Fractional Deviations in Precursor Stoichiometry Dictate the Properties, Performance and Stability of Perovskite Photovoltaic Devices

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1 Electronic Supplementry Mteril (ESI) for Energy & Environmentl Science. This journl is The Royl Society of Chemistry 2018 Frctionl Devitions in Precursor Stoichiometry Dictte the Properties, Performnce nd Stility of Perovskite Photovoltic Devices Pul Fssl 1, Vincent Lmi 1, Alexndr Busch 1, Zhiping Wng 2, Mtthew T. Klug 2, Henry J. Snith 2 nd Yn Vynzof 1* 1 P. Fssl, V. Lmi, A. Busch, Prof. Y. Vynzof, Kirchhoff-Institut für Physik nd Centre for Advnced Mterils, Ruprecht-Krls-Universität Heidelerg, Im Neuenheimer Feld 227, 690 Heidelerg, Germny. *Emil: vynzof@uni-heidelerg.de 2 Dr. Z. Wng, Dr. M. T. Klug, Prof. H. J. Snith, Clrendon Lortory, Deprtment of Physics, University of Oxford, Oxford OX1 3PU, UK. Supplementry Note 1: For the PAc2 recipe, the chnges in stoichiometry were performed y dding MAI/DMF stock solution into precisely known volumes of MAI:PAc2/DMF perovskite solution. In order to perform controlled stoichiometric chnges, it is necessry to know the totl solid concentn in ech of the solutions. Therefore, the density of the two solutions ws determined in the following fshion: For the MAI/DMF stock solution, mg MAI ws dissolved in 1 ml DMF (29 wt%). The density of this solution ws determined (y weighing exctly known volumes) to e 1145 mg/ml, i.e. solid concentn of mg/ml MAI. For the perovskite solution, crefully weighed mounts of MAI nd PAc2 were dissolved in DMF t 42 wt% with certin stoichiometry y. After ddition of HPA (6.43 ul / 1 ml DMF) with density of mg/ml, the weight percentge of perovskite (MAI+PAc2) decreses to 41.8%. The density for vrious stoichiometries ws determined y weighing.

2 Once the density of the perovskite solution nd the perovskite weight percentge re known, the solid concentn s well s the mount of PAc2 per smple (40 µl) cn e clculted for ech stoichiometry y. The weight of PAc2 in the remining precursor solution ws clculted for ech step nd suitle mounts of MAI/DMF stock solution were dded to perform the desired chnges in stoichiometry y (see Tle S1 for n exmple clcultion, nd Fig. S13, S14 nd S16 for the results using this recipe). This is y fr the most reproducile nd ccurte method to perform controlled stoichiometric chnges nd furthermore it llows the use of the sme precursor solution for complete tch. While chnging the precursor stoichiometry is in principle lso possile y weighing slightly different mounts of ech of the precursors into individul solutions, our experience shows tht humn error when weighing vrious compounds in the sme vil is often underestimted nd tht even with extreme cre nd ttention, this method is less ccurte thn the one descried ove nd used in this study. As the initil precursor weighing nd the pipetting using clirted Gilson Pipetmn Micromn E pipettes (for determining the density s well s performing chnges in stoichiometry) oth introduce smll errors on the order of ~0.1% nd ~1%, respectively, we were le to estimte the systemtic error for ech stoichiometry y. As n exmple, using the vlues in Tle S1, even in the extreme cse tht the ctul volume per smple ws 38 µl insted of the supposed 40 µl (5% reltive error), the finl stoichiometry fter 5 vrition steps would still e y = insted of y = 3.06 which represents n error of only y = (see Tle S2). For the MAPI solvent quenching recipe, gin the density of the perovskite solution nd the stock solution, contining PI2:MAI (5:1) nd MAI (oth dissolved in DMF/DMSO (4:1, v:v), with wt % nd 40 wt %, respectively), ws determined y weighing to clculte the solid concentn in the solutions. Suitle mounts of stock solution were dded to the strting perovskite solution to vry the stoichiometry z = PI2:MAI s presented in Fig. S13 nd Fig. S15 (ESI ).

3 For the CsFAMA recipe, the prmeter spce for performing stoichiometric vritions round the stoichiometric Cs0.05(FA3MA0.17)5P(IBr0.1)3 (with z = Cs0.05(FA3MA0.17)5I:P(I5Br0.15)2 = 1) is even igger. In order to keep it simple, we vried the mount of excess P(I5Br0.15)2, which represents the typicl led excess in mny pulictions, or the mount of excess FAI3MAI0.17 in the precursor solution. Sme s for the PAc2 recipe, the density of the two stock solutions contining P(I5Br0.15)2 nd FAI3MAI0.17 s well s tht of the perovskite precursor solution Cs0.05(FA3MA0.17)5P(IBr0.1)3 (ll dissolved in DMF/DMSO (4:1, v:v) with 31.2 wt%, 18 wt% nd 43.3 wt%, respectively) ws determined y weighing. Suitle mounts of stock solution were dded to the perovskite solution to vry the stoichiometry z = Csx(FA3MA0.17)100-xI:P(I5Br0.15)2 s presented in Fig. S16. The mount of Cesium is slightly decresing upon the ddition of the Cs-free stock solutions, however still remins etween < x < 0.05 for ll investigted stoichiometries. The finl nominl compositions cn e written s Cs0.05(FA3MA0.17)5P(IBr0.1)3, z1 P(I5Br0.15)2 for n excess of led or Cs0.05(FA3MA0.17)5P(IBr0.1)3, z2 (FAI3MAI0.17) for n excess of orgnic with z1/2 representing the respective molr excess. Supplementry Note 2: Modified Willimson-Hll method for microstrin estimtion Brodening nd shifts in the XRD pek cn e cused y either reduction in the grin size (Scherrer rodening) nd/or non-uniform strin (microstrin). We note tht Scherrer rodening will only e significnt when the grins re in the rnge of or less thn 100nm, nd s we will discuss lter we do not expect this to e significnt contriution here. Strin is the reltive chnge in size of n oject with respect to its idel size (or size efore experiencing n externl force). The microstrin in crystlline mteril is result of smll fluctutions in the lttice spcing, induced y crystl imperfections/structurl defects including disloctions, vcncies, stcking fults, interstitils, twinning, nd grin oundries. 1,2,3 By simply considering Brggs lw for scttering of light of wvelength λ, nλ = 2dsinθ, it is cler tht smll fluctutions in d (i.e. d) will

4 result in smll fluctutions, or rodening, in θ when mesuring the X-ry diffrction from the mteril. We quntify the extent of microstrin in our perovskite crystls y nlysing the pek rodening in the diffrction ptterns ccording to the modified Willimson-Hll method. 2,4 The effective oserved d- spce rodening ( dos) determined from the XRD pek width rodening, is convoluted function of the Gussin full width hlf mximum rodening in the 2θ scn due to the instrument response ( dins), the grin size ( dsize) nd the microstrin ( dε). These cn e de-convoluted from the oserved rodening, vi, d 2 os = d 2 ε + d 2 ins + d 2 size (1) where the unit-less microstrin ε is defined s ε = ( dε/d), where d is the men d-spcing. For single crystls, the size effect induced pek width rodening cn e neglected, hence if d 2 size << d 2 os 19 nd we cn write, ( d 2 os - d 2 ins) 1/2 εd. (2) Therefore, the slope of ( d 2 os - d 2 ins) 1/2 versus d, gives the mgnitude of the microstrin, ε, in the crystls. References: 1. Roinson, I. & Hrder, R. Coherent X-ry diffrction imging of strin t the nnoscle. Nt. Mter. 8, (2009). 2. A. Prmnick, X. P. Wng, C. Hoffmnn, S. O. Dillo, M. R. V. Jørgensen, X.-L. W. Microdomin dynmics in single-crystl BTiO3 during prelectricferroelectric phse trnsition mesured with time-of-flight neutron scttering. Phys. Rev. B 92, (2015). 3. Willimson, G.. & Hll, W.. X-ry line rodening from filed luminium nd wolfrm. Act Metll. 1, (1953). 4. Zho, Y. & Zhng, J. Microstrin nd grin-size nlysis from diffrction pek width nd grphicl derivtion of high-pressure thermomechnics. J. Appl. Crystllogr. 41, (2008).

5 Supplementry Tles: Supplementry Tle 1: Exmple clcultion for performing chnges in stoichiometry: At the strt, the solution with stoichiometry of y = 2.96 contins 200 mg PAc2 with perovskite (MAI+PAc2) weight percentge of 41.8%. For ech stoichiometry 2 smples (80 µl) re prepred. The density nd the perovskite wt% ws determined eforehnd for ll stoichiometries. Thus, the remining weight of PAc2 fter ech vrition step nd with tht the required mount of MAI/DMF stock solution (with mg/ml MAI nd 29 wt%) to otin the desired stoichiometry could e clculted. Stoichiometry, y = Amount of PAc2 [mg] Density [mg/ml] Perovskite content [wt %] PAc2 per 1 ml [mg] PAc2 per smple [mg] Amount of MAI to chnge y y 0.02 [mg] MAI/DMF solution to dd [µl] MAI:PAc2 Strt: Mix 200 mg PAc2 with mg MAI nd dd µl DMF nd 4.22 µl HPA 2.96 (x2) (x2) (x2) (x2) (x2) (x2) Supplementry Tle 2: Error estimtion using the sme clcultion s in Supplementry Tle 1, ut ssuming tht the ctul volume per smple ws (y error) 38 µl insted of 40 µl. The reltive error for ech step is clculted, yielding totl error of y = for the lst stoichiometry y = Stoichiometry, y = Amount of PAc2 [mg] Actul mount [mg] PAc2 per smple [mg] Actul mount [mg] Aimed for stoichiometry Actul stoichiometry Reltive error for this step y MAI:PAc2 Strt: Mix 200 mg PAc2 with mg MAI nd dd µl DMF nd 4.22 µl HPA 2.96 (x2) (x2) (x2) (x2) (x2) (x2) Totl error

6 Supplementry Tle 3: EDX nlysis t 10 kv. Atomic percentges of Glss/ITO/PEDOT:PSS/MAPI3 with vrious stoichiometries determined y EDX t n ccelen voltge of 10 kv. The vlues present n verge of minimum 8 different spots on one smple nd ll perovskite lyers were fricted in the sme tch. The signls from the 40 nm PEDOT:PSS (S), the ~150 nm ITO lyer (Sn nd In) nd the glss sustrte (Si) re clerly visile, showing tht the whole perovskite ulk is proed. The corresponding I:P tomic s re presented in Fig. S7. Element O K S L Si K Sn L C K [%] N K [%] In L [%] / y [%] [%] [%] [%] I L [%] P M [%] Supplementry Tle 4: EDX nlysis t 6 kv. Atomic percentges of Glss/ITO/PEDOT:PSS/MAPI3 with vrious stoichiometries determined y EDX t n ccelen voltge of 6 kv. The vlues present n verge of minimum 8 different spots on one smple nd ll perovskite lyers were fricted in the sme tch. No signl for Si, In nd Sn is detected, showing tht the interction volume of the electron em only just reches the thin PEDOT:PSS lyer nd the min signl comes from regions of the perovskite lyer closer to the film surfce. The corresponding I:P tomic s re presented in Fig. S7. Element Si K In L Sn L P M C K [%] N K [%] O K [%] S L [%] I L [%] / y [%] [%] [%] [%]

7 EQE (%) Asornce (.u.) Current density (ma/cm 2 ) Current density (ma/cm 2 ) Supplementry Figures: Voltge (V) Figure S1. () Representtive J-V curves in reverse (solid lines) nd forwrd (dotted lines) scn directions from the tch shown in the min text mesured under simulted AM1.5G solr irrdince t 100 mwcm 2. () Representtive drk J-V curves over the whole rnge of investigted stoichiometries Voltge (V) Wvelength (nm) Wvelength (nm) Figure S2. () Representtive EQE spectr of the devices shown in the min text. () UV-Vis mesurements of vrious stoichiometries.

8 XPS pek position (ev) Ioniztion potentil (ev) MA + pek w.r.t. VL (ev) Normlized FF Normlized PCE Normlized V OC c 5 0 Btch1 Btch2 Btch3 Btch4 Btch MAI:PAc d Normlized J SC MAI:PAc MAI:PAc MAI:PAc 2 Figure S3. Normlized photovoltic prmeters of five tches with vrying stoichiometry. The trend in () VOC, () JSC, (c) FF nd (d) PCE with chnging stoichiometry ws oserved in severl tches. The solute dt of Btch 3 (lue) is shown in Fig.1 in the min text d = P4f 7/2 d = N1s d = I3d 5/ MAI:PAc d = d = MAI:PAc 2 Figure S4. () Shift in XPS pek positions of P4f7/2, N1s nd I3d5/2 with chnging stoichiometry nd corresponding fits with slope d. () Ioniztion potentil (left) nd position of the MA+ pek w.r.t. to vcuum level (right) over stoichiometry nd corresponding fits with slope d.

9 I/P tomic Mx ELQE (%) Opticl power (uw) Understoichiometric Stoichiometric Overstoichiometric EF,e EF,e EF,e VOC, mx VOC, mx VOC, mx EF,h EF,h EF,h Figure S5. Schemtic illustn of the possile qusi Fermi level splitting nd the mximum otinle VOC for understoichiometric, stoichiometric nd overstoichiometric perovskite lyers. The position of the qusi Fermi level for holes, EF,h, remins unchnged s the PEDOT:PSS/perovskite interfce is not ffected. The position of the qusi Fermi level for electrons, EF,e, rises in energy s it is limited y the Fermi level of the metl electrode, which in our cse is lwys t lest n energetic distnce elow the conduction nd t the surfce. Reference 37 (min text) contins further informtion on the effect of such surfce energetics chnges on the device uilt-in potentil nd the open circuit voltge ~200 ma/cm 2 1E-4 MAI:PAc Wvelength (nm) Figure S6. () Mximum ttinle ELQE from ITO/PEDOT:PSS/MAPI3/PCBM/BCP/Ag devices with vrious stoichiometries y. () EL spectr t fixed current density of 200 ma/cm EDX 10 kv EDX 6 kv Fit to XPS dt MAI:PAc 2 Figure S7. I/P tomic determined y EDX mesurements performed t 10 kv (lck) nd 6 kv (red) s well s fit to the corresponding XPS dt in Fig. 2d.

10 SPO fter 297 dys (%) Hysterese index Normlized FF Normlized PCE Normlized V OC c e Time (s) Normlized J SC d f = PCE Reverse /PCE Forwrd Figure S8. Trcking of the photovoltic prmeters in reverse scn with drk storge in mient tmosphere of Btch 2 in Fig. S3, () VOC, () JSC, (c) FF, (d) PCE, (d) SPO fter 297 dys nd (e) hysteresis index.

11 SPO fter 245 dys (%) Hysterese index Normlized FF Normlized PCE Normlized V OC Normlized J SC c d e f Time (s) 0.5 = PCE Reverse /PCE Forwrd Figure S9. Trcking of the photovoltic prmeters in reverse scn with drk storge in mient tmosphere of Btch 1 in Fig. S3, () VOC, () JSC, (c) FF, (d) PCE, (d) SPO fter 245 dys nd (e) hysteresis index.

12 Bndgp (ev) Asorption coefficient (cm -1 ) d os 2 - dins 2 ) 1/2 (Å) Microstrin (%) Pek intensity / 10 3 (.u.) FWHM ( ) Lttice constnt (Å) (110) plne (110) plne (220) plne MAI:PAc 2 MAI:PAc 2 Figure S10. () Pek intensity nd full width hlf mximum (FWHM) of the (110) pek for vrious stoichiometries. () Lttice constnt over stoichiometry for the (110) nd (220) plnes d spcing (Å) MAI:PAc 2 Figure S11. () A modified Willimson-Hll plot of ( dos 2 - dins 2 ) 1/2 versus d spcing extrcted from the corresponding diffrction profiles shown in Fig. 4c in the min text. The slopes of the solid lines from liner fits indicte the extent of microstrin for the different stoihciomteries. () Clculted microstrin of smples with different precursor s A = A 0 exp(e/e u ) from Tuc-plot fit MAI:PAc 2 Energy (ev) Figure S. () Bndgp of ITO/PEDOT:PSS/MAPI3 with vrious stoichiometries clculted from Tuc-plots. () PDS mesurements of Glss/MAPI3 with vrious stoichiometries. The Urch energy (Eu) is determined y fitting the exponentil decy of the sornce with the eqution shown in the grph.

13 FF (%) PCE (%) V OC (V) J SC (ma/cm 2 ) c MAI:PAc 2 MAI:PAc 2 d MAI:PAc 2 10 MAI:PAc 2 Figure S13. ()-(d) PV prmeters in reverse scn of FTO/Poly-TPD/PAc2/PCBM/BCP/Ag solr cells with vrition of the y = MAI:PAc2. See Experimentl Section nd Supplementry Note 1 for more detils on solution nd device prepn.

14 FF (%) PCE (%) V OC (V) J SC (ma/cm 2 ) c MAI:PI 2 d MAI:PI MAI:PI MAI:PI 2 Figure S14. ()-(d) PV prmeters in reverse scn of FTO/Poly-TPD(F4-TCNQ)/MAPI/PCBM/BCP/Ag solr cells with vrition of the z = MAI:PI2. See Experimentl Section nd Supplementry Note 1 for more detils on solution nd device prepn.

15 FF (%) PCE (%) V OC (V) J SC (ma/cm 2 ) 8 c MAI:PAc 2 68 MAI:PAc 2 d MAI:PAc 2 MAI:PAc 2 Figure S15. ()-(d) PV prmeters in reverse scn of ITO/SnO2-NP/PAc2/Spiro-OMeTAD/Ag solr cells with vrition of the y = MAI:PAc2. See Experimentl Section nd Supplementry Note 1 for more detils on solution nd device prepn.

16 FF (%) PCE (%) V OC (V) J SC (ma/cm 2 ) c MAI:PI MAI:PI 2 d MAI:PI MAI:PI 2 Figure S16. ()-(d) PV prmeters in reverse scn of FTO/SnO2/PCBM/MAPI/Spiro-OMeTAD/Ag solr cells with vrition of the z = MAI:PI2. See Experimentl Section nd Supplementry Note 1 for more detils on solution nd device prepn.

17 PCE (%) PCE (%) Current density (ma/cm 2 ) FTO/Poly-TPD/CsFAMA z = 085 Reverse z = 085 Forwrd Reverse/Forwrd: PCE = 18.3/18.5 % FF = 74.3/74.4 % J SC = 23.0/23.2 ma/cm 2 V OC = 8/8 V SPO = 18.6 % Cs 100-x (FA 3 /MA 0.17 ) x I:P(I 5 Br 0.15 ) Voltge (V) Figure S17. () PCE (verge of the reverse nd forwrd scn) of FTO/Poly-TPD (F4- TCNQ)/CsFAMA/PCBM/BCP/Ag solr cells with Cs0.05(FA3MA0.17)5P(IBr0.1)3 s ctive lyer nd vrition of the z = Cs100-x(FAI3MAI0.17)x : P(I5Br0.15)2 nd () corresponding J-V curve of the chmpion pixel with z = 085. See Experimentl Section nd Supplementry Note 1 for more detils on solution nd device prepn dy 1 dy 3 dy MAI:PAc 2 Figure S18. Preliminry storge stility investigtion of PAc2-sed devices in the stndrd rchitecture (Fig. S15), showing tht the initilly most efficient overstoichiometric devices with y = 3.03 re degrding much fster when compred to the very stle devices with y ~ The unencpsulted devices were stored in ir in drk (~40-50% RH) etween the mesurements.