**3.2 Frontier molecular orbitals (FMOs) analysis**

The analysis of Frontier molecular orbitals (FMOs) gives a description of the electron delocalization as well as the electron transport capacities within the conjugated skeleton. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) strongly determine the optoelectronic properties of conjugated compounds, pointedly on the photovoltaic properties of donor materials. Largely, donor compound should tend to have a deep HOMO level to assure a high open circuit voltage VOC and a suitable LUMO energy level with respect to that of the acceptor unit [47–49].

The FMOs of the considered materials are carried out based on DFT/B3LYP method at 6-311 g(d,p) and listed in **Table 1**. The FMOs contour plots are illustrated in **Figure4**.


#### **Table 1.**

*Solar Cells - Theory, Materials and Recent Advances*

acceptor units. The bridge bond lengths are found around 1.45 Å for P-CPDTBT3 and 1.43 Å for SM-CPDTDPP. The obtained values are higher than the regular C=C bond length (1.34 Å) and smaller than the regular C-C bond length (1.54 Å) which

*Ground state optimized structures of P-CPDTBT3 and SM-CPDTDPP at DFT/B3LYP/6-311 g(d,p) level of theory.*

indicates that these bonds are still found to have double-bond character.

*Molecular electrostatic potential (MEP) of the considered compounds.*

**68**

**Figure 3.**

**Figure 2.**

*Electronic properties for studied materials obtained at DFT/B3LYP 6-311 g(d,p) level of theory.*

**Figure 4.** *FMOs contour plots at the optimized ground state of the considered materials.*

The simultaneous interactions of donor and acceptor groups are the responsible of the electron delocalization and thus producing the electronic charge distribution within the HOMOs and LUMOs. As it can be seen from **Figure 4**, there is considerable discrepancy of molecular orbital distributions resulting from the particular molecular configurations of P-CPDTBT3 and SM-CPDTDPP.

The spatial distribution of the HOMO orbital of P-CPDTBT3 is dominantly localized over the main conjugated backbone. While, that of SM-CPDTDPP is mainly located on the central part of the conjugated framework. The LUMOof P-CPDTBT3 is dispersed over the central CPDT unit indicating a high steric hindrance rising from the strong electron withdrawing group effect of dicyanomethylene group [50]. In the case of SM-CPDTDPP, the LUMO is centered over the DPP substituted group and the thiophene π-spacer units. These distributions may increase the π → π\* electronic transitions and reinforce the ICT ability. Besides, these materials dispose narrow band gap energies (1.62 eV) for P-CPDTBT3 and 1.42 eV for SM-CPDTDPP) that lead to improve the electron transition and light harvesting. The 2D molecular electrostatic maps of studied materials have been simulated to better understand the intra-molecular interactions (See **Figure 5**). As revealed from **Figure 5**, the central part is the most conjecturable zone into the conjugated framework of the studied molecules that is in good agreement with the FMOs analysis.

Ionization potential (IP) and Electron Affinity (EA) were calculated from the neutral, cation and anion optimized structures. IP and EA describe the barrier injection energies of electron and hole, respectively. The application of the considered materials in OCSs requires relevant IP and EA in order to promote the electron injection and hole transport. Thus, it is revealed from the FMOs analysis the significant effect of building blocks on the electronic properties that are related to the charge delocalization.

**71**

**Figure 6.**

*level of theory.*

*Designing Well-Organized Donor-Bridge-Acceptor Conjugated Systems Based…*

λ

The optical absorption spectrum in the solar spectral zone also its intensity are the main factors that influence the value of short-circuit current density (JSC) of OSCs [51]. Fundamentally, the JSC is a function of the external quantum efficiency

*J q EQE S d SC* = . (

Where, EQE presents the product of light harvesting efficiency (ηλ), exciton diffusion efficiency (ηED), charge separation efficiency (ηCS), and charge collection efficiency (ηCC). As revealed from the following expression, the donor material absorption capability remains a crucial parameter for increasing the organic solar cell efficiency. The light harvesting efficiency (ηλ) is related to the oscillator strength (f) of the maximum optical absorption wavelength as expressed

1 10 *<sup>f</sup>*

In order to explore the photo-physical properties of the considered compounds,

the optical absorption spectra were simulated using TD-DFT approach as cost-

*Optical absorption spectra of P-CPDTBT3 and SM-CPDTDPP simulated at TD-DFT//B3LYP/6-311 g(d,p)* 

ηλ λ λ

) coveringall the frequencies providedfrom the

) ∫ (1)

<sup>−</sup> = − (2)

*DOI: http://dx.doi.org/10.5772/intechopen.94874*

(EQE) with the photon number *S*(

solar spectrum, as above [52]:

above [53]:

effective method [54, 55].

**3.3 Optical properties**

**Figure 5.** *2D molecular electrostatic maps of studied materials.*

*Designing Well-Organized Donor-Bridge-Acceptor Conjugated Systems Based… DOI: http://dx.doi.org/10.5772/intechopen.94874*
