Giant switchable photovoltaic effect in organometal trihalide perovskite devices

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1 LETTERS PUBLISHED ONLINE: 8 DECEMBER 214 DOI: 1.138/NMAT415 Gint switchle photovoltic effect in orgnometl trihlide perovskite devices Zhengguo Xio 1,2, Yongo Yun 1,2, Yuchun Sho 1,2, Qi Wng 1,2, Qingfeng Dong 1,2, Cheng Bi 1,2, Pnkj Shrm 2,3, Alexei Gruvermn 2,3 nd Jinsong Hung 1,2 * Orgnoled trihlide perovskite (OTP) mterils re emerging s nturlly undnt mterils for low-cost, solutionprocessed nd highly efficient solr cells 1 9. Here, we show tht, in OTP-sed photovoltic devices with verticl nd lterl cell configurtions, the photocurrent direction cn e switched repetedly y pplying smll electric field of <1 V µm 1. The switchle photocurrent, generlly oserved in devices sed on ferroelectric mterils, reched 2.1 ma cm 2 under one sun illumintion in OTP devices with verticl rchitecture, which is four orders of mgnitude lrger thn tht mesured in other ferroelectric photovoltic devices 1,11. This field-switchle photovoltic effect cn e explined y the formtion of reversile p i n structures induced y ion drift in the perovskite lyer. The demonstrtion of switchle OTP photovoltics nd electric-field-mnipulted doping pves the wy for innovtive solr cell designs nd for the exploittion of OTP mterils in electriclly nd opticlly redle memristors nd circuits. The verticl structure device hs lyered structure of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(4- styrenesulphonte) (PEDOT:PSS)/perovskite/Au, s shown in Fig. 1, where the methylmmonium led iodide (MAPI 3 ) perovskite lyer ws formed y the interdiffusion of led iodide (PI 2 ) nd methylmmonium hlide (MAI) stcking lyers 12. The s-prepred devices with 3 nm thick perovskite films hve J SC of 8.5 ma cm 2 nd V OC of.18 V (Supplementry Fig. 1), despite the work functions of PEDOT:PSS nd Au electrodes eing lmost the sme. This might e due to the different interfcil electronic structures t the perovskite/pedot:pss nd perovskite/au contcts. A lrge memristive effect from these devices ws identified, oth in the drk nd under illumintion, which showed scnning-history-dependent current output (Supplementry Fig. 2). The diode direction cn e switched during the scnning process in the drk, s shown in Fig. 1. The on/off rtios of the drk current re >1 3 nd >1 2 t ises of 1. V nd.7 V, respectively. The devices cn lso e poled y constnt is pulse with durtion of 5 3 s (Supplementry Fig. 3,). The device with perovskite lyer thickness of 1,15 nm ws switched y smll is of 1. V (Supplementry Fig. 3c), which corresponds to miniml poling electric field of <1 V µm 1. The switchle diode in the drk resulted in gint switchle photovoltic effect under illumintion. In contrst to switchle photovoltics using ismuth ferrite mterils, with J SC typiclly of the order of µa cm 2 (refs 1,11), J SC for the OTP devices switched etween 18.6 nd 2.1 ma cm 2 (Fig. 1c) under one sun illumintion, which is comprle to tht of optimized perovskite solr cells 1 5,13. This indictes tht most of the photogenerted excitons dissocited to free chrges in the perovskite lyer, even without n electron- or hole-ccepting lyer, nd the free chrges were efficiently collected y the electrodes regrdless of the diode directions. The photocurrents showed vriety of hysteresis loops with chnged scnning rtes, wheres J SC remined lmost unchnged nd V OC ws round.15 V lower t lower scnning rte of.25 V s 1, s shown in Supplementry Fig. 2 d. The photocurrent direction fter poling remined unchnged fter storing the device for two months under mient illumintion in the gloveox (Supplementry Fig. 4). The vlue of V OC ws switched etween.42 V nd.73 V. There is vrition in J SC nd V OC from different devices; some of them showed lrger V OC, close to.9 V, ut with reltively smller J SC of 8 ma cm 2 (Supplementry Fig. 5). The verge V OC nd J SC of the verticl structure devices were ±.65 V nd ±18.5 ma cm 2, respectively (Supplementry Fig. 5). (electric field pointing from PEDOT:PSS to Au) resulted in positive V OC. A typicl device ws switched more thn 75 times with the V OC in the first ten nd lst ten poling cycles shown in Fig. 1d. The photocurrent direction of the device cn survive fter 75 poling cycles. After positive poling, V OC remined lmost constnt t.42 V, wheres fter negtive poling, V OC showed switching ftigue from.73 V to.21 V. The switchle photovoltic phenomenon ws universlly demonstrted using other OTP mterils, such s CH 3 NH 3 PI 3 x Cl x, HC(NH 2 ) 2 PI 3 nd CH 3 NH 3 PBr 3 (Supplementry Fig. 6), nd with mny other top electrodes, including nickel, gllium nd pltinum (Supplementry Fig. 7), ut not luminium or silver, owing to the severe chemicl rection of perovskite with luminium nd silver. We moved on to demonstrte the switchle photovoltic effect in lterl structure devices ecuse of the very low electric field of 1 V µm 1 needed for poling nd the sence of electrode selectivity shown in the verticl structure devices. Fig. 2 shows schemtic of photovoltic device with lterl symmetric structure of Au/OTP/Au/OTP/..., where tens to hundreds of cells re connected in series. Fig. 2 shows the opticl imge of Au electrode pttern fricted y photolithogrphy, where the electrode spcing is 8 µm. As shown in Fig. 2c, the non-poled device showed no photovoltic effect with zero V OC ecuse of the symmetricl electrodes, wheres the poled single cell showed V OC of.48 V under.25 sun illumintion fter poling t 1 V for 1 s. The photovoltic direction fter poling ws consistent with tht of the verticl structure devices. The photovoltic direction ws flipped y reversed poling is. In contrst to ferroelectric photovoltics 14, 1 Deprtment of Mechnicl nd Mterils Engineering, University of Nersk-Lincoln, Lincoln, Nersk , USA. 2 Nersk Center for Mterils, Nnoscience, University of Nersk-Lincoln, Lincoln, Nersk , USA. 3 Deprtment of Physics nd Astronomy, University of Nersk-Lincoln, Lincoln, Nersk , USA. These uthors contriuted eqully to this work. *e-mil: jhung2@unl.edu NATURE MATERIALS VOL 14 FEBRUARY Mcmilln Pulishers Limited. All rights reserved

2 LETTERS NATURE MATERIALS DOI: 1.138/NMAT415 Au MAPl 3 ITO/PEDOT:PSS Current density (ma cm 2 ) c d Poling pulses V oc Current density (ma cm 2 ) ,6 29,8 Time (s) Figure 1 Verticl structure photovoltic devices nd their switching ehviour., Schemtic of the verticl structure device.,c, Drk current () nd photocurrent hysterisis curves (c) of the devices under continuous current sweeping t rte of.14 V s 1 etween 2.5 V nd 2.5 V. The rrows in the figures show the scnning direction. d, Open circuit voltge of the device recorded fter repeted poling y ±2.5 V is for more thn 75 cycles. Only the first ten nd lst ten cycles re shown. The red dots refer to V OC of the device fter positive nd negtive poling. Au Sunlight.3 mm + MAPI 3 Glss d = 8 µm d = 8 µm Au c.12 d Current density (ma cm 2 ) Negtive poling Without poling Current density (ma cm 2 ) cells 1 cell 25 cells 2 cells 125 cells Figure 2 Lterl structure photovoltic devices nd their switching ehviour., Schemtic of the lterl structure devices., A Au stripe rry under microscope (reflective mode), where the electrode spcing, d, shown in the enlrged imge elow, is 8 µm. Lterl photovoltic devices connected in series were fricted y depositing uniform perovskite films (3 nm) on pre-formed Au stripe electrodes with spcings etween 8 nd 1 µm. c, Photocurrents of the device efore nd fter negtive nd positive poling for single cell mesured t sweeping rte of.5 V s 1 under.25 sun illumintion; the rrows in the figure indicte the scnning direction. d, Photocurrents of lterl photovoltic devices connected in series mesured t sweeping rte of.5 V s 1 for ech cell. The poling of the lterl photovoltic devices ws conducted either y poling ech cell individully or y simply poling the whole re etween the first nd lst electrodes. 194 NATURE MATERIALS VOL 14 FEBRUARY Mcmilln Pulishers Limited. All rights reserved

3 NATURE MATERIALS DOI: 1.138/NMAT415 LETTERS E Negtive poling E ITO PEDOT:PSS p-doped + n-doped Au ITO PEDOT:PSS n-doped + p-doped Au c WF (n) WF (p) E vcuum E C d Au Conducting tip E F Perovskite n p E V e + Poling perovskite 181. mv f Poling perovskite 181. mv Au Au mv 4 µm 4 µm mv g Normlized intensity ITO From ITO side Without poling Positive poling Negtive poling h Normlized intensity From Au (25 nm) side Without poling Positive poling Negtive poling Wvelength (nm) Wvelength (nm) Figure 3 Switchle photovoltic model nd mechnism study.,, Schemtics of ion drift in perovskite during positive nd negtive poling, respectively, showing tht ccumulted ions in the perovskite ner the electrodes induced p- nd n-doping. c, Energy digrm of the p i n structure fter poling. WF, workfunction. d, Schemtic imge of device with prt of the Au electrode peeled off. The scnned re is mrked s dshed rectngle. e,f, KPFM potentil imge of the perovskite/au res fter positive nd negtive poling, respectively, of the perovskite lyers (3 nm). g,h, Photoluminescence from the thin perovskite lyers close to either PEDOT:PSS electrode or Au electrode. A thin gold electrode (25 nm) ws used s the top electrode so the photoluminescence emission from the perovskites close to oth electrodes cn e mesured. Blue light (45 nm) ws used to excite only the 25 nm thick perovskite lyer close to the electrodes (estimted y the extinction coefficient) of the 1,15 nm thick device. The photoluminescence mesurement ws conducted in situ during the poling process to exclude other fctors ffecting photoluminescence. The smll extr pek t 71 nm in g is due to the ITO. the V OC of single device fter poling remins lmost constnt t round.5 V for rnge of electrode spcings from 8 to 1 µm (Supplementry Fig. 9). The J SC vlue of the device with n 8 µm electrode spcing is.1 ma cm 2. The smller J SC in the lterl structure devices is due to the electrode spcing eing much lrger thn the chrge diffusion length in perovskite mterils 15,16. The sttistics of the performnce of the lterl structure devices is shown in Supplementry Fig. 8. The lrgest solute V OC nd J SC vlues reched.88 V nd.11 ma cm 2, nd the verge V OC nd J SC vlues of the lterl structure devices were ±.5 V nd ±.75 ma cm 2, respectively. As shown in Fig. 2d, the V OC vlue of lterl photovoltic devices connected in series is the sum of ech unit cell, wheres the J SC remins lmost constnt, indicting uniform performnce of ech unit cell. A lrge V OC of 47 V ws oserved for devices with 125 unit cells connected in series under illumintion of 25 mw cm 2. To determine the origin of the switchle photovoltic effect in OTP devices, we exmined three possile mechnisms which hve een reported for switchle photovoltic or memristor ehviour, which ws lso speculted to result in photocurrent hysteresis with chnged photocurrent scnning direction nd scnning rte in some perovskite photovoltic devices 17 : ferroelectricity of the photoctive lyer 18 ; chrge trps in the ctive lyer s surfce 17,19 ; nd motion nd ccumultion of ions inducing doping effect 1,2. Some theoreticl clcultions hve predicted strong ferroelectricity for MAPI 3, with polriztion chrge density of the order of 38 C m 2 (refs 21,22). However, no ferroelectric polriztion ws detected from these devices with comprle voltge scnning rtes NATURE MATERIALS VOL 14 FEBRUARY Mcmilln Pulishers Limited. All rights reserved

4 LETTERS NATURE MATERIALS DOI: 1.138/NMAT Microscope Au + MAPI 3 Light d = 75 µm Glss min 45 min 55 min 11 min Figure 4 In situ monitoring of the mteril chnge during the poling process., Illustrtion of the set-up used for in situ monitoring of the poling process using lterl structure device., Snpshots of the in situ recorded video, showing chnged perovskite mteril close to the node side during the poling process. The electricl field pplied on the perovskite film ws 1.2 V µm 1. to those in the photovoltic study when they were mesured t oth room temperture nd 77 K (Supplementry Fig. 1). Piezoresponse force microscopy (PFM) imging nd hysteresis loop mesurements showed nothing resemling ferroelectric ctivity in the MAPI 3 perovskite films tht is, there were wek PFM mplitude nd phse signls nd no hysteresis switching ctivity, despite the ppliction of much higher is thn in the switchle photovoltic studies. Furthermore, the ferroelectric photovoltic effect cn e excluded y the unchnged photovoltge with respect to the electrode spcing in the lterl structure devices, (Supplementry Fig. 9) nd diminished switching ehviour t reduced temperture (Supplementry Fig. 14). Our finding does not, in principle, rule out ferroelectricity in MAPI 3, s more mesurements (such s temperture-dependent dielectric nd structurl testing) re required to clrify this issue. The chrge-trpping mechnism cn lso e excluded ecuse it cnnot explin flipping in the direction of the photovoltge nd photocurrent, or the persistence of the photocurrent output long fter poling, s shown in Supplementry Fig. 11. It ws previously reported tht similr hlide-contining perovskites, such s CsPCl 3 nd CsPBr 3, re good hlideion vcncy conductors t elevted tempertures 23. We scrie switchle photovoltics to ion drift under the electric field in the perovskite lyer. Theoreticl clcultions predicted tht negtively chrged P nd MA vcncy (V P nd V MA ) could result in p-type doping, wheres positively chrged I vcncy (V I ) results in n-type doping in MAPI 3 (refs 24,25), which we verified experimentlly in study of composition-dependent self-doping ehviour in MAPI 3 (ref. 26). In this scenrio, the electric field cuses the drift of chrged V I, V P nd/or V MA, which hve low formtion energies in MAPI 3 (refs 24,25), to the re ner the electrode nd forms p i n structure. Our scenrio for the switchle photovoltics is illustrted in Fig. 3,, using verticl structure devices s n exmple. The positively chrged ions or vcncies moved to the Au side during positive poling nd ccumulted there, leding to n-doping in perovskite on the Au side. Similrly, the remining negtive spce chrge lyer cn p-dope the perovskite lyer close to PEDOT:PSS, forming p i n homojunction structure. A reverse is cn flip the p i n structure to n i p y forcing ions or ion vcncies to drift in the opposite direction. The memristive drk- nd photocurrent hysteresis cn lso e explined well y the time-dependent drift of ions under the field 27. To test this scenrio, we first mesured the doping-induced ndending in perovskite close to the top Au electrode fter poling in the verticl structure devices y mens of Kelvin proe force microscopy (KPFM). The p- or n-doping should induce n increse or reduction of the work function for the perovskite top surfce, respectively, s illustrted y the energy digrm in Fig. 3c. The long retention of the diode direction indictes tht the poling-induced chnges in composition nd work function re stle fter poling. Au electrodes could e esily peeled off y Scotch tpe to expose the poled perovskite films. Fig. 3d illustrtes the films used for the KPFM study, where some unpeeled Au res were intentionlly left s work function reference. As shown in Fig. 3e,f, the work function of the perovskite films ws.22 V lower thn tht of Au fter positive poling, nd comprle to tht of Au fter negtive poling which grees well with the energy digrm in Fig. 3c nd supports the proposed doping mechnism. The surfce topogrphy showed no ovious chnge in the poling res (Supplementry Fig. 12), thus excluding the effect of topogrphy on the surfce potentil mesurement. The discrepncy of the chnge in perovskite work function with V OC cn e explined y possile surfce contmintion y residues of Au fter peeling nd/or y moisture/oxygen, s the KPFM mesurement ws conducted in ir. Finlly, semiconductor doping generlly cuses photoluminescence quench, which ws lso oserved for the p- nd n-doped perovskite regions close to oth the Au nd PEDOT:PSS sides, s shown in Fig. 3g,h. Another conclusive piece of evidence for the drift of ions during the poling process comes from the oservtion of composition nd morphology chnges during poling of lterl structure device. In this study, the device ws intentionlly poled for much longer time (2 h) thn ws needed for the doping effect, such tht the chnge in perovskite composition/morphology ws discernile. The trnsprency of the perovskite film ws monitored in situ under n opticl microscope, s schemticlly shown in the mesurement setup in Fig. 4. The dynmic process ws recorded on video using time-ccelerted mode (Supplementry Movie 1), severl snpshots of which re shown in Fig. 4. The perovskite stripe re close to the node ecme incresingly trnsprent, nd the morphology in this re ws completely different from other res, with mny pinholes ppering (Supplementry Fig. 13), indicting the drift of ions from the node side. The loss of perovskite mteril on the node side indicted tht the drifting ions were V P nd/or V MA. The extensive study of the dynmic poling process for devices under vrying electric field, temperture, nd perovskite film morphology gve support to mechnism of field-driven ion drift s eing responsile for the photovoltic switching, s shown in Supplementry Fig. 14. Here, trin of voltge pulses with durtion of.95 s ws pplied to the devices nd J SC ws recorded fter ech pulse. An elevted temperture nd/or pplied electric field (y chnging either the pplied is or perovskite film thickness) ccelerted the poling process (Supplementry Fig. 14,), wheres poling ws lmost frozen t tempertures elow C (Supplementry Fig. 14c). The extent of the photovoltic switching effect ws influenced y the morphology, stoichiometry nd film qulity of the perovskite films owing to their influence on the defect concentrtion. For exmple, poling ecme more 196 NATURE MATERIALS VOL 14 FEBRUARY Mcmilln Pulishers Limited. All rights reserved

5 NATURE MATERIALS DOI: 1.138/NMAT415 difficult (Supplementry Fig. 14d f) in device with much lrger perovskite grin size which ws formed y solventnneling process. This cn e explined y the reduced vcncy concentrtion in solvent-nneled perovskite films due to fewer grin oundries resulting from the solvent-nneling process incresing the grin size from 3 nm to rnge 6 1, nm (ref. 28). The sttistics of device performnce during the dynmic poling process under different mesurement conditions nd with different film morphologies re shown in Supplementry Fig. 14g. These results confirmed tht the switchle photovoltic ehviour of the devices is due to ion drift. The est device prmeters in the single-lyer verticl structure devices fter poling pproched those of optimized multi-lyer devices with oth electron- nd hole-trnsporting lyers. This work indictes new direction for perovskite solr cell design using the controlled doping of perovskite for homojunction solr cells, thus reducing device friction complexity. The lterl structure device is prticulrly interesting ecuse it elimintes the need for trnsprent electrodes, ut its efficiency is still limited y the chrge crrier diffusion length in present perovskite mterils. Further improvements in crystl qulity nd surfce pssivtion techniques will help to resolve this issue. The perovskite memristors reported here cn e red out not only y electricl pulses ut lso y opticl pulses 18. This work pves the wy towrds new pproch to doping perovskite using electric pulses, such tht doping pttern cn e progrmmed nd directly written using scnning proe microscopy for memristor rry friction. This might open up new pplictions for perovskite mterils in optoelectronic computtionl devices, s memristors re eing incresingly pursued for computing 29,3. Methods CH 3 NH 3 I precursor synthesis. Methylmmonium iodide (CH 3 NH 3 I) ws synthesized using previously descried method 1. A concentrted queous solution of hydroiodic cid (HI) (15. ml, 57 wt% in wter, Alf Aesr) ws rected with methylmine (CH 3 NH 2 ) (13.5 ml, 4 wt% in queous solution, Alf Aesr) t C for 2 h with constnt stirring under nitrogen tmosphere. Methylmmonium iodide ws crystllized y removing the solvent using rotry evportor. The white powder produced ws wshed with diethyl ether (Alf Aesr) three times nd dried in vcuum overnight. Film formtion nd device friction. Poly(3,4-ethylenedioxythiophene): poly(4-styrenesulphonte) (PEDOT:PSS) (Bytron-P 483) ws spin-coted on clen indium tin oxide (ITO) sustrtes t speed of 3, revolutions per minute (r.p.m.). The films were then nneled t 15 C for 3 min. PI 2 nd MAI were first dissolved in dimethylformmide (DMF) nd 2-propnol, respectively. The MAPI 3 films were formed y spin coting PI 2 (4 mg ml 1 in DMF) nd (45 mg ml 1 in 2-propnol) sequentilly t 6, r.p.m. for 35 s, respectively, followed y therml nneling t 1 C for 2 h. The MAPI 3 x Cl x films were formed y spin coting PI 2 (4 mg ml 1 in DMF) nd MAI.8 Cl.2 (45 mg ml 1 in 2-propnol) sequentilly t 6, r.p.m. for 35 s, respectively, followed y therml nneling t 11 C for 1 h. The FAPI 3 films were formed y spin coting PI 2 (4 mg ml 1 in DMF) nd FAI (45 mg ml 1 in 2-propnol) sequentilly t 6, r.p.m. for 35 s, respectively, followed y therml nneling t 12 C for 1 h. The MAPBr 3 films were formed y spin coting PBr 2 (6 mg ml 1 in DMF) nd MABr (65 mg ml 1 in 2-propnol) sequentilly t 6, r.p.m. for 35 s, respectively, followed y therml nneling t 1 C for 1 h. After spin coting the ove inorgnic mterils, the films were dried t 7 C for 3 min efore spin coting the orgnic component. For the solvent nneling of perovskite films, pproximtely 1 µl DMF ws introduced into the Petri dishes during the therml-nneling process. The devices were completed y the therml evportion of 5 nm gold (Au) s n electrode. For lterl photovoltic devices, the Au stripe rrys were first fricted on glss sustrtes y photolithogrphy tht is, the 5 nm thick Au lyer ws thermlly deposited on pre-ptterned photoresist lyer, then the photoresist ws removed y cetone. The positive photoresistor, Shipley S-1813, ws used. The resulting Au stripe ptterns hve lengths of 3 mm, nd spcings of 8 µm (Fig. 2). The totl width of lterl photovoltic devices connected in series comprising 125 cells is 1.4 mm. Perovskite films were then fricted on these sustrtes using the sme method s for verticl device friction. The resulting structure of the lterl devices is shown schemticlly in Fig. 2. The thickness of the perovskite films for oth verticl nd lterl device structures is 3 nm, except where specificlly lelled. LETTERS Film nd device chrcteriztion. The stedy-stte photocurrent curves were mesured under simulted AM 1.5G irrdition (1 mw cm 2 ) using Xenon-lmp-sed solr simultor (Oriel 675, 15 W Solr Simultor). A Schott visile-colour glss-filtered (KG5 colour-filtered) Si diode (Hmmtsu S1133) ws used to clirte the light intensity efore photocurrent mesurements. A Keithley 42 semiconductor nlyser ws used to pply scnning is nd test the output current simultneously. All the electricl tests for verticl devices were conducted in gloveox. Lterl solr cell mesurements were conducted in proe sttion chmer under vcuum of 1 3 Torr, wherein light (25 mw cm 2 ) ws incident through qurtz window. A Keithley 24 High Voltge Supply with mximum voltge output of 1,2 V ws used for the poling process. The electricl field pplied on the perovskite film is 1.2 V µm 1 for 1 s. The poling of the cells ws conducted either y poling ech cell individully or y simply poling the whole re etween the first nd lst electrodes. For lterl devices, the poling is lsted for 1 s, period longer thn in the verticl devices, owing to the much greter ion drift distnce. After poling, the I V curves were mesured using Keithley 24. Oservtion of the in situ poling process under microscope. In situ oservtion of the ion drift under n electric field ws crried out y locting the smples under n opticl microscope (Olympus BX61) coupled to high-resolution chrge-coupled device (CCD) cmer (Photometrics, CoolSNAP-cf). The opticl microscope worked in the trnsmission mode with the smple illuminted from ottom. The smples were kept in stedy N 2 flow during the poling process to prevent the sorption of oxygen nd moisture. KPFM, PFM nd tomic force microscopy (AFM). KPFM nd AFM mesurements were crried out with Dimension Icon (Bruker) in ir nd in the drk. Pltinum iridium-coted conductive proes (SCM-PIT, Bruker) were used in the KPFM nd AFM mesurements. The PekForce KPFM mode, comining the tpping mode AFM with frequency modultion KPFM cn mesure the topogrphic nd surfce potentil signls of the sme re. The scnning re nd tip velocity were 2 µm 2 µm nd 81.4 µm s 1, respectively. The lift height for KPFM mesurements ws 8 nm for ll smples. PFM ws performed y pplying high-frequency modultion voltge (2 6 khz, V) to the Pt Ti-coted silicon (Mikromsch) or Au-coted SiN tips (Olympus). Locl piezoelectric hysteresis loops were mesured in fixed loctions on the film s function of d.c. is superimposed on the.c. modultion voltge. For smple preprtion, the verticl structure devices were first poled in gloveox. Susequently, most res of the top gold electrodes were peeled off y Scotch tpe, ut some smll res of the Au electrode were purposely left s the reference for the KPFM surfce work function. Ferroelectricity chrcteriztion. The ferroelectric polriztion mesurements were crried out using Precision Premier II from Rdint Technologies, oth t room temperture nd 77 K, (y soking the smples in liquid nitrogen for 1 min efore the mesurements). The voltge scnning rte pplied ws.8 V s 1, which is comprle to or slower thn the rte of photocurrent scnning for the switchle photovoltic effect study. The dt ws collected using the softwre pckge Vision which is integrted in the Precision Premier II. Received 11 July 214; ccepted 28 Octoer 214; pulished online 8 Decemer 214; corrected online 7 Jnury 215 References 1. Lee, M. M. et l. Efficient hyrid solr cells sed on meso-superstructured orgnometl hlide perovskites. Science 338, (212). 2. Burschk, J. et l. Sequentil deposition s route to high-performnce perovskite-sensitized solr cells. Nture 499, (213). 3. Liu, M., Johnston, M. B. & Snith, H. J. Efficient plnr heterojunction perovskite solr cells y vpour deposition. Nture 51, (213). 4. Liu, D. & Kelly, T. L. Perovskite solr cells with plnr heterojunction structure prepred using room-temperture solution processing techniques. Nture Photon. 8, (213). 5. Heo, J. H. et l. Efficient inorgnic orgnic hyrid heterojunction solr cells contining perovskite compound nd polymeric hole conductors. Nture Photon. 7, (213). 6. Edri, E. et l. Elucidting the chrge crrier seprtion nd working mechnism of CH 3 NH 3 PI 3 -xcl x perovskite solr cells. Nture Commun. 5, 3461 (214). 7. Kojim, A., Teshim, K., Shiri, Y. & Miysk, T. Orgnometl hlide perovskites s visile-light sensitizers for photovoltic cells. J. Am. Chem. Soc. 131, (29). NATURE MATERIALS VOL 14 FEBRUARY Mcmilln Pulishers Limited. All rights reserved

6 LETTERS 8. Im, J-H. et l. 6.5% efficient perovskite quntum-dot-sensitized solr cell. Nnoscle 3, (211). 9. Kim, H-S. et l. Led iodide perovskite sensitized ll-solid-stte sumicron thin film mesoscopic solr cell with efficiency exceeding 9%. Sci. Rep. 2, 591 (212). 1. Choi, T. et l. Switchle ferroelectric diode nd photovoltic effect in BiFeO 3. Science 324, (29). 11. Grinerg, I. et l. Perovskite oxides for visile-light-soring ferroelectric nd photovoltic mterils. Nture 53, (213). 12. Xio, Z. et l. Efficient, high yield perovskite photovoltic devices grown y interdiffusion of solution-processed precursor stcking lyers. Energy Environ. Sci. 7, (214). 13. Chen, Q. et l. Plnr heterojunction perovskite solr cells vi vpor ssisted solution process. J. Am. Chem. Soc. 136, (213). 14. Yun, Y., Xio, Z., Yng, B. & Hung, J. Arising pplictions of ferroelectric mterils in photovoltic devices. J. Mter. Chem. A 2, (214). 15. Strnks, S. D. et l. Electron hole diffusion lengths exceeding 1 micrometer in n orgnometl trihlide perovskite sorer. Science 342, (213). 16. Xing, G. et l. Long-rnge lnced electron- nd hole-trnsport lengths in orgnic inorgnic CH 3 NH 3 PI 3. Science 342, (213). 17. Snith, H. J. et l. Anomlous hysteresis in perovskite solr cells. J. Phys. Chem. Lett. 5, (214). 18. Guo, R. et l. Non-voltile memory sed on the ferroelectric photovoltic effect. Nture Commun. 4, 199 (213). 19. Andersson, P., Roinson, N. D. & Berggren, M. Switchle chrge trps in polymer diodes. Adv. Mter. 17, (25). 2. Yi, H. et l. Mechnism of the switchle photovoltic effect in ferroelectric BiFeO 3. Adv. Mter. 23, (211). 21. Frost, J. M. et l. Atomistic origins of high-performnce in hyrid hlide perovskite solr cells. Nno Lett. 14, (214). 22. Stoumpos, C. C., Mlliks, C. D. & Kntzidis, M. G. Semiconducting tin nd led iodide perovskites with orgnic ctions: Phse trnsitions, high moilities, nd ner-infrred photoluminescent properties. Inorg. Chem. 52, (213). 23. Mizuski, J., Ari, K. & Fueki, K. Ion-conduction of the Perovskite-type Hlides. Solid Stte Ion. 11, (1983). NATURE MATERIALS DOI: 1.138/NMAT Yin, W-J., Shi, T. & Yn, Y. Unusul defect physics in CH 3 NH 3 PI 3 perovskite solr cell sorer. Appl. Phys. Lett. 14, 6393 (214). 25. Kim, J., Lee, S-H., Lee, J. H. & Hong, K-H. The role of intrinsic defects in methylmmonium led iodide perovskite. J. Phys. Chem. Lett. 5, (214). 26. Wng, Q. et l. Qulifying composition dependent p nd n self-doping in CH 3 NH 3 PI 3. Appl. Phys. Lett. 15, (214). 27. Yng, J. J. et l. Memristive switching mechnism for metl/oxide/metl nnodevices. Nture Nnotech. 3, (28). 28. Xio, Z. et l. Solvent-nneling of perovskite induced crystl growth for photovoltic device efficiency enhncement. Adv. Mter. 26, (214). 29. Yng, J. J., Strukov, D. B. & Stewrt, D. R. Memristive devices for computing. Nture Nnotech. 8, (213). 3. Borghetti, J. et l. Memristive switches enle stteful logic opertions vi mteril impliction. Nture 464, (21). Acknowledgements We thnk the Ntionl Science Foundtion for its finncil support under Awrds ECCS nd ECCS , the Deprtment of Energy under Awrd DE-EE679 nd the Defense Thret Reduction Agency under wrd HDTRA Author contriutions J.H. conceived nd supervised the project. Z.X. fricted nd mesured the verticl structure device. Y.Y. nd Q.W. fricted nd mesured the lterl structure device. Y.S. conducted the KPFM mesurement. Q.D. synthesized the MAI mteril. P.S. nd A.G. conducted the PFM mesurement. All uthors nlysed the dt nd wrote the mnuscript. Additionl informtion Supplementry informtion is ville in the online version of the pper. Reprints nd permissions informtion is ville online t Correspondence nd requests for mterils should e ddressed to J.H. Competing finncil interests The uthors declre no competing finncil interests. 198 NATURE MATERIALS VOL 14 FEBRUARY Mcmilln Pulishers Limited. All rights reserved

7 CORRIGENDUM Gint switchle photovoltic effect in orgnometl trihlide perovskite devices Zhengguo Xio, Yongo Yun, Yuchun Sho, Qi Wng, Qingfeng Dong, Cheng Bi, Pnkj Shrm, Alexei Gruvermn nd Jinsong Hung Nture Mterils (214); pulished online 8 Decemer 214; corrected online 7 Jnury 215. In the version of this Letter originlly pulished online, the (ʹ) nd ( ) symols where reversed. In keeping with the Kröger Vink nottion, the (ʹ) should indicte negtive chrge nd ( ) positive chrge, thus the following sentences should hve red Theoreticl clcultions predicted tht negtively chrged P nd MA vcncy (V Pʹ nd V MAʹ) could result in p-type doping, wheres positively chrged I vcncy (V I ) results in, In this scenrio, the electric field cuses the drift of chrged V I, V Pʹ nd/or V MAʹ, which hve low formtion energies nd The loss of perovskite mteril on the node side indicted tht the drifting ions were V Pʹ nd/or V MAʹ. These errors hve now een corrected in ll versions of the Letter. 215 Mcmilln Pulishers Limited. All rights reserved

8 SUPPLEMENTARY INFORMATION DOI: 1.138/NMAT415 Gint Switchle Photovoltic Effect in Orgnometl Trihlide Perovskite Devices Zhengguo Xio 1,2, Yongo Yun 1,2, Yuchun Sho 1,2, Qi Wng, 1,2 Qingfeng Dong, 1,2 Cheng Bi 1,2, Pnkj Shrm 2,3, Alexei Gruvermn 2,3 nd Jinsong Hung 1,2 * 1 Deprtment of Mechnicl nd Mterils Engineering, University of Nersk-Lincoln, Lincoln, Nersk , USA. 2 Nersk Center for Mterils, Nnoscience, University of Nersk-Lincoln, Lincoln, Nersk , USA. 3 Deprtment of Physics nd Astronomy, University of Nersk-Lincoln, Lincoln, Nersk , USA *Correspondence to: jhung2@unl.edu NATURE MATERIALS Mcmilln Pulishers Limited. All rights reserved.

9 SUPPLEMENTARY INFORMATION DOI: 1.138/NMAT415 Figure S1. J-V chrcteristics of the s-prepred verticl structure devices with the structure of (ITO)/poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonte) (PEDOT:PSS) /Perovskite (3 nm) /Au. Scnning rte ws.14 V/s nd the scnning direction ws leled in the figure. Fig. S1 shows current density (J)-voltge (V) chrcteristics of the s-prepred device with 3 nm perovskite lyers. The rrow in the figure depicts the scnning direction. Here, the scnning direction for J-V chrcteristics ws from zero to positive voltge to rule out the possiility for high J SC cused y the un-intentionl poling process during device test. It s impressive tht the device with gold electrode, which usully serves s contct for hole-only device, hs high short current density (J SC ) of 8.2 ma/cm 2. The result indictes the junction hs een prtilly formed efore poling. The reson for junction formtion efore poling should e relted to different interfce structures etween perovskite/pedot:pss nd perovskite/au contcts, ecuse the J SC from the s-fricted device using gold s oth node nd cthode is much smller. 2 NATURE MATERIALS Mcmilln Pulishers Limited. All rights reserved.

10 DOI: 1.138/NMAT415 SUPPLEMENTARY INFORMATION Current density(ma/cm 2 ) c V OC (V) V/s.1 V/s.25 V/s Forwrd scnning Reverse scnning Scnning rte (V/s) J SC (ma/cm 2 ) d V/s.1 V/s.25 V/s Forwrd scnning Reverse scnning Scnning rte (V/s) Figure S2. Drk current () nd photocurrents () of the verticl structure devices with the structure of ITO/PEDOT:PSS /Perovskite (3 nm)/au tested under different scnning rte nd directions.(c-d) V OC (c) nd J SC sttistics sed on five devices with different scnning rtes from.5 V/s to.25 V/s. Forwrd scnning refers scnning from negtive to positive is. In ll the mesurements, ITO ws connected s the cthode nd Au ws connected s the node. Fig. S2 shows the drk current nd photocurrent hysteresis of the sme ITO/PEDOT:PSS /Perovskite (3 nm)/au verticl structure device with different scnning rtes nd directions. The rrows in the figure depict the is scnning directions. A lrge memristive effect from the NATURE MATERIALS Mcmilln Pulishers Limited. All rights reserved.

11 SUPPLEMENTARY INFORMATION DOI: 1.138/NMAT415 devices ws oserved ecuse the drk- nd photocurrent curves hve strong dependence on the scnning history. c Time (s) V poling Time (s) Poling is= -1V Mesured device V OC s 11.3 s Accumulted.2 s 1.5 s 4.2 s poling time Time (s) V poling Figure S3. -, Drk current of the ITO/PEDOT:PSS/Perovskite (3 nm)/au verticl structure devices during positive nd negtive poling process, c, Dynmic poling process of the ITO/PEDOT:PSS/Perovskite/Au verticl structure device with 1,15 nm thick perovskite lyer using -1. V pulse. As shown in Fig. S3 -, in the positive poling (+2.5 V) process, the current density is very smll t the eginning of the poling process, which corresponds to the reverse-ised drk current of the n-i-p structure device (region I). The current incresed quickly with incresed 4 NATURE MATERIALS Mcmilln Pulishers Limited. All rights reserved.

12 DOI: 1.138/NMAT415 SUPPLEMENTARY INFORMATION poling time, nd current pek showed up, which should e due to the ion motion in the perovskite under reverse is (region II). The current egun to reduce fter tens of seconds poling due to the depletion of esily moile ions, nd the current cme to plteu (region III) with high current density, indicting the perovskite film ws switched to p-i-n structure nd the device worked t forwrd is with lrge injection current. The negtive poling showed short poling time thn positive poling, indicting the non-symmetricl composition/morphology profile long the verticl direction. Fig. S3c shows the dynmic poling process of the device with 1,15 nm thick perovskite lyer. Here trin of -1. V pulses with different width were pplied on the device, fter which the V OC of the device ws mesured. The ccumulted poling time, poling is nd mesured device V OC were lso mrked in the figure. As shown in the figure, the device ws switched fter 11.3 s ccumulted poling. Normlized J SC (.u.) J SC (After negtive poling) J SC (After positive poling) Time (h) 5 NATURE MATERIALS Mcmilln Pulishers Limited. All rights reserved.

13 SUPPLEMENTARY INFORMATION DOI: 1.138/NMAT415 Figure S4. The stility test of short circuit current for the ITO/PEDOT:PSS/Perovskite (3 nm)/au verticl structure device fter positive nd negtive poling for 2 s t ±2 V. (The J SC were normlized y the vlue mesured 24 h fter the friction of the device (18.2 ma/cm 2 nd -19. ma/cm 2 ). Fig. S4 shows the stility test result of the short circuit photocurrent for devices fter positive nd negtive poling. The photocurrent direction fter poling remined unchnged fter two months. Two devices were negtively nd positively poled, respectively, under stedy is of 2.5 V for out ten seconds. After tht, the devices were kept under mient illumintion in glove ox nd no further poling ws pplied during the test. In order to eliminte the influence of scnning voltge, we only test the J SC to identify the stility of current direction. As illustrte in the figure, the J SC for oth directions chnged smll rte of ~1% for the first 1 hours. It s mzing no ovious reduction ws identified for device with negtive current fter 1 hours. 6 NATURE MATERIALS Mcmilln Pulishers Limited. All rights reserved.

14 DOI: 1.138/NMAT415 SUPPLEMENTARY INFORMATION c Count Negtive poling Negtive poling V OC (V) J SC (ma/cm 2 ) d Negtive poling V OC (V) 7 6 Negtive poling J SC (ma/cm 2 ) Count Figure S5. Performnce vrition of the ITO/PEDOT:PSS/Perovskite (3 nm)/au verticl structure devices. Scnning rte ws.14 V/s. Photovoltic performnce sttistics of the verticl structure devices. The verticl structure devices were poled t 6 V/µm for 2 s. The photocurrents were mesured under 1 sun illumintion t sweep rte of.14 V/s,, Distriution of the photovoltic performnce of verticl structure devices in the J SC -V OC coordinte. c, V OC distriution of the poled verticl structure devices; d, J SC distriution of the poled verticl structure device; It is found tht there is lrge vrition for the performnces of the ITO/PEDOT:PSS /Perovskite (3 nm)/au verticl structure devices. Some devices showed lrger photocurrent NATURE MATERIALS Mcmilln Pulishers Limited. All rights reserved.

15 SUPPLEMENTARY INFORMATION DOI: 1.138/NMAT415 while some others showed lrger V OC. Fig. S5 shows one device with lrge switchle photovoltge etween.87 V nd -.75 V. 2 Negtive poling 15 1 MAPI x Cl 3-x c Negtive poling Negtive poling HC(NH2) 2 PI MAPBr Figure S6. Switchle photovoltic effect of the ITO/PEDOT:PSS/Perovskite (3 nm)/au verticl structure devices with different perovskite mterils,, CH 3 NH 3 PI 3-x Cl x,, HC(NH 2 ) 2 PI 3 nd c, CH 3 NH 3 PBr 3. The devices were scnned etween ±2.5 V t scnning rte of.14 V/s. 8 NATURE MATERIALS Mcmilln Pulishers Limited. All rights reserved.

16 DOI: 1.138/NMAT415 SUPPLEMENTARY INFORMATION In order to confirm the ion drift induced photovoltic switching mechnism, we exmined other three orgnoled trihlide perovksite mterils, CH 3 NH 3 PI 3-x Cl x, HC(NH 2 ) 2 PI 3 nd CH 3 NH 3 PBr 3 (Fig. S6). It is found ll the devices with these mterils s ctive lyers showed field switchle photovoltic ehvior Negtive poling Pt cthode Positive Poling G cthode c Negtive poling Negtive poling Ni cthode Figure S7. Switchle photovoltic effect of the ITO/PEDOT:PSS/Perovskite (3 nm)/metl verticl structure devices with different metl contcts of Pt, Ni nd G. The devices were scnned etween ±2.5 V t scnning rte of.14 V/s. In order to exmine whether gold is required for the switchle OTP devices, we tested other metls like luminum (Al), silver (Ag), pltinum (Pt), gllium (G) nd nickel (Ni) using the 9 NATURE MATERIALS Mcmilln Pulishers Limited. All rights reserved.

17 SUPPLEMENTARY INFORMATION DOI: 1.138/NMAT415 verticl device structure. Metl electrodes including Al, Ag, Pt, were deposited y therml evportion or sputtering, metl electrode of G ws formed y directly drop the G liquid on perovskite films, nd metl electrode of Ni ws formed using Ni conductive tpes. None of the devices were optimized to hve high performnce. The devices with inert metl electrodes like Pt nd Ni, nd low work function metl of G lso show switchle photovoltic effect s shown in Fig. S7. It ws found tht the thermlly-evported Al nd Ag rected with the perovskite ecuse the deposited electrodes were lck without metllic color, nd the device performnce did not mke ny sense. The difference in short circuit current density cn e explined y the different ctive mterils, electrode mterils, nd contcts etween the electrodes nd the perovskite lyers, ecuse of the different electrode mterils used nd different friction methods to form these electrodes. J SC (ma/cm 2 ) Lterl structure devices Negtive poling V OC (V) Count c Count 1 Negtive poling V OC (V) 6 Negtive poling J SC (ma/cm 2 ) 1 NATURE MATERIALS Mcmilln Pulishers Limited. All rights reserved.

18 DOI: 1.138/NMAT415 SUPPLEMENTARY INFORMATION Figure S8. Photovoltic performnce sttistics of the lterl structure devices. The lterl structure devices were poled under 1.2 V/µm for 1 s, nd photocurrent were mesured under qurter sun illumintion with sweep rte of.5 V/s., distriution of the photovoltic performnce of lterl structure devices in the J SC -V OC coordinte., V OC distriution of the poled lterl structure devices; c, J SC distriution of the poled lterl structure devices d=8 m Normlized J (.u.) 1 d=5 m d=1 m Figure S9. J-V chrcteristics of the Au/perovskite (3 nm)/au lterl structure devices with different electrode spcing. The devices were poled under 1.2 V/µm for 1 s, nd photocurrent were mesured under qurter sun illumintion with sweep rte of.5 V/s. Fig. S9 shows the normlized photocurrent curves with respect to the electrode spcing in the Au/perovskite/Au lterl structure devices. In contrst to ferroelectric photovoltic devices NATURE MATERIALS Mcmilln Pulishers Limited. All rights reserved.

19 SUPPLEMENTARY INFORMATION DOI: 1.138/NMAT415 whose V OC is proportionl to the electrode spcing, the V OC of single lterl structure device keeps lmost constnt t round.5 V regrdless the electrode spcing vrition from 8 µm to 1 µm. This result cn exclude the contriution of potentil ferroelectric property of the perovskite, if it hs, to the switchle photovoltic effect oserved. 4 3 Remnent Polriztion 4 3 Remnent Polriztion Polriztion ( C/cm 2 ) Polriztion ( C/cm 2 ) c -3-4 Scnning rte:.8v/s t room temperture d -3-4 Scnning rte:.8v/s t 77K e f 12 NATURE MATERIALS Mcmilln Pulishers Limited. All rights reserved.

20 DOI: 1.138/NMAT415 SUPPLEMENTARY INFORMATION Figure S1. Ferroelectric polriztion loops mesured t room temperture () nd t 77 K () scnned t the sme frequency of photovoltic switch process. c, Piezoresponse force microscopy (PFM) topology (c), mplitude (e) nd phse (f) imges of the perovskite (3 nm). d, Representtive PFM hysteresis loops (phse nd mplitude) signl for ny loction on the film surfce. Theoreticl clcultion predicted ferroelectric property of MAPI 3 with spontneous polriztion of 38 µc/cm 2 (1). We mesured the ferroelectric hysteresis using the Precision Premier Ⅱ from the Rdint technologies, Inc.. However we did not find ny ferroelectric polriztion within the mesurement rnge of the equipment from these devices oth t room temperture nd t 77 K nd using the sme frequency of photovoltic switch process. This results further exclude the contriution of potentil ferroelectric property of the perovskite, if it hs, to the switchle photovoltic effect oserved. 13 NATURE MATERIALS Mcmilln Pulishers Limited. All rights reserved.

21 SUPPLEMENTARY INFORMATION DOI: 1.138/NMAT415 Normlized J SC (.u.) Negive poling Time (s) Figure S11. J SC output of the ITO/PEDOT:PSS/Perovskite (3 nm)/au verticl structure devices fter negtive nd positive poling with time. The J SC were normlized y the vlue mesured t the strting point. The devices were poled t 6 V/µm for 2 s. Fig. S11 shows the normlized J SC output mesured overtime. It is ovious tht, fter positive or negtive poling, the device cn output persistent photocurrent under light, which excluded the contriution of chrge trps to the switchle photovoltic effect. 14 NATURE MATERIALS Mcmilln Pulishers Limited. All rights reserved.

22 DOI: 1.138/NMAT415 SUPPLEMENTARY INFORMATION + poling perovskite 12.1 nm - poling perovskite nm Au Au nm nm c + poling perovskite d + poling perovskite Figure S12. AFM topogrphy imge (), surfce potentil imge (c) nd dhesion imge (d) of the perovskite film fter positive poling. AFM topogrphy imge of nother film fter negtive poling (). The Au electrode in topogrphy imge shown in Fig. S12 is not cler ecuse the thickness of the Au electrode is only out 5 nm, much smller thn the roughness (~15 nm) of the perovskite thin films formed on glss. Nevertheless, we oserved cler difference of Au region nd perovskite region s well s the seprtion line etween these two regions y exmining the dhesion mpping recorded during AFM scnning. The dhesion mpping of the exct sme re of Fig. 3e is shown in Fig. S12d. Adhesion is defined s the minimum tension ( pull-off ) forces 15 NATURE MATERIALS Mcmilln Pulishers Limited. All rights reserved.

23 SUPPLEMENTARY INFORMATION DOI: 1.138/NMAT415 required for retrcting the AFM tip from the smple surfce. Mny different types of interction forces, minly vn der Wls nd/or electrosttic force, ttriute to dhesion. Becuse the interction etween tips with different mterils hve different dhesion, which constitutes the contrst, this technology hs een pplied to wide vriety of mterils to investigte the surfce heterogeneity or mteril distriutions. As shown in Fig. 3e nd Fig. S12d, the dhesion mpping grees well with the KPFM mpping. Figure S13. A SEM imge of Au/perovskite/Au lterl structure device fter poling It shows the morphology chnge of perovskite fter poling ner the node re. Compred to the re fr wy from the node, the re close to the Au electrode showed lot of pin-holes formed. 16 NATURE MATERIALS Mcmilln Pulishers Limited. All rights reserved.

24 DOI: 1.138/NMAT415 SUPPLEMENTARY INFORMATION d f V pulse Thickness 3 nm t RT V pulse V pulse Time (S) 5 Solvent nneling -2.5 V pulse Therml nneling -5 e 1µm 1µm Time (s) c J SC (ma/cm 2 ) 2 Thickness: 3 nm -2.5 V pulse At RT 1 25 Temperture of mesurement: 2 RT 6 o C Film tretment: Therml nneling -15 Solvent nneling -2 Therml nneling Accumulted poling time (s) Figure S14. Dynmic poling process of the ITO/PEDOT:PSS/Perovskite (3 nm)/au verticl structure devices t () vried electricl field, (-c) temperture nd (d-f) with different film morphology. The thickness of the films in d-f is 1,15 nm. (g) J SC versus poling time of the devices with different film nneling processes nd mesurement tempertures. The devices were mesured under.1 sun when mesured t 6 C. The error r showed the device performnce At 6 o C Time (s) 1. Negtive poling g K Negtive poling K J SC (ma/cm 2 ) NATURE MATERIALS Mcmilln Pulishers Limited. All rights reserved.

25 SUPPLEMENTARY INFORMATION DOI: 1.138/NMAT415 vrition sed on the sttistics of five devices of ech ctegory. And the scnning rte in Fig. S14c ws.14 V/s. The drift of ions is expected to depend on temperture, electric field nd the film morphology. We studied the influence of these fctors on the poling process y pplying trin of short voltge pulse (.95 S) on the device nd mesured the device t J SC fter ech pulse to void dditionl poling during the mesurement. As shown in Fig. S14-c, lrger electric field or higher poling temperture results in fster switching of the device. The switching of photovoltic ws frozen under low temperture elow C, where the photocurrent direction cnnot e switched y the sme is of ±2.5 V. It should e noted the dt shown in Fig. 14 were mesured under 1 sun illumintion, nd the photocurrent in Fig. S14 were mesured under round.1 sun illumintion due to the proe sttion setup limittion. Photocurrents in Fig. S14c were mesured with incident light penetrting the thin opque Au electrodes, therefore the short circuit current densities were much smller thn other cses. Our recently developed solvent-nneling process resulted in lrger grin size in the perovskite films thn the therml-nneling, s shown in the cross-section scnning electron microscopy imges (Fig. S14d-e). We lso tested the dynmic poling process of the devices with perovskite films fricted y solvent-nneling nd therml-nneling. The perovksite film with lrger grin size hd fewer grin oundries nd thus less ion vcncies, which lso resulted in smller ion drift velocity. As shown in Fig. S14e, the device with the solvent-nneled perovskite films took much longer time to e switched thn the therml-nneled devices. 18 NATURE MATERIALS Mcmilln Pulishers Limited. All rights reserved.