**1. Introduction**

104 Organic Light Emitting Devices

[73] Suyama T, Okuno Y, Matsuda T. Enhancement of TM-TE mode Conversion Caused by Excitation of Surface Plasmons on a Metal Grating and its Application for Refractive

[74] Feng J, Okamoto T, Kawata S. Enhancement of Electroluminescence Through a Twodimensional Corrugated Metal Film by Grating-induced Surface-plasmon Cross

[75] Reinke N. A, Ackermann C, Brütting W. Light Extraction via Leaky Modes in Organic

Index Measurement. Prog. Electromagn. Res. 2007;72: 91-103.

Light Emitting Devices. Opt. Commun. 2006;266: 191-197.

Coupling. Opt. Lett. 2005;30: 2302-2304.

Organic semiconductors hold the combined properties of inorganic semiconductors such as silicon and more desirable properties of plastics [1,2]. Since, the inception of the field of plastic electronics, various organic semiconductors including conjugated polymers and small molecules have been synthesized, studied, and applied to optoelectronic semiconductor device structures in order to improve efficiency, reduce cost or realize new applications that are difficult to achieve with silicon-based technology [3,4].

Recently, the exploitation of polymer as an active layer in organic electronic displays has received a particular attention. In this direction, greater efforts have been devoted to seek new possibilities for use in optoelectronic devices such as Polymer Light Emitting Diodes (PLEDs) [5-11], Polymer Photovoltaic Cells (PPCs) [12-23] and Polymer Field Effect Transistors (PFET) [24-33]. The field of PLEDs is still an active research area since the first conjugated conducting or semiconducting polymeric material, poly(p-phenylene-vinylene) (PPV), was reported by Burroughes et al. in 1990 [34]. In fact, only polymers can enable manufacturing of large-area light-emitting displays. These electronic devices need special polymers with specific and adapted properties. Since then, there have been increasing interests and research activities in synthesis and design of new polymeric materials for organic electronic devices. However, their properties and those of the related devices are still poorly understood.

© 2012 Alimi et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Alimi et al., licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

One of the requirements for efficient PLEDs is balanced charge injection from the two electrodes and efficient transport of both holes and electrons within luminescent layer in the device structure [35]. More recently, much effort has been devoted to develop wide band gap conjugated polymers for application in light emitting diodes. Then, a number of conjugated polymers including poly(p-phenylene-vinylene) (PPV) [36,37], poly(pphenylene) (PPP) [38-41], polythiophene (PT) [42,43] and polyfluorene (PF) [44] have been widely used as light-emitting materials in devices. However, one major problem with these polymers is that they are -excessive in nature and hence are much better at accepting and transporting holes than electrons. Another series of polymers containing -deficient heterocycles like pyridine [45] and oxadiazoles [46] show greater tendency to transport electrons than holes [47].

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

In this context, two new alternating donor-acceptor conjugated copolymers, both of which may be used in organic electronics, are investigated here. The first one is a copolymer containing thienylene-dioctyloxyphenyle-thienylene (TBT) and bipyridine (BIPY) units as shown in Fig. 1 that can be used as an active layer in PLEDs. It is constructed with dioctyloxy substituted phenylene incorporated between two electron-rich-thiophene units, abbreviated as TBT unit, and a bipyridine (BIPY) unit (Fig. 1). It was obtained by the Stille reaction method and the detailed synthesis procedures and characterization have already been reported [74,75]. The soluble copolymer has a well-defined structure and exhibits excellent optical properties. The number average (Mn) and weight average (Mw) molecular weights of the copolymer, determined by gel permeation chromatography (GPC) using polystyrene as standard, are obtained as 3098 and 3477, respectively. The corresponding polymerization degree, DPn, is found to be 5 corresponding to 25 cycles of number. Photophysical properties of copolymer including Raman scattering, UV-Visible optical absorption

<sup>S</sup> <sup>S</sup> <sup>N</sup> <sup>N</sup>

TBT BIPY

Introducing long alkoxy pendants at 2 and 5 positions of the phenyl ring improves the solvent processability which is a prerequisite for fabricating organic light-emitting diodes

The second part of this chapter concerns a composite based on Benzothiadiazole mixed with carbazole, or hexylthiophene that can be used for fabricating Polymer Solar Cells (PSCs). PSCs based on the bulk heterojunction (BHJ) structure have attracted broad attentions in recent years [76,77]. The requirements for the structure and properties of polymeric donors are low band gap, broad absorption range, high mobility and appropriate HOMO and LUMO levels [78]. Among the polymers tested for suitability as an active layer, poly(3 hexyl-thiophene) (P3HT) and poly(carbazole) (PCz) have emerged as promising candidates for applications in optoelectronic devices because of their exceptional properties [79,80]. However, alternative copolymers of [2,1,3]-benzothiadiazole (BT) acceptor units with various donor units have attracted particular attention for using them in high performance PSCs [81-83]. To optimize the material properties, conjugated polymers with alternating electron-rich and hole-rich units along their backbone have been extensively developed because their absorption spectra and band gap can be readily tuned by controlling the intra-

However, in these linear D-A polymers, the molecular interactions and packing orientation of the conjugating moieties need to be carefully controlled to ensure proper process ability

n m

OC8H17

C8H17O

molecular charge transfer (ICT) from donors to acceptors [84].

**Figure 1.** Chemical structure of TBT-BIPY copolymer.

(OLEDs) by the spin coating method.

and emission are studied.

Applications in Polymer Light Emitting Diodes (PLEDs) and Polymer Photovoltaic Cells (PPCs) 107

To tune the emission properties of PLEDs, sophisticated control of the polymer luminescence color, efficiency, and charge transport properties are required. The emission wavelength depends on the extent of conjugation/delocalization, and can be controlled by the modification of the configuration or conformation of the polymer and by interactions with the local environment [48,49]. This can be achieved by grafting functional moieties such as electron donor or acceptor groups, which allow the modulation of the electronic structure of the conjugated backbone [50,51]. Donor–acceptor (D–A) organic molecules are among the most important conjugated polymers, that produce low bad gap useful in technological fields novel materials, by adjusting the HOMO and LUMO levels [52-54]. The low optical band gaps of the compounds should result by alternating the electron-rich unit of donor segments and the strong electron-deficient unit of acceptor segments in the structure. Then, superior transport properties in organic materials can be achieved with planar and highly conjugated chains [55-57]. Many investigations have proven that conjugated D-A type polymers play important roles in their balanced charge transporting properties and show unique optical properties. The HOMO and LUMO energy levels of these systems are important for understanding charge injection processes in the luminescent devices [58-60].

On the other hand, due to their interesting electrical, optical and optoelectronic properties, conjugated oligomers represent a prominent class of compounds from the viewpoint of theory, synthesis, and applications in materials science [61-65]. Moreover, they are model compounds for the corresponding polymers [66,67]. In parallel to recent experimental work on oligomers, theoretical efforts have also begun complementing the experimental studies in the characterization of the nature and the properties of their ground- and lowest electronic excited states [68-73]. In addition, these approaches have provided significant insight into the electronic and optical properties of conjugated polymers. In the absence of structural information, the experimental measurement, in conjunction with molecular orbital theory, is a valuable tool in analyzing the electronic structure of polymers. This enables an estimate not only of the relative energies of the electronic levels but also of their detailed distribution over the whole molecule. The ionization Potential (IP), electron affinity (EA), molecular electronic structure of the ground and lowest excited states as well as the nature of absorption and photoluminescence obtained through quantum calculations are of great interest prior to fabricating organic devices.

In this context, two new alternating donor-acceptor conjugated copolymers, both of which may be used in organic electronics, are investigated here. The first one is a copolymer containing thienylene-dioctyloxyphenyle-thienylene (TBT) and bipyridine (BIPY) units as shown in Fig. 1 that can be used as an active layer in PLEDs. It is constructed with dioctyloxy substituted phenylene incorporated between two electron-rich-thiophene units, abbreviated as TBT unit, and a bipyridine (BIPY) unit (Fig. 1). It was obtained by the Stille reaction method and the detailed synthesis procedures and characterization have already been reported [74,75]. The soluble copolymer has a well-defined structure and exhibits excellent optical properties. The number average (Mn) and weight average (Mw) molecular weights of the copolymer, determined by gel permeation chromatography (GPC) using polystyrene as standard, are obtained as 3098 and 3477, respectively. The corresponding polymerization degree, DPn, is found to be 5 corresponding to 25 cycles of number. Photophysical properties of copolymer including Raman scattering, UV-Visible optical absorption and emission are studied.

**Figure 1.** Chemical structure of TBT-BIPY copolymer.

106 Organic Light Emitting Devices

than holes [47].

devices [58-60].

interest prior to fabricating organic devices.

One of the requirements for efficient PLEDs is balanced charge injection from the two electrodes and efficient transport of both holes and electrons within luminescent layer in the device structure [35]. More recently, much effort has been devoted to develop wide band gap conjugated polymers for application in light emitting diodes. Then, a number of conjugated polymers including poly(p-phenylene-vinylene) (PPV) [36,37], poly(pphenylene) (PPP) [38-41], polythiophene (PT) [42,43] and polyfluorene (PF) [44] have been widely used as light-emitting materials in devices. However, one major problem with these polymers is that they are -excessive in nature and hence are much better at accepting and transporting holes than electrons. Another series of polymers containing -deficient heterocycles like pyridine [45] and oxadiazoles [46] show greater tendency to transport electrons

To tune the emission properties of PLEDs, sophisticated control of the polymer luminescence color, efficiency, and charge transport properties are required. The emission wavelength depends on the extent of conjugation/delocalization, and can be controlled by the modification of the configuration or conformation of the polymer and by interactions with the local environment [48,49]. This can be achieved by grafting functional moieties such as electron donor or acceptor groups, which allow the modulation of the electronic structure of the conjugated backbone [50,51]. Donor–acceptor (D–A) organic molecules are among the most important conjugated polymers, that produce low bad gap useful in technological fields novel materials, by adjusting the HOMO and LUMO levels [52-54]. The low optical band gaps of the compounds should result by alternating the electron-rich unit of donor segments and the strong electron-deficient unit of acceptor segments in the structure. Then, superior transport properties in organic materials can be achieved with planar and highly conjugated chains [55-57]. Many investigations have proven that conjugated D-A type polymers play important roles in their balanced charge transporting properties and show unique optical properties. The HOMO and LUMO energy levels of these systems are important for understanding charge injection processes in the luminescent

On the other hand, due to their interesting electrical, optical and optoelectronic properties, conjugated oligomers represent a prominent class of compounds from the viewpoint of theory, synthesis, and applications in materials science [61-65]. Moreover, they are model compounds for the corresponding polymers [66,67]. In parallel to recent experimental work on oligomers, theoretical efforts have also begun complementing the experimental studies in the characterization of the nature and the properties of their ground- and lowest electronic excited states [68-73]. In addition, these approaches have provided significant insight into the electronic and optical properties of conjugated polymers. In the absence of structural information, the experimental measurement, in conjunction with molecular orbital theory, is a valuable tool in analyzing the electronic structure of polymers. This enables an estimate not only of the relative energies of the electronic levels but also of their detailed distribution over the whole molecule. The ionization Potential (IP), electron affinity (EA), molecular electronic structure of the ground and lowest excited states as well as the nature of absorption and photoluminescence obtained through quantum calculations are of great Introducing long alkoxy pendants at 2 and 5 positions of the phenyl ring improves the solvent processability which is a prerequisite for fabricating organic light-emitting diodes (OLEDs) by the spin coating method.

The second part of this chapter concerns a composite based on Benzothiadiazole mixed with carbazole, or hexylthiophene that can be used for fabricating Polymer Solar Cells (PSCs). PSCs based on the bulk heterojunction (BHJ) structure have attracted broad attentions in recent years [76,77]. The requirements for the structure and properties of polymeric donors are low band gap, broad absorption range, high mobility and appropriate HOMO and LUMO levels [78]. Among the polymers tested for suitability as an active layer, poly(3 hexyl-thiophene) (P3HT) and poly(carbazole) (PCz) have emerged as promising candidates for applications in optoelectronic devices because of their exceptional properties [79,80]. However, alternative copolymers of [2,1,3]-benzothiadiazole (BT) acceptor units with various donor units have attracted particular attention for using them in high performance PSCs [81-83]. To optimize the material properties, conjugated polymers with alternating electron-rich and hole-rich units along their backbone have been extensively developed because their absorption spectra and band gap can be readily tuned by controlling the intramolecular charge transfer (ICT) from donors to acceptors [84].

However, in these linear D-A polymers, the molecular interactions and packing orientation of the conjugating moieties need to be carefully controlled to ensure proper process ability and charge transporting properties [85]. A fundamental understanding of the ultimate relations between structure and properties of these materials is necessary for using them in photovoltaic cells. A number of studies demonstrate that the interplay between theory and experiment is very important in providing useful insights in understanding the molecular electronic structure of the ground and excited states as well as the nature of absorption and photoluminescence [86]. To rationalize our theoretical results, the simulated data are compared with the available experimental data [87].

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

better understanding of the relationship between the structure and resulting properties. Finally, the parameters that influence the photovoltaic efficiency are elucidated. We think that the presented study of structural, electronic, optical, and charge transfer properties for this compound will help the design more efficient functional photovoltaic copolymers.

The objective of the presented result here is not to develop or optimize any applications, but to understand why and how the combined theoretical and experimental studies on copolymers can be conducted in developing optimized Polymer Light Emitting Diodes

All molecular calculations are performed in the gas phase using Density Functional Theory (DFT) implemented in the GAUSSIAN (03) program [91]. We have used the B3LYP (Becke three-parameter Lee-Yang-Parr) exchange correlation functional [92,93] with 3-21G\* and 6- 31G\* as basis sets. In the first part, the calculation of conformational characteristics has been done by varying the torsion angle in steps of 20° from = 0° to = 180°. For each increment, the dihedral angle is held fixed while the remainder of the molecule is optimized. The energy differences in electronic states are always calculated relative to the corresponding absolute minimum conformation and then the relative potential energy surfaces are drawn. In the optimization procedure of these compounds, the alkyl chains at the N-9 positions of carbazole (Cz) motifs and dioctyloxy groups in TBT-BIPY copolymer are replaced by methyl and methoxy groups, respectively. This has been proven that the presence of alkyl/alkoxy groups does not significantly affect the equilibrium geometry and hence the electronic and the optical properties [94]. Hexyl groups in 3HT motifs are then replaced by methyl groups. The optimization of the composite (P3HT2BTCz: PCBM) is done in two steps. First optimization with PM3 semi-empirical method was carried out, then the resulting structure was re-optimized by DFT/B3LYP/3-21G\* to find the equilibrium geometrical structure.

Optical absorption spectra are calculated using the Time-Dependant Density Functional Theory (TDDFT) [95] based on optimized ground state geometries [96]. Theoretically the transition energies and their respective intensities in a given configuration interaction (CI)

**S S S**

**S N**

**C6H13 C6H13 N**

**Figure 3.** Chemical structure of P3HT2BTCz compound.

(PLEDs) and Photovoltaic Cells (PPCs).

**2. Theoretical methodology** 

Applications in Polymer Light Emitting Diodes (PLEDs) and Polymer Photovoltaic Cells (PPCs) 109

**N**

**C C6H13 6H13**

**S**

In what follows, we elucidate the photophysical properties of the benzothiadiazole derivative compounds with structures as shown in Fig. 2 (a,b). These two D-A polymers provide a basis for a more comprehensive study of the backbone ring, heteroatom and fused ring effects on polymer properties. Therefore, it is of practical significance to extend our previous work to a comprehensive theoretical investigation on these two types of BTDbased derivatives. Moreover, poly(3-hexyl-thiophene) (P3HT) units have relative higher charge mobility in comparison with other conjugated polymers and have been widely used as π-conjugating spacers [88,89]. Its insertion in the polymer backbone serves the dual purpose of transporting carriers and providing sites for exciton dissociation [90]. Moreover, the incorporation of electron-withdrawing moieties (3HT) as side chains leads to some useful properties which can further widen the absorption spectrum.

**Figure 2.** Chemical structure of compounds under study: (a): P3HTBT, (b): PCzBT.

Recently, the conjugated P3HT2BTCz compound, built as carbazole-thiophenebenzothiadiazole, has been copolymerized onto the backbone of the copolymer as shown in Fig. 3. This compound has been synthesized and experimentally characterized, using only photoluminescence and optical absorption spectroscopy. Their related intense and broad absorption bands as well as favorable excited-state energy levels make them good candidates for fabricating PSCs. Thus, if P3HT2BTCz compound is blended with [6,6] phenyl-C61-bytric acid methyl ester (PCBM) fullerene derivative into BHJ photovoltaic devices [87], then the conversion efficiency may be increased.

Here further investigations of geometrical parameters, electronic structures, photo-physical and vibrational properties of these compounds are carried out, on the basis of quantumchemical calculations, providing a reasonable interpretation of the experimental results and better understanding of the relationship between the structure and resulting properties. Finally, the parameters that influence the photovoltaic efficiency are elucidated. We think that the presented study of structural, electronic, optical, and charge transfer properties for this compound will help the design more efficient functional photovoltaic copolymers.

**Figure 3.** Chemical structure of P3HT2BTCz compound.

The objective of the presented result here is not to develop or optimize any applications, but to understand why and how the combined theoretical and experimental studies on copolymers can be conducted in developing optimized Polymer Light Emitting Diodes (PLEDs) and Photovoltaic Cells (PPCs).
