**1. Introduction**

## **1.1 DSSCs design**

Dye Sensitized Solar Cells (DSSCs) have drawn the attention of renewable energy scientists, since the proof of concept done by O'Regan and Grätzel in 1991 [1]. In that concept, it was shown that an adsorbed photosensitizer on a low-cost low bandgap semiconductor can generate electricity with a reasonable efficiency from the incident sun light. Such a process was a breakthrough at the time, despite the low efficiency of the utilized sensitizer, as only highly crystalline semiconductor, such as Si, was believed to be the only way to capture the sunlight and convert it into electricity. In a typical Si solar cell, the light is absorbed by the crystalline Si atoms and the energetic charges are generated within the bandgap of the semiconductor, which later can be extracted by the external circuit [2]. However, in the DSSC, the light is absorbed by the photosensitizer "adsorbed dye" and then transfers its energetic charge to the low-cost semiconductor that is responsible for transferring the charge to the external circuit. The first utilized photosensitizer was based on metal complex, a Ru-complex, thus, many metal-based complexes were tested later on the best performances in DSSCs [3]. The main excited state charge dynamics for metal-based complexes for DSSCs are based on a MLCT (metal to ligand charge transfer process) state, in which the incident light moves an electron from the metal core to the surrounding ligands in the complex, then the charge hops from the ligand to the CB (conduction band of the semiconductor) via a triplet state. Thus, heavy metal ions

with low oxidation potentials were utilized such as Ru atoms [4]. Later on, plenty of attempts have been done to replace these costly metal photosensitizers by the metalfree photosensitizers, organic dyes, to further reduce the cost of the working cell [5].
