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1 Graded bandgap perovskite solar cells Onur Ergen, 1,3,4 S.Matt Gilbert 1, 3,4,,Thang Pham 1, 3,4,Sally J. Turner, 1,2,4, Mark Tian Zhi Tan 1, Marcus A. Worsley 1, 3,4 and Alex Zettl 1 Department of Physics, University of California at Berkeley, Berkeley, California 947, USA 2 Department of Chemistry, University of California at Berkeley, Berkeley, California 947, USA 3 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 947, USA 4 Kavli Energy Nanosciences Institute at the University of California, Berkeley, and the Lawrence Berkeley National Laboratory, Berkeley, California 947, USA Physical and Life Sciences Directorate, Lawrence Livermore National Laboratory, Livermore, CA 940, USA Corresponding Author: Prof. Alex Zettl (azettl@berkeley.edu) Supporting Information Content A) Material Characterization Absorption and Photoluminescence measurements SEM-EDAX study Top view SEM Images XRD patterns B) Electrical Characterization Current density vs. Time Hall Effect mobility measurements I-V curve of champion cell The reverse and forward sweep of perovskite cells Ohmic contact behavior of GaN NIR-PL spectra of graded band gap perovskite cells based solar cell with only GA modification EQE&IQE data of champion cell with integrated photocurrent Back surface pits on the GaN suface Mott-Schottky measurements GaN/Perovskite interface NATURE MATERIALS 1

2 A) Material Characterization Concentration variation Absorbance (a.u.) Sn(I 3-x ), x=0 Sn(I 3-x ), x=1, x=0, x=0.04, x=0., x=0.6, x=1 x=1 x= Wavelength (nm) x=1 x=0 Photoluminescence (a.u.) 7 3 Sn(I 3-x ), x=0 Sn(I 3-x ), x=1, x=0, x=0.04, x=0., x=0.6, x= Wavelength (nm) Figure S 1 UV-visible light absorption spectra of SnI 3-x and Pb(I 1 - x, with varying iodide concentration x, b) Photoluminescence (PL), spectra of perovskite cells, SnI 3-x and Pb(I 1 - x, by varying iodide concentration x. 2 NATURE MATERIALS

3 SUPPLEMENTARY INFORMATION The role of the graphene aerogels (c) (d) Figure S 2 Photoluminescence analysis of perovskite cells in air (only PbI 3 - x ). without GA. with GA modification. (c) Bandgap changing by time with and without GA layer. (d) EDAX line mapping for oxygen signature of a perovskite with and without GA modification. Graphene aerogel encapsulation acts as a barrier for oxygen penetration and moisture ingress. 3 Figure S 3 Top view SEM image of a perovskite sample after peeling off GA layer. The line formations arise due to interfacial adhesion. Top view SEM image of samples without GA improvement. The scale bars in the SEM images are µm. NATURE MATERIALS 3

4 7 Intensity (a.u.) 3 1 W/ GA θ (degree) 7 Intensity (a.u.) 3 W/O GA θ (degree) Figure S 4 XRD diffraction patterns of the perovskite layers of a) W/ GA and b) W/O GA. 4 NATURE MATERIALS

5 SUPPLEMENTARY INFORMATION The role of the h-bn Figure S Cross sectional SEM-EDAX analysis of perovskite cells EDAX signal for cell with h-bn, over the area outlined by red box in the inset SEM image. Line mapping of cell with h-bn modification (dashed line indicates the position of h-bn). The scan is along the vertical red line (from top to bottom) shown in the inset SEM image. (c) Line mapping of cell without h-bn modification. The scan is along the red vertical line (from top to bottom) shown in the inset SEM image. Scale bar for inset of a) and b) is 0nm; scale bar for inset of c) is 100nm. NATURE MATERIALS

6 B) Electrical Characterization Stability under illumination Current Density (ma/cm 2 ) Complete cell (W/h-BN, W/GA) 0.6 W/h-BN, W/O GA 0.4 W/O h-bn, W/O GA 0.2 Open Circuit Voltage (V ) Time (min) PCE (%) Time (min) Figure S 6 a) Time dependent Current density (solid lines) and Voc (dashed lines) is shown. The cells without h-bn and GA exhibit faster degradation under constant illumination compared to the complete cell with h-bn and GA. (solid lines is Jsc and dashed lines is V oc ) b) Power conversion efficiency of the cells with h-bn and GA (red), cell w/h-bn and W/O GA (black), W/O h-bn and W/O GA (green). Complete cells show a very stable behavior under constant illumination. Even though a decrease was observed in the current density, there is a constant increase in open circuit voltage indicating that efficiency becomes stable with time. 6 NATURE MATERIALS

7 SUPPLEMENTARY INFORMATION The role of the graphene aerogels on mobility 300 Hall Mobility (cm 2 V -1 s -1 ) Complete cell Complete cell (W/O GA) Crystallization Temperature ( 0 C) Figure S 7 Hall effect measurement. The mobility plotted against the annealing temperature of double layered perovskite cells (re-crystallization temperatures). NATURE MATERIALS 7

8 Figure S 8 J-V curves for 21.7% PCE graded band gap perovskite cell with (red) and without (blue) light illumination. 8 NATURE MATERIALS

9 SUPPLEMENTARY INFORMATION Forward Reverse Current Density (ma/cm 2 ) Voltage (V) R F Counts 8 6 R F R F 4 2 R F F R R F R F PCE (%) Figure S 9 Reverse and forward sweep (<0.01V/s) J-V for a typical graded band gap perovskite device. Histogram of solar cell efficiencies with reverse and forward sweep, after 1h illumination in air. NATURE MATERIALS 9

10 Figure S 10 Ohmic contact behavior illustrated by current-voltage (I-V) plots. The GaN contact paths are made from Ti/Al/Ni/Au (30/100//10 nm). 10 NATURE MATERIALS

11 SUPPLEMENTARY INFORMATION Pre Illumination Post Illumination (1mWcm -2 ) Post Illumination (60mWcm -2 ) Post Illumination (100mWcm -2 ) Photoluminescence (a.u.) Wavelength (nm) Figure S 11 Near infrared photoluminescence (NIR-PL) spectra of graded band gap perovskite solar cells, with both h-bn and GA modifications. Under constant illumination an additional PL peak forms near 1300nm and grows with increasing light intensity. NATURE MATERIALS 11

12 Current Density (ma/cm 2 ) 1 10 /GA (10nm thick ) (Freshly illuminated) /GA(10nm thick ) (Under illumination 10min) /GA (300nm thick ) (Freshly iluminated) /GA (300nm thick ) (Under illumination 10min) (Freshly iluminated cell) (Under illumination 10min) GaN GA/HTM Back Contact GaN HTM Back Contact Current Density (ma/cm 2 ) Voltage (V) Type I- -(Ref.) Type I- -(This work) Type II-MASnIBr 2 -(Ref.13) Type II-MASnIBr 2 -(This Work) Name Jsc (ma/cm 2 ) V oc (V) FF (%) PCE (%) Type I-(Ref.) Type II- (Ref. 13) Type I-(This work) Type II-(This work) Voltage (V) Figure S 12 J-V measurement of based solar cells with and without GA. Devices prepared with GA show better stability in air. (All devices prepared in air). J- V measurement of based devices which are fabricated by following the same procedures as shown in refs. [] and [13], Type I and Type II respectively. Type I and Type II cells have the similar architecture (FTO/d-TiO 2 /mp-tio 2 / /spiro- OMETAD/Au), but different ETL, HTL and Au thicknesses. The table shows the detailed comparison of our cells prepared and cells reported in the literature. 12 NATURE MATERIALS

13 SUPPLEMENTARY INFORMATION 100 "!! *! W/h-BN,W/GA Silicon Ref. Cell 1 Silicon Ref. Cell EQE (%) 80 )! (! 60 '! &! 40 %! $! Current Density (ma/cm 2 ) #! 10 "!! $!! &!! (!! *!! ""!! "$!! "&!! "(!! "*!! Wavelength (nm) The integrated current Density(Jsc) = ma/cm 2 Max possible Jsc with 100% absorption= ma/cm IQE(%) GaN (3.3eV) x=0 x=0.3 x=0.02 x=0.29 x=0.79 x=1 (1.22 ev) (1.4 ev) (1.6 ev) (1.77 ev) (1.98 ev) (2.2 ev) -x MA HTM (Spiro- OMeTAD) Reflective absorption (%) h-bn Wavelength (nm) Figure S 13 External quantum efficiency of the champion cell with integrated photocurrent (thick black line). Maximum possible J sc, if the QE is 100% over the spectrum, is 49.4mA/cm 2 and the expected J sc is 42.32mA/cm 2. The EQE spectrum for reference silicon cells is also shown under A.M 1.. The plot of reflective absorbtion and internal quantum efficiency (IQE) versus wavelength. The inset shows the composition profile and approximate band diagram of the cell. NATURE MATERIALS 13

14 Pits Figure S 14 Back surface pits on the GaN surface after etching. The cells display excellent light trapping properties due to these textured surface properties. 14 NATURE MATERIALS

15 SUPPLEMENTARY INFORMATION GaN C -2 (x10 1 F -2 ) 1 10 SnI Voltage (V) Figure S 1 Mott-Schottky analysis for the GaN/ SnI 3 interface. The dotted line is the linear fit to experimental data. The doping density of the perovskite film is found to be 1.4x10 17 cm -3. The inset shows the cross sectional SEM image of the GaN/ SnI 3 device (scale bar is 0nm). The depletion width within the perovskite is calculated to be ~11nm. NATURE MATERIALS 1