**5.1 Electron injection**

Electron injection process is the transfer of a charge such as an electron from the excited dye to the CB of the semiconductor after light absorption, and it is considered the first beneficial process for high performance in DSSCs. The electron injection rate depends on the coupling strength between the adsorbed dye and the semiconductor, which includes the energy alignments of both the excited state of the dye and the fermi level of the semiconductor. For a long time, the electron injection time scale was trusted to be only in the range of 100 fs, however, this is not the case for all organic dyes as shown later by showing slower electron injections lifetimes [26, 27, 31]. The detection of slow electron injection in the picosecond time scale was mainly achieved by utilizing the IR (infrared) probe light in the fs-TA instead of the visible probe light [27, 29, 32, 33]. The advantage of using the IR versus the visible probe was mainly attributed the sole sensitivity of the IR to the vibrations of the electrons in the CB of the semiconductor, while the visible probe interacts with several species at the semiconductor surface such as the oxidized dye and the redox couple [8, 33, 34]. Famous organic indoline dyes were measured on TiO2 mesoporous surfaces using fs-TA in the IR region centered at 5000 nm, and multi-exponential injection rates were detected including fast lifetimes of 100 fs and slow ones in the range of tens of ps [27, 33]. **Figure 6** shows the captured data for various indoline dyes, in which the D131 dye shows a fast injection lifetime of 100 fs, while other dyes (D102, D149, and D205) show additional slow injection lifetimes that can reach to 30 ps as in the case of D149 dye. These slow injection rates are connected to large scale motions on mesoporous surfaces as shown later on, such as isomerization. The presence of slow injection rates is thought to be beneficial to the overall efficiency of the DSSC, due to the expected minimized charge recombination afterwards [26, 31].
