COLLEGE OF NATURAL & COMPUTATIONAL

SCIENCE
DEPARTMENT OF PHYSICS

OXIDES & NANO COMPOSITE AS CHARGE TRANSPORT

LAYER IN ORGANIC SOLAR CELL
A Research Proposal Submitted Debre Birhan University in partial
fulfillment for the Requirement of MSC in Physics

PREPARED BY: NURI WOLIYU

ADVISOR: BIZUNEH G/MICHAEL (PhD)

January, 2020

Debre Birhan, Ethiopia

ACKNOWLEDGEMENT
I greatly owe my heartfelt gratitude to my dedicated Advisor Dr. Bizuneh G/Michael for his
incredible guidance that let me finish this proposal.

TABLE OF CONTENTS

I
ACKNOWLEDGEMENT..................................................................................................................I

LIST OF TABLE.............................................................................................................................IV

LIST OF FIGURES..........................................................................................................................V

CHAPTER ONE................................................................................................................................1

INTRODUCTION.............................................................................................................................1

1.1. GENERAL BACKGROUND......................................................................................4

1.2. OBJECTIVES OF THE STUDY............................................................................................5

1.2.1. THE GENERAL OBJECTIVES OF THE STUDY.............................................5

1.2.2. THE SPECIFIC OBJECTIVES OF THE STUDY...................................................5

CHAPTER TWO...............................................................................................................................6

REVIEW OF LITERATURE............................................................................................................6

2.1. History of Perovskite based solar cells........................................................................6

2.2. Perovskite materials.....................................................................................................7

2.3. Working Principle of Perovskite Solar Cells...............................................................8

2.4. Organic Solar Cells (OSC) working principle........................................................................9

2.5. Structure and materials of OSC..................................................................................12

2.6. Perovskite Solar Cell Structure..................................................................................14

2.7. Metal oxides in Perovskite solar cells........................................................................16

2.7.1. TiO2 and ZnO metal oxides TiO2......................................................................16

CHAPTER - THREE.......................................................................................................................17

MATERIALS AND METHODS....................................................................................................17

3.1. Material......................................................................................................................17

3.1.1. The material we will use.....................................................................................17

3.1.2. For simulation work................................................................................................17

II
 3.2. Methods......................................................................................................................17

3.3. Method of simulation.................................................................................................17

CHAPTER 4....................................................................................................................................18

TIME SCHEDULE AND BUDGETALLOTMENT......................................................................18

4.1. Time Schedule............................................................................................................18

4.2. Budget Allotment.......................................................................................................18

REFERENCE..................................................................................................................................19

III
LIST OF TABLE
Table 1: Time Plan………………………………………………………………….18

Table 2: Budget Plan………………………………………………………………..18

LIST OF FIGURES
Figure 1.1 Classification of different generations of solar cells ……………………..……….2

IV
Figure 1.2 Best research-Cell Efficiencies ……………………………………………..……..3
Figure 1.3 Solar efficiency of Silicon and Perovskite based Solar Cells. Taken from “Enabling
Breakthroughs in Solar Technology” …………………………………………………..……..4
Figure 2.1 Typical crystal structure (unit cell) of perovskite material. ……………….………7
Figure 2.2 Schematic of the basic working principle of perovskite solar cells. ………….…...8
Figure 2.3 Schematic diagram of photo-generated charge transfer and recombination
process in perovskite solar cells (Marchioro et al., 2014). ……………………………………9
Figure 2.4: A photon with energy hν generates an exciton that separates into a positive and
negative polaron. The charges are then collected at the electrodes. ………………...……….10
Figure 2.5: Irradiance of the solar spectrum measured on Earth as a consequence less
collected carriers and a lower efficiency. Therefore, in order to optimize the device efficiency,
a compromise exists between VOC and ISC. …………………………………..……………12
Figure 2.6(a): Chemical structure of common organic compounds employed in this work. ...13
Figure 1.7: Typical structure of a bulk heterojunction solar cell. ……………………...…….14
Figure 2.4 Device architecture of mesoporous (a) and planar (b) perovskite based solar cell.
…………………………………………………………………………………………...……14
Figure 2.5 Device architecture of regular n-i-p (a) and inverted p-i-n (b) perovskite based
solar cell. …………………………………………………………………..…………………15

V
CHAPTER ONE

INTRODUCTION
The increasing energy demands and environmental concerns due to the use of fossil fuels in the
21st century have motivated researchers and policy makers to explore clean and environmental
friendly renewable sources of energy. Apart from environmental pollution the limited resources
of fossil fuels are going to be consumed soon and therefore alternative sources of energy are
required for future energy demands. These alternative sources of energy to overcome the energy
crisis include hydroelectricity, tides, geothermal, wind and solar energy. Among all these
sources of energy, solar energy is the center of interest due to the enormous amount of energy
provided by the sun. Also, solar energy has the most extractable potential with the least
environmental effects which can meet stringent energy needs. The sun provides much more
energy per day than the energy consumption requirement of the current population for the whole
year. Photovoltaic (PV) provide a direct conversion of the incident solar radiation into electricity.
This process does not have any side product such as noise and pollution which makes the PV
technology a robust, reliable and long lasting renewable source of energy.
The first working photovoltaic device demonstrated in the 1950s with the efficiency
of 3% using silicon. Today the performance of such devices is rapidly increasing with efficiency
as high as 25% (Jeon et. al. 2014). The first major boost in research and development on solar
cells received from the space industry in the 1960s. These are solar cells which were more
expensive than solar cells we have today. The main attention on photovoltaic occurred after the
oil crisis in 1970s. In this era the photovoltaic were investigated and promoted as an alternative
energy resource to overcome the energy crisis. It was quickly recognized that the PV can supply
power to "remote" areas and hence prompted to terrestrial photovoltaic industry. Today
photovoltaic are classified in three generations based on materials used as shown in Figure 1.1.
Today photovoltaic are found to be one of the effective technologies in overcoming
the shortage of energy. Solar cells are divided into three different generations on the
basis of materials used as shown in Figure 1.1.

1
Figure 1.1 Classification of different generations of solar cells.
The up to date efficiencies of the different generation solar cells are shown in Figure 1.2. The
first generation of photovoltaic are the silicon based solar cells. Silicon based solar cells have
high power conversion efficiency but the high volume of materials and manufacturing cost are
the major issues with this type of solar cells. Silicon based solar cells are divided into mono-
crystalline (Mono c-Si), polycrystalline (Poly c-Si) and amorphous silicon cells. The second
generation of solar cells is thin film solar cells including CIGS, CdTe and CIS. These second
generation thin film solar cells, couldn’t meet the requirements due to the use of indium and
tellurium etc. The third generation of solar cells consisting of dye sensitized solar cells (DSSCs),
Copper zinc tin sulphide (CZTS), organic solar cells (OPVs), quantum dot solar cells and
perovskite based solar cells (PSCs) are the most promising photovoltaic technology due to their
high efficiency at low processing cost. These third generation solar cells are mostly solution-
processed using organic semiconductors, hybrid composites, or inorganic semiconducting
materials. A key role has been played by the solution-processed dye by producing photo-
generated current. However, the low range of power conversion efficiency (PCE) for DSCs,
OPVs and CZTS has limited their commercialization. For widely use of the third generation of
solar cells, a technology that produces durable, high efficiency and low cost solar cells is needed
(Sum and Mathews, 2014).

2
Figure 1.2 Best research-Cell Efficiencies
Currently perovskite solar cells have become the focus of research, due to the tremendous optical
and electrical properties of the perovskite materials. The perovskite semiconducting material has
attracted the attention of scientists and researchers because of its low binding energy (Tanaka et
al 2003), long diffusion length, long carrier life time and a strong light absorption in broad
absorption range from visible to near infrared spectrum with a direct tuneable band gap of 1.2-
2.7 eV (8-10). Since the introduction of organic lead halide perovskite semiconductor
photovoltaic device by Miyasaka and his group in 2009, a huge progress has been made in the
design and optimization of perovskite solar cells (Kojima et al., 2009). High efficiency
perovskite solar cells (PSCs) can be produced at low cost using simple processing methods
(Yang et al., 2017; Jeon et al., 2018). As perovskites have excellent light absorbing property,
they require a thin layer of about 300-500 nm which minimizes the material cost (Huang et al.,
2014). Furthermore, by introducing low temperature processable inorganic metal oxide
engineering the chemical composition of the perovskite materials can alter a range of properties
including optical and electronic properties that are useful for enhancing the performance and
stability of the solar cells.
Today PSCs have shown high power conversion efficiencies (PCE) of over 23% (Jeon et al.,
2013; Kojima et. al. 2009; Jeon et. al. 2018; Lee et. al. 2012; Green et. al. 2014). As shown in
Figure 1.3, a sharp increase in the efficiency of PSCs is observed over very short period of time.
In 2011 the Park group improved the perovskite solar cells efficiency from its initial value of
3.8% to 6.5 %. In 2012 the collaborated work of Gratzel and Park increased the efficiency
further to 9.7% (Kim et al., 2012). Yang’s group reported 19.3% efficiency for planar structure
in 2015. Furthermore Seok group have certified efficiency of 20.1% in 2015 (Wiegrebe, 2008).
Recently the efficiency reached to 23.2% for perovskite solar cells in 2018 (Jeon et al., 2018).
However, for widely use of this type of solar cells, a technology that produces durable and low
cost that is competitive with the Si solar cell technology is needed.

3
Figure 1.3 Solar efficiency of Silicon and Perovskite based Solar Cells. Taken from “Enabling
Breakthroughs in Solar Technology”

1.1. GENERAL BACKGROUND

Perovskite based hybrid solar cells emerged in the last decade as potential alternative devices for
the development of PV technology. However, important issues need to be solved before
perovskite based solar cells become a commercialized product. Stability of perovskite and use of
toxic lead (Pb) heavy matal are big challenges for commercialization of perovskite solar cells.
Also, high processing temperature and lower conductivity of the inorganic metal oxide used as
ETL and poor stability and high materials cost of the organic HTL are some of the main
challenges. The currently used ETMs need high temperature post-treatment to increase the
crystallinity and conductivity. Similarly, the organic HTMs have extensively used in
high efficiency PSCs which are often expensive and un-stable upon exposing to the ambient
environment. Therefore using high temperature processable ETLs and organic charge extraction
layer may cause problems in the future commercialization of PSCs. In this regard, low
temperature processable, stable and low cost ETL and HTL materials are required for PSCs. The
stability and performance of perovskite solar cells can also be affected by the underlying n-type
layers. It is proposed that optimization of the stoichiometry and morphology of ETLs using
appropriate processing methods would enhance the stability and performance of perovskite solar
cells. Low temperature processable metal oxides such as WO3 and SnO2 may also be tuned to
obtained good electron transport properties and comparable energy levels that can substitute
TiO2 blocking layer and as electron transport materials to enhance the overall performance of
PSCs. Inorganic p-type metal oxide semiconductors such as NiO, CuOx, and MoO3 are the most
suitable replacement as hole transport materials due to their excellent chemical stability, higher
charge mobility and high transparency. Different metal oxides such as MoO3 and NiO as HTM
have been explored for perovskite solar cells.

4
The objective of this study is to explore new, stable and inexpensive ETL that can be processed
at low temperatures. For example one of the aims is to study ETL that can be processed at low
temperature using PVD (sputtering and e-beam evaporation). Once the suitability of SnOx as
ETL using sputtering process is explored, then a thorough and systematic study of the ETL will
be performed. Similar approach will be used for the other materials. The research will further
investigate the optical and electrical properties of the ETL by tuning the composition of the
material for achieving high PCE. Similarly, this study will explore HTL metal oxides to replace
the expensive and unstable organic hole transport layer. This research strategy will not only
provide stable and high performance solar cell device but will also reduce the cost of the
material.

1.2. OBJECTIVES OF THE STUDY

1.2.1. THE GENERAL OBJECTIVES OF THE STUDY

The general objective of this research is to modify the properties of different metal oxides as
ETL and HTL by tuning their electronic bands and align with the band of the perovskite absorber
and thereby improve the performance of the PSC device.

1.2.2. THE SPECIFIC OBJECTIVES OF THE STUDY

The specific objectives of this proposed research are:

 To investigate metal oxide charge transport layer
 Study ETLs which can be processed at low temperature
 Optimize the properties of the ETLs
 Apply new metal oxide HTLs to perovskite solar cells
 Exploring PVD (sputtering & e-beam) techniques for deposition of these materials

5
CHAPTER TWO

REVIEW OF LITERATURE
2.1. History of Perovskite based solar cells

Perovskite based solar cells evolution Some authors dated back to the early 1990 for the
beginning of concerted efforts in the investigations of perovskite as solar absorber. Green et. al.
have recently published an article on the series of events that lead to the current state of solid
perovskite solar cell (Green MA and Ho-Baillie, 2014). The year 2006 regarded by many as a
land mark towards achieving perovskite based solar cell when 2.2% power conversion e fficiency
was reported from dye-sensitized solar cell using MAPbBr3 as sensitizer by a researcher in Japan
(Mahmood K et al., 2015). The first perovskite-sensitized TiO2 solar cells were fabricated using
liquid electrolytes based on iodide or bromide solutions. Kojima et al reported, in 2009, the first
organic lead halide compounds CH3NH3PbBr3 and CH3NH3PbI3 as sensitizers in photo
electrochemical cells. They measured an improved PCE of 3.81% for the CH3NH3PbBr3 and
3.13% for the CH3NH3PbI3-based devices, respectively (Im J-H et al., 2011). In 2011, the same
group reported a PCE of 6.5% still using the CH3NH3PbI3-based iodide liquid electrolyte
contact, but with improved preparation conditions. However, the perovskite nanocrystals
decomposed in the iodide liquid electrolyte that lead to rapid device degradation, lasted only
about 10 min (Niu G et al., 2015). The need for solid contact was a necessary condition to
improve device stability because of the adverse effect that polar iodine has on the perovskite
structure emanating from iodine liquid electrolyte. By mid-2012, a solid state DSSCs was
fabricated with exceptionally high PCE of 9.7% using spiro OMeTAD as a HTM and perovskite
solar absorber (CH3NH3PbI3). The use of a solid-state HTM dramatically improved the devices
stability compared to liquid electrolyte, however, the stability issue still remains the main
challenge for mass production and commercialization of perovskite solar cells (Fan J et al.,
2014). On the other hand, the power conversion efficiency of PSCs was growing to a new level
which was reported in 2013, using a sequential deposition method, for the formation of the
perovskite pigment within the porous metal oxide film that resulted in improved film
morphology and PCE as high as 15% (Burschka J et al., 2013). In March 2013, Sang II Seok and
his coworkers reported in Nano Letters promising results from using mixed halide perovskite
solar cell through optimization of the halides in CH3NH3Pb(I Br ) 1 3 -x x compound (Noh JH,
et al., 2013). This pave the way for unprecedented growth in PCE over the years, with a highly
efficient solid-state solar cells produced with PCE of 12.3% (Assadi MK et al., 2017), 15% in
2013, 19.3% at first half of 2014 (Tong X et al., 2016). A PCE of 20.1% was reported by Korean
Research Institute of Chemical Technology (KRICT), by W. S. Yang and co-workers using
Formamidinium lead iodide (FAPbI3) as active layer which was later certified by National
Renewable Energy Laboratory (NREL) in late 2014 (Green MA and Ho-Baillie, 2017). Another
break though was reported in the early 2016 jointly by (KRICT)/ Ulsan National Institute of
Science and Technology (UNIST) with PCE 22.1% (www.nrel.gov). Methods of device

6
preparations and the PSCs performances achieved by various research groups through varying
material optimizations, device architectural designs, interfacial engineering and improved
fabrication conditions have been duly reported and highlighted (Mahmood K et al., 2015).

2.2. Perovskite materials

Those materials possessing the crystalline structure of Calcium Titanium Oxide (CaTiO3) are
known as perovskite materials. The basic crystal structure of perovskite materials with a
chemical formula of ABX3 is shown in Figure 2.1. As discussed in chapter 1, because of the
extraordinary electrical and optical properties of perovskites, these materials have attracted a
tremendous attention in the past few years. The high optical absorption coefficient (as high as
104 cm-1) (Xing et al., 2013), tunable band gap (Noh et al., 2013) and low exciton binding
energy (Sun et al., 2014) and a long carrier mobility of up to 1 µm (Stranks et al., 2013) make
perovskite materials the best choice for photovoltaic applications.
Perovskite materials used for photovoltaic applications are hybrid organic and inorganic metal
halide compounds consisting of organic ammonium cations such as CH3NH3+ (MA+) or
NH2CHNH2+ (FA+), inorganic cations such as Pb2+ or Sn2+ and halogen anions Cl-, Br- or I-.
In the molecular structure shown in Figure 2.1, the organic cation is represented with A site, the
inorganic cation with B and the anion positioned at X.

Figure 2.1 Typical crystal structure (unit cell) of perovskite material.

7
 2.3. Working Principle of Perovskite Solar Cells
The different types of perovskite solar cells have been described in chapter 1. The light
absorbing perovskite layer is sandwiched between ETL and HTL of the solar cell. As shown in
Figure 2.2, upon exposure of the solar cell to sunlight, the perovskite absorbs the light to produce
the excitons (electrons and holes). These excitons then form free carriers because of the
difference in the binding energy of perovskite materials and generate current. These generated
free electrons and holes are then separated at the ETL and HTL interfaces by the respective
electron and hole transporting layers. Electrons from perovskite material are then transferred to
electron transport layer (ETL) and holes are transferred to hole transporting layer (HTL). Finally,
the electrons are collected by TCO from ETL and hole collected by metal back

Figure 2.2 Schematic of the basic working principle of perovskite solar cells.
electrode. The TCO and metal back electrode are connected to create a photocurrent in the outer
circuit. Due to the high carrier mobility and long diffusion length of the perovskite materials, the
PSCs have superior photovoltaic performance.

8
Figure 2.3 Schematic diagram of photo-generated charge transfer and recombination
process in perovskite solar cells (Marchioro et al., 2014).
As explained by Marchioro et al. (2014) the charge transport is achieved by the charge
separation at the ETL/perovskite and perovskite/HTL interfaces and charge injection to ETL and
HTL from perovskite (process i and ii in Figure 2.3). Process i and ii are the required charge
transfer procedures. At the same time, undesirable processes which are detrimental to the
performance of the perovskite solar cells also occur. These undesirable processes include exciton
annihilation (process iii), non-radiative recombination, reverse transmission of electrons and
holes (processes iv and v) and the carrier recombination at ETL/perovskite interface (process vi).
This whole process of charge transport in the ETL/perovskite/HTL contributes to the
performance of the PSCs.

2.4. Organic Solar Cells (OSC) working principle

One of the main differences between organic and inorganic solar cells is that in the former a
photon is absorbed by an organic material, which gives rise to an electron-hole pair called an
exciton. An exciton needs some energy in order to be separated into a free electron and a hole
pair. Such a separation can occur thanks to the blend of two materials having a proper chemical
potential difference, that provides the necessary electrical field to break the electron-hole bond.
In particular, the material which is donating an electron when separating the exciton is called a
donor, and is characterized by a high LUMO (lowest unoccupied molecular orbital and

9
equivalent to the conductive band), while the material which is receiving an electron when
separating the exciton is called an acceptor, and is characterized by a low HOMO (highest
occupied molecular orbital and equivalent to the valence band). The basic working principle of
an organic solar cell, schematically illustrated in Fig. 1.4, is the following:
 A photon with an energy EP H > EGAP is absorbed by the active layer, forming an
exciton.
 The exciton can diffuse (on the order of the tens of nanometres before recombining) until
it reaches a donor-acceptor interface.
 The exciton can separate thanks to the potential at the donor-acceptor interface, forming
two free carriers, one positive and one negative (also called polarons).

Figure 2.4: A photon with energy hν generates an exciton that separates into a positive and
negative polaron. The charges are then collected at the electrodes.
 The charge carriers can diffuse across the materials driven by chemical potential toward
the respective electrodes.

The amount of photons that once reaching the solar cell are converted into excitons, and are
effectively separated and collected at the electrodes defines the solar cell quantum efficiency. In
particular, the ratio of the collected electrons divided by the number of incident photons, for a
given energy, defines the External Quantum Efficiency (EQE), while the ratio of the collected
electrons divided by the number of photons absorbed by the active layer, for a given energy,
defines the Internal Quantum Efficiency (IQE):

(2.1)

10
More generically, the ratio of the output power divided by the input power is the photo
conversion efficiency η, also called Power Conversion Efficiency (PCE), see eq. 2.1. The
efficiency depends on the architecture of the active layer, which deeply affects the probability
of separating an exciton.
The most basic architecture is a bilayer device, which was used by Tang in 1986 in order to
make the first demonstration of a working organic solar cell [C. W. Tang, 1986]. However, this
structure suffered from low efficiency due to the short diffusion length of the exciton in an
organic solar cells. Approximately ten years later, G. Yu et al. introduced the bulk heterojuction
(BHJ) solar cell. In such a device, the interpenetrating mix of donor and acceptor provided large
interfacial areas that increased the exciton dissociation rate by reducing the distance that the
carriers needed to travel before dissociation [G. Yu et al., 1995]. Later, it was realised that
domain sizes of donor and acceptor can be further optimized with additives, such as 1,8-
octanedithiol (ODT) and diiodooctane (DIO) [J Peet et al., 2007]. Another advantage of a BHJ is
that it can be prepared at once by coating a well-blended solution, without the need of any mask.
The efficiency η is defined by:

(2.2.)
where PIN (W) is the power irradiating the solar cell, PM (W) is the maximum power at the
output with VM (V) and IM (A) respectively the voltage and current at the Maximum Power
Point (MPP), ISC (A) is the short circuit current, VOC (V) is the open circuit voltage and
FF is the fill factor. The FF is defined as the ratio between the power at the MPP and the
product between the open circuit voltage and short circuit current, see eq. 2.3. below:

(2.3.)
A common quantity used to describe the irradiating light is the irradiance, measured in W/m2,
which must be multiplied by the active area of the device in order to obtain the input power
PIN. The OSC active area is the area where the top and bottom electrodes overlap. Figure
1.5 shows the solar irradiance plotted as a function of the wavelength (λ).
The efficiency depends on several material properties which affect for instance the VOC. In
fact, the open circuit voltage depends on the value of LUMO of the acceptor and on the value
of HOMO of the donor, as indicated in Fig. 1.4 and reported in eq. 2.4.:

(2.4.)

11
where q = 1.6 · 10-19 C is the elementary charge, while ELUMO(acceptor) and EHOMO(donor)
are the energy levels of the acceptor LUMO and of the donor HOMO respectively. Since
VOC is directly proportional to the PCE of the device (see eq. 1.4), a lot of research groups
have tried to synthesize donors with a low HOMO, in order to increase the value of the VOC.
However, lowering the HOMO also causes a larger EGAP of the donor, since the position of
the LUMO cannot be moved much in order to guarantee a good exciton separation [Hae Jung
Son et al., 2011]. A larger EGAP implies less solar spectrum absorbed (since less photons can
satisfy eq. ) and

Figure 2.5: Irradiance of the solar spectrum measured on Earth [39] as a consequence less
collected carriers and a lower efficiency. Therefore, in order to optimize the device efficiency, a
compromise exists between VOC and ISC.

2.5. Structure and materials of OSC

Polymers used in OPV are characterized by being conjugated: the conjugation of a polymer
refers to the presence of a backbone chain of alternating double and single bonds. The sp2
hybridized carbon centres have a valence electron in the pz orbital (π) which is orthogonal to
the other σ bonds. The overlapping of π/π bonds determines the creation of delocalized energy
states within the structure, which promotes intermolecular transport, and allows for transport
of charge.
The active layer used in polymer solar cells consists of a donor-acceptor blend, where a
conjugated polymer acts as a donor, and generally a fullerene derivative acts as an acceptor. The
most common materials used in this work for the active layer are Poly (3-hexylthiophene)
(P3HT) and Phenyl-C61-butyric acid methyl ester (PCBM), whose chemical composition is
reported in Fig. 1.6(a-b). Advanced polymers can be synthesized to improve the PCE of a
solar cell, for example it is possible to combine two units in the structure, one that acts as
an acceptor and one that acts as a donor, by forming a donor-acceptor (D-A) copolymer. In

12
these polymers, the HOMO and LUMO mostly depend on the HOMO of the donor and on the
LUMO of the acceptor, respectively. This helps optimize the efficiency by engineering both the
EGAP and the LUMO alignment with the fullerene, by choosing the appropriate donoracceptor
unit [Luyao Lu et al., 2015]. Moreover, to help the extraction of charge in OSC by reducing
charge recombination and improving exciton dissociation, two additional functional layers are
used: a Hole Transport Layer (HTL) which is characterized by a good hole mobility that tends to
act as an electron blocker, and an Electron Transport Layer (ETL) which conversely has a good
electron mobility and tends to act as a hole blocker. The HTL used in this thesis is Poly (3,4
ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) (see Fig. 1.6(c)), while the ETL
is zinc oxide (ZnO)

Figure 2.6(a): Chemical structure of common organic compounds employed in this work.

Two possible architectures of a bulk heterojunction solar cell are shown in Figure 2.6. The first
type of polymer solar cells employed Indium Tin Oxide (ITO) as a transparent anode. In such a
structure, the back6 electrode is the cathode which has a low work function, and can easily
oxidise due to its direct air exposure. This configuration is called the normal structure and it is
still largely used, see Fig. 1.7(a). In another structure, the cathode and anode are flipped in the
positions, resulting in placement of the high work function anode at the back of the cell. This
approach was developed in order to be able to utilize high work function electrodes, such as
silver, as the back electrode. The latter can be printed from a liquid paste and therefore, such
architecture enables the possibility of scalable processing of devices using roll-to-roll machinery
[Frederik C. Krebs et al., 2009]. It was later shown that silver was more resistant towards humid
environments compared to aluminium and therefore could significantly improve the device
stability under certain test conditions [Mikkel Jørgensen et al., 2012]. Such a structure is called
an inverted structure, see Fig. 2.6(b).

13
Figure 1.7: Typical structure of a bulk heterojunction solar cell.

2.6. Perovskite Solar Cell Structure

Because of the ambipolar nature of the perovskite, various architectures are possible for
perovskite solar cells. Basically two device structures are constructed, the mesoporous structure
(Burschka et al.,2013) and the planar hetero-structure (Liu et al.,2013). Both the mesoporous and
planar structures are shown in Figure 2.4 a & b, respectively. The performance of the solar cells
can be improved by efficient separation of the charges, then transporting the charges to
respective charge transporting layer and efficiently collected them at the electrodes.

Figure 2.4 Device architecture of mesoporous (a) and planar (b) perovskite based solar cell.
Because of the large specific surface area (~1000 m2/g) and high porosity, the mesoporous
structure has been intensively used in perovskite solar cells (Zhou et al., 2018). The efficiency of

14
the perovskite solar cells is increased by allowing the light absorbing layer to have good
adhesion with the mesoporous charge transport layer which increases the light receiving area of
the light absorbing layer. By introducing a mesoporous layer the perovskite can infiltrate into it
and can have a better contact and larger area which helps in the charge generation and mobility.
The most used mesoporous material for perovskite solar cells is TiO2, where the perovskite
penetrates into the pores and forms an interconnected layer. All solid state perovskite solar cells
with a mesoporous architecture was reported by Kim et al. and Burschka et al. achieving
efficiencies of 9.7 % and 15.0%, respectively. The PCE of the mesoporous perovskite solar cell
increased to 20% at the end of 2014 (10). Recently the highest efficiency of 23.2% on laboratory
scale was reported by Jaemin Lee & Jangwon Seo using TiO2 based mesoporous architecture.
High PCE values have been achieved using mesoporous architecture, however, the high sintering
temperature required for TiO2 mesoporous layer is a barrier to the commercialization of
perovskite solar cells on flexible device and low cost solar cells (Kim et al., 2012; Michael et al.,
2012, Jeon et al., 2014). Also, the high processing temperature adds complications to the process
by increasing the processing time and cost of making the solar cell devices. Snaith and co-worker
substituted the mesoporous TiO2 with insulating Al2O3 and the device was still working quite
well (Michael et al., 2012). This give an indication the perovskite can be used both as light
harvesting material and electron transporting layer and hence the PSC device can be completed
without the mesoporous layer. After the realization of the ambipolar nature and the longer
diffusion length of perovskite materials, more and more interest was developed into the planar
hetrojunction perovskite solar cells (Xing, et al., 2013; Stranks et al., 2013). These properties of
perovskite materials opened the possibilities of removing the high temperature sintered
mesoporous layer and make a simple planar solar cell structure, which can be processed at lower
temperature (Liu et al., 2013). A planar hetrojunction perovskite solar cell without the
mesoporous layer is shown in Figure 2.4b. Planar devices are basically of two types, the regular
structure (n-i-p) and inverted structure (p-i-n) depending on the order on the transporting layers
(Meng et al, 2016). In ni-p structure a TCO is coated with an ETL followed by perovskite and
then an HTL and finally a metal electrode as shown in Figure 2.5a while in a p-i-n structure TCO
is coated with a HTL then perovskite which is followed by ETL and a metal back contact as
shown in Figure 2.5b. Regular planar structures are the most explored PSCs due to the high
performance compared to the inverted structures and the ease of fabrication and simpler
architecture.

Figure 2.5 Device architecture of regular n-i-p (a) and inverted p-i-n (b) perovskite based solar
cell.

15
 2.7. Metal oxides in Perovskite solar cells
The low cost of manufacturing is not enough for the OPVs to compete with the traditional
devices, and more attention should be paid to the device low efficiencies and poor stability.
These two problems can be solved by introducing metal oxides into OPVs, with the expectation
to improve both of the performance and the life time of devices. Besides the usage of WO3,
MoO, V2O5, etc. as an electron buffer layer to improve the device performance (Schmidt, H., et
al., 2009), and the insertion of a TiOx layer as an optical spacer, metal oxides also show
attractive potential in DSSC, reaching a measured efficiencies as high as 12.3%. In addition,
wide bandgap inorganic semiconductor materials such as TiO2, ZnO are commonly applied in
hybrid solar cells. In this type of devices, light absorption and exciton generation take place in
the organic materials, while the metal oxides are used as an acceptor for electron transport. These
hybrid solar cells can benefit both from the high absorption coefficient of polymer in the visible
range, and the higher charge carrier mobility of electrons transport of inorganic materials.

2.7.1. TiO2 and ZnO metal oxides TiO2

As an attractive n-type semiconductor, TiO2 shows fundamental and practical advantages,
and is widely applied in the industry, such as photocatalysts, gas sensors, pigments, cosmetics
and solar cell (Wang et al., 2016). One dimensional titania nanostructures are even broaden the
applications due to their novel properties depending on its crystalline phase state, dimensions
and morphology (Yu et al., 2016).
ZnO

ZnO is a remarkable technological material with many attractive properties such as strong
piezoelectric and pyroelectric properties, antibacterial property and its bactericidal efficacy (Rao
et al., 2016). Due to the reduction in size, the one-dimensional ZnO shows novel electrical,
mechanical, chemical and optical properties, which are beneficial to be the application of surface
acoustic wave filters, photodetectors, light emitting diodes, gas sensors, optical modulator
waveguides, and solar cells (ibid).
Both of TiO2 and ZnO are used in OPVs because of their similar optical and electrical
properties. These two metal oxides are transparent to visible light, due to their respective wide
bandgaps of 3.2 eV and 3.37 eV, only absorbing in UV range of solar light. Their high electron
affinity, which stems from the position of their conduction bands, allows them to match with the
LUMO of almost all organic semiconductors (Perera et al., 2015). This high exciton binding
energy in TiO2/ZnO-organic photovoltaic device allows efficient exciton emission at room
temperature. In additional, the mobility of TiO2 presents with a range of 20 to 10-6 cm2V-1s-1,
highly depending on its structure (Irwin et al., 2008), and an even higher value of mobility can be
obtained by doping ZnO with Al (Jeng et al 2014). Since the mobility of metal oxides is much
higher than of the organic semiconductor, it provides an ultra-fast pathway for electron transfer
and generally improves the charge transport in the devices.

16
CHAPTER - THREE

MATERIALS AND METHODS

In this thesis we need to design using ternary blend system with two donors and one acceptors,
these two donors helps to maximizes absorption width of active photo layers of a device.

3.1. Material
3.1.1. The material we will use
 Different detergents (Acetone, alcohol cleaners, tape water isopropanols, Chloroform
(Solvent))
 Ultrasonic cleaner /ultrasonic bath/
 Conducting ITO glass.
 Cutter, meter tape,multi meter, soft paper.
 Differentsized sample holder/lml -10ml/
 Sucker, ITO slid holder. Spine coater, Glove. Ovum,
 UV-VIS
 Poly(3-hexylthiophene) (P3HT) Polythienothiophene/ benzodithiophene (PTB7), [6,6]-
phenyl C61-butyric acid methyl ester (PCBM)

3.1.2. For simulation work

We need also the software programs for simulation work. These are:
General purposed photovoltaic device model (gpvdm).
Scaps

3.2. Methods
In the optical and electrical characterization of ternary bulk hetero junction binary blend
structure of the form D1:D2:A o PTB7 and P3HT will be the donor part where photo active
layer and PTB7 is the acceptor polymer. This polymer thin film will be prepared by spine
coating method.

3.3. Method of simulation

Both electrical and electrical characterization will be done by General purposed photo
voltaic software program.
The graphical work will be done using origin software.

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CHAPTER 4

TIME SCHEDULE AND BUDGETALLOTMENT

4.1. Time Schedule
R. Activity Estimated time in G.C
No.
1 Reviewing literatures Dec.1st/2018 –Jan. 25/2019

2 Proposal development Jan. 26/2019 – Feb. 12/2019

3 Proposal defense Feb. 13/2019 – Feb.17/2019

4 simulation work Feb.18/2019 – Mar.3/2019

5 Collecting materials (Setup experiment) Mar. 4/2018 – Mar. 10/2019

6 Working on the experiment Mar.11/2019 – Mar. 20/2019

7 data analysis Mar. 21/2019 – Apr.4/2019

8 Thesis Writing Apr.5/2019 – May20/2019

9 Submission of the Thesis May.21/2019 – May31/2019

10 Presentation of the Jun1/2019 –

4.2. Budget Allotment

R. No. Description Cost in birr

1 Stationery materials 1000

2 Internet access 1000

3 Thesis printing 500

4 Transportation and allowance 1500

5 Laboratory payment 3000

6 Materials for device preparation 18000

Total 0

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