*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


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**.

## *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.

**Figure 3.** *Schematic of SILAR depositions. Source: Ref. [9].*

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.

#### *Quantum Dots Sensitized Solar Cell DOI: http://dx.doi.org/10.5772/intechopen.107266*

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.

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.

#### **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 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,

$$\text{Pb}^{2+} + 2\text{(NO}\_3\text{)}^{-} + 2\text{Na}^{+} + \text{S}^{2-} \underset{\text{aq.}}{\text{PbS} + 2\text{NaNO}\_3}$$

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

**Figure 5.** *Absorption spectra for synthesized lead sulfide quantum dots. Source: Ref. [15].*

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.

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].

#### **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.

#### **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].*

## **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

#### **Figure 8.**

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

(Quantum dots), the thermodynamic limits will get altered. For QDSSC, 44% [17] is the maximum projected thermodynamic efficiency.
