**3. DSSCs working mechanism**

Different than Ru-complexes, the organic dyes in DSSCs inject the energetic electrons from the singlet states as the triplet state population has mostly a very low quantum yield [12]. As the spin state for both the excited and the ground state of the organic dyes is the same, various deactivation mechanisms can occur for the adsorbed dyes on semiconductor surfaces. These deactivation processes include large scale motions such as isomerization [13], twisting [14], and local chemical interactions such as interactions with electrolyte components surface species [15–17].

**Figure 2** summarizes the main processes for exciting an adsorbed dye on lowband gap semiconductor such as TiO2. There processes are such as follow:


#### **Figure 1.**

*Schematic representation for the successful design of organic photosensitizers for utilization in DSSCs based on D-L-A strategy, readapted from reference [8].*

**369**

*Excited-State Dynamics of Organic Dyes in Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.94132*

3.**Electron Injection:** The excited electron is transferred from the dye to the CB

*Schematic representation for electron dynamics in DSSCs. Each process has its number that is mentioned in the main text. Red numbers are for deactivating processes and blue numbers are for favorable processes, readapted* 

4.**Electron Diffusion:** The injected electron is diffused through the mesoporous semiconductor area reaching the conducting glass such as FTO (Fluorine

5.**Electron Regeneration:** The oxidized dye recovers its electron from the

utilized electrolyte in the DSSCs, which has a low oxidation potential, such as

6.**Electron Recombination to Redox:** The injected electron in the CB diffuses backward to the adsorbed species on the semiconductor surfaces such as, the

7.**Electron Recombination to dye:** The injected electron in the CB diffuses backward to the adsorbed species on the semiconductor surfaces such as, the

All these processes contribute both positively and negatively to the overall performance of the DSSC. These processes are marked in different colors in **Figure 2**, depending on their role. However, due to the sake of this chapter, I will mainly be focusing on the exited state dynamics of organic dyes that improve or reduce the total performance of the DSSC. However, before presenting these dynamics, I will illustrate in the following section the main optical tools utilized for investigating

Several optical spectroscopic tools have been utilized to follow the charge dynamics for organic dyes in DSSCs. These common tools include TCSPC

of the semiconductor, leaving an oxidized adsorbed dye behind.

doped Tin Oxide).

) redox couple.

oxidized redox couple.

oxidized dye.

these processes in DSSCs.

**4. Optical spectroscopic tools**

(I− /I3 −

**Figure 2.**

*from reference [8].*

*Excited-State Dynamics of Organic Dyes in Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.94132*

#### **Figure 2.**

*Solar Cells - Theory, Materials and Recent Advances*

**2. Organic dyes and strategic designs**

**3. DSSCs working mechanism**

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].

Several synthetic strategies have been implemented for optimizing the metal-free, pure organic photosensitizers for working conditions in DSSCs [6]. One of the successful approaches for building organic photosensitizers is based on **D-L-A** (Donor-Linker-Acceptor) approach [7]. In this approach, the D unit is an electron-rich moiety, the L unit is typically a single or several consecutive π-bonds, then the A unit is an electrondeficient moiety that is connected by an anchoring group, such as COOH (carboxylic acid), which binds to the low-band gap semiconductor, see **Figure 1** for a graphical illustration of the organic dye. The ultimate dye should absorb most of the incident solar spectrum especially in the visible and the infrared regions, with high oscillator strength [3]. One of the most successful organic photosensitizers in DSSCs is the indoline family, which is based on Indoline moiety as a D unit [9–11]. While other acceptor groups (A) have been utilized such as rhodanine and cyanoacrylic moieties [9–11].

Different than Ru-complexes, the organic dyes in DSSCs inject the energetic electrons from the singlet states as the triplet state population has mostly a very low quantum yield [12]. As the spin state for both the excited and the ground state of the organic dyes is the same, various deactivation mechanisms can occur for the adsorbed dyes on semiconductor surfaces. These deactivation processes include large scale motions such as isomerization [13], twisting [14], and local chemical interactions such as interactions with electrolyte components surface species [15–17]. **Figure 2** summarizes the main processes for exciting an adsorbed dye on low-

1.**Excitation:** The adsorbed dye absorbs part of the incident solar spectrum and an electron is transferred from the ground state to the excited state instantaneously.

2.**Decay:** The populated electron recombines back again to the ground state due

*Schematic representation for the successful design of organic photosensitizers for utilization in DSSCs based on* 

band gap semiconductor such as TiO2. There processes are such as follow:

to various excited state processes within the dye.

**368**

**Figure 1.**

*D-L-A strategy, readapted from reference [8].*

*Schematic representation for electron dynamics in DSSCs. Each process has its number that is mentioned in the main text. Red numbers are for deactivating processes and blue numbers are for favorable processes, readapted from reference [8].*


All these processes contribute both positively and negatively to the overall performance of the DSSC. These processes are marked in different colors in **Figure 2**, depending on their role. However, due to the sake of this chapter, I will mainly be focusing on the exited state dynamics of organic dyes that improve or reduce the total performance of the DSSC. However, before presenting these dynamics, I will illustrate in the following section the main optical tools utilized for investigating these processes in DSSCs.
