**4. Part II: Donor-acceptor polymers for photovoltaic cell devices**

## **4.1. Results and discussion**

#### *4.1.1. Conformational study*

As first step, an accurate representation of the bond rotations in the chain is extremely important, since the properties of such polymers depend strongly on the conformational statistics of polymer chains [119]. Besides, the geometries obtained for the most stable conformations are used as input data for full optimization calculations. DFT/B3LYP calculations are performed on the following three model compounds, poly(3 hexylthiophene)-benzothiadiazole (P3HTBT), poly (carbazole-benzothiadiazole) (PCzBT) and poly(3-hexylthiophene)- di-benzothiadiazole-carbazole (P3HT2BTCz).

Photophysical Properties of Two New Donor-Acceptor Conjugated Copolymers and Their Model Compounds:

**(a)**

**(b)**

copolymer.

summarized.

compounds.

AD+=A-

**Figure 18.** DFT/B3LYP/3-21G\* optimized structure of: (a) (P3HTBT)4 copolymer and (b) (PCzBT)4

Based on these optimized structures, the principal physico-chemical parameters of the two copolymers are collected in Table 6. Along with the torsional angle () (the deviation from co-planarity between the donor and acceptor units), intra-molecular charge transfer (DCT) (the summation of all charges for the donor unit 3-hexyl thiophene (3HT) and carbazole (Cz)), bridge length (LB) (the bond length between the donor and acceptor) are

P3HTBT 1.458 0.038 0.0 PCzBT 1.482 0.063 147.0

Considering the most stable conformation, we can deduce that the optimized structure of the PCzBT appears under a twisted configuration with a large torsional angle ( = 147.0°). This suggests that a strong steric hindrance effect exists between the donor and acceptor moieties, whereas the P3HTBT structure has perfectly a planar structure. Moreover, compared to PCzBT, the order of the LB of P3HTBT remains smaller indicating the formation of the mesomeric structures induced by intra-molecular charge transfer, that is, D-

probably originating from the nitrogen atoms with high electronegativity in the main backbone of Cz donor group. DCT significantly enhances the -electron delocalization

which is largely dependent on rather than on the acceptor strength.

. The large intra-molecular charge (DCT) of PCzBT copolymer backbone is

**Table 6.** The optimum characteristic parameters (LB, DCT, and ) of P3HTBT and PCzBT model

LB (Å) DCT (e) (°)

Applications in Polymer Light Emitting Diodes (PLEDs) and Polymer Photovoltaic Cells (PPCs) 125

In conformational part, two basis sets 3-21G\* and 6-31G\* have been used for the sake of comparison. We note that the results derived from these two basis sets are almost similar. The relative energy for the first model (Fig. 17) shows two local minima in both sides of the spectrum (0° and 180°) and a maximum at about 90°.

**Figure 17.** Potential energy curves of: (a) P3HTBT and (b) PCzBT monomer simulated at DFT/B3LYP level with () 3-21G\* and () 6-31G\* basis sets.

The results indicate that the P3HTBT is completely planar with the inter-ring torsion angle 0°. It's obvious that this planarity is caused by intra-molecular repulsion between sulphur atoms in the main polymer backbone. In the case of benzothiadiazole copolymerized with carbazole, the conformational behaviour is completely different. The twisted conformations have two torsion angles 1 and 2 at around 40° and 140°, respectively. The latter conformation (140°) is slightly more stable by about 0.5 kcal.mol-1.

#### *4.1.2. Structural and characteristic parameters*

The fully optimized structures with DFT/B3LYP/3-21G\* method, with the respect to the torsion angles of both P3HTBT and PCzBT copolymers are shown in Fig. 18.

Photophysical Properties of Two New Donor-Acceptor Conjugated Copolymers and Their Model Compounds: Applications in Polymer Light Emitting Diodes (PLEDs) and Polymer Photovoltaic Cells (PPCs) 125

124 Organic Light Emitting Devices


**(a)**

Relative energy **(**kcal.mol-1**)**

**4.1. Results and discussion** 

*4.1.1. Conformational study* 

**4. Part II: Donor-acceptor polymers for photovoltaic cell devices** 

and poly(3-hexylthiophene)- di-benzothiadiazole-carbazole (P3HT2BTCz).

spectrum (0° and 180°) and a maximum at about 90°.

0 20 40 60 80 100 120 140 160 180 200

conformation (140°) is slightly more stable by about 0.5 kcal.mol-1.

torsion angles of both P3HTBT and PCzBT copolymers are shown in Fig. 18.

**Torsion angle (degree)**

level with () 3-21G\* and () 6-31G\* basis sets.

*4.1.2. Structural and characteristic parameters* 

As first step, an accurate representation of the bond rotations in the chain is extremely important, since the properties of such polymers depend strongly on the conformational statistics of polymer chains [119]. Besides, the geometries obtained for the most stable conformations are used as input data for full optimization calculations. DFT/B3LYP calculations are performed on the following three model compounds, poly(3 hexylthiophene)-benzothiadiazole (P3HTBT), poly (carbazole-benzothiadiazole) (PCzBT)

In conformational part, two basis sets 3-21G\* and 6-31G\* have been used for the sake of comparison. We note that the results derived from these two basis sets are almost similar. The relative energy for the first model (Fig. 17) shows two local minima in both sides of the

**(b)**

Relative energy **(**kcal.mol-1**)**

**Figure 17.** Potential energy curves of: (a) P3HTBT and (b) PCzBT monomer simulated at DFT/B3LYP

The results indicate that the P3HTBT is completely planar with the inter-ring torsion angle 0°. It's obvious that this planarity is caused by intra-molecular repulsion between sulphur atoms in the main polymer backbone. In the case of benzothiadiazole copolymerized with carbazole, the conformational behaviour is completely different. The twisted conformations have two torsion angles 1 and 2 at around 40° and 140°, respectively. The latter

The fully optimized structures with DFT/B3LYP/3-21G\* method, with the respect to the


**Torsion angle (degree**)

**Figure 18.** DFT/B3LYP/3-21G\* optimized structure of: (a) (P3HTBT)4 copolymer and (b) (PCzBT)4 copolymer.

Based on these optimized structures, the principal physico-chemical parameters of the two copolymers are collected in Table 6. Along with the torsional angle () (the deviation from co-planarity between the donor and acceptor units), intra-molecular charge transfer (DCT) (the summation of all charges for the donor unit 3-hexyl thiophene (3HT) and carbazole (Cz)), bridge length (LB) (the bond length between the donor and acceptor) are summarized.


**Table 6.** The optimum characteristic parameters (LB, DCT, and ) of P3HTBT and PCzBT model compounds.

Considering the most stable conformation, we can deduce that the optimized structure of the PCzBT appears under a twisted configuration with a large torsional angle ( = 147.0°). This suggests that a strong steric hindrance effect exists between the donor and acceptor moieties, whereas the P3HTBT structure has perfectly a planar structure. Moreover, compared to PCzBT, the order of the LB of P3HTBT remains smaller indicating the formation of the mesomeric structures induced by intra-molecular charge transfer, that is, D-AD+=A- . The large intra-molecular charge (DCT) of PCzBT copolymer backbone is probably originating from the nitrogen atoms with high electronegativity in the main backbone of Cz donor group. DCT significantly enhances the -electron delocalization which is largely dependent on rather than on the acceptor strength.

The optimized structure of the resulting P3HT2BTCz composite and its main geometrical parameters (torsion angle and interring bond length) are illustrated in Fig. 19. The inspection of these data reveals that the resulting composite shows an almost non planar conformation which is more underlined on both sides of carbazole units to reach the values of 45° and 49°. Moreover, compared to those of P3HTBT and PCzBT, the central bonds connecting the two neighbouring central rings are slightly shorter, showing that this compound is more conjugated to extend the delocalization on all the chain backbone.

Photophysical Properties of Two New Donor-Acceptor Conjugated Copolymers and Their Model Compounds:

By using the linear extrapolation technique [120], it can be seen from Fig. 20 that this value decreases with increasing the chain length from monomer to quatermer. Moreover, the theoretical data resulting from the two considered basis sets are very close and no significant changes are noticed when going from 3-21G\* to 6-31G\* basis set calculations.

**PCzBT**

0,0 0,2 0,4 0,6 0,8 1,0 1,2

1/n (n: monomer number)

**Figure 20.** Representation of the band gap energy (Eg) as function of inverse chain length (1/n) for

The band gap of P3HTBT is found to be around 1.55 and 1.61 eV with 6-31G\* and 3-21G\* basis sets, respectively. These values are lower than that of pristine P3HT (1.90 eV) [121], due to the presence of benzothiadiazole in the main backbone copolymer. In parallel, a wide band gap for PCzBT is estimated to be 2.44-2.52 eV (Fig. 20). Nevertheless, the band gap of resulting composite P3HT2BTCz is found to be 2.31 eV which is in agreement with the experimental values Eg 1.97 eV (derived from the UV-visible absorption spectrum in chloroform solution) [87]. These results are in close agreement with the experimental data by taking into account the packing effects (interchain interaction) in the solid state [122]. The HOMO level energy is estimated to be - 4.9 eV making this copolymer photo-chemically

The TDDFT method was applied on the basis of the ground state optimized geometry of different compounds under study. As shown in Fig. 21, the absorption spectrum of the P3HT2BTCz composite seems to be the superposition of the two absorption spectra of P3HTBT and PCzBT copolymers. Compared to PCzBT and P3HTBT polymers, the absorption spectra is broader due the red shifted absorption, which may be attributed to the much better conjugation along the polymer backbone. Besides, the simulated absorption spectra show that the P3HT2BTCz compound absorbs from the UV at a wavelength of 600 nm, with two main absorption peaks centred at 478 and 319 and a weak peak at 260 nm. The

P3HTBT and PCzBT calculated by DFT/B3LYP with 6-31G\* and 3-21G\*basis sets.

**P3HTBT**

 DFT/B3LYP/6-31G\* DFT/B3LYP/3-21G\* DFT/B3LYP/6-31G\* DFT/B3LYP/3-21G\*

1,4 1,6 1,8 2,0 2,2 2,4 2,6 2,8 3,0 3,2 3,4 3,6

Energy (eV)

stable.

Applications in Polymer Light Emitting Diodes (PLEDs) and Polymer Photovoltaic Cells (PPCs) 127

**Figure 19.** DFT/B3LYP/3-21G\* optimized geometric structure of the resulting P3HT2BTCz composite.
