**4.3. Organometallic or colloidal (Wet Chemistry) synthesis method**

Organometallic or colloidal (Wet Chemistry) synthesis method is one of bottom-up methods. It's also a self-assembly technique. It's relatively inexpensive and fast growth method to synthesis large quantity of quantum dots all at once. The growth could occur at non-extreme conditions. Figure 10 show a schematic of an example of the setup used in synthesis of colloidal quantum dots.

**Figure 10.** Schematic of a common setup used in synthesis of colloidal quantum dots.

As an example recipe given by [16] to synthesis colloidal Lead Sulfide PdS quantum dots: Heat together in a flask at 150 °C for 1 hr under N2 flow (to make lead oleate):

**•** 220 mg lead(II) oxide

**4.1. Electron-beam lithography**

312 Solar Cells - New Approaches and Reviews

**4.2. Self-assembly growth**

required.

Figure 9

quantum dots.

Electron-beam lithography is one of the top-down methods. Electron beam system is used to etch well defined patterns on a bulk semiconductorand or superlattice, then a semiconductor

Self-assembly growth is one of the bottom-up methods. Molecular Beam Epitaxy (MBE) and Metal Organic Chemical Vapor Deposition (MOCVD) are widely used in the growth of the superlattice with different materials of different lattice constants.Semiconducting compound with a smaller lattice constant is grown (deposited atom by atom) on the surface of a com‐ poundwith a larger lattice constant. The relaxation of the grown layer after specific growth thickness due to lattice mismatch results in nucleation of islands of random shapes and controllable size. This growth mode is known as Stranski-Krastanov growth (see Figure 9).MBE method, beside it is sophisticated and slow, it is expensive. However, growth control of quantum dots is precise and multi-layers of quantum dots is possible. MOCVD is used in mass production of sample wafers and in contrast to MBE the growth of crystals is by chemical reaction and not physical deposition. In this approach high vacuum and temperature are

layers are grown according to a well-established protocol in micro-technology.

**Figure 9.** Schematic of InAs island formation on GaAs surface by Stranski-Krastanow growth.

Organometallic or colloidal (Wet Chemistry) synthesis method is one of bottom-up methods. It's also a self-assembly technique. It's relatively inexpensive and fast growth method to synthesis large quantity of quantum dots all at once. The growth could occur at non-extreme conditions. Figure 10 show a schematic of an example of the setup used in synthesis of colloidal

**4.3. Organometallic or colloidal (Wet Chemistry) synthesis method**


After cooling to 90‐100 °C, quickly inject into the reaction flask 5 ml of 0.1 M hexamethyldisi‐ lathiane (0.5 mmol) dissolved in ODE. Then immediately cool with an ice bath.

Precipitate (methanol/butanol mixture), centrifuge and resuspend in nonpolar solvent (e.g., toluene, tetrachloroethylene, or hexanes).

$$\begin{aligned} \text{PbO} + 2\text{C}\_{18}H\_{34}O\_2 - \text{(}N\_2 \text{ }ODE \{ 122 \text{ }^\circ \text{C} \text{)} \rightarrow \text{Pb} \{ }^{\text{eff} \text{)}}\_{\text{oil-se}} + H\_2O \\ \text{Pb} \{ }^{\text{eff} \text{)}}\_{\text{oil-se}} + \{ \}\_{\text{c} \text{ }^{\text{f}}\_{\text{c}} \text{S} \text{} \text{ }^{\text{(} 22 \text{ }^\circ \text{C)} \rightarrow \text{PbS} \text{ } \text{quantum dots exposed with OA} \end{aligned}$$

To manipulate functionality, alter the charge and reactivity of the surface, synthesized quantum dots usually caped with a shell from different composition. Also, the shell can enhance the stability and dispersability of the colloidal core. Magnetic, optical, or catalytic functions may be readily imparted to the dispersed colloidal core. In fact, encasing colloids in a shell of different composition may perhaps protect the core from extraneous physical and chemical changes.

Generally speaking, colloidal quantum dots could be categorized as Type-I (e.g., CdSe/ZnS) and Type-II QDs (e.g., CdTe/CdSe) as shown in Figure 11:


**Figure 11.** Illustration of (a) Type-I quantum dots (b) and Type-II quantum dots.

In fact, many researchers investigating quantum dot sensitized solar cells adopt other growth strategies to assemble quantum dots on electrode surface. We used the most direct and easiest one, the drop casting (electrode is socked with quantum dots solution drop by drop). Chemical bath deposition CBD has been used by [17], Electrophoritic deposition has been adopted by [18], successive ionic layer adsorption and reaction (SILAR) has been used by [19], and the used of bifunctional linker was demonstrated by Subramanian and coworkers [20].
