**2. Computational methods**

*Solar Cells - Theory, Materials and Recent Advances*

modified [12–14].

based BHJ-OSCs.

BHJ-OSCs [31, 32].

Recently, polymers and small molecules organic semiconductors have received

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 materials-

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

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

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

**66**

**Figure 1.**

*Molecular structures of investigated compounds.*

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 efficiencies (PCE) were estimated using Scharber diagram.
