**5.2 Deactivation processes**

• Aggregation

Aggregation is a common problem for adsorption of organic dyes on mesoporous surfaces, in which the dyes are stacking in various ways very close to each other

#### **Figure 6.**

*Time resolved transient absorption for the electron injection process of indoline dyes adsorbed on TiO2. (A) Comparison between various dyes indicated in the legend. (B) Comparison between D205 on TiO2 versus impeding the dye in PMMA, readapted from reference [27].*

**373**

**Figure 7.**

*reference [13].*

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

thus, reduces the overall efficiency of the cell.

*trans* equilibrium on the mesoporous surfaces [13, 40].

• Isomerization

• Twisting

due to the high concentration utilized during the adsorption process, resulting of side deactivation pathways that hindered the charge transfer processes in DSSCs [13, 35, 36]. Reducing the dye aggregations can happen by utilizing co-adsorbent agents such as CDCA (cheno-deoxycholic acid) [37], or by impeding organic dyes in the MOF-ZIF8 structures, which increases the dye's emission lifetime by putting the dyes at far distances from each other [38]. **Figure 7** presents the appearance of fast emission lifetime components for the D149 dye upon the presence of aggregation. However, upon using low concenrtation of the D149 dye, the short lifetimes disappears due to the absence of aggregation. In DSSCs, the presence of aggregation reduces the amount of charges transferred to the CB of the semiconductor, and

The local movement of adsorbed organic dyes was overlooked for a long time due to the expected well-packed order of adsorbed dyes, and many argued that isomerization is not a competing process with the electron injection as the latter is very fast. However, as electron injection process can be slow as well, the isomerization and the change of local arrangements of molecules on surfaces can reduce the DSSC efficiency due to uncontrolled deactivation processes [13, 39, 40]. **Figure 8** shows the absorption spectra changes of L0Br organic dye labeled by heavy bromine atom on the mesoporous ZrO2 surfaces under photo-irradiation [40]. The changes in absorption spectra along the NMR measurements revealed the formation of *cis-*

Isomerization of organic dyes in DSSCs is not always spectroscopically detectable especially when the resulted isomers such as *cis* and *trans* isomers are chemically identical [14]. However, this is not the case for many organic molecules containing subunits such as phenyl groups that can rotate or twist without spectroscopic signatures. For instance, the parent molecule of the organic dye D149 dye has a diphenyl groups attached to a double bond. In solution, the lifetime of the parent molecule is very short of ca. 20 ps, and upon impeding this parent molecule in polymer matrix PMMA (Poly methyl methacrylate), the lifetime is extended to 2.5 ns, the time-resolved data for the parent molecule is shown in **Figure 9**. This ultrafast deactivation process is present in the derived dyes utilized in solar cells and it can

*Time-resolved emission for D149 organic dye inside PMMA matrix showing the effect of concentration and the aggregation formation on the appearance of fast emission lifetime components (to the left), readapted from* 

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

due to the high concentration utilized during the adsorption process, resulting of side deactivation pathways that hindered the charge transfer processes in DSSCs [13, 35, 36]. Reducing the dye aggregations can happen by utilizing co-adsorbent agents such as CDCA (cheno-deoxycholic acid) [37], or by impeding organic dyes in the MOF-ZIF8 structures, which increases the dye's emission lifetime by putting the dyes at far distances from each other [38]. **Figure 7** presents the appearance of fast emission lifetime components for the D149 dye upon the presence of aggregation. However, upon using low concenrtation of the D149 dye, the short lifetimes disappears due to the absence of aggregation. In DSSCs, the presence of aggregation reduces the amount of charges transferred to the CB of the semiconductor, and thus, reduces the overall efficiency of the cell.

• Isomerization

*Solar Cells - Theory, Materials and Recent Advances*

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].

Aggregation is a common problem for adsorption of organic dyes on mesoporous

surfaces, in which the dyes are stacking in various ways very close to each other

*Time resolved transient absorption for the electron injection process of indoline dyes adsorbed on TiO2. (A) Comparison between various dyes indicated in the legend. (B) Comparison between D205 on TiO2 versus* 

*impeding the dye in PMMA, readapted from reference [27].*

**5. Excited state dynamics**

**5.2 Deactivation processes**

• Aggregation

**5.1 Electron injection**

**372**

**Figure 6.**

The local movement of adsorbed organic dyes was overlooked for a long time due to the expected well-packed order of adsorbed dyes, and many argued that isomerization is not a competing process with the electron injection as the latter is very fast. However, as electron injection process can be slow as well, the isomerization and the change of local arrangements of molecules on surfaces can reduce the DSSC efficiency due to uncontrolled deactivation processes [13, 39, 40]. **Figure 8** shows the absorption spectra changes of L0Br organic dye labeled by heavy bromine atom on the mesoporous ZrO2 surfaces under photo-irradiation [40]. The changes in absorption spectra along the NMR measurements revealed the formation of *cistrans* equilibrium on the mesoporous surfaces [13, 40].

• Twisting

Isomerization of organic dyes in DSSCs is not always spectroscopically detectable especially when the resulted isomers such as *cis* and *trans* isomers are chemically identical [14]. However, this is not the case for many organic molecules containing subunits such as phenyl groups that can rotate or twist without spectroscopic signatures. For instance, the parent molecule of the organic dye D149 dye has a diphenyl groups attached to a double bond. In solution, the lifetime of the parent molecule is very short of ca. 20 ps, and upon impeding this parent molecule in polymer matrix PMMA (Poly methyl methacrylate), the lifetime is extended to 2.5 ns, the time-resolved data for the parent molecule is shown in **Figure 9**. This ultrafast deactivation process is present in the derived dyes utilized in solar cells and it can

#### **Figure 7.**

*Time-resolved emission for D149 organic dye inside PMMA matrix showing the effect of concentration and the aggregation formation on the appearance of fast emission lifetime components (to the left), readapted from reference [13].*

#### **Figure 8.**

*Two organic dyes, L0, and L0Br were utilized to investigate the isomerization process on ZrO2 surfaces under 400 nm photo-irradiation, readapted from reference [40].*

#### **Figure 9.**

*The time-resolved emission data for the parent molecule of D149 in solution and in PMMA matrix (left). fs-TA data for the parent molecule in toluene (right), readapted from reference [14].*

potentially compete with the electron injection process, minimizing the amount of charges extracted through the DSSC.

#### • **TICT (Twisted Intramolecular Charge Transfer)**

Although the previous large scale motions of organic dyes apparently compete with the electron injection process, the TICT process of some studied organic dyes seems to help boosting the DSSC efficiency through an indirect pathway [31, 41]. Upon comparing organic dyes with the twisting ability on mesoporous semiconductors surfaces with the corresponding ones that do not show such a process, both the electron dynamics and the DSSC efficiency have been correlated [26, 31]. An organic dye named L1 dye shows the TICT process in solution as depicted in **Figure 10**. This dye shows a high performance in DSSCs of ca. 5.5% [26]. While the modified dye L1Fc that do not show any TICT state, instead shows a LCT (local charge transfer state), its efficiency in DSSCs was lower L1 of ca. 1.1% [26, 31].

Using fs-TA in the infrared region to investigate the electron dynamics in the CB of TiO2 revealed that the presence of TICT state allows for slower electron injection from the L1 dye to the TiO2, and due the structural rearrangements of the L1 dye on the mesoporous surfaces, the back electron recombination is hindered allowing for

**375**

efficiency.

**Figure 11.**

**Figure 10.**

*readapted from reference [31].*

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

high performance in DSSCs. However, for the L1Fc case along with the L1/PMMA case, the TICT state is blocked and thus the electron injection was faster from the LCT state, but the electron recombination was order of magnitudes faster than in the L1 dye case, resulting of poor efficiency in DSSCs. **Figure 11** shows the time resolved data for electron injection for the discussed three cases. Thus, although the presence of TICT process can consume some energy to populate the TICT state, the benefit of reducing the charge recombination process is much larger on the DSSC

*(A) False 2D plot for the electron injection of the L1 dye to the CB of TiO2 in the infrared. (B) Normalized* 

*kinetic traces for L1, L1Fc, and L1/PMMA on TiO2, readapted from reference [31].*

*Chemical structures of L1 and L1Fc dyes along with their absorption and emission data in acetonitrile,* 

Traditionally, the utilized electrolyte in DSSCs is solely assumed to regenerate the adsorbed oxidized dye on the mesoporous surface after the electron injection. This regeneration process is typically in the pico- to nano- second time scale [42–44]. However, just recently, it has been shown that the utilized electrolyte can form ground state interactions with the adsorbed dye on the surface that both affect the electron injection and recombination processes [15]. These effects will have detrimental effects on the performance of organic dyes in DSSCs. The formation of ground state complexes have been confirmed by using steady state absorption and emission measurements. **Figure 12** shows the kinetic traces for the electron dynamics of adsorbed organic dye D149 on TiO2 in contact with different components of the

• Chemical Interactions with the Redox Couple

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

#### **Figure 10.**

*Solar Cells - Theory, Materials and Recent Advances*

*400 nm photo-irradiation, readapted from reference [40].*

potentially compete with the electron injection process, minimizing the amount of

*The time-resolved emission data for the parent molecule of D149 in solution and in PMMA matrix (left).* 

*Two organic dyes, L0, and L0Br were utilized to investigate the isomerization process on ZrO2 surfaces under* 

Although the previous large scale motions of organic dyes apparently compete with the electron injection process, the TICT process of some studied organic dyes seems to help boosting the DSSC efficiency through an indirect pathway [31, 41]. Upon comparing organic dyes with the twisting ability on mesoporous semiconductors surfaces with the corresponding ones that do not show such a process, both the electron dynamics and the DSSC efficiency have been correlated [26, 31]. An organic dye named L1 dye shows the TICT process in solution as depicted in **Figure 10**. This dye shows a high performance in DSSCs of ca. 5.5% [26]. While the modified dye L1Fc that do not show any TICT state, instead shows a LCT (local charge transfer

Using fs-TA in the infrared region to investigate the electron dynamics in the CB of TiO2 revealed that the presence of TICT state allows for slower electron injection from the L1 dye to the TiO2, and due the structural rearrangements of the L1 dye on the mesoporous surfaces, the back electron recombination is hindered allowing for

charges extracted through the DSSC.

• **TICT (Twisted Intramolecular Charge Transfer)**

*fs-TA data for the parent molecule in toluene (right), readapted from reference [14].*

state), its efficiency in DSSCs was lower L1 of ca. 1.1% [26, 31].

**374**

**Figure 8.**

**Figure 9.**

*Chemical structures of L1 and L1Fc dyes along with their absorption and emission data in acetonitrile, readapted from reference [31].*

#### **Figure 11.**

*(A) False 2D plot for the electron injection of the L1 dye to the CB of TiO2 in the infrared. (B) Normalized kinetic traces for L1, L1Fc, and L1/PMMA on TiO2, readapted from reference [31].*

high performance in DSSCs. However, for the L1Fc case along with the L1/PMMA case, the TICT state is blocked and thus the electron injection was faster from the LCT state, but the electron recombination was order of magnitudes faster than in the L1 dye case, resulting of poor efficiency in DSSCs. **Figure 11** shows the time resolved data for electron injection for the discussed three cases. Thus, although the presence of TICT process can consume some energy to populate the TICT state, the benefit of reducing the charge recombination process is much larger on the DSSC efficiency.

• Chemical Interactions with the Redox Couple

Traditionally, the utilized electrolyte in DSSCs is solely assumed to regenerate the adsorbed oxidized dye on the mesoporous surface after the electron injection. This regeneration process is typically in the pico- to nano- second time scale [42–44]. However, just recently, it has been shown that the utilized electrolyte can form ground state interactions with the adsorbed dye on the surface that both affect the electron injection and recombination processes [15]. These effects will have detrimental effects on the performance of organic dyes in DSSCs. The formation of ground state complexes have been confirmed by using steady state absorption and emission measurements. **Figure 12** shows the kinetic traces for the electron dynamics of adsorbed organic dye D149 on TiO2 in contact with different components of the

**Figure 12.**

*Effect of chemical interactions between the D149 organic dye and the redox couple electrolyte (Iodide, iodine, tri-iodide) on the electron dynamics of D149 dye on mesoporous TiO2, the rise of the signal is due to electron injection, while the signal decay is due to the electron recombination, readapted from reference [15]. (A) Comparison between D149 and complexes of D149 with tri-iodide, and iodine. (B) Comparison between D149 and complexes of D149 with iodide, and full electrolyte.*

traditional iodide electrolyte used in various DSSC sets [45]. For the case of D149/ TiO2, slower electron injection and recombination processes have been observed. However, upon adding I3 − , I− , or I2, the electron injection process was much faster of ca. 100 fs, and more importantly the electron recombination was increased dramatically, due to the adsorbed complexes species on the surface [15]. Thus, the chemical interactions between the chemical substances should be considered upon optimizing the DSSC efficiency.
