**2.3 Characteristics of bulk heterojunction solar cells**

Conjugated polymer thin films sandwiched between two metal electrode are usually described using a metal-insulator-metal (MIM) picture (Parker, 1994). The different operating regimes the MIM device due to externally applied voltages is shown in Figure 3. As illustrated in Figure 3(a), the vacuum levels (*E*vac) of the stacked materials shall align

themselves (Shottky-Mott model).

Figure 3(a) indicates the energy diagram of a bulk heterojunction solar cell in open circuit condition. The *E*vac of the different materials are aligned as explained above, and no electrical field is present within the device. Figure 3 (b) represents the short circuit condition. The Fermi levels of the two electrodes align themselves and a built-in field appears in the bulk, resulting in a constant slope for the HOMO and LUMO levels of the donor and acceptor (respectively, HD, LD, HA, and LA) and for the *E*vac.

absorption photons, they can diffuse over a length of approximately 5-15 nm (Haugeneder et al., 1999). Since the exciton diffusion lengths in conjugated polymers are less than the photon absorption length, the efficiency of a bilayer cell is limited by the number of photons that can be absorbed within the effective exciton diffusion range at the polymer/electron interface. This limits drastically the photocurrent and hence the overall efficiency of the organic bilayer solar cells. To overcome this limitation, the surface area of the donor/acceptor interface needs to be increased. This can be achieved by creating a mixture of donor and acceptor materials with a nanoscale phase separation resulting in a threedimensional interpenetrating network: the "bulk heterojunction solar cells" (Figure 2).

Fig. 2. (a) Schematic device structure and (b) energy diagram for an organic bulk

diffuse and/or be driven to the respective electrodes (Dennler & Sariciftci, 2005).

donor and acceptor (respectively, HD, LD, HA, and LA) and for the *E*vac.

**2.3 Characteristics of bulk heterojunction solar cells** 

themselves (Shottky-Mott model).

The discovery of 1-(3-methoxycarbonyl)propyl-1-phenyl[6]C61 (PCBM) (Hummelen et al., 1995), a soluble and processable derivative of fullerene C60, allowed the realization of the first organic bulk heterojunction solar cell by blending it with poly(2-methoxy-5-(2'-ethyl-hexoxy)- 1,4-phenylene-vinylene) (MEH-PPV) (Yu et. Al., 1995). Figure 2(b) demonstrates the schematic energy diagram of an organic bulk heterojunction solar cell. Contrary to Figure (1b), excitons experience dissociation wherever they are generated within the bulk. Indeed, the next interface between donor and acceptor phases is present within the exciton diffusion length everywhere in the device. After having been generated throughout the bulk, the free carriers have to

Conjugated polymer thin films sandwiched between two metal electrode are usually described using a metal-insulator-metal (MIM) picture (Parker, 1994). The different operating regimes the MIM device due to externally applied voltages is shown in Figure 3. As illustrated in Figure 3(a), the vacuum levels (*E*vac) of the stacked materials shall align

Figure 3(a) indicates the energy diagram of a bulk heterojunction solar cell in open circuit condition. The *E*vac of the different materials are aligned as explained above, and no electrical field is present within the device. Figure 3 (b) represents the short circuit condition. The Fermi levels of the two electrodes align themselves and a built-in field appears in the bulk, resulting in a constant slope for the HOMO and LUMO levels of the

heterojunction solar cell

Fig. 3. MIM picture for a polymer diode under different operating modes. (a) open circuit condition, (b) short circuit condition, (c) forward bias, (d) reverse bias.

When polarized in the forward direction (high work function electrode (ITO) connected to (+) and low work function electrode (Al) connected to (-)) as in Figure 3 (c), electrons can be injected from the Al electrode to ITO electrode and holes from ITO electrode to Al electrode. The effective field in the device will ensure the drift of electrons from Al electrode to ITO electrode and hole from ITO electrode to Al electrode. Finally, when the device is polarized in the reverse direction (ITO connected to (-) and Al connected to (+)) (Figure 3 (d), charge injection is hindered by the field present in the device (Dennler & Sariciftci, 2005).

Organic Bulk Heterojunction Solar Cells Based on Poly(*p*-Phenylene-Vinylene) Derivatives 421

well as tunability of their optical and electronic properties through chemical modifications. Since the first report of electroluminescence from PPV, a great research attention has been focused on these types conjugated polymers (Burroughes et al., 1990). This focus was moreover up heaved after the discovery of an ultrafast photoinduced charge transfer from alkoxy-substituted PPV to the buckminsterfullerene (Sariciftci et al., 1992). PPV and its derivatives remain the most popular conjugated polymers for this application and continue to generate considerable interest and much research for photovoltaic applications (Cheng et

The pure PPV is insoluble, intractable, and infusible and therefore difficult to process. Solution processability is desirable as it allows polymeric materials to be solution cast as thin films for various applications. A general methodology to overcome this problem is to develop a synthetic route that involves a solution-processable polymer precursor. First synthetic route for high quality PPV films with high molecular weights was first introduced by Wessling (Figure 5) allowed the synthesis of soluble precursors, which can be processed into thin films prior to thermal conversion to PPV (Wessling & Zimmerman, 1968; Wessling, 1985). A potential drawback of the precursor routes is the limited control over poly-

Some of the drawbacks of this precursor approach include the generation of toxic side products during the solid state elimination process, structural defects arising from incomplete thermal conversion or oxidation, and undefined molecular weights and distribution (Papadimitrakopoulos et al., 1994). By incorporating long alkyl or alkoxy chains into the phenylene ring before polymerization to ensure the solubility, a one step approach can be applied to make processable PPV derivatives which can then be cast into thin films directly without conversion for device fabrication (Braun & Heeger, 1991). To date, the most widely used method for the preparation of PPV derivatives is the Gilch route (Gilch, 1966). A typical Gilch route to the synthesis of a representative solution-processable poly(2 methoxy-5-((2′-ethylhexyl)oxy)-1,4-phenylene-vinylene) (MEH-PPV, **P2**) is represented in Figure 6 (Neef & Ferraris, 2000). By following the same synthetic route, poly(2-methoxy-5- ((3′,7′-dimethyloctyl)oxy)-1,4-phenylenevinylene) (MDMO-PPV, **P3**) can also be synthesized

dispersity and molecular weight of the resulting polymer.

Fig. 5. Synthesis of PPV **(P1)** via the Wessling Route

al., 2009).

Fig. 4. First and fourth quadrant of a typical *J-V* curve observed for a Glass/ITO/PEDOT:PSS/MDMO-PPV:PCBM(1:4)/Al solar cell. Shown are the short circuit current (*I*SC), the open circuit voltage (*V*OC), the current (*I*mpp) and voltage (*V*mpp) at the maximum power point (*P*max)

Solar cells are operated between open circuit and short circuit condition (fourth quadrant in the current-voltage characteristics), as shown in Figure 4. In the dark, there is almost no current flowing, until the contacts start to inject heavily at forward bias for voltages larger than the open circuit voltage. Under illumination, the current flows in the opposite direction than the injected currents. The overall efficiency of a solar cell can be expressed by the following formula:

$$\eta = \frac{V\_{oc}.I\_{sc}.FF}{P\_{\text{as}}} \tag{1}$$

where *VOC* is the open circuit voltage, *SC I* is the short circuit current, and *Pin* is the incident light power. The fill factor ( *FF* ) is given by

$$FF = \frac{I\_{mp}.V\_{mp}}{V\_{oc}.I\_{sc}}\tag{2}$$

where *mpp I* and *Vmpp* represent the current and voltage at the maximum power point ( *P*max ) in the fourth quadrant, respectively (Figure 4).

### **3.** *p***-phenylene-vinylene based conjugated polymers**

#### **3.1 Poly(***p***-phenylene-vinylene) and its derivatives**

Poly(*p*-phenylene-vinylene)s (PPVs) and its derivatives are one of the most promising classes of conjugated polymers for organic solar cells due to their ease of processability as

Fig. 4. First and fourth quadrant of a typical *J-V* curve observed for a

maximum power point (*P*max)

light power. The fill factor ( *FF* ) is given by

the fourth quadrant, respectively (Figure 4).

**3.** *p***-phenylene-vinylene based conjugated polymers** 

**3.1 Poly(***p***-phenylene-vinylene) and its derivatives** 

following formula:

Glass/ITO/PEDOT:PSS/MDMO-PPV:PCBM(1:4)/Al solar cell. Shown are the short circuit current (*I*SC), the open circuit voltage (*V*OC), the current (*I*mpp) and voltage (*V*mpp) at the

Solar cells are operated between open circuit and short circuit condition (fourth quadrant in the current-voltage characteristics), as shown in Figure 4. In the dark, there is almost no current flowing, until the contacts start to inject heavily at forward bias for voltages larger than the open circuit voltage. Under illumination, the current flows in the opposite direction than the injected currents. The overall efficiency of a solar cell can be expressed by the

> . . *OC SC in V I FF P*

where *VOC* is the open circuit voltage, *SC I* is the short circuit current, and *Pin* is the incident

where *mpp I* and *Vmpp* represent the current and voltage at the maximum power point ( *P*max ) in

Poly(*p*-phenylene-vinylene)s (PPVs) and its derivatives are one of the most promising classes of conjugated polymers for organic solar cells due to their ease of processability as

. . *mpp mpp OC SC I V*

(1)

*V I* (2)

*FF*

well as tunability of their optical and electronic properties through chemical modifications. Since the first report of electroluminescence from PPV, a great research attention has been focused on these types conjugated polymers (Burroughes et al., 1990). This focus was moreover up heaved after the discovery of an ultrafast photoinduced charge transfer from alkoxy-substituted PPV to the buckminsterfullerene (Sariciftci et al., 1992). PPV and its derivatives remain the most popular conjugated polymers for this application and continue to generate considerable interest and much research for photovoltaic applications (Cheng et al., 2009).

The pure PPV is insoluble, intractable, and infusible and therefore difficult to process. Solution processability is desirable as it allows polymeric materials to be solution cast as thin films for various applications. A general methodology to overcome this problem is to develop a synthetic route that involves a solution-processable polymer precursor. First synthetic route for high quality PPV films with high molecular weights was first introduced by Wessling (Figure 5) allowed the synthesis of soluble precursors, which can be processed into thin films prior to thermal conversion to PPV (Wessling & Zimmerman, 1968; Wessling, 1985). A potential drawback of the precursor routes is the limited control over polydispersity and molecular weight of the resulting polymer.

Fig. 5. Synthesis of PPV **(P1)** via the Wessling Route

Some of the drawbacks of this precursor approach include the generation of toxic side products during the solid state elimination process, structural defects arising from incomplete thermal conversion or oxidation, and undefined molecular weights and distribution (Papadimitrakopoulos et al., 1994). By incorporating long alkyl or alkoxy chains into the phenylene ring before polymerization to ensure the solubility, a one step approach can be applied to make processable PPV derivatives which can then be cast into thin films directly without conversion for device fabrication (Braun & Heeger, 1991). To date, the most widely used method for the preparation of PPV derivatives is the Gilch route (Gilch, 1966). A typical Gilch route to the synthesis of a representative solution-processable poly(2 methoxy-5-((2′-ethylhexyl)oxy)-1,4-phenylene-vinylene) (MEH-PPV, **P2**) is represented in Figure 6 (Neef & Ferraris, 2000). By following the same synthetic route, poly(2-methoxy-5- ((3′,7′-dimethyloctyl)oxy)-1,4-phenylenevinylene) (MDMO-PPV, **P3**) can also be synthesized

Organic Bulk Heterojunction Solar Cells Based on Poly(*p*-Phenylene-Vinylene) Derivatives 423

Fig. 8. Chemical structures of PPV and its different alkoxy-substituted derivatives

Fig. 9. Chemical structure of BBL **P4**

The substituent on the phenylene ring of PPV allows for the tuning of the band gap. Enhanced electron affinity of substituted PPVs is reflected by higher absorption coefficients, lower-lying HOMO (highest occupied molecular orbital) levels, and higher open circuit voltages from fabricated organic solar cells. The HOMO and LUMO levels of unsubstituted PPV are reported to be ca. -5.1 and -2.7 eV, respectively, with a band gap around 2.4 eV. Introducing two alkoxy groups into the phenylene ring to perturb the molecular orbitals lowers the LUMO to -2.9 eV with an essentially unchanged HOMO level. Hence, the band gap is reduced to 2.2 eV (Alam & Jenekhe, 2002). As a consequence, PPV emits a green-yellow light, while MEH-PPV exhibits a yellow-orange emission. Because PPV derivatives are the earliest conjugated polymers developed for organic electronics application, they were also frequently used as the active materials in organic bilayer solar cells before the concept of bulk heterojunction configuration was widely accepted. For organic bilayer solar cells, PPV and MEH-PPV serve as the electron donor in conjunction with a poly(benzimidazobenzophenanthroline ladder) (BBL, **P4**) as the electron acceptor (Figure 9). The photovoltaic parameters of the bilayer device with the configuration ITO/PPV/**P4**/Al showed a *J*sc of 2.3 mA/cm2, a *V*oc of 1.06 V, an FF of 47%, and power conversion efficiency (PCE) value of 1.4%. In case of using MEH-PPV as the electron donor, the device reached a PCE of 0.8% under the same conditions (Alam & Jenekhe, 2004). The better photovoltaic performance of PPV over MEH-PPV can be accredited to the greater crystallinity and structural order of the PPV main chain compared to alkylated MEH-PPV.

N N

**P4**

O O

N

n

(Figure 7). This route involves mild polymerization conditions, and the molecular weights of the polymers obtained are generally high (Cheng et al., 2009).

Fig. 6. Synthesis of MEH-PPV (**P2**) via the Gilch Route

Fig. 7. Chemical structure of MDMO-PPV (**P3**)

Solution-processable PPV derivatives were first reported by Wudl et al. (Wudl et al., 1991; Braun & Heeger, 1991) and by Ohnishi et al. (Doi et al., 1993). The solubility of the materials was achieved by grafting of long alkoxy chains, which cause some conformational mobility of the polymers. Consequently the soluble derivatives have lower glass transition temperatures than pure PPV. Poly[(2,5-dialkoxy-1,4-phenylene)-vinylene]s including long alkoxy side chains are soluble in common organic solvents such as chloroform, toluene, chlorobenzene, dichloromethane, tetrahydrofuran. Fig. 8 shows the chemical structures of PPV and its various alkoxy-substituted derivatives. The solubility increases from left to right, whereby the solubility of the much studied MEH-PPV and MDMO-PPV is enhanced by the branched nature of their side chains (Braun & Heeger, 1991).

PPV derivatives are predominantly hole conducting materials with high-lying LUMO (lowest unoccupied molecular orbital) levels. The unbalanced charge carrier transport properties and the relatively high barrier for electron injection from electrode metals such as aluminium limit the efficiency of the photovoltaic devices.

Several approaches have been explored to improve the electron affinity of PPVs. The insertion of weak electron-withdrawing triple bonds (―C≡C―) within the PPV backbone can be regarded as one of the most original approach of enhancing the electron affinity of PPVs Brizius et al., 2000; Egbe et al., 2001).

(Figure 7). This route involves mild polymerization conditions, and the molecular weights of

O

Br

Solution-processable PPV derivatives were first reported by Wudl et al. (Wudl et al., 1991; Braun & Heeger, 1991) and by Ohnishi et al. (Doi et al., 1993). The solubility of the materials was achieved by grafting of long alkoxy chains, which cause some conformational mobility of the polymers. Consequently the soluble derivatives have lower glass transition temperatures than pure PPV. Poly[(2,5-dialkoxy-1,4-phenylene)-vinylene]s including long alkoxy side chains are soluble in common organic solvents such as chloroform, toluene, chlorobenzene, dichloromethane, tetrahydrofuran. Fig. 8 shows the chemical structures of PPV and its various alkoxy-substituted derivatives. The solubility increases from left to right, whereby the solubility of the much studied MEH-PPV and MDMO-PPV is enhanced

n

excess *t*-BuOK THF

MeO

O

MEH-PPV **(P2)**

n

O

MDMO-PPV **(P3)**

PPV derivatives are predominantly hole conducting materials with high-lying LUMO (lowest unoccupied molecular orbital) levels. The unbalanced charge carrier transport properties and the relatively high barrier for electron injection from electrode metals such as

Several approaches have been explored to improve the electron affinity of PPVs. The insertion of weak electron-withdrawing triple bonds (―C≡C―) within the PPV backbone can be regarded as one of the most original approach of enhancing the electron affinity of PPVs

the polymers obtained are generally high (Cheng et al., 2009).

Br

Fig. 6. Synthesis of MEH-PPV (**P2**) via the Gilch Route

MeO

(CH2O)n HBr

MeO

O

Fig. 7. Chemical structure of MDMO-PPV (**P3**)

by the branched nature of their side chains (Braun & Heeger, 1991).

MeO

aluminium limit the efficiency of the photovoltaic devices.

Brizius et al., 2000; Egbe et al., 2001).

Fig. 8. Chemical structures of PPV and its different alkoxy-substituted derivatives

The substituent on the phenylene ring of PPV allows for the tuning of the band gap. Enhanced electron affinity of substituted PPVs is reflected by higher absorption coefficients, lower-lying HOMO (highest occupied molecular orbital) levels, and higher open circuit voltages from fabricated organic solar cells. The HOMO and LUMO levels of unsubstituted PPV are reported to be ca. -5.1 and -2.7 eV, respectively, with a band gap around 2.4 eV. Introducing two alkoxy groups into the phenylene ring to perturb the molecular orbitals lowers the LUMO to -2.9 eV with an essentially unchanged HOMO level. Hence, the band gap is reduced to 2.2 eV (Alam & Jenekhe, 2002). As a consequence, PPV emits a green-yellow light, while MEH-PPV exhibits a yellow-orange emission. Because PPV derivatives are the earliest conjugated polymers developed for organic electronics application, they were also frequently used as the active materials in organic bilayer solar cells before the concept of bulk heterojunction configuration was widely accepted. For organic bilayer solar cells, PPV and MEH-PPV serve as the electron donor in conjunction with a poly(benzimidazobenzophenanthroline ladder) (BBL, **P4**) as the electron acceptor (Figure 9). The photovoltaic parameters of the bilayer device with the configuration ITO/PPV/**P4**/Al showed a *J*sc of 2.3 mA/cm2, a *V*oc of 1.06 V, an FF of 47%, and power conversion efficiency (PCE) value of 1.4%. In case of using MEH-PPV as the electron donor, the device reached a PCE of 0.8% under the same conditions (Alam & Jenekhe, 2004). The better photovoltaic performance of PPV over MEH-PPV can be accredited to the greater crystallinity and structural order of the PPV main chain compared to alkylated MEH-PPV.

Fig. 9. Chemical structure of BBL **P4**

Organic Bulk Heterojunction Solar Cells Based on Poly(*p*-Phenylene-Vinylene) Derivatives 425

*t*-BuOK

Hexane OHC P(OEt)3

MeO

OR

Br

OR

MeO <sup>n</sup>

regioregular MDMO-PPV **(P7)**

MeO

RO

regiorandom MDMO-PPV **(P8)**

n

OMe

n

150oC

Fig. 11. Synthesis of regioregular and regiorandom MDMO-PPVs (**(P7)** and **(P8)**)

EtO

MeO

P O

OR

P OEt O

EtO OEt OR

*t*-BuOK

photovoltaic device performance with regioregular MDMO-PPV (Cheng et al., 2009).

1.8%, respectively.

Br

R = 3,7-dimethyloctyl

OHC

OHC

MeO

MeO

OR

CO +

MeO

OR

OR

O

P OEt OEt

Br

MnO2

Miscellaneous physics and engineering aspects have been investigated for devices based on the MDMO-PPV/PCBM bulk heterojunction active layer system: photooxidation (Pacios et al., 2006), stacked cells (Kawano et al., 2006), active layer thickness (Lenes et al., 2006), NMR morphology studies (Mens et al., 2008), and insertion of a hole-transporting layer between PEDOT and the active layer (Park et al., 2007). Besides the PCBM organic acceptor, inorganic electron acceptors (van Hal et al., 2003; Beek et al., 2005; Boucle et al., 2007; Sun et al., 2003) such as metal oxides or quantum dots are also under active development and have been combined with MDMO-PPV to prepare hybrid (organic-inorganic) bulk heterojunction solar cells. Optimized photovoltaic devices using blends of MDMO-PPV:ZnO (Beek et al., 2005) or MDMO-PPV:cadmium selenide (Sun et al., 2003) showed moderate PCE values of 1.6% and

Regioregularity in MDMO-PPV also plays an important role in determining the photovoltaic device performance. A fully regioregular MDMO-PPV (**P7**) by the Wittig-Hornor reaction of a single monomer comprised of aldehyde and phosphonate functionalities was synthesized (Tajima et al., 2008). Regiorandom MDMO-PPV (**P8**), from dialdehyde and diphosphnate monomers, was also prepared for comparison (Figure 11). The device achieved a PCE of 3.1%, a *J*sc of 6.2 mA/cm2, a *V*oc of 0.71 V, and an FF of 70% with regioregular MDMO-PPV (**P7**). This is the highest efficiency reported for the PPV:PCBM system so far. But the device based on regiorandom MDMO-PPV (**P8**)/PCBM only achieved a PCE of 1.7%. It is concluded that higher crystallinity of the polymer for higher hole mobility and better mixing morphology between the polymer and PCBM contribute to the improvement of

Soluble MEH-PPV was also combined with PCBM for organic solar cell applications. The organic bilayer solar cell with the configuration ITO/PEDOT:PSS/MEH-PPV/PCBM/Al showed a *J*sc of 2.1 mA/cm2, a *V*oc of 0.75 V, an FF of 23%, and a PCE value of 0.46% (Zhang et al., 2002). An organic bulk heterojunction solar cell including MEH-PPV/PCBM as the photoactive layer showed better PCE values in the range 1.1-1.3% than bilayer solar cells. Furthermore, by stacking two independent single organic solar cells together with the help of the transparent cathode LiF/Al/Au, the PCE of the multiple-device stacked structure can be dramatically improved to 2.6% (Shrotriya et al., 2006). The devices can be stacked together and connected either in parallel or in series, resulting in doubled *J*sc or *V*oc, respectively, compared to those of a single device (Cheng et al., 2009).

Fig. 10. Chemical structures of C60 derivatives **P5** and **P6** 

Two alternative soluble methanofullerene derivatives **P5** and **P6** have been developed to serve as electron acceptors and combined with MEH-PPV to produce organic solar cells. The chemical structures of the C60 derivatives are shown in Figure 10. Due to a better compatibility of **P5** with MEH-PPV, the MEH-PPV/**P5** system shows a better device PCE of 0.49% than the MEH-PPV/**P6** system, which has a PCE of 0.22% (Li et al., 2002). In addition to C60 derivatives, different types of titanium oxide (TiO2) were blended with MEH-PPV for photovoltaic device applications (Breeze et al., 2001; Song et al., 2005; Wei et al., 2006; Neyshtadt et al., 2008; Shim et al., 2008). However, their device performances were generally low, with PCE values lower than 0.5% (Cheng et al., 2009).

In organic bulk heterojunction solar cells, MDMO-PPV is the most widely used PPV derivative to serve as the electron donor in combination with C60 electron acceptor derivatives. Organic solar cells based on combined MDMO-PPV:PCBM (1:4, w/w) were fabricated by Shaheen and co-workers (Shaheen et al., 2001). It was found that when chlorobenzene was used as the casting solvent instead of toluene to deposit the active layer, an optimal morphology with suppressed phase segregation and enhanced microstructure was obtained, resulting in increased charge carrier mobility for both holes and electrons in the active layer. And this device achieved a *J*sc of 5.23 mA/cm2, a *V*oc of 0.82 V, and a high PCE of 2.5% (Shaheen et al., 2001).

Soluble MEH-PPV was also combined with PCBM for organic solar cell applications. The organic bilayer solar cell with the configuration ITO/PEDOT:PSS/MEH-PPV/PCBM/Al showed a *J*sc of 2.1 mA/cm2, a *V*oc of 0.75 V, an FF of 23%, and a PCE value of 0.46% (Zhang et al., 2002). An organic bulk heterojunction solar cell including MEH-PPV/PCBM as the photoactive layer showed better PCE values in the range 1.1-1.3% than bilayer solar cells. Furthermore, by stacking two independent single organic solar cells together with the help of the transparent cathode LiF/Al/Au, the PCE of the multiple-device stacked structure can be dramatically improved to 2.6% (Shrotriya et al., 2006). The devices can be stacked together and connected either in parallel or in series, resulting in doubled *J*sc or *V*oc,

O

O

O

O

O

O

O

O

O

O

Two alternative soluble methanofullerene derivatives **P5** and **P6** have been developed to serve as electron acceptors and combined with MEH-PPV to produce organic solar cells. The chemical structures of the C60 derivatives are shown in Figure 10. Due to a better compatibility of **P5** with MEH-PPV, the MEH-PPV/**P5** system shows a better device PCE of 0.49% than the MEH-PPV/**P6** system, which has a PCE of 0.22% (Li et al., 2002). In addition to C60 derivatives, different types of titanium oxide (TiO2) were blended with MEH-PPV for photovoltaic device applications (Breeze et al., 2001; Song et al., 2005; Wei et al., 2006; Neyshtadt et al., 2008; Shim et al., 2008). However, their device performances were generally

**P6**

In organic bulk heterojunction solar cells, MDMO-PPV is the most widely used PPV derivative to serve as the electron donor in combination with C60 electron acceptor derivatives. Organic solar cells based on combined MDMO-PPV:PCBM (1:4, w/w) were fabricated by Shaheen and co-workers (Shaheen et al., 2001). It was found that when chlorobenzene was used as the casting solvent instead of toluene to deposit the active layer, an optimal morphology with suppressed phase segregation and enhanced microstructure was obtained, resulting in increased charge carrier mobility for both holes and electrons in the active layer. And this device achieved a *J*sc of 5.23 mA/cm2, a *V*oc of 0.82 V, and a high

respectively, compared to those of a single device (Cheng et al., 2009).

O

O O

O

O O

O

**P5**

O

O

O

Fig. 10. Chemical structures of C60 derivatives **P5** and **P6** 

low, with PCE values lower than 0.5% (Cheng et al., 2009).

PCE of 2.5% (Shaheen et al., 2001).

Fig. 11. Synthesis of regioregular and regiorandom MDMO-PPVs (**(P7)** and **(P8)**)

Regioregularity in MDMO-PPV also plays an important role in determining the photovoltaic device performance. A fully regioregular MDMO-PPV (**P7**) by the Wittig-Hornor reaction of a single monomer comprised of aldehyde and phosphonate functionalities was synthesized (Tajima et al., 2008). Regiorandom MDMO-PPV (**P8**), from dialdehyde and diphosphnate monomers, was also prepared for comparison (Figure 11). The device achieved a PCE of 3.1%, a *J*sc of 6.2 mA/cm2, a *V*oc of 0.71 V, and an FF of 70% with regioregular MDMO-PPV (**P7**). This is the highest efficiency reported for the PPV:PCBM system so far. But the device based on regiorandom MDMO-PPV (**P8**)/PCBM only achieved a PCE of 1.7%. It is concluded that higher crystallinity of the polymer for higher hole mobility and better mixing morphology between the polymer and PCBM contribute to the improvement of photovoltaic device performance with regioregular MDMO-PPV (Cheng et al., 2009).

Miscellaneous physics and engineering aspects have been investigated for devices based on the MDMO-PPV/PCBM bulk heterojunction active layer system: photooxidation (Pacios et al., 2006), stacked cells (Kawano et al., 2006), active layer thickness (Lenes et al., 2006), NMR morphology studies (Mens et al., 2008), and insertion of a hole-transporting layer between PEDOT and the active layer (Park et al., 2007). Besides the PCBM organic acceptor, inorganic electron acceptors (van Hal et al., 2003; Beek et al., 2005; Boucle et al., 2007; Sun et al., 2003) such as metal oxides or quantum dots are also under active development and have been combined with MDMO-PPV to prepare hybrid (organic-inorganic) bulk heterojunction solar cells. Optimized photovoltaic devices using blends of MDMO-PPV:ZnO (Beek et al., 2005) or MDMO-PPV:cadmium selenide (Sun et al., 2003) showed moderate PCE values of 1.6% and 1.8%, respectively.

Organic Bulk Heterojunction Solar Cells Based on Poly(*p*-Phenylene-Vinylene) Derivatives 427

These monomers were all prepared by Knoevenagel condensations to construct cyanovinylene linkages. The electron-rich nature of thiophene units in these polymers makes them good candidates to serve as electron donors in organic bulk heterojunction solar cells. For example, both **P11**/PCBM- and **P12**/PCBM-based solar cells achieved a PCE of around 0.14%. Optimization of these devices by thermal annealing showed a slight increase of PCE to 0.19%. Reynold et al. also reported synthesizing a range of CN-PPV derivatives (**P14-P17)** containing dioxythiophene moieties in the polymer main chain (Figure 14) (Thompson et al., 2005; Thompson et al., 2006). The best photovoltaic device based on these

H13C6 C6H13

S

O O

NC

O O

S

n

CN

S

n

O O

H13C6 C6H13

**P15**

NC

**P17**

S

O O

CN-PPV derivatives with PCBM as the active layer achieved a PCE of 0.4%.

CN

n

O

n

S

C6H13

Fig. 14. Chemical structures of **P14-P17** 

**P16**

H25C12O

H13C6 C6H13

S

NC

OC12H25 <sup>O</sup> <sup>O</sup>

H13C6 C6H13

S

O O

NC

**P14**

H25C12O

OC12H25

wt%)/LiF/Al (Cheng et al., 2009).

**3.3 Acetylene-substituted poly(***p***-phenylene-vinylene)s** 

CN

O

H13C6

Acetylene-substituted PPV derivatives can be synthesized via the Wittig – Horner Reaction (Figure 15). The coplanar and rigid nature of the acetylene moiety in the polymer chain may have the potential to obtain a higher degree of packing and thus improve the photovoltaic performance of such devices. Having coplanar electron-rich anthracene units and triple bond bridges, **P18** exhibits broader absorption, a lower HOMO level, and a smaller optical band gap of 1.9 eV, compared to MDMO-PPV. A device with the configuration of ITO/PEDOT/**P18**:PCBM (1:2, w/w)/LiF/Al, achieved a PCE value of up to 2% with a high *V*oc of 0.81 V. Figure 16 shows the chemical structures of a series of acetylene-substituted PPV derivatives synthesized by similar procedures. For polymers **P21** and **P22** (Egbe et al., 2007), the introduction of a thiophene ring into the polymer backbone showed an improvement in the PCE ranging from 1.2% to 1.7%. This is higher than those for **P19** and **P20** (Egbe et al., 2005), based on the same device configuration of ITO/PEDOT/polymer:PCBM (1:3,

### **3.2 Cyano-substituted poly(***p***-phenylene-vinylene)s**

Cyano-substituted poly(*p*-phenylene-vinylene)s (CN-PPV) with electron deficient cyano groups on the vinyl units are synthesized by Knoevenagel polycondensation polymerization of terephthaldehyde and 1,4-bis(cyanomethyl)benzene in the presence of the base *t*-BuOK (Figure 12). Hence, the LUMO and HOMO levels of PPV derivatives can also be tuned by incorporating electronic substituent into the vinylene bridges (Cheng et al., 2009).

Fig. 12. Synthesis of CN-PPV copolymer **P9** via a Knoevenagel Polycondensation

CN-PPVs show high electron affinity to reduce the barrier to electron injection and good electron-transport properties as a result of the electron-withdrawing effect of the cyano side group and suitable electron acceptors in organic photovoltaic devices (Granström et al., 1998; Halls et al., 1995; Gupta et al., 2007). To effectively reduce the band gap of CN-PPV below 2 eV, electron-rich thiophene units with lower aromaticities have been incorporated into the main chain to form a D-A arrangement. A series of copolymers based on the bis(1 cyano-2-thienylvinylene)pheniylene structures with different alkyl or alkoxy side chains on the thiophene rings were reported by Vanderzande et al. (Colladet et al., 2007) (Figure 13).

Fig. 13. Chemical structures of **P10-P13** 

These monomers were all prepared by Knoevenagel condensations to construct cyanovinylene linkages. The electron-rich nature of thiophene units in these polymers makes them good candidates to serve as electron donors in organic bulk heterojunction solar cells. For example, both **P11**/PCBM- and **P12**/PCBM-based solar cells achieved a PCE of around 0.14%. Optimization of these devices by thermal annealing showed a slight increase of PCE to 0.19%. Reynold et al. also reported synthesizing a range of CN-PPV derivatives (**P14-P17)** containing dioxythiophene moieties in the polymer main chain (Figure 14) (Thompson et al., 2005; Thompson et al., 2006). The best photovoltaic device based on these CN-PPV derivatives with PCBM as the active layer achieved a PCE of 0.4%.

Fig. 14. Chemical structures of **P14-P17** 

426 Solar Cells – New Aspects and Solutions

Cyano-substituted poly(*p*-phenylene-vinylene)s (CN-PPV) with electron deficient cyano groups on the vinyl units are synthesized by Knoevenagel polycondensation polymerization of terephthaldehyde and 1,4-bis(cyanomethyl)benzene in the presence of the base *t*-BuOK (Figure 12). Hence, the LUMO and HOMO levels of PPV derivatives can also be tuned by

incorporating electronic substituent into the vinylene bridges (Cheng et al., 2009).

CN

O

Fig. 12. Synthesis of CN-PPV copolymer **P9** via a Knoevenagel Polycondensation

CN-PPVs show high electron affinity to reduce the barrier to electron injection and good electron-transport properties as a result of the electron-withdrawing effect of the cyano side group and suitable electron acceptors in organic photovoltaic devices (Granström et al., 1998; Halls et al., 1995; Gupta et al., 2007). To effectively reduce the band gap of CN-PPV below 2 eV, electron-rich thiophene units with lower aromaticities have been incorporated into the main chain to form a D-A arrangement. A series of copolymers based on the bis(1 cyano-2-thienylvinylene)pheniylene structures with different alkyl or alkoxy side chains on the thiophene rings were reported by Vanderzande et al. (Colladet et al., 2007) (Figure 13).

*t*-BuOK *t*-BuOH, THF

> S NC

S NC

O C14H29

O

H21C10O

H21C10O

**P11** C8H17

CN S

CN S

**P13**

OC10H21

O

OC10H21

O

MeO NC

MeO

n

CN

n

O

**P9**

n

O

C14H29

C8H17

**3.2 Cyano-substituted poly(***p***-phenylene-vinylene)s** 

NC

+

MeO

Fig. 13. Chemical structures of **P10-P13** 

CN S

CN S

OC10H21

O

**P10**

**P12**

OC10H21

n

n

O

S NC

OHC CHO

MeO

O

S NC

O

O

H21C10O

H21C10O

### **3.3 Acetylene-substituted poly(***p***-phenylene-vinylene)s**

Acetylene-substituted PPV derivatives can be synthesized via the Wittig – Horner Reaction (Figure 15). The coplanar and rigid nature of the acetylene moiety in the polymer chain may have the potential to obtain a higher degree of packing and thus improve the photovoltaic performance of such devices. Having coplanar electron-rich anthracene units and triple bond bridges, **P18** exhibits broader absorption, a lower HOMO level, and a smaller optical band gap of 1.9 eV, compared to MDMO-PPV. A device with the configuration of ITO/PEDOT/**P18**:PCBM (1:2, w/w)/LiF/Al, achieved a PCE value of up to 2% with a high *V*oc of 0.81 V. Figure 16 shows the chemical structures of a series of acetylene-substituted PPV derivatives synthesized by similar procedures. For polymers **P21** and **P22** (Egbe et al., 2007), the introduction of a thiophene ring into the polymer backbone showed an improvement in the PCE ranging from 1.2% to 1.7%. This is higher than those for **P19** and **P20** (Egbe et al., 2005), based on the same device configuration of ITO/PEDOT/polymer:PCBM (1:3, wt%)/LiF/Al (Cheng et al., 2009).

Organic Bulk Heterojunction Solar Cells Based on Poly(*p*-Phenylene-Vinylene) Derivatives 429

The efficiency of organic solar cells is increasing steadily by means of interdisciplinary approach. Extensive efforts are currently carry out by chemists in order to create new low bandgap materials to harvest more photons and increase the power conversion efficiency. Furthermore, processability of conjugated polymers that can be deposited from liquid solutions at low temperature make them suitable for large scale production on flexible substrates at low cost roll-to-roll process. To integrate new advanced device concepts and the nanostructure engineering of the morphology are also important in bringing high efficiency and low cost organic solar cells one step closer to successful commercialization.

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**4. Conclusion** 

**5. References** 

pp.9505-9516.

1251.

**P18**

Fig. 15. Synthesis of acetylene-containing **P18** via a Wittig – Horner reaction

Fig. 16. Chemical structures of acetylene-containing PPV derivatives **P19-P22**
