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iii. Ruthenium/Rhenium Complexes Containing Conjugated Polymers

iii. Ruthenium/Rhenium Complexes Containing Conjugated Polymers

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Applications of Metal Containing Polymers in Organic Solar Cells


these polymers, the ruthenium complex moieties can play the different functions, such as sensitizers, charge transporting species, and light-emitting

groups. Other ruthenium containing polymers with a nonconjugated main

chain—such as polyimide (12),67 polyamide (13),66 and polystyrene (14)68—

derivatives were also synthesized. The photocharge generation and charge

transport properties of these polymers were investigated.

All these polymers with ruthenium terpyridine/bipyridine complexes

contain ionic ruthenium complex on the polymer main chain. Other than the

Metal Containing Polymers in Solar Cells


conventional spin-casting technique for polymer film formation, the polymers

can be fabricated into multilayer films by the layer-by-layer (LbL) electrostatic

self-assembly method.69,70 This film forming technique involves the dipping of

a suitable substrate into a polycation and a polyanion solution alternatively.

This is a simple and versatile approach for fabricating multilayer polymer films

because the polymer film thickness can be controlled accurately and the loss of

material in the film forming process can be minimized. This technique has been

adopted in the fabrication of photovoltaic cells by few research groups. 71,72 In

these examples, pure organic polyelectrolytes were employed.

In 2004, Chan et al. reported the fabrication of photovoltaic cells based

on polymer multilayers formed by ruthenium containing conjugated polymer

15 and partially sulfonated polyaniline (SPAN).73 Scheme 8 shows the structures of the polymers and the schematic diagram of the deposition process. The

design rationale was that SPAN functioned as the hole transport polymer and

polymer 15 functioned as the sensitizing and electron transport polymers. It

had been previously shown by the same group that some conjugated polymers

functionalized with ruthenium complexes exhibited bipolar charge transport

character.62 In addition, in polymer multilayer thin films fabricated by the LbL

process, the polymers formed interpenetrating polymer networks instead of

stratified layers. Therefore, the polymer film may be considered to be a bulk

heterojunction photovoltaic cell. Devices with the structure ITO/(SPAN/15)20/

Al and with an active layer thickness ranging from 9 to 58 nm were fabricated.74 The thickness could be changed by varying the deposition conditions,

such as the pH of the solution, the addition of different electrolytes, the concentration of salt in the solution, and the post film forming annealing processes. Multilayer films obtained from different conditions also showed

significant differences in surface morphology. The best photovoltaic device

showed a power conversion efficiency of 4.4 3 1023%., Although these devices

exhibit lower efficiency compared to other bulk-heterojunction polymer based

devices, this method provides a simple alternative approach for in the fabrication of polymeric photovoltaic cells based on ionic polymers.

Rinsing with solvents

ITO glass

Polycation in DMF

Rinsing with solvents


SPAN in water


Applications of Metal Containing Polymers in Organic Solar Cells

Besides ruthenium complexes, rhenium complexes were also used as the

photosensitizers in photovoltaic cells. Bulk heterojunction photovoltaic cells

fabricated from sublimable rhenium complexes exhibited a power conversion

efficiency of 1.7%.75,76 The same rhenium complex moiety was incorporated

into conjugated polymer chains such as polymer 16aÀc (Scheme 9). Fabrication of devices based on conjugated rhenium containing polymers 17aÀc and

SPAN by the LbL deposition method was reported.77 The efficiencies of the

devices are on the order of 1024%.

The synthesis of a series of polyfluorene 18 incorporated with chlorotricarbonyl rhenium(I) 2,2-bipyridine complexes was reported (Scheme 10).78

Suzuki cross-coupling reactions were used to construct the main chain, and the



Metal Containing Polymers in Solar Cells


complexes were introduced into the polymer after the polymerization reaction.

For the polymer with 50% rhenium complex, a lower energy absorption band

centered at 420 nm was observed. The role of rhenium complexes in the photosensitization process was studied by measuring the field induced surface

photovoltaic spectra. The generation of photovoltage was assigned to the π-π*


iv. Hyperbranched Polymers

Besides liner polymers, hyperbranched metal containing polymers can

also be used in the multilayer film deposition by the LbL process. Hyperbranched polymers 19À21 were synthesized from the corresponding AB2 type

monomer by the coordination reaction between the activated rhenium center

and the pyridine N-donor ligand (Scheme 11). It was observed from AFM that

the polymer molecules exhibited a globular shape when dispersed on a substrate.79 The polymer can be used in the modification of the surface of nano- to

micro-size ZnO tetrapods.80 In the devices fabricated by the LbL process,

sulfonated polythiophene derivative PTEBS was used as the hole transport

polyanion.81 A device with the structure ITO/(polymer 19/PTEBS)80/Al

revealed a power conversion efficiency of 6 31023%. Detailed investigation of

the transient photocurrent generated by the device revealed that the photocurrent rise and decay time constants are on the order of tens of seconds, which

is much longer than those devices based on pure organic polymers. Due to the

presence of counterions in the polymer multilayer films, the devices can be

considered as capacitors (capacitance B 0.16 μF). In other devices fabricated

from polymers 20 and 21 by the LbL process, power conversion efficiencies of

the same order of magnitude were obtained.82

v. Conjugated Polymers with Pendant Metal Complexes

One of the approaches in improving light harvest efficiency is to broaden

the absorption spectrum of the active layer. This can be achieved by either

designing a new compound with very broad absorption spectrum or by

incorporating different photosensitizing units into the polymer. Chan et al.

reported the synthesis of conjugated polymers 22À23 incorporated with a

pendant ruthenium black dye analogue (Scheme 12) by a conventional palladium catalyzed coupling reaction.83 Unlike other ruthenium/rhenium complex

containing polymers that were synthesized from the metal complex monomers,

the trithiocyanato ruthenium complex moieties were introduced after the

synthesis of the polymer because the thiocyanato ligands are more labile in

the presence of polar solvents. The polymer main chains are based on poly

(phenylene-thiophene) or poly(fluorene-thiophene). The conjugated systems

and the ruthenium complex absorb in different regions (400À550 nm and

600À700 nm, respectively), yielding polymers with very broad optical

absorption in the visible to near IR regions (Fig. 5).

Photovoltaic cells with the simple device structure ITO/polymer/C60/Al

were fabricated. The power conversion efficiencies of the devices fabricated


Applications of Metal Containing Polymers in Organic Solar Cells


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.

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