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IV. Metal Containing Polymers in Solar Cells

IV. Metal Containing Polymers in Solar Cells

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166



Applications of Metal Containing Polymers in Organic Solar Cells



in the second part, the use of metal containing polymers in heterojunction or

bulk-heterojunction photovoltaic cells is discussed. To limit the scope of the

discussion, only polymeric materials functionalized with metal complexes

are addressed; the use of organic polymers doped with metal complexes is not

included in the section.



A. Dye-Sensitized Solar Cells

Several organic polymers have been used as the sensitizers in DSSCs.43

Examples of metal containing polymers for DSSC are also mainly based on

ruthenium complexes containing conjugated polymers. Conjugated polymers

with π2π* electronic transitions generally have larger absorption crosssections than that of metal complexes (commonly MLCT transitions). The

combination of metal complex and conjugated polymer can enhance not only

the charge transport properties of the photovoltaic devices but also the

absorption range (both absorption from the MLCT transition of the complex

and π2π* transition of the conjugated polymer). Coordination polymers 1-3

(Scheme 2) comprise a poly(p-phenylenevinylene) (PPV) building block with a

donor or acceptor character and a ruthenium(II) bis(terpyridyl) chromophore

as a spacer.44 Fluorescence spectroscopy revealed the energy harvested by the

polymer segments was efficiently transferred to the complex. Photovoltaic

performances of two different types of solar cells, all-polymer solar cells

obtained by spin-coating and DSSC with TiO2 crystalline film, based on these

polymers were compared. All the DSSC devices for the three different coordination polymers showed better performance than the all-polymer-based solar

cells. Device efficiency close to 0.1% was obtained under AM 1.5 illumination

(device configuration: FTO/TiO2(anatase)/TiO2(nanoporous)/polymer 3/electrolyte/Pt/FTO) in spite of the lack of anchoring groups on the polymer to

attach on the surface of TiO2.45 The use of Co(btp)3ClO4-based electrolyte

[btp5(4,4-di-tert-butyl-2,2-dipyridyl)] instead of I-/I3- avoids polymer degradation and thus increases cell performances.44,46

Similar studies have been carried out on a zwitterionic ruthenium bis(terpyridine) dye covalently linked to a thiophene conjugated polymer (Scheme 2,

polymer 4). The zwitterionic ruthenium dye was functionalized with a phosphonic acid group on one of the terpyridine ligands, while another terpyridine

ligand was conjugatively linked to the regioregular poly(hexylthiophene) by the

Stille coupling reaction. The ruthenium dye anchored to the TiO2 anatase

enabled efficient electron injection, and the conjugated polymer served as lightharvesting antenna for the ruthenium complex and hole transport materials.

Although the device efficiency was far from satisfaction (ηp,0.05 %), the

material design demonstrated the efficient light-harvesting ability and energy

transfer in the combination of Ru dye and polythiophene.47

In another report, layer-by-layer (LbL) self-assembly deposition was

employed to prepare the acceptor-sensitizer dyad polyelectrolyte film in

DSSC.48 The viologen-containing polymer film with bromide as the counterions



Metal Containing Polymers in Solar Cells



167



SCHEME 2.



of polypropylviologen (PPVG), which was coated on the ITO substrate by

dipping. Poly(ethylenedioxythiophene)-polystyrene sulfonate (PEDOT:PSS)

was deposited on top of the PPVG by the same method (Fig. 4). Successive

layering of these two polymers alternatively formed a multilayer polyelectrolyte. The photosensitizer N3 dye was incorporated into the multilayer by

ion exchange with the bromide counterions of PPVG. A 4.5 bilayer device

generated a photocurrent of Isc51.2 μA/cm2 under AM 1.5 conditions, with a

device configuration of ITO/multilayer/electrolyte (0.3 M LiI and 30 mM I2

in CH3CN)/Pt/ITO.

DSSCs with rodlike polymers as the sensitizer generally have poor efficiency due to their rigidity and incomplete infiltration into the void of the TiO2

nanostructures. The loose contact between the polymer and the TiO2 surface

gives rise to poor electron injection and an increase in charge recombination. In

situ photo-electrochemical polymerization (PEP) of hole conducting materials

not only can provide better contact with sensitizers but also can replace the

liquid electrolyte. Scheme 3 shows a series of novel ruthenium bipyridyl

complexes functionalized with electropolymerizable hole transporting

groups.49À52 In situ PEP was carried out in the void of mesoporous structure

after the ruthenium dyes were anchored on the labyrinthine surface of TiO2.



168



Applications of Metal Containing Polymers in Organic Solar Cells

(a)

*

n



*

O

* N+



N+



*



n

PPVG



O



S



*

n



SO3−Na+



PEDOT:PSS



(b)



LBL deposition

ITO



Ion-exchange



ITO

Step 1: PPVG

Step 2: PEDOT:PSS



ITO



Ru



= PPVG



= PEDOT:PSS



= N3 dye



FIGURE 4. (a) Structures of PPVG and PEDOT:PSS. (b) The fabricating of the

polyelectrolyte-organometallic multilayer film by the layer-by-layer deposition process.



SCHEME 3.



Metal Containing Polymers in Solar Cells



169



The dyed TiO2 film was immersed in an acetonitrile solution containing pyrrole

or bis-EDOT monomer and lithium perchlorate. The applied potential of

the film was referenced to the Ag/Ag1 electrode and irradiated by a 500 W Xe

lamp (22 mW/cm2, λ . 500 nm).

A solid-state solar cell was assembled with an ionic liquid—1-ethyl-3methylimidazolium bis(trifluoromethanesulfone)amide (EMITFSA) containing

0.2 M lithium bis(trifluoromethanesulfone)amide and 0.2 M 4-tertbutylpyridine—as the electrolyte and Au or Pt sputtered film as the cathode.51,52

The in situ PEP of polypyrrole and PEDOT allows efficient hole transport

between the ruthenium dye and the hole conducting polymer, which was facilitated by the improved electronic interaction of the HOMO of the ruthenium dye

and the conduction band of the hole transport material. The best photovoltaic

result (ηp50.62 %, Isc5104 μA/cm2, Voc50.716 V, and FF50.78) was obtained

from the ruthenium dye 5 with polypyrrole as the hole transport layer and

the carbon-based counterelectrode under 10 mW/cm2 illumination. The use of

carbon-based materials has improved the electric connectivity between the hole

transport layer and the electrode.51

The same approach was adapted in a later study. An efficient solid-state

DSSC was fabricated using hybridized ruthenium dye 8. The hole conducting

PEDOT was formed in situ via PEP. The thickness of the mesoporous TiO2

layer of the solar cell was varied. The highest efficiency (2.6% under 100 mW/

cm2 illumination) was achieved by using a 5.8- μm-thick TiO2 layer.52

Recently, a tris(2,2-bipyridyl) ruthenium(II) complex containing cyclodextrin (CD) supramolecule was applied to DSSC (Scheme 4).53 Comparing to



SCHEME 4.



170



Applications of Metal Containing Polymers in Organic Solar Cells



the ruthenium complex without cyclodextrin moiety, the photovoltaic performance of polymer 9 revealed a 40% enhancement in efficiency (Isc54.21 mA/

cm2, Voc50.59 V, FF50.59, ηp51.6%). It was proposed that the CD moiety

was able to act as a mediator and fine-tune the photoelectrode-electrode

interface. There is a binding between the redox I-/I3- couple and the CD cavity,

resulting in the improvement in dye regeneration.



B. Organic Thin Film Solar Cells

i. Polyferrocenylsilanes

Polyferrocenylsilanes represent a major type of metal containing polymers

with several interesting potential applications, such as ceramic materials, magnetic

materials, redox activity, and sensing. On the other hand, relatively little work has

been conducted regarding their applications in photovoltaic cells. Manners et al.

have fabricated photovoltaic devices with the active layer composed of a blend of

poly(ferrocenylmethylphenylsilane) (PFMPS; Scheme 5)54 and PCBM or C60 with

the device configuration of ITO/PFMPS:fullerene(PCBM or C60)/Mg/Ag/Au

[PCBM51-(3-methoxycarbonyl)propyl)-1-phenyl[6.6]C61] with various fullerene

contents. In the absence of fullerene, the device did not exhibit measurable photocurrent response. When the polymer was doped with 5 mol % C60 and a bias of À1

V was applied, a photo to dark current ratio of 15 was observed. Phase separation

was observed when the C60 content was . 8 mol %, while for PCBM, no observable phase separation was found even when the concentration was increased to

12 mol %. The generation of photocurrent was attributed to an intermolecular

photoinduced electron charge transfer from the ferrocene donors to the fullerene

acceptors. For the best device, the Isc, Voc, and FF measured were 3.8 nA, 0.44 V,

and 0.28, respectively (incident light power5160 mW/cm2).

ii. Polymeric Metal Complexes

A series of polymeric complexes with the general formula {M(dmb)2Y}n

À

À

À

(M5Cu(I), Ag(I); dmb51,8-diisocyano-p-menthane; Y ¼ BFÀ

4 ; NO3 ;PF6 ; CIO4 )

55

was prepared by Harvey et al. (Scheme 6). The structures of some of the complexes



SCHEME 5.



Metal Containing Polymers in Solar Cells



171



H3C

H3C



CH3



CN



CN

dmb



SCHEME 6.



were characterized by X-ray crystallography.56 The silver complexes [Ag(dmb)2]

À

À

Y (Y ¼ BFÀ

4 ; NO3 ; CIO4 ) exhibit a polymeric tubular structure in the solid state,

and the silver atoms act as the bridge linking different dmb ligands in a distorted

tetrahedral configuration. The weight average molecular weights of the copper

complex were in the range of 1.27 3105 to 1.92 3105, while the silver complexes

were suggested to be in oligomeric form. The counteranions may be replaced by

tetracyanoquinodimethane(TCNQ), giving electrically insulating materials.

Further doping with neutral TCNQ yielded conducting materials, of which the

photoconducting properties were studied. A photoinduced electron transfer

from the metal center to TCNQ was proposed.57 Preliminary results on the

photovoltaic properties of these materials were obtained by fabrication of photovoltaic cells based on [Cu(dmb)2]TCNQn by depositing the complex on tin

oxideÀcoated glass substrates. Replacement of TCNQ by C60 yielded a lower

photocurrent response, which was attributed to the poor π-stacking.

iii. Ruthenium/Rhenium Complexes Containing Conjugated Polymers

Despite the extensive application of ruthenium complexes in DSSC,

transition metal containing polymers have received relatively little attention in

the fabrication of polymeric photovoltaic cells. Most of the early works on

ruthenium containing polymers were focused on the light-emitting properties.58À60 Several examples of ruthenium terpyridine/bipyridine containing

conjugated polymers and their photoconducting/electroluminescent properties

were reported.61,62

Bisterpyridyl ruthenium(II) complex containing conjugated polymers

were synthesized by the Heck coupling reaction. In the initial studies, it was

found that the ruthenium complex could function as the photosensitizer. The

photocurrent response of the polymer can be enhanced by the ruthenium

complex. In addition, the metal complexes can also act as charge transport

units, and the hole and electron carrier mobilities of the polymer depended on

the metal content.60,62,63 It was suggested that the charge transport process was

due to the presence of both ruthenium-centered oxidation and the terpyridine/

bipyridine ligandÀcentered reduction in the complex. The effect of metal

complex content to the carrier mobilities was also observed in other examples

of ruthenium containing polymers,60,64À66.

Conjugated polymers incorporated with different heterocyclic moieties

were reported. Examples of these conjugated main chains are poly(phenylene

vinylene) (10)60 and polybenzoxazole/polybenzthiazole (11)64 (Scheme 7). In



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