Metal Oxide Nanocomposite Thin Films for Optoelectronic Device Applications E-Book
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- Sprache: Englisch
METAL OXIDE NANOCOMPOSITE THIN FILMS FOR OPTOELECTRONIC DEVICE APPLICATIONS The book provides insight into the fundamental aspects, latest research, synthesis route development, preparation, and future applications of metal oxide nanocomposite thin films. The fabrication of thin film-based materials is important to the future production of safe, efficient, and affordable energy as the devices convert sunlight into electricity. Thin film devices allow excellent interface engineering for high-performance printable solar cells as their structures are highly reliable and stand-alone systems can provide the required megawatts. They have been used as power sources in solar home systems, remote buildings, water pumping, megawatt-scale power plants, satellites, communications, and space vehicles. Metal Oxide Nanocomposite Thin Films for Optoelectronic Device Applications covers the basics of advanced nanometal oxide-based materials, their synthesis, characterization, and applications, and all the updated information on optoelectronics. Topics discussed include the implications of metal oxide thin films, which are critical for device fabrications. It provides updated information on the economic aspect and toxicity, with great focus paid to display applications, and covers some core areas of nanotechnology, which are particularly concerned with optoelectronics and the available technologies. The book concludes with insights into the role of nanotechnology and the physics behind photovoltaics. Audience The book will be an important volume for electronics and electrical engineers, nanotechnologists, materials scientists, inorganic chemists in academic research, and those in industries, exploring the applications of nanoparticles in semiconductors, power electronics, and more.
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Table of Contents
Cover
Table of Contents
Series Page
Title Page
Copyright Page
Preface
Part I: Nanotechnology
1 Synthesis and Characterization of Metal Oxide Nanoparticles/Nanocrystalline Thin Films for Photovoltaic Application
1.1 Present Status of Power Generation Capacity and Target in India
1.2 Importance of Solar Energy
1.3 Evolution in Photovoltaic Cells and their Generations
1.4 Role of Nanostructured Metal Oxides in Production, Conversion, and Storage in Harvesting Renewable Energy
1.5 Synthesis of Nanostructured Metal Oxides for Photovoltaic Cell Application
1.6 Characterization of Metal Oxide Nanoparticles/Thin Films
1.7 Conclusion and Future Aspects
References
2 Experimental Realization of Zinc Oxide: A Comparison Between Nano and Micro-Film
2.1 Introduction
2.2 Approaches to Nanotechnology
2.3 Wide Band Semiconductors
2.4 Zinc Oxide (ZnO)
2.5 Properties of Zinc Oxide
2.6 Thin Film Deposition Techniques
2.7 Procedure of Experimental Work
2.8 Calculation of Thickness of Thin ZnO Films
2.9 Structural Analysis
2.10 Optical Characterization
2.11 Electrical Characterization
2.12 Applications of Zinc Oxide
2.13 Conclusions and Future Work
References
3 Luminescent Nanocrystalline Metal Oxides: Synthesis, Applications, and Future Challenges
3.1 Introduction
3.2 Different Types of Luminescence
3.3 Luminescence Mechanism in Nanomaterials
3.4 Luminescent Nanomaterials Characteristic Properties
3.5 Synthesis and Shape Control Methods for Luminescent Metal Oxide Nanomaterials
3.6 Characterization of Nanocrystalline Luminescent Metal Oxides
3.7 Applications of Nanocrystalline Luminescent Metal Oxides
3.8 Conclusion and Future Aspects of Nanocrystalline Luminescent Metal Oxides
References
4 Status, Challenges and Bright Future of Nanocomposite Metal Oxide for Optoelectronic Device Applications
Abbreviations
4.1 Introduction
4.2 Synthesis of Nanocomposite Metal Oxide by Physical and Chemical Routes
4.3 Characterization Techniques Used for Metal Oxide Optoelectronics
4.4 Optoelectronic Devices Based on MOs Nanocomposites
4.5 Advantages of Pure/Doped Metal Oxides Used in Optoelectronic Device Fabrication
4.6 Parameters Required for Optoelectronic Devices Applications
4.7 Conclusion and Future Perspective of Metal Oxides-Based Optoelectronic Devices
Acknowledgement
References
Part II: Thin Film Technology
5 Semiconductor Metal Oxide Thin Films: An Overview
5.1 Introduction
5.2 Conclusion and Outlook
Acknowledgement
References
6 Thin Film Fabrication Techniques
6.1 Introduction
6.2 Thin Film - Types and Their Application
6.3 Classification of Thin-Film Fabrication Techniques
6.4 Methodology
6.5 Advantages of CVD Process
6.6 Comparison Between PVD and CVD
6.7 Conclusion
References
7 Printable Photovoltaic Solar Cells
7.1 Introduction
7.2 Working Principle of Printable Solar Cells
7.3 Wide Band Gap Semiconductors
7.4 Metal Oxide-Based Printable Solar Cell
7.5 What is Thick Film, Its Technology with Advantages
7.6 To Select Suitable Technology for Film Deposition by Considering the Economy, Flexibility, Reliability, and Performance Aspects
7.7 Procedures for Firing
7.8 Deposition of Thin Film Layers via Solution-Based Process
Conclusion
References
8 Response of Metal Oxide Thin Films Under Laser Irradiation
8.1 Introduction
8.2 Interaction of Laser with Material
8.3 Radiation Causes Modification
8.4 Application Laser Irradiated Films
8.5 Wavelength Range of Radiation
8.6 Laser Irradiation Mechanism
8.7 Experimental Procedure
References
Part III: Photovoltaic and Storage Devices
9 Basic Physics and Design of Photovoltaic Devices
9.1 Introduction: Solar Cell
9.2 Semiconductor Physics
9.3 Carrier Concentrations in Equilibrium
9.4 p-n Junction Formation
9.5 Process of Carrier Production and Recombination
9.6 Equations for Poisson’s and Continuity Equation
9.7 Photovoltaic (Solar Power) Systems
9.8 Types of Photovoltaic Installations and Technology
9.9 Electrical Characteristics Parameters
9.10 Basic p-n Junction Diode Parameters
9.11 Conclusion
References
10 Measurement and Characterization of Photovoltaic Devices
10.1 Introduction
10.2 Electrical and Optical Measurements
10.3 Current-Voltage (I-V) Characterization
10.4 Quantum Efficiency
10.5 Hall Effect Measurements
10.6 Photoluminescence Spectroscopy and Imaging
10.7 Electroluminescence Spectroscopy and Imaging
10.8 Light Beam Induced Current Technique (LBIC)
10.9 Electron Impedance Spectroscopy (EIS)
10.10 Characterization by Ellipsometry Spectroscopy
10.11 Conclusion
References
11 Theoretical and Experimental Results of Nanomaterial Thin Films for Solar Cell Applications
11.1 Introduction
11.2 Literature Survey
11.3 Theoretical and Experimental Results
11.4 Experimental Results of Optical Properties
11.5 Conclusions
Acknowledgement
References
12 Metal Oxide-Based Light-Emitting Diodes
12.1 Introduction
12.2 Structure of LEDs
12.3 Working Principle of LEDs
12.4 Selection of Material for Construction of LEDs
12.5 Basic Terminology Involved in Fabrication of LEDs
12.6 LEDs Based on ZnO (Zinc Oxide)
12.7 Transition Metal Oxide-Based LEDs
12.8 Lanthanide-Based OLEDs
12.9 Conclusion
References
13 Metal Oxide Nanocomposite Thin Films: Optical and Electrical Characterization
13.1 Introduction
13.2 Nanocomposite Thin Films (NCTFs)
13.3 Materials Used for Preparation of NCTFs
13.4 Methods of Preparation of NCTFs
13.5 Applications
13.6 Examples
13.7 Laser Irradiation Sources
13.8 Functional Characterization Techniques
13.9 Conclusion
References
14 Manganese Dioxide as a Supercapacitor Material
14.1 Introduction
14.2 Supercapacitor Components
14.3 Methods for MnO
2
Nanoparticles
14.4 Doped-MnO
2
Materials
14.5 MnO
2
with Polymer Composites
14.6 Nanocomposites
14.7 Conclusion
References
Index
End User License Agreement
List of Tables
Chapter 2
Table 2.1 Different structural parameters.
Chapter 10
Table 10.1 Comparison of metal oxide-based PV devices.
Table 10.2 Comparison of metal oxide-based PV devices efficiency and fill fact...
Table 10.3 Metal oxide mobility parameter.
Chapter 11
Table 11.1 Generations of solar cells of various types.
Table 11.2 The efficiency of the power conversion solar cells made amidst
Z
n
O
/
Table 11.3 Efficiency of solar cells made of tin oxide in converting energy.
Table 11.4 Efficiency of nickel oxide-based solar cells in converting power co...
Table 11.5 Fitting of Sellmeier’s connection model parameters for zinc oxide [...
Table 11.6 The values of Cauchy’s constraints for ZnO [108, 110].
Chapter 14
Table 14.1 Comparison between basic parameters of MnO
2
and their composites.
List of Illustrations
Chapter 1
Figure 1.1 3D-π chart for installed power capacity (MW) of fuels for 2021–22 i...
Chapter 2
Figure 2.1 Approaches to nanotechnology.
Figure 2.2 Crystal structures of ZnO.
Figure 2.3 Band structure of zinc oxide [14].
Figure 2.4 Experimental setup for coated films [17].
Figure 2.5 XRD representation of ZnO films with comparison of peaks.
Figure 2.6 Scanning electron microscope pictures for nano- and microfilm [17].
Figure 2.7 Absorbance spectra of ZnO films [17].
Figure 2.8 Pl spectra along with color representation of nano and micro ZnO fi...
Figure 2.9 DC conductivity behaviors of nano and micro ZnO films [17].
Chapter 3
Figure 3.1 Schematic of processes in luminescence phenomena.
Figure 3.2 Different types of luminescence.
Chapter 4
Figure 4.1 Techniques for the synthesis of MOs.
Figure 4.2 Shows the Sol-Gel process.
Chapter 5
Figure 5.1 Illustrative representation of physical vapor deposition (PVD) meth...
Figure 5.2 Schematic diagram of sputter deposition method for thin film format...
Figure 5.3 Schematic diagram of chemical vapor deposition (CVD) method for thi...
Chapter 6
Figure 6.1 Schematic diagram of physical vapor deposition (PVD) method for thi...
Figure 6.2 Schematic diagram of molecular beam epitaxy (MBE) method for thin f...
Figure 6.3 Schematic diagram of sputter deposition method for thin film deposi...
Figure 6.4 Schematic diagram of chemical vapor deposition (CVD) method for thi...
Chapter 7
Figure 7.1 Band alignment of the representative metal oxide electron transport...
Figure 7.2 Schematic illustration of the low-temperature process through collo...
Figure 7.3 Screen printing method.
Figure 7.4 Schematic representation of the screen printing process.
Chapter 8
Figure 8.1 Laser-induced phenomena in metal oxide films and their applications...
Figure 8.2 (a) Wavelengths used for lasers in the electromagnetic spectrum of ...
Figure 8.3 Comparison of optical penetration depths of laser radiation of diff...
Figure 8.4 Laser set up for irradiation on ZnCdO coated films.
Figure 8.5 (a) Absorbance and (b) reflectance spectra of ZnCdO films under las...
Figure 8.6 Absorption coefficient vs. wavelength of coated films.
Figure 8.7 First derivative (d
R/
d
λ
) plot of the reflectance spectra.
Figure 8.8 Arhenius plot of DC conductivity of ZnCdO coated films.
Chapter 9
Figure 9.1 A simple typical solar cell diagram. This diagram shows how electro...
Figure 9.2 A condensed energy band diagram for a semiconductor with a direct b...
Figure 9.3 Shows how the Fermi function changes with temperature [7].
Figure 9.4 A broad classification of photovoltaic technologies.
Figure 9.5 I-V plot of a TiO2/CuO/Cu2O solar cell develop by AFORS-HET softwar...
Chapter 10
Figure 10.1 Classification of PV characterization domain.
Figure 10.2 Types of defects associated with PV devices.
Figure 10.3 Schematics of current-voltage measurement system for PV device. Re...
Figure 10.4 I–V profiles of transparent solar cell (NiO/TiO2/FTO/Glass) for br...
Figure 10.5 Setup of quantum efficiency measurement system. Reprinted with per...
Figure 10.6 Hall effect outline view.
Figure 10.7 Schematic of LBIC measurement.
Chapter 11
Figure 11.1 Model of the transparent thick glass substrate with thin absorbent...
Figure 11.2 (a) Absorbance spectra and (b)Absorption coefficient with thicknes...
Figure 11.3 (a) Tauc’s plots for determining the energy band-gap (b) 1
st
deriv...
Figure 11.4 (a) ZnCdO film absorbance spectra (b) Tauc’s Plot ZnCdO films were...
Figure 11.5 Figure shows the ZnCdO films sintered at 500°C, at 550°C, and at 6...
Figure 11.6 Shows the variations(a) real (ε′) and (b) imaginary (ε″) component...
Chapter 12
Figure 12.1 Basic construction of LEDs.
Figure 12.2 Schematic representation of working of LED.
Figure 12.3 Crystal structure of ZnO.
Figure 12.4 The arrangement of conduction band (CB) and valence band (VB) in M...
Chapter 13
Figure 13.1 Metal oxide thin films and their uses.
Figure 13.2 Crystal structure of some metal oxide nanocomposites.
Figure 13.3 Temperature dependence of DC Conductivity of (ZnO) thick film.
Figure 13.4 Log (σ
dc
) vs. 1000/T of CdO thick film.
Figure 13.5 Absorption Spectra of
Z
nO thick film.
Figure 13.6 Energy band gap determination of ZnO thick film.
Figure 13.7 Optical transmission spectrum of CdO thick film.
Figure 13.8 hν vs. (αhν)
2
for determination of CdO thick film band gap energy.
Chapter 14
Figure 14.1 Ragone plots for various energy-storage technologies.
Figure 14.2 Different types of supercapacitor.
Figure 14.3 Shows different types of electrolytes of supercapacitors.
Figure 14.4 Structure of supercapacitor.
Figure 14.5 Shows synthesis of MnO
2
.
Guide
Cover Page
Series Page
Title Page
Copyright Page
Table of Contents
Begin Reading
Index
WILEY END USER LICENSE AGREEMENT
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Scrivener Publishing100 Cummings Center, Suite 541JBeverly, MA 01915-6106
Publishers at ScrivenerMartin Scrivener ([email protected])Phillip Carmical ([email protected])
Metal Oxide Nanocomposite Thin Films for Optoelectronic Device Applications
Edited by
Rayees Ahmad Zargar
Department of Physics, Baba Ghulam Shah Badshah University, Rajouri (J&K), India
This edition first published 2023 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA and Scrivener Publishing LLC, 100 Cummings Center, Suite 541J, Beverly, MA 01915, USA© 2023 Scrivener Publishing LLCFor more information about Scrivener publications please visit www.scrivenerpublishing.com.
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Library of Congress Cataloging-in-Publication Data
ISBN 978-1-119-86508-7
Cover image: Pixabay.ComCover design by Russell Richardson
Preface
Metal Oxide Nanocomposite Thin Films for Optoelectronic Device Applications provides insight into the fundamental aspects, latest research, synthesis route development, preparation, and future applications of metal oxide nanocomposite thin films.
The fabrication of thin film-based materials will be important to the future production of safe, efficient, and affordable energy. Such devices are used to convert sunlight into electricity. Thin film devices allow excellent interface engineering for high-performance printable solar cells. Their structures are highly reliable, stand-alone systems that can provide megawatts. They have been used as power sources in solar home systems, remote buildings, water pumping, megawatt-scale power plants, satellites, communications, and space vehicles. With these mass applications, the demand for photovoltaic devices skyrockets every year.
Alternately, nanocomposite film coating has also revolutionized the fields of materials science and applied physics, thus progressing as a unified discipline for scientific industries. For the first time, this comprehensive handbook presents the emerging scenario of deposited techniques for the synthesis of metal oxide nanocomposites. The handbook is divided into three sections (Nanotechnology; Thin Film Technology; Photovoltaic and Storage Devices). It covers important topics like different metal oxide properties, scale-up processes, low-cost synthesis methods, various characterizations, and their respective physics. It will be an important volume for academic researchers and those in industries, exploring the applications of nanoparticles in semiconductors, power electronics, and more.
This book covers the basics of advanced nanometal oxide-based materials, their synthesis, characterization, and applications, and all the updated information on optoelectronics. Topics discussed include the implications of metal oxide thin films, which are critical for device fabrications. It provides updated information on the economic aspect and toxicity, with great focus paid to display applications, and covers some core areas of nanotechnology, which are particularly concerned with optoelectronics and the available technologies. The book concludes with insights into the role of nanotechnology and the physics behind photovoltaics.
My thanks go to Wiley and Scrivener Publishing for their continuous support and guidance.
Rayees Ahmad Zargar
Part INANOTECHNOLOGY
1Synthesis and Characterization of Metal Oxide Nanoparticles/Nanocrystalline Thin Films for Photovoltaic Application
Santosh Singh Golia1, Chandni Puri1,2, Rayees Ahmad Zargar3 and Manju Arora1*
1CSIR-National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi, India
2Academy of Scientific and Innovative Research (AcSIR), Ghaziabad, India
3Department of Physics BGSBU, Rajouri (J&K), India
Abstract
The rapid depletion of natural resources especially fossil fuels due to excessive consumption has raised the severity of problem which encouraged researchers to explore existing and new advanced nanomaterials for application in non-renewable energy devices. The promising photovoltaic devices are suitable alternative for meeting energy demand as compared to other water, wind, and tidal energy resources. The extensive work is going on the economic, lightweight, and flexible organic dye sensitized and inorganic-organic hybrid type solar cell materials like nanosized pure and doped transition metal oxides or their nano-composites because of their excellent stability, electronic, high resistance, and thermal properties. They can be easily prepared by different chemical and physical methods and plays role of light absorbers, transparent electrodes, and hole and electron transport layers in solar cells. The large intrinsic mobility of charge carriers in pure metal oxides has been utilized as charge collectors which improve the charge selectivity of electrodes due to their particular band gap energy. Metal oxides in nanocrystalline thin film form helps in optimizing light distribution within devices for efficient light absorption. In this chapter briefly introduced the present status of power generation capacity and target in India, importance of solar energy, evolution of photovoltaic cell, nanostructured metal oxides synthesis, characterization and application in photovoltaic cells is discussed.
Keywords: Nanoparticles, synthesis, characterizations, solar cell applications
1.1 Present Status of Power Generation Capacity and Target in India
India has emerged as the world’s third-largest energy consuming country owing to enormous rise in the usage of domestic consumer products as well as at workplace electronic/electrical gadgets to make life easier. The energy consumption has been doubled since 2000 as reported in India Energy Outlook 2021 [1]. This demand increases with sharply with time due to enormous rise in population, industrialization, and development of new smart cities with time. These are the key factors which encourage addition/demand of more consumer products/appliances and at offices in day-to-day life. Recently, Government of India, Ministry of Power has published the status of power at a glance received from Central Electricity Authority (CEA) which states that at present the total installed power generation capacity 3,99,497 (MW) generation is fulfilled 2,36,109 MW (59.1%) by Fossil Fuel and 1,63,388 MW (40.9 %) by Non-Fossil Fuel sources [2]. Coal, lignite, gas and diesel come under fossil fuel resources, while new and renewable energy (hydro (big and small), wind, solar, biomass, waste to energy) and nuclear energy resources are categorized under non-fossil fuel. The new and renewable energy resources contribute in the production of 1,56,608 MW power i.e. 39.2% and nuclear power plants produce 6780 MW (1.7%) of the total installed power generation setups. These details of the installed power generation capacity (MW) from each fossil and non-fossil fuel are presented in the following Figure 1.1.
The electricity generation target fixed for 2021–22 financial year was 1356 Billion Unit (BU) in fossil fuels through thermal energy produced 1155.200 BU, hydro plants: 149.544 BU; Nuclear plants: 43.020 BU; and 8.236 BU was import from Bhutan to overcome the demand from different Sectors of Society.
1.2 Importance of Solar Energy
India receives ∼5,000 trillion kWh per annum energy from Sun over its land with many parts have an exposure of 4–7 kWh.m-2 per day [3]. Solar energy is effectively captured for large/small scale power generation all over India in villages, remote areas, urban and upcoming smart cities as an alternative to fuel sources owing to its plenty availability. Government of India has categorized under National mission to cope up with the rising electricity demand by people with time. National Institute of Solar Energy has reported that ∼ 748 GW solar power can be produced by utilizing 3% of the unused/barren land for the installation Solar PV modules along with grid connected power generation capacity within stipulated time schedule over the years. This clean, and eco-friendly renewable energy resource needs initial investment to start i.e. installation of solar panels and storage system setup depending upon the power requirement of individual/society/ offices/village/city etc. with merits of off-grid/on-grid distribution at low/ normal temperature. The first off-grid practical application of photovoltaic solar panels/cells is to provide power for orbiting satellites, spacecraft in space and pocket calculators etc. But now, the photovoltaic modules connected to grid for large scale power generation are in demand which require inverter to convert the direct current (DC) electricity obtained from photovoltaic cells modules(PCMs) to AC electricity. The off-grid power generation by PCMs is suitable for lighting remote areas, lamp posts along roads/streets, emergency telephones, remote sensing applications. They require minor maintenance of cleaning panels from time to time for efficient absorption of sunlight and battery/storage devices contacts cleaning. At present, the awareness and need of people has started using solar energy driven lighting on roads/homes/offices/hospitals/industries, heating and cooling appliances in some regions/states of India with no harmful emissions. The availability of electricity in turn simplifies the life of people especially residing in villages/remote areas. The people have started their own skill/ancestral work/business/initiate existing or new small and medium scale industries at their own places as a source of income which in turn has improved their standard of living and thinking. So, just by providing electricity to villages/towns/remote areas, one can restrict the migration of people to cities in search of job and forced to live away from their families.
Figure 1.1 3D-π chart for installed power capacity (MW) of fuels for 2021–22 in India.
1.3 Evolution in Photovoltaic Cells and their Generations
Photovoltaic Cells (PCs) are the basic constituent of solar power technology which convert sunlight to direct current (DC) electricity and utilized directly or through batteries to power devices. The solar cell is like a p-n junction diode, having two layers i.e. p-type and n-type of doped semiconductors and available in different sizes as well as shapes to utilize the maximum effective surface area and minimize contact resistance [4–7]. n-type semiconductor layer coated with antireflection coating for sufficient absorption of sunlight and embedded with metal which acts as front negative electrode and p-type semiconductor with conductive oxide layer play role of back positive electrode. When, these p-type and n-type semiconductor layers are placed one over another to establish contact between them for inducing electric potential. Electrons from n-type semiconductor layer moves across the junction and diffuses to the p-type semiconductor layer that create a static positive charge in n-type layer. Similarly at the same time, the holes from p-type layer diffuse across the junction and develop a static negative charge. Both these escaped free electrons and holes pair up and disappear. At a particular stage, the depletion zone is formed at the p-n junction which restricts the migration of free charge carriers. These separated static positive and negative charges create an electric field across the depletion zone. The built-in electric field supplies voltage required for driving the unidirectional current through an external circuit. When the semiconductor layer absorbs photon energy equivalent or more from sunlight than its band gap, valence electrons from valence band (VB) are excited to conduction band (CB) and leave behind positively charged vacancies i.e. holes in VB. Hence, these transitions lead to the flow of electrons toward the negative end and holes toward the positive direction. The atoms collide and initiate free electrons migration for generating the current of electricity. The major issue for PCs is the very high cost of materials and installation units. This problem encouraged researchers for the development of new alternative economic materials which can markedly enhance the efficiency, durability, technical viability of solar cells/modules/ panels. With the evolution in new materials for solar cell fabrication are classified into four generations.
1.3.1 First Generation Photovoltaic Cell
The conventional first generation photovoltaic cells belong to wafer technology in which silicon (Si) or gallium arsenide (GaAs) semiconductors doped with different element are used for photovoltaic cell development. Si has an indirect energy band gap value about 1.12 eV, while GaAs has direct energy band around 1.43 eV at ambient temperature with outstanding optical properties. Thicker single crystalline (Float Zone (FZ)) and polycrystalline (Czochralski (CZ)) Silicon wafers doped with group III (B, Al) for p-type and group V (P, As) for n-type periodic table elements wafers have been used in photovoltaic cells for efficient absorption of sunlight, while highly efficient GaAs photovoltaic cell requires lesser material. The developed photovoltaic cells have maximum efficiency around 28%, 21%, and 16% respectively. GaAs-based photovoltaic cells have maximum efficiency ∼44%. The extensive R&D work has been reported on the synthesis of these materials and the commercially available technologies for photovoltaic cell fabrication. Both these substrates are very costly owing to their processing cost. The huge abundance and cost effectiveness of Si as compared to rare elements Ga and As has encourages wide range usage of polycrystalline silicon photovoltaic cell panels for large scale production to meet about 90% [8] of demand in spite of the low photovoltaic cell efficiency.
1.3.2 Second-Generation Photovoltaic Cell
These photovoltaic cells are based on thin film technology in which the two heterojunction layers are deposited in between two contact layers which require material of little micrometer thickness deposited on very economic transparent conductive oxide coated glass/ceramic substrate. Extensive work was carried out during 1990 to 2000 period on these type photovoltaic cells and cheaper than first generation photovoltaic cells. The copper-indium-gallium-diselenide (CIGS) and CdTe chalcopyrite structured thin films have been used for the photovoltaic cells fabrication and reported the highest efficiency of photovoltaic cell 22.6% and 22.1% [9–11]. Copper-zinc-tin-sulfide/selenide (CZTSSe) photovoltaic cells have achieved higher efficiency of 12.6%. However, for reducing the cost and utilization of abundant, nontoxic material with rare-metal free absorber layer, a substitute for CIGS is necessary [9–13]. When compared with silicon photovoltaic cells, CZTSSe absorbers are highly efficient due to reduced energy losses and can harvest more photons from the sun. However, the actual efficiency levels in comparison with silicon are still lagging. These generations of photovoltaic cells, made up of the above-mentioned thin films exhibit a high photovoltaic absorption coefficient, requiring a layer thickness of ∼2.5 μm, compared to ∼170 to ∼250 μm for silicon photovoltaic cells [14]. CIGS-based photovoltaic cell consists of a multilayered thin film deposited onto a substrate made of glass, metal, or polymer foil. The p- and n-type semiconductor materials used in this PV cell are CIGS and cadmium sulfide, respectively. They are sandwiched between the molybdenum positive electrical back contact and the transparent conductive oxide which acts as negative electrical contact. Alternatively, Cu2ZnSnS4 (CZTS) is another prominent thin-film photovoltaic cell material which is an inexpensive, abundant, nontoxic material, and also a rare-metal free low-cost absorber layer suitable to be used as a substitute for CIGS absorber layer in the thin-film photovoltaic cells [15]. The economic amorphous and micro-crystalline Si thin films are also considered under second generation photovoltaic cells.
The front illumination side of photovoltaic cell consists of an intrinsic a-Si:H passivation layer and a p-doped amorphous silicon layer as emitter. These layers are deposited by plasma enhanced chemical vapor deposition (PECVD) technique and an antireflective transparent conductive oxide (TCO) top layer by physical vapor deposition (PVD) method. The charge collection layer is screen-printed through metallic contacting grid. While, on the back side electron collecting stack has been used, which comprises of PECVD derived an intrinsic a-Si:H passivation layer, n-type amorphous silicon along with PVD deposited TCO layer and metallic contacting layer. The minimal material requirement, low temperature processing, light weight, flexibility of films for fitting on curved surface/panels, light weight and flexible textile or polymeric sheets and lower cost/Watt are some of the advantages of the second generation photovoltaic cells. While, the low efficiency of these photovoltaic cell in comparison to first generation photovoltaic cell is their disadvantage.
1.3.3 Third Generation Photovoltaic Cell
This generation photovoltaic cells are different from the earlier two first and second generations because they use different nanomaterials and the high surface to volume ratio of nanomaterials as well as their tendency to acquire different morphological shapes e.g. quantum dots, nanotubes, nanowires, nanopillars, nanocrystals, etc. This category is comprised of the organic dye sensitized, photoelectrochemical, conjugated conducting polymer, and polymer PVCs which are used in various space and territorial applications. Their design is different from the normal semiconductor p-n junction device for the separation of photogenerated charge carriers. These PVCs have multiple energy levels which means multiple carrier pair generation and these carriers can be captured before thermalization process [16, 17]. That’s why third generation PVCs are also known as hot carrier solar cells with the advantages of very high limiting efficiencies. The fundamental operational principle of these PVCs, carrier thermalization processes minimize or restrict the energy loss and use the excessive thermalization energy as increased voltage in the external circuit and sustain the high absorption as well as the consequent external current of a small bandgap material. So, by tuning bandgap energy, on can capture light from different parts of the solar energy. These PVCs are developed on flexible substrates to achieve better interactions among molecule-to-molecule [18].
Dye sensitized solar cells (DSSCs) are considered as alternative for conventional silicon solar cells owing to their high throughput, ease of fabrication and cost effectiveness. In the photochemical cell, the dye molecules semiconductor layer is formed in between photosensitized anode and electrolyte. The extensive work has been carried on these type solar cells by using nanocrystalline metal oxide and their composites. The non-hazardous and stable Anatase phase TiO2 nanoparticles porous layer has been used as anode layer and dye molecules absorbs the incident sunlight to promote electron transport from a lower level to an excited one. The excited electron is injected by the dye into the semiconducting TiO2 anode layer and the chemical diffusion of electrons from the TiO2 layer to the conductive indium tin oxide (ITO)/tin oxide (SnO2) layer forms an outer circuit. The electrons come back to the cell to complete the circuit and for returning the dye back to its “normal” state via electrolyte solution and maintains the movement of electrons through the cell. This cell is sandwiched in between the two conducting overlapped glass plates. Platinum electrode act as the cathode part and a mixture of iodide/tri-iodide redox couple plays the role of electrolyte. The usage of nanomaterials in DSSC helps in harvesting the excessive energy in the absorbed photons for multiple exciton generation which in turn markedly enhances the energy conversion efficiency of a solar cell. Hence the broad band semiconducting oxide nanomaterials play a vital role in energy conversion efficiency and control the energy sustainability of solar PV. The major disadvantage of DSSC is the synthesis and chemical stability of the dye and liquid organic electrolytes. A lot work has been reported on the DSSCs which involves the use of different nanocrystalline pure, mixed and functionalized metal oxides [19–35].
To overcome the limitations of DSSCs, the economic organic-inorganic halide i.e. perovskites and QDs used in-place of dyes. First time, these perovskite-based PV solar cells were fabricated by Miyasaka et al. in 2009 [36] with maximum power conversion efficiency of ∼ 3.8%. Now the efficiency reported for such nano solar cells is more than 20% which is greater than that of organic thin film solar cell. The highest efficiencies reported for perovskite solar cells have been mainly obtained with methyl ammonium lead halide materials. For example, lead halide perovskite materials, methyl ammonium lead iodide with band gap of about 1.5-1.6 eV with a light absorption spectrum up to a wavelength of 800 nm has been measured and widely used as a light capturer in solar cells. The perovskite-based 3rd generation solar cells are categorized into two types (i) comprised of compact TiO2 and mesoporous TiO2 layers and (ii) vapor-deposited perovskite solar cells based on small molecule organic solar cell and having only compact TiO2 layer. Perovskite solar cells contain∼ 40 nm compact TiO2 layer on transparent conducting oxide. When the perovskite (CH3CH2NH3PbX3) in the porous layers absorb light, electron hole pairs are produced with electrons on the conduction band (3.93 eV) and holes on the valence band (-5.43 eV). The electron hole pairs are separated swiftly by introducing electrons into TiO2 (-4.0 eV) and transporting holes to the platinum counter electrode (-5.0 eV) forming a photocurrent in the device. The third-generation devices can be easily fabricated using simple industrial technologies which are capable of fabricating polymer solar cells. Though stability and performance of this generation of solar cell devices are limited and still at lab-scale, they have great potential and can be easily commercialized in the near future. The concentrated solar cell is also one of the promising technologies used in 3rd generation PVCs fabrication. These cells work on the principle of concentrating large amount of solar radiation on its small area. Though the semiconducting material cost is very high, this way one can drastically reduce the cost. This setup needs very good quality integrated perfect optical system with condensing levels from about 10–1000 suns. This technology will definitely emerge as very attractive acceptable technique in future.
1.3.4 Fourth Generation Photovoltaic Cell
These photovoltaic cells involve composite technology. They use mixture of polymers with nanoparticles to form single thin multispectral layer which are further stacked to produce economic with enhanced efficiency multi-spectrum photovoltaic cells [37]. Such, tandem PV cell consists of upper and lower solar cells with buffer layer in between them and they try absorbing the maximum or entire wavelength region of solar spectrum. The charge carriers produced by the absorption of sun light upper cell are collected by the electrodes and photocurrent moves through the buffer layer present in between these two PV cells [38]. The tandem cell contains at least two p-n junctions with cells composed of materials that absorb different photon energies. The upper cell absorbs the higher energies and the lower cell would absorb the lower energies which are not absorbed by the upper cell. The tandem cells have higher efficiency because they absorb more photons of the photovoltaic spectrum for energy conversion. This photovoltaic technology has been used for space application. Tandem photovoltaic cells are composed of the periodic table groups III and V elements compounds e.g. GaAs, InP, GaSb, GaInP, and GaInAs etc. The highest reported efficiency of these PV cells containing two and three cells are around 43% and 48% respectively.
1.4 Role of Nanostructured Metal Oxides in Production, Conversion, and Storage in Harvesting Renewable Energy
Nanomaterials have drawn considerable interest in the field of renewable energy production, conversion, and storage devices e.g. PV cells, fuel cells, batteries, thermal plants, hydrogen generation and new hi-tech storage devices etc. due to their high catalytic, mechanical, electrical, and optical performance as compared to bulk materials [39]. The particle size, morphological shape, large surface-to-volume ratio, and grain boundaries of nanomaterials have markedly enhanced efficiency of energy harvesting devices [40]. Nowadays, by the incorporation of nanotechnology and nanomaterials high power, stable energy generation, conversion, storage setups for large- and small-scale production as well as power distribution are in use all over the world. The nanostructured wide band metal oxides in thin/thick film form are used as active (transport) layer or passive (transparent conducting oxide electrode) layer in PV cells [41, 42]. Such nano dimensional metal oxides remarkably increase the visible region energy absorption via multiple reflections from the normal sunlight for efficient conversion to electricity, reduces electron-hole recombination process and ease of tuning band gap for their dual usage as absorber or window layer in thin films/heterojunction PV cells [43]. They minimize discharging rate of storage devices. The wide band gap energy semiconducting metal oxides exhibit optoelectronic properties and used as charge transport layers (CTLs) in PV devices. The n-type wide band gap ZnO, TiO2, SnO2 metal oxides with low work-function allows electron transport in the device and blocks hole transport. They are used as electron transport layers (ETLs). While, p-type wide-bandgap metal oxides like NiO, MoO3, V2O5 and WO3 etc. with high work function permit hole transport and blocks electron. They can be utilized as hole transport layer (HTL) in solar cell. But, some of these oxides suffer with a problem of not blocking electron transport efficiently. So, one needs very stable metal oxides for CTLs application which work efficiently throughout PV cells life. It’s active layer/ETL, active layer/ HTL, cathode/ETL, and anode/HTL interface contacts should be very stable and they should remain intact for long-time operations. The last one is that during PV cell working, the active layers are in the excited states and they are mostly sensitive to surrounding atmospheric oxygen or moisture. Hence, the effect of environmental stability on device performance can be greatly enhanced if CTLs also play role as barriers for oxygen and moisture diffusion. It means the optoelectronic properties of nanostructured metal oxides as CTLs viz. work function, band structure, conductivity, intragap states and optical properties parameters should be optimized for exploiting their usage in PV cells application.
1.5 Synthesis of Nanostructured Metal Oxides for Photovoltaic Cell Application
The various chemical and physical methods are employed for the synthesis of nanostructured metal oxides with desired stoichiometry, structure, particles shape (e.g. nanospheres, nanorods, nanowires, nanotubes, nanoflakes, nanoribbons, nanoflowers, nanosheets etc.) and size, surface morphology in random and regular ordered arrangements of agglomerated ensembles in thin films etc. as per device requirement. The chemical methods are widely used as compared to physical methods owing to their low cost, ease of controlling stoichiometry by optimizing synthesis parameters, appropriate selection of reactants and synthesis process. In physical methods, the cost of substrates, instrumentation and their maintenance are very high. Chemical methods involved in the deposition of nanocrystalline metal oxides thin films are summarized under two categories (i) evaporation, sputtering and gas-phase-transport-based techniques and (ii) the solution deposition method. The chemical vapor deposition (CVD) [44–47], metal organic chemical vapor deposition (MOCVD) [48–52], plasma enhanced chemical vapor deposition (PECVD) [53–55], spray pyrolysis (SP) [56–61], atomic layer epitaxy (ALE) and atomic layer deposition (ALD) [62–69] comes under gas-phase methods. While, chemical co-precipitation [70–75], sol-gel [76–81], solvothermal [75, 82–87]/hydrothermal [88–90], microemulsion [91–95], microwave assisted [96–101], ultrasonic/sonochemical [102–104], green chemistry [106–111] methods are used for obtaining nanoparticles of desired size and morphology whose dispersions are used in spin [112–114] and dip-coating [77, 115, 116] techniques for depositing thin films. Physical vapor deposition [117–124] methods are pulsed laser deposition (PLD) [125–128], radio frequency (RF) sputtering [129–132] and dc magnetron sputtering [133–136]. The key difference between PVD and CVD methods is in the vapor used for film deposition. In PVD, the of vapor consists of physically discharged atoms and molecules which merely condenses on the substrate. While for CVD, the vapor undergoes a chemical reaction on the substrate to form a thin film. In addition to these methods, the other methods like chemical bath deposition (CBD) [137–142], electron beam evaporation (EBE) [143–147], thermal or vacuum evaporation [148–156], electrochemical deposition [157–161] and anodic oxidation [162–167]. The screen printing [168–174] technique has been used for the deposition of metal oxide thick films on different substrates.
1.5.1 Chemical Vapor Deposition Method
Chemical Vapor Deposition (CVD) method undergoes series of chemical reaction viz. gas phase reaction chemistry, thermodynamics, kinetics and transport phenomena, film nucleation and growth process and reactor setup at specific process parameters like temperature, pressure and reaction rates for mass as well as energy transport for thin film deposition [44–47]. These factors control the chemical reaction between the reactant and substrate to obtain good quality of films of desired composition and physical properties. The tentative CVD reactions are pyrolysis, reduction, oxidation, compound formation, disproportionation, and reversible transfer etc. The basic steps followed in all types of CVD methods are listed below:
Convection or diffusion of reactant in a gas phase to the reaction chamber
Chemical and gas phase reactions produce reactive species and by-products
Transport of the reactants via boundary layer to the substrate surface
Chemical and physical adsorption of the reactants on the surface of substrate
Heterogenous reactions at the surface results in the formation of a solid film
The volatile by-products are desorbed by diffusion through the boundary layer to main gas stream
The gaseous by-products are removed from the reactor via convection and diffusion process.
CVD methods are categorized on the basis of energy sources (thermal, laser, photon) and reactors used for initiating the chemical reaction. The device application decides the selection of appropriate process/reactor as per substrate material requirement and coating materials, surface morphology, film thickness, uniformity, availability of precursors, and their cost. Nanocrystalline TiO2 and Sb doped SnO2 thin films prepared by CVD method for application in DSSCs was reported by P.S. Shinde and C.H. Bhosle [46] and J.I. Scott et al. [47] respectively.
1.5.2 Metal Organic Chemical Vapor Deposition Method
Metal organic chemical vapor deposition (MOCVD) method is also known as metal organic vapor phase epitaxy (MOVPE) and generally used for synthesizing III-V and II-IV semiconducting compounds [48]. The III-IV compounds are widely in various optoelectronic and electronic devices for large scale production. Now, the epitaxial growth of thin films by MOCVD technique has been also used in the preparation of metals, dielectrics and multifunctional nanomaterials for many new state-of-art advanced technologies. In MOCVD setup, all sources are vaporized and moved into the reactor with carrier gas. MOCVD reactors have the following three different geometries (i) high-speed rotation vertical reactor, close-coupled showerhead (CCS) reactor and planetary rotation horizontal reactor in its system. The injected gases should be ultra-pure and have finely controlled entry into the system. The reaction steps involved for producing III-V semiconductors are given as:
Vaporization of the group III and group V precursors into gas phase to transport them towards substrate for diffusion via the phase interface
Adsorption of molecules/atoms at the surface of substrate
Molecules/atoms are directed to crystallization zones for chemical reaction between group III and V salts
Decomposition and/or desorption of the by-products by diffusion and convection out methods from the boundary layer
Removal of the by-product out from the reactor by evacuation process.
The work on MOCVD processed textured ZnO, CdO and TiO2 thin films for solar cells applications have been reported [49–52].
1.5.3 Plasma-Enhanced CVD (PECVD) Method
Plasma-enhanced CVD (PECVD) method deposit a thin film from gaseous state to a solid state on the substrate. The chemical reaction takes place after the creation of a plasma in the reactor chamber which subsequently deposit thin film on the substrate surface. PECVD uses an electrical source of energy to generate plasma to undergo reaction process. The electrical energy produces ionic species and radicals through homogeneous chemical reactions to actuate heterogeneous chemical reactions for thin films formation on the substrate. The main advantage of PECVD over thermal CVD method is the very low operating temperature i.e. near to the ambient temperature [53]. The use of plasma to activate the gas phase chemistry opened many new reaction routes for the film deposition at low temperatures. H. Huang et al. [54] and L. Mazzarella et al. [55] deposited SnO2 and nanocrystalline silicon thin films by PECVD method to use then in solar cells.
1.5.4 Spray Pyrolysis Method
In spray pyrolysis method, the thin film deposition involves the spraying of precursor solution onto a heated surface of substrate for the reaction of chemical constituents to produce desired chemical compound [56, 57]. This process completes by following anyone of the three ways:
The first way, the droplets of the solution sprayed by atomizer fall on the hot surface, the solvent evaporates and components then react in the dry state.
The second way, the solvent evaporates on the way when the drops are moving towards the hot surface and the dry solid gets attached to the hot surface by decomposition.
The third way, the processes involved are the vaporization of solvent as the droplets are transported towards the substrate and consequently undergoes heterogeneous reaction with the solution components. The substrate temperature, carrier gas flow rate, nozzle-to-substrate distance, and the solution amount along with its concentration are some of the parameters which controls the quality of deposited film. Out of these, the substrate temperature is found to be the key parameters for depositing thin film by spray pyrolysis method because it is involves the drying of droplets, decomposition, crystallization, and grain growth processes. The main part of the spray pyrolysis deposition setup is the atomizer to form aerosol from the precursor solution. The chemical reactants are chosen in such a manner that the products other than the desired compound should evaporate at the deposition temperature. In spray pyrolysis, the reaction in the vapor phase at moderate high temperature takes place in normal air atmosphere for depositing transparent conducting thin films of nanocrystalline metal oxides in aqueous and non-aqueous medium such as Sb-doped SnO
2
[
58
], CdO [
59
], porous TiO
2
[
60
] and Ce-doped CdO [
61
]as compact layers on the substrate surface having conductive or non-conductive films for solar cells application. Spray pyrolysis method has been widely used by the researchers owing to its simplicity and cost effectiveness.
1.5.5 Atomic Layer Deposition or Atomic Layer Epitaxy Method
Atomic Layer Deposition (ALD) or Atomic Layer Epitaxy (ALE) Method is also known as pulsed CVD. This process is based on the principle of self-limiting surface chemical reactions between two gaseous precursors and permits the layer-by-layer deposition of ultrathin thin films even up to atomic levels [62–64]. Such deposited conformal films have high aspect ratio structures, uniform thickness over large surface areas, at comparatively low temperatures and at moderate pressure. This method can be used in fabrication of hybrid nanostructures for many applications like in magnetic recording heads, optoelectronic, complementary metal-oxide-semiconductor (CMOS) transistors, integrated circuits (ICs), micro-electro-mechanical systems (MEMS micro-electro-mechanical systems (MEMS) and passivation/antireflection coatings etc. ALD method has been successfully used in the preparation of high-k metal gate transistors which markedly reduces the size of ICs chips. By this method, the ultrathin layers of ZnO [65–69] and V2O5 [60] are deposited in solar cells. The metal oxide thin layers deposited on CdTe/ITO interface imparts the role of barrier layer in solar cells [66].
1.5.6 Chemical Co-Precipitation Method
In the chemical co-precipitation methods, the sparingly soluble products from aqueous solutions (or other solvents) separates out after mixing reactant solutions followed by thermal decomposition to obtain the desired metal oxide/oxides. This method undergoes the nucleation, growth, Ostwald ripening, and/or agglomeration processes simultaneously [70–74]. Under the high-supersaturating condition, at first very fine large number of small crystallites as sparingly soluble precipitates appear due to initiation of nucleation process which swiftly agglomerate to form bigger sized thermodynamically stable nanoparticles under growth process. During this process, the smaller particles have been used to form bigger sized nanoparticles via Ostwald ripening. In this step, the particle size, morphology, surface/volume ratio and properties of the final products are optimized in the absence of stabilizer. The synthesis of desired stoichiometric metal oxide nanoparticles (MONPs), the precipitated nanoparticles are subjected to drying, calcination/sintering and annealing processing. While in some cases, MONPs are directly obtained by precipitation reaction. The agglomeration of MONPs has been drastically reduced by the following two ways (i) steric repulsion by capping ligands and (ii) Electrostatic (van der Waals) repulsions via chemisorption process. In steric repulsion the MONPs are capped with bigger sized organic molecules/polymeric chains as surfactants which encapsulate each particle and restricts agglomeration process. On the other hand, in electrostatic repulsion process, the charged species such as H+, OH- or other ionic species present at the surface of MONPs are involved in stabilization and reducing agglomeration. But MONPs capped with insulating surfactant organic molecules hampers it’s charge transport layer property owing to the variations in the energy barriers for charge carrier transport layers [71].
D. Ouyang et al. [72] studied the usage of cost-effective metal oxide nanocrystals synthesized by solution-method as Carrier Transport Layers in organic and perovskite solar cells for improving their efficiency and life. M. Bhogaita and D. Devaprakasam [73] investigated the hybrid photoanode based on TiO2-ZnO prepared by co-precipitation route as better alternative to individual TiO2 or ZnO photoanode for dye-sensitized solar cell using phyllanthus reticulatas pigment sensitizer for effectively enhancing device efficiency by reducing charge recombination process. M. Yin et al. [74] prepared low cost non-hazardous Cu2O by co-precipitation method from cuprous acetate with an aim for photovoltaic applications because of its high optical absorption coefficient, low band gap i.e. ∼ 2.2 eV. While, M.M. Rashad et al. reported nano-powdered SnO2 synthesis by chemical co-precipitation and solvothermal methods and their use in Dye Sensitized Solar Cells (DSSCs) for increasing solar cell efficiency.
1.5.7 Sol-Gel Method
Sol-gel one of the extensively used method in which metal alkoxides or metal salts as reactants are used to undergo hydrolysis reaction to form the “sol” i.e. uniform dispersion of fine particles and then follows by the condensation process to get “gel” a cross-linked network of particles [76–78]. The gel is generally aged for several hours to days and dried to obtain porous Metal Oxide hydroxide/MONPs [78, 79]. Such crystalline porous MONPs are subjected to calcination at high temperature in the temperature controlled furnace to achieve the desired structure, morphological shape and size of MONPs. However, the calcination at high temperature increases the particle size and decreases the surface area of the nanoparticles because of the heat induced enlargement effect.
The conventional aqueous sol–gel approach is suffered from high reaction rates and unable to control the hydrolysis and aggregation processes. In addition to this, the aqueous sol–gel approach needs annealing at high temperature to form crystalline phase of sample. On the other hand, non-aqueous sol–gel approach was used to overcome these problems [78, 80, 81]. It follows various chemical reactions, like hydrolysis, alcoholysis, aminolysis, and halide elimination to prepare colloidal oxide nanocrystals. As per the reaction mechanism pathways, metal precursors and their concentration, activation reagents, and reaction parameters such as temperature (includes pre- and post- heat treatment of the materials used), pH of solution, duration and nature of solvents were cautiously chosen to control the reaction kinetics and crystallization processes.
M. Szindler et al. [76] reported the synthesis of ZnO nanopowder by sol-gel method and characterized by various analytical techniques with an aim to use this as photoanode in DSSCs. S. Thiagarajan et al. [77] discussed the synthesis of metal oxide nanoparticles by sol-gel route and described in detail the synthesis of ZnO, Al doped ZnO, Iron oxides, tin oxide, and copper oxide by this method for application in the energy conversion devices. B. Ludi et al. described the mechanism behind the formation of ZnO nanoparticles in benzyl alcohol by the non-aqueous sol-gel method [81].
1.5.8 Solvothermal/Hydrothermal Method
In the solvothermal method, the precursor solutions are treated in closed reaction vessel/autoclave at temperatures more than the boiling points of solvents used. The pressure developed in the vessel by the solvent vapors raises the boiling point of the solvent and form highly crystalline materials [82–84]. Solvothermal as well as hydrothermal methods are the most powerful and widely used methods to synthesize nanomaterials because of the high reproducibility, simple procedure, and the easy scale-up merits. The large production of the nanomaterials is possible by using continuous reactors. However, the solvothermal methods generally require long reaction time ranging from several hours to days. Therefore, simple and rapid synthetic method for the preparation of metal oxide nano-assemblies always attracts researchers. In this context, C.T. Dinh [84], X. Liang et al. [85], Z. Li [86] and R. Krishnapriya [87] prepared by solvothermal method shape controlled, nanocrystalline TiO2 particles, colloidal metal oxide as charge transport layer in LEDs and solar cells, WO3 as hole transport layer in perovskite solar cells and ZnO as photoanodes in DSSCs respectively. While, J. Beusen et al. [88], I.T. Papadas et al. [89], and X.D. Ai [90] prepared TiO2 nanoparticles amorphous layer, CuGaO2 nanoparticles and ZnO whiskers shaped particles respectively by hydrothermal method for application in solar cells.
1.5.9 Microemulsions Method
The microemulsion method deals with thermodynamically stable transparent solution with low viscosity formed spontaneously from the surfactant, water, and oil mixed solution. A water-in-oil microemulsion is a solution of nano sized water droplets stabilized by surfactant molecules monolayer [91–93]. The microemulsion has been used as a nanoreactor for preparing uniform size nanoparticles via chemical reactions confined within the aqueous core. Hence, the nanoparticles formed by this method are stabilized by the surrounding surfactant layer acting and controls the aggregation of as prepared nanoparticles [91]. By the addition of solvent, such as acetone or ethanol, the precipitate are formed by adding acetone or ethanol which are collected either by filtration or centrifuging the mixture. The droplet size has diameter in about 2–20 nm range which is far more than by the surfactant monolayer thickness. This reveals that the enlargement in water or oil droplet size takes place during the chemical reaction of reactants for nanoparticles formation. The particle size can be easily controlled by changing water concentration and different morphologies are obtained by varying surfactant concentration. It means, in microemulsions monodispersed droplets exist in a dynamic state which undergo continuous collisions with each other to form bigger sized droplets followed by their breakup. This spontaneous reaction results in the formation of micelles with precipitate in it. The monodispersed metal oxide nanoparticles derived by microemulsion method have very fine in nature [92]. Nanostructured ZnO for solar cell applications are also prepared by microemulsion method [94, 95].
1.5.10 Microwave-Assisted Method
Microwave (MW) assisted method utilizes MW irradiation to proceed for chemical reactions. In this method, the reactants are heated through microwave dielectric heating which depends upon material characteristic property to absorb MW energy and transforms it into heat. In the liquid phase synthesis of nanomaterials, MW irradiation initiates heating either by dipolar polarization or by ionic conduction. The dipolar polarization mechanism is governed by the rotation of polar solvent molecules or reactant salts constituting dipoles in the reaction mixture. While, charged particles like free ions or ionic species present in material follows ionic conduction. When irradiated at MW frequencies, the dipoles in the sample try to align themselves in the direction of the applied electric field. As the electric field oscillates, the molecular dipoles accept the alternating electric field streamlines and loses energy as heat by the molecular friction and dielectric loss [96, 97
