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v. Conjugated Polymers with Pendant Metal Complexes

v. Conjugated Polymers with Pendant Metal Complexes

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176



Applications of Metal Containing Polymers in Organic Solar Cells



SCHEME 11.



from polymers 22 and 23 were 0.12 and 0.084%, respectively. The hole carrier

mobilities of the polymers were measured to be on the order of 1024 cm2/Vs.

This explains the modest efficiencies exhibited by these devices given the simple

device structure. After photoexcitation, electrons are captured by the C60 near

the interface, while holes migrate to the conjugated main chain. Unlike other

polymer photovoltaic cells, insertion of a PEDOT:PSS layer between the ITO

and polymer did not improve the device performance significantly.



Metal Containing Polymers in Solar Cells



177



SCHEME 12.



1

Current (mA/cm2)



4



Absorbance (a.u.)



0.8



3

2

1



−0.1



0.6



0

−1



0.1



0.2



0.3



0.4



Voltage (V)



−2

−3



0.4

−4



0.2



0

300



400



500



600



700



800



900



Wavelength (nm)



FIGURE 5. UV-VIS absorption spectra of polymer 22 (- - - - -) and 23 (———). Inset:

The current-voltage characteristics of the ITO/polymer 22 and 23/C60/Al devices under

illumination with simulated solar light (100 mW/cm2).



178



Applications of Metal Containing Polymers in Organic Solar Cells



vi. Platinum Acetylide Containing Conjugated Polymers

Platinum(II) bridged polyynes have been promising candidates for nonlinear optical materials.84,85 They are characterized by the presence of 2CCPt-CC units on the polymer chains (Scheme 13). This functional unit has also

been employed in the construction of supramolecules with various dimensions,

shapes, and sizes.86 In these types of polymers, π-conjugation is preserved due

to the partial overlapping between the π-orbitals of the conjugated ligand and

the Pt 5d orbitals. Early works on Pt-acetylide polymers were mainly focused

on the synthetic methodology and the photophysical properties. Examples

include conjugated Pt-acetylide polymers based on phenylene, thiophene,87

pyridine, fluorene (polymer 25),88 and bithiazole (polymer 26)89 (Scheme 14).

The photosensitizing properties of some of these polymers under visible

light irradiation were studied briefly.89 Schanze et al. investigated the photophysical properties of the Pt-acetylide oligomers and polymers in more

detail. A series of oligomer 24-n (n 5 2-5, 7) was synthesized by the successive coupling between the Pt-Cl moieties and the acetylide terminated

building block using the convergent approach. These oligomers exhibit very

rapid S1-T1 intersystem crossing, and it was suggested that the triplet excited

state (T1) was more localized than the singlet state (S1).90 Both experimental

and theoretical results indicated that the triplet excited is localized within a

single À[PtL2-CC-C6H4-CC-PtL2] unit.91 In another report, Pt-acetylide

polymer 28 and its model compound 27 with thiophene moieties on the main

chain were synthesized by similar methods. The polymer was blended with

PCBM and fabricated into bulk heterojunction devices. The design rationale

for devices based on these Pt based polymers is that the formation of a longlived triplet state can lead to more efficient charge separation. Efficient

quenching of the 28 emission from the triplet state by PCBM was observed,

suggesting an electron (or energy) transfer process between these two moieties. Devices with the structure ITO/PEDOT:PSS/28:PCBM (1:4)/LiF/Al

with different active layer thickness were fabricated. Under illumination of

simulated solar light (AM1.5, 100 mW/cm2), the highest power conversion

efficiency was 0.27 %, and the Isc, Voc, and FF measured were 0.99 mA/cm2,

0.64 V, and 0.43, respectively. In a later report, the synthesis of a thiophenebased Pt-acetylide 29 end capped with two C60 moieties was presented.92

Results from transient absorption spectroscopy suggests that upon photo



SCHEME 13.



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,



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