**4. Synthesis of quantum dots**

reaching the bottom of the conduction band. However, in a quantum dot the hot carrier undergoes impact ionization process (carrier multiplication). Therefore, absorption of a single photon generates multiple electron-hole pairs.This phenomena is called multiple exciton generation MEG. Therefore, absorption of UV photons in quantum dots produces more

**(a) Bulk Semiconductor (b) Quantum Dot**

**Figure 6.** Thermalization of hot carriers in a bulk semiconductor (a) and a quantum dot (b), from [12].

After excitation of charge carriers the timing of the processes leading to generation of multiple charge carriers are detailed in [14] and schematically illustrated in Figure 7. After absorption of photon it takes hot carries 0.1 ps to go for impact ionization, then after 2 ps carries cool down. It takes around 20 ps for Auger recombination and finally 2ps are needed for carries to cool

Experimentally carrier multiplication process-one photon generates more than one exciton via impact ionization or inverse of Auger recombination-in quantum dot has been investigated and quantified using the transient absorption spectroscopy technique. In short, [5] laser pulses are directed on the sample one for excitation and another for absorption. Measuring time of relaxation indicates whether there are single excitons recombining radiatively or biexcitons

It has been discovered that there is linear proportionality between absorption change for number of generated electron-hole pairs less than 3 and number of generated electron-hole pairs. For example, investigations using PbSe and PbS QDs; confirmed 300% photo-generated carrier density (quantum yield QY)in PbSe QDs by a photon with energy of 4 times the energy spacing between the HOMO and LUMO (Eg) of the quantum dot [5, 15] as shown in Figure 8 a.Further investigations by Schaller and his coworkers [6] reported higher carrier multiplica‐ tion efficiencies of 700% of photo-generated carrier density at higher photon energy to bandgap

electrons than near infrared photons.

310 Solar Cells - New Approaches and Reviews

down and become ready for new excitation.

recombining via Auger recombination.

There are many methods or techniques used in fabrication or synthesis of quantum dots that can be classified as top-down (carving large piece of a material to the desired nanostructures) or bottom-up (assembling atoms to form the desired nanostructures) methods. Generally speaking, some of these techniques are practical in solar cells applications. A summary of each synthesis method is given below.
