Ruthenium photosensitizers with 2,2  -bipyridine-based ancillary ligands

Sincethedevelopmentofrutheniumsensitizers,N3[9,10],N719 [10,13]andblackdye[14],whichshowexcellentDSCperformance, manyresearchgroupshaveattemptedtomodifytheirstructures withthegoalofimprovingphotovoltaicperformance.The molec-ularstructureoftheN3dyeconsistsoftwoanchoringligandsfor connectingtotheTiO2surfaceandtwoNCSforbalancingthecharge oftheRumetal(Fig.2).Inanefforttoimprovethelight-harvesting abilityofthephotosensitizer,severalmodificationsweremadein the2,2^-bipyridine(bpy)moiety,bothinanchoringaswellas ancil-laryligands.ThevariationofsubstituentsinthebpyligandofRu sensitizersandtheircorrespondingDSCphotovoltaicproperties arelistedinTable1andtheeffectsofstructuralengineeringon

Chart1.Theproportionaldistributionofrutheniumphotosensitizerswithexamplesfromeachcategoryandtheirphoton-to-currentconversionefficiency.

fundamentalproperties,suchasabsorptionandredoxbehaviorand photovoltaicparameterswithrespecttostandardN3and/orN719 dyearediscussed.

ThetwoacidicprotonsoftheN3dyemaydissociateatapKa

of1.5andtheresultingdianionicsaltisreferredtoastheN719 dye.RelativetotheN3dye,theN719dyeexhibitsahighεvalue andtheRu(III/II)redoxpotentialisnegativelyshiftedduetothe replacementofH⁺withatetraalkylammoniumcation(Table2), andalsothedoublyprotonatedformofN3issuperiortotheneutral N3dyeforsensitizationofnanocrystallineTiO₂films[13].

AseriesofamphiphilicheterolepticRu-bpysensitizerdyes sub-stitutedwithalkylchainsofvariouslengthsatthe4,4^-positions ononeofthebpyligandsweredeveloped(1–5).ThelowestMLCT bandofthesecomplexes(1–5)wasblue-shiftedwithlowerε val-uescomparedwiththehomolepticN3dye(Table2)[15,16].For instance,dyes2(N820)and4(Z907)showablue-shiftof360and 280cm⁻¹,respectively,relativetotheN3dyealongwithalower εvalue.Thisisduetoelectron-donatingnatureofthealkylgroups atthe4,4^-positionsofthebpyligandresultingthe␲*orbitalof theligandsatahigherenergycomparedwiththatofdcbpy. Conse-quently,Ru(III/II)oxidationpotentialofN3complexishigherthan thealkylchainsubstitutedcomplexesreflectingstrong electron-withdrawingnatureofthedcbpyligand.

Withtheaimof protectingthedyelayeragainstthe ingres-sionofwaterfromtheelectrolyteand hencetoenhancedevice stability,thesehydrophobicdyeswereexaminedunderidentical conditionsof cellfabricationand measurement[17].Such alkyl chainsfunctionasanelectricalinsulatingbarrierlayerbetweenthe

N N N

N N N Ru HOOC

HOOC

C C

S S

COOH

COOH

N3

Fig.2. MolecularstructureoftheN3dye.

sensitizerdyeandthehole-transportingmedium,thereby reduc-inginterfacialchargerecombination(CR)lossesandincreasingthe opencircuitpotential(V_OC)andshort-circuitphotocurrent(J_SC).

Hence,the VOC of theDSCs is in thedescendingorder of alkyl chainlengthC13(N621)≈C9(Z907)>C6(N820)≈C1(KD1)(2–5) [16],whichisconsistentwiththeefficiencytrendwithimproved devicestabilityunderthermalstressandlightsoaking[18]. How-ever,theC18dyedeviatesfromtheseriesforthefollowingreasons:

(a)retardationoftheregenerationreactionwithincreasingchain lengthandinparticular,C18dyeshows700-foldlower regenera-tionrateconstant[16];(b)slowerchargerecombinationbetween theelectrolyteandtheinjectedelectron[17];and(c)faster recom-bination rate between the dye and the injected electron [18].

Thesedrawbackswerereflectedinthesignificantlyreduceddevice performanceoftheC18dye.Furthermore,theinhomogeneous dye-loadingontheTiO₂surfaceduetothefoldingofthelongC18chains along withincompleteswellingof thelongC18chains,thereby reducestherecombinationblockingeffectofthealkylchainspacer [19].

A series of N3-related Ru(II)-sensitizers that contained 4,4^ -di(p-X-phenyl)-2,2^-bipyridine (X=CN, F, H, OMe, NMe2) as an ancillary ligandshowed a systematicbathochromic shiftin the MLCTbandontraversingfromelectron-withdrawingto electron-donatingcongeners(7–12).Incomparisonwiththe1(KD1)dye, substitution with phenyl group allows 7 (dye-1) to exhibit a 350cm⁻¹red-shiftedMLCTabsorptionwithhigherεvalue, result-ing in higher cell , under identical conditions [15]. For the complexes 9–11 and N3, the cell  increases with increasing electron-donatingpower:F(9)<COOH(N3)<H(10)<OMe(11).In contrast,strongestelectron-donating(NMe2-based)andstrongest electron-withdrawing(CN-based)devicesshowedinferiordevice performance.TheCRkineticsbetweentheinjectedelectronsand the oxidized dye is in the order CN (8)>COOH (N3)>F (9)>H (10)>OMe(11).ThisfastCRinCN-baseddeviceiscausedbythe electronic characteristics,resulting in poordevice performance andfortheNMe₂-baseddye,isprobablyduetofastCR dynam-ics betweentheinjected electronsand the oxidizedelectrolyte andalsotherutheniumsensitizerintheexcitedstate(Dye^•+)is quenchedbyI⁻[20].IthasbeenshownthataminesinteractwithI₂ toformchargetransfercomplexandthismightleadtoanincrease intheamountofoxidizedelectrolytethatcomesintocontactwith thedyes,therebyenhancingCR.

Table1

Molecularstructuresofrutheniumphotosensitizerswithvariousbipyridine-basedancillaryligandsandphotovoltaicpropertiesofcorrespondingDSCs.

Generalformula: N

N N

N N N Ru HOOC

HOOC

C C

S S

X

X

No X(code) JSC,(mAcm⁻²) VOC(V) FF (%) Ref.

N3 COOH 18.2 0.72 0.73 10.0^a [10]

1 H(KD1) 10.4 0.53 0.62 4.5^b [15]

2

CH

3

(N820)

^{14.7 0.70 – 6.7c [16]}

3

C

6

H

13 ^{15.5 0.70 – 7.4c [16]}

4

C

₉

H

₁₉

(Z907)

^{16.0 0.74 0.67 8.4c [16]}

5

C

₁₃

H

₂₇

(N621)

^{16.2 0.74 0.72 8.6c [16]}

6

C

₁₈

H

₃₇ 3.5 0.67 0.56 1.3^d [17]

7

(dye-1)

12.6 0.57 0.58 5.5^b [15]

8

CN

^{10.0 0.67 0.74 4.9e [20]}

9

F

^{14.3 0.72 0.73 7.6e [20]}

10

H

^{15.2 0.71 0.72 7.8e [20]}

11

OMe

^{16.0 0.72 0.72 8.3e [20]}

12

NMe

₂ ^{8.4 0.69 0.73 4.2e [20]}

13

O

(Z910)

17.2 0.78 0.76 10.2^f [22]

14

O C

⁶

H

₁₃

(K-19)

14.6 0.71 0.67 7.0^f,g [23]

15

O (K77)

19.2 0.78 0.72 10.5^f [24]

Table1(Continued)

Generalformula: N

N N

N N N Ru HOOC

HOOC

C C

S S

X

X

No X(code) JSC,(mAcm⁻²) VOC(V) FF (%) Ref.

16

O

O (N945)

16.5 0.79 0.72 9.6^f [26]

17

N

H O

15.8 0.69 0.66 7.0^f [27]

18

O NHHN O O

CH3

H H H CH₃

15.5 0.68 0.71 7.4^f [27]

19 H

O N HN O

O

CH3

H H H CH3

16.1 0.68 0.7 7.6^f [27]

20 ¹¹ OH ^{17.5 0.70 0.72 8.8f [27]}

21

H N C

₈

H

₁₇

O (A597)

11.8 0.78 0.78 7.3^h [28]

22

S C

₆

H

₁₃

(TG6)

14.0 0.75 0.55 5.8ⁱ [29]

23

O O

3

(K51)

^{15.4 0.74 0.69 7.8}

j [30]

24

O O

3

(K60)

16.9 0.73 0.69 8.4^f [33]

25

R

_f

= C

₂

F

₄

H ( CT4 )

R

_f

= C

₃

F

₇

(CT7) O R

_f

R

f

= C

4

F

8

H (CT8)

13.3 0.67 0.70 6.2^k [34]

26 15.4 0.68 0.66 6.9^k [34]

27 15.0 0.68 0.67 6.8^k [34]

Table1(Continued)

Generalformula: N

N N

N N N Ru HOOC

HOOC

C C

S S

X

X

No X(code) JSC,(mAcm⁻²) VOC(V) FF (%) Ref.

28

O

O

O

O (DCSC13)

10.1 0.73 0.69 5.1^l [35]

29

N

(Ru-bpy–T PA)

– – – – [36]

30

N

(IJ-1)

17.6 0.80 0.73 10.3 [37]

31

O N

O

O

– – – – [38]

32

(N845)

^m

N C

₄

H

₉

C

₄

H

₉

(D5)

^m

10.8 0.63 0.68 4.6^c,n [40]

Table1(Continued)

Generalformula: N

N N

N N N Ru HOOC

HOOC

C C

S S

X

X

No X(code) JSC,(mAcm⁻²) VOC(V) FF (%) Ref.

33

N

C

4

H

9

C

₄

H

₉

11.7 0.63 0.66 4.8^c,n [40]

34

(D6)

S C

₆

H

₁₃

(HRS-1)

20.0 0.68 0.69 9.5^o [41]

35

S S C

8

H

17

(CYC-B1)

23.9 0.65 0.55 8.5^p [42]

36

S C

8

H

17

(CYC-B3)

15.7 0.67 0.71 7.4^q [43]

37

S C

₈

H

₁₇

O O (SJW-E1)

21.6 0.67 0.63 9.0^q [43]

38

S

O O (Ru-EDOT)

19.1 0.66 0.72 9.1 [44]

39

S S S C

₆

H

₁₃

(CYC-B11)

20.1 0.74 0.77 11.5 [46]

40

S C

₆

H

₁₃

(C101)

18.6 0.74 0.75 10.5 [47]

41

O C

6

H

13

(C102)

17.8 – – 9.5 [47]

Table1(Continued)

Generalformula: N

N N

N N N Ru HOOC

HOOC

C C

S S

X

X

No X(code) JSC,(mAcm⁻²) VOC(V) FF (%) Ref.

42

S

S C

₈

H

₁₇

(C104)

17.9 0.76 0.78 10.5 [48]

43

S O O

C

₆

H

₁₃

(C103)

18.3 0.76 0.75 10.4 [49]

44

S S C

6

H

13

O O

O O

(C107)

19.2 0.74 0.75 10.7 [49]

45

Se C

₆

H

₁₃

(C105)

18.7 0.75 0.75 10.6 [50]

46

S S

C

₆

H

₁₃

(C106)

19.2 0.78 0.76 11.3 [51]

47

S S

S (3T)

15.5 0.68 0.70 7.4 [52]

48

O S (LXJ-1)

16.5 0.72 0.75 8.8^r [53]

49

S C

₆

H

₁₃

(JK-188)

18.6 0.72 0.71 9.5^s [54]

Table1(Continued)

Generalformula: N

N N

N N N Ru HOOC

HOOC

C C

S S

X

X

No X(code) JSC,(mAcm⁻²) VOC(V) FF (%) Ref.

50

S S C

₆

H

₁₃

(JK-189)

18.9 0.63 0.73 8.7^s [54]

51

S

N

(JK-55)

17.6 0.64 0.72 8.2^t [55]

52

S N

(JK-56)

^m

17.5 0.71 0.73 9.2^t [55]

53

S N

(CYC-B6S)

19.8 0.78 0.63 9.7^u [56]

Table1(Continued)

Generalformula: N

N N

N N N Ru HOOC

HOOC

C C

S S

X

X

No X(code) JSC,(mAcm⁻²) VOC(V) FF (%) Ref.

54

S N

C

₇

H

₁₅

C

₇

H

₁₅

(CYC-B6L)

18.2 0.78 0.63 9.0^u [56]

55

S N

O O (CYC-B13)

10.3 0.73 0.68 5.1^v [57]

56

S S N

C

7

H

15

C

₇

H

₁₅

(CYC-B7)

17.4 0.79 0.65 9.0 [58]

䊉Indicatesthepointofattachmentofthesubstituenttotheancillaryligand.

aConditions:96.0mW/cm²simulatedAM1.5solarradiation;inamixtureof(90:10,v/v)acetonitrileand3-methyl-2-oxazolidinone,0.3MLiIand0.03MI2.

b Conditions:75.0mW/cm²simulatedAM1.5solarradiation;inamixtureof(1:1,v/v)acetonitrileandpropylenecarbonate,0.5MLiIand0.05MI2;cellarea:0.15cm².

c StandardglobalAM1.5solarradiation;inamixtureof(1:1,v/v)acetonitrileandvaleronitrile,0.6MN-methyl-N-butylimidazoliumiodide,0.05MLiI,0.05MI2,0.5M tert-butylpyridine.

d UnderidenticalconditionN621shows=3.2%.

eConditions:100mW/cm²simulatedAM1.5solarradiation,underidenticalconditionN3shows=7.8%.

f StandardglobalAM1.5solarradiation;inamixtureof(3:1,v/v)acetonitrileandvaleronitrile,0.6M1-propyl-3-methylimidazoliumiodide(PMII),0.13MGuNCS,0.03M I2,0.5Mtert-butylpyridine.

gUnderidenticalconditionZ907andN719show=6.0and6.7%,respectively.

h UnderidenticalconditionZ907shows=8.3%.

i UnderidenticalconditionN719shows=5.5%.

j UnderidenticalconditionZ907shows=6.6%.

k UnderidenticalconditionN719shows=7.3%.

l UnderidenticalconditionN820shows=4.4%.

mThestructurecontainsonlyonesubstituentforX,anotherXisCH3.

n UnderidenticalconditionN3shows=4.1%.

o UnderidenticalconditionN719shows=8.9%.

p UnderidenticalconditionN3shows=7.7%.

q UnderidenticalconditionN3shows=8.4%.

r UnderidenticalconditionZ907shows=8.4%.

s UnderidenticalconditionZ907shows=9.0%.

tUnderidenticalconditionZ907shows=8.8%.

u UnderidenticalconditionN3shows=8.5%.

vUnderidenticalconditionCYC-B6SshowsJSC=9.2,VOC=0.73,FF=0.70,=4.7%.

Table2

Absorptionandelectrochemicaldataforstandarddyesandentries1–5.

No Code max(nm)(ε(10⁴M⁻¹cm⁻¹))^a EoxofRu(III/II)(Vvs.SCE)^b Ref.

Absorptionandelectrochemicaldataforstandarddyeandentries13–16.

No Code max(nm)(ε(10⁴M⁻¹cm⁻¹))^a EoxofRu(III/II)(Vvs.SCE) Ref.

LightabsorptioncanbeenhancedinheterolepticRu(II) com-plexesbyendowingthemwithelectron-donatingalkoxygroups, whichaccomplishnewamphiphilicsensitizers(13–16)withbroad coverageofthevisiblespectrum(Table3).Theεandcellfollow theorder:Z910>Z907>N719[21,22].

Furtherimprovementshavebeenmade,withcell>10%,bythe substitutionofa OMegroupoftheZ910dyewithmore hydropho-bicanalogsviz. OC₆H₁₃ 14(K-19)and O^tBu15(K-77)group atstyrylmoieties oftheamphiphilicheteroleptic Ru-complexes [23–25].Theε valuefor MLCTabsorptionof the14 (K-19)dye is higherthan that of thestandard Z907, N719and Z910 dyes (Table3).Thisfosteredapplicationsanddevicesbasedon14 (K-19)exhibitedbettercellcomparedwiththeZ907andN719,along withbetterstabilityunderthermalstressandlightsoaking,dueto thestrongeradsorptionofthe14(K-19)dyebythepresenceofthe longalkylchains[23].Amoststrikingbreakthrough,however,was achievedinthecaseofthe15(K-77)dye.Lightharvestingbythe lowestenergyMLCTbandwasimprovedfurthercomparedwith14 (K-19)dyeandthecellreached10.5%[24].

Excited-statedirectionalityi.e.efficientelectrontransferfrom the Dye^•+ to the TiO₂ conduction band is another essen-tial requirement for DSCs. For example, the monoanionic Bu4N[Ru(Hdcbpy)(L¹)(NCS)2]16(N945H)dyeshoweda830cm⁻¹ red-shiftedabsorptioncomparedwithZ910andtheεvalue fol-lowstheorder:N945H>Z910>N3(Table3)[26].Thephotovoltaic dataforthedianionic16(N945)dyeshowedasuperiorcell com-paredwiththestandarddianionicdyeN719.Incomparisonwith the14(K-19)dye,16(N945)exhibitsahighercellvalue.Thisis probablyduetotheincorporationofdonorgroupsintheorthoand metapositionsoftheextended␲conjugatedsystemthatincreased theLUMOenergy levelwithoutdisturbingtheHOMOlevel and therebyincreasingthedrivingforceforelectroninjectionintoTiO₂ byadjustingtheelectrondensitiesofthedonormoieties.

InasearchformoreamphiphilicheterolepticRu(II)dyes, amide-functionalizedligandsL²,L³,L⁴ and alkylhydroxyligandL⁵ and correspondingsensitizers(17–20) were synthesized[27]. How-ever,allofthedyesshowa lowerε valueforMLCTabsorption (Table4)along withalowcellcomparedwithhomolepticN3

dyeandamongthesedyes,dye20yieldedabettercellvalueof 8.8%.

HallettandJones[28]synthesized21(A597),aruthenium sen-sitizer,comprisedofadoabpyasanancillaryligand.Although21 (A597)exhibitedfavorableabsorptionandredoxbehaviorin solu-tion (Table4), thecell waslower than Z907,under identical conditions.Thereasonforthelowercellcouldbeduetothepoor lightharvestingofthedyeafteranchoredontoTiO₂filmsand/ora fasterCRphenomenon.

The publication by O’Regan, Ghaddar and co-workers [29]

describesthe22(TG6)dye,witha SC₆H₁₃groupastheelectron donoralongwithstyryl␲conjugationatthe4,4^-positionsofthe bpyligand.Thismonoanionicdyeshowedbettervisiblelight har-vestingcomparedwiththe OC₆H₁₃substituted14(K-19)dye.The presenceofasulfuratominthealiphaticchainmaybeappreciated bycomparingtheHOMO–LUMOenergygapof22(TG6)(2.50eV) withthatoftheN3(2.60eV)determinedbydensityfunctional theo-reticalcalculationsandafastelectroninjectiondynamicsintoTiO₂ CBisseen,astheexcited-stateoxidationpotential(E^∗ox)ofthe22 (TG6)was−0.13VmorenegativecomparedwiththatoftheN719.

Furthermore,interfacialrecombinationbetweenaninjected elec-tronandtheelectrolytewasincreasedusing22(TG6)dye,maybe duetotheextended␲-conjugationandthiscouldbevalidforany dyewithanextended␲-conjugation.

Auniqueclassofsolarcellscomprisedofion-coordinating sen-sitizers,induceastrikingimprovementincomparedwithanon ion-coordinationanalog[30–32].Forinstance,the23(K51)dyeis ananalogofZ907,inwhichhydrophobicalkylchainshavebeen replacedwithion-coordinatingoxyethylenesidechains.Bothdyes showedsimilarlightabsorptionbehavior.However,thecellvalue of23(K51)is7.8%,whichishigherthanthatofZ907,duetofaster rateofdyeregenerationfortheformer[30].Whenlithiumionswith concentrationequaltothedyeadsorbedonthesurfaceoftheTiO2

surfacewereaddedtotheliquidelectrolyte,theZ907dyeexhibited adecreaseinV_OCandanincreasedJ_SCandacounterbalanceofthe parametersresultedinthesameefficiency.Incontrast,insucha situation,the23(K51)dyeshowsLi⁺“ion-trapping”functionality andinhibitstheadsorptionofLi⁺ontheTiO₂surfaceandshows

Table4

Absorptionandelectrochemicaldataforentries17–21.

No Code max(nm)(ε(10⁴M⁻¹cm⁻¹))^a EoxofRu(III/II)(Vvs.SCE)^a Ref.

Absorptionandelectrochemicaldataforentries29–31.

No Code max(nm)(ε(10⁴M⁻¹cm⁻¹))^a EoxofRu(III/II)(Vvs.SCE)^a Ref.

potentialinvarianceontheTiO₂conductionband,withastriking improvementinJSCandVOCvalues.AthigherLi⁺concentration,the excessLi⁺isadsorbedontheTiO2surfaceresultinginaslightdrop inV_OCvalue,buttheJ_SCvalueisincreasedastheTiO₂surfaceadapts toagloballypositivechargeandhenceagreaterlocalconcentration ofI⁻andincreaseddyeregenerationrate[31].

However,the23(K51)dyehadpoorstabilitytowards continu-ousthermalstressat80^◦Cduetodesorptionintotheelectrolyte, becauseofthepresenceofoxyethylenechains,which conferred an increase in solubility. In order to reduce its solubility, the dye[Ru(H₂dcbpy)(L⁶)(NCS)₂] 24(K60), whichhasa ligandwith moreextended␲-conjugationandion-coordinatingoxymethylene chains,wasdeveloped[33].TheeffectofLi⁺iononthephotovoltaic parametersofthedye24(K60)issimilartothatofthedye23(K51).

Thedevicecontainingthe24(K60)dyeexhibitedgoodstabilityand maintainedmorethan93%oftheinitialphotovoltaicperformance uponagingat80^◦Cunderdarkconditionsorat60^◦Cundervisible lightsoaking(100mW/cm²).

Linandco-workers[34]reportedonthesynthesisofaseriesof fluorousamphiphilicrutheniumsensitizers25(CT4),26(CT7)and 27(CT8).The26(CT7)and27(CT8)dyesexhibitcellvaluesthat arecomparablewiththatofstandardN719,andoutperformedthe Z907dye.Themainreasonforthesesuperiorefficienciesisdue tothepresenceofhydrophobicfluorouschains,whichallowthese dyestoadsorbontheTiO2surfacemorestronglyandprovidehigher dyedensitythanalkylchainsinthecaseoftheZ907dye.

The[Ru(H₂dcbpy)(L⁷)(NCS)₂]28(DCSC13) dyeillustratesthe incorporationofL⁷ donorgroupenhancesεofMLCTabsorption bandrelativetothatof2(N820)dye.ItalsoinhibitstheI3−inthe electrolytefromrecombiningwithe⁻(TiO₂),resultinginreduced interfacialrecombinationandhence,increasedV_OCvaluecompared withthatoftheN820dye.Thedrawbackofthe28(DCSC13)dyeis thefactthatbulkygroupsmakethedyesizebiggerandreducethe densityofthedyeontheTiO₂surface.Nevertheless,underidentical conditions,boththeVOCandcellofthe28(DCSC13)sensitizerare higherthan2(N820),besides40%lessadsorptionofmoleculesonto theTiO₂fortheformer[35].

Anewseriesof“donor–acceptor”dyesinvolving␲-conjugation witha covalently attached pendant triphenylamine (TPA) hole acceptingmoietyhavebeenextensivelystudiedinDSCs[36–39].A comparisonbetweendyes29(Ru-bpy–TPA)and2(N820)indicates that,althoughthevaluesforεandcellhavebeengreatlyenhanced fortheformer(Table5),theCRkineticsarenotinfluencedbythe TPAunits[36].Similarto29(Ru-bpy–TPA),amethylsubstituted

TPA-donorantennadye30(IJ-1)alsoshowedenhancedεandcell

valuescomparedwith2(N820)[37].

However,whenanarylamine-basedsecondaryelectrondonor group(L⁸)is employedin thedye, [Ru(H₂dcbpy)(L⁸)(NCS)₂] 31 (N845),an interfacebetweentheholeand theTiO₂ nanostruc-turesisdeveloped[38].The31(N845)dyedisplayedrecombination dynamicsthreeordersofmagnitudelessthan theN719dye.In addition, the long distance between the hole and TiO₂ results along-livedcharge-separatedpair,whichishighlydesirablefor efficientDSCs.Incidentally, SCNfreearyl-TPA-basedN3-analogs havealsoshownsimilarlong-livedcharge-separationstates[39].

Extended␲-conjugationofoligophenylenevinylenegroupsat the4,4^-positionofthebpyin32(D5)and33(D6),allowsthese twodyestoexhibitbetterlightabsorptioncomparedwithN3dye (Table6)andtheJSCandcellvaluesfollowtheorder:D6>D5>N3.

Incontrast,VOCofthe32(D5)and33(D6)dyesremainsame com-paredwithN3dye.Thisisprobablyduetothefactthatsensitizers 32(D5)and33(D6)areunabletoformhydrophobiclayersaround TiO2andthusarenotabletominimizethebackelectrontransfer frome⁻(TiO₂)toI₃⁻.Nevertheless,32(D5)and33(D6)dyesadhere morestronglyontheTiO₂surfacethantheN3dye,henceshowing long-termstability[40].

Yanagida and co-workers [41] employed a Ru sensitizer 34 (HRS-1), consisting of a hybrid of long alkyl chain containing thienyl-vinyl-conjugated bpy ligand as an ancillary ligand. The hydrophobiccharacteristicsofthelonghexylchainsaidin(i) reduc-ingtheinterfacialrecombinationofe⁻(TiO₂)→I₃⁻;(ii)preventing water from reachingthesurface of the TiO2 and dyeleaching, whichtherebyincreasesthestabilityofthedevices.Furthermore, 34(HRS-1)dyeexhibitsanenhancedcellthanthatofN719.

Thepotentialofthiophene-basedancillaryligandsonRu-based DSCswasdevelopedbyWuandco-workers[42].Abis-thiophene substitutedefficientelectrondonorgroupcontainingtheRu sen-sitizer,35 (CYC-B1),showeda dramaticeffectonphotophysical properties(Table7)anddeviceperformance.A100mVcathodic shiftofE₁^Ru(III_/2 ^/II)in35(CYC-B1)reflectsthemoreelectron-donating propertyofthealkylbis-thiophene-bpycomparedwiththatforthe dcbpyligandinN3.Suchrichfundamentalpropertiesmakethe per-formanceofthe35(CYC-B1)dyemoreefficientintheDSCthanN3, underidenticalconditions.

Thedye36(CYC-B3)[43],whichcontainsonelessthiophene unitthan35(CYC-B1),causesdiminishedlightabsorption(Table7) alongwithalowersolubility,makingthisdyeinferiortoward pho-tovoltaiccells,incomparison withthat ofthe35 (CYC-B1).The

Table6

Absorptiondataforentries32–33.

No Code max(nm)(ε(10⁴M⁻¹cm⁻¹))^a Ref.

␲–␲*ord␲–␲* d␲–␲*

32 D5 441(4.03) 527(2.60) [40]

33 D6 449(7.85) 539(3.43) [40]

aInDMF.

Table7

Absorptionandelectrochemicaldataforstandarddyeandentries34–37.

No Code max(nm)(ε(10⁴M⁻¹cm⁻¹)) EoxofRu(III/II)(Vvs.SCE) Ref.

␲–␲* ␲–␲*ord␲–␲* d␲–␲*

34 HRS-1 371 (4.24) 542 (1.87)^a 0.80^a [41]

35 CYC-B1 312(3.58) 400(4.64) 553(2.12)^b 0.76^b [42]

36 CYC-B3 320(5.23) 370(3.30) 544(1.57)^b – [43]

37 SJW-E1 546(1.87)^b – [43]

aInEtOH.

bDMF.

Table8

Absorptionandelectrochemicaldataforentries38–39.

No Code max(nm)(ε(10⁴M⁻¹cm⁻¹))^a EoxofRu(III/II)(Vvs.SCE)^a Ref.

␲–␲* ␲–␲*ord␲–␲* d␲–␲*

38 Ru-EDOT 538(1.60) 0.65 [44]

39 CYC-B11 320(4.55) 380(5.40) 554(2.42) 0.72 [46]

aInDMF.

introduction of electron-donating EDOTinstead of a thiophene moietyin37(SJW-E1)dyeresultedinanenhancedεofMLCTband comparedwith36(CYC-B3)[43].

In summary, the relative cell  followsthe order:35 (CYC-B1)>37 (SJW-E1)≈34(HRS-1)>N3>36 (CYC-B3)dyes.Ofnote, theMLCTεvalueoftheN3dyeislowerthanthatof36(CYC-B3),but thecellisstillhigher.Electrochemicalimpedancespectroscopy (EIS)dataintheformofaBodephaseplotshowthatthelifetime ofthee⁻(TiO2)is,indescendingorder:N3>37(SJW-E1)>36 (CYC-B3).ThisprobablyaccountsforthehigherofN3althoughitshows moderatelight-harvestingability.

Inthecaseofthe38(Ru-EDOT)dye[44],thefollowingchanges havebeenmade:anincreaseintheelectron-donatingabilityof thethiophenemoietybytheEDOTgroupalongwithextending␲ conjugationandthesimultaneousremovalofthelongalkylchain.

ThehigherεofMLCTbandandcathodicallyshiftedE^Ru(III_1/2 ^/II)values for38(Ru-EDOT)relativetothoseofN3dyereflectstheinfluence ofthenew,electron-richligand(Table8).Thisdyeexhibitsasimilar ion-coordinatingeffectwhenLi⁺ionsareadded[45].

Replacementofthehexyl-terminalchainin35(CYC-B1)with anelectron-richhexylthio-terminalchainin39(CYC-B11),causes thehighestεvalue(2.42×10⁴M⁻¹cm⁻¹)oftheRu(II)-sensitizers containingthiophenemoietiesdiscussedsofar.A40mVcathodic shiftinE^Ru(III/II)_1/2 of39(CYC-B11)relativetothatof35(CYC-B1), reflectstheinfluenceofthesulfuratomsonelectrondonationand

␲-conjugationtothebpyancillaryligandandonthemetalcomplex

(Table8)[46].Adevicefabricatedusingthesensitizer39(CYC-B11), inthepresenceofavolatileliquidelectrolyteshowedanexcellent cellvalueof11.5%.

Thedye40(C101),containedapendanthexylchainina thio-phene,whichwasattachedtoabpyunitshowslong-termstability [47]. The MLCTabsorption bandof 40 (C101) is 803cm⁻¹ red-shiftedrelativetoZ907withenhancedεvalue(Table9)andithas aremarkablecellof11%.

Uponsubstitutionofthethiophenegroupin40(C101)withthe furanin41(C102),thecellvaluewasdiminished[47].Thisis prob-ablyduetotheloweradsorbeddyedensityofthelattercompared withtheformeronaTiO2 surface.Hence,41(C102)dyecoated cellsaremoreexposedtotheelectrolyteandthustheCRrateis enhanced,resultinginalowercellvalue.Thesensitizer42(C104) [48],whichcontainedathieno[3,2-b]thiophenemoiety,exhibited amuchhigherεattheMLCTbandcomparedwiththatofZ907and thus,thecellvalueishigherthanthatofthelattercomplex,under identicalconditions.

Wangandco-workers[49]developedtheruthenium sensitiz-ers,43(C103)and44(C107),containingEDOTandO-EDOTunits, respectively,conjugatedatthe4,4^-positionsofabpyligand.The εandcellvaluesfollowthetrend:44(C107)>43(C103)>Z907 (Table10).However,V_OCfollowsthereverseorderasbecauselower dyedensityofthelargeancillaryligandcontaining44(C107)dye, hence,CRis fasterand thusVOC valueislowerthan 43(C103).

Thedye45(C105) [50]containsa selenopheneunit conjugated withabpyligandcoordinatedwithRu(II).Theεvalueincreasesin

Table9

Absorptiondataforentries40–42.

No Code max(nm)(ε(10⁴M⁻¹cm⁻¹))^a Ref.

␲–␲*ord␲–␲* d␲–␲*

40 C101 407(1.80)547(1.75) [47]

41 C102 407(1.76)547(1.68) [47]

42 C104 312(5.50) 368(4.75)553(2.05) [48]

aInDMF.

Table10

Absorptiondataforentries43–46.

No Code max(nm)(ε(10⁴M⁻¹cm⁻¹))^a Ref.

Absorptiondataforentries47–50.

No Code max(nm)(ε(10⁴M⁻¹cm⁻¹)) Ref.

theorderof45(C105)>40(C101)>41(C102),consistentwiththe electropositivityandthesizeoftheheteroatoms(Se>S>O) and photovoltaicparametersof41(C102)areparalleltothatofthe40 (C101).Thesameeffectonphotophysicalandphotovoltaic param-etershasbeenobservedbyreplacingthehexyl-terminalchainin40 (C101)withanelectron-richhexylthio-terminalchainin46(C106), asobservedfor35(CYC-B1)and39(CYC-B11).TheεvalueofMLCT bandfor46(C106)ishigherthanthatoftheZ907and40(C101) dyes(Table10),andencouragingly,ithasacell=11.3%[51].

Adendriticterthiophenefunctionalizedrutheniumsensitizer, 47(3T),wasdevelopedbyGrätzel,Bäuerle,andco-workers[52].

Although47 (3T)showedbetterabsorptionbehavior thanZ907 (Table11),thenon-planarconfigurationandˇ-substituentpattern onterthiophenerestrictfurtherenhancementinεthan40(C101).

Although47 (3T)showedbetterabsorptionbehavior thanZ907 (Table11),thenon-planarconfigurationandˇ-substituentpattern onterthiophenerestrictfurtherenhancementinεthan40(C101).

在文檔中 Structure optimization of ruthenium photosensitizers for efficient dye-sensitized solar cells - A goal toward a "bright" future (頁 2-15)