This report is relate to topic of Flexible Solar Cell. In this report you get content is introduction, introduction to flexible solar cell, types of solar cell, types of flexible solar cell, application n etc.

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CHAPTER-1
INTRODUCTION
Mechanically flexible solar cells could drastically change the way energy is generated in the future.
Some of the applications include use in high altitude and space environments for
telecommunication purposes, integrated cells for building energy, use as the primary energy source
in soft robotics, and even on clothing to charge a smartphone.
To create a more flexible solar cell there needs to be a compromise between thickness, mechanical
resilience, and durability. Efforts in advancing the technology of solar cell devices have been
primarily concerned with cost and efficiency of the cells. High device cost and preparation required
to fabricate inorganic solar cells, which are most frequently used, have limited the overall impact
that solar energy can have.
The most common inorganic solar cell type is made using crystalline silicon as the semiconductor
layer, which is separated into two layers of different types, positive and negative (p and n). The
semiconductor layer of this cell is sandwiched between a top cathode and bottom anode layer,
where the cathode has metal connections placed onto it and the anode layer is attached to a metal
contact, so that the cell can be wired into a circuit. This basic construction is constant for all major
cell types, including CdTe, CIGS, CIS, dye sensitized, polymer, and perovskite cells. Because of
how broad improving cost and effectiveness is for making better solar cells, many avenues of
composition and construction have been researched for all cell types.
An alternative way of making solar more widely accessible is to create a versatile solar cell that
can be implemented in more places. The inorganic solar cells we created is a type of thin film solar
cell that can be used in mechanically flexible applications, creating further options where solar
cells can be used. Furthermore, because our cell is completely inorganic it has increased stability.
This type of solar cells differs from silicon solar cells first in that the cell layers are constructed
using deposition, creating a thinner, lighter, and as previously stated flexible cell. Secondly this
cell type is different because the p and n type layer are made from different classes of material,
with the p-type being organic and the n-type inorganic. This helps to create a simpler cell
construction overall which aids in creating a more flexible device. Although much research has

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been done on improving the semiconductor layer, changes to the other layers in the cell structure
have been considered less thoroughly and can likely be improved to increase flexibility and
efficiency. Thin film solar cell or flexible solar cells are considerably less expensive to
manufacture than the traditional Photovoltaics, and thus opened a new era of photovoltaic business.
Thus, the old fragile, heavy are more expensive glass-coated silicon panels are being replaced by
flexible solar cells. Actually, photovoltaics and the flexible solar cells are advancement of Nano
chemistry. It was forecasted that; thin film solar cells are the ultimate future of industrial
photovoltaics by the inventors of silicon solar cells in 1954.
Fig. 1.1. Global annual PV production (source: http://www.webcitation.org/6SFRTUaBS)
Fig. 1.2. Annual global market shares of thin-film technologies.
(source:http://www.webcitation.org/6SFRTUaBS)

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In this context, the installation of thin-film systems more than doubled last year and they now
account for some 12% of solar installations around the world. Thin-film (TF) photovoltaic modules
are less expensive to manufacture than traditional polysilicon-based panels and have considerably
lowered the barrier to entry into the photovoltaic energy business. The sector is thus rapidly
switching from the heavy, fragile glasscoated silicon panels to thin-film technologies which use a
number of different photovoltaic semiconductors, and the revenue market share of TFPVs is
expected to rise to 20% of the total PV market by 2010 (Figure 1.3).
Fig 1.3:- Forecast of the photovoltaic market with breakdown per technology, pointing to an annual growth
rate of 70% for thin-film photovoltaics from 2007 to 2010 (source: Yole).Legend (from top to bottom): organic
(pale green), DSSCs (brown), CdTe (pale blue), IIIV (orange), CIS/CIGS (midblue), a-Si/m-SI (purple), a-Si
(green), Si thin wafer (red), Si wafer based (dark blue).

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CHAPTER-2
LITERATURE SURVEY
A photovoltaic power generation system consists of multiple components like cells, mechanical
and electrical connections and mountings and means of regulating and/or modifying the electrical
output. These systems are rated in peak kilowatts (kWp) which is an amount of electrical power
that a system is expected to deliver when the sun is directly overhead on a clear day. A grid
connected system is connected to a large independent grid which in most cases is the public
electricity grid and feeds power into the grid. They vary in size from a few kWp for residential
purpose to solar power stations up to tens of GWp. This is a form of decentralized electricity
generation. Poponi assessed the prospects for diffusion of photovoltaic (PV) technology for
electricity generation in grid-connected systems by the methodology of experience curves that is
used to predict the different levels of cumulative world PV shipments required to reach the
calculated break-even prices of PV systems, assuming different trends in the relationship between
price and the increase in cumulative shipments. The following papers have been referred for this
seminar and report drafting:-
[1]. Rehman et al. utilized monthly average daily global solar radiation and sunshine duration
data to study the distribution of radiation and sunshine duration over Saudi Arabia and also
analyzed the renewable energy production and economical evaluation of a 5 MW installed capacity
photovoltaic based grid connected power plant for electricity generation.
[2]. Al-Hasan et al. discussed optimization of the electrical load pattern in Kuwait using grid
connected PV systems as the electric load demand can be satisfied from both the photovoltaic
array and the utility grid and found during the performance evaluation that the peak load matches
the maximum incident solar radiation in Kuwait, which would emphasize the role of using the PV
station to minimize the electrical load demand and a significant reduction in peak load can be
achieved with grid connected PV systems.
[3]. Ito et al. studied a 100 MW very large-scale photovoltaic power generation (VLS-PV) system
which is to be installed in the Gobi Desert and evaluated its potential from economic and

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environmental viewpoints deduced from energy payback time (EPT), life-cycle CO2 emission rate
and generation cost of the system.
[4]. Zhou et al. performed the economic analysis of power generation from floating solar chimney
power plant (FSCPP) by analyzing cash flows during the whole service period of a 100 MW plant.
[5]. Muneer et al. explored the long-term prospects of large-scale PV generation in arid/semi-arid
locations, around the globe and its transmission using hydrogen as the energy vector.
[6]. Curnow et al. described the megawatt plant at the new Munich Trade Fair Centre that
represents a significant advance in large PV plant technology, both in terms of system technology
and the components employed, operational control and costs.
[7]. Bhuiyan et al. studied the economics of stand-alone photovoltaic power system to test its
feasibility in remote and rural areas of Bangladesh and compared renewable generators with
nonrenewable generators by determining their life cycle cost using the method of net present value
analysis and showed that life cycle cost of PV energy is lower than the cost of energy from diesel
or petrol generators in Bangladesh and thus is economically feasible in remote and rural areas of
Bangladesh.
[8]. Alazraki and Haselip assessed the impact of small-scale PV systems installed in homes,
schools and public buildings over the last six years under the PERMER (Renewable Energy Project
for the Rural Electricity Market) co-funded by a range of public and private sources and the
structure of financial subsidies has enabled these remote rural communities to receive an electricity
supply replacing traditional energy sources.
[9]. Kivaisi presented the installation and use of a 3 kWp photovoltaic (PV) plant at Umbuji
village, in Zanzibar, Tanzania that was intended to provide power supply for a village school,
health centre, school staff quarters, and mosques.
[10]. Bansal et al. developed an integration of solar photovoltaics of 25 kWp capacity in an
existing building of the cafeteria on the campus of the Indian Institute of Technology, Delhi by
creating a solar roof covering with the photovoltaic array inclined at an angle of 15◦ from the
horizontal and faces due south.

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[11]. Ubertini and Desideri studied a 15 kWp photovoltaic plant and solar air collectors coupled
with a sun breaker structure that was installed on the roof of a scientific high school.

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CHAPTER-3
PHOTOVOLTAIC TECHNOLOGY: A REVIEW
Photovoltaic (PV) systems convert sunlight into electricity. Once an exotic technology used almost
exclusively on satellites in space, photovoltaic has come down to earth to find rapidly expanding
energy markets. Many thousands of PV systems have been installed around the globe. PV devices
can be made from many different materials in many different designs. The diversity of PV
materials and their different characteristics and potentials demonstrate the richness of this growing
technology. They also explained about PV effect. Because PV occurs through PV effect.
Primary unit of PV system solar cell, it is known as PV cell. PV effect was observed in 1839 by
the French scientist Edmund Becquerel. Most PV cells in use today are silicon-based, cells made
of other semiconductor materials are expected to surpass silicon PV cells in performance and cost
and become viable competitors in the PV market place. PV technology uses the semiconductor
materials to design the PV system, solar cells are collectively arranged into modules and modules
are arranged together to form panels or arrays. Mainly three types of PV technology such as
crystalline, thin film and nano- technology. PV technology is and is suited to a broad range of
application and can contribute substantially to our future energy needs. The basic principles of PV
were discovered in the 19th
century. It was not before the 1950s and 1 adfr960s that solar cells
found practiced use as electricity generators, a development that came about through early silicon
semi9conductor technology for electronic applications.
PV technology describes through the generations. First generation used crystalline silicon, second
generation used the thin film and third generation used conductive organic molecules to design
organic cell.
The aim to continuous development of PV technology through the generations is not only to
improve the efficiency of the solar cells but also to reduce the production cost of the modules and
arrays. Moreover such variety in technology is needed to enhance the deployment of solar energy
for a greener and clean environment.

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3.1 Concept of Photovoltaic Technology:-
A solar cell (also called photovoltaic cell or photoelectric cell) is a solidstate electrical
device that converts the energy of light directly into electricity by the photovoltaic effect. Which
is a physical and chemical phenomenon. It is a form of photoelectric cell, defined as a device
whose electrical characteristics, such as current, voltage or resistance, vary when exposed to light.
Since the first solar cell was produced by Bell Labs in the 1950s, solar photovoltaic (PV)
technology has been gradually evolving. The work resulted in the development of a compound
which is formed of semiconductor elements found in the periodic table and the synthesis of
an organic solar cell. Broadly, photovoltaic technologies are now classified as: crystalline silicon
solar cells, thin-film solar cells, and organic solar cells. In the following paragraphs, an overview
of various concepts in photovoltaic technology based on crystalline silicon wafers are briefly
described. Such concepts were used from the early 1990s to deliver relatively high-efficiency solar
modules for the market. As the $/watt of a solar panel is dropping, the evolution in photovoltaic
technology is also progressing.
Fig 3.1.1:- Schematic diagram of elemental photovoltaic solar cell

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The following are the different types of solar cells.
 1st
Generation Solar Cell:- The cell consists of a large-area, single-crystal, single layer p-
n junction diode, capable of generating usable electrical energy from light sources with the
wavelengths of sunlight. In the past, the overwhelming majority of cells have been fabricated using
silicon wafers, as used in microelectronics, as the starting material and a screen printing technology
for depositing the metal contact, giving the final cell structure shown in figure 3.1.2.
Fig 3.1.2:- Standard Screen Printed Solar Cell
The cells are typically made using a diffusion process with silicon wafers. There are following
types of 1st
generation solar cells:-
 Monocrystalline solar cell (mono-Si)
 Polycrystalline solar cell (multi-Si)

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A survey of the manufacturers nominal efficiency of a range of first generation commercial
Modules is shown in 3.3.1.
Fig. 3.1.3:- Survey of first generation module efficiencies (from manufacturer’s data at standard test
conditions)
 2nd
Generation Solar Cell:- A thin-film solar cell is a second generation solar cell that
is made by depositing one or more thin layers, or thin film (TF) of photovoltaic material on a
substrate, such as glass, plastic or metal. Amorphous silicon technology and wafer-based
technology. Cell efficiencies are similar to the production values of buried contact cells, although
module efficiency is higher due to denser packing in themodule.
Fig 3.1.4:- Structure of Second Generation Solar Cell (Taken as Reference)

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Since the thickness of the semiconductor material required may only be of the order of lm,
almost any semiconductor isinexpensive enough to be a candidate for use inthe cell(silicon
is one of the few that is cheap enoughtobeusedasaself-supportingwafer-basedcell). Many
semiconductors have been investigated, with five thin-film technologies now the focus of
commercial development.
Fig.3.1.5:- Survey of efficiency of second generation thin-film solar modules (from manufacturer’s data at
standard test conditions)
There are following types of 2nd
generation solar cell:-
 Amorphous Silicon solar cell (a-Si)
 Thin-film solar cell (TFSC)
 Cadmium telluride solar cell (CdTe)
 Copper indium gallium selenide solar cells (CI(G)S)
 3rd
Generation Solar Cell:- Third generation solar panels include a variety of thin film
technologies but most of them are still in the research or development phase. Some of them
generate electricity by using organic materials, others use inorganic substances (CdTe for
instance).
There are following types of 3rd
generation solar cell:-
 Biohybrid solar cell
 Concentrated PV cell (CVP and HCVP)
 Multi-junction solar cell (MJ)

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The basic structure of 3rd
generation solar cell is shown in following figure:-
Fig 3.1.6:- Structure of 3rd
Generation Solar Cell
Fig3.1.7:- Efficiency and cost projections for first-, second-, and third-generation photovoltaic technology
(wafers, thin films, and advanced thin films, respectively).

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3.2 Flexible Solar Cell:-
A flexible solar cell which is also known as thin film solar cell that is made by depositing very
thin layers of photovoltaics material on any kind of substrate, such as, paper, tissue, plastic, glass
or metal. It is one of the most revolutionary and epoch making technologies in the sector of solar
energy. The significance of the word “flexible” is that, these kind of solar cells are not like those
traditional big, bulky solar panels which is very common nowadays, these are literally flexible,
very thin, lightweight, have very little installation cost and can be installed anywhere without going
much trouble. Thickness of a typical cell varies from a few nanometers to few micrometers,
whereas its’s predecessor crystalline-silicon solar cell (c-Si) has a wafer size up to 200
micrometers.
Fig. 3.2.1 Flexible solar cells being manufactured in a 3-D printer. (screenshot ©youtube.com)

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These flexible solar panels have a lot of advantages over their counterpart (cSi). Being thin and
lightweight, considerably more flexible panels could be installed within a definite area with respect
to the traditional bulkier panels without thinking much about any extra
installation and the cost. That is, flexible solar panels can be installed without any extra brackets
and mounting devices making it more aesthetically pleasing. Again, since the flexible solar panels
can cover every inch of a surface and are installed directly on that surface so, these are less prone
to wind damage.
Flexible solar cells are cheaper but less efficient than the c-Si technology. The reason behind being
cheap is that, flexible solar cells require much less silicon and other necessary materials to produce
a panel that is capable of producing the same amount of energy as a c-Si panel. Though their
performance had been enhanced manifold in the last couple of years in comparison to c-Si. Even
it has been possible to obtain efficiencies more than 21% in the laboratory which is more than
multi c-Si. Multi c-Si is the leading technology that’s been being used in most solar PV systems.
Even with lesser efficiency than c-Si technology, flexible solar cells costs less amount of money
to buy, and also can be installed in more within the same area. That is, maybe not more but the
same amount of energy can be obtained from a definite area without thinking much about the
installation cost.

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3.3 Flexible Photovoltaic Cell: Their Working Principle & Fabrication
In order to design the best possible flexible solar cell, the basic working principles of the cell
must first be understood. The methods of light absorption, charge separation, and charge transfer
must be determined to be able to take advantage of all the aspects needed to increase efficiency
and flexibility. Beyond these basic working principles, the various state of the art flexible panels
must be investigated. The methods being used by others in the field will give insight into options
that work well,and ones that do not, and ones that have been explored thoroughly.
3.3.1 Working Principle:-
All types of solar panels follow a basic layout that consists of a series of layers that work
together to allow electrons to flow through a functional circuit. The cathode is the conductor closest
to the side of the p-type semiconductor layer, and is usually made from a metal in a grid like
pattern, although our cathode is a solid metal film of Au/Pd. Below this layer lies the two
semiconductor layers. The semiconductors are typically separated into two layers, called n and p
type semiconductors, n standing for negative and p for positive. Usually to obtain the two layers
of a semiconductor the material will need to be doped. There are also materials that act as intrinsic
n or p type semiconductors that do not need to be doped. Doping introduces a small amount of an
alternate element into the main semiconductor material. To make the n type layer of the
semiconductor the element that is introduced into the main structure has more valence electrons to
create free electrons and the p type semiconductor layer has less valence electrons, in order to
create vacancies ("holes") for the free electrons to occupy. These are the layers where the charge
is separated and transported. These layers can consist of a variety of materials which differ in many
of the major types of solar cells that exist today. A common example is crystalline silicon in which
one layer is doped to promote charge movement and the other layer is doped to become a charge
receiver. The back- contact acts as the anode and finishes off the circuit. The anode layer of thin
film solar cells is especially important in flexible solar cells, because it can often be the limiting
flexible layer. Two of the primary options are Indium Tin Oxide (ITO) and Aluminum Doped Zinc
Oxide (ZnO-Al).

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3.3.2 Working of Flexible Solar Panels:-
Function and structures are very closely linked with each other. So, let’s have a look on the
structures and functions of a flexible solar cell. The main working principle behind the flexible
cells are more or less similar to the traditional silicon cells.
Semiconductors (such as silicon) are the main theme of photovoltaic cells. They are crystalline
and amorphous with distinct electrical properties [19]. Usually with a very high resistance, they
are insulators in their pure state. But, they can conduct electricity depending on its temperature or
when they’re fused with other materials. This fusion or mixing of semiconductors is known as
doping. Traditional solar cells use silicon as the semiconductor. In its crystalline form silicon has
some special characteristics. It has 14 electrons arranged in three different shells. The electron
distribution in three shells are 2,8,4 respectively. The last shell being half filled, in order to fulfill
its octet, silicon needs four more electrons. They do so by sharing 4 electrons from other atoms
.
Fig 3.3.2.1 i) Structure of Silicon
ii) Silicon Covalent bond Structure
The structure looks like that; each silicon atom has four hands joined to four other atom’s hands.
It is the crystalline structure. But, there’s a problem. Pure crystalline silicon is a poor conductor

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due to its complete octet. There are no free electrons to flow. So, in order to increase its
conductance, some impurities are added, which changes the way everything works. We know that,
this is known as doping. When doped with phosphorus, which has five electrons in its last shell,
the four electrons are normally shared with four other of the neighboring silicon atom. But, the
remaining electron of each phosphorus atom acts as free electrons thus developing a lot of free
electrons. And when sufficient energy is added, the single electrons from each phosphorus can
break free from its shell creating a hole. These electrons are known as free carrier which flows
inside the crystal lattice and looks for another hole to cover and thus creating electricity. It is
known as an n-type semiconductor.
Similarly, when doped with materials like, boron, then excess numbers of positively charged Holes
are created. To be more specific, holes are just spaces that accepts electrons, that is holes give a
scope for the electrons to flow. It is known as p-type semiconductor.
Now, an interesting thing happens when these n-type and p-type silicon come in contact. An
electric field forms and all the electrons from the n-type part to fill the holes of the p- type
semiconductor. But, all the free electrons can’t fill all the holes. They form a barrier at the junction
of the two semiconductors, which doesn’t let all the electrons to cross. This fusion of ptype and n-
type is known as a diode, which only allows electrons to flow from p-type to n-type and blocks in
the other. It is just like a check valve.
Whenever light, i.e. Photon hits a solar cell, it breaks the electron-hole pairs. That is each photon
will release one electron leaving the wholes. Then the electrons are sent to the N-side and holes to
the p-side. Thus further disruptions are created to the electrical neutrality and if any external path
is provided, then the electron will flow through the path to join the holes in the P-side. Since,
electron flow is current so, thus electricity is produced. The electric field of the cell provides the
voltage.
Above was the description of a traditional solar cell. Thin film solar cells and traditional
photovoltaic cells are similar in structure and function. The difference is in the arrangement of
various layers in the cells of each type and the semiconductors of the cell. Flexible solar cell uses
thin layers of either cadmium telluride (CdTe) or copper indium gallium deselenide (CIGS) or,
amorphous silicon (a-Si) instead of crystalline silicon (c-Si). Flexible solar cells (CIGS) on a glass

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substrate requires an extra layer of Molybdenum to make an effective electrode, but, cells with
metal substrate do not need this layer as they act as electrode. A layer of Zinc-oxide at the top acts
as the other electrode. Between these two are the semiconductor and Cadmium sulfide layer, which
acts as the p-type and n-type material. In case of the CdTe cells, a layer of carbon paste fused with
copper and Tin Oxide (SnO2) or, Cadmium Stannate (Cd2SnO4) acts as the two electrodes. And
in place of CIGS the semiconductor used is Cadmium Sulfide (CdS). The other semiconductor is
the same as that in case of CIGS.
Fig.3.3.2.2:- On the Left-Thin film solar cell (CIGS) on a metal substrate (source: © 2008 HowStuffWorks)
On the right, thin film solar cell on a Kapton (Polyamide film) substrate (source: © 2015 Circuits Today)

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3.3.3 Fabrication / Manufacturing Process:-
The manufacturing process of the traditional c-Si solar panels are very time-consuming and
complex and it drives-up the per-watt cost of electricity. Whereas, the manufacturing of flexible
solar cells is comparatively easier. A company named Nanosolar produces flexible solar cells by
the application of a process named, offset printing. It is one kind of printing technique where an
inked image is transferred from a plate to a rubber plate, then to the required printing surface. The
process followed by the Nanosolar company is more or less as described below:
1.Reams of Al (aluminum) foils come out through very large presses which are similar to those
used in newspaper printing. The foils are really long in size making them much more versatile in
case of application.
2.Thin layer of semiconductors is deposited on the aluminum foil by a printer in an open
environment. This open environment printing has advantages over the CIGS-on-glass or CdTe cell
manufacturing in which the printing is done on a vacuum chamber. This vacuum printing is both
expensive and time consuming. Presses used in this step are very easy to handle and very little of
the printing material is wasted which increases the overall efficiency.
Fig. 3.3.3.1. An engineer working at the press and printing the semiconductor.(©NANOSOLAR)

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Fig. 3.3.3.2. Schematic of the manufacturing process of a thin film solar cell. (source: ©2017 Nanosolar
Corporation)
3.The second semiconductor i.e., CdS is printed by another press followed by the printing of the
ZnO layer.
4.Finally, the foil is cut according to required sizes and shape.
Fig 3.3.3.3:- manufacturing thin films through rolling

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3.4 Classification of Flexible Photovoltaic Cells:-
Below we will discuss some of the various semiconductors in use in the field.
3.4.1 Crystalline Silicon Cells:-
Crystalline silicon cells make up 90% of the solar panels that exist in the world today. Within
the distinction of crystalline cells there are two types, which are monocrystalline and
polycrystalline. Monocrystalline cells are formed into a single crystal from ingots and have
efficiencies ranging from 15%- 20%, while polycrystalline panels are simpler to manufacture they
also have low efficiencies, ranging from 13%-16%. On top of being more efficient,
monocrystalline panels are also more space efficient, longer lasting when compared with other
types of cells, and more efficient in higher temperatures. The last type of silicon cell is amorphous,
which while being the least common silicon panel type, is most suitable for thin filmed flexible
panels. These types of cells are formed from using vapor deposition to create a thin layer of silicon
on a substrate made of metal, glass, or plastic. The thickness of these cells is roughly 1/300 the
size of a monocrystalline silicon cells. The efficiency is around 7% due to the difference in
structure of the silicon.1
Fig 3.4.1:- Crystalline Solar Cell

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3.4.2 Copper Indium Gallium Selenide:-
Cells Copper Indium Gallium Selenide (CIGS) is one of the many options being explored in
the field of flexible cells. The CIGS layer acts as the p-type semiconductor in the panel, where it’s
supreme photon absorption coefficient helps to trap as much energy as possible, and is coupled
with Cadmium Sulfide (CdS) as the n-type semiconductor. Besides its good absorption properties,
what makes CIGS so good for flexible solar panels is how thin it can be layered. This ability to
make the semiconductor only 2-4 μm thick allows the panels to be super flexible, when printed
on a flexible material. MiaSolé, a company specializing in CIGS panels, prints their panels on a
thin stainless steel back sheet. This allows the panels to be very flexible in all directions, however,
there is still the possibility of putting a crease in the stainless-steel sheet if bent too far. MiaSolé’s
panels are some of the most efficient on the market currently though, achieving almost 17%
efficiency.
Fig 3.4.2:- Copper Indium Galium Selenide Solar Cell
3.4.3 Cadmium Telluride (CdTe) :-
Cells Cadmium Tellurium cells are the second most common solar cells after silicon cells.
These cells are relatively easy to manufacture and have lower costs than silicon solar cells. The
efficiency of these cells can reach over 22% and in commercial applications have been reported at

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over 16% efficiency.Most cells are produced as a single p-n heterojunction structure with a p-type
CdTe layer and most often an n-type layer of cadmium sulfide (CdS). These materials are direct-
band gap materials, with a band gap energy of close to 1.45 eV, which allows for high absorption
rates because the band gap energy is well matched with the solar spectrum. CdTe cells can be
produced via thin film deposition thus allowing for a more flexible solar cell. Construction of CdTe
cells is typically completed by adding a TCO and a copper back contact. Although the copper back
contact is frequently added Cu atoms can diffuse into the semiconductor and accumulate in the p/n
junction, creating defects within the cell layering. The highest efficiencies of CdTe Cells have
been reached through Cadmium Chloride (CdCl2) vapor treatment environment at temperatures
well above 300 °C. Although this vapor treatment has led to much greater efficiencies, for flexible
applications of this panel type annealing at temperatures above 100°C would lead to ruining the
substrate, and nixing the efficiency of the cell.. This process takes place before the back contact is
added on and in an oxygen-rich.
Fig 3.4.3:- Cadmium Telluride Solar Cell
3.4.4 Perovskite Cells :-
Beginning in 2009, methylammonium lead halide perovskites were synthesized and after
testing, were found to have good absorber qualities for use in solar cells. Perovskite was initially
heralded as an attractive option because of the ease with which it could be prepared and processed.
Originally used as a sensitizer in dye-sensitized cells, an efficiency of only 3.8% was obtained.

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The reason for the efficiency being low in these tests were because some of the perovskite would
form solids within the electrolyte.8 Perovskite solar cells have an average efficiency of 7% under
laboratory conditions. In some cases, certain solar cells have reached efficiencies of over 20%.
They can be made with PET conductive substrates to complete the inner circuit of the cell. All
manufacturing processes except for the annealing of buffer layers, manufacturing can be done with
a roll to roll process as well. The manufacturing of these cells is relatively easy compared to some
other types of cell. The perovskite material is also flexible; the cell declines only 0.1% after fifty
bending cycles. Despite the obvious benefits to this cell type there are still safety issues due to the
cells lead content and questions of its stability in actual devices. With stability as a major concern,
applying this type of cell for mechanically flexible applications could lead to durability issues and
drastically lower efficiencies after bending.
Fig 3.4.4 Pervoskite Solar Cell
3.4.5 Bi2S3 Solar Cells Bi2S3
Nanocrystals act as the n type semiconductor in thin filmed cells. The efficiency of these cells
are roughly one percent making them less than ideal, but this cell type is being researched and
improved constantly. These nanocrystals are relatively easy to manufacture, when compared to the
single crystal solar cells that exist today. These cells can be produced via solution processing so
that the thin film may act as the electron acceptor in the solar cell.One of the reasons that Bi2S3 is
gaining attention in solar, is due to its relatively large band gap.Depending on the structure and
processing of Bi2S3, the band gap can be up to about 1.8 eV. The band gap gets lower as the Bi2S3

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layer gets thicker, and is lower if Bi2S3 is in its crystalline form. It was observed that the band gap
decreased from 1.8 eV for a 50-nm thick, amorphous layer to 1.5 eV for a 200-nm thick, semi-
crystalline layer.16 This larger band gap at a thinner, amorphous stage could be promising for
flexible applications, as the thinner profile will help with flexibility. Additionally, not needing to
anneal the cell is important as high temperature processing methods could cause damage to any
flexible substrate or other thin layer that is being used.
Some work has been done recently on Bi2S3 solar cells by D. B. Salunkhe et al., where TiO2/
Bi2S3 thin film panels were created using Successive Ionic Layer Absorption and Reaction
(SILAR). These cells were created by first layering TiO2 onto Fluorine Tin Oxide (FTO) sheets,
and then depositing Bi2S3 nanoparticles onto the TiO2 using the SILAR method; a polysulphide
electrolyte and a platinum coated FTO sheet finished off the cell structure. SEM images indicated
that there was complete coverage of the TiO2 by the Bi2S3, indicating that there should be optimal
ohmic contact. The coverage improved as more SILAR cycles were performed, and efficiency
increased as well. The best efficiency achieved, after undergoing 25 cycles, was only 0.148%. This
low efficiency was attributed to either poor charge transport between the TiO2 and Bi2S3 layers,
or to the non-suitability of the polysulfide electrolyte that was used.
Fig 3.4.5:- Bi2S3 Solar Cell
3.4.6 Bulk Heterojunction Solar Cells (BHJ) :-
P3HT:PCBM solar cells are a type of bulk heterojunction cell where there is no separation
between the n and p-type layers of the semiconductor. Rather the n and p layers of the cell are a
random mix of the two semiconductors, with more of the n type material at the top of the
semiconductor and more of the p type material at the bottom. The full names of P3HT and PCBM

26. 26
are poly-3-hexylthiophene and [6,6]-phenyl C61 butyric acid methyl ester, respectively. Per one
study in France, a weight ratio of 1:1 of the polymers yielded the best efficiencies in the cells.
These cells are made on an ITO substrate and the substrate is spin coated with 50 nm of
poly(3,4ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS). Then the active layer of
P3HT:PCBM is deposited by spin-casting from n anhydrous chlorobenzene solution and then the
cathode is deposited on. From there the cells go through thermal annealing at 100°C.18
Crystallinity is an important factor that dictates the optical absorption of the P3HT nanodomains,
this is shown when the cell is annealed properly as the efficiency of the solar cell is greatly
increased.19 To add to this, there are many other factors that can affect the efficiency of these
cells, including structure and ratio of P3HT to PCBM. The efficiency of this organic solar cell
highly depends on the 3D morphology of the P and N layers that create the charge separation. The
ideal layout for a solar cell with P3HT and PCBM is to create a . fingered morphology shown in
Figure 2b, whereas Figure 2c is a more realistic result. However, since these cells are just
nanometers in thickness, it is difficult to create the desired effect. With electron tomography, the
blend morphology and crystallinity can be studied to create more efficient cells.
Fig 3.4.6:- Bulk Hetrojunction Solar Cell

27. 27
Table 3.4.1:- Cell efficiencies measured under the global AM1.5 spectra (1000 W/m2
) at 25 °C (IEC60904-3:
2008, ASTM G-173-03 global).
Classification
Efficiency
(%)
Area
(cm2
)
Fill
Factor*
(%)
Test Center
And date
Manufacturer and
substrate
CIGS (cell) 20.5 ± 0.6 0.752 77
NREL
(3/14)
Solibro, on glass
[31]
CIGS (mini- module) 18.7 ± 0.6 15.892 75.6
FhG-ISE
(9/13)
Solibro, 4 serial
cells [32]
CdTe (cell) 21.0 ± 0.4 1.0623 79.4
Newport
(8/14)
First Solar, on
glass [33]
Si (amorphous) 10.2 ± 0.3 1.001 69.8 AIST (7/14) AIST [34]
Si (microcrystalline) 11.4 ± 0.3 1.046 73.1
AIST (7/14) AIST [35]
Dye 11.9 ± 0.4 1.005 71.2 AIST (9/12) Sharp [36]
Dye (mini- module)
10.0 ± 0.4 24.19 67.7
AIST (6/14) Fujikura/Tokyo U.
Science [37]
Organic thin-film 11.0 ± 0.3 0.993 71.4 AIST (9/14) Toshiba [38]
*fill factor is the measure of the quality of a cell. It can be found by comparing the maximum Power to
the theoretical power that is the output at open circuit voltage and short circuit current

28. 28
3.5 Research & Development
3.5.1 Feather-Light Solar Cells:
The endless possibilities of the flexible solar sector are astounding. From large scale embedded
PV fabrics to micro granular cells, all are within our grasp.
In the recent times, the researchers at MIT have discovered demonstrated solar cells so lightweight
and thin, it can stay stable even on a helium balloon. This discovery has led many to believe that
efficient yet light solar cells might be possible after all.
The new process is described in a paper by MIT professor Vladimir Bulović, research scientist
Annie Wang, and doctoral student Joel Jean, in the journal Organic Electronics.
The future of solar energy depends on the innovations and applications of new and old
technologies. If photovoltaic (PV) devices that turn light into electricity could be mass produced
with printing presses and eligible for the mass people, as if they were newspapers or banknotes or
any kind of paper or fabric products, they could be affordable and ubiquitous.

29. 29
3.5.2 Solar Fabrics
The process of turning solar panels into attires has already started. Several clothing lines have
started to market their own “Solar Fabrics”. Konarka Technologies produce a thin film polymer
based PV cell, as a flexible film stitched onto a fabric. The ability to make these cells even smaller
is dependent on further research into nano-crystal PV cells. In theory nano-technology could
provide a way to expand the range of photons a cell could collect, increasing its efficiency while
becoming smaller. Konarka, in partner Leading Swiss University, is working on this.
The days of silicon based solar cells are almost over. They were expensive due to their
Conventional, silicon-based, solar panels are rigid, expensive and hard to handle. Small, thin and
flexible PV devices on films are already being made that are lightweight and translucent. These
material can generate electricity in low light, even indoors. Integrating them into phones and
watches, as well as walls and windows, would transform the world's energy generation, reduce
pollution and near future the solution for global warming.
Now-a-days organic PV cells are up and coming. They are extremely light weight and
easilymanageable. They do not need special expertise to be installed or altered. It’s a technology
for the mass people.

30. 30
3.5.3 Multi Layered Thin Films
Now a single layer of depleted protons or free electrons transport less electricity than a multi-
layered cell. The main idea is to stack up cells to increase the band gap spectrum. So that if the
upper levels miss any spectrum due to its band length, the lower level will suck up the spectrum.
So the best possible result can be got when multiple layers of cells will work at an optimal energy
spectrum. The connection of the layers can be controlled according to demand It can be series or
parallel or others.
To create a multi-layered thin film solar cell, the lattice structure in each of the solar cell needs to
be identical. Otherwise there will be losses. The research on developing lower quality materials
that that use cheaper deposition process are being done. These devices might not be as efficient as
expensive ones, but the price, size and power ratio are cost effective. The mass production of these
devices are much feasible.
3.5.4 Multiplicity of Phonon Modes
The Brillouin zone, the minimal area where a super lattice allows a wider range of phonons to
become optically active at or very close to the center zone causing dispersion of hot electrons
causing a rise in the voltage.

31. 31
One of the most important points of the hot carrier cells is the collection of carriers through ESC
of a narrow width. The complex structure of the lattices enable the hot carrier cells to remain hot
for a longer period of time.
3.5.5 Slowed Carrier Cooling:-
Through slow carrier cooling the hot holes or electrons can be cooled slowly thus maintaining
more time to utilize the converted heat. Also due to thermal decay the voltage drpos considerably.
So it’ll be helpful to slow down the thermal decay and to keep the carrier hot for a long time.

32. 32
3.6 Expected Characteristics
Due to the ever decreasing amount of fossil fuels and organic resources, people have turned to
renewable energy. But the efficiency of these technologies are quite below par. But with
continuous research we can see the ray of hope at the end of the road.
Figure 3.6.1 :- Expected Characteristics of FPV

33. 33
3.7 Recent Development
3.7.1 A Gratzel Cells:-
Also known as Dye-sensitized solar cell(DSSC,DSC). This marvelous discovery has brought
professor Gratzel of Switzerland 2010 Millenium Technology Grand Prize.
The main reason this technology will burst into the renewable market is because of its low cost.
It’s price to performance ratio is excellent. This technology is also called “Artificial
Photosynthesis” and it is a potential candidate as the technology that’s going to replace the standard
silicon photovoltaics.
Fig 3.7.1:- Photocurrent-voltage curve of a DSC at different light intensities.
The conversion efficiency in full AM 1.5 sunlight was 11.04%. It increased to 11.18% at 65% full
sunlight. A heat-resistant quasi-solid-state electrolyte based on imidazolium iodide was
introduced. When used in conjunction with the amphiphilic ruthenium dye Z-907, it was possible
to pass for the first time the critical 1000-h stability test at 80 °C with a DSC.47 Other laboratories
have also presented systematic verifications of the cell stability carried out independently.

34. 34
Fig 3.7.2:- Dye Sensitized Solar Cell: Recent Development
During recent years, industrial interest in the DSC has surged and the first commercial products
have appeared. A number of industrial corporations, such as Konarka (www.konarkatech.com) in
the U.S.A., Aisin Seiki in Japan, RWE in Germany, and Solaronix in Switzerland, are actively
pursuing the development of new products. Particularly interesting are applications in building
integrated photovoltaic elements such as electric-power-producing glass tiles. The Australian
company Sustainable Technologies International (www.sta.com.au) has produced such tiles on a
large scale for field testing and the first building has been equipped with a wall of this type.
3.7.2 Perovskite Solar Cell:
Perovskite cells are the cells that are composed of both organic and inorganic substances and has
lead or halide based material as the photo-sensitive material. It is a kind of a hybrid solar cell.
Perovskite materials such as Methylammonium Lead Halides are much cheaper and much more
simpler to manufacture. The name perovskite comes from the perovskite structure. The efficiency
of the basic standard cell varies from 1% to 3% whereas perovskite cells can achieve up to almost
22% efficiency.
As for manufacturing process, standard solar cells require expensive, multistage processes that
needs such optimum temparature, pressure or environment that’s hard to achieve.But Perovskite

35. 35
cells can be manufactured through simple wet chemistry techniques and doesn’t need any
exaggerating lab facility.
Fig 3.7.3:- Illustration of Perovskites films made through vapur assisted solution process
As this is a new technology, it is not without any fault. The main problem with this is the stability
of the cell. Environmental factors may cause variation and unstable situation. Thermal influence,
heating under applied voltage, photo influence etc cause the instability. Though the solubility of
the organic materials cause the real haphazard, it can be solved temporarily with polymer
insulation or structural or architectural change of the polymers. These remedies are not permanent
though. A strong hydrophobic barrier can prevent the organic parts from decaying and a strong
front coating can prevent the UV lights from negatively interacting with the PSC stacks.
Under simulated solar illumination the perovskite cells show more hysteric behavior in the I-V
curve more than other basic or advanced solar cells. Different causes have been hold liable for this
hysteresis such as Ion movement, polarization, filling of trap states etc. But the actual source of
this hysteresis is yet to discover. To solve this problem, extremely slow voltage scan or Stabilized
Power output have been proposed as solutions. Though it is being guessed that the surface may
have a hand to play in this matter. In the inverted architecture an organic n-type is used instead of
normal metal oxide. And it showed promising results. Almost little to no hysteresis was observed.
3.7.3 Quantum Dot Solar Cells:
To remove the bulk materials from the basic solar cells, to cut down the extra weight and to enable
a large band spectrum Quantum dot solar cells are used. Also known as Graphene cells. The
efficiency varies from 7% to 8%. Quantum Dots are referred to as “Artificial Atoms”. The energy
levels or the band gaps are tunable to a wide range. The tuning is done in typical and simple wet
chemistry preparations.

36. 36
Fig 3.7.4:- TEM of colloidal lead selenide PbSe quantum dot
Fig3.7.5:- Variation of quantum dot energy band gap vs. dot size for some common semiconductors.
3.7.4 Light Trapping and Increased Absorption :
Usually in standard solar cells light get dispersed and almost hardly 30% light comes to of work.
So to concentrate the ray of light and to scatter it amongst the plasmonic solar panel is known as
light Trapping. Through this process the percentage of containing the supplied light almost
increases to 95%. the structure can also entrap the light into one wavelength that is far superior
than silicon based solar panels. If the trapped light can be used and absorbed, the efficiency goes
higher automatically.

37. 37
To aid the absorption of light different technologies are being used. Such as, by placing metal
wires on the surface the dispersion, scattering and absorption increases drastically.

38. 38
3.8 Testing Methods of Flexible Photovoltaics
Once the cells have been manufactured, various tests will be conducted to determine flexibility,
life time, and efficiency of the panel. While the main goal of this project is produce a flexible cell,
achieving a moderate efficiency is also important. The testing methods for each of these factors
will be outlined below.
3.8.1 Flexibility:
The flexibility of the cell is important as a flexible cell is only good if it can be repeated and does
not greatly affect the cell performance. To test flexibility, the final cell and each individual layer
will be bent at a fixed radius. We will bend the cell for several cycles and use a Scanning Electron
Microscope (SEM) to determine if any nano-level deformation has occurred. Electrical resistance
of certain layers will also be compared before and after bending.
3.8.2 Efficiency:
We will perform normal solar cell efficiency tests, observing efficiency at 1 sun and determining
short circuit current and open circuit voltage. The efficiency will also be tested again after the
panel has been bent to its ultimate radius, and again after it has undergone cyclic loading. In this
way, we can see if flexing has any effect on long term efficiency, possibly indicating deformation
as well. Comparing efficiencies between low temperature and high temperature annealed cells will
also be done.

39. 39
3.9 Advantages & Limitation of Flexible Photovoltaic Technology
3.9.1 Advantages of Flexible Photovoltaic Technology:
 The greatest advantage of flexible solar cells is their agility factor.
 They are lightweight and can easily fit into spaces where conventional solar panels cannot.
For instance, if your house fails the roof test for the installation of solar shingles or panels
owing to structural issues, you can always opt for ultra-thin flexible solar cells.
 Another advantage of flexible solar panels is that they can be easily attached to unusual
places such as laptops, mobile phones, cameras, to name a few. A great example of this is
the solar roof of Fisker Karma where the flexible solar panel is integrated perfectly to align
with the curved roof of the car.
 The cost of installing flexible solar panels is much less compared to regular solar panels
since they require less labor and effort to be installed and being lightweight, they can be
easily carried
 As the panels can be glued on the roof, there’s no need for mounting racks, which makes
the installation more cost effective.
 As paper costs approximately a thousandth of glass, solar cells using printing processes
can be much cheaper than conventional solar panels.
 Also other methods involving coating papers with materials include first coating the paper
with a smooth material to counter-act the molecular scale roughness of paper. But in this
method, the photovoltaic material can be coated directly onto untreated paper.
 Performance in Low-light:- Many thin solar panels have better energy production in low-
light and shading situations

40. 40
3.9.2 Limitation of Flexible Photovoltaic Technology
 They are not suitable for large-scale solar projects that require sturdy and more reliable
solar panels. The efficiency of these flexible solar panels ranges between 11-13% which is
much less compared to the effectiveness of monocrystalline or polycrystalline panels that
have the efficiency range between 14-17%.
 Complex structure
 Need to be very careful in handling
 Can’t be used in astronomical devices.
 Heat Retention:- Because thin film solar is usually applied directly to a surface, they can
retain more hear. Traditional panels are generally installed with a standoff, meaning there
is space between the panel and the supporting surface, allowing for air to cool the panels.
Thin film solar may retain more heat, creating a balance act between this and its benefit of
better performance at higher temperatures.
 Space Needed:- With the efficiencies currently available, you would need approximately
50% more room with thin film solar to produce the same electricity as a traditional solar
setup.
 Durability:- Since the technology is fairly new, there are some questions about how long
these cells will last. But many early-adopters have reported their cells lasting 15 years and
more. These cells do not require the glass and aluminum casings of traditional cells because
the materials within them are flexible and malleable, not brittle like crystalline silicon. This
means they will likely take more abuse and last longer.
 Fewer Defects:- Because the manufacturing process is simpler, there are often fewer
defects. The highly technical method of building traditional solar panels, sometimes
compared to computer chip manufacturing, involves a lot of detailed soldering. This has

41. 41
been historically a place where the traditional panels experienced a lot of warranty issues.
Not so with solar film. The process is closer to printing and therefore is subject to fewer
defect issues.

42. 42
3.10 Applications of Flexible Photovoltaic Technology
Though solar energy can be utilized in many ways. The most efficient way to use solar energy is
to use the heat and the light. Thus the solar panels. Solar panels have gone through significant
changes and generations. It came from heavy, expensive and rigid solar panels to flexible ultra thin
films. Thin film solar cells have revolutionize the idea of solar cells. It has the potential to
compliment our everyday chores and routines. It has become versatile as well as relatively more
efficient. The possible applications of the thin film solar panels are:
1. Flexible solar panels are portable solar power systems which can be used on-the-go, for
RV’s, autos and boats.
2. They can be used to charge solar batteries.
3. Flexible panels are low-cost off-grid PV systems for homes and cabins.
4. Incorporation into new solar energy consumer products.
5. These types of panel are useful for various expeditions such as hiking, cycling, kayaking
and climbing. Roll-able and flexible panels are also useful on boats, which can reduce the
energy demands on the boat motor.
Fig.3.10.1:- It is known as a HeLi-on phone charger which uses organic thin film technology where the
solar cells are printed onto a flexible plastic film. It can be unraveled easily while charging and rolled
away during storage. (source: infinitypv.com)

43. 43
6. They can be used as wall paper and window shades for producing electricity from room
lighting.
7. They can also be manufactured on clothing, which can in turn be used to charge portable
electronic devices like mobile phones and media players

44. 44
CHAPTER- 4
FUTURE SCOPE & CONCLUSION
4.1 Future Scope of Flexible Photovoltaic Technology
This report herein will detail the steps taken to create a solar cell where the main goal is flexibility,
rather than efficiency or cost. The general construction of the cell will be an inorganic hybrid
heterojunction solar cell, where the basic layering of the cell will be
(PET/Glass)/ITO/TiO2/Sb2S3/CuSCN/(Au/Pd ).
Figure 4.1:- Final Cell Layout
In this structure, the study will center around the effects that cell layers have on flexibility and
efficiency, by modifying the ZnO layer and changing what is used as the n-type semiconductor:
namely; Antimony Sulfide, or Sb2S3. The ZnO layer will be constructed of nanowires, and we
will investigate the effect of changing the wire concentration, shape, length, and thickness. Both
the inorganic n-type semiconductor, Sb2S3, as well as the p-type semiconductor, CuCSN, will be
altered by tuning the morphology, ratio of the p and n-type layers, and thickness. Fatigue and
bending tests will be applied to each cell, to observe the effect of modifications to the structures
flexibility. Traditional efficiency tests will also be applied before and after bending, but not during
due to difficulties calculating area and applying bending during testing. Lastly given appropriate
time and sufficient testing of the individual cells, mechanical design ideas will be applied to the
structure of the solar panel to investigate covering larger areas while still maintaining flexibility
and good efficiency.

45. 45
4.2 Conclusion for Flexible Photovoltaic Technology
This study showed us that the combination of PET, ITO, Sb2S3, CuSCn, and gold/palladium is a
viable flexible solar cell that should be studied in more depth to increase the efficiency of the cell.
Other material options, material deposition processes, and cell designs should also be explored to
attempt to create a more resilient solar cell.
To perform many of the processes that were involved in the creation of these cells our group had
to work together to plan and schedule. Often with our work there were setbacks in the laboratory
that would delay us for days. Other times when we were met with several failures in a row we had
to learn to innovate and overcome the obstacle we faced. Learning to work with other people
outside of our group was also important, as we relied on the help of graduate students for learning
lab practices and for obtaining SEM images. With proper planning and communication, we were
able to work very well together.
Creating flexible solar cells is an important step in the future of the energy industry. The
applications of solar power will be greatly increased with the advancement and implementation of
flexible solar cells.
.

46. 46
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