**1. Introduction**

Third-generation photovoltaics are able to produce high efficiency photon to electricity conversion devices at a cheaper production cost. Solar cells based on pure Si forms were the first-generation devices with an efficiency of ~27%. Due to the high production cost, researchers searched for new processes and materials that led to the second-generation solar cells comprising copper indium diselenide, amorphous silicon, and polycrystalline solar cells. Production was still expensive, as the fabrication process required a large amount of energy. Production of the third-generation solar cell is cheaper and the cells are reasonably efficient. There are several technologies classified as third-generation solar cell technologies. These include solar cells sensitized by a dye material, solar cells sensitized by quantum dots (QDs) and perovskite-sensitized solar cells. These solar cells have a similar structure consisting of a

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

photoanode, counter electrode (CE) and a medium for charge transport. The working principle is also similar. Work on sensitized photovoltaics started during the 1970s with the use of organic dyes as the sensitizer. Organic dyes can be natural or synthetic. Natural organic dyes can be obtained from plant sources but the performance is poor and the efficiency is low. Apart from natural organic dyes, synthetic organic dyes can give efficiency as high as 13%. Ruthenium based dye is one of the synthetic organic dyes and is known to give good performance with current density about 20 mA cm-2. As development in dye-sensitized solar cells (DSSCs) continues, an idea to replace organic dyes with inorganic sensitizers resulted in the emergence of quantum dot-sensitized solar cells (QDSSCs) that utilize quantum dots or nano-sized semiconductor crystals with a short band gap and a high extinction coefficient. Later, since 2009, researchers have begun to use perovskite materials as sensitizers. Perovskite works very well with the solid-state hole transfer material and until now its efficiency has reached 21%. However, perovskites are very moisture-sensitive materials and fabrication must be done in very clean and controlled conditions. In sensitized solar cells, the photoanode is a very crucial component because this is where the electrons are generated by the sensitizer. Photoanodes will absorb photons, excite and transport electrons when illuminated. On exiting the photoanode, the electrons will be sent to the cathode and returned to the sensitizer via a hole conductor or a redox mediator in the electrolyte. For DSSCs, the photoanode components are the dye sensitizer, a mesoporous semiconducting oxide layer and a transparent conducting oxide (TCO). Photoanodes for QDSSC and perovskite solar cells have similar components with DSSCs except that quantum dot nano-sized semiconductor crystals and perovskite materials act as the sensitizer. Another difference between them is the redox mediator used in the electrolyte. QDSSC works well with the polysulphide electrolyte instead of the iodide based electrolyte (as in DSSCs) because the iodide-based electrolyte will cause rapid degradation in photocurrent due to the corrosive nature of the iodide ion on many semiconductor materials including quantum dots. Perovskite solar cells use hole conductors instead of a redox mediator electrolyte. **Figure 1** illustrates progress of third-generation devices.

**Figure 1.** Graphs showing progress of third-generation photovoltaics.
