High Photovoltage of 1 V on a Steady-State Certified Hole Transport Layer-Free Perovskite Solar Cell by a Molten-Salt Approach

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1 Supporting Information High Photovoltage of 1 V on a Steady-State Certified Hole Transport Layer-Free Perovskite Solar Cell by a Molten-Salt Approach Lukas Wagner 1, Sijo Chacko 1, Gayathri Mathiazhagan 1, Simone Mastroianni 1,2, and Andreas Hinsch 1,* 1 Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstraße 2, D Freiburg, Germany 2 Freiburg Materials Research Center FMF, Albert-Ludwigs-University Freiburg, Stefan- Meier-Straße 25, D Freiburg, Germany Fabrication of perovskite-melt precursor Figure S1a shows the processing steps for the molten-salt based precursor solution as described in the manuscript. PbI 2 /MAI (1:1) - DMF Complex was purchased from TCI (99.99%, trace metals basis) and acetonitrile from Carl Roth ( 99.9 %). Methylamine gas was obtained by drying methylamine 40 % solution in water (Merck) on calcium oxide powder. As can be seen from Figure S1b, displaying the XRD diffractograms of the heat-treated DMFcomplex powder as well as theoretical calculations of the DMF-complex and pure MAPbI 3, by heating the perovskite DMF complex at 100 C under nitrogen atmosphere, the powder can be converted to pure MAPbI 3 powder without residues of the DMF-complex. For the XRD measurements, powdered samples were enclosed in glass capillaries (0.3 mm diameter) and investigated at room temperature in Debye Scherrer geometry on a STOE Stadi P powder diffractometer (equipped with a Mythen-1K detector) with Ge(111) monochromatized Mo-Ka radiation. Structural data for the calculation of the theoretical diffractograms have been obtained from references [1] (MAPbI 3 DMF) and [2] (MAPbI 3 ).

2 Figure S1: a) Photographs of the processing steps for the molten-salt based precursor solution. b) Powder XRD diffractograms of the thereby obtained perovskite powder and theoretical values of DMF-complex and pure MAPbI 3 for comparison. Solar cell fabrication FTO TEC 7 glass plates with dimension of 100x100 mm 2 were patterned using laser ablation. The glass plates were treated in an ultrasonic bath containing 3% Deconex OP153 at 60 C followed by rinsing with DI water. An ultrasonic treatment at 60 C for 1 min in DI water was further operated to ensure removal of any detergent residual. Glass plates were then blown dry using nitrogen. A compact TiO 2 layer of nm thickness was deposited by spray pyrolysis using a solution comprising 0.05 M of titanium disisopropoxy (bis) acetyl acetone (Sigma Aldrich) diluted with ethanol (>= 99.5% from Carl Roth). Mesoporous layers of TiO 2 (paste prepared via mixing Dyesol DSL-18NRT with terpineol in 1:0.75 weight ratio) and ZrO 2 (Solaronix Zr-Nanoxide ZT/SP) of thickness 900 nm and 1600 nm respectively were screen printed and sintered at 500 C for 30 min. Screen printing a layer of approximately 9 µm of graphite as counter electrode (Solaronix Elcocarb B/SP), followed by sintering at 400 C for 30 min, completes the architecture of the cell. The cells were then filled with perovskite precursor solution as described below under Batch type device fabrication.

3 Solar cell characterization SEM/EDX maps were obtained with a Zeiss Auriga 60 electron microscope and a Bruker EDX system. Figure S2a shows a SEM image of the entire FIB cut of approximately 45 μm width, demonstrating the homogeneous perovskite filling of the porous contact layers. Additionally, in Figure S2b, a (non-polished) cross-section obtained from mechanical cleaving of the cell is shown. a) Graphite ZrO 2 TiO 2 FTO b) Figure S2: SEM image of a) a broad, FIB-polished cross section and b) a non-polished part of the cross section. I-V characteristics were measured under a class A LED solar simulator (Wavelabs) with a potentiostat (Ivium CompactStat). Therefore, the cell was thoroughly patterned with a photomask to 8.76 mm² to avoid any other incoming light except for the measured area. Transient simultaneous PL and electrical measurements were obtained with a blue LED (405 nm) that was fully illuminating the masked area under an optical microscope. The PL signal was captured with an scmos camera. Figure S3 shows the response of the V OC and PL upon toggling of the light source. The V OC decay in Figure S3 indicates a fast and a long-lived time regime. As can be seen from the simultaneously measured PL decay, the fast partial V OC decay corresponds to fast decay of short-lived primary charge carriers. The long-lived portion of the V OC signal exists for time regimes in the order of 1-10 seconds, whereas the PL has already completely vanished. This points towards long lived carriers in traps which do not contribute to the photocurrent. The long-lived photovoltage also demonstrates that the device is practically shunt-free.

4 Additionally, Figure S4 shows an I SC measurement similar to the one from Figure 4 in the manuscript but illuminated for a longer time span. Figure S3: V OC and PL response to toggling of the light source. Figure S4: I SC response to toggling of the light source as reported in Figure 4 but illuminated over a longer time-scale.

5 Batch type device fabrication and reproducibility The authors are convinced that to demonstrate the proof-of-principle of a novel solar cell fabrication method, it is sufficient to show the certified stabilized performance of a single device. Nonetheless, to give more information on reproducibility, we show the cell performances of a range of 23 devices with a larger illuminated active area. For the I-V characteristics, measured with a potentiostat (Ivium CompactStat), the cells were illuminated continuously with a class A xenon arc lamp solar simulator. The cells were not patterned with a shadow mask and for calculation of the active area, the overlap of the electrodes was assumed, resulting in an active area of 0.4 cm². For comparison with the data presented by other groups, first a reverse (1) and forward (2) I-V sweep was carried out, followed by a steady-state measurement of the V OC (3) and power conversion efficiency at a fixed voltage (4). The I-V sweeps were performed with a sweep-rate of 50 mv/s, whereas reverse refers to a sweep direction from 1.1 V to -0.1V and forward to the respective opposite direction. The presented steady-state performance refers to the values the devices reached after they were left for 60 s under short circuit (V OC ) or a fixed volt corresponding to the maximum power point as derived from the previous reverse sweep. We note that the cells were not cooled during these measurements and that for air-cooled measurements, slightly higher performances have been observed. Between each measurement, the cell was left unilluminated in open circuit until the very start of each measurement. The contact scaffold of the devices was produced as described above. For the infiltration of the perovskite precursor, the cells were placed in a petri-dish with the minimal possible size which was closed with parafilm. With a micro-syringe, the parafilm was then punctured and the cell was filled with the molten-salt solution. After infiltration, the puncture hole was immediately closed with a second layer of parafilm. The solution then quickly spreads throughout the porous layers and remains in the yellow, liquid phase while the dish remains closed. To allow sufficient time for infiltration in the pores, the cell was left inside the closed container for 15 minutes. Afterwards, the parafilm was removed, leading to a rapid drying and darkening of the perovskite precursor. The cell was left drying for 15 minutes before it was annealed on a hotplate. All processing steps were carried out in air, i.e. outside a glovebox. The cells were stored in ambient conditions for two days before the measurements. Figure S5 shows the performance of these cells for which a range of processing conditions were varied as listed in Table S1, namely the volume of the molten salt solution (between 2 and 8 µl), the annealing time (between 10 and 40 minutes at 100 C) and the annealing temperature (between 50 and 100 C for 30 minutes). No significant influence of these processing parameters on the device performance could be observed, indicating that the presented method is very robust to variations in the manufacturing conditions. With the processing conditions of 4 µl of solvent and an annealing step of 30 minutes at 100 C, steady-state efficiencies of 9.4 % and photovoltages of 950 mv could be obtained. This is still below the reported certified record value, however it should be kept in mind that this result was obtained on a larger active area.

6 Table S1: Processing parameters of the cells shown in Figure S5. Experiment group Solution volume [µl] Annealing time [min] Annealing temperature [ C] A A A A B B B C C Figure S5: I-V characteristics of a range of devices produced under different processing conditions as reported in Table S1. REFERENCES (1) Hao, F.; Stoumpos, C. C.; Liu, Z.; Chang, R. P. H.; Kanatzidis, M. G. Controllable Perovskite Crystallization at a Gas Solid Interface for Hole Conductor-Free Solar Cells with Steady Power Conversion Efficiency over 10%. J. Am. Chem. Soc., 2014, 136, (2) Stoumpos, C. C.; Malliakas, C. D.; Kanatzidis, M. G. Semiconducting Tin and Lead Iodide Perovskites with Organic Cations: Phase Transitions, High Mobilities, and Near- Infrared Photoluminescent Properties. Inorg.Chem. 2013, 52,