**1. Introduction**

One of the foremost challenges in solar cells industry is reducing the cost/watt of delivered solar electricity. In conventional microstructures (bulk) single junction solar cells, photons with energies less than semiconductor bandgap are not harvested while those with energies much larger than the bandgap produce hot-carries and upon cooling down (thermalization) the excess energyget wasted as heat. Therefore, novel materials or structures with tunable bandgap or intermediate band that can be tuned to match the spectral distribution of solar spectrum arecrucial. Quantum dots (QDs) have the advantage of tunable bandgap as a result of size variation as well as formation of intermediate bands. In contrast to traditional semiconductor materials that are crystalline or amorphous, quantum dots can be molded into a variety of different types, in two-dimensional (sheets) or three-dimensional arrays. They can be proc‐ essed to create junctions on inexpensive substrates such as plastics, glass or metal sheets.They can easily be combined with organic polymers and dyes.

Quantum dots are a special class of semiconductors, which are nanocrystals, composed of periodic groups of II-VI, III-V, or IV-VI materials and can confine electrons (quantum con‐ finement). When the size of a QD approaches the size of the material's exciton Bohr radius, quantum confinement effect becomes prominentand electron energy levels can no longer be treated as continuous band, they must be treated as discrete energy levels. Hence, QD can be considered as an artificial molecule with energy gap and energy levels spacing dependent on its size (radius). The energy band gap increases with a decrease in size of the quantum dot, as shown in Figure 4. As the size of a QD increases its absorption peak is red shifted due to shrinkage of its bandgap (see Figure 5). The adjustable bandgap of quantum dots allow the construction of nanostructured solar cell that is able to harvest more of the solar spectrum.QDs have large intrinsic dipole moments, which may lead to rapid charge separation. Quantum dots have been found to emit up to three electrons per photon dueto multiple exciton gener‐

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ation (MEG), as opposed to only one for standard crystalline silicon solar cell.Theoretically, this could boost solar power efficiencyfrom 20 % to as high as 65 %.

Generally speaking, there are three important parameters that characterize the performance of a photovoltaic cell. These are the open-circuit voltage (*Voc*), the short circuit current (*Isc*), and the fill factor (*F F*). However, the fill factor is also a function of *Voc* and *Isc*. Therefore, these last two parameters are the key factors for determining the cell's power conversion efficiency. Under ideal conditions, each photon incident on the cell with energy greater than the band gap will produce an electron flowing in the external circuit. The fill factor is determined from the maximum area of the I-V characteristics under illumination and the short circuit current and open circuit voltage, or

$$FF = \frac{V\_{mp}\,\mathrm{I}\_{\mathrm{m}\,p}}{V\_{oc}\,I\_{sc}}\tag{1}$$

where Vmp and Imp are the operating point that will maximize the power output. In this case, the energy conversion efficiency is given by:

$$\eta = \frac{V\_{\text{oc}} \text{ I}\_{\text{sc}} \text{ FF}}{P\_{\text{in}}} \tag{2}$$

where *Pin* is the input power.

This chapter main objective is to give an introductory coverage of a more sophisticated subject. After we review the physics, designs, structures, and some growth/synthesis techniques of quantum dots. We will give a comprehensive description of some architectures of QD solar cells (e.g., Schottky cell, p-i-n configuration, depleted heterojunction, and quantum dots sensitized solar cell. Also, challenges and opportunities of quantum dots solar cells will be discussed.
