**1. Introduction**

The organic photovoltaic solar cells have gained most attention compared to the inorganic counterparts thanks to their exclusive characters such as the flexibility, the light weight, the transparency and the low-cost of fabrication [1–6]. Bulk heterojunction organic solar cells (BHJ-OSCs), have been largely emerged regarding the several advantages especially their impressive photo-physical properties. Introducing a high performance material remains a challenge for researchers [7–11].

Recently, polymers and small molecules organic semiconductors have received great attention to be used in BHJ-OSCs, because of their well-defined molecular structure, simple synthesis, high mobility and the structure could be easily modified [12–14].

Particularly, π-conjugated systems incorporating donor-acceptor (D-A) and donor-π-acceptor-π-donor (D-π-A-π-A) configurations have been emerged as promising category of materials for photovoltaic applications. In fact, thanks to the high electron delocalization and the intra-molecular charge transfer (ICT) that takes place within the conjugated skeleton, D-A materials have shown interesting optoelectronic properties and high charge carrier mobility [15, 16]. These particular characteristics of these kinds of π-conjugated systems leaded to further improve the conjugated arrangement for more increasing the device performance of materialsbased BHJ-OSCs.

Polymers and small molecules based on cyclopentadithiophene (CPDT) were widely used in organic electronic applications thanks tothe high rigidity, planarity and charge transfer ability delivered by CPDT group [17–23]. Further, Benzothiadiazole (BT) [24–27] and Diketopyrrolopyrrole (DPP) [28–30] have been extensively utilized as electron deficient acceptor patterns in conjugated systems. These acceptor groups offer the advantages of the high electron withdrawing ability, the excellent electro-optical properties and the important carrier mobility within conjugated materials. Hence, a rational design incorporating donor and acceptor moieties may improves the optoelectronic properties to assure high performance BHJ-OSCs [31, 32].

This study aims to investigate two donor materials as illustrated in **Figure 1**, the first one is a copolymer based on CPDT and BT with donor-acceptor (D-A) configuration, namely P-CPDTBT3. The choice of the conjugated chain length of this copolymer with n = 3 is based on the simulated results in our previous work [33]. The second one is a small molecule based on CPDT and DPP with D-π-A-π-D configuration, namely SM-CPDTDPP, in which thiophene was used as a potential π-spacer building block regarding its high electron abilities [34–36]. These compounds are desired to be blended with fullerene-based acceptor material to form

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

efficiencies (PCE) were estimated using Scharber diagram.

active layer within BHJ-OSC architecture. A computational investigation applying the DFT approach based on the effects of tuning the acceptor building block

together with the molecular configuration on the optoelectronic properties has been

All the calculations have been performed by means of GAUSSIAN 09 software package [37]. The ground state geometric optimizations were carried out using density functional theory (DFT) with the B3LYP hybrid functional method at 6-311 g(d,p) basis set in gaseous phase. The frontier molecular orbitals (FMOs) properties (HOMO, LUMO and Eg) were determined based on the optimized ground state geometries. The electronic parameters including the ionization potential (IP) and electron affinity (EA) were calculated from neutral, cation and anion optimized structures. The optical absorption characteristics were investigated using time dependent DFT (TD-DFT) method at B3LYP/6-311 g(d,p) level of theory [38, 39]. Charge transfer properties were investigated based on the reorganization energies of hole and electron within the studied molecules. Transition density matrix (TDM) plots were carried out using Multiwfn [40] to understand the electron–hole coherence correlation and the exciton dissociation at the first excited state. Finally, photovoltaic parameters were computed and power conversion

The studied conjugated molecules are constructed based on CPDT units as donors with BT and DPP as acceptor units. Hence, these compounds are of ″pushpull″ type conjugated molecules [41, 42]. Both P-CPDTBT3 and SM-CPDTDPP were optimized in the ground state using DFT//B3LYP/6-311 g(d,p) method. This study aims to examine the effect of the conjugated molecular design on the optoelectronic and photovoltaic properties. Here, we have maintained the CPDT donor building block and we have tuned the acceptor moieties based on BT and DPP units. Besides, we are looking to reveal the difference of behavior between polymer and

As it can be seen from **Figure 2**, both compounds exhibit a high planar optimized geometry. The dihedral angles are almost 0°, as observed from the side view of these molecules. These planar configurations are arising from the intramolecular non-covalent interactions of S---H, N---H and S---N types that take place within the conjugated framework [43]. These non-covalent bonds are found smaller than the sum of Van der Waals radii of the considered atoms. The planar backbone structure is one of the key factors to enhance the conjugation degree and accord-

The bridge bonds are described as the bonds that link between the distinct building blocks such as electron donating units, electron acceptor units and π-spacer within the conjugated backbone. The interest of examining the bridge bond length is to get an idea about the interactions among the different building blocks. Where, the shorter bridge bond length leads to stronger intra-molecular interactions and higher charge transfer [44, 45]. For the studied compounds, the bridge bond defines the bond C-C between the CPDT donor and BT or DPP

ingly increasing the π-staking for more charge transfer capability.

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

**2. Computational methods**

**3. Results and discussions**

small molecule.

**3.1 Ground-state geometry optimizations**

reported.

**Figure 1.** *Molecular structures of investigated compounds.*

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

active layer within BHJ-OSC architecture. A computational investigation applying the DFT approach based on the effects of tuning the acceptor building block together with the molecular configuration on the optoelectronic properties has been reported.
