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vi. Platinum Acetylide Containing Conjugated Polymers

vi. Platinum Acetylide Containing Conjugated Polymers

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Metal Containing Polymers in Solar Cells



179



SCHEME 14.



excitation, a charge separated state is produced by photoinduced charge transfer

process:

C60À ½Pt2 ThŠ* À C60 -C60 ẵPt2 Thỵ C

60



3ị



Photovoltaic devices based on pure 29 and 29:PCBM blend as the active

layer is fabricated. It was interesting to observe that a device with pure 29

showed a higher power conversion efficiency (0.05%) compared to that consisting of a 29/PCBM blend (0.0.0024% to 0.041%). The good performance in

the photovoltaic cells with 29 only was attributed to the efficient charge

separation process and that the material exhibits efficient hole and electron

transport. The C60 moieties facilitated that electron transport, while the holes

are transported via the hopping between Pt2-thiophene units.

Another series of Pt acetylide based polymer 30 was synthesized by Wong

et al. (Scheme 15).93 The conjugated system is based on bithienyl benzothiadiazole,



180



Applications of Metal Containing Polymers in Organic Solar Cells



and this polymer exhibits an strong absorption band centered at 554 nm, which is

significantly lower than other phenylene or thiophene based Pt-acetylide polymers

[24-n (n57) exhibits an absorption peak maximum at 371 nm]. This absorption

band was assigned to be the electronic transition to the charge transfer excited

state, in which the --Pt-- unit was the donor and the benzothiadiazole was the

acceptor. As a result, it is possible to harvest the solar light in the red to near IR

region more efficiently. Figure 6 shows the absorption and emission spectra of 30.

The polymer was blended with PCBM and fabricated into PV cell with the

structure ITO/PEDOT:PSS/30:PCBM/Al. The current-voltage curve of the device

is shown in Figure 7. A power conversion efficiency of 4.93% was measured

(Isc515.43 mA/cm2, Voc50.82 V, FF50.39). The high efficiency was attributed to

the enhancement in photosensitization in the longer wavelength region. This was

confirmed by the plot of external quantum efficiency of the device as the function of

wavelength (Figure 8).



SCHEME 15.



1.2



Absorbance/PL Intensity (a.u.)



1



0.8



0.6



0.4



0.2



0

300



400



500



600



700



800



900



Wavelength (nm)



FIGURE 6. UV-VIS absorption and photoluminescence spectra of polymer 30.



Metal Containing Polymers in Solar Cells



181



10



5



−0.2



0



0.2



0.4



0.6



Voltage (V)



−5

Current (mA/cm2)



1



0.8



−10



−15



−20



0.6



120



0.5



100



0.4



80



0.3



60



0.2



40



0.1



20



0

300



EQE (%)



Absorbance (a.u.)



FIGURE 7. Current-voltage characteristic of the device ITO/PEDOT:PSS/polymer 30:

PCBM/Al under illumination with simulated AM 1.5 solar light.



0

400



500

600

Wavelength (nm)



700



FIGURE 8. The external quantum efficiency of the device ITO/PEDOT:PSS/polymer

30:PCBM/Al as a function of the incident light wavelength. The absorption spectrum of

the device is shown for comparison.



182



Applications of Metal Containing Polymers in Organic Solar Cells



The photovoltaic properties of other Pt-acetylide polymers 31 were

published by the same group (Scheme 16). The aromatic linking units were

modified, and the power conversion efficiency of the devices were in the range

between 0.21% and 2.66%.94 A review article dedicated to the optical properties of this group of Pt acetylide polymers has been published.95

vii. Other Metal Containing Polymers with Potential Photovoltaic

Applications

In this section, some ruthenium complex containing polymers incorporated with charge transport functionalities are presented. Being incorporated

with both photosensitizing and charge transport units in the same polymer

molecule, they are considered promising candidates for polymeric photovoltaic

cells. However, the photovoltaic properties have not been reported so far.

Polymer 32 (Scheme 17) was synthesized by the Stille coupling reaction

between bis(tributyltin)quarterthiophene and the dibromo substituted ruthenium



SCHEME 16.



SCHEME 17.



Metal Containing Polymers in Solar Cells



183



complex.96 The polymer exhibits broad absorption band spanning from 400 to

600 nm, which was explained to be due to the efficient delocalization of p-electrons

on the main chain. Electrochemical experiments also revealed electronic interactions between the conjugated backbone and the ruthenium complexes.

Triphenylamine derivatives are known to be efficient hole transport

materials and are widely used in organic light-emitting devices. Thelakkat et al.

reported the synthesis of a 2,2-bipyridine ligand capped with poly(vinyltriphenylamine) at both ends.97 The polymer chain was synthesized by

the atom transfer radical polymerization of 4-bromostyrene using 4,4-bis

(chloromethyl)bipyridine as the initiator (Scheme 18). The bromide groups

were then replaced by diphenylamine in the presence of palladium catalyst.

Polymer 33 was then obtained by the metalation reaction.

Schubert98 proposed the potential use of several ruthenium containing

polymers in photovoltaic devices. A ruthenium containing poly(ethylene glycol)

derivative 34 was synthesized by the functionalization of 4-(3-aminopropyl)-4methyl-2,2-bipyridine with poly(ethylene glycol) (Mn52800, PDI51.05), which

was activated with N,N-carbonyldiimidazole (Scheme 19).99 Applications in

solid electrolytes for DSSC was proposed. Polyester 35 was incorporated with



SCHEME 18.



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