*4.1.3. Optical properties and electronic structures*

As firstly discussed, the oligomer geometries and their corresponding band gap are calculated using DFT/B3LYP method with 3-21G\* and 6-31G\* basis sets. The band gap is estimated as the difference between the HOMO and LUMO energies. In our case, the band gap of (P3HTBT)n and (PCzBT)n (n = 1-4) oligomers are listed in Table 7.


**Table 7.** Band gap energy Eg of (P3HTBT)n and (PCzBT)n (n: from 1 to 4 units).

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.

126 Organic Light Emitting Devices

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.

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

As firstly discussed, the oligomer geometries and their corresponding band gap are calculated using DFT/B3LYP method with 3-21G\* and 6-31G\* basis sets. The band gap is estimated as the difference between the HOMO and LUMO energies. In our case, the band

> 3.23 2.67 2.11 1.94 1.61

> 3.10 2.80 2.71 2.68 2.52

Band gap energy (Eg) (eV)

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

3.26 2.45 2.12 1.96 1.55

3.09 2.82 2.63 2.61 2.44

gap of (P3HTBT)n and (PCzBT)n (n = 1-4) oligomers are listed in Table 7.

*4.1.3. Optical properties and electronic structures* 

monomer

**Table 7.** Band gap energy Eg of (P3HTBT)n and (PCzBT)n (n: from 1 to 4 units).

Polymer Number of

P3HTBT

PCzBT

**Figure 20.** Representation of the band gap energy (Eg) as function of inverse chain length (1/n) for P3HTBT and PCzBT calculated by DFT/B3LYP with 6-31G\* and 3-21G\*basis sets.

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 stable.

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 band located at 319 nm arises from the delocalized \* transition in the polymer and the visible absorption peak located at longer wavelength centered at 478 nm could be assigned to the intra-molecular charge transfer transition between the Cz donor moiety and the BT acceptor unit [123].

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

In order to study the emission properties of P3HT2BTCz compounds, the TD/B3LYP method was applied to the geometry of the lowest singlet excited state optimized at the CIS level with 3-21G\* basis set [124]. The normalized photoluminescence (PL) spectrum of P3HT2BTCz (Fig. 22) shows a maximum at 649 nm with strongest intensity (f = 0.8415), compared to 630 nm in experimental spectrum as indicated in Table 9. This may be regarded as an electronic transition reverse of the absorption corresponding mainly from LUMO to HOMO. Moreover, the observed red-shifted emission in the PL spectra is found to be in reasonable agreement with the experimental one by taking into account the packing effects

Oscillator

S1S0 649 15400 0.8415 HOMOLUMO 75% 630a

We also find relatively high values of Stokes Shift (SS) in P3HT2BTCz (0.62 eV (172 nm))

**Figure 22.** Experimental and TD-DFT calculated normalized absorption and emission spectra of

Based on the above results, the energy band structures are plotted in Fig. 23. When carbazole (Cz) is replaced by 3-hexylthiophene (3HT), the energy of HOMO level increases,

**Table 9.** Emission energy of P3HT2BTCz obtained by the TDDFT/B3LYP/3-21G\* method.

(inter-chain interaction) in the solid state (0.49 eV (124 nm)) [87].

Emission energy (cm-1)

Electronic transition

(Fig. 22).

P3HT2BTCz.

**<sup>a</sup>**in chloroform solution [87]

Emission wavelength (nm)

Applications in Polymer Light Emitting Diodes (PLEDs) and Polymer Photovoltaic Cells (PPCs) 129

Strength (f) MO/character Coefficient Experimental

value (nm)

**Figure 21.** TD/B3LYP/3-21G\* simulated UV-Visible optical absorption spectra: of PCzBT, P3HTBT and P3HT2BTCz.

The vertical excitation energy and their corresponding oscillator strength along the main excitation configuration are listed in Table 8. The first optically allowed electronic transition of P3HT2BTCz populates the HOMOLUMO excitation with high oscillator strength (f = 1.0898). The two other transitions are mainly assigned respectively to HOMOLUMO+1 and HOMO-1LUMO+3 excitations. All intermediate states with low oscillator strength, so-called dark states, have intra-molecular charge transfer (ICT) character. Through this study, it is found that the calculated results reproduce very well the corresponding experimental data [87].


**<sup>a</sup>**in chloroform solution [87] **<sup>b</sup>**in solid film [87]

**Table 8.** Main electronic transitions in P3HT2BTCz composites and their assignments.

In order to study the emission properties of P3HT2BTCz compounds, the TD/B3LYP method was applied to the geometry of the lowest singlet excited state optimized at the CIS level with 3-21G\* basis set [124]. The normalized photoluminescence (PL) spectrum of P3HT2BTCz (Fig. 22) shows a maximum at 649 nm with strongest intensity (f = 0.8415), compared to 630 nm in experimental spectrum as indicated in Table 9. This may be regarded as an electronic transition reverse of the absorption corresponding mainly from LUMO to HOMO. Moreover, the observed red-shifted emission in the PL spectra is found to be in reasonable agreement with the experimental one by taking into account the packing effects (inter-chain interaction) in the solid state (0.49 eV (124 nm)) [87].


**<sup>a</sup>**in chloroform solution [87]

128 Organic Light Emitting Devices

acceptor unit [123].

P3HT2BTCz.

experimental data [87].

**<sup>a</sup>**in chloroform solution [87] **<sup>b</sup>**in solid film [87]

Wavelength (nm)

Oscillator

Electronic transition 0,0

0,2

0,4

**Normalized Absorbance**

0,6

0,8

1,0

band located at 319 nm arises from the delocalized \* transition in the polymer and the visible absorption peak located at longer wavelength centered at 478 nm could be assigned to the intra-molecular charge transfer transition between the Cz donor moiety and the BT

> **PCzBT P3HTBT P3HT2BTCz**

200 300 400 500 600

**Figure 21.** TD/B3LYP/3-21G\* simulated UV-Visible optical absorption spectra: of PCzBT, P3HTBT and

The vertical excitation energy and their corresponding oscillator strength along the main excitation configuration are listed in Table 8. The first optically allowed electronic transition of P3HT2BTCz populates the HOMOLUMO excitation with high oscillator strength (f = 1.0898). The two other transitions are mainly assigned respectively to HOMOLUMO+1 and HOMO-1LUMO+3 excitations. All intermediate states with low oscillator strength, so-called dark states, have intra-molecular charge transfer (ICT) character. Through this study, it is found that the calculated results reproduce very well the corresponding

S0S1 478 1.0898 HOMOLUMO 80% 504a

S0S2 319 0.6912 HOMOLUMO+1 51% 327a

S0S3 260 0.1905 HOMO-1LUMO+3 54% ----

**Table 8.** Main electronic transitions in P3HT2BTCz composites and their assignments.

Strength (f) Main MO/character Coefficient Experimental

value (nm)

518b

338b

**Wavelength (nm)**

**Table 9.** Emission energy of P3HT2BTCz obtained by the TDDFT/B3LYP/3-21G\* method.

We also find relatively high values of Stokes Shift (SS) in P3HT2BTCz (0.62 eV (172 nm)) (Fig. 22).

**Figure 22.** Experimental and TD-DFT calculated normalized absorption and emission spectra of P3HT2BTCz.

Based on the above results, the energy band structures are plotted in Fig. 23. When carbazole (Cz) is replaced by 3-hexylthiophene (3HT), the energy of HOMO level increases, while that of LUMO decreases. This change on the electronic structure facilitates both the hole and electron-transporting ability. The electronic structure differs greatly from one model to another, showing the effect of donor units in D-A architecture polymer and it results from the coupling behaviour of 3HT, Cz and BT in the main backbone. Moreover, further insights are obtained comparing the DFT calculated density of states (DOS) of the P3HTBT and PCzBT with that of P3HT2BTCz composite. This comparison is showed in Fig. 23 (at the right). Two striking things immerge from DOS diagram: 1) the ground state interaction between the donor and acceptor units and 2) this interaction induces intra-gap charge transfer states lying inside the gap of the PCzBT. As a result, P3HT2BTCz composite orbitals are shifted towards higher energies compared to the isolated PCzBT orbitals and towards lower energies compared to the isolated P3HTBT orbitals.

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

**HOMO LUMO** 

**HOMO-1 LUMO+1** 

**HOMO-2 LUMO+2** 

The most intense calculated bands of the infrared absorption (IR) of these compounds, shown in Fig. 25 are collected in Table 10 together with their corresponding assignments.

1080 s 1024 s 1037 w 1037 vw S-N stretching (BT) + Rocking CH3 (P3HT and

Cz).

**Figure 24.** Contour plots for the main HOMO and LUMO molecular orbitals of P3HT2BTCz

**PBT P3HTBT PCzBT P3HT2BTCz Assignments**

956 s 961 w 960 vw 947 vw S-N Scissoring (BT).

707 m 725 w 728 w **- -** Out of plan C-H wagging (BT + Cz). - - 957 w **----** Ring breathing (BT and P3HT).

*4.1.4. Vibrational study and force constant analysis* 

**υ (cm-1) I υ (cm-1) I υ (cm-1) I υ (cm-1) I**

compound.

Applications in Polymer Light Emitting Diodes (PLEDs) and Polymer Photovoltaic Cells (PPCs) 131

**Figure 23.** Electronic structure and DOS diagram of P3HTBT, PCzBT and P3HT2BTCz, simulated using DFT/B3LYP/3-21G\* method.

The electron density iso-contours of HOMO and LUMO of P3HT2BTCz compound are plotted in Fig. 24. It can be seen that an asymmetric character within the rings and between subunits prevails for the HOMO orbital of this copolymer. Moreover, the localization of electronic charge lies mainly in the side part of HOMO orbital, which is typically expected due to the chain-end effects, which changes the shape of LUMO orbital. Due to the nonplanarity observed for the P3HT2BTCz compound geometry, in its ground state, electrons are mainly localized on the benzothiadiazole units, as result of the weak interactions between the two building blocks. This fact is particularly noticeable in the LUMO orbital with a symmetric character between the subunits.

According to our calculations, electron densities in the first excited state namely LUMO and LUMO+1 are delocalized on BT and P3HT units with a symmetric character. Whereas, for higher energy levels, e.g., LUMO+2 levels take part in electron transitions on the P3HT and Cz units. Yet, the charge density of HOMO, HOMO-1 shows that the charge density spreads over the main chain of the compound to become much concentrated around the P3HT and Cz units in HOMO-2.

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) 131

**Figure 24.** Contour plots for the main HOMO and LUMO molecular orbitals of P3HT2BTCz compound.

#### *4.1.4. Vibrational study and force constant analysis*

130 Organic Light Emitting Devices


DFT/B3LYP/3-21G\* method.

Cz units in HOMO-2.

Energy (eV)

while that of LUMO decreases. This change on the electronic structure facilitates both the hole and electron-transporting ability. The electronic structure differs greatly from one model to another, showing the effect of donor units in D-A architecture polymer and it results from the coupling behaviour of 3HT, Cz and BT in the main backbone. Moreover, further insights are obtained comparing the DFT calculated density of states (DOS) of the P3HTBT and PCzBT with that of P3HT2BTCz composite. This comparison is showed in Fig. 23 (at the right). Two striking things immerge from DOS diagram: 1) the ground state interaction between the donor and acceptor units and 2) this interaction induces intra-gap charge transfer states lying inside the gap of the PCzBT. As a result, P3HT2BTCz composite orbitals are shifted towards higher energies compared to the isolated PCzBT orbitals and

0,0

0,5

1,0

**DOS (a.u.)**

**Figure 23.** Electronic structure and DOS diagram of P3HTBT, PCzBT and P3HT2BTCz, simulated using

The electron density iso-contours of HOMO and LUMO of P3HT2BTCz compound are plotted in Fig. 24. It can be seen that an asymmetric character within the rings and between subunits prevails for the HOMO orbital of this copolymer. Moreover, the localization of electronic charge lies mainly in the side part of HOMO orbital, which is typically expected due to the chain-end effects, which changes the shape of LUMO orbital. Due to the nonplanarity observed for the P3HT2BTCz compound geometry, in its ground state, electrons are mainly localized on the benzothiadiazole units, as result of the weak interactions between the two building blocks. This fact is particularly noticeable in the LUMO orbital

According to our calculations, electron densities in the first excited state namely LUMO and LUMO+1 are delocalized on BT and P3HT units with a symmetric character. Whereas, for higher energy levels, e.g., LUMO+2 levels take part in electron transitions on the P3HT and Cz units. Yet, the charge density of HOMO, HOMO-1 shows that the charge density spreads over the main chain of the compound to become much concentrated around the P3HT and

1,5

**P3HT2BTCz**

**PCzBT**

**P3HTBT**

2,0


**Energy (eV)**

HOMO

LUMO

towards lower energies compared to the isolated P3HTBT orbitals.

**Eg Eg = 2.31 eV = 2.68 eV Eg = 1.94 eV**

**(PCzBT)4 (P3HTBT) P3HT2BTCz <sup>4</sup>**

with a symmetric character between the subunits.

The most intense calculated bands of the infrared absorption (IR) of these compounds, shown in Fig. 25 are collected in Table 10 together with their corresponding assignments.



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

A large number of bands appear with very high peaks due to an induced strong dipolar moment. All characteristics of infrared bands in P3HT [125], PCz and BT vibration modes

The inspection of these spectra shows that after combining the two copolymers to obtain the P3HT2BTCz composite, some bands undergo slight changes in their positions and intensities. The main vibrational modes of PBT persist following the addition of 3HT, Cz groups in the P3HT2BTCz composite. Firstly, a down shift of the band assigned to C-H stretching in benzothiadiazole unit is observed at high frequencies with strong intensity located at 3330 cm-1 in (P3HTBT)4 and (PCzBT)4.. The band at 1785 cm-1 assigned to the antisymmetric C=C stretching mode becomes clear in the other PCzBT and P3HT2BTCz compounds and the C-C bending vibrational mode located at 1621 cm-1 in PBT becomes clearly pronounced in PCzBT and P3HT2BTCz with a high energy shift of about 20 cm-1.

The signal attributed to the S-N stretching at 1073 cm-1 completely disappears in the functionalized composite following a significant interaction of different groups. This effect is also confirmed by the shift (from 1486 to 1550 cm-1 and from 1443 to 1513 cm-1) observed in IR bands ascribed to symmetric and anti-symmetric C=C stretching, respectively, as a consequence of the presence of more conjugated backbone. The band at 707 cm-1, ascribed as out-of-plane C-H wagging of PBT polymer, decreases in intensity in the first two copolymers and disappears completely in the case of the P3HT2BTCz composite. This effect is due to a significant interaction between donor Cz as donor and BT as acceptor acceptor groups. Thus, this analysis highlights the effective charge transfer in the main backbone of

In order to support the above discussed results further, the force constant analysis of benzothiadiazole unit in P3HTBT, PCzBT copolymers and P3HT2BTCz composite, have

Generally, the bond stretch depends on two main parameters, the bonding energy (E0) and the force constant k. The latter represents the potential energy surface (PES) curvature near

covalent bond [126]. Considering the BT unit as shown in Fig. 26, one can deduce that the bond length variation marks important changes in benzothiadiazole bonding, depending on the electronic configuration and hence force constant. One can also notice that the inter-ring force constants (F1 and F12) increase dramatically with a significant decrease in their corresponding bond length, leading to a more conjugated composite compared to P3HTBT and PCzBT. It clearly shows that the intra-ring delocalization is larger for P3HT2BTCz than for P3HTBT or PCzBT. In addition, the modifications on benzene moiety are clearly seen through the force constants F5 and F7. On the thiadiazole part, force constants such as F8, F9, F10 and F11 are very similar but a slight variation can be noticed in the case of the composite because of the presence of Cz and BT together in the main backbone structure with 3HT as spacer. This configuration can enhance the charge transfer between donor and acceptor

units. These observations are consistent with the above discussed properties.

���)����. The force constant (k) is proportional to the strength of the

these compounds targeted for photovoltaic applications.

been investigated as shown in Fig. 26.

the minimum�� � ����

are observed.

Applications in Polymer Light Emitting Diodes (PLEDs) and Polymer Photovoltaic Cells (PPCs) 133

**Table 10.** Main selected infrared modes of PBT, P3HTBT, PCzBT and P3HT2BTCz and their corresponding assignments (υ: frequency, I: intensity, s: strong, vs: very strong, m: medium, w: weak, vw: very weak).

**Figure 25.** Theoretical infrared spectra of: (a) PBT, (b) P3HTBT, (c) PCzBT and (d) P3HT2BTCz.

A large number of bands appear with very high peaks due to an induced strong dipolar moment. All characteristics of infrared bands in P3HT [125], PCz and BT vibration modes are observed.

132 Organic Light Emitting Devices

vw: very weak).

Infrared Absorbance (a.u.)

1234 vw 1234 w 1231 w 1224 w C-H Rocking (3HT + Cz) + C-H wagging (BT)

**- - - -** 1582 m **- -** Aromatic C-H and C-C stretching (Cz and BT).

**- -** 1350 w **- -** 1347 w CH3 Wagging (3HT). **- - - -** 1372 w 1372 **-** CH3 Scissoring (Cz).  **--** 1432 w 1429 w C=C stretching (Cz). 1445 s 1445 vw - - - - Ring vibration.

1792 m 1787 m **- -** - - C-C Bending (BT).

3330 vs 3310 S 3312 s - - C-H stretching (BT).

1514 vw 1516 m - - 1516 m Ring vibration + C-H Rocking.


 **--** 1792 s 1791 s Symmetric C-C stretching (Cz). **- - - -** 1841 m 1841 w C-N Scissoring and C-C stretching (Cz).

**Table 10.** Main selected infrared modes of PBT, P3HTBT, PCzBT and P3HT2BTCz and their

corresponding assignments (υ: frequency, I: intensity, s: strong, vs: very strong, m: medium, w: weak,

**(d)**

**(c)**

**(b)**

**(a)**

400 800 1200 1600 3200 3400

Wavenumber **(**cm

**Figure 25.** Theoretical infrared spectra of: (a) PBT, (b) P3HTBT, (c) PCzBT and (d) P3HT2BTCz.



The inspection of these spectra shows that after combining the two copolymers to obtain the P3HT2BTCz composite, some bands undergo slight changes in their positions and intensities. The main vibrational modes of PBT persist following the addition of 3HT, Cz groups in the P3HT2BTCz composite. Firstly, a down shift of the band assigned to C-H stretching in benzothiadiazole unit is observed at high frequencies with strong intensity located at 3330 cm-1 in (P3HTBT)4 and (PCzBT)4.. The band at 1785 cm-1 assigned to the antisymmetric C=C stretching mode becomes clear in the other PCzBT and P3HT2BTCz compounds and the C-C bending vibrational mode located at 1621 cm-1 in PBT becomes clearly pronounced in PCzBT and P3HT2BTCz with a high energy shift of about 20 cm-1.

The signal attributed to the S-N stretching at 1073 cm-1 completely disappears in the functionalized composite following a significant interaction of different groups. This effect is also confirmed by the shift (from 1486 to 1550 cm-1 and from 1443 to 1513 cm-1) observed in IR bands ascribed to symmetric and anti-symmetric C=C stretching, respectively, as a consequence of the presence of more conjugated backbone. The band at 707 cm-1, ascribed as out-of-plane C-H wagging of PBT polymer, decreases in intensity in the first two copolymers and disappears completely in the case of the P3HT2BTCz composite. This effect is due to a significant interaction between donor Cz as donor and BT as acceptor acceptor groups. Thus, this analysis highlights the effective charge transfer in the main backbone of these compounds targeted for photovoltaic applications.

In order to support the above discussed results further, the force constant analysis of benzothiadiazole unit in P3HTBT, PCzBT copolymers and P3HT2BTCz composite, have been investigated as shown in Fig. 26.

Generally, the bond stretch depends on two main parameters, the bonding energy (E0) and the force constant k. The latter represents the potential energy surface (PES) curvature near the minimum�� � ���� ���)����. The force constant (k) is proportional to the strength of the covalent bond [126]. Considering the BT unit as shown in Fig. 26, one can deduce that the bond length variation marks important changes in benzothiadiazole bonding, depending on the electronic configuration and hence force constant. One can also notice that the inter-ring force constants (F1 and F12) increase dramatically with a significant decrease in their corresponding bond length, leading to a more conjugated composite compared to P3HTBT and PCzBT. It clearly shows that the intra-ring delocalization is larger for P3HT2BTCz than for P3HTBT or PCzBT. In addition, the modifications on benzene moiety are clearly seen through the force constants F5 and F7. On the thiadiazole part, force constants such as F8, F9, F10 and F11 are very similar but a slight variation can be noticed in the case of the composite because of the presence of Cz and BT together in the main backbone structure with 3HT as spacer. This configuration can enhance the charge transfer between donor and acceptor units. These observations are consistent with the above discussed properties.

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

shown in Fig. 27, where the substituents in the case of PCBM play the role of the spacers between the donor and C60 acceptor units. This stable configuration is governed by the interaction of oxygen of the PCBM with the sulphur atom of thiophene units on both sides

**Figure 27.** Optimized geometric structure of P3HT2BTCz:PCBM (1:1) simulated by DFT/B3LYP/3-21G\*

Based on the comparison between the donor and the acceptor compounds, the resulting composite shows some interesting electronic properties, such as a low band gap of 1.93 eV and a lower HOMO energy level of -5.32 eV which indicate that this composite can be used as an active layer in photovoltaic cells. The corresponding structure of a photovoltaic device is schematically presented in Fig. 27. The difference in the LUMO energy levels of P3HT2BTCz and PCBM is close to 1.0 eV, suggesting that the photo-excited electron transfer from P3HT2BTCz to PCBM may be sufficiently efficient in photovoltaic devices [129-130]. Energetically, in comparing different anode and cathode metals as shown in Fig. 28 one can notice that the transparent ITO (Indium Tin Oxide) anode and Al, Ag or Mg (Aluminium, Silver or Magnesium) as cathode are the most suitable metals for for effective charge

of carbazole motives.

method.

collection on two electrodes.

Applications in Polymer Light Emitting Diodes (PLEDs) and Polymer Photovoltaic Cells (PPCs) 135

**Figure 26.** Main force constants and bond lengths of equivalents benzothiadiazole sites.

#### **4.2. Photovoltaic properties**

Favorable values of HOMO and LUMO levels, band gaps, and strong absorptions in the visible region suggest that the P3HT2BTCz may be used as active layer in PSCs devices when blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), which is the most broadly used acceptor in solar cell devices [127-128]. After an optimization procedure via the semi-empirical PM3 method, the resulting blend geometrical structure of the composite (P3HT2BTCz:PCBM) in weight ratio of 1:1 is then re-optimized by DFT/B3LYP/3-21G\* as shown in Fig. 27, where the substituents in the case of PCBM play the role of the spacers between the donor and C60 acceptor units. This stable configuration is governed by the interaction of oxygen of the PCBM with the sulphur atom of thiophene units on both sides of carbazole motives.

134 Organic Light Emitting Devices

1,30

4,0 4,5 5,0 5,5 6,0 6,5 7,0 7,5 8,0 8,5

**4.2. Photovoltaic properties** 

**Force constant (Millidyne/Angoström)**

1,35

1,40

1,45

1,50

**Bond length (Å)**

1,55

1,60

1,65

 PCzBT P3HTBT P3HT2BTCz

 PCzBT P3HTBT P3HT2BTCz

0 1 2 3 4 5 6 7 8 9 10 11 12 13

N S N

> **F4 F12 F8**

**Force index** 

Favorable values of HOMO and LUMO levels, band gaps, and strong absorptions in the visible region suggest that the P3HT2BTCz may be used as active layer in PSCs devices when blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), which is the most broadly used acceptor in solar cell devices [127-128]. After an optimization procedure via the semi-empirical PM3 method, the resulting blend geometrical structure of the composite (P3HT2BTCz:PCBM) in weight ratio of 1:1 is then re-optimized by DFT/B3LYP/3-21G\* as

**F6 F7 F5**

**F10 F9**

**F1 F2 F3**

**Figure 26.** Main force constants and bond lengths of equivalents benzothiadiazole sites.

**F11**

1,70

**Figure 27.** Optimized geometric structure of P3HT2BTCz:PCBM (1:1) simulated by DFT/B3LYP/3-21G\* method.

Based on the comparison between the donor and the acceptor compounds, the resulting composite shows some interesting electronic properties, such as a low band gap of 1.93 eV and a lower HOMO energy level of -5.32 eV which indicate that this composite can be used as an active layer in photovoltaic cells. The corresponding structure of a photovoltaic device is schematically presented in Fig. 27. The difference in the LUMO energy levels of P3HT2BTCz and PCBM is close to 1.0 eV, suggesting that the photo-excited electron transfer from P3HT2BTCz to PCBM may be sufficiently efficient in photovoltaic devices [129-130]. Energetically, in comparing different anode and cathode metals as shown in Fig. 28 one can notice that the transparent ITO (Indium Tin Oxide) anode and Al, Ag or Mg (Aluminium, Silver or Magnesium) as cathode are the most suitable metals for for effective charge collection on two electrodes.

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

The first part of this chapter shows how important it is to combine thienylene, dialkoxyphenylene and bipyridine fragments to obtain compounds with a strong electronic delocalization. As a result, analysis of the results obtained in the gas phase has allowed us to understand the crucial role played by the intra-molecular S--O and N--H interactions in determining the planarity of the compound. This leads to the formation of a donor–acceptor type of arrangement within the polymer backbone and an intra-molecular charge transfer for the TBT-BIPY copolymer model compound. In addition, we have presented the optical and emission properties of these compounds by studying the ground and first excited states

In the second part, we have used the density functional theory DFT/B3LYP to investigate the photo-physical properties of some copolymers in alternate donor-acceptor structure. In fact, the modification of chemical structures can greatly modulate and improve the electronic and optical properties of pristine copolymers. Hence, added to benzothiadiazole units, the introduction of carbazole motives in the copolymer backbone results in a better overlap of the absorption spectrum with the solar spectrum. In addition, the hexylthiophene linkage is found not only as a conjugated bridge but also it reduces the steric interaction between aromatic rings and thus enhances the effective charge transfer between donor and acceptor units.

In fact, the obtained theoretical data derived from DFT/B3LYP/3-21G\* method are in good agreement with the available experimental data. The resulting optimized BHJ active layer shows a -stacking configuration governed by a Wander walls interaction. A model energy band diagram is introduced, simulating the energy behaviour of this active layer. Based on this design concept, the PSC using the blend of P3HT2BTCz with fullerene derivatives, exhibit a promising performance with a PCE up to 5%. This approach provides great flexibility in fine-tuning of the absorption spectra and energy levels of the resultant

Finally, these results clearly indicate that these new compounds with alternating donoracceptor structures are promising materials for application in optoelectronic devices. Devices fabrication and characterization are in progress and will be published elsewhere.

*Research Unit: New Materials and Organic Electronic Devices (UR 11ES55), Faculty of Sciences of* 

*UMIM, Polydisciplinary Faculty of Taza, University Sidi Mohamed Ben Abdellah, Taza, Morocco* 

This work has been supported by the cooperation Tunisio-Marocain (CMPTM No.

11/TM/72) and Tunisian-French cooperative action (CMCU/07G1309).

of copolymer models.

**Author details** 

M. Bouachrine

 \*

**Acknowledgement** 

Corresponding Author

S. Ayachi, A. Mabrouk and K. Alimi\*

*Monastir, University of Monastir, Tunisia* 

polymers for achieving high device performance.

Applications in Polymer Light Emitting Diodes (PLEDs) and Polymer Photovoltaic Cells (PPCs) 137

**Figure 28.** Schematic energy diagram of the proposed bulk heterojunction solar cell.

The photovoltaic efficiency performance data of the photovoltaic cell (power conversion efficiency (PCE) values, including the open circuit voltage (Voc), short circuit current (Jsc), fill factor (FF) and incident-light power (Pin), are derived from the following equation: PCE = Voc. Jsc. FF/Pin. The maximum open circuit voltage (Voc) of the BHJ solar cell is related to the difference between HOMO of the electron donor and LUMO of the electron acceptor, taking into account the energy lost during the photo-charge generation [131-133]. The theoretical values of open-circuit voltage Voc have been calculated from the following expression [134]:

$$V\_{OC} = |E(HOMO)^{donor}| - |E(LUMO)^{acceptor}| - 0.3$$

Based on this formula, it can be seen that the Voc value of P3HT2BTCz: PCBM is about 0.97 V but it depends on the difference of the output of the electrodes [135]. Starting from the above results, P3HT2BTCz composite seems to be a good candidate for photovoltaic application due to its high Voc and wider absorption range broader than the range of absorption of other copolymers. Based on Scharber model [136], the maximum power-conversion efficiency of the photovoltaic solar cell, with P3HT2BTCz:PC61BM (1:1) composite as active layer can be up to 5%.
