SOLAR CELLS

A Eu3+-Eu2+ ion redox shuttle impartsoperational durability to Pb-Iperovskite solar cells

Ligang Wang1, Huanping Zhou1*, Junnan Hu1, Bolong Huang2, Mingzi Sun2,Bowei Dong1, Guanghaojie Zheng1, Yuan Huang1, Yihua Chen1, Liang Li1, Ziqi Xu1,Nengxu Li1, Zheng Liu1, Qi Chen3, Ling-Dong Sun1*, Chun-Hua Yan1*

The components with soft nature in the metal halide perovskite absorber usually generatelead (Pb)0 and iodine (I)0 defects during device fabrication and operation. These defectsserve as not only recombination centers to deteriorate device efficiency but alsodegradation initiators to hamper device lifetimes. We show that the europium ion pairEu3+-Eu2+ acts as the “redox shuttle” that selectively oxidized Pb0 and reduced I0 defectssimultaneously in a cyclical transition. The resultant device achieves a power conversionefficiency (PCE) of 21.52% (certified 20.52%) with substantially improved long-termdurability. The devices retained 92% and 89% of the peak PCE under 1-sun continuousillumination or heating at 85°C for 1500 hours and 91% of the original stable PCE aftermaximum power point tracking for 500 hours, respectively.

Device lifetime and power conversion ef-ficiency (PCE) are the key factors deter-mining the final cost of the electricitythat solar cells generate. The certifiedPCE of perovskite solar cells (PSCs) has

rapidly reached 23.7% over the past few years(1–9), which is on par with that of polycrystallinesilicon and Cu(In,Ga)Se2 solar cells, but poordevice stability (10–12) under operating condi-tions prevents the perovskite photovoltaics fromoccupying even a tiny market share (13, 14).Generally, commercial solar cells come witha warranty of a 20- to 25-year lifetime with aless than 10% drop of PCE, which corresponds toan average degradation rate of ~0.5% per year(15). Compared with those inorganic photo-voltaic materials—e.g., silicon (IV group) andCIGS (I-III-VI group) (16)—the elements or com-ponents are mostly large and more polarized inorganic-inorganic halide perovskite materials,such as I–, methylammonium (MA+), and Pb2+.They construct a soft crystal lattice prone todeform (17) and vulnerable to various agingstresses such as oxygen, moisture (18, 19), and

ultraviolet (UV) exposure (20, 21). By encapsula-tion (22–24), interface modification (13, 25–29),and UV filtration, the device lifetime can beprolonged by the temporary exclusion of theseexternal environmental factors.However, some aging stresses cannot be

avoided during device operation, including lightillumination, electric field, and thermal stress,upon which both I– and Pb2+ in perovskitesbecome chemically reactive to initiate the de-composition even if they are well encapsulated(30). Because of the soft nature of I–, Pb2+ ions,and Pb-I bonding, intrinsic degradation wouldoccur in perovskite materials upon various ex-citation stresses, which finally induce PCE de-terioration. On one hand, I– is easily oxidizedto I0, which not only serve as carrier recombi-nation centers but also initiate chemical chainreactions to accelerate the degradation in perov-skite layers (31). On the other hand, Pb2+ is proneto be reduced to metallic Pb0 upon heating orillumination, which has been observed in Pbhalide perovskite films (32, 33).Pb0 is a primary deep defect state that severely

degrades the performance of perovskite opto-electronic devices (34, 35), as well as their long-term durability (36). Furthermore, most softinorganic semiconductors are suffering similarinstability, such as PbS (37), PbI2 (38, 39), andAgBr (40), among others. Several attemptshave been reported to eliminate either Pb0 orI0 defects, like optimizing film processing (41)and additive engineering (42–44). To date, theseadditives are mostly sacrificial agents spe-cific for one kind of defects, which diminishsoon after they take effects. Long-term opera-tional durability requires the simultaneous elim-ination of both Pb0 and I0 defects in perovskitematerials in a sustainable manner.

We demonstrated constant elimination ofPb0 and I0 simultaneously in PSCs over theirlife span, which leads to exceptional stabilityimprovement and high PCE through incorpo-ration of the ion pair of Eu3+ (f6) ↔ Eu2+ (f7) asthe redox shuttle. In this cyclic redox transition,Pb0 defects could be oxidized by Eu3+, while I0

defects could be reduced by Eu2+ at same time.The Eu3+-Eu2+ pair is not consumed duringdevice operation, probably because of itsnonvolatility and the suitable redox potentialin this cyclic transition. Thus, the championPCE of the corresponding device was pro-moted to 21.52% (certified, 20.52%) withnegligible current density-voltage (J-V) hys-teresis. Devices with the Eu3+-Eu2+ ion pairexhibited excellent shelf lifetime and thermaland light stability, which suggests that thisapproach may provide a universal solution tothe inevitable degradation issue during deviceoperation.The reaction between Pb0 and I0 is thermo-

dynamically favored and has a standard molarGibbs formation energy for PbI2(s) of −173.6 kJ/mol(45), which provides the driving force for elim-inating both defects. However, simply mixingmetallic Pb and I2 powder only led to limitedformation of PbI2, which suggests the presenceof kinetic barriers at room temperature. To en-able elimination of Pb0 and I0 defects in PSCssimultaneously across device life span, we proposethe “redox shuttle” to oxidize Pb0 and reduce I0

independently, wherein they can be regeneratedduring the complete circle. It requires selectivelyoxidizing Pb0 and reducing I0 defects withoutintroducing additional deep-level defects. Afterfinely screening many possible redox shuttleadditives, the rare earth ion pair of Eu3+-Eu2+

was identified as the best candidate, mostlyowing to their appropriate redox potentials.Eu3+ could easily be reduced to Eu2+ with thestable half-full f7 electron configuration to formthe naturally associated ion pair. The redoxshuttle can transfer electrons from Pb0 to I0

defects in a cyclical manner, wherein the Eu3+

oxidizes Pb0 to Pb2+ and the formed Eu2+ sim-ultaneously reduces I0 to I– (Fig. 1F). Thus, eachion in this pair is mutually replenished duringdefects elimination.The proposed redox shuttle eliminates cor-

responding defects on the basis of the fol-lowing two chemical reactions:

2Eu3+ + Pb0 → 2Eu2+ + Pb2+ (1)

Eu2+ + I0 → Eu3+ + I– (2)

We first explored the feasibility of the Eu3+-Eu2+

ion pair to promote electron transfer from Pb0 toI0 in solution (Fig. 1A) by dispersing I2 (25 mg)powder andmetallic Pb powder (25mg) in 2ml ofN,N-dimethylformamide (DMF) and isopropanol(IPA) that had a volume ratio of 1:10 as areference solution. The Eu3+-Eu2+ ion pairwas incorporated by further adding europiumacetylacetonate [Eu(acac)3] (11 mg) into the 2-mlsolution. Under continuous stirring at 100°C,

RESEARCH

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1Beijing National Laboratory for Molecular Sciences, StateKey Laboratory of Rare Earth Materials Chemistry andApplications, PKU-HKU Joint Laboratory in Rare EarthMaterials and Bioinorganic Chemistry, Key Laboratory for thePhysics and Chemistry of Nanodevices, Beijing KeyLaboratory for Theory and Technology of Advanced BatteryMaterials, Department of Materials Science and Engineering,College of Engineering, College of Chemistry and MolecularEngineering, Peking University, Beijing 100871, P.R. China.2Department of Applied Biology and Chemical Technology,The Hong Kong Polytechnic University, Hung Hom, Kowloon,Hong Kong SAR. 3Department of Materials Science andEngineering, Beijing Institute of Technology, Beijing 100081,P.R. China.*Corresponding author. Email: happy_zhou@pku.edu.cn (H.Z.);yan@pku.edu.cn (C.-H.Y.); sun@pku.edu.cn (L.-D.S.)

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the sample solution gradually turned from blackto colorless with a large amount of yellow precipi-tates after 60 min, whereas the reference solu-tion remained dark brown with little evidence ofyellow precipitates.UV-visible (UV-vis) spectra of the reference

solution exhibited an absorption peak at ~370 nm(Fig. 1B), which we attributed to the presenceof an I0 species (36) that was absent in thesample solution, which had an absorption peakat ~290 nm that we attributed to a PbIx species.Both the I0 and Pb0 species were effectivelyconverted to I– and Pb2+ upon Eu3+ addition. Anx-ray diffraction (XRD)measurement on the pre-cipitates revealed both PbI2 (12.7°, 25.9°, 39.5°)and metallic Pb (31.3°, 36.2°, 52.2°) species inboth cases (Fig. 1C). In the sample, the char-acteristic peak intensity ratio of PbI2 to me-tallic Pb was larger than that of the reference.This result further confirmed that Eu3+ could

accelerate the conversion of Pb0 and I0 to Pb2+

and I–, respectively.When we added Eu(acac)3 to the CH3NH3I

solution of water/chloroform, we observed noI0 species absorption peak in the correspond-ing UV-vis spectrum (Fig. 1D), showing thatEu3+ selectively oxidizes Pb0 rather than I–.The stronger oxidizing agent of Fe3+ oxidizedI– species, and the absorption peak of I0 waspresent. We verified that Eu3+ was reduced toparamagnetic Eu2+ in CH3NH3PbI3 (MAPbI3)perovskite filmswith 1% (Eu/Pb,molar ratio) Eu3+

incorporated, which showed a strong signal inelectron paramagnetic resonance (EPR) measure-ments (Fig. 1E) that was absent in Eu2O3 and inthe reference MAPbI3 film.We compared the effect of Eu3+ by studying

other ions, including redox-inert Y3+ and strongoxidizing Fe3+, by preparing film samplesincorporated with 1% metal ions (M/Pb, molar

ratio) and performed high-resolution x-rayphotoelectron spectroscopy (XPS) analysis toelucidate the potential effects on both Pb0 and I0

defects. As shown in Fig. 2A, the binding energy(BE) at 142.8 and 137.9 eV were assigned to 4f5/2,4f7/2 of divalent Pb2+, respectively, and the twoshoulder peaks at 141.3 and 136.4 eV aroundlower BE were associated with metallic Pb0. Wecalculated the intensity ratio of Pb0/(Pb0 + Pb2+)for three metal-incorporated samples and thereference to observe a notable tendency (Fig. 2, Aand D, and table S1). The Pb0 intensity ratio inreference reached 5.4%, which is comparableto that of Y3+-incorporated film. This ratio inthe perovskite film with oxidative Eu3+ andFe3+ additives was reduced to nearly 1.0%,indicating that metallic Pb0 was successfullyoxidized.With respect to I0 species, it is difficult to ob-

tain I0/(I0 + I–) ratio by peak fitting accurately

Wang et al., Science 363, 265–270 (2019) 18 January 2019 2 of 6

Fig. 1. Eu3+-Eu2+ ion pair promotes the conversion of Pb0 andI0 to Pb2+ and I– in solution and perovskite film. (A) I0 and Pb0 powderdispersed in mixed DMF/IPA solvent (volume ratio 1:10) with orwithout Eu3+ [Eu(acac)3], and the solutions were stirred at 100°C.(B) The UV-vis absorption spectra of the upper solution and (C) XRDpatterns of the bottom precipitation from the sample and referencesolutions (after 60 min) shown in (A). (D) The representative solution and

the absorption spectra of bottom layer in which MAI mixed with Eu3+

or Fe3+ dissolved in water/chloroform. (E) EPR spectra of MAPbI3 filmwith or without Eu3+ incorporation and Eu2O3 sample, for which thevalue of proportionality factor (g-factor) is 2.0023. (F) Proposedmechanism diagram of cyclically elimination of Pb0 and I0 defects andregeneration of Eu3+-Eu2+ metal ion pair. a.u., arbitrary units;Ref, reference.

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because I0 species are volatile during the anneal-ing process of perovskite film preparation. Thus,we examined the ratio of I/Pb and BE shift tomonitor the iodine evolution indirectly. As shownin Fig. 2, B and E, and table S1, we observed thesimilar I/Pb ratio in the reference and the Y3+-incorporated sample but a much lower ratio inthe Fe3+ sample. Incorporation of Fe3+ likely gen-erated I0 species that were released. A higher I/Pbratio was observed in the Eu3+ sample comparedwith the reference, possibly indicating less vola-tile I0 species produced in the correspondingfilm. Furthermore, the BE of I 3d3/2 further con-firmed the argument, wherein it shifted toward ahigher value of 0.3 eV in Fe3+ sample but lower0.2 eV in Eu3+ sample as compared with the ref-erence. Given the lower BE of I–, it clearly showedthat I− was well preserved in the Eu3+ sample. Inaddition, Eu2+ was 36% of the total Eu content,which further confirmed the Eu3+-Eu2+ ion pairworking as a redox shuttle (Fig. 2C).According to the charge conservation rule, the

amount of I0 should be twice that of Pb0 involvedin the entire redox reaction. Iodine species (HIand I2) are all volatile, which follows the 1:2molarratio (33). We checked the total change in theamount of iodine (DI) and lead (DPb0) in thefilm upon the addition of Eu(acac)3, whereinDI/DPb0 was calculated to be 3.5 (see table S1and supplementary text). The change in theamount of iodine (DI) was about three times

that of lead (DPb0) during the degradationprocess, indicating that the amount of I0 speciespreserved was twice that of Pb0 species con-sumed upon redox shuttle addition. In thecontext of a redox reaction, the standard elec-trode potential (Eq) is often used as a referencepoint to rationally predict the occurrence ofthe reaction. According to the Eq of each halfreaction involved (which may deviate in solidmaterials) (table S2), Fe3+ is too oxidative andoxidizes Pb0 and I– simultaneously. On the con-trary, Eu3+ exhibited the suitable Eq to selec-tively oxidize Pb0 without I– oxidation, whilethe reduction product of Eu2+ reduced I0 to I–

at same time. Thus, the constant elimination ofPb0 and I0 defects still preserved the Eu3+-Eu2+

ion pair.We examined the effectiveness of Eu3+-Eu2+

redox shuttle in the film.Metallic Pb0 is themajoraccumulated defect in aged perovskite films be-cause of its nonvolatility (33). The content of Pb0

is a measure of the extent of decomposition inthe perovskite film. When the sample was sub-jected to 1 sun illumination or 85○C aging condi-tion for more than 1000 hours, the Pb0/(Pb0 +Pb2+) ratio in films with redox shuttle were 2.5%or 2.7%, compared with 7.4% or 11.3% in thereference film, respectively, as shown in fig. S1and table S3. The redox shuttle can preserve theI/Pb ratio in the aged film. Meanwhile, the cor-responding I/Pb ratio in Eu3+-incorporated film

was 2.68 or 2.57 as compared with that of ref-erence 2.30 or 2.13, indicating the perovskite filmwas well preserved.We also examined the crystallographic and

optoelectronic properties perovskite films withthe redox shuttle. According to XRD results, thephase structure was retained in the perovskitefilms with improved crystallinity upon Eu3+ ad-dition (figs. S2 to S4). No residual acetylacetonateanionwas detected by XPS and Fourier transforminfrared spectroscopy measurement (figs. S5 andS6). The Eu3+-Eu2+ ions were concentrated nearthe film surface, wherein the detected Eu/Pb ratiowasmuchhigher than the precursor ratio (table S1).When the Eu(acac)3 was introduced from 0.15 to4.8%,we observed neither extra diffraction peaksnor an obvious shift of diffraction peaks in theXRD patterns (figs. S2 to S4), which indicatesthat Eu3+-Eu2+ ions may not necessarily accom-modate in the crystal lattice.Given the similar radius of Eu2+ [117 pm (46)]

and Pb2+ (119 pm), however, we cannot confi-dently rule out the possibility that Eu2+ replacesPb2+ at B site, wherein direct evidence is ex-pected. In addition, europium-iodine–basedorganic-inorganic perovskite (47) and lanthanideions doped CsPbX3 perovskite nanocrystals werefound in previous reports (48). The morphologyand grain size of the perovskite filmwith the tinyamount redox shuttle remained similar to thereference (Fig. 3A and fig. S7). Also, we did not

Wang et al., Science 363, 265–270 (2019) 18 January 2019 3 of 6

Fig. 2. High-resolution XPS spectra of Pb 4f, I 3d, and the Eu 3d of perovskite films with the incorporation of 1% M/Pb different acetylacetonatemetal salts [M(acac)3, M = Eu3+, Y3+, Fe3+]. (A) Pb 4f spectra, the insertions are the enlarged spectra of Pb0 4f. (B) I 3d spectra. (C) Eu 3d spectra.(D) Fitted results of the Pb0/(Pb0+Pb2+) ratio. (E) Fitted results of I/Pb ratio.

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observe obvious orientation variation by synchro-tron grazing-incidencewide-angle x-ray scattering(GIWAXS) analysis (Fig. 3B and fig. S8).In addition, the optical bandgap of the

perovskite film upon Eu3+ addition was calcu-lated to be 1.55 eV, similar to that of the reference(fig. S9). The photoluminescence (PL) intensity(fig. S10) and carrier lifetime (Fig. 3C) increasedin the perovskite film with the incorporation ofEu3+, indicating the decrease of nonradiative re-combination centers from defects elimination.The improvement of the morphology and grainsize could also lead to the increased PL lifetime,so the defects reduction should be further con-firmed by other methods. We used the spacecharge–limited current (SCLC) measurement toquantify the defect density Ndefects of 5.1 × 1015

and 1.5 × 1016 cm−3 for Eu3+-incorporated sam-ples and the reference, respectively (Fig. 3D).We studied the influence of the Eu3+-Eu2+

ion pair on the formation energies of redox reac-tion, lattice stability, and energy band structureby density functional theory (DFT) calculations.To construct the model, a small fraction of metal

ions (Eu3+) was intercalated into two adjacentlattices (Fig. 3E), given the observation that Euwas concentrated at surfaces and grain bounda-ries. The formation energies for defects elim-ination (Eqs. 1 and 2) were calculated (Fig. 3F).For both reference and Eu3+-incorporated sys-tems, the half reactions related to Pb0 eliminationrequired a substantially high potential energy asthe main barrier, whereas the I0 elimination halfreactions were comparably favorable. However,after introducing Eu species at the interface, thebarrier in Pb0 elimination half reactions wasgreatly decreased, but the barrier for I0 elimi-nation half reactions decreased only slightly.Withthe assistance of Eu species at the interface, theoverall redox potential energy has beenmuch low-ered, representing an energetical stabilization trendfor the charge-transfer reaction (Fig. 3F).We also compared the thermodynamic prop-

erties for reference and Eu-incorporated systems.Figure 3G shows that the MAPbI3 with Eu in-corporation has a steeper slope in change of freeenergy DG than in that of reference, meaningthat Eu-incorporated MAPbI3 shows a more

energetically favorable physicochemical trendthan pureMAPbI3 does. Additionally, it revealsEu incorporation in MAPbI3 materials did notbring in obvious electronic disorders as extratraps (fig. S11).We incorporated the perovskite absorber

equipped with the redox shuttle in two deviceconfigurations. One is based on ITO/TiO2/perovskite/spiro-OMeTAD/Au, wherein spiro-OMeTAD refers to 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene, withMAPbI3(Cl). The other is based on ITO/SnO2/perovskite/spiro-OMeTAD (modified)/Au for higherPCE and stability, with (FA,MA,Cs)Pb(I,Br)3(Cl),in which FA is formamidinium. Both perovskiteswere deposited bymeans of a traditional two-stepmethod, during which Eu(acac)3 or other additiveswere added in PbI2/DMF precursor solution.The two devices showed similar trends (Fig. 4Aand fig. S12). The Eu3+-incorporated devices ex-hibited thebest PCE,whereas theFe3+-incorporateddevices suffered from themarkedly decreased PCE.The average PCE increased from 18.5 to 20.7% inthemixed perovskite upon Eu3+ addition (Fig. 4A),

Wang et al., Science 363, 265–270 (2019) 18 January 2019 4 of 6

Fig. 3. Influence of morphology, orientation, electronic structure,carrier behaviors of Eu3+-incorporated perovskite film, and resultsof DFTcalculations. The characterization of reference and 0.15%Eu3+-incorporated perovskite film: (A) scanning electron microscopy images;(B) GIWAXS data; (C) time-resolved photoluminescence spectra; (D) J-Vcharacteristics of devices (ITO/perovskite/Au), used for estimating theSCLC defects concentration (Ndefects = 2ee0VTFL/eL

2, e and e0 are the

dielectric constants of perovskite and vacuum permittivity, L is thethickness of the perovskite film, and e is the elementary charge).(E) The interface ultrathin Eu clustering-layer-incorporated structuralmodel. (F) Left: half-reaction potential barriers; right: overall redoxcharge-transfer reaction barrier for Eu incorporated at the interface.(G) The summary of DG between MAPbI3 and MAPbI3 incorporated withEu at the interface.

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which is attributed to the effective defectselimination. We attributed the decreased PCEin Fe3+-incorporated devices to the additional I0

defects introduced by oxidation.One of the optimized devices achieved the PCE

of 21.52% (reverse 21.89%, forward 21.15%) (Fig.4B) with negligible hysteresis (certified reverse20.73%, forward 20.30%, average 20.52%, certificateattached in fig. S13). The measured stable outputatmaximumpoint (0.97 V) was 20.9%. Integratingthe overlap of the incident-photon-to-current-efficiency spectrum of Eu3+-incorporated PSCsunder the AM 1.5-G solar photon flux generatedthe current density of 23.2 mA·cm−2 (fig. S14).The stabilized J-V performance of PSCs wasevaluated as follows (49): parameters are mea-sured under a 13-point IV sweep configurationwherein the bias voltage (current for open circuitvoltageVOC determination) is held constant untilthe measured current (voltage for VOC) was deter-mined to be unchanging at the 0.05% level. Theoriginal, stabilized, and poststabilized efficiency

of Eu3+-incorporated PSCs tested by third-partycertification institution were similar, which in-dicates the stable characteristics of the devices(fig. S15).The shelf lifetime of the corresponding devices

was investigated, wherein the PCE evolution wasdescripted for solar cells stored in an inert en-vironment (Fig. 4C). With the Eu3+-Eu2+ redoxshuttle incorporated, the devices maintained90% of the original PCE even after 8000 hoursstorage because of improved long-term VOC,short-circuit current density (JSC) and fill factor(FF) stability (fig. S16). Although the stability ofY3+-incorporated PSCs was comparable to thereference, Fe3+-incorporated PSC showed severelydeteriorated stability, which lost the photoelectricconversion capability completely after merely2000 hours of storage.To estimate the stability of Eu3+-incorporated

PSCs under operational conditions, half solarcells were subjected to either continuous 1 sunillumination or 85°C aging condition, respectively

(Fig. 4D), in which the top charge-transfer ma-terials and electrode were deposited after agingtest. Improved long-term VOC and FF stability(fig. S17) allowed the devices, after 1000 hours,to retain 93% of the original PCE continuous1 sun illumination or 91% after heating at 85°C.Several previous studies showed that small-molecule spiro-OMeTADwould crystallize underthermal stress and create pathways that allowfor an interaction of the perovskite and themetal electrode (50, 51). By modifying the hole-transport materials (spiro-OMeTAD) with con-ductive polymer poly(triarylamine), the fulldevices incorporated with the Eu3+-Eu2+ ionpair maintained 92% and 89% of the originalPCE because of obvious long-term VOC and FFstability improvement (fig. S18) under the samelight or thermal stress for 1500 hours, respectively(Fig. 4E). Furthermore, the Eu3+-incorporated fulldevices could maintain 91% of the original stablePCE tracked at maximum power point (MPP) for500 hours (Fig. 4F).

Wang et al., Science 363, 265–270 (2019) 18 January 2019 5 of 6

Fig. 4. Long-term stability and original performance evolution of PSCs.(A) Original performance evolution based on (FA,MA,Cs)Pb(I,Br)3(Cl)perovskite with the incorporation of 0.15% different M(acac)3 (M = Eu3+,Y3+, Fe3+). (B) The J-V curve, stable output (measured at 0.97 V), andparameters of 0.15% Eu3+-incorporated champion devices. (C) Long-termstability of PSCs based on MAPbI3(Cl) perovskite absorber with theincorporation of 0.15% different [M(acac)3 (M = Eu3+, Y3+, Fe3+)], stored in

inert condition. The PCE evolution of Eu3+-Eu2+-incorporated and referencedevices under 1 sun illumination or 85°C aging condition: (D) half PSCs(original PCE: 0.15% Eu3+ incorporated PSCs, 19.21 ± 0.54%; referencePSCs, 18.05 ± 0.38%) and (E) full PSCs (original PCE: 0.15%Eu3+ incorporated PSCs, 19.17 ± 0.42%; reference PSCs, 17.82 ± 0.30%).Scanning speed is 20 mV/s. (F) The MPP tracking of 0.15% Eu3+-incorporateddevice, measured at 0.97 V and 1-sun illumination.

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ACKNOWLEDGMENTS

The manuscript was improved by the insightful reviews ofanonymous reviewers. We thank Y. Yang (University of California,Los Angeles), Y. Li (Beijing Institute of Technology), H. Xie (CentralSouth University), Q. Bao (East China Normal University), and

J. Xiao (Beijing Institute of Technology) for insightful data analysisand valuable discussion. We also thank the third certificationinstitutions National Institute of Metrology (China) and NewportTechnology and Application Center PV Lab (USA) forauthentication tests; beamline BL14B1 (Shanghai SynchrotronRadiation Facility, SSRF) for providing beam time and help duringthe experiments; and Enli technology Co., Ltd. for help with PVefficiency and EQE measurement. Funding: This work wassupported by National Natural Science Foundation of China(nos. 91733301, 51672008, 51722201, 21425101, 21331001, and21621061), MOST of China (2014CB643800), National KeyResearch and Development Program of China (grant nos.2017YFA0206701 and 2017YFA0205101), Beijing Natural ScienceFoundation (4182026), National Key Research and DevelopmentProgram of China (grant no. 2016YFB0700700), NationalNatural Science Foundation of China (51673025), Beijing MunicipalScience and Technology Project (no. Z181100005118002), andYoung Talent Thousand Program. Author contributions: L.W. andH.Z. conceived the idea and designed the project. H.Z., C.-H.Y.,and L.-D.S. directed and supervised the research. L.W. fabricatedand characterized devices. Y.H., Y.C., L.L., Z.X., and N.L. alsocontributed to device fabrication. L.W. performed the SEM, PL,UPS, UV-vis, XPS, and XRD measurements. GIWAXS wasperformed and analyzed by G.Z., supported by BL14B1 beamline ofSSRF. B.D. and Z.L. performed EPR. M.S. and B.H. carried out DFTcalculation. L.W. drafted the manuscript; Q.C. and H.Z. revised andfinalized the manuscript. Competing interests: The authors have nocompeting interests. Data and materials availability: All data areavailable in the main text or the supplementary materials.

SUPPLEMENTARY MATERIALS

www.sciencemag.org/content/363/6424/265/suppl/DC1Materials and MethodsSupplementary TextFigs. S1 to S18Tables S1 to S3References (52–57)

24 June 2018; resubmitted 25 September 2018Accepted 28 November 201810.1126/science.aau5701

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ion redox shuttle imparts operational durability to Pb-I perovskite solar cells2+-Eu3+A Eu

Chen, Liang Li, Ziqi Xu, Nengxu Li, Zheng Liu, Qi Chen, Ling-Dong Sun and Chun-Hua YanLigang Wang, Huanping Zhou, Junnan Hu, Bolong Huang, Mingzi Sun, Bowei Dong, Guanghaojie Zheng, Yuan Huang, Yihua

DOI: 10.1126/science.aau5701 (6424), 265-270.363Science 

, this issue p. 265Scienceaging conditions.incorporating this redox shuttle maintained more than 90% of their initial power conversion efficiencies under various

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arising from reactions with oxygen and water, but light, electric-field, and thermal stresses can lead to metastable Device longevity is a key issue for organic-inorganic perovskite solar cells. Encapsulation can limit degradation

A redox road to recovery

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MATERIALSSUPPLEMENTARY http://science.sciencemag.org/content/suppl/2019/01/16/363.6424.265.DC1

REFERENCES

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