**2. Material and methods**

#### **2.1 Method theoretical**

To explore and understand the electronic and optoelectronic properties of photosensitized materials with application in OPV technology, many theories have emerged. One of the most important and common theories is the theory of functional density (DFT), which is a tool that allowed to establish any property used in photosensitized materials, quantum state of atoms, molecules and solids, making modeling and simulation possible of complex systems with millions of degrees of freedom. At present, DFT has grown tremendously and has become one of the main tools in theoretical physics and molecular chemistry. Modeling in the framework of computational chemistry of photosensitized systems made up of electron donors and electron acceptors ultimately influences photo induced electron transfer and energy reactions. Numerous studies using the Density Functional Theory (DFT) methodology to design, evaluate and predict photovoltaic properties of photoactive materials with application in OPV have been published. The approximation of the theory of functional density (DFT) implemented was Gaussian 09 together with the functional correlation (B3LYP) and the base set 6-31g (d, 2p). This calculation allows optimization of geometry without symmetry restrictions for stationary points. In addition, it provided information on the harmonic frequency analysis, which allows the optimized minimum to be verified. The local minimum is identified when the number of imaginary 32frequencies is equal to zero.

The analysis of the changes in electron density for a given electronic transition was based on the electron density difference maps (EDDMs) constructed using the GaussSum suite of programs. Gásquez and co-workers had proposed two different electronegativities (X) for the charge transfer process: one that describes fractional

negative charge donation *X*� whereas the other gives the fractional negative charge

*Ligands and Coordination Compounds Used as New Photosensitized Materials…*

Thus, the construction of a so-called donor-acceptor map (DAM) has been suggested. A DAM graphic can be constructed by plotting the values of (y-axis) and

The photovoltaic properties are calculated according to the Scharber model, which is an empirical model for predicting the PCE of organic cell solar. HOMO-LUMO as orbital border under solar irradiation with AM 1.5 G (ASTM G173). The PCE was expressed by the following Eqs. (3) and (4), in where Voc is the open

*PCE* ¼ *FF Jsc*

(q = elementary charge, EQE = external quantum efficiency, ϕ = irradiation flow

On the other hand, the valor corresponding ΔEGAP was calculated as, Eq. (5):

**LHE** (light capture efficiency determinate), (f = oscillator strength) and Es1 =

The excitation energies (Es1) presented in **Table 4** were relatively small for the PDT molecule ligand, which indicate a shift to visible region in relationship with λmax. The *p*-PDT showed the lowest value for Es1, which is directly correlated with the conversion energy (PCE). On the other hand, the cycle size generated for *o*-PDT

ð*<sup>λ</sup>max λmin*

with AM 1.5 G, and λ = wavelength), and Pinc (incident light power).

*Voc*

*X*� ¼ 0*:*25 3ð Þ *I* þ *A* (1)

*X*<sup>þ</sup> ¼ 0*:*25 ð Þ *I* þ 3*A* (2)

*Pinc* � � (3)

*EQE <sup>ϕ</sup>AM* <sup>1</sup>*:*<sup>5</sup> *<sup>G</sup>*ð Þ*<sup>λ</sup> <sup>d</sup><sup>λ</sup>* (4)

Δ*E* ¼ *E* ð*LUMO*Þ � *E* ð*HOMO*Þ (5)

*LHE* <sup>¼</sup> <sup>1</sup> � <sup>10</sup>�*<sup>f</sup>* (6)

acceptance *X*þ, Eqs. (1) and (2):

*Geometry optimization for (a) T and (b) TZn.*

*DOI: http://dx.doi.org/10.5772/intechopen.92268*

**Figure 2.**

*X*<sup>þ</sup> (x-axis) for each molecule of interest.

circuit voltage, and Jsc is short circuit current.

where *FF* is a fill factor of 0.75, Eq. (4):

Excitation energy for λmax, Eq. (6).

**3.1 Optical properties of macrocycle molecules**

**3. Results and discussion**

**133**

**Jsc** ¼ q

**Figure 1.** *Chemical structure for (a)* p*-PDT, (b)* m*-PDT and (c)* o*-PDT.*

*Ligands and Coordination Compounds Used as New Photosensitized Materials… DOI: http://dx.doi.org/10.5772/intechopen.92268*

**Figure 2.** *Geometry optimization for (a) T and (b) TZn.*

negative charge donation *X*� whereas the other gives the fractional negative charge acceptance *X*þ, Eqs. (1) and (2):

$$X^{-} = \mathbf{0}.\mathbf{25}(\mathbf{3}I + A) \tag{1}$$

$$X^{+} = \mathbf{0}.25 \, (I + \mathbf{3A}) \tag{2}$$

Thus, the construction of a so-called donor-acceptor map (DAM) has been suggested. A DAM graphic can be constructed by plotting the values of (y-axis) and *X*<sup>þ</sup> (x-axis) for each molecule of interest.

The photovoltaic properties are calculated according to the Scharber model, which is an empirical model for predicting the PCE of organic cell solar. HOMO-LUMO as orbital border under solar irradiation with AM 1.5 G (ASTM G173). The PCE was expressed by the following Eqs. (3) and (4), in where Voc is the open circuit voltage, and Jsc is short circuit current.

$$\text{PCE} = \text{FF}\left(J\_{\text{sc}} \, \frac{V\_{\text{ac}}}{P\_{\text{inc}}}\right) \tag{3}$$

where *FF* is a fill factor of 0.75, Eq. (4):

$$\mathbf{J\_{sc}} = \mathbf{q} \int\_{\lambda \text{min}}^{\lambda \text{max}} EQE \ \phi^{AM} \ ^{1.5 \text{ } G} (\lambda) d\lambda \tag{4}$$

(q = elementary charge, EQE = external quantum efficiency, ϕ = irradiation flow with AM 1.5 G, and λ = wavelength), and Pinc (incident light power).

On the other hand, the valor corresponding ΔEGAP was calculated as, Eq. (5):

$$
\Delta E = E \left( LUMO \right) - E \left( HMOO \right) \tag{5}
$$

**LHE** (light capture efficiency determinate), (f = oscillator strength) and Es1 = Excitation energy for λmax, Eq. (6).

$$LHE = 1 - 10^{-f} \tag{6}$$

#### **3. Results and discussion**

#### **3.1 Optical properties of macrocycle molecules**

The excitation energies (Es1) presented in **Table 4** were relatively small for the PDT molecule ligand, which indicate a shift to visible region in relationship with λmax. The *p*-PDT showed the lowest value for Es1, which is directly correlated with the conversion energy (PCE). On the other hand, the cycle size generated for *o*-PDT

**Figure 1.**

**132**

*Chemical structure for (a)* p*-PDT, (b)* m*-PDT and (c)* o*-PDT.*

*Stability and Applications of Coordination Compounds*


#### **Table 4.**

*Optical properties for (a)* o*-PDT, (b)* m*-PDT and (c)* p*-PDT.*

and *m*-PDT systems is smaller, but this does not guarantee a better transfer. On the contrary, there is less efficient in the electronic transport.

The LHE values were 2.0217, 2.8755 and 0.07 for *o*-PDT, *m*-PDT and *p*-PDT, respectively. This indicates that *o*-PDT and *m*-PDT had a similar sensitivity to sunlight and will reflect higher values of LHE compared to *p*-PDT.

The visible light absorption ability may benefit from absorbing more photons and generating high photocurrent, which is a strong advantage of T derivatives. In the previous reports, PD spacers that cannot absorb visible light were observed. It is necessary that T derivate linked to the PD fragment enhances the electronic coupling in the excited state, which operates as a gated wire in π-conjugated systems, as is observed for *o*-PDT, *m*-PDT and *p*-PDT (**Figure 3**). The isomeric effect is greatly correlated to geometric distortion *o*-PDT and *m*-PDT molecules, which were dramatically affected in relationship to its planarity. The cavity between linear molecules is small, but the torsion affects the electronic properties.

#### **3.2 Geometry study for macrocycles with Lewis acid.**

The effect of Lewis acid on macrocycle stabilization is shown below. The geometric environment of the metallic center was tetrahedral, considering two positions to the electro donator atoms corresponding to linear and macrocycle molecule; and two water molecules. The incorporation of the metal into the linear chain (ZnT) generates a decrease in the value for GAP around 1.72 eV, a value located in the visible region. However, the effect is more severe when incorporated into the macrocycle, in where; its addition generated a decrease in GAP still 1.55 eV (*p*-ZnPDT). The DAM graphic for these systems indicated a significant improvement in donor capacity. These criteria are important to electronically activate the photovoltaic cell (**Figure 4**).

**Figure 3.**

**Figure 4.**

**135**

*Theoretical spectra electronic for (a)* o*-PDT* m*-PDT.*

*Ligands and Coordination Compounds Used as New Photosensitized Materials…*

*DOI: http://dx.doi.org/10.5772/intechopen.92268*

*DAM graphic for T,* p*-PDT, ZnT and* p*-ZnPDT.*

The spectra in **Figure 5** showed a similar profile for TZn, and *p*-ZnPDT with the incorporation of Lewis acid in the structure, which have an intense main band to 568 nm, and 516 nm respectively. This band corresponds to the dominant electron transition from HOMO to LUMO, that is, from the π molecular orbital (chromophore fragments-π-linker) to the π\* orbital (acceptor fragment), and this process can be ascribed to the intramolecular charge transfer.

#### **3.3 Photovoltaic properties of macrocycle molecules**

The results showed in **Table 5** suggested decreased the ΔEGAP in relationship with PCE. These results are congruent with the optical, and electronic properties observed previously. The *p*-PDT presented the best photovoltaic properties. The metal ion generates a symmetrical tension in the system, and this could explain its behavior. The Jsc increased in function of decreased the ΔEGAP, concluding that the preferential isomer for the construction of this family macrocycles is the *p*-PDT, considering theoretical models in the gas phase.

*Ligands and Coordination Compounds Used as New Photosensitized Materials… DOI: http://dx.doi.org/10.5772/intechopen.92268*

**Figure 3.** *Theoretical spectra electronic for (a)* o*-PDT* m*-PDT.*

**Figure 4.** *DAM graphic for T,* p*-PDT, ZnT and* p*-ZnPDT.*

and *m*-PDT systems is smaller, but this does not guarantee a better transfer. On the

*o*-PDT 432.68 2.874 2.0217 0.9905 *m*-PDT 425.37 2.923 2.8755 0.9987 *p*-PDT 465 2.67 0.07 0.14

**Es1 (eV)** *F* **LHE**

The LHE values were 2.0217, 2.8755 and 0.07 for *o*-PDT, *m*-PDT and *p*-PDT, respectively. This indicates that *o*-PDT and *m*-PDT had a similar sensitivity to

The visible light absorption ability may benefit from absorbing more photons and generating high photocurrent, which is a strong advantage of T derivatives. In the previous reports, PD spacers that cannot absorb visible light were observed. It is necessary that T derivate linked to the PD fragment enhances the electronic coupling in the excited state, which operates as a gated wire in π-conjugated systems, as is observed for *o*-PDT, *m*-PDT and *p*-PDT (**Figure 3**). The isomeric effect is greatly correlated to geometric distortion *o*-PDT and *m*-PDT molecules, which were dramatically affected in relationship to its planarity. The cavity between linear mole-

The effect of Lewis acid on macrocycle stabilization is shown below. The geometric environment of the metallic center was tetrahedral, considering two positions to the electro donator atoms corresponding to linear and macrocycle molecule; and two water molecules. The incorporation of the metal into the linear chain (ZnT) generates a decrease in the value for GAP around 1.72 eV, a value located in the visible region. However, the effect is more severe when incorporated into the macrocycle, in where; its addition generated a decrease in GAP still 1.55 eV (*p*-ZnPDT). The DAM graphic for these systems indicated a significant improvement in donor capacity. These criteria are important to electronically activate the photo-

The spectra in **Figure 5** showed a similar profile for TZn, and *p*-ZnPDT with the incorporation of Lewis acid in the structure, which have an intense main band to 568 nm, and 516 nm respectively. This band corresponds to the dominant electron transition from HOMO to LUMO, that is, from the π molecular orbital (chromophore fragments-π-linker) to the π\* orbital (acceptor fragment), and this process

The results showed in **Table 5** suggested decreased the ΔEGAP in relationship with PCE. These results are congruent with the optical, and electronic properties observed previously. The *p*-PDT presented the best photovoltaic properties. The metal ion generates a symmetrical tension in the system, and this could explain its behavior. The Jsc increased in function of decreased the ΔEGAP, concluding that the preferential isomer for the construction of this family macrocycles is the *p*-PDT,

contrary, there is less efficient in the electronic transport.

**λmax**

*Stability and Applications of Coordination Compounds*

*Optical properties for (a)* o*-PDT, (b)* m*-PDT and (c)* p*-PDT.*

**Molecule Wavelength**

**Table 4.**

sunlight and will reflect higher values of LHE compared to *p*-PDT.

cules is small, but the torsion affects the electronic properties.

**3.2 Geometry study for macrocycles with Lewis acid.**

can be ascribed to the intramolecular charge transfer.

**3.3 Photovoltaic properties of macrocycle molecules**

considering theoretical models in the gas phase.

voltaic cell (**Figure 4**).

**134**

**Conflict of interest**

*DOI: http://dx.doi.org/10.5772/intechopen.92268*

**Author details**

**137**

Yenny Patricia Avila Torres

Cali, Santiago de Cali, Colombia

provided the original work is properly cited.

\*Address all correspondence to: yavilatorres@gmail.com

Research Group QUIBIO, Facultad de Ciencias Básicas, Universidad Santiago de

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

The authors declare that they have no conflict of interests.

*Ligands and Coordination Compounds Used as New Photosensitized Materials…*

#### **Figure 5.**

*Theoretical spectra electronic for TZn and* p*-ZnPDT.*


#### **Table 5.**

*Photovoltaic parameters for T,* p*-PDT, ZnT and* p*-ZnPDT.*

### **4. Summary and future perspectives**

The purpose of this review of DSSC materials was to compile the information reported to: synthetized ruthenium complexes, porphyrins, and metal-free organic dyes. For researchers, it is important to know parameters such as: PCE, Jsc, and Voc; which help you to diffuse between structures, and propose synthesis strategies that make possible new materials in this field application. Principles for the future development of new molecules can be analyzed and likewise it is interesting support to follow up structure families as a function of time. Although many structures are shown here, there is still a need to optimize the chemical, and physical properties to promote improved solar cells. On the other hand, in this work, the best photovoltaic parameters were described for *p*-PDT with PCE 26.18%, Jsc = 14.79 mA/cm<sup>2</sup> , and ΔE = 2.66 eV such as macrocycle. The metal ion influences the electronic properties, and decreases the ΔEGAP. The incorporation of Lewis acid in the structure macrocycle to increase of the optical properties, which allows rigidity that can benefit planarity.

### **Acknowledgements**

This work was supported by Universidad Santiago de Cali—DGI Grants 63661. The author acknowledgment to Melissa Suarez for technical support and Hoover Valencia for data acquisition.

*Ligands and Coordination Compounds Used as New Photosensitized Materials… DOI: http://dx.doi.org/10.5772/intechopen.92268*
