*3.3.3. Electronic transitions*

116 Organic Light Emitting Devices

predicting the optical and emission properties.

By combining the experimental data (optical band gap and Raman frequencies) with DFT calculations, two units of TBT-BIPY copolymer were considered as model structure for

For better understanding of the optical and emission processes, we have firstly computed the bond lengths of the ground and excited states, where the changes of bond lengths can be compared. The values of bond lengths for the 2-TBT-BIPY copolymer, in their ground- and excited-states are shown in Fig. 10. It can be seen that some bond lengths increase and some decrease in the excited state. Furthermore, we find that all the bond lengths of two bipyridine as well as those of C-O-C are shortened. Whereas, in the left TBT unit, the C-C single bond of thiophene rings as well as that connecting the thiophene ring to phenylene and bipyridine rings increase. In addition, in the second TBT unit, double bonds of thiophene rings and single/double bonds of substituted phenylene rings also increase.

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70

S

3738 39 40 42 41

**Figure 10.** Bond length variation of ground (a) and excited (b) states of 2-TBT-BIPY copolymer as well as the difference in bond length between the excited and ground states (in Å) (c). The horizontal axis labels represent the bonds between adjacent atoms in the numbering sequence shown in Figure from

<sup>64</sup> S

Label of Bonds

<sup>43</sup><sup>44</sup> 45 46 47 48 49

52 53

O

CH3

50 51

O

H3C

N <sup>25</sup>

54 55 56 57 <sup>59</sup> <sup>58</sup> <sup>60</sup> <sup>61</sup>

S

62 63

65 66

N

<sup>67</sup><sup>68</sup> <sup>70</sup> 69

> 71 72 73

**(a)**

**(b)**

**(c)**

1,4 1,5 1,6 1,7 1,8

27 28 29

N

<sup>30</sup> <sup>31</sup> 32 33 34 35 36

N

1,3 1,4 1,5 1,6 1,7 1,8



Bond Length (Å)

13 14

O

H3C

17 18 19 20 <sup>22</sup> <sup>21</sup> <sup>23</sup><sup>24</sup> <sup>26</sup>

S

1 2 3 4 5 <sup>6</sup> <sup>7</sup> 8 9 10 11 12

the bottom.

15 16

O

CH3

Bond Length (Å)

Bond Length Difference (Å)

We have applied a variety of theoretical approaches, including CIS/3-21G\*, TD-B3LYP/3- 21G\* and ZINDO methods to study the optical and emission properties of TBT-BIPY copolymers. The theoretical results thus obtained are compared with the experimental ones. All the energy levels calculated using the Time Dependent Density Functional Theory (TD-DFT), the CIS/3-21G\* and the semi-empirical quantum-chemical ZINDO are used to predict the optical absorption and emission spectra of the ground (S0) and first excited (S1) optimized structures. The assignment of electronic transitions and their oscillator strengths are also calculated using these three methods.

From theoretical calculations, the wavelength of transitions from the ground to the first excited state (S0S1) and from the first excited state to ground state (S1S0) having the largest oscillator strength as well as their corresponding molecular orbital character for 2- TBT-BIPY are listed in Table 3. The corresponding experimental optical absorption and emission wavelengths measured in TBT-BIPY copolymer in chloroform solution are also listed in the same table. Clarke et al [112] suggest that the importance of the HOMO-LUMO transition may be easily understood from the spectral distribution of molecular orbitals. Accordingly, to a first approximation, a significant overlap found between HOMO and LUMO implies an intense transition between HOMO to LUMO and vice-versa. Here, the vertical S0S1 transition dominates the HL excitation by 60-81%.


(H=HOMO, L=LUMO, L+1=LUMO+1, etc.), f: Oscillator strength

**Table 3.** The vertical transition energies (nm) and their oscillator strengths of absorption from the ground to the first excited state (S0S1) and emission from the first excited to ground (S1S0) states of TBT-BIPY copolymers calculated by CIS/3-21G\*, TD//B3LYP/3-21G\* and ZINDO methods.

#### *3.3.4. Frontier molecular orbitals (HOMO and LUMO)*

To gain insight into the excitation properties and the ability of electron or hole transport, we have shown in Fig. 11 HOMO and LUMO together known as frontier molecular orbitals which contribute significantly to the electronic transitions between the ground and excited states in 2-TBT-BIPY.

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

orbitals is rather delocalized over the molecule. Changing to the excited state geometry, they

A schematic representation for the intra-molecular charge transfer (CT) in the ground and excited states of 2-TBT-BIPYcopolymer, calculated as the average of the summation of Mulliken charge distribution of the TBT and BIPY units, is displayed in Fig.12. In general, intra-molecular charge transfer is generated through the alternating donor-acceptor conjugated systems [117]. From this figure, we think that the alternating TBT (positively charged) and BIPY (negatively charged) can be used as donor and acceptor, respectively. We have separately examined their HOMO and LUMO levels, which indicates that for the TBT unit, the HOMO is at -4.29 eV and the LUMO at -1,29 eV and for bipyridine unit we get -6.52 eV for the HOMO and -1.33 eV for the LUMO. Although the LUMO levels for both are quite similar, a weak intra-molecular charge transfer in these molecules can established. Based on the comparison between ground and excited-state geometries for 2-TBT-BIPY, we deduce that the charge distributions are predominantly restricted to the substituted phenylene and

**Figure 12.** Illustration of the 2-TBT-BIPY copolymer structure with Mulliken charges distributions for TBT and BIPY units at the ground and excited states. All segments presented with dotted line separate

We can also predict the geometrical structure changes between the ground (S0) and singlet excited (S1) from the molecular orbitals. Therefore, to better understand the excitation

the sub-units involved in the copolymer structure.

become more localized.

thiophene units.

*3.3.5. Mulliken charge distribution for TBT-BIPY* 

Applications in Polymer Light Emitting Diodes (PLEDs) and Polymer Photovoltaic Cells (PPCs) 119

**Figure 11.** Contour plots for the HOMO and LUMO molecular orbitals which contribute significantly to the electronic transitions in 2-TBT-BIPY copolymers: (a) absorption from ground to excited and (b) emission from excited-to ground states.

We have examined and found that the presence of methoxy side chain does not have a significant effect on the molecular orbital distribution. In the HOMO, the C=C segments are -bonding and have anti-bonding character with respect to their neighboring C=C units. Whereas, in the case of LUMO, the C=C units are anti-bonding and bonding in the bridge single bond. In general, excitation of a -electron from HOMO to LUMO leads to increase the localization of electron density on the acceptor part of the molecule. Here, the promotion of one electron from HOMO to LUMO is explained by the frontier molecular orbital. For the TBT-BIPY copolymer, the LUMO favors the inter-ring mobility of electrons, while the HOMO only promotes the intra-ring mobility of electrons [116]. As outlined before, in the excited state of copolymer, both the HOMO and LUMO frontier molecular orbitals topology are significantly affected, particularly in the left TBT unit indicating their contribution to the excitation processes. In fact, in the ground state, the spatial distribution of the molecular 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) 119

orbitals is rather delocalized over the molecule. Changing to the excited state geometry, they become more localized.

#### *3.3.5. Mulliken charge distribution for TBT-BIPY*

118 Organic Light Emitting Devices

states in 2-TBT-BIPY.

emission from excited-to ground states.

*3.3.4. Frontier molecular orbitals (HOMO and LUMO)* 

To gain insight into the excitation properties and the ability of electron or hole transport, we have shown in Fig. 11 HOMO and LUMO together known as frontier molecular orbitals which contribute significantly to the electronic transitions between the ground and excited

> **HOMO 237a**

**HOMO-1 236a**

**HOMO-2 235a**

**(a) (b)**

**Figure 11.** Contour plots for the HOMO and LUMO molecular orbitals which contribute significantly to the electronic transitions in 2-TBT-BIPY copolymers: (a) absorption from ground to excited and (b)

We have examined and found that the presence of methoxy side chain does not have a significant effect on the molecular orbital distribution. In the HOMO, the C=C segments are -bonding and have anti-bonding character with respect to their neighboring C=C units. Whereas, in the case of LUMO, the C=C units are anti-bonding and bonding in the bridge single bond. In general, excitation of a -electron from HOMO to LUMO leads to increase the localization of electron density on the acceptor part of the molecule. Here, the promotion of one electron from HOMO to LUMO is explained by the frontier molecular orbital. For the TBT-BIPY copolymer, the LUMO favors the inter-ring mobility of electrons, while the HOMO only promotes the intra-ring mobility of electrons [116]. As outlined before, in the excited state of copolymer, both the HOMO and LUMO frontier molecular orbitals topology are significantly affected, particularly in the left TBT unit indicating their contribution to the excitation processes. In fact, in the ground state, the spatial distribution of the molecular

**LUMO 238a**

**LUMO+1 239a**

**LUMO+2 240a**

A schematic representation for the intra-molecular charge transfer (CT) in the ground and excited states of 2-TBT-BIPYcopolymer, calculated as the average of the summation of Mulliken charge distribution of the TBT and BIPY units, is displayed in Fig.12. In general, intra-molecular charge transfer is generated through the alternating donor-acceptor conjugated systems [117]. From this figure, we think that the alternating TBT (positively charged) and BIPY (negatively charged) can be used as donor and acceptor, respectively. We have separately examined their HOMO and LUMO levels, which indicates that for the TBT unit, the HOMO is at -4.29 eV and the LUMO at -1,29 eV and for bipyridine unit we get -6.52 eV for the HOMO and -1.33 eV for the LUMO. Although the LUMO levels for both are quite similar, a weak intra-molecular charge transfer in these molecules can established. Based on the comparison between ground and excited-state geometries for 2-TBT-BIPY, we deduce that the charge distributions are predominantly restricted to the substituted phenylene and thiophene units.

**Figure 12.** Illustration of the 2-TBT-BIPY copolymer structure with Mulliken charges distributions for TBT and BIPY units at the ground and excited states. All segments presented with dotted line separate the sub-units involved in the copolymer structure.

We can also predict the geometrical structure changes between the ground (S0) and singlet excited (S1) from the molecular orbitals. Therefore, to better understand the excitation

process in TBT-BIPY copolymer, we have investigated the molecular orbitals involved in the electronic transition.

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

**300 350 400 450 500 550 600**

PL

Wavelength (nm)

**Figure 13.** The simulated optical absorption and emission spectra of 2-TBT-BIPY copolymer with CIS/3-

Ue

*ES ER*

EVE

*GS ER*

**Figure 14.** Schematic representation of the potential energy surface (PES) of the ground (Ug) and excited (Ue) states along with their normal mode coordinates of 2-TBT-BIPY copolymer calculated by CIS/3-21G\* (a) and TD//B3LYP/3-21G\* (b) methods. The parameters indicated are the absorption energy

> ��� �� ��).

Ug

398,7

Abs

481,6

**(b)** 508,8

120000 519,2 **(c)** 439,4 Abs

**(a)** 357,2

Abs

21G\* (a), TD-B3LYP/3-21G\* (b) and ZINDO (c) methods.

0,785

EVA

**(a)**

0,979


(EVA), emission energy (EVE) and relaxation energy ���






Energy (eV)

0 1 2

Molar absorbance (mol-1.L.cm-1)

Applications in Polymer Light Emitting Diodes (PLEDs) and Polymer Photovoltaic Cells (PPCs) 121

PL

PL

Ue

**(b)**

Normal Coordinate, q




U g

PL Intensity (arb. unit)

#### *3.3.6. Simulated optical and emission spectra for TBT-BIPY copolymer*

In Fig.13, we have depicted the simulated results of the optical absorption and emission spectra for 2-TBT-BIPY copolymer using the above three methods. To select the accurate method for predicting these optical properties, we show the potential energy surface (PES) of the ground (Ug) and excited (Ue) states along with their normal coordinates for 2-TBT-BIPY copolymer in Fig. 14 and Table 4. In Fig. 14, the two potential energies surfaces (PES) are plotted along with their normal coordinate q and the absorption and fluorescence spectra obtained from the transition between these two PES, using CIS/3-21G\* and TD-DFT, respectively. The optical absorption energy (EVA), emission energy (EVE) and the relaxation energy (�� ��� �� ��) are presented in Table 5. The Stokes shift (SS), which is defined as the difference between the absorption and emission energies (EVA-EVE), is usually related with the band widths of both the absorption and emission bands [118] and it is a measure of the energy loss due to the molecular relaxation. It can be expressed as: �� = �� �� � �� �� = ��� −EVE. From the results given in table 5, we show that the SS calculated by CIS/3-21 G\* is about two times higher than that calculated by TD-DFT. Accordingly, due to the neglect of the effects of electron correlation and higher order excitations, the geometrical relaxation after the excitation contributes much to the Stokes shift calculated by CIS/3-21 G\*. It is well known that the absorption energy (EVA) is usually considered to be maximum in the absorption spectrum, but it must be corrected for the zero-point vibrational energy (ZPE). In our case, compared with the results given in Table 4, SS energies calculated by CIS/3-21G\* and TD-DFT methods as given in Table 5 deviate only by 0.084 eV and 0.088 eV, respectively. This difference of about 0.08 eV probably represents the value that needs to be used to correct the theoretical data. By such correction to the experimental value an excellent agreement is obtained with the result calculated by ZINDO method as shown in Table 4 for 2-TBT-BIPY copolymer.


**Table 4.** The optical absorption energy (EVA), emission energy (EVE), relaxation energy ��� ��� �� ��) and Stokes shift (SS) calculated by CIS/3-21G\* and TD//B3LYP/3-21G\* methods for 2-TBT-BIPY.

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) 121

120 Organic Light Emitting Devices

electronic transition.

relaxation energy (��

2-TBT-BIPY copolymer.

��

��

��� ��

process in TBT-BIPY copolymer, we have investigated the molecular orbitals involved in the

In Fig.13, we have depicted the simulated results of the optical absorption and emission spectra for 2-TBT-BIPY copolymer using the above three methods. To select the accurate method for predicting these optical properties, we show the potential energy surface (PES) of the ground (Ug) and excited (Ue) states along with their normal coordinates for 2-TBT-BIPY copolymer in Fig. 14 and Table 4. In Fig. 14, the two potential energies surfaces (PES) are plotted along with their normal coordinate q and the absorption and fluorescence spectra obtained from the transition between these two PES, using CIS/3-21G\* and TD-DFT, respectively. The optical absorption energy (EVA), emission energy (EVE) and the

as the difference between the absorption and emission energies (EVA-EVE), is usually related with the band widths of both the absorption and emission bands [118] and it is a measure of

��� −EVE. From the results given in table 5, we show that the SS calculated by CIS/3-21 G\* is about two times higher than that calculated by TD-DFT. Accordingly, due to the neglect of the effects of electron correlation and higher order excitations, the geometrical relaxation after the excitation contributes much to the Stokes shift calculated by CIS/3-21 G\*. It is well known that the absorption energy (EVA) is usually considered to be maximum in the absorption spectrum, but it must be corrected for the zero-point vibrational energy (ZPE). In our case, compared with the results given in Table 4, SS energies calculated by CIS/3-21G\* and TD-DFT methods as given in Table 5 deviate only by 0.084 eV and 0.088 eV, respectively. This difference of about 0.08 eV probably represents the value that needs to be used to correct the theoretical data. By such correction to the experimental value an excellent agreement is obtained with the result calculated by ZINDO method as shown in Table 4 for

CIS/3-21G\* TD//B3LYP/3-21G\*

EVA (eV) 7.613 2.899 EVE (eV) 7.169 2.681

��(eV) 0.25 0.099

��(eV) 0.194 0.119 SS (eV) 0.444 0.218

**Table 4.** The optical absorption energy (EVA), emission energy (EVE), relaxation energy ���

Stokes shift (SS) calculated by CIS/3-21G\* and TD//B3LYP/3-21G\* methods for 2-TBT-BIPY.

the energy loss due to the molecular relaxation. It can be expressed as: �� = ��

��) are presented in Table 5. The Stokes shift (SS), which is defined

�� � ��

��� ��

��) and

�� =

*3.3.6. Simulated optical and emission spectra for TBT-BIPY copolymer* 

**Figure 13.** The simulated optical absorption and emission spectra of 2-TBT-BIPY copolymer with CIS/3- 21G\* (a), TD-B3LYP/3-21G\* (b) and ZINDO (c) methods.

Normal Coordinate, q

**Figure 14.** Schematic representation of the potential energy surface (PES) of the ground (Ug) and excited (Ue) states along with their normal mode coordinates of 2-TBT-BIPY copolymer calculated by CIS/3-21G\* (a) and TD//B3LYP/3-21G\* (b) methods. The parameters indicated are the absorption energy (EVA), emission energy (EVE) and relaxation energy ��� ��� �� ��).

For understanding better the results optical absorption and emission spectra calculated by ZINDO method experimental results are presented in Fig. 15. All curves are normalized to unity at their respective maximum. Prior to comparing the results calculated by ZINDO with those of obtained from experiments, it may be noted that no solvent effects have been taken into account in the ZINDO calculation. Keeping this in mind and comparing the spectra shapes, we believe that ZINDO results are in agreement with the experimental ones.

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

light. For this electrode ITO coated glass substrates are frequently used. As for the counter

The energy barriers between the emitting polymer and electrodes can be estimated by comparing the work function of the electrodes with HOMO and LUMO energy levels of emitting polymer. Thus, the hole-injection barrier is Eh = EHOMO-4.8 eV, where 4.8 eV is the work function of the ITO anode and the electron-injection barrier is Ee = X - ELUMO, where X is the work function of cathode. The difference between the electron- and hole-injection barriers (Ee-Eh) is a useful parameter to evaluate the balance in electron and hole injection. Lower the (Ee-Eh) better the injection balance of electrons and holes from the cathode and anode, respectively. For TBT-BIPY copolymer, we have shown in Fig.16 the energy level

HOMO


**Figure 16.** Energy-level diagrams of a single-layer PLED (ITO/TBT-BIPY/AL, Mg or Ca).

**Table 5.** Parameters to evaluating the balance in electron and hole injections in PLED.

ITO "Anode" TBT-BIPY X "Cathode"

The ionization potentials (IP) and electron affinity (EA) are calculated by DFT/B3LYP/3- 21G\*on the geometry of the neutral, cationic and anionic states to estimate the energy barrier for the injection of both holes and electrons into TBT-BIPY copolymer. The calculated values are obtained as 5.62 eV and 1.35 eV, respectively. From Table 5, we showed that low work function metals such as Mg or Ca are typically used to minimize the barrier and then to

> X X (eV) Eh (eV) Ee (eV) Ee-Eh (eV) Al 4.2 0.12 2.05 1.93 Mg 3.6 0.12 1.45 1.33 Ca 2.8 0.12 0.65 0.53

LUMO

Eg = 2.77 eV *Ee*

*Electron Injection*

*<sup>X</sup>*

Vacuum

*IP*


electrode aluminum is used mostly.

diagrams of a single-layer PLED.

*ITO*

provide for an ohmic contact.

*Eh*

*Hole Injection*

Applications in Polymer Light Emitting Diodes (PLEDs) and Polymer Photovoltaic Cells (PPCs) 123

**Figure 15.** Normalized experimental optical absorption and photoluminescence spectra of TBT-BIPY copolymer () and those calculated by ZINDO method for 2-TBT-BIPY copolymer (- - -).

### **3.4. PLEDs architecture**

In general, conjugated organic materials have smaller hole injection barriers than electron injection barriers due to the electron richness in a -conjugated system, leading to poor electron transport ability in these materials. There are two possible approaches to improve this poor electron-transporting ability in organic materials used for fabricating LEDs. The most straightforward modification is to deposit a low work function (WF) metals such as Mg or Ca as cathode by high vacuum sublimation. However, the sensitivity of these metals towards oxygen and moisture limits their practical applications. The other more practical approach is to design or invent a material with lower LUMO energy by increasing its electron affinity, so that LUMO is to WF of the cathode material.

The electron injection energy barrier (Ee) is determined by the electron affinity (EA) or by the difference between LUMO and WF of the cathode (c), while the hole injection energy barrier (Eh) is determined by the difference between IP or HOMO and WF of the anode (a). In the most simple case, a single organic layer OLED, the organic layer is sandwiched between two electrodes of different work functions, one of which has to be transparent to light. For this electrode ITO coated glass substrates are frequently used. As for the counter electrode aluminum is used mostly.

122 Organic Light Emitting Devices

0,0

**3.4. PLEDs architecture** 

0,2

0,4

Normalized Absorbance

0,6

0,8

1,0

For understanding better the results optical absorption and emission spectra calculated by ZINDO method experimental results are presented in Fig. 15. All curves are normalized to unity at their respective maximum. Prior to comparing the results calculated by ZINDO with those of obtained from experiments, it may be noted that no solvent effects have been taken into account in the ZINDO calculation. Keeping this in mind and comparing the spectra shapes, we believe that ZINDO results are in agreement with the experimental ones.

Abs PL

350 400 450 500 550 600 650 700

Wavelength (nm)

In general, conjugated organic materials have smaller hole injection barriers than electron injection barriers due to the electron richness in a -conjugated system, leading to poor electron transport ability in these materials. There are two possible approaches to improve this poor electron-transporting ability in organic materials used for fabricating LEDs. The most straightforward modification is to deposit a low work function (WF) metals such as Mg or Ca as cathode by high vacuum sublimation. However, the sensitivity of these metals towards oxygen and moisture limits their practical applications. The other more practical approach is to design or invent a material with lower LUMO energy by increasing its

The electron injection energy barrier (Ee) is determined by the electron affinity (EA) or by the difference between LUMO and WF of the cathode (c), while the hole injection energy barrier (Eh) is determined by the difference between IP or HOMO and WF of the anode (a). In the most simple case, a single organic layer OLED, the organic layer is sandwiched between two electrodes of different work functions, one of which has to be transparent to

**Figure 15.** Normalized experimental optical absorption and photoluminescence spectra of TBT-BIPY

copolymer () and those calculated by ZINDO method for 2-TBT-BIPY copolymer (- - -).

electron affinity, so that LUMO is to WF of the cathode material.

0,0

0,2

0,4

0,6

Normalized PL Intensity

0,8

1,0

The energy barriers between the emitting polymer and electrodes can be estimated by comparing the work function of the electrodes with HOMO and LUMO energy levels of emitting polymer. Thus, the hole-injection barrier is Eh = EHOMO-4.8 eV, where 4.8 eV is the work function of the ITO anode and the electron-injection barrier is Ee = X - ELUMO, where X is the work function of cathode. The difference between the electron- and hole-injection barriers (Ee-Eh) is a useful parameter to evaluate the balance in electron and hole injection. Lower the (Ee-Eh) better the injection balance of electrons and holes from the cathode and anode, respectively. For TBT-BIPY copolymer, we have shown in Fig.16 the energy level diagrams of a single-layer PLED.

**Figure 16.** Energy-level diagrams of a single-layer PLED (ITO/TBT-BIPY/AL, Mg or Ca).

The ionization potentials (IP) and electron affinity (EA) are calculated by DFT/B3LYP/3- 21G\*on the geometry of the neutral, cationic and anionic states to estimate the energy barrier for the injection of both holes and electrons into TBT-BIPY copolymer. The calculated values are obtained as 5.62 eV and 1.35 eV, respectively. From Table 5, we showed that low work function metals such as Mg or Ca are typically used to minimize the barrier and then to provide for an ohmic contact.


**Table 5.** Parameters to evaluating the balance in electron and hole injections in PLED.
