*5.2.1 Photovoltaic cells*

In photovoltaic (PV) cells/solar cells sunlight is directly converted into electricity. Generally, a PV cell is fabricated by two or more thin layers of semiconducting material. On illumination of sun light, the PV cells generate electrical charges that are conducted away by metal contacts. The PV cells have minimal maintenance cost and have a long life. Another important advantage of PV cells is that they generate solar electricity without emitting greenhouse or any other environmentally hazardous gases. Single PV cells generally provide very small amount of current. In order to obtain a demandable current and voltage output, a number of PV cells are connected together in series and confined with a glass cover, called solar cover glass and plastic sheet to form a PV panel.

Basically, it is a p-n junction diode. Under the exposure of light into the p-n junction, number of electron-hole pairs are generated and separated to produce electricity [70]. Up to date, three types of photovoltaic cells are available such as the first generation, second generation, and third generation PV cells. Crystalline silicon wafers as p-n junction diodes are the first generation cells. It is noted that the silicon solar cells has better efficiency but these are very expensive. The second generation solar cells are based on thin films of crystalline or amorphous silicon and CuInSe2-based cells. It is found that the third generation cells such as polymerbased solar cells, nanocrystals based solar cells, dye-sensitized solar cells, quantum *Indium Oxide Based Nanomaterials: Fabrication Strategies, Properties, Applications, Challenges… DOI: http://dx.doi.org/10.5772/intechopen.94743*

dot sensitized solar cells, perovskite solar cell and concentrated solar cells are very potential to harvest the solar energy (**Figure 3**) [70]. Recently, the inexpensive and flexible polymer thin films with stable inorganic nanostructures as fourth generation solar cells are developed to improve the efficiency [70]. In this regard, different indium oxide based nanomaterials are used for fabrication of advanced solar cells that can efficiently convert light energy into electricity [71, 72].

## *5.2.2 Photoelectrochemical cell*

Photoelectrochemical (PEC) cell is a typical device where solar energy is converted into chemical energy in the form of fuel. Generally, it is made with photoactive semiconductor electrodes (photocathode and photoanode). The electrodes are immersed in a suitable electrolyte solution and the semiconductor-electrolyte junction is illuminated with a light source that has higher energy compared to the BGE of the semiconductor (**Figure 4**). As a result, the electrons and holes are

**Figure 3.** *Schematic presentation of a solar cell.*

**Figure 4.** *Schematic diagram displaying the basic principle with key parameters of PEC water splitting.*

generated and separated in the space charge region [4, 6, 11]. Now-a-days, various indium oxide based nanomaterials are used as both photoanode and photocathode materials for PEC water splitting application. Under light illumination, the photogenerated minority carriers (holes) in photoanode reach at the interface of electrolyte-semiconductor whereas majority carriers (electrons) accumulate at the interface of semiconductor-conducting substrate and transported with the help of a connecting wire to the counter electrode. These photogenerated charge carriers react with electrolyte solution to produce O2 and H2 [73]. In this regard, Cao *et al.* [11] fabricated a 3D hierarchically porous In2O3/In2S3 heterostructure array onto fluorine-doped tin oxide glass substrate *via* an ion exchange–induced synthesis and used the heterostructure film as photoanode in PEC cell with incident photon-tocurrent conversion efficiency of 76% at 400 nm.
