Supporting Information

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1 Supporting Information Enhanced Thermal Stability in Perovskite Solar Cells by Assembling 2D/3D Stacking Structures Yun Lin 1, Yang Bai 1, Yanjun Fang 1, Zhaolai Chen 1, Shuang Yang 1, Xiaopeng Zheng 1, Shi Tang 1, Ye Liu 1, Jingjing Zhao 1, and Jinsong Huang 1,2 * 1 Department of Mechanical and Materials Engineering, University of Nebraska-Lincoln, Lincoln, Nebraska 68588, USA 2 Department of Applied Physical Science, University of North Carolina, Chapel Hill. North Carolina, 27514, USA. * To whom correspondence should be addressed-mail: jhuang@unc.edu S1

2 Experimental Section Preparation of Perovskite solar cells: Indium tin oxide (ITO) coated glass substrates were cleaned sequentially in acetone and isopropyl alcohol (IPA) under sonication for 30 min, twice for each step. After drying in a vacuum oven, the substrates were treated by UV ozone for 15 min and then transferred to nitrogen glovebox for use. Methylammonium iodide (MAI) and lead iodide (PbI 2 ) were mixed at a molar ratio of 1:1, then Dimethylformamide (DMF)/ Dimethyl sulfoxide (DMSO) mixed solvent (9:1 in volume ratio) was added into it to form the perovskite precursor solution with a concentration of 1.3M. After completely stirring for about 2 hours, the prepared precursor solution was filtered by a 0.2 µm polytetrafluoroethylene filter to get the clear solution. Poly(bis(4-phenyl)(2,4,6-trimethylphenyl)amine) (PTAA) layers as the hole transport layer were firstly deposited on ITO substrates by spin-coating PTAA solution (0.2 wt.%) at 6000 rpm for 35 s, followed by thermal annealing at 100 for 10 min. Perovskite layers were prepared by a one-step method, the precursor solution was spin-coated at 2000 rpm for 2 s and 4000 rpm for 20 s, respectively. At about 10 seconds after the beginning of the second spin-coating step, 130 µl of toluene as anti-solvent was dropcasted on the substrates. The films were annealed at 100 for 10 min and then cooled down to room temperature. For the control devices, PCBM as the electron transport layers were directly deposited on top of perovskite layers by spin-coating the solution (20 mg/ml in 1,2-Dichlorobenzene(DCB)) at 6000 rpm for 35 s and afterwards annealed at 100 for 30 min. C60 (buffer layers) and bathocuproine (BCP, hole blocking layers) were thermally evaporated sequentially, which is followed by the thermal evaporation of Cu electrode. S2

3 For 2D perovskites formation on top of 3D perovskite materials, two methods were adopted in this study. The perovskite layers were further treated in the following procedures before the deposition of PCBM comparing to the control devices. For one method, n-butylamine (BA) solutions (dissolved in Chlorobenzene with concentrations of 0.1 % v/v, 0.3 % v/v and 1 % v/v) were dropped on top of perovskite layers to fully cover them, within three seconds the substrates were spun at 6000 rpm for 35s. For the other method, n-butylammonium iodide (BAI) solutions (dissolved in IPA with concentrations of 1 mg/ml, 2 mg/ml and 4 mg/ml) were dropped on the surface of perovskite layers, the substrates were spun at 6000 rpm for 35s within 3 seconds. Both resulting thin films were annealed at 100 for 10 min. Film characterization: The XRD experiments were performed by a Bruker D8 Discover Diffractometer utilizing Cu Kα1 radiation. The SEM images were obtained with a Quanta 200 FEG ESEM scanning electron microscope. The absorption spectra were recorded by an Evolution 201/220 UV/Visible Spectrophotometer. Time-resolved Photoluminescence (TRPL) measurements were carried out by a HORIBA Scientific ihr320 Imaging Spectrometer. Device characterization: The photocurrent density-voltage curves of the photovoltaic devices were recorded by a Keithley 2400 Source-Meter with homemade testing software. The devices were exposed to a xenon-lamp based solar simulator (Oriel 67005, 150 W) under AM 1.5 G irradiation (100 mw/cm2), the light intensity was calibrated by a Si photodiode (Hamamatsu S1133). The J-V tests were swept with scan rate of 0.1 V/s and S3

4 delay time of 100 ms. Capacitance-frequency tests were performed using an E4980A Precision LCR Meter from Agilent. For stability test, the unmodified 3D perovskite devices and the devices with 2D/3D stacking structures were prepared in configuration shown in Figure 3c. In thermal stability tests, the non-encapsulated devices were put on a hotplate with setting temperature of 95 in nitrogen-filled glovebox under dark condition, but were removed from hotplate for the J-V tests. S4

5 (a) RMS=32.51nm (b) RMS=16.87nm (c) RMS=18.95nm Figure S1. AFM images with RMS surface roughness values in a scan area of µm 2 of (a) MAPbI 3 films, (b) MAPbI 3 films with BA treating and (c) MAPbI 3 films with BAI treating. S5

6 Figure S2. The J-V characteristics of BA (a) and BAI (b) treated devices with concentration optimization. Table S1: Photovoltaic performance values of the control, BA and BAI treated devices with concentration optimization. J sc (ma/cm 2 ) V oc (V) FF PCE (%) BA (0.1 % v/v) BA (0.3 % v/v) BA (1 % v/v) BAI (1 mg/ml) BAI (2 mg/ml) BAI (4 mg/ml) S6

7 (a) Count (c) Count Efficiency (%) Voc (V) (b) Count (d) Count FF Jsc (ma/cm 2 ) Figure S3. Comparison of histograms of photovoltaic parameters: efficiency (a), fill factor (b), V oc (c) and J sc (d) for the perovskite solar cells based pristine, and MAPbI 3 films (statistics from 40 samples for each fabrication condition). Table S2: Statistical analysis of J sc, V oc, FF and PCE for devices based on pristine, BAtreated and MAPbI 3 films. Average J sc (ma/cm 2 ) J sc s.d. (ma/cm 2 ) Average V oc (V) V oc s.d. (V) Average FF FF s.d. Average PCE (%) PCE s.d. (%) BAtreated BAItreated S7

8 Table S3: Photovoltaic performance values for the best cells of the control, and devices. J sc (ma/cm 2 ) V oc (V) FF PCE (%) Table S4: Photovoltaic performance values for the best cells of the control, BA and BAI treated devices measured by forward and reverse scanning at 0.1 V/s. J sc (ma/cm 2 ) V oc (V) FF PCE (%) Reverse Forward Reverse Forward Reverse Forward S8

9 (a) 1.00 J SC (norm.) (c) 1.0 FF (norm.) Time (hours) Time (hours) (b) V OC (norm.) Time (hours) (d) PCE (norm.) Time (hours) Figure S4. The comparison of (a) normalized J sc, (b) normalized V oc, (c) normalized FF and (d) normalized PCE of control, and devices in the thermal stability test. 7 samples were tested for each condition, each point represents the mean value of each set of data and error bars are standard deviations. S9