Quantum Dots Sensitized Solar Cell

Abstract

In the modern and automated twenty-first century, with technological advancements, the scientific society has gifted a new alternative clean energy source (dye/quantum dot sensitized solar cells) to mankind as one of the applications of nanomaterials. Nowadays, the world is using a tremendous amount of fossil fuel for energy creation, the solar energy by using nanomaterials in the form of solar cells is a perfect alternative. In the present chapter, the emphasis has been given on the different techniques used by the researcher for synthesis of nanoparticles. The synthesis of quantum dots by simple cost-effective technique is covered with respect to PbS quantum dots. The working of quantum dot sensitized solar cells is also explained with its basic components. The narrow-band-gap semiconducting materials, which are suitable for solar cell application, are also listed in this context.

1. Introduction

The large existing need of the global energy is accomplished by burning fossil fuels. However, the inherent problems associated with the use of fossil fuel are their limited availability and the environmental issues. This forces mankind to look for new, more sustainable long-term energy solutions to provide the future energy resource. As the major renewable energy source, i.e., solar energy has the potential to become an essential component of future global energy production. The solar cell converts sunlight directly into electricity. In recent year, the concept of utilizing nanomaterial-based architectures in light energy conversion devices has emerged as an alternative to single-crystalline-based photovoltaic devices.

Besides the silicon solar cells, quantum dots sensitized solar cell (44%) and dye sensitized solar cells (33%) are two promising alternatives due to their cost-effective concepts for solar to electric energy conversion. The configuration of QDSSC consists of Photoanode (TiO₂, ZnO, ZrO₂, SnO₂, etc., nanoparticle thin films), quantum dots (PbS, PbSe, CdS, CdSe, etc.), electrolyte (polysulfide, cobalt complex, etc.), and counter electrode (MoO₃, CuS, CuSe, Pt, carbon black, etc.). Upon the illumination of light, the QDs generate the exciton (electron-hole pairs). Consequently, electron is injected into the photoanode to generate photocurrent, the hole is transported to the counter through redox couple toward electrode (Cathode). The overall power conversion efficiency (PCE) of a QDSSC is governed by the light harvesting efficiency, quantum yield for charge injection, and charge collection efficiency.

The chapter focuses on different approaches of preparation of nanomaterials. Synthesis of Quantum Dots (PbS) by simple cost-effective route. The structure of Quantum Dot Sensitized Solar Cell is given with requirement of basic components. The use of narrow-band-gap semiconductor, the materials suitable for the solar cell are also discussed in this topic.

2. Methods of preparation of nanomaterials

Numerous methods of nanomaterials synthesis have been developed. Generally, the two approaches top-down and bottom-up are used.

1. Top-down approach: In this approach, the etching and ball milling techniques are generally used to crush down large structure into small one, till the desired size is achieved.

2. Bottom-up approach: This approach uses the atomic or molecular precursors to form nanostructure; the nanoparticles are formed atom by atom or layer by layer. This approach is more widespread and generally used to synthesis of nanomaterials having different morphologies and properties. Hence, bottom-up approach has given a stress for further important discussion.

The method of synthesis of nanoparticle is given in Figure 1.

Figure 1.

Methods of synthesis of nanomaterials.

Synthesis of nanoparticles:

There are four methods used for the synthesis of nanoparticles:

1. Physical

2. Chemical

3. Biological

4. Hybrid

2.1 Physical methods

It is broadly divided into two types, namely mechanical and vapor-based methods [1, 2, 3].

2.1.1 Mechanical methods

In this process, containers are used to make nanoparticles, hard balls of tungsten carbide, zirconia, or steel are plunged in the containers along with the materials whose nanoparticles are to be made. So this high-energy ball milling is a type of mechanical method.

2.1.2 Vapor-based methods

The nanostructures are formed by evaporating different material on various types of substrates. It is divided into a. Physical vapor deposition (PVD) and b. Chemical vapor deposition (CVD).

2.1.2.1 Physical vapor deposition (PVD) method

It is general term used to describe any of the variety of methods to deposit thin films by the condensation of vaporized form of the desired films material on to various working surfaces (e.g., onto semiconductor wafers). PVD is used in the manufacturing of semiconductor devices.

2.1.2.1.1 LASER ablation

LASER ablation is the process of removing material from a solid (or occasionally liquid) surface by using pulse of LASER beam. At low LASER flux, material is heated by the absorbed energy and evaporates or sublimates, and at high flux, the material is converted to plasma. Single-wall carbon nanotubes are synthesized by LASER ablation method.

2.1.2.1.2 Biological method or green synthesis

Synthesis of nanomaterial using biological ingredient, which is divided by using microorganism such as fungi, yeasts, and bacteria, plants extracts or enzymes and use of templates such as DNA, membrane, viruses, diatoms.

2.1.2.1.3 Hybrid method

Hybrid method utilized for the preparation of nanoparticles involves the process such as electromechanical, chemical vapor deposition, particle arresting in glass or zeolites or polymers micro-emulsion.

2.1.2.2 Chemical vapor deposition

There are techniques depending on the source of excitation and the conditions of deposition, which include Atmospheric-Pressure CVD (APCVD), Low-Pressure CVD (LPCVD), Ultrahigh-Vacuum CVD (UVCVD), Metal Organic CVD (MOCVD), and Hot Filament CVD (HFCVD). In this method, when a substrate is exposed to one or two volatile precursors, they chemically react in vapor phase or decompose on its surface under thermal, plasma, or laser excitation and form nanostructures of high quality.

2.2 Chemical methods

These are less expensive, simple, and easily scalable techniques and used to synthesize nanostructures of different shape, morphologies, and particle sizes.

Various chemical methods are discussed as follows:

2.2.1 Colloidal synthesis

Colloids are the class (few nm sizes) of material where two or more phases exist with a size less than a micron. Particles are generally suspended in some host matrix. The colloidal particles are stabilized by columbic repulsion or stearic hindrance against aggregation.

Colloidal synthesis is carried out generally in a three-neck flask and to avoid processes such as oxidation of products; the reaction is carried out in inert atmosphere.

2.2.2 Chemical bath deposition

The high-quality metal chalcogenide nanocrystalline thin films (metallic or non-metallic substrate) are fabricated by using this technique. This requires the metal and chalcogen ions. In other way, deposition of the film is possible if the ionic product should exceed the solubility product.

Thin film deposition proceeds as nucleation on the surface of the substrate, i.e., formation of stable second phase by the combination of a minimum number of ions or molecules in contact with a solution. The adsorbed cations/anions deposited on the substrate act as nucleation centers, and subsequent deposition takes place through adsorption of more ionic species present in the solution that results in the homogeneous and uniform film. The film deposition takes place ion by ion or cluster by cluster. The former results into uniform, adherent thin film, whereas latter thick, powdery, and diffusely reflecting films. The pH is also one of the deciding factors, at higher pH values, solubility product exceeds ionic product, i.e., unfavorable condition for deposition. Also as the temperature increases, metal ion gets dissociated from the complexing agent and film deposits through nucleation. Moreover, concentration, type of complexing agent, stirring rate of the solution also decide the quality of films [4, 5].

Doctor blade method:

Doctor blade method is very simple technique for coating of substrate, in this method precursor, solution is dropped/placed on the substrate. The film is produced by moving the blade over the substrate. Drying and annealing steps are used to form a solid film on the substrate. Various steps involved in doctor blade method are illustrated in Figure 2.

Figure 2.

Doctor blade technique for the fabrication of photoanode.

In doctor blade technique, well-mixed slurry of nanoparticles along with other constituents such as binder, dispersants, etc., are used. The slurry is spread over on a substrate and thin layer is produced, thickness of films is depending on the distance between blade and substrate [6, 7]. The advantages of Doctor Blade method are

• Formation of large area film

• No wastage of material

• Uniform film deposition

2.2.3 Successive ionic layer and adsorption reaction (SILAR) method

SILAR is also known as modified chemical bath deposition technique. It has advantages such as

1. It is the simplest method for deposition of any element in any proportion.

2. SILAR method does not require very good quality substrate, target, and vacuum like closed vapor deposition method.

3. Thickness and size of film are easily controlled by changing deposition cycles.

4. Deposition is carried out at room temperature.

It is mostly based on the phenomenon, i.e., adsorption and reaction of the ionic species. In which, the substrate is dipped in the separate solution for the fixed interval of time. One SILAR cycle is completed when substrate is dipped in both the precursors once successively. In order to avoid the precipitation, after every withdrawal of the films, it is followed by rinsing the substrate by alcohol or water.

A large number of metal chalcogenides (PbS, CdS, CdSe) [8] and metal oxides are prepared using this method. It is the simplest technique with which QD size and coverage can be controlled. The schematic of SILAR [9] deposition is given in Figure 3.

Figure 3.

Schematic of SILAR depositions. Source: Ref. [9].

2.2.4 Electrodeposition

The binder-free and inexpensive technique used to produce nanoparticle with controlled morphology, size, and composition. In this technique, varying operating parameters of deposition such as concentration of solution, operating voltage, deposition potential and electric current flow, the thickness and its crystallographic orientation can be controlled.

The films produced by this technique are uniform, adherent. As per the reports, the synthesis of nanoparticles, nanowires, metal oxides, and metal nanoparticles is possible using electrodeposition [10].

2.2.5 Sol-gel method

This technique generally has hydrolysis of precursors, condensation followed by polycondensation to form particles, gelation, and drying process. In this technique, sol is generated and converted into a viscous gel, and then it gets converted into a solid material.

This technique is commonly used to synthesize ceramic or metal oxides [11].

2.2.6 Langmuir-Blodgett method (L-B)

L-B method is used to deposit the molecular monolayers and multilayers. It is suitable to deposit various organic materials salts and fatty acids [12]. This technique transfers organic layer at air liquid interface on to solid substrate in which amphiphilic long-chain molecule like oleic epoxide. Although the layers of L-B films are ordered, there is only Vander Waals interaction between these layers. Thus, even large number of layer present in the film preserves its two dimensional properties.

2.2.7 Spray pyrolysis

This is very simple and cost-effective deposition technique to deposit thin and thick films of metal oxides, metal sulfides [13, 14], etc. The dense, porous, multilayer films can be prepared by this technique.

The spray pyrolysis unit consists of atomizer, which sprays a metal salt solution onto a heated substrate; the droplets sprayed on undergo a thermal decomposition. There are also other factors such as precursor solution, temperature controller, and substrate heater influencing the decomposition.

The synthesis of PbS quantum dot by simple facile technique without capping agent and clumsy vacuum techniques is discussed here.

3. Synthesis of lead sulfide quantum dots

QDs possess size-dependent and discrete electronic energy spectra due to quantum confinement effect [15, 16]. Quantum dots such as CdS, CdSe, InP, PbS, PbSe, etc., are synthesized by researchers using a methods mentioned above and have a wide range of applications. Among metal chalcogenides, lead chalcogenides, especially PbS and PbSe QDs have been interesting nanostructures due to their characteristic property to display Multiple Exciton Generation, where a single photon can yield three excitons; hence, it is useful in highly efficient photovoltaic conversion. Also, PbS quantum dot sensitized solar cell gave a very high photocurrent. In view of these, the synthesis and application of PbS have assumed great importance. PbS is a IV–VI semiconductor with Bohr excitonic radius of 18 nm. It has a bulk band gap of 0.41 eV that can be tuned up to 1.5 eV at the QD level, and hence, it shows a strong quantum confinement effect. PbS has its applicability in sensors, photography, IR detector (due to absorption near IR region), solar absorber, etc.

To synthesize QDs, hydrothermal, sono-chemical, micro-emulsion, and organometallic techniques have been developed by the researchers. The organometallic method gives a better size distribution of PbS nanocrystals, but the formation involves hazardous and unstable chemicals such as (TMS)₂S, trioctylphosphine, etc.; therefore, it is significant to find a simple route. Previously PbS QDs resulted in either polycrystalline or single crystalline with the help of a coordinating agent, but Bhalekar and Pathan synthesized PbS quantum dots by sono-chemical technique using precursors such as lead nitrate Pb(NO₃)₂ and sodium sulfide Na₂S in aqueous media, without any harmful element.

The experimental procedure involved in the synthesis is as follows:

About 0.01 M of Pb(NO₃)₂ and Na₂S (very well dispersed) solutions were prepared in double distilled water (DDW) at room temperature separately. These solutions are then added to 400 ml DDW drop-wise and the color variation is observed. The solutions are sonicated further using probe sonicator (ENUP-250A). The procedural steps [15] are schematically shown as in Figure 4. The solutions with color variation are named from A to D. Sonication is an important because it produces cavitation in the medium, which is equivalent to pressures of few hundreds of atmospheres. The hot spots in the aqueous solution are due to ultrasonication, and it de-agglomerates/slows down the rate of agglomeration of the particles in the medium.

Figure 4.

Synthesis procedure. (a) Addition of precursors in the aqueous bath. (b) Mixing of the solution using glass rod. (c) Ultrasonication of the solution using probe sonicator. (d) Actual photograph of synthesized quantum dots. Source: Ref. [15].

The sonication creates the conditions such as high temperature and pressure, which is not possible by other techniques. The rate of reaction increases with the rate of agglomeration. Y. Dong et al. have used sodium sulfide along with oleic acid as coordinating agent for controlling growth, stabilizing the resulting colloidal dispersion, and electronically passivating the semiconductor surface. Bhalekar and Pathan [15] controlled the growth only by large aqueous bath and sonication. Thus, growth is not rapid to form bulk; hence, it results in well-defined PbS QDs. The drop-wise addition of precursors changes the color of the solution from colorless to faint yellow [15] as in Figure 4 and with further addition changes it from yellow to faint brown and at last to brown. A rapid nucleation occurs when there is rapid injection, which turned the solution immediate black color, in this condition, it is difficult to achieve narrow size distribution.

The reaction in the bath is, as initially Pb(NO₃)₂ and Na₂S produces Pb²⁺, (NO₃)⁻, Na⁺ and S²⁻ free ions respectively in aqueous bath and in a large bath it yield as,

$$P{b^2}^{+}+2{\left(N{O}_3\right)}^{-}+2N{a}^{+}+S^2{{}^{-}}_{\overrightarrow{aq.}}PbS+2NaN{O}_3$$

The solubility of the Pb(NO₃)₂ in aqueous solution can be given as,

The reaction involved in the aqueous bath proceeds as per the Hard and Soft Acids and Bases (HSAB) Theory. In which Na⁺ is a hard acid and (NO₃)⁻ is a hard base and hence they combine. Pb²⁺ is a border line acid and S²⁻ is a soft base and due to prominent interaction PbS is formed.

Optical Properties of Lead Sulfide Quantum Dots are as absorption is observed in QDs due electron-hole pair generation persuaded by absorption of photons. The optical absorption spectra [15] in Figure 5 reveals that, initially when the color of the solution is faint yellow, the peak observed at 1155 nm in near infrared region known as first excitonic peak indicating the formation of PbS QDs. The quantum confinement effect is also evident from the large blue shift in absorption spectra (shown in the inset). In the main spectrum, the second and third excitonic peaks are at 965 nm and 812 nm respectively. The average radius and effective band gap energy of PbS QDs are 2.55 nm and 1.073 eV respectively. The peak positions for sample B, C are same as that of sample “A” except sample “D” in which the second and third excited peaks are not visible due to large particle size. Also, from the sample “A” onward, sharpness of peaks decreases with the color variation. This specifies a broader particle size distribution.

Figure 5.

Absorption spectra for synthesized lead sulfide quantum dots. Source: Ref. [15].

TEM images [15] of synthesized lead sulfide quantum dots as in Figure 6 are taken at 500 nm, 200 nm, 50 nm, and 20 nm, and it confirms the formation of QDs from [15] HRTEM image Figure 7a and selected area electron diffraction (SAED) pattern Figure 7b. The regular circular particles as observed are in between 3 and 8 nm and the average mean radius of PbS QD was 6 nm as evident from particle size distribution curve [15], i.e., histogram as in Figure 8. Since the inter-planar spacing (0.347 nm) observed here is in conformity with the standard PbS data. Figure 7b, i.e., SAED pattern of the sample “A” confirms the formation of predominantly single nanocrystalline structure. This diffraction fringes of QDs matches with the cubic phase of PbS and labeled rings have been identified to the (220), (222), (400), (440), (511) planes [JCPDS no.: 05-0592].

Figure 6.

Transmission electron microscopic images of lead sulfide quantum dots at various magnifications. Source: Ref. [15].

Figure 7.

(a) HRTEM image and (b) SAED pattern of lead sulfide quantum dots. Source: Ref. [15].

Figure 8.

Particle size distribution of lead sulfide quantum dots. Source: Ref. [15].

3.1 Nanostructured solar cells: the QD class

Quantum dot sensitized solar cells are excitonic solar cells, the basic idea behind the emergence of third-generation photovoltaics is to design solar cells with efficiency that exceeds the limit proposed by Shockley and Quiesser. QDs with their unique characteristics are widely used to improve the efficiencies of QDSSC. They are the structures having properties such as size-dependent optical band gap, high molar extinction coefficients, high intrinsic dipole moments, which giving rise to good charge separations and also the multiple charge carriers are created by using a single photon, in order to build a stable solar cell.

3.2 Quantum dot sensitized solar cells: an excitonic class

The cost-effectiveness and simple method of fabrication make the Dye Sensitized Solar Cell (DSSC) the popular customer among scientists. The maximum thermodynamic photoconversion efficiency is 31%, which is smaller than the Shockley Quiesser limit. The light harvesting capabilities and band of absorption are disadvantages of DSSC. Solar spectrum may not be completely utilized by the dye due to the limited absorption band. Dye absorption duration reduces the performance of the solar cell due to aggregation. When the dyes are replaced by zero-dimensional structures (Quantum dots), the thermodynamic limits will get altered. For QDSSC, 44% [17] is the maximum projected thermodynamic efficiency.

3.3 Basic architecture of QDSSC

QDSSC [16] consists of three essential components, namely photoanode, electrolyte containing redox couple, and counter electrode. Figure 9 shows the schematic of QDSSC including the electron transfer processes.

Figure 9.

Schematic of quantum dot sensitized solar cells. Source: Ref. [16].

3.4 Working of QDSSC

When the light is incident on the solar cell through a transparent conducting oxide, photons are absorbed to generate excitons. As the band positions between the Conduction Band (CB) of the Photoelectrode and the CB of QD is greater than the binding energy of the exciton, then it gets separated at the Metal Oxide (MO)/QD interface into electrons and holes. The electron moves into the CB of MO to reach the transparent Conducting Oxide (TCO) surface before it reaches the band. In the other way, the electrolyte reduces the reductant species (Re), i.e., hole created in the QD. The Re turns into Oxidant species (Ox) after losing electron to the hole, which diffuses toward the Counter Electrode (CE) to receive an electron coming from the external circuit. With the hole reduction, the QD is ready to absorb another photon for the creation of an exciton.

Thus, a photovoltaic effect converting light into electricity does useful work in the external load. The constant photoconversion process is continual till the light is incident on the active area of the solar cell. The photovoltaic processes discussed above are summarized as,

$$QD+hv\to Q{D}^{\ast }\left(\mathrm{formation}\mathrm{of}\mathrm{exciton}\mathit{due}\mathrm{t}\mathrm{o}\mathrm{absorption}\mathrm{of}\mathrm{photon}\right)$$E1

$$Q{D}^{\ast }+MO\to Q{D}^{+}+M{O}^{-}\left(\mathrm{Electron}\mathrm{tranfer}\mathrm{from}QD\mathrm{t}\mathrm{o}MO\right)$$E2

$$Q{D}^{+}+Re\to QD+\mathit{Ox}\left(\mathrm{charged}QD\mathrm{getting}\mathrm{reduced}\mathrm{t}\mathrm{o}\mathrm{neutral}QD\right)$$E3

$$\mathit{Ox}+{\mathrm{e}}^{-}\to Re\left(\mathrm{Reduction}\mathrm{of}\mathrm{oxidant}\mathrm{frome}-\mathrm{supplied}byCE\right)$$E4

Components of QDSSC:

Photoelectrode: Electronic structure of metal oxide and its effect on electron injection.

The wide-band-gap semiconductors are needed for transmitting maximum solar spectrum (visible and infrared region) for effective power conversion efficiency. The conduction band of Photoanode should be more positive with respect to the conduction band of the sensitizer. That creates a band counterbalance between the QD-MO and has electron injection from QD to MO.

In addition, electrical band gap, the electronic mobility, and electronic structure are taken into consideration. For smooth passage of the carriers through the metal oxide semiconductors SnO2 and ZnO are preferred materials as photoelectrodes. In spite of that, TiO2 has appeared as a model photoanode semiconducting system, which demonstrated the best of the efficiencies in both DSSC and QDSSC.

The electron transfer from sensitizer to MO in both DSSC and QDSSC is directed by Marcus theory [18, 19]. According to this theory, the rate of electron injection is directly proportional to Density of States (DOS) in the conduction band of metal oxide. But, the DOS depends on the effective mass of the electron (me*). me* of CB electrons in titania is around 5–10 me (me is the mass of the electron) and about 0.3 me in ZnO and SnO₂ [20]. TiO₂ has two orders of magnitude densities of vacant states greater than in ZnO and SnO₂. Some of the MO and their band positions with respect to vacuum level are shown in Figure 10 [21].

Figure 10.

Band positions of metal oxides with respect to vacuum. Source: Ref. [21].

The effective area offered by the photoelectrode or photoanode surface is many times greater than its geometrical area. This assists the wider coverage of sensitizer for better light harvesting capability in the electrode. Various studies have proven the influence of particle size of photoanode on electron transport properties such as charge transport resistance (Rt) and charge recombination resistance Rr. Improved charge transport was observed in particles with greater sizes leading to longer diffusion length and minimal collisions at grain boundaries.

(*The grain boundaries defect (the interface between two grains) reduces the electronic transport through a material)

With a porosity of 60%, the calculated electron diffusion length in a mesoporous titania photo electrode is 15–20 μm. The use of one-dimensional nanostructure improves electron transport properties such structures includes nano tubes, nano rods, nano wires, and nano fibers.

Some of the metal oxides are listed below:

3.4.1 Titanium dioxide (TiO₂)

Titanium dioxide is in general n-type a wide direct band gap semiconductor. It is inexpensive and nontoxic. This metal oxide has three morphologies, namely Anatase, Rutile, and Brookite having band gap 3.2 eV, 3.05 eV, and 3.26 eV, respectively.

Rutile is the most stable morphology of the titania, it is used in device application when light scattering is important. The Fermi level of anatase is 100 mV, which is higher than rutile; due to this anatase is attaining higher open-circuit voltages when used as electrode in solar cell applications.

In addition, anatase has greater surface area and hence large amount of sensitizer loads on it, rather than rutile, which results in to greater photocurrents when used in DSSC/QDSSC.

3.4.2 Zinc oxide (ZnO)

Zinc Oxide (ZnO) is wide-band-gap semiconducting material II–VI having direct band gap 3.2 eV, and bond energy is 60 meV at room temperature. ZnO is frequently used as the domain of solar energy conversion due to its stability against photo corrosion and photochemical property and alternative material to TiO2 owing huge surface area and high catalytic activity. It has a great benefit to be applied in a catalytic reaction process. Zinc oxide crystallizes in three main forms, hexagonal wurtzite, cubic zinc blende, and rocksalt. The wurtzite structure is most stable at environmental conditions, and thus, it is most common. ZnO is used in gas sensing, photocatalyst, solar cell, ultraviolet light emitting materials, field effect transistors, and transparent conductors.

3.4.3 Zirconium dioxide ZrO₂

The white crystalline oxide ZrO₂ adopts monoclinic crystal structure at room temperature and transitions to tetragonal and cubic at higher temperatures. The band gap of ZrO₂ depends on the phase (cubic, tetragonal, monoclinic, or amorphous) and synthesis method, due to trap states, its band gap decreases up to 2.8–3.7 eV.

ZrO₂ is used as a photoelectrode for the fabrication of solar cell, due to its high refractive index, wide band gap, low absorption, and dispersion in the visible and near infrared spectral region. Along with these materials, tin oxide, cerium oxide are also used for fabrication of photoanodes.

3.4.4 Photoelectrode sensitization with QDs

The photoelectrode (mesoporous nanocrystalline) is the means of electron transport in solar cells, after the dissociation of exciton at the MO/QD interface. But it stands unworthy unless it is sensitized by the QDs and the photovoltaic process cannot be initiated. The QDs need to absorb maximum part of solar spectrum to create better exciton density at the interface. In order to have better charge transportation, the mere absorption is not sufficient for power conversion, but this demands proper band positioning between MO and QD, i.e., band offset should be greater than the exciton binding energy. Different types of QDs are involved in QDSSC; these includes CdS, CdSe, CdTe, ZnSe, PbS, PbSe, Bi2S3, Bi2Se3, Sb2S3, Sb2Se3, and InAs ternary compounds such as CuInS₂, CuInSe₂.

The band position, for example, lead chalcogenide QD system is shown in Figure 11 with favorable band edges with respect to MO [16].

Figure 11.

Energy band positions of TiO2, ZnO, ZrO2 with respect PbS. Source: Ref. [16].

Multiple exciton generation: Under the illumination condition, a high-energy photon excites the electron from the valence band (VB) and jumps to conduction band (CB), and it generates one electron-hole pair, i.e., exciton.

As the energy of light (Photon) is twice the band gap of QD, the exciton has a very high kinetic energy, which can be used to excite another electron hopping from the VB to CB by impact ionization. Hence, one photon creates two excitons permitting the internal quantum efficiency to drive beyond 100%. Indeed, more excitons are produced, if the energy of the photon is sufficiently high. Multiple exciton generation is observed in PbS PbSe and CdSe/ZnS QDs. This property of QDs is useful to improve the efficiency of QDSSC and light-emitting devices. In PbS-based QDSSC, it is observed that an incident photon to current conversion efficiency (IPCE) is over 100%, which leads to the Multiple Exciton Generation.

3.4.5 Surface passivation of quantum dots: Influence on QDSSC performance

QDs have defect states, due to high surface to volume ratio. This results in low photocurrents obtained in QDSSC. Passivation of these surface or defect states using organic or inorganic passivating agents, which include ZnS, ZnSe, Cu-ZnS, and halides, is seen to have immensely enhanced the performance of QDSSC. The original trapped states on QDs are reduced by such treatments and also reduce the recombination of charge carriers. ZnS is a very often used as surface passivator in QDSSC. It is observed that ZnS coating of MO besides QD has further improved the cell efficiency by improving the electrode/electrolyte interface [22]. Combined treatment of ZnS/SiO₂ on photoelectrode sensitized with CdSe_xTe_1-x yielded a relatively very high efficiency in QDSSC, of around 8.2% [23]. Thus, a surface treatment of QDs as well as photoanodes improves the performance of QDSSC.

3.4.6 Electrolyte

Electrolyte (redox) consists of species of which is an electron donor called reductant and an electron acceptor called oxidant. Dye/Quantum dot sensitized solar cell shows a photovoltaic action due to the circulation of charge carriers through an electrolyte. Polyiodide is the most favorable electrolyte with respect to DSSC, due to its electron transfer kinetics, which justifies their high photoconversion efficiencies. Though, the I-/I3- (Iodide/polyiodide) electrolyte displays amiable charge transfer kinetics due to its better hole evacuation capability, it is corrosive in case of QDSSC where the photocurrent degrades continuously.

Sulfide/polysulfide (S²⁻ /Sn²⁻) is used as the electrolyte in QDSSC due to its stability with the semiconductor QDs and solar cell performance. The other redox couples such as Fe³⁺/Fe²⁺, Co²⁺/Co³⁺ and Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ [24, 25, 26, 27] are employed. Polysulfide (PS) has a high redox potential, which supports in faster and better hole evacuation, but it reduces the open-circuit voltage and has poor fill factor (FF).

The stability of QDSSC is a question as long as liquid electrolytes are employed. Electrolytes volatilization, permeation of oxygen and water vapor from the atmosphere lead to quick degradation of the device performance. Rather than all disadvantages of the PS as an electrolyte, it has reported the best of efficiencies for QDSSC so far.

3.4.7 Counter electrode (CE)

The collection of the electrons from the external circuit and the reduction of the oxidant species in the electrolyte are done with the help of a component known as counter electrode. This completes the circuit and with the electrolyte, it makes the device ready for a constant photovoltaic action. Thus, a counter electrode reduces the oxidant species.

Pt and Au are the most popular CE in DSSC, and they are highly efficient and compatible with Iodide/Polyiodide redox species [28, 29, 30, 31]. In QDSSC, Pt results in a poor device performance due to the chemical activity, which reduces chemisorption of sulfide species. The charge transfer at the CE/Electrolyte is also affected due to poisoning effect of electrolyte on the surface of CE leading to reduction in FF.

With their very good electronic mobility and corrosion resistance for electrolyte species, carbonaceous materials such as graphene are considered as CE with Graphene/CoS, graphene/PbS, Carbon nanofiber/CuS, Carbon black/PbS, multiwall carbon nanotubes (MWCNT)/Cu2ZnSnSe(CZTSe), etc.

4. Conclusion

The chapter focused on different types of nanomaterial synthesis techniques such as physical, chemical, etc. In the latter part, it covers the synthesis of PbS quantum dots, by hazardous-chemical-free simple technique using ultrasonication and without capping agent. This technique produces quantum dots of 6 nm size, and it is confirmed from UV spectra, HRTEM images, and SAED pattern. In order to understand the structure of solar cells, the QDSSC is discussed in detail with its essential components and materials used in the architecture.