**2.1. DSSC structure**

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

8 Nanostructured Solar Cells

electrolyte. **Figure 1** illustrates progress of third-generation devices.

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

**Figure 2** shows the structure of a DSSC. The photoanode consists of a TCO substrate on the top of which is deposited a semiconducting oxide layer (usually TiO2) and the dye sensitizer. Actually, there are two TiO2 layers. The first TiO2 layer is a blocking layer to suppress electron recombining with the ionized dye and/or the mediators. The second layer is mesoporous TiO2 of 20–30 nm thickness. These particles are larger than the blocking layer particles. The mesoporous TiO2 layer thickness is about 10 µm. A colloidal TiO2 paste for the

**Figure 2.** Schematic diagram of the DSSC structure.

second layer can be prepared by grinding TiO2 of 21 nm size with nitric acid, a polymer of low molecular mass (e.g. polyethylene glycol of molecular mass 200 g/mol) with a little surfactant. This paste will be deposited over the blocking TiO2 layer and heated at ~450°C for 30 min. To ensure the dye adheres to the mesoporous TiO2 layer, the TiO2 films are soaked in the dye solution overnight. The larger surface area of the mesoporous TiO2 area allows a greater amount of dye to be adsorbed on its surface. An electrolyte usually with an iodide/ triiodide couple is needed for DSSC. The electrolyte can be in liquid or gel form. A catalytic active material (usually platinum) is required as the counter electrode to reduce the triiodide ion (I 3 <sup>−</sup>) to the iodide ion (<sup>I</sup> −).
