLCs and video cameras. Provides reliability and low maintenance level
[8,17,29–32].
Water desalination Desalination (transformation of seawater into drinking water) is done using batteries charged during the day
with photovoltaic panels
[8,32].
Satellites Solar panels used in satellites are composed of solar cells located on the outer parts of satellites that can be
attached to the satellite body or open and oriented to the Sun. Three-junction solar cells are currently used in
series (called a triple junction)
[8,17]
With germanium base. Because of their location they are able to receive even more photons than the panels
installed on Earth and produce even more energy to keep the electrical equipment on the satellite running.
Weather monitoring The solar panel provides the energy required to power all measuring equipment, weather sensors, processing
and communication
[29,30].
Fig. 4. Typical System of photovoltaic solar energy.
P.G.V. Sampaio, M.O.A. González
Renewable and Sustainable Energy Reviews 74 (2017) 590–601
593
suitable for large production volumes; good photovoltaic conversion
efficiency; long-term stability. A material fulfilling all the requirements
has not yet been found
[18,19,40].
4.2.1. Silicon cells
Silicon is the most popular material in commercial solar cell
modules, accounting for about 90% of the photovoltaic cell market.
This success is due to several beneficial characteristics of silicon: (1) is
abundant, being the second most abundant element on Earth; (2) is
generally stable and non-toxic; (3) bandgap of 1.12 eV, almost ideally
adapted to the terrestrial solar spectrum, that is, the silicon is
sensitized within the range of electromagnetic spectrum emitted by
the sun; And (4) silicon photovoltaic cells are readily compatible with
the silicon-based microelectronics (transistor and integrated circuits
manufacturing) industry
[14,17,38].
The monocrystalline (m-Si), polycrystalline (p-Si, also referred to as
multicrystalline, mc-Si) cells are cells under the aegis of crystalline
silicon structures
[24,41]. Cells from a single silicon crystal are
cultured by the Czochralski process
[19,22]. These cells have excellent
conversion efficiency, however, they have high manufacturing costs,
higher energy requirements during their life cycle, longer energy return
time, and require the use of very pure materials (solar grade silicon)
and with the perfect crystal structure
[1,3,19,22,23,39,42].
The efforts of the photovoltaic industry to reduce costs and increase
the rate of production led to the development of new crystallization
techniques. In this way the cells based on multicrystals appeared. Such
technology is becoming more attractive because the cost of production
is lower, even though these cells are somewhat less efficient than
monocrystalline cells
[23]. In addition to lower manufacturing costs,
polycrystalline cells offer other advantages compared to monocrystal-
line cells, such as: better aesthetic appearance, less energy consumed
during its life cycle, shorter energy return time, lower Greenhouse
effect, requires less energy in its manufacture, the crystal structure does
not have to be perfect
[1,3,19,22,23,39,42].
Silicon cells are not restricted only to cells based on the crystal
structure. There are also silicon nanowire cells (SiNWs), which are
under intense investigation for photovoltaic applications, as they can
allow a new way of converting solar to electric energy with high
efficiency and low cost. This attractiveness is attributed to its original
geometric characteristics
[14].
Firstly, SiNW solar cells exhibit better optical absorption of the
solar spectrum, ie in comparison to other traditional technologies, it
requires less silicon to obtain the same amount of absorption. The
energy losses that occur when light passes through a photovoltaic cell
without being absorbed is smaller in silicon nanowire cells. Second,
SiNW solar cells allow the use of silicon of inferior quality to solar
grade silicon. Thirdly, SiNWs can be produced with excellent electrical
characteristics. These advantages can substantially reduce the cost of
production of SiNW-based solar cells by keeping these cells competitive
[14].
4.2.2. Thin film cells
In the search for cost reduction, the need for research on thin film
solar cells has arisen. Thin-film solar cells require much less material
from the semiconductor to be manufactured in order to absorb the
same amount of sunlight, up to 99% less material than crystalline solar
cells
[39]. The use of this technology has increased in recent years due
to its high fl
exibility, easy installation, diffuse light efficiency of
approximately 12% and a service life of 25 years
[16]. The main
approaches are based on amorphous silicon cells (a-Si);
Microamorphic silicon (a-Si / μc-Si); Cadmium telluride (CdTe);
Copper indium selenide (CIS) and copper, indium and gallium-
diselenide (CIGS).
The manufacturing methods are similar to those used in the
production of flat panel monitors for computer monitors, cell phones
and televisions. A thin photoactive film is deposited on a substrate,
which may be either glass or a transparent film. Then the film is
structured into cells. Unlike crystalline modules, thin film modules are
manufactured in one step. Thin film systems generally cost less to be
produced than crystalline silicon systems, but have substantially lower
efficiency rates. On average, thin film cells convert from 5% to 13% of
solar radiation into electricity, compared to 11–20% for crystalline
silicon cells. However, since thin films are relatively new, they may
offer greater opportunities for technological improvement
[39].
The first amorphous silicon (a-Si) publications relevant to the
manufacture of solar cells appeared after the 1960s. The first amor-
phous silicon solar cell was reported by Carlson in 1976. In the market
the same arose in 1981. The high expectation In this material was
contained by the relatively low efficiency obtained so far and by the
initial degradation induced by light
[17,18,43].
This technology diverges from crystalline silicon in the fact that the
silicon atoms are located at random with each other. This randomness
in the atomic structure has an important effect on the electronic
properties of the material, causing a larger gap (1.7 eV) while that of
crystalline silicon is 1.1 eV
[23].
Another configuration is the microamorph silicon cells, which
combine two different types of silicon, amorphous and microcrystal-
line, one on top of the other in a single device, where the upper layer
consists of an ultra thin layer of a-Si, which converts The shorter
wavelengths of the visible solar spectrum and the lower layer have the
microcrystalline silicon which is most effective in converting the longer
wavelengths. This results in higher efficiency gains of about 8–9% more
than amorphous silicon cells depending on the cell structure and the
thickness of the layers
[23,44].
One of the most promising approaches to manufacturing low cost
and high efficiency involves the use of cadmium telluride. The CdTe has
been known to have the ideal gap (1.45 eV) with a high coefficient of
absorption of the solar spectrum being one of the most promising
photovoltaic materials for thin
film cells. However, the toxicity of
cadmium (Cd) and environmental issues related to the use of this
material pose a problem for this technology. Therefore, First Solar, one
of the world's largest manufacturers of photovoltaic solar modules, has
launched a recycling program for deactivated PV cells, extremely
popular in the field of thin films because of the efficiency of its process,
which has the capacity to reduce the Cost of production to make the
cost of this technology more competitive. The other potential problem
is the availability of Te, which can lead to scarcity of raw materials, thus
affecting the cost of the modules
[17,23,43–46].
Copper and indium diselenide (CuInSe
2
) or indium copper selenide
(CIS), as is sometimes known, and copper-indium-gallium selenide
(CIGS) are photovoltaic devices containing semiconductor elements of
groups I, III and VI of the periodic table which are Beneficial because of
their high optical absorption coefficients and their electrical character-
istics that allow the adjustment of the device [17,23,44]. Some of the
major challenges of these technologies have been limited ability to
expand the process of high yield and low cost, degradation under wet
conditions, as it promotes changes in the properties of the material and
the shortage of Indian in nature
[23,42,47].
4.2.3. Organic photovoltaic cells
Organic photovoltaic cells offer the long-term potential of achieving
the goal of a PV technology that is economically viable for large-scale
power generation
[3], since organic semiconductors are a less expen-
sive alternative to Than inorganic semiconductors, such as silicon. In
addition, organic molecules can be processed by simpler techniques
that are not suitable for crystalline inorganic semiconductors [21,48].
Almost all organic solar cells have a flat layered structure, wherein
the light absorbing layer is sandwiched between two different electro-
des. One of the electrodes has to be (semi) transparent, the indium tin
oxide (ITO) is normally used, however a thin layer of metal can also be
used. Calcium, magnesium, gold and aluminum can also be used as
electrodes, the latter being the most used
[21,48].
P.G.V. Sampaio, M.O.A. González
Renewable and Sustainable Energy Reviews 74 (2017) 590–601
594