**2. Dye-sensitized solar cells (DSSCs)**

DSSCs employ oxide semiconductors with wide band gaps and sensitizers that absorb electromagnetic (EM) waves in the visible light. DSSC was first developed in 1972 as a chlorophyll-sensitized zinc oxide (ZnO) electrode solar cell [1]. In 1976, an amorphous silicon photovoltaic was reported for the first time by Carlson and Wronski, and its efficiency was 2.4% [2]. Subsequently, solar energy researchers began to give attention to DSSCs. However, the main dilemma was that a single layer of dye molecules on the surface allowed only 1% incident sunlight absorption that delayed further progress [3]. The breakthrough in DSSC research was in 1991 [4]. The efficiency was 7.1%. About 80% of photons absorbed were converted into electrical current. The cheap cost of production and the simple structure inspired many researchers worldwide to improve the efficiency to a level deemed acceptable for commercialization.

The DSSC operating principle may be compared to the process of photosynthesis with the dye functioning as chlorophyll [4]. In DSSCs, the transport of charges (electrons) to the external circuit begins when electrons exit the semiconducting network layer and ends when the redox mediator in the charge transport medium returns them to the sensitizers. The purity of the semiconducting material is not as crucial as in the earlier generation solar cells.
