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 Voc 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).

image471image472

f-BuOK

image473

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

 

OC8H17

 

OR1

 

OC8H17

OR1

 

X

 

X

image474

 

R1O

 

X

 

H17C8O

 

X

 

n

 

OR2

 

image475

image018

P19 X=H, R-| = octadecyl, R2 = octyl X= H, R1 = dodecyl, R2= octyl X = octyloxy, R! = R2 = octadecyl X = octyloxy, R-| = R2 = octyl X = octyloxy, R-| = R2 = 2-ethylhexyl

P20 R1 = R2 = octadecyl R1 = R2 = octyl R., = R2 = 2-ethylhexyl R1 = 2-ethylhexyl, R= methyl

image476

image477

P22 R = 2-ethylhexyl ormethyl

 

P21 R = 2-ethylhexyl or methyl

2. Conclusion

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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