LETTER. All-solid-state dye-sensitized solar cells with high efficiency

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1 doi:1.138/nture1167 All-solid-stte dye-sensitized solr ells with high effiieny In Chung 1, Byunghong Lee 2, Jiqing He 1, Roert P. H. Chng 2 & Merouri G. Kntzidis 1 Dye-sensitized solr ells sed on titnium dioxide (TiO 2 ) re promising low-ost lterntives to onventionl solid-stte photovolti devies sed on mterils suh s Si, CdTe nd CuIn 12x G x Se 2 (refs 1, 2). Despite offering reltively high onversion effiienies for solr energy, typil dye-sensitized solr ells suffer from durility prolems tht result from their use of orgni liquid eletrolytes ontining the iodide/tri-iodide redox ouple, whih uses serious prolems suh s eletrode orrosion nd eletrolyte lekge 3. Replements for iodine-sed liquid eletrolytes hve een extensively studied, ut the effiienies of the resulting devies remin low 3 9. Here we show tht the solutionproessle p-type diret ndgp semiondutor n e used for hole ondution in lieu of liquid eletrolyte. The resulting solid-stte dye-sensitized solr ells onsist of CsSnI 2.95 F.5 doped with SnF 2, nnoporous TiO 2 nd the dye N719, nd show onversion effiienies of up to 1.2 per ent (8.51 per ent with msk). With ndgp of 1.3 eletronvolts, enhnes visile light sorption on the red side of the spetrum to outperform the typil dye-sensitized solr ells in this spetrl region. Photovoltis is promising renewle energy tehnology tht onverts sunlight to eletriity, with rod potentil to ontriute signifintly to solving the future energy prolem tht humnity fes. To dte, semiondutor solr ells dominte ommeril mrkets, with rystlline Si hving n 8% shre; the remining 2% is mostly thinfilm solr tehnology, suh s CdTe nd CuIn 12x G x Se 2 (ref. 1). The former is n indiret ndgp semiondutor typilly requiring 3-mm-thik sorption lyer, nd mteril nd proessing osts re very high. The ltter ontins elements tht re toxi nd of low undne in the Erth. However CuIn 12x G x Se 2 forms the est performing thin-film solr devies, exhiiting n effiieny of,2%, ut is more thn 1.4 times s expensive s CdTe nd morphous Si. A lowost nd environmentlly friendly lterntive to these solid-stte devies is the dye-sensitized solr ell (DSC) 1,2. It is inexpensive to prepre, nd the light-weight thin-film strutures re omptile with utomted mnufturing. Conventionl DSCs (Grätzel ells) onsist of self-ssemled monolyer of moleulr dye t the interfe etween mesoporous wide-ndgp semiondutor oxide nd liquid eletrolyte 1,2. The most ommonly used redox ouple is iodide/tri-iodide (I /I 3 )inn orgni liquid eletrolyte however, it is highly orrosive, voltile nd photoretive, interting with ommon metlli omponents nd seling mterils. Consequently, it dversely ffets long-term performne nd durility 3. Alterntive liquid eletrolytes free of the I /I 3 redox ouple hve een long-term gol in this field 5,6,11. The ultimte solutions would e purely solid-stte ells, given the inevitle prolems of ny liquid eletrolyte, suh s lekge, hevy weight nd omplex hemistry. Efforts hve foused on using solidstte orgni or p-type onduting polymer hole-trnsport mterils (HTMs), ut their onversion effiieny remins modest. Solr ells using spiro-ometad (refs 9, 12) nd is-edot (ref. 7) exhiit the highest onversion effiienies mong orgni nd onduting polymer mterils of 6.8% nd 6.1%, respetively. (TAD is 2,29,7,79-tetrnis(N,N-di-p-methoxyphenyl-mine)9,99-spiroifluorene, nd EDOT is 2,29-is(3,4-ethylenedioxythiophene).) A generl prolem of solid HTMs in DSCs is poor filling of the nnoporous TiO 2 lyer; this interrupts the hole-onduting pth etween the HTM nd the dye moleule dsored on TiO 2 (ref. 1). Despite mny ntiipted dvntges, inorgni HTMs re unommon. CuI, CuSCN nd NiO re exmples, ut their moilities re very low. The effiieny of CuI-sed DSCs ws found to initilly reh out 3%, ut rpidly photodegrded 1. Cells of CuSCN (ref. 13) nd p-type NiO prtiles 14 showed low effiienies. Here we report new type of ll-solid-stte, inorgni solr ell system tht onsists of the p-type diret ndgp semiondutor nd n-type nnoporous TiO 2 with the dye N719 (isdiisothioynto-is(2,29-ipyridyl-4,49-dioxylto) ruthenium(ii) is-(tetrutylmmonium)). We show tht is well fitted for this purpose euse of its energy gp of 1.3 ev nd remrkly high hole moility of m h m 2 V 21 s 21 t room temperture. We found tht is solule in polr orgni solvents, suh s etonitrile, N,N,-dimethylformmide nd methoxyetonitrile. Consequently, it is solution-proessle nd n e trnsferred into TiO 2 pores t moleulr level to mke intimte ontts with dye moleules nd TiO 2. We present results showing tht doping of with F nd 1 m A (α 1 ) d σ (S m 1 ) σ S 4 2 E g = 1.3 ev E (ev) T (K) Figure 1 Crystl struture nd optil nd eletril trnsport properties of., Distorted three-dimensionl perovskite struture of t room temperture. Red polyhedron, [SnI 6/2 ] 2 ; yellow sphere, Cs., Shrp sorption edge t 1.3 ev of. A, sorption in units of S 21, where is the sorption oeffiient nd S the sttering oeffiient. E, energy in ev. E g, the vlue of the energy gp., A typil ingot of grown in Bridgmn furne. d, Temperture dependene of eletril ondutivity (s, filled squres) nd Seeek oeffiient (S, filled irles) S (μv K 1 ) 1 Deprtment of Chemistry, Northwestern University, Evnston, Illinois 628, USA. 2 Mterils Siene nd Engineering, Northwestern University, Evnston, Illinois 628 USA. 486 NATURE VOL MAY 212

2 RESEARCH SnF 2 drmtilly improves the photourrent density (J SC ) nd power onversion effiieny (g). At n optimum molr onentrtion of 5% F nd 5% SnF 2, the ell exhiits the highest effiieny so fr reported for solid-stte solr ell equipped with dye-sensitizer: g 5 1.2% under the stndrd ir mss 1.5 (AM 1.5) irrdition (1 mw m 22 ), nd g % with msk. The oserved vlue is lose to tht of the highest reported performne N719-dye-ontining Grätzel ell (g < 11%) 15. In our ells, reples the entire liquid eletrolyte. Optil sorption spetrosopy nd inident photon-to-urrent onversion effiieny (IPCE) mesurements show tht our ells exhiit redshifted sorption edge (t 789 nm) ompred to tht of the typil Grätzel ell nd outperform it in the red nd infrred spetrl regions. We note tht poor performne of DSCs in this spetrl rnge hs een one of their min drwks. dopts distorted three-dimensionl perovskite struture tht rystllizes in the orthorhomi Pnm spe group t room temperture (Fig. 1) 16. The ompound is diret ndgp semiondutor with shrp optil sorption edge t 1.3 ev (Fig. 1). Hll effet mesurements on thin film showed p-type ondution ehviour nd very high hole moility of m h m 2 V 21 s 21 t room temperture. To onfirm this, we prepred lrge, ule-free nd rk-free polyrystlline ingots (Fig. 1), nd exmined the eletril properties of the ingot. Thermoeletri power mesurements gve positive Seeek oeffiients over the entire temperture rnge with liner dependene on temperture, suggesting p-type ondution (Fig. 1d). These hole moilities re two to three orders of mgnitude lrger thn those of ny orgni polymer HTM. A shemti digrm of the reltive energy levels of,tio 2 nd the N719 dye is shown in Fig. 2. The positions of the vlene nd mximum (VBM) nd the ondution nd minimum (CBM) of were determined y the work funtion (4.92 ev) ording to ultrviolet photoemission spetrosopy nd the energy gp (1.3 ev). The energy levels of TiO 2 nd the N719 dye re tken from the literture 5,17. The digrm vlidtes the exellent fit of in our new solid-stte solr ell system, giving perfet hrge seprtion nd repling liquid eletrolytes. The CBM lies nerly in the sme energy level s the lowest unoupied moleulr oritl (LUMO) of the N719 dye nd ove the CBM of TiO 2. The VBM lies muh higher thn the highest oupied moleulr oritl (HOMO) of the dye. From onventionl view of DSCs, eletrons generted y the dye re trnsported to n-type Energy versus vuum (ev) Vuum TiO ev ΔE = 3.2 ev 7.46 ev e N ev 3.62 ev 6.1 ev h + ΔE = 1.3 ev 4.92 ev Figure 2 Energy levels of the omponents of the solid-stte solr ell. The vlene nd mximum (VBM; ornge olour) nd the ondution nd minimum (CBM; lue olour) of TiO 2 nd re represented in ev, long with the energy differene etween the edges. The ground (HOMO; ornge) nd exited sttes (LUMO; lue) of N719 dye is lso shown. The energy sle is referened to the vuum level. DE,e 2 nd h 1 represent the vlue of the energy gp, eletron nd hole, respetively. semiondutor TiO 2 nd the oxidized dye is redily regenerted y p-type semiondutor euse it is fst hole trnsporter. The ells we report here re likely to operte y mehnism tht is different from tht of onventionl DSCs inorporting liquid eletrolyte. For exmple, solid-stte ells mde with 2x F x nd TiO 2 without the dye worked s photovolti devies ut with low effiieny,,.2% (Supplementry Informtion). This demonstrtes tht 2x F x n t s its own dye nd photogenerted holes nd eletrons n e seprted, ut it is not very effiient y itself. So, if in our dye-ontining ells sors the light insted of the dye, eletrons nd holes would e generted therein. Eletrons would then e trnsferred to the CBM of TiO 2. At this stge, the N719 dye ould t onduit forfsthrgetrnsferfrom filitting rpid hrge seprtion through hole trnsfer to, resulting in high effiieny. Therefore, we presume tht dye s well s my tke prt in light sorption nd sensitiztion in onerted wy, providing synergi effet. This issue will e the fous of our future studies. The file nd inexpensive frition proedure of our ells is simple ompred to other reported methods for photovolti devies. Powdered (out 123 mg) ws dissolved in polr orgni solvents (1.5 ml) to give trnsprent yellow solutions t room temperture. The solutions were injeted onto the nnoporous TiO 2 eletrode y miropipette nd dried under nitrogen. Complete evportion of the solvent ws onfirmed with infrred spetrosopy. The unique dvntge of is tht solution phse of this semiondutor mteril n therey diffuse into the nnopores of TiO 2 nd e stilized s solid phse on solvent removl. There re no yproduts. Aordingly, neither high-temperture, omplex hemil retions nor high-vuum deposition proesses re required, ontrry to most onventionl semiondutor solr ells. Cross-setionl snning eletron mirosopy (SEM) imges of our ells revel tht homogeneously permetes deep into the nnoporous TiO 2 sustrte (Fig. 3, ). The elementl mpping on the Cs Sn I Ti O d TiO 2 Si A B B 1 μm C C e 2 μm Figure 3 Cross-setionl eletron mirosopy imge of /TiO 2 ell on Si., Cross-setionl SEM imge. Thikness is,1 mm., Elementl mpping y energy dispersive spetrosopy for the retngulr re indited y lue ox in, showing homogeneous infiltrtion of into nnoporous TiO 2., Bk-sttering ross-setionl SEM imge, showing no disernile ontrst differene. d, Cross-setionl HRTEM imge tken ner the ottom of the ell. A, nd TiO 2 overlpped; B, high (hkl) index re showing unler lttie fringes; C, TiO 2. e, Left pnel, experimentl eletron diffrtion pttern, inditing TiO 2 (ring ptterns) nd (spots indited y white rrows etween ring ptterns); right pnel, theoretil lultion of ntse TiO 2. Si 24 MAY 212 VOL 485 NATURE 487

3 RESEARCH LETTER retngulr re shown in Fig. 3 demonstrtes exellent distriution of Cs, Sn nd I toms throughout the 1-mm-thik nnoporous TiO 2 (Fig. 3). The ross-setionl k-sttering eletron imge shows no disernile ontrst differene over the exmined re, suggesting tht homogeneously spreds over the entire TiO 2 re (Fig. 3). The ross-setionl high-resolution trnsmission eletron mirosopy (HRTEM) imge tken t the ottom prt of the TiO 2 eletrode indites tht effetively fills the TiO 2 pores, nd rystllizes on the surfe of the TiO 2 (Fig. 3d). The nd TiO 2 phses re oserved to overlp with eh other, s shown in re A in Fig. 3d. Are B hs too high (hkl) index to give ler lttie fringes. Are C in Fig. 3d indites TiO 2. Figure 3e shows experimentl eletron diffrtion ptterns (left) in omprison with tht of the theoretil TiO 2 (right). The ring ptterns re ssigned to ntse TiO 2, nd the spots (indited y the white rrows) etween the rings of TiO 2 orrespond to. To effiiently operte DSCs, the dye sensitizer tht is dsored on the surfe of the nnoporous TiO 2 trnsfers n eletron to TiO 2 nd is then regenerted y n eletrolyte or p-type ondutor 1,2. Consequently, intimte ontt etween the ltter nd dye sensitizer is essentil for perfet hrge seprtion. However, homogeneous infiltrtion of p-type solid HTMs throughout the n-type nnoporous mterils is the min hllenge for solid-stte DSCs 1,3. Orgni polymer HTMs do not effiiently infiltrte the mirometre-thik nnoporous TiO 2,degrding performne. When monomers re used for etter penetrtion into the pores, undesirle omplex hemil retions, suh s photoeletrohemil polymeriztion, re required 2,1.Thekeyhrteristis of in our solr ells is tht (1) it is solution-proessle, nd thus permetes throughout the entire TiO 2 struture, llowing file hrge seprtion nd hole removl, nd (2) it exhiits very lrge hole moilities. To ompre the optil response of the CsSnI 2.95 F.5 ell with tht of onventionl Grätzel ell, the ltter ws prepred with N719 dye nd showed,1% effiieny 18. The optil sorption spetrum of the CsSnI 2.95 F.5 -ontining ell, otined in trnsmission mode, revels well-defined edge t 789 nm, whih is signifintly red-shifted from tht of the Grätzel ell 18 with N719 dye, t 667 nm (Fig. 4). This oservtion indites tht our ell sors red nd ner-infrred light more effiiently thn the Grätzel ell. Note tht lk of sunlight sorption in the red nd ner-infrred regions hs een hllenge for typil ruthenium-sed dyes. Figure 4 ompres the inident photon-to-urrent onversion effiieny (IPCE) s funtion of exittion wvelength for the CsSnI 2.95 F.5 ell with the Grätzel ell. The IPCE spetrum is mesure of the light response of photovolti Asorne (.u.) CsSnI 2.95 F.5 /N719/TiO 2 N719/TiO , 1,2 Wvelength (nm) IPCE (%) CsSnI 2.95 F.5 ell Grätzel ell Wvelength (nm) Figure 4 Optil response of the CsSnI 2.95 F.5 ell nd onventionl Grätzel ell., Optil sorne spetr of the devies onsisting of CsSnI 2.95 F.5 /N719 dye/tio 2 (red line) nd N719 dye/tio 2 (liquid eletrolyte ws not dded here) (lk line)., The IPCE spetrum s funtion of the wvelength of monohromti light tht impinges on the CsSnI 2.95 F.5 ell (filled irles) in omprison with tht of the N719-dye-ontining Grätzel ell (filled squres). devies, whih is diretly relted to the short-iruit urrent. In the nm spetrl rnge, our ell produes higher nd roder photourrent density in the externl iruit under monohromti illumintion (per photon flux). Note tht the upper limit of our IPCE mesurement setting is 7 nm, resulting in shrp drop eyond 67 nm. The pristine ell gve very good photourrent density voltge (J V) hrteristis open-iruit voltge (V OC ), fill ftor (FF), short iruit urrent density (J SC ) nd overll power onversion effiieny (g) s solid-stte solr ell: V OC V, FF %, J SC ma m 22, g % (g 5 V OC 3 J SC 3 FF) (Fig. 5). To improve J V hrteristis, we studied the effet of fluorine doping on. The optimum molr onentrtion of 5% fluorine doping gve remrkle inrese in J SC, whih rehed 12.2 ma m 22, resulting in 1.5-fold lrger vlue of g, 5.62%. Further improvement ws otined y introduing SnF 2 into CsSnI 2.95 F.5. The SnF 2 doping proess ws simple. The desired mounts of SnF 2 nd CsSnI 2.95 F.5 powders were dded to polr orgni solvents with stirring. The resulting solutions were injeted onto the nnoporous TiO 2 eletrodes. The CsSnI 2.95 F.5 smple doped with 2% SnF 2 provided 29% nd 21% inrese in J SC (15.7 ma m 22 )ndg (6.81%), respetively, ompred to the CsSnI 2.95 F.5 smple. The optimum molr onentrtion of SnF 2 doping in CsSnI 2.95 F.5 ws found to e 5%. For the orresponding ell, the TiO 2 nnoporous film ws pre-treted y fluorine plsm ething proess to inrese the size of the nnopores nd nnohnnels s desried in the literture 18. This lso possily helps to redue surfe sttes nd hrged prtile reomintion 18. The resulting ell showed very good J V hrteristis: J SC ma m 22 ; V OC 5.73 V; FF %, g %. To fully employ the photon flux sored, we pplied two lyers of the three-dimensionl inverse photoni rystl ZnO (ref. 19) over the ounter eletrode of the sme ell. Eh lyer of the photoni rystl hd different hole dimeter vlues of 375 nm nd 41 nm were used. The orresponding ell exhiited J SC ma m 22, V OC V, FF % nd g 5 1.2%. When msk ws pplied on the ell, g % ws oserved (Supplementry Informtion). The oserved effiieny is the highest mong ny kind of dyesensitized solr ell free of liquid eletrolyte, nd is lose to tht of J (ma m 2 ) CsSnI 2.95 F.5 CsSnI 2.95 F.5 + 2% SnF 2 CsSnI 2.95 F.5 + 5% SnF 2 CsSnI 2.95 F.5 + 5% SnF 2 with ZnO CsSnI 2.95 F.5 + 1% SnF Photovoltge (V) Figure 5 Photourrent density voltge (J V) hrteristis of the solr ell devies under irrdition of 1 mw m 2 simulted AM 1.5 sunlight. These solr ells inorported nd CsSnI 2.95 F.5 doped with SnF 2. Fluorine nd SnF 2 doping inrese photourrent density, resulting in n improved power onversion effiieny (g). Applition of three-dimensionl ZnO photoni rystl lyers further enhnes the photourrent density, nd gives the highest vlue of g (1.2%) for the ell of CsSnI 2.95 F.5 doped with SnF NATURE VOL MAY 212

4 RESEARCH the high performne Grätzel ell (g < 11%). The smples with 1% SnF 2 resulted in redution in J SC (mesured vlue, 13.6 ma m 22 ) nd g (mesured vlue, 5.46%). Compounds of the type CH 3 NH 3 PX 3 (X 5 Br, I), whih re isostruturl with, hve een used s visile light sensitizers dsored on the TiO 2 surfe for photovolti ells. These ells, however, employed orgni liquid eletrolyte ontining LiX/X 2 redox ouples, nd exhiited low effiieny 2. We hve demonstrted the superior performne of the p-type inorgni high-hole-moility semiondutor 2x F x nd its ility to reple the prolemti orgni liquid eletrolytes in dyesensitized solr ells (DSCs). We refer to these solid-stte solr ells s solid-stte DSCs (SSDSCs). Unlike liquid eletrolytes with their omplex hemistry, rystlline inorgni solids promise long-term stility, nd lso enle theoretil lultions sed on eletroni struture nd further improvement of their photovolti properties. The new solr ells desried here re ll-solid-stte, inorgni systems. The 2x F x ompounds onsist of inexpensive, undnt elements nd re solution-proessle t room temperture, llowing for lowost proessing. Our new ell rehes onversion effiieny of,1.2% (8.51% with msk), nd is the first exmple of n ll solid-stte dyesensitized solr ell system tht my eventully exeed the performne of liquid eletrolyte Grätzel ell. With further optimiztion nd new dyes 11, muh higher effiienies re likely. This work opens up the possiility of semionduting solid mterils eoming stte of the rt nd promoting muh higher effiienies thn hve een possile with onventionl DSCs. METHODS SUMMARY Synthesis of 2x F x. Pure 2x F x ( # x # 1) ompounds were hieved y heting stoihiometri mixture of CsI, SnI 2 nd SnF 2 in n evuted Pyrex or fused sili tue t 45 uc for 3 min, followed y quenhing to room temperture. The ground powders (,1 35 mg) were dissolved/dispersed in nhydrous polr orgni solvents (1.5 ml): N,N-dimethylformmide, etonitrile nd methoxyetonitrile. For SnF 2 doping, pproprite rtios of CsSnI 2.95 F.5 nd SnF 2 powders were stirred in the sme orgni solvents. TiO 2 eletrode preprtion nd devie ssemly. TiO 2 eletrode preprtion, fluorine plsm ething proess, nd devie ssemly re desried in the literture 18. The solutions of 2x F x (with SnF 2 if neessry) were injeted into the ell y miropipette nd dried. The ZnO photoni rystls were prepred s desried in the literture 19. They were tthed on the top of the ounter-eletrode if neessry. Full Methods nd ny ssoited referenes re ville in the online version of the pper t Reeived 6 Ferury; epted 8 Mrh Hgfeldt, A., Boshloo, G., Sun, L. C., Kloo, L. & Pettersson, H. Dye-sensitized solr ells. Chem. Rev. 11, (21). 2. Grätzel, M. Reent dvnes in sensitized mesosopi solr ells. A. Chem. Res. 42, (29). 3. Yngid, S., Yu, Y. H. & Mnseki, K. Iodine/iodide-free dye-sensitized solr ells. A. Chem. Res. 42, (29). 4. Koh, J. K., Kim, J., Kim, B., Kim, J. H. & Kim, E. Highly effiient, iodine-free dyesensitized solr ells with solid-stte synthesis of onduting polymers. Adv. Mter. 23, (211). 5. Deneke, T. et l. High-effiieny dye-sensitized solr ells with ferroene-sed eletrolytes. Nture Chem. 3, (211). 6. Wng, M. K. et l. An orgni redox eletrolyte to rivl triiodide/iodide in dyesensitized solr ells. Nture Chem. 2, (21). 7. Liu, X. Z. et l. An effiient orgni-dye-sensitized solr ell with in situ polymerized poly(3,4-ethylenedioxythiophene) s hole-trnsporting mteril. Adv. Mter. 22, E15 E155 (21). 8. Jing, K. J. et l. Photovoltis sed on hyridiztion of effetive dye-sensitized titnium oxide nd hole-ondutive polymer P3HT. Adv. Funt. Mter. 19, (29). 9. Bh, U. et l. Solid-stte dye-sensitized mesoporous TiO 2 solr ells with high photon-to-eletron onversion effiienies. Nture 395, (1998). 1. Bisquert, J. Dilemms of dye-sensitized solr ells. ChemPhysChem 12, (211). 11. Yell, A. et l. Porphyrin-sensitized solr ells with olt (II/III)-sed redox eletrolyte exeed 12 perent effiieny. Siene 334, (211). 12. Ci, N. et l. An orgni D-p-A dye for reord effiieny solid-stte sensitized heterojuntion solr ells. Nno Lett. 11, (211). 13. O Regn, B., Lenzmnn, F., Muis, R. & Wienke, J. A solid-stte dye-sensitized solr ell frited with pressure-treted P25-TiO 2 nd CuSCN: nlysis of pore filling nd IV hrteristis. Chem. Mter. 14, (22). 14. Bndr, J. & Weersinghe, H. Solid-stte dye-sensitized solr ell with p-type NiO s hole olletor. Sol. Energy Mter. Sol. Cells 85, (25). 15. Kroon, J. M. et l. Nnorystlline dye-sensitized solr ells hving mximum performne. Prog. Photovolt. Res. Appl. 15, 1 18 (27). 16. Shum, K. et l. Synthesis nd hrteriztion of thin films. Appl. Phys. Lett. 96, (21). 17. Grätzel, M. Photoeletrohemil ells. Nture 414, (21). 18. Lee, B., Buhholz, D. B., Guo, P. J., Hwng, D. K. & Chng, R. P. H. Optimizing the performne of plsti dye-sensitized solr ell. J. Phys. Chem. C 115, (211). 19. Lee, B. et l. Mterils, interfes, nd photon onfinement in dye-sensitized solr ells. J. Phys. Chem. B 114, (21). 2. Kojim, A., Teshim, K., Shiri, Y. & Miysk, T. Orgnometl hlide perovskites s visile-light sensitizers for photovolti ells. J. Am. Chem. So. 131, (29). Supplementry Informtion is linked to the online version of the pper t Aknowledgements The uthors knowledge support for this ollortive reserh: NSF-DMR for R.P.H.C.; DOE Energy Frontier Reserh Center, ANSER, DE-SC159 for B.H.L., J.H. nd M.G.K.; the Inititive for Energy nd Sustinility t Northwestern (ISEN) for I.C. Devie testing ndmesurements were done in the ANSER Filities nd mterils hrteriztion ws performed in the NSFMRSEC Filities (DMR ). Author Contriutions I.C. nd M.G.K. oneived nd designed the experiments nd prepred the mnusript. I.C. synthesized mterils. R.P.H.C. nd B.L. designed nd frited the solr ells, I.C. nd B.L. performed mesurements. J.H. olleted TEM dt. I.C., B.L., R.P.H.C. nd M.G.K. disussed the results nd wrote the mnusript. Author Informtion Reprints nd permissions informtion is ville t The uthors delre no ompeting finnil interests. Reders re welome to omment on the online version of this rtile t Correspondene nd requests for mterils should e ddressed to M.G.K. (m-kntzidis@northwestern.edu). 24 MAY 212 VOL 485 NATURE 489

5 RESEARCH LETTER METHODS TiO 2 eletrode preprtion. High-purity ntse TiO 2 nnoprtiles nd their pste form were prepred s desried in the literture 18. The otined pste ws spred on the fluorine-doped SnO 2 glss sustrte (FTO, Pilkington, TEC 8 glss, 8 V per squre, 2.3 mm thik) using dotor lde, followed y grdul lintion to remove residul polymers under n ir flow t 15 uc for 15 min, t 32 uc for 1 min nd t 5 uc for 3 min, giving pure ntse TiO 2 nnoprtile film with fltnd smoothsurfe. A fluorine plsm ething proess ws pplieds desried in the literture if neessry 18. The film thikness ws determined y surfe profiler (Tenor P-1). The ZnO photoni rystls were prepred s desried in the literture 19. They were tthed on the top of the ounter-eletrode if neessry. Devie ssemly. The TiO 2 eletrode ws immersed in n ethnol solution ontining purified M (is-diisothioynto-is(2,29-ipyridyl-4,49- dioxylto) ruthenium(ii) is(tetrutylmmonium)) (N719, Solronix) for 18 h t room temperture, followed y rinsing with ethnol nd drying under nitrogen flow. The ounter-eletrode ws prepred y oting FTO glss with thin lyer of 5 mm H 2 PtCl 6 solution (in isopropnol), followed y heting t 4 uc for 2 min. The two eletrodes were seled together with therml melt polymer film (Surlyn, DuPont). The typil tive re of the ell ws,.2.3 m 2. The ext re of photonode ws lirted y n optil snner under resolution of 6 dots per inh. The 2x F x without SnF 2 solutions were injeted into the ell y miropipette nd dried. Devie hrteriztion. The devies were evluted under 1 mw m 22 stndrd ir mss 1.5 glol (AM 1.5G) sunlight simultion with lss A solr ell nlyser (Spetr Nov Teh.). A silion solr ell fitted with KG3 filter tested nd ertified y the Ntionl Renewle Energy Lortory (NREL) ws used for lirtion. The KG3 filter ounts for the different light sorptions of the dye-sensitized solr ell nd silion, nd ensures tht the spetrl mismth orretion ftor pprohes unity. Inident photon-to-urrent onversion effiieny ws mesured using monohromtor (Join-Yvon fluoresene spetrometer) nd potentiostt (CH Instruments 122). Eletrohemil impedne spetrosopy ws performed under the sme light illumintion with n impedne nlyser (Solrtron 126) nd potentiostt (Solrtron 1287) when the devie ws tested t its V OC.An dditionl low mplitude modultion sinusoidl voltge of 1 mv rms ws lso pplied etween the node nd thode of the devie over the frequeny rnge of.5215 khz. Eletron mirosopy. SEM imges were tken with Hithi S-48II SEM. TEM investigtions were rried out in JEOL 21F mirosope operting t 2 kev elerting voltge. hrteriztion. Hll effet mesurement. The smple ws sujeted to flow of nitrogen gs to protet it from oxidtion during mesurements. A d.. urrent soure (622 Keithley Instruments) ws used to supply onstnt 1 ma urrent through the smple while n externl mgneti field up to 61.4 T ws pplied in steps of.25 T. The voltge response, oth in the positive nd negtive diretions of the mgneti field, ws reorded t eh respetive step y nnovoltmeter (2182A Keithley Instruments) nd the hlf-differene of the two signls ws reorded s the Hll voltge. Chrge-trnsport properties. The ingot ws ut nd polished into retngulr shpe with dimensions of,2 mm3 3 mm3 8 mm under N 2 tmosphere. Eletril ondutivity nd Seeek oeffiient were mesured simultneously under He tmosphere from room temperture to 65 K on ULVAC-RIKO ZEM-3 instrument system. The mehnil eletrodes mde of Rh/Pt nd Pt were diretly ontted to the ingot with no use of hemil pste. Optil sorption spetrosopy. Optil diffuse refletne mesurements were performed t room temperture using Shimdzu UV-311 PC spetrometer operting in the 2 2,5 nm region. The refletne versus wvelength dt generted were used to estimte the ndgp of the mteril y onverting refletne to sorption dt ording to the Kuelk-Munk eqution: /S 5 (1 R) 2 (2R) 1, where R is the refletne nd nd S re the sorption nd sttering oeffiients, respetively.