CHAPTER 1

Introduction and Scope of the Thesis

Contents

1.1 Solar Cell ............................................................................................................... 3

1.1.1 Origin of Solar Cells ....................................................................................... 4

1.1.2 Generation of Solar Cells ............................................................................... 5

1.2 Perovskite Solar Cell ............................................................................................. 8

1.2.1 Perovskite Material ......................................................................................... 9

1.2.1.1 Crystal Structure.................................................................................... 10

1.2.1.2 Optoelectronic Properties ...................................................................... 11

1.2.2 Working Principle of Perovskite Solar Cells................................................ 13

1.2.3 Numerical Modelling of Perovskite Solar Cells ........................................... 17

1.3 Fabrication Process for Perovskite Solar Cells .................................................... 21

1.3.1 Electrodes ..................................................................................................... 22

1.3.2 Photoabsorber Layers ................................................................................... 23

1.3.2.1 One Step Deposition via Spin Coating.................................................. 24

1.3.2.2 Two Step Deposition via Spin Coating ................................................. 25

1.3.2.3 Vapor assisted deposition technique ..................................................... 26

1
 1.3.3 Charge Transport Layers............................................................................... 26

1.4 Characterization Techniques for Perovskite Solar Cells ...................................... 29

1.4.1 Surface Characterization ............................................................................... 29

1.4.1.1 Atomic Force Microscopy ..................................................................... 29

1.4.1.2 Scanning Electron Microscopy .............................................................. 30

1.4.1.3 Transmission Electron Microscopy ....................................................... 30

1.4.1.4 X-Ray Diffraction .................................................................................. 31

1.4.2 Optical Characterizations .............................................................................. 32

1.4.2.1 Absorbance ............................................................................................ 32

1.4.2.2 Photoluminescence ................................................................................ 32

1.4.3 Optoelectronic Characterizations .................................................................. 33

1.5 Literature Review ................................................................................................. 35

1.5.1 Review of Perovskite Based Solar Cells ....................................................... 35

1.5.2 Review of TiO2 nanorods Based Perovskite Solar Cells .............................. 38

1.5.3 Review of ZnO Nanorod Based Perovskite Solar Cell ................................. 39

1.5.4 Major Observation from the Literature Survey ............................................ 41

1.6 Issues and Challenges in Perovskite Solar Cells .................................................. 42

1.7 Motivation and Problem Definition ..................................................................... 43

1.8 Scope of the Thesis .............................................................................................. 44

2
 CHAPTER 1

Introduction and Scope of the Thesis

1.1 Solar Cell

Modernization and urbanization of societies and industries have significantly

increased the use of energy in everyday life. Over the past several years, increased

consumption of non-renewable resources like coal, petroleum, and gases will

exhaust all the available natural sources [1]. The limited resources of fossil fuels

and environmental pollution caused by the burning of fossil fuels have primarily

encouraged the researchers for clean and green energy harvesting through

renewable energy sources to meet the high demand for energy in the various

electrical and electronic applications [2]. The major non-renewable and renewable

sources are shown in Figure 1.1.

Solar energy is considered one of the most important renewable sources to

solve the present and future energy problems because of its abundant availability

and pollution-free generation [3]. Solar energy is converted to electrical energy by

a typical solar cell, which has the edge of resolving large energy demand, less

effect on climate change, control global warming, clean and unlimited energy

source [3]. Various types of solar cells are fabricated using different photoactive

materials, namely silicon, germanium, gallium arsenide, cadmium telluride, dyes,

conducting polymers, perovskites, etc. [4]. The power conversion efficiency

(PCE) of the solar cell primarily depends on photoactive materials and the device

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Chapter 1 Introduction and Scope of the Thesis

structures.

Figure 1.1: Various non-renewable and renewable energy sources.

1.1.1 Origin of Solar Cells

The Concept of the photovoltaic effect was first experimentally discovered by

French physicist Edmond Becquerel in 1839 [5]. Later, Willoughby Smith passed an

electric current through selenium and found the effect of light on electric current in

1873 [6]. He described that the light is striking a photosensitive semiconductor material

result in electrical energy. This remarkable discovery is considered as the beginning of

the new era for the conversion of solar energy into electrical energy using a solar cell

[7]. In 1883, Charles Fritts invented the first solid-state photovoltaic (solar) cell using

the thin film coating of gold on light-sensitive material selenium to form the junctions

[8]. Although this device had only 1% efficient, it paved the way for future solid-state

photovoltaic devices [8]. In 1905, Albert Einstein proposed a new theory of quantum

physics and described the photoelectric effect [9]. The research on photovoltaic cells

4
Chapter 1 Introduction and Scope of the Thesis

was revolutionized after experimental validation of Einstein‟s photoelectric theory in

1916 and the Nobel Prize to Albert Einstein in 1921 [9].

Further, p-n junction based solar cells using Cu2O and Ag2S was reported by

Vadim Lashkaryoy in 1941. The modern device structure of the junction based solar

cells was proposed by Russell Ohl [10] in a patent filed in 1946. The first commercial

standard solar cell having 6% power conversion efficiency (PCE) was developed using

inorganic materials by Daryl Chapin, Calvin Fuller, and Gerald Pearson in Bell

Laboratory in 1954 [11]. Later, the production of solar cells was started for commercial

use. The first silicon-based solar cell panel was incorporated into US satellite Vanguard

1 in 1958 [Internet Source]. Several satellites such as Explorer III, Vanguard II, and

Sputnik-3 were launched with solar cell-powered system onboard in the next few years.

1.1.2 Generation of Solar Cells

Silicon (Si) is the most widely used material in the electronic industry for the last

seven decades. Fabrication of Si-based solar cells requires high-temperature processing

and various sophisticated requirements, including nanofabrication facilities, ultra-high

vacuum deposition process, etc. Further, the preparation of Si from silica is a high

energy draining method, which makes very low energy payback time (EPBT) of the

silicon-based solar cells. Moreover, the management of extremely large e-wastes

resulting from the worldwide use of solar cells made of Si and other inorganic

semiconductors have become a challenge for environment and water management

systems. Thus, rigorous research is focusing on developing organic and hybrid

semiconductors for environment-friendly and low-cost solar cells. The organic material

was first introduced in the solar cell by Calvin in the 1960s [12]. The idea of a multi-

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Chapter 1 Introduction and Scope of the Thesis

junction solar cell came in 1970, and the layer of different semiconductor material was

used in a solar cell to absorb a broad spectrum. The real breakthrough in the field of

photovoltaic devices came with the introduction of two different photosensitive

semiconductors instead of one semiconductor material by Tang in 1986 [13]. The

German and French scientists built a multi-junction solar cell and recorded 46% power

conversion efficiency in 2014 [14]. The solar cell development using various types of

material in the different periods is categorized into four generations of the solar cells, as

illustrated in Figure 1.2.

The present day‟s solar market is dominated by the first two generations [15], [16].

The first generation includes eminent and medium-cost technologies (i.e., mono or

polycrystalline Si and GaAs based solar cells), which results in moderate yields. The

second-generation mainly comprises thin film (TF) technology-based solar cells that are

cheaper to manufacture but have lower efficiency [16]. Copper Indium Gallium

Selenide (CIGS) and CdTe/CdS based solar cells are examples of the second generation.

Then, the third-generation solar cells are very efficient but expensive as they explore the

usage of novel materials and the variability of designs [17]. They involve technologies

based on newer compounds, stacked or tandem multilayers of III-V materials

(inorganics based), Quantum Dots (QDs), Dyed Sensitized Solar Cells (DSSCs), etc.

Finally, the fourth generation is currently under investigation and is also recognized

as “Inorganics-in-Organics”. It mainly syndicates the flexibility or low cost of polymer

TFs with the efficient and stable novel inorganic nanostructures (such as metal oxides

and nanoparticles) along with organic-based nanomaterials (like graphene or its

derivatives, carbon nanotubes) [18]. The recent development in solar cell manufacturing

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Chapter 1 Introduction and Scope of the Thesis

includes solar cells made of several organic, inorganic, and organometallic halide

perovskite materials. The exceptional improvement in the power conversion efficiency

from 3.8% to 25.2% in the perovskite solar cell (PSC) has been observed in a very short

span of time. Moreover, the cost to manufacture a perovskite solar cell is a fraction of

the cost of other thin-film technology with almost equivalent performance. Although

PSC has lower stability over time, it has several advantages, such as low-temperature

solution processing and the ability to produce a flexible solar cell. All these advantages

and the availability of a wide range of perovskite materials can be used to fabricate

more exciting and appropriate solar cells.

Figure 1.2: Different generations of solar cells.

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Chapter 1 Introduction and Scope of the Thesis

1.2 Perovskite Solar Cell

Perovskite solar cell comes under the most recent generation of the solar cell. It has

the inherent advantage of high efficiency, low cost, and easy fabrication process. The

PSCs are made using perovskite structured material as photoactive. The efficiency of

the PSC depends mainly on the perovskite material used. The achieved efficiency for

perovskite solar cells is comparable to the Si and other materials-based solar cells‟

efficiency, as shown in Figure 1.3. The improvement in the PCE has been achieved by

optimizing the thin-film processing technology and bandgap engineering of the

perovskite film in the solar cells [19]. Efficiency has also been enhanced by choosing a

suitable wide bandgap organic or inorganic material for electron transport layers

(ETLs), hole transport layers (HTLs), and bandgap alignment in the device structure.

Moreover, the defects such as pinholes and grain boundaries created during the

fabrication process and the material properties such as extinction coefficient, carrier

mobility, diffusion length, bandgap, etc., affect the PCE of the solar cells.

Figure 1.3: Comparative rapid growth in PCE for perovskite-based solar cells [20].

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Chapter 1 Introduction and Scope of the Thesis

1.2.1 Perovskite Material

Perovskite material was first discovered by the Russian mineralogist Gustav Rose

in 1839 in a piece of chlorite rich skarn. The mineral was CaTiO3, named after the

legendary Russian mineralogist Count Lev A. Perovskiy [21]. Later, the name

perovskite was also referred for the three-element metal oxides such as PbTiO3, BiFeO3,

BaTiO3, etc., having a perovskite structure with the formula ABO3. These metal oxides

find use in several ferroelectric, dielectric, pyroelectric, and piezoelectric applications.

Nowadays, the name perovskite does not only refer to metal oxides but also to halides,

which have halide anions (F-, Cl-, Br-, I-) in place of oxide anions (ABX3; A = cation, B

= divalent metal cation, X = halogen anion). The halide perovskite was discovered by

Well et al. in 1893 during their experiments on the synthesis of cesium-based lead

halide compounds, CsPbX3 (X = Cl, Br, I) [22]. The real breakthrough for the

perovskite occurred in 1957, when C. K. Møller, a Danish researcher, reported that

CsPbCl3 has the perovskite structure [23].

The first three-dimensional organic-inorganic hybrid perovskite was discovered by

replacing cesium in CsPbX3 (X = Cl, Br or I) with methylammonium cations (MA =

CH3NH3+) by Dieter Weber in 1978 [24]. The organic-inorganic hybrid perovskite

(CH3NH3PbI3) is most commonly used as photoactive material for making highly

efficient photovoltaic and optoelectronic devices. The lead halide perovskite materials

possess unique and much suitable semiconductor properties such as high absorbance

coefficient, direct bandgap, large diffusion length, etc., which allow them to manifest

into efficient photovoltaic cells and other optoelectronic applications such as light-

emitting diodes (LEDs), photodetectors, X-ray detectors, and so on. Despite their

eccentric electronic and optoelectronic properties, the two major issues are that stability

9
Chapter 1 Introduction and Scope of the Thesis

and toxicity of the perovskite materials have held up their commercial applications in

the current scenario. In this thesis, 3D organic-inorganic lead halide perovskites have

been analyzed for photovoltaic applications.

1.2.1.1 Crystal Structure

The crystal structure of hybrid perovskite compounds is similar to CaTiO3 or in

more general form ABX3. Typically, the „A‟ atoms are larger than the „B‟ atoms. The

„A‟ and „B‟ cations coordinate with 12 and 6 „X‟ anions, respectively, to form

cuboctahedral and octahedral geometries, as shown in Figure 1.4.

Table 1.1: Comparison of optical properties of perovskite with other materials.

The halide perovskites have semiconducting properties that are highly desirable for

photovoltaic (PV) and other optoelectronic applications. The essential parameters of

perovskite are compared with other common materials used for solar cell fabrication

and listed in Table 1.1.

10
Chapter 1 Introduction and Scope of the Thesis

Figure 1.4: (a) Structure of ABX3 perovskite (b) Cubic unit cell of CH3NH3PbI3 [25].

1.2.1.2 Optoelectronic Properties

The ionic nature of organic-inorganic halide perovskite materials and their

semiconducting properties, i.e., allow a free tuning of absorption edge wavelength

(bandgap) and optical absorption by varying or combining halide ions (Cl, I, Br),

thereby developing mixed-halide solid solutions. Methylammonium lead iodide

(MAPbI3) is a unique intrinsic semiconductor [26] showing superior mobility of both

photogenerated holes and electrons. Figure 1.5 illustrates the band diagram for MAPbI3,

where the valence band (VB) incorporates nearly ~25% Pb 6s2 orbitals (lone pair) and

70% I 5p orbitals, while the conduction band (CB) contains a mixture of 6p2 orbitals of

Pb and several other orbitals. In such a case, there exists strong coupling in VB orbitals

between I 5p orbitals and Pb lone-pair 6s2 [27]. The structure of MAPbI3 is highly

symmetric, which results in the direct bandgap in this material. Moreover, Pb s orbital

lone pair enables p-p electronic transitions from VB to CB. Therefore, the combination

of these factors imparts extraordinarily high optical absorption coefficients to MAPbI3

(~105 cm−1) [28]. The acclaimed defect tolerant properties of perovskite material are

credited to its ionic characteristics, strong I p-Pb s anti-bonding coupling, and weak I p-

Pb p coupling [27]. This property is replicated by the MAPbI3 having large carrier

11
Chapter 1 Introduction and Scope of the Thesis

diffusion lengths. Moreover, as MAPbI3 is an intrinsic semiconductor, it has ambipolar

carrier mobility owing to its ionic crystal. On a final note, recombination between

electrons and holes is suppressed in halide perovskites due to a charge-screening effect

against Coulombic interaction, which is assisted by the high ionic density of the

perovskite.

Figure 1.5: Band structure of MAPbI3 [29].

The optical bandgap of the hybrid perovskite material can be tuned by the changing

in “A” site cation and “B” site cation [30]. Perovskite quantum dots (QDs) offers

tunability with size, which can be controlled using changing the concentration of the

capping agents or surfactants (octyl amine and oleic acid) [31]. CH3NH3PbI3 also has

the ability to be used in photodetector because it shows the high gain of the

photoinduced current, which exceeds 100%. Nowadays, perovskites have potential use

for color image sensors in digital cameras. The perovskite-based color sensors and

photodetectors take advantage of the low-cost and printable semiconductors and are

expected to be commercialized soon. Perovskites are also used for the detection of X-

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Chapter 1 Introduction and Scope of the Thesis

Rays in the medical diagnosis. It works well because lead-based perovskites have high

absorption coefficients for X-ray radiation. Samsung‟s research group and Park et al.

reported the detection of an actual X-Ray image by using a 2D patterned image sensor

[32]. Organic lead halide perovskite-based optoelectronic semiconductor devices may

also be used as radiation sensors for space explorations owing to their high defect

tolerance nature.

1.2.2 Working Principle of Perovskite Solar Cells

The perovskite-based solar cells are commonly made in p-i-n structure, as shown in

Figure 1.6. The organic-inorganic hybrid perovskites (i.e., CH3NH3PbI3, CH3NH3PbBr3,

CH3NH3PbI3, etc.) are used as intrinsic (i) layer and work as photoabsorber. The wide

bandgap conducting polymers (i.e., spiro-OmeTAD, PTAA, etc.) and metal oxides (i.e.,

TiO2, ZnO, etc.) are used for the p and n layer, respectively. The photovoltaic operation

in the p-i-n structured perovskite solar cell is based on three basic concepts: (a)

Generation of charge carrier in absorber (i) layers, (b) Separation of charge carrier by

transport (p and n) layers, and (c) Collection of charge carrier at electrodes. When the

solar cell is illuminated with sunlight having photon energy (hv) greater than the

bandgap (Eg), the photon is absorbed by the absorber layer, and the charge carrier is

generated. Due to the internal electric field, the electron-holes pair are separated by

consecutive electron and hole transport layers. If these charge carriers are not separated,

they will recombine shortly. Finally, the charge carrier is collected by the top and

bottom electrodes of the solar cell and creates the photovoltage across the device. The

polarity of the output voltage is the same as the “forward bias” direction of the device,

but the photocurrent is opposite to the direction of the forward current through the

13
Chapter 1 Introduction and Scope of the Thesis

device under dark condition.

Solar light causes a current (I) to flow from the solar cell to the load. The magnitude

of this current (I) is the algebraic sum (without sign) of generated current (IPH), the

current flowing in the non-linear junction (ID), and the current passing through shunt

resistance (ISH). The equivalent electrical circuit of the solar cell has been shown in

Figure 1.7. The I-V characteristics equation for the equivalent electrical circuit of the

solar cell under illuminating condition is given as,

where IPH is photogenerated current, IS is reverse saturation current, and VT is the

thermal voltage.

Top Electrode

HTL

Perovskite (Absorber Layer)

ETL

TCO

Glass

Figure 1.6: General structure of perovskite solar cell.

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Chapter 1 Introduction and Scope of the Thesis

Figure 1.7: Electrical equivalent model of the perovskite solar cell [16].

It is evident from the I-V characteristic equation that solar cell parameters are

greatly affected by series and shunt resistance. The impact of series resistance and shunt

resistance on solar cell parameters are analyzed by the equivalent circuit shown in

Figure 1.7. The series resistance RS, represented in the equivalent circuit, can arise due

to the contact resistance, resistance of semiconducting layers, and the contact resistance

of electrodes. The shunt resistance, RSH, can similarly be caused by various factors, such

as surface leakage along the edge boundaries, crystal defects, or pinholes in the surface.

The thickness of the different layers is also a factor that changes the resistance of the

device and affects the performance of the solar cell. It can be observed that the zero

value of RS and the infinite value of RSH gives the best performance (ideal case).

The performance of the solar cell is specified using four main solar cell parameters,

namely open-circuit voltage (VOC), short circuit current (JSC), fill factor (FF), and power

conversion efficiency (ƞ). The electrical equivalent model, shown in Figure 1.7, and the

current density-voltage (J-V) curve, shown in Figure 1.8, are used to analyze these

characteristics of the solar cell. For all the values of J and V, the product of these two

quantities gives the power density (P=J×V), and the product has a maximum value

(called Pmpp) at a particular current and voltage value (called Jmpp and Vmpp). The

short circuit current density and open-circuit voltage are described as:

15
Chapter 1 Introduction and Scope of the Thesis

I (at V=0) = ISC and

V (at I = 0) = VOC

Or

ISC = Im = Iℓ and VOC = Vm for forward-bias power quadrant

Figure 1.8: J-V curve for PSC device where the red curve is for under illumination and black is for
dark. The area in the shade gives maximum achievable power.

On the other hand, the fill factor (FF) is evaluated by comparing the maximum

power to the theoretical power (PT). The fill factor also reflects the J-V curve‟s

squareness, which represents the effects of resistive and recombination losses. The

fill factor depends on open circuit voltage and short circuit current density, so there

can also be a reduction in short circuit current below the value of the photocurrent

due to the forward biasing across the junction as a result of the voltage drop across

the series resistance (RS). Finally, power conversion efficiency (ɳ) is defined as the

ratio of output energy of the solar cell (PMPP) to the incident optical power density

from the sun (Pin). PCE can also be written as,

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Chapter 1 Introduction and Scope of the Thesis

1.2.3 Numerical Modelling of Perovskite Solar Cells

A numerical simulation-based modeling approach is performed to develop a

highly efficient solar cell, obtain optimum solar cell parameters, and understand the

device physics. The fabrication cost can be minimized by optimizing device

characteristics using technology computer-aided design (TCAD) simulation. Several

TCAD simulation tools, such as SCAPS-1D, Lumerical, AMPS, SETFOS (Fluxim),

etc., are available for the electrical and optical simulation of organic and hybrid

perovskite solar cells [33]. Among these two popular tools, SCAPS-1D and SETFOSTM

(Fluxim) are used in the present thesis work for the validation of the experimental

results.

The numerical simulation model for the DSSC, silicon solar cell and other

hybrid solar cells explains the behavior of the charge movement and the factor that

affects the performance of the device. The performance of DSSC is based on the redox

level in electrolytes, so the model includes a solvent that conducts ions and current.

Similarly, the simulation models of the silicon-based photovoltaic device involve the

doping that forms the p-n junction. The working principle of the perovskite solar cell is

different from DSSC and silicon solar cells. The redox level and doping level is not

required for the PSCs simulation model. Electrons and holes contribute to the current

generated by photons energy in the PSC. So the numerical simulation plays a vital role

in understanding carrier transportation mechanism and device performance.

17
Chapter 1 Introduction and Scope of the Thesis

The SCAPS-1D simulator can efficiently simulate the CIGS, CdTe, and

crystalline solar cell (Crystalline Si and GaAs ), which comprises up to 7 layers,

whereas SETFOS can also be used for simulation of organic LED, solar cell as well as

tandem solar cell structure with different layers. The simulation and modeling of hybrid

perovskite solar cells are reported first time by Agrawal et al. [34] in 2015. The

SETFOS simulation tool includes four different types of models, such as drift-diffusion,

absorbance, advanced optics, and emission for a solar cell with a fitting and

optimization algorithm. The Emission module deals with dipole emission, full-

spectrum, color filter, and substrate optics, whereas the scattering module helps in

improving the optical efficiency of the material. The charge generation, recombination,

and transportation profile are managed by the absorption and drift-diffusion modules.

The basic parameters required for electrical simulation are electron affinity (X),

Bandgap (Eg), dielectric constants, the density of states of the conduction band and

valence band (NC and NV), mobility of electrons and holes (µe and µh), the thermal

velocity of electrons and holes, doping concentration of acceptor and donor (NA and

ND), absorption coefficient, defect density (Nt) of different layers and also working

temperature.

In these simulation tools, there is the flexibility to choose parameters either

graded or different profiles. Optically we can separately define reflection, transmission,

and other parameters. It also provides the option to vary the illumination spectra such as

AM1.5G, AM0, AM1.5D, etc. SCAPS-1D provides generation- recombination status,

energy band, quantum efficiency, and J-V characteristics, which is used for the

calculation of open-circuit voltage (Voc), short circuit current (Jsc), power conversion

efficiency (PCE), and fill-factor. The sweep function is also available in the tools,

18
Chapter 1 Introduction and Scope of the Thesis

which is very useful for optimizing the performance of the solar cell by varying the

material parameters such as thickness, defect density, doping, etc., of the different

layers.

The simulation tools solve Poisson‟s equation for the electric field, which

depends upon charge flow and trap centers in the material. Poisson‟s equation for a

semiconductor can be stated as [35]:

The current density due to electron or hole at any point in the device must be identical at

all the points under the steady-state condition in dark condition. However, generation

and recombination affect the current density under illumination. The continuity equation

for electrons and holes can be inscribed as [35]:

Also, the total currents are calculated from drift and diffusion components as [35]:

19
Chapter 1 Introduction and Scope of the Thesis

Where Jn and Jp are current densities due to electron and hole, G is generation rate R is

recombination rate, ɸ is electrostatic potential, e is electric charge,

ɛo is vacuum permittivity, ɛr is relative permittivity, p and n are electron and hole

concentration, NA and ND are charged impurities of donor and acceptor and is defect

charge density.

The recombination models commonly used are band-to-band recombination,

SRH (Shockley-Read-Hall) recombination, and Auger recombination, as illustrate in

Figure 1.9. Band to band (rediative recombination) and Auger recombination are

unavoidable because these are intrinsic material properties. Band-to-band recombination

is the process in which the electrons fall back from the conduction band to the valence

band to recombine with holes. The expression for the band to band recombination is

given as [35]:

Where is the recombination coefficient. The SRH (Shockley-Read-Hall)

recombination takes place due to the impurities and defects in a material. These defects

create trap states, which are essential for this recombination; hence it is also called trap-

assisted recombination. The rate of this recombination can be expressed as:

20
Chapter 1 Introduction and Scope of the Thesis

Where and are electron and hole lifetime, ni is intrinsic carrier concentration.

Figure 1.9: Band-to-band (radiative recombination), SRH, and Auger recombination.

SRH recombination occurs extensively when defects between crystal grains are

present and cause increased trap states. Therefore, single-crystal materials have

generally lesser SRH recombination due to extrinsic impurities and dangling bonds at

the surface. In Auger recombination, the energy released by a band to band

recombination of an electron-hole pair is given to another carrier. It can be expressed as

[35]:

Where and are constants.

1.3 Fabrication Process for Perovskite Solar Cells

The perovskite solar cell is a multilayered thin-film device. The different layers,

namely electrodes (anode and cathode), photo absorber, hole transport layer, electron

transport layer, and other interface layers, are composed of different materials in the

21
Chapter 1 Introduction and Scope of the Thesis

solar cell structure. Some key processing steps should be considered for high-

performance solar cells. First of all, a suitable bandgap material (absorber material) is

chosen to absorb the maximum solar light spectrum, the next one is selecting the wide

bandgap electron and hole transport material to separate maximum charge carriers, and

finally, the suitable work function material for electrodes are required to collect the

maximum charge carriers. This section briefly discussed the various layers of perovskite

solar cells.

1.3.1 Electrodes

The suitable top and bottom electrodes are used for better collection of charge carriers

to get high-performance solar cells. Different physical vapor deposition (PVD)

techniques such as sputtering, thermal, electron beam, etc., are commonly used to obtain

a thin film of metals (Al, Ag, Cu, Au, Pd, Pt, etc.) and doped semiconductors (ITO,

FTO, etc.) for the electrodes.

Figure 1.10: Block diagram of physical vapor deposition.

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Chapter 1 Introduction and Scope of the Thesis

A block diagram of a typical PVD system is illustrated in Figure 1.10. The ITO/FTO is

primarily deposited by the sputtering method and used as transparent conducting oxide

electrodes at the bottom of the solar cell so that maximum light can pass through it.

Noble metals, Au and Pd, with high work function (~4.8 to ~5.1 eV) are the most

suitable top electrodes (avoiding fast oxidation and degradation) for perovskite solar

cell is commonly deposited by thermal and electron beam evaporation. Thermal

evaporation is also employed to deposition high purity Ag/Al film as a top electrode on

both the solution-processed and thermally deposited metal oxide ETL.

1.3.2 Photoabsorber Layers

The photoabsorber layer is the most crucial layer, and perovskite material is used

for this layer. The LUMO and HUMO level of hybrid perovskite material can be easily

modified by using the composition of different halide anions (i.e., the composition of

Cl, Br, and I in hybrid perovskite), and thus the absorbance coefficient changes. In the

last few years, the power conversion efficiency of perovskite solar cells has been

remarkably increased by gradual development in deposition techniques of perovskite

thin film. The film coating controls the morphology and crystallinity of hybrid

perovskite material that affects the absorbance coefficient of perovskite material and

significant impact on short circuit current density. The stability of perovskite film also

depends on the growth and crystallinity of perovskite material.

The high solar cell performances achieved in this short period are primarily due to

the extensive efforts that have been invested over the years to develop and optimize

these perovskite thin film deposition procedures [27]. These deposition methods include

single step deposition [36], sequential deposition [37], vapor assisted deposition (via

23
Chapter 1 Introduction and Scope of the Thesis

single source and dual source) [38], screen printing, and anti-solvent methods [39].

Among these, sequential deposition and anti-solvent methods are two of the most

widely used (methods) for the fabrication of solar cells. The PCE of hybrid PSC is

achieved over 25.2% via sequential deposition technique [40]. In general, the spin

coating deposition technique has enormous potential for the development of highly

efficient perovskite solar cells for large scale production. Therefore, these achievements

are very promising for the scale up of perovskite photovoltaics in the near future [27].

The three common deposition methods are discussed in following sub-sections:

Figure 1.11: Deposition methods for the perovskite thin film: (a) One step deposition, (b) Two step
deposition, and (c) Physical vapor deposition [41].

1.3.2.1 One Step Deposition via Spin Coating

The perovskite thin-film is initially deposited via spin coating in a single step on

mesoporous metal oxide films using an equimolar ratio of PbX2 and CH3NH3X in a

common solvent [42]. Polar solvents, such as dimethylformamide (DMF),

24
Chapter 1 Introduction and Scope of the Thesis

dimethylsulfoxide (DMSO), g-butyrolactone(GBL), or N-methyl-2-pyrrolidone (NMP),

are commonly used [43]. Spin coating is followed by heating the film at 70–150oC to

evaporate the high-boiling point solvent and in order to increase the grain size and

crystallinity of hybrid perovskite (MAPbX3) thin film [43]. The perovskite deposited

substrate is converted from light yellow to dark brown after cooling at ambient

temperature and confirmed the formation of perovskite film [42]. The problem with

these deposition methods is that non-uniformity and uncontrolled grain size. Typically,

it leads to uncontrolled precipitation of the perovskite with large morphological

variations in photovoltaic performance in the resulting devices.

1.3.2.2 Two Step Deposition via Spin Coating

A sequential deposition method for perovskite thin layer is first employed by Liang et

al. [28] to enhance the performance of PSC. For the preparation of organic-inorganic

lead halide (CH3NH3PbI3) thin film, the solution of organic compound (PbI2) is

infiltrated in the mesoporous layer of Al2O3 or TiO2, and the film is dried by

evaporating the solvent. In the 2nd step, the PbI2 deposited substrate is dipped into the

solution of organic compound (CH3NH3I). Conversion of perovskite thin-film occurs

within the mesoporous when both the compound organic and inorganic comes into

contact, permitting much better control over the perovskite morphology. The sequential

deposition technique is further optimized for performance improvement of hybrid PSCs

by Burschka et al. [37]. This significantly increased the reproducibility of device

performance and achieved a PCE of approximately 15%. Further, with the development

of technology, the anti-solvent technique has been employed for enhanced performance

parameters of perovskite solar cells. Jeon et al. [44] have used an anti-solvent method

25
Chapter 1 Introduction and Scope of the Thesis

during deposition of perovskite thin film to fabricate highly efficient hybrid perovskite

solar cells.

1.3.2.3 Vapor assisted deposition technique

Vapor assisted deposition process is a novel techniques for the deposition of high

quality and uniform perovskite thin film. In this method, the first step includes the

deposition of the inorganic compound via solution process followed by deposition of

physical vapor of organic compound from a single source. This technique provides full

surface coverage and moisture stability in a non-vacuum solution. The vapor assisted

solution process is distinct from other traditional solution deposition techniques because

it decelerates nucleation and permits robust reorganization of thin-film growth. Ping fan

et al. reported a PCE of 10.90% for perovskite solar cells based on the single-source

physical vapor deposition method [32]. The vapor deposition via dual-source process is

used for well-defined grain structure, extremely uniform deposition, and full surface

coverage of perovskite thin-film without post-heating treatment. But the simultaneous

control of evaporation of both the organic and inorganic compound is very difficult to

handle with dual-source evaporation methods.

1.3.3 Charge Transport Layers

Wide bandgap hole blocking and electron blocking materials are commonly used

for ETL and HTL, respectively, in the PSCs. The structural and material engineering of

the ETL and HTL plays an important role in the performance improvement of the PSCs.

Efficiency can be enhanced by choosing a suitable wide bandgap material for ETL,

HTL, and bandgap alignment in the device structure [45]. The primary function of the

ETL and HTL layer is to extract the electron and hole from the absorber layer and

26
Chapter 1 Introduction and Scope of the Thesis

transport them to electrodes. The energy band alignment between HTL (ETL) and

absorber layer plays an essential role for transportation of hole (electron) and blocking

of the electron (hole) to minimize charge carrier recombination.

The structural, chemical, electrical, and optical properties of the charge transport layers

(ETL/HTL) strongly depend on the deposition techniques and the environment under

which the deposition of the materials is performed on the desired substrate. Various

methods such as spin coating, thermal evaporation, electrochemical, hydrothermal,

spray coating, screen printing, vapor deposition, chemical bath deposition, etc., are

employed for the deposition of ETL and HTL of the solar cell. The comparison of

various methods is listed in Table 1.2. The spin coating and screen printing (Blade

coating, slot die coating), spray coating, hydrothermal, etc., are chemical solution-

based, low-cost techniques. Metal oxide electron transport layers such as TiO2 and ZnO

are deposited by all techniques. However, the hydrothermal method is mostly used for

the nanostructures (nanorods, nanowires, nanotubes, etc.) based thin film of TiO2, ZnO,

and other metal oxides at low cost and low temperature. The hydrothermal process

provides a controlled size and shape of the nanostructures with highly crystallized and

uniform.

27
Chapter 1 Introduction and Scope of the Thesis

Table 1.2: Comparison of different deposition techniques for charge transport layers.

Methods Advantages Disadvantages

Sputtering: Thin-film is deposited by 1. Large surface area 1. It may be surface damage
ion bombardment of the source (target) deposition in the substrate used.
by generated plasma on the desired 2. Good reproducibility 2. A high-cost vacuum
substrate. chamber is required

E-beam evaporation: The electron 1. Good for liftoff 1. Some CMOS processes
beam is used to deposition thin-film by 3. Highest purity sensitive to radiation and
the transformation of atoms into the 4. High precision of film heat
gaseous phase. thickness 2. Difficult for alloys
5. Ease of operation 3. Poor step coverage and
6. Excellent material decomposition
utilization 4. A high-cost vacuum
chamber is required
Chemical bath deposition (CBD): The 1. Simple and low-cost 1. Lack of reproducibility
thin film layer is deposited by dipping experimental set up requires compare to other chemical
substrates in solutions with ions of 2. Easy Control of the deposition methods
interest (particularly metal ions). thickness of the film
Sol-Gel: This method is based on 1. Excellent composition 1. Film thickness control
inorganic polymerization reactions in control 2. Optimization for the
which colloidal solution is used for 2. Can be used for large scale particle size of the colloidal
deposition of the thin film of metal production. solution required
oxide. It includes four steps: hydrolysis, 3. Low cost and low- 2. Surrounding environment
polycondensation, drying, and thermal temperature technique affects the film
decomposition. 3. Porosity control is
difficult
4. Multiple thin film layer
may cause cracks

Chemical Vapor deposition (CVD): 1. High purity 1. High temperatures

Deposition by chemical reaction of 2. Relatively high deposition requirement
reactants at the substrate surface and rates 2. The precursors material
energy for the reaction is provided by 3. High quality, high- should be volatile at room
heat. performance thin film temperature
PECVD: The chemical vapor deposition 1. Less temperature required 1. In some cases, toxic
the technique uses plasma to improve 2. Good material properties of precursors are needed
the yield and performance of the the deposited thin film. 2. High-cost equipment
synthesis 3. Plasma could damage the
deposited thin films
Spray Pyrolysis: The traditional 1. Simple and low– cost 1. Size of droplets of the
multisource CVD process can be method initial solution is not the
converted to a single-source deposition 2. Suitable for large scale same
process by spraying a solution onto a deposition which can lead to
surface at an adequate temperature inhomogeneity in the
material
Hydrothermal deposition: This is a 1. Better control of size 1. Optimization is required
simple deposition technique that requires and shape of thin film for the process
a specific growth temperature under high 2. It is possible to obtain high 2. Problems in
pressure to synthesize single-crystalline crystallized nanostructures reproducibility.
material using an aqueous solution. This
is commonly used for the deposition of
TiO2/ZnO nanorods.

28
Chapter 1 Introduction and Scope of the Thesis

1.4 Characterization Techniques for Perovskite Solar Cells

The different types of characterization techniques are commonly used to analyze

and verify different layers in the solar cell structure. The characterization includes
surface, optical, and optoelectronic for the thin films and devices, as follows:

1.4.1 Surface Characterization

Some essential surface characterizations are performed to understand the basic

properties of the synthesized perovskite and other associated materials in the solar cell
structure. The brief discussion on these characterization tools are discussed in the
following subsections:

1.4.1.1 Atomic Force Microscopy

Atomic force microscopy (AFM) is a surface imaging technique used to find the

surface topography of the deposited film. It is a type of scanning probe microscopy

(SPM) and is widely used for height, surface roughness, and magnetism measurements.

The resolution of the AFM is in the order of a fraction of nanometer that is 1000 times

higher than the optical diffraction limit. Surface imaging through AFM is taken place by

measuring the force between the tip of the probe and the sample under process, as

depicted in Figure 1.12 (a). Three modes of operations of the AFM are as follow:

Contact mode: The raster scanning of the sample with respect to the tip of the probe

for a minimal area is taken place to obtain the image.

Non-contact mode: In this mode of operation, the probe oscillates at the resonance

frequency, the sample under process is kept standstill, and the force between probe and

sample is measured to get the exact image.

29
Chapter 1 Introduction and Scope of the Thesis

Tapping mode: It is somewhere between contact and non contact mode of operation

and takes advantage of the above two. The sample is escaped from being damaged by

incorporating intermittent contact.

1.4.1.2 Scanning Electron Microscopy

Scanning electron microscopy (SEM) is a powerful tool to get the morphological,

topographical, and compositional information of the sample through a focused beam of

electrons. High resolution (better than 1 nanometer) image is produced by raster

scanning of the electron beam and combining the beam‟s position with the detected

signal, as depicted in Figure 1.12 (b). The most common mode of operation of the SEM

is the detection of the secondary electron emitted by the atoms excited by the electron

beam. The topographical image of the sample is produced by detecting the number of

secondary electrons emitted by atoms excited by the electron beam. Energy dispersive

X-ray (EDX) attached with SEM is used to find the elemental composition of the

sample. Data produced by EDX analysis shows spectra consist of unique peaks

corresponding to the elemental composition of the material.

1.4.1.3 Transmission Electron Microscopy

Transmission Electron Microscopy (TEM) is a technique to study the topography,

morphology, composition, and crystallography of the material. TEM utilizes high

energy electrons and electromagnetic lenses for characterization, as depicted in Figure

1.12 (c). TEM generates high-resolution images with a maximum resolution near 0.5Å.

The selected area electron diffraction (SAED) pattern is primarily used for

crystallographic measurement utilizing the wave nature of electrons instead of particle

30
Chapter 1 Introduction and Scope of the Thesis

nature. A high energy (~100-400 keV) parallel beam of the electron is passed through

the sample. The electron beam has a wavelength in the nanometer range compared to

atom spacing in the prepared sample. Electrons are diffracted by atoms of the material,

and the scattering angle defines the crystal structure of the sample.

1.4.1.4 X-Ray Diffraction

X-ray Diffraction (XRD) is a widely used tool to extract information about crystal

structure, phase, texture, and other parameters such as average grain size, defects, etc.,

of the nanomaterial. Data produced by XRD analysis is based on the diffraction pattern,

as depicted in Figure 12 (d). The X-Ray strikes on each set of lattice planes of the

sample at a specific angle in the process of the extraction of the sample information.

Figure 1.12: Meaurement setup for (a) AFM (b) HRSEM, (c) TEM, and (d) XRD.

31
Chapter 1 Introduction and Scope of the Thesis

1.4.2 Optical Characterizations

The optical characterization technique is a non-contact and non-destructive type

analysis. It is commonly used to find the parameters such as absorbance, reflectance,

transmittance, emission, film thickness, crystal structure, optical constant, etc., of the

sample under investigation.

1.4.2.1 Absorbance

The absorbance technique is an extensively used technique for the evaluation of

various parameters, namely absorbance, reflectance, transmittance, film thickness,

crystal structure, optical constant, etc. The absorbance is also measured in reflectance

and transmittance mode. It is also used to study the spectral composition of the light

reflected from the surface with respect to the angularly dependent intensity and the

composition of the light initiated from the light source. This study provides the film

thickness as well as refractive index, coating homogeneity, and the other optical

constant. Absorbance is primarily measured using the UV-Vis spectroscopy technique,

as depicted in Figure 1.13 (a). In this technique, UV/visible light is passed through the

sample, and the difference in intensity after transmission gives information about the

optical characteristics of the sample.

1.4.2.2 Photoluminescence

The photoluminescence technique is a powerful non-contact and non-destructive

type technique widely used to extract information about the electronic structure of the

semiconducting materials. Under light illumination, photoexcitation occurs by

absorbing the light and imparting the excess energy into the material. This excess

energy can be dissipated in the form of light emission called photoluminescence, as

32
Chapter 1 Introduction and Scope of the Thesis

depicted in Figure 1.13 (b). Data generated by this analysis provides excitation and

emission peaks corresponding to the materials, and based on these peaks electronic

structure of the corresponding material is extracted.

Figure 1.13: Measurement setup for (a) Photoluminescence spectrometer and (b) UV-Vis absorption
spectroscopy.

1.4.3 Optoelectronic Characterizations

Electrical characterization of semiconducting materials or devices plays an essential

role in examining its suitability for the integrated circuit used in different kinds of

electronic devices such as mobile phones, computers, digital cameras, etc. The electrical

characterization of electronic devices is called electronic characterization, and the

measurement of change in electronic characteristics under optical illumination is called

33
Chapter 1 Introduction and Scope of the Thesis

optoelectronic characterization. The complete experimental setup for the measurement

of various electrical and optoelectronic characteristics is depicted in Figure 1.14.

Current (I)-voltage (V) characteristic of any device shows how the current flowing

through the device is being changed with respect to the voltage applied on terminals. An

I-V characteristic is very crucial to evaluate various device parameters such as ideality

factor, barrier height, etc. Capacitance (C)-voltage (V) characterization of any device

represents the relationship between junction capacitance and the voltage applied across

the terminal of the device. C-V characteristics are junction dependent and are used in

the calculation of barrier height, carrier concentration, and depletion width. Impedance

spectroscopy is used to study the resistance or capacitance properties of the device by

applying the AC sinusoidal excitation signal. On the other hand, the I-V characteristics

under dark and solar light illumination are used for the evaluation of various solar cell

parameters.

Figure 1.14: Optoelectronic characterization setup for perovskite solar cells.

34
Chapter 1 Introduction and Scope of the Thesis

1.5 Literature Review

This section aims to summarize the gradual development of perovskite solar cells and

the detailed discussion on hybrid perovskite-based solar cells. First of all, the perovskite

solar cell with different hole transport materials and some additives have been reviewed.

Later, some crucial works on TiO2 and ZnO nanorods based hybrid perovskite solar

cells have been reviewed.

1.5.1 Review of Perovskite Based Solar Cells

The inorganic-organic hybrid perovskite is the emerging photosensitive material for

photovoltaic applications due to its high absorbance coefficient and suitable bandgap

[46]. The journey of perovskite solar cells began in 2006 with a power conversion

efficiency (PCE) of 2.2% by Miyasaka et al. [47]. They have also reported the PCE of

3.1% and 3.8% by using CH3NH3PbBr3 and CH3NH3PbI3 sensitizers, respectively, in

the dye-sensitized solar cell (DSSC) [48]. Considerable efforts have been made in the

last few years to improve the PCE of the hybrid perovskite solar cells (PSCs) to near

25% [49]-[53], as depicted in Figure 1.15.

Perovskite sensitizer used with liquid electrolyte faced serious stability issues in the

solar cell structure. In 2008, Miyaska et al. have employed a solid-state hole transport

layer in PSC and fabricate carbo/conductive polymer composite based first solid-state

PSC with PCE of 1% [54]. Later, Gratzel et al. reported Sb2S3 and poly(3-

hexylthiophene) (P3HT) based solid-state perovskite photovoltaic device with 5.13%

PCE [55]. Park et al. fabricated the first perovskite solar cell based on perovskite

quantum dot (an equimolar mixture of the organic and inorganic compound in g-

35
Chapter 1 Introduction and Scope of the Thesis

butyrolactone solvent) and achieved PCE of 6.5% in 2011 [56]. Later, Park et al. made a

full solid-state device by employing nanoparticles (NPs) of CH3NH3PbI3 as a light

sensitizer on mesoporous ETL and recorded the PCE of 9.7% in 2012 [57]. Snaith et

al. fabricated solar cells by incorporating a new deposition technique (dual-source vapor

deposition) for perovskite thin film and get an excellent PCE of over 15% in 2013 [58].

In 2014, Juan et al. investigated the impedance spectroscopy measurement for

CH3NH3PbI3-XClX solar cell and studied the physical parameters of carrier transport and

recombination in compositions based perovskite thin film [59].

Figure 1.15: Progress in the power conversion efficiency of PSCs from 2006 to 2018 [60]

Further, Luo et al. [61] reported first-time perovskite nanowire-based hybrid

solar cell and recorded 14.71 % PCE in 2015. Madhavan et al. [62] reported more than

20% PCE in 2D/3D perovskite-based PSC using CuSCN as a hole transport layer. They

have also reported a photovoltaic device with photo-electrochemical systems that

employ mesoscopic forms of semiconducting oxides as light absorbers. In 2015, Niu et

36
Chapter 1 Introduction and Scope of the Thesis

al. [63] investigated the chemical stability of perovskite material in different

environmental conditions such as oxygen, moisture, UV-light, etc. The charge carrier

transport behavior of quantum dots in perovskite material has been explained by Ning et

al. [64], whereas the optical and electrical properties of perovskite material for

photovoltaic and optoelectronic applications have been reported by Jung and Park [65]

in 2015. Yang et al. [66] have investigated different electron transport layer-based

perovskite solar cells in 2017. The performance of organic HTL and ETL based hybrid

PSCs with a different annealing temperature for the perovskite layer have been studied

by Jhong et al. in 2017 [67]. Hayase has reported a compositional mix of PbI2 and SnI2

based hybrid and toxic-free hybrid perovskite solar cell [68].

Furthermore, the performance of CH3NH3SnI3-based solar cells with several

types of HTM layers such as CuSCN, Cu2O, and spiro-OMeTAD are optimized using

the TCAD simulation [69]. The ZNRs are employed for ETL in all the PSC structures

and achieved remarkable PCE of 18.34%, 20.23%, and 20.21% for CuSCN, Cu2O, and

spiro-OMeTAD based HTL. Li et al. [70] have grown ZNRs on the aluminum-doped

ZnO seed layer and fabricated PSCs with the following structure: ITO/AZO seed-

layer/ZnO-NRs/perovskite/spiro-MeOTAD/Au. They analyzed the surface preheating

effect on parameter performance of ZNRs based PSCs and achieved optimum J sc of

21.43 mA/cm2, Voc of 0.84 V, FF of 57.42%, and PCE of 10.34% with reduced

pinholes in perovskite layer due to substrate preheating. P.S.Chandrasekhar et al. [71]

demonstrated the performance of nitrogen-doped graphene/ZnO nanorod

nanocomposites based PSC with improved device parameters photocurrent of 21.98

mA/cm2 and PCE of 16.82% compare to without graphene-based PSCs.

37
Chapter 1 Introduction and Scope of the Thesis

1.5.2 Review of TiO2 nanorods Based Perovskite Solar Cells

Various types of organic and inorganic materials such as PCBM, C60, TiO2, ZnO,

SnO2, and Al2O3 have been reported as ETL in the PSCs [72], [73]. Among them, TiO2

is the most promising material for the ETL due to its large bandgap, high stability, and

desired band bending with perovskite material [74]. The TiO2 based ETL improves the

stability of the PSCs and hence is mostly preferred over other metal oxides [73].

Different TiO2 nanostructures such as nanotube, nanosheets, nanoparticles, nanorods,

and nanowires have been used as ETL in highly efficient PSCs [75]-[78]. However,

TiO2 nanorod is preferred over other nanostructures in the PSCs due to its better charge

transportation and robustness with perovskite materials [74].

Alberti et al. and Lozhkina et al. [79], [80] have developed thin solid-state hybrid solar

cells composed of mesoscopic TiO2, based solar cells using solution-printable

processes. They have also developed other perovskite cell structures toward the

realization of full printable technology of high-performance hybrid solar cells. The

perovskite layer in PSC structure FTO/TiO2/Perovskite/SpiroOMeTAD/Au was

fabricated via blade coating, and ETM/HTM material was deposited using thermal

evaporation technique with an active solar cell area of 100 cm2 by Razza et al. [77] and

achieved 4.3 % PCE. Recently, Wan et al. [81] have improved the efficiency of CdS-

sensitized TiO2 solar cells by hydrothermal etching treatments of TiO2 nanorods

(TNRs). Priyadarshi et al. [82] introduced another deposition technique, drop cast for

perovskite material for large area (70 cm2) fabrication of PSCs structure of

FTO/TiO2/ZrO2/Carbon/Perovskite and recorded 10.74% PCE. The PSC with 11.32%

PCE based on an active area of 1.47 cm2 was fabricated by Gao [83] using 50 nm

38
Chapter 1 Introduction and Scope of the Thesis

compact layer of TiO2, spiro-OmeTAD based HTM layer by spin coating, and 300 nm

thin perovskite layer via knife coating method.

1.5.3 Review of ZnO Nanorod Based Perovskite Solar Cell

The ZnO nanomaterials are among the most promising wide bandgap material for solar

cell applications, LED, and photodetectors. ZnO is a widely used electron transport

material due to its high mobility, easy synthesis, abundant availability, and high stability

[84], [85]. In general, ZnO nanostructure is preferred in the ETL over its bulk

counterpart due to the larger surface-to-volume ratio. The surface to volume ratio plays

a vital role in photocatalytic activity [86]. Although 0D material (colloidal quantum dot)

has a large surface to volume ratio, its use in perovskite solar cell structure is very less

due to poor charge transportation. One dimensional material (ZnO nanorods) offers

increased photocatalytic efficiency compared to bulk ZnO. Compared to other

nanostructures, ZnO nanorods have fast electron transportation, a large conduction

band, and high electron density. Son et al. [86] have reported the maximum PCE of

14.35% using (NH4)2TiF6 treated ZnO NRs based ETL in the PSC structure. Xu et al.

[87] have obtained the PCE of 9.15% using ZnO NR (ZNR) arrays synthesized by

modified solvothermal method.

Liu et al. have reported PCE of 15.4%% using ZnO ETL and vapor deposited

hybrid perovskite material [58]. From the last two decades, there was some

modification in the quality of single-crystalline ZNRs to improve the performance of

optoelectronic and photovoltaic devices [88], [89]. Several techniques such as seed

layer via spin coating, sputtering, chemical bath deposition (CBD), electrostatic

spraying, atomic layer deposition (ALD) have been used for uniform growth of ZNRs

39
Chapter 1 Introduction and Scope of the Thesis

for minimizing the defects and traps [87], [90]–[92]. The bandgap of ZnO

nanomaterials can be tuned by doping with some metal dopants like Al, Mg, and P.

[88], [93], [94]. Shirazi et al. have also reported the ZNRs based HTL free hybrid PSC

and achieved efficiency improvement by increasing the conductivity of ZNRs by Al

doping [93]. Jeon et al. found the uniform and defect-free layer of perovskite when the

toluene has been used as anti-solvent during spin coating deposition of perovskite thin

film. The better phase formation and crystallinity of PbI2 with MAI (methylammonium

iodide) are achieved when DMSO has been used as a solvent for PbI2 precursor instead

of DMF [44].

Peng et al. [95] minimized the decomposition of the hybrid perovskite layer by

insertion of the SnO2 layer on ZnO nanorods. The core-shell SnO2-ZnO nanostructures

have been used to increase the oxygen vacancies at the ZnO-hybrid perovskite interface

[95] for stabilization of perovskite later. The power conversion efficiency of hybrid PSC

is significantly improved from 6.92% to 12.17%, and hysteresis is eliminated from the

J-V curve by taking the core-shell nanostructure of the SnO2-ZnO layer.

Zhang et al. [96] used a relatively fast and low temperature processed electrochemical

method for deposition of ZnO layer and demonstrated low J-V curve hysteresis with

PCE of 11% in the fabricated ZnO nanostructures based PSCs on ITO coated flexible

substrate. The charge carrier mobility and charge recombination mechanism have also

been investigated by Zhang et al. [97] for ZnO thin film deposited via electrospraying

technique-based PSCs. They have also reported the performance of ZnO

nanostructured based PSCs for perovskite thin film deposition via one-step and two-step

deposition methods.

40
Chapter 1 Introduction and Scope of the Thesis

Later, several ZnO nanostructures deposited via different techniques such as pulsed

laser deposition [98], electrophoretic deposition [99], PECVD [100], magnetic

sputtering [101] have been explored for the fabrication of different types of PSCs

structures, including planner [102], inverted planner [103], and mesoporous [97].

Kumar et al. [104] fabricated the perovskite/Spiro-OMeTAD/Au based PSCs with four

different planner structure for ETL, namely FTO/ZnO(CL), FTO/ZnO(CL)/ZNRs,

PET/ITO/ZnO(CL), and PET/ITO/ZnO(CL)/ZNRs. They have synthesized ZNRs using

chemical bath deposition technique on FTO/ITO substrate with and without ZnO seed

layer and recorded PCE of 8.90% for FTO substrates and 2.62% for flexible PET/ITO

substrate-based PSCs.

1.5.4 Major Observation from the Literature Survey

The milestones achieved in the area of perovskite solar cells during their development

are interesting to note [60]. It is observed that the period of 2013-2015 was focused

primarily on the development of high-efficiency cells by using appropriate material

selection and deployment of methods for high-quality perovskite film formation. This

period also addressed issues regarding hysteresis and interfacial engineering. In 2016-

2017, compositional engineering is applied widely, which resulted in improvement in

both structural stability and efficiency. Extensive efforts are made to develop lead-free

perovskites too. Now, after developing and optimizing methods of synthesis, the

attention of researchers is on making the perovskite solar cell more stable for long-term

use by addressing issues regarding light, moisture, oxygen, and thermal instability.

Currently, attempts are made to scale up the production of solar cells for meeting

industrial requirements, and the methods such as mechanochemical synthesis are

41
Chapter 1 Introduction and Scope of the Thesis

gaining popularity due to their solvent-free nature. Also, extensive research is being

carried out in the area of all-inorganic halide based perovskites solar cells.

1.6 Issues and Challenges in Perovskite Solar Cells

The commercialization of hybrid PSC is still challenging because of toxicity issues

(during fabrication, deployment, and disposal), long term chemical and phase stability,

and cost-effectiveness. The perovskite material pertains to its stability and degradation

due to moisture, oxygen, UV radiation, and temperature. The three major areas of thrust

in perovskite research are: improving the material stability like phase stability, thermal

stability, and replacement of the toxic heavy metal (Pb+2). One possible modification

has come through the substitution of cations, metal ions, and halogen to solve these

challenges. Tin (Sn) is a candidate for the replacement of lead (Pb2+) cation and can

help in bandgap tuning of the perovskite material.

Compositional engineering is the leading interest in perovskite research for

optimizing structure and usability governing factors. The hybrid perovskite decomposed

in methylammonium, hydrogen iodide, and lead iodide in the presence of water. The

prominent approach is the substitution of a methyl group with a bulky organic cation,

leading to a 2D perovskite structure. The 2D layered and 3D bulk perovskite material

can then be used to alternatively coat layers to prevent moisture impingement due to the

hydrophobic nature of the alkyl group and avoid further degradation; however, with its

own challenges such as disorientation, grain size improvement. Though, to enhance air

stability, some butyl halide groups could be used. The compositional change by n-butyl

ammonium iodide in the methyl group can yield towards low dimensionality. So,

Goldschmidt tolerance factor will change accordingly based on the composition of

42
Chapter 1 Introduction and Scope of the Thesis

methyl and butyl group. A suitable tolerance factor will make a more stable perovskite

material against moisture and oxygen.

1.7 Motivation and Problem Definition

It is discussed that the perovskite solar cells have some inherent challenges for

commercialization. The identified challenges and their possible solutions are listed as

follows:

 The 3rd generation of solar cells (organic solar cells) has low efficiency.

 The absorbance coefficient of perovskite material is relatively more so efficiency

can be improved by incorporating perovskite material.

 The low energy photons of solar spectrum (having energy less than the bandgap of

active material) incident on photovoltaic device can‟t be converted into electricity

and lost while the excess energy of high energy photon (having energy greater than

the bandgap of active material) have been lost in heat form.

 This can be easily overcome by changing the “A” cation of the perovskite

material, and the suitable bandgap material (quasi-2D/3D perovskite) can be

employed in PSCs for wide bandgap absorption.

 The efficiency of planar PSCs is less compare to mesoscopic PSCs.

 The efficiency can be enhanced using TiO2 and ZnO nanostructure in PSCs. The

mesoporous layer (nanorods, nanowires, nanotubes, etc.) permits the perovskite

photo absorber to penetrate mesoporous framework material, enhancing the

absorption due to the high surface volume ratio of the perovskite material. So the

incorporation of mesoporous material into PSCs improves the efficiency of PSCs.

 The planar perovskite solar cell has less stability

43
Chapter 1 Introduction and Scope of the Thesis

 By the incorporation of TiO2 and ZnO NRs in PSCs, the stability can be

improved. The monohydrate phase forms rapidly when the hybrid perovskite

layer is deposited on a compact layer, but the decomposition rate of hybrid

perovskite material is reduced when mesoporous scaffolds are employed.

Therefore, in this thesis, we focussed on optimizing the mesoporous n-type electron

transport layer to improve the efficiency and stability of the perovskite solar cell.

1.8 Scope of the Thesis

The present thesis deals with the fabrication, characterization, and TCAD

simulation of some CH3NH3PbI3 hybrid perovskite-based PSCs using TiO2 nanorods

(TNRs)/ ZnO nanorods (ZNRs) as the ETL and Spiro-OMeTAD/PTAA as the HTL in

the device. All the PSCs considered in the present thesis are of conventional n-i-p

mesoporous device structure where the n-region represents the ZNRs/TNRs based ETL,

i-region includes CH3NH3PbI3 hybrid perovskite-based active layer, and the p-region

represents the Spiro-OMeTAD or PTAA based HTL in the PSCs. The thesis consists of

SIX chapters, including the present chapter. The outline of the remaining FIVE chapters

is briefly described as follows:

Chapter 2 reports the fabrication, simulation, and characterization of FTO/TiO2

Nanorods/CH3NH3PbI3 (Hybrid-Perovskite)/PTAA/Pd structure-based perovskite solar

cells (PSCs) where the FTO (fluorine-doped tin oxide) is the substrate, TiO2 nanorods

(NRs) layer acts as the electron transport layer (ETL), hybrid perovskite (CH3NH3PbI3)

is the active layer, PTAA is the hole transport layer (HTL), and Pd film acts as the

contact electrode in the device. The TNRs are grown by the hydrothermal process

followed by TiCl4 treatment for enhancing the performance of the device. The

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Chapter 1 Introduction and Scope of the Thesis

perovskite thin-film active layer and PTAA based HTL are deposited using the spin-

coating technique. The effect of the ETL thickness on the performance parameters of

the PSCs has been investigated and compared with the simulation data. The fabrication

and characterization have been performed in open atmospheric conditions. The

measured electrical and optical characteristics for three devices with three different ETL

thicknesses were compared with the TCAD simulation results for comparing

performance parameters of the proposed structure under real operating conditions and

theoretically ideal conditions.

Chapter 3 investigates the effects of solvothermal etching and TiCl4 treatment of

the TiO2 NRs based ETL on the performance of the FTO/TiO2 NRs/CH3NH3PbI3

/Spiro-OMeTAD/Pd PSCs. This study is carried out to basically show that not only the

thickness of the ETL but also its surface morphology plays an important role in the

performance optimization of the proposed PSC. Three different devices have been

studied: The first device contains hydrothermally grown TNRs based ETL, the second

device uses a TiCl4 treated TNRs based ETL, and the third device uses solvothermally

etched and TiCl4 treated TNRs ETL, maintaining remaining parts of the device same as

used in Chapter-2. The growth of other layers in the device is the same as considered in

Chapter-2. Finally, the electrical and optical parameters are compared for all three PSC

devices mentioned above.

Chapter 4 deals with the investigation of the effect of ZnO NRs ETL (grown on

four different types of ZnO seed layers by hydrothermal method) on the performance of

FTO/ZnO NRs/CH3NH3PbI3/PTAA/Au based PSC structures. Four different types of

seed layers of drop-casted ZnO film, spin-coated colloidal ZnO nanoparticles (NPs)

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Chapter 1 Introduction and Scope of the Thesis

film, spin-coated colloidal ZnO quantum dots (QDs) film, and solvothermally grown

ZnO NRs film were deposited on four FTO substrates. Then the seed layer coated FTO

substrates were processed for growing ZnO NRs (of four different morphologies on four

different seed layers) by the hydrothermal method. The CH3NH3PbI3 perovskite active

layer, PTAA based HTL, Au were successfully deposited for fabricating four different

PSC devices under study. The surface morphologies of the four different types of ZnO

NRs ETLs were studied by XRD and SEM analyses. The electrical and optical

characteristics of the four PSCs with four different morphologies of ZnO NRs based

ETLs have been studied in detail. The PSCs with ZnO NRs ETL grown on the ZnO

QDs based seed layer showed better electrical and optical characteristics over the other

devices.

Chapter 5 investigates the effects of doped and undoped Spiro-OMeTAD based

HTL on the performance of FTO/ZnO NRs/CH3NH3PbI3/Spiro-OMeTAD/Pd PSCs.

The two types of hybrid-PSCs with doped and undoped HTLs were fabricated and

characterized. Measured results were compared with the TCAD simulation data to

validate the results measured under open atmospheric conditions.

Finally, Chapter-6 summarizes the major observations and findings of the present

thesis. Some future scopes of research related to this thesis have been outlined at the

end of this chapter.

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