**5.1. Metal-semiconductor junction solar cell**

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

Generally speaking, colloidal quantum dots could be categorized as Type-I (e.g., CdSe/ZnS)

**•** In Type-I QDs, all charge carriersare confined in the core material inwhich radiative

**•** In Type-II QDs, charge carriers aresegregated in the core and shell;radiative recombination

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

There are many realistic quantum dots based solar cells. In this section we classify them in seven strategies just as an approach to fulfill the main objective of this chapter that is intro‐ ducing quantum dot solar cells. For example, [21] metal-semiconductor junction, polymersemiconductor, and semiconductor-semiconductor systems are well know strategies of quantum dots based solar cell. Moreover; quantum dots attached to n-C60 composite clusters

used of bifunctional linker was demonstrated by Subramanian and coworkers [20].

and Type-II QDs (e.g., CdTe/CdSe) as shown in Figure 11:

occursacross the material interface (see Figure 11-b).

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

**5. Quantum dots solar cells**

recombinationoccurs (see Figure 11-a).

chemical changes.

314 Solar Cells - New Approaches and Reviews

Figure 11

Figure 12-a schematically shows the structure of the metal-semiconductor junctionwhich also called Schottky barrier quantum dotsbased solar cell. Itis basically fabricated from quantum dots layers (Nanocrystals film) sandwiched between metallic electrode and ITO counter electrode deposited on transparent glass substrate to act as photo-electrode.In the band diagram shown in Figure 12-b, a depletion region is due tocharge transfer to QD film. Because of high electron density in metal (~ 1022 cm-3 Figure 12 ), the depletion is negligible on its side of the cell.

**Figure 12.** (a) Schematic of Schottky barrier quantum dots based solar cell, (b) band diagram of Schottky solar cell. Adopted from [24].

Layer‐by‐layer fabrication of quantum dots film is prepared by layer‐by‐layer dip coating of the substrate. For example, in layer by layer fabrication of PbSe quantum dots films, quantum dots film prepared by dip-coating, alternating between PbSe NCs in hexane and then 0.1 M EDT in anhydrous a cetonitrile, allowing the film to dry between each layer [25]. Device with 140 nm‐thick QD film achieved an efficiency of ~2.2 % [26] heterojunction solar cell has been investigated by sputtering 150 nm layer of ZnO and growing layers of PbS quantum dots fabricated in air (4.1 mg/mL concentration of PbS 820 nm NC in hexanes;1 mM solution of 1,2 ethanedithiol in a cetonitrile layer-by-layer deposition for 20 cycles (~ 120nm PbS film thickness). Tested devices showed good air stability over ~1,000 hrs with cell power efficiency of 1.53 %.

It has been found [24] that the open circuit voltage in Schottky cell is constrained due to pinning of Fermi level resulted from the formed defect states at the metal-semiconductor inter‐

**Figure 13.** Hybrid silicon/PbS QD film solar cells. From [28].

face.Moreover, as illustrated in Figure 12-a, photoelectrons are firstly generated near the ITO (ohmic contact) and need to pass through long pathwayto the metallic electrode. Hence, many of the photoelectrons are encountering high recombination probability.Also on the onset of photovoltaic action the hole current injection to metallic contact is not reduce because the barrier to hole injection become less effectiveas shown in Figure 12-b. Gao J. et. al. [27] reported a method to eliminate such disadvantage by inserting a layer of a transition metal oxide (TMO) between the quantum dot (QD) layer and metal electrode. An n-Type transition metal oxide such asmolybdenum oxide (MoOx) and vanadium oxide (V2Ox) work as a hole extraction layer in PbS quantum dot solar cells resulted in power conversion efficiency =4.4%. The formation of dipole at the interface of MoOx and PbS enhances the band binding and hence allowing efficient hole extraction from the PbS film valance band by the MoOx.
