Supplementary Information Polarization and Dielectric Study of Methylammonium Lead Iodide Thin Film to Reveal its Nonferroelectric Nature under Solar Cell Operating Conditions Md Nadim Ferdous Hoque, 1 Mengjin Yang, 2 Zhen Li, 2 Nazifah Islam, 1 Xuan Pan, 3 Kai Zhu, 2* and Zhaoyang Fan 1* 1 Department of Electrical and Computer Engineering and Nano Tech Center, Texas Tech University, Lubbock, Texas, 79409, USA 2 Chemistry and Nanoscience Center, National Renewable Energy Laboratory, Golden, Colorado 80401, USA 3 Beijing Institute of Hydrogeology and Engineering Geology, Haidian, Beijing, 100195, China * Contact email: kai.zhu@nrel.gov; zhaoyang.fan@ttu.edu 1
Experimental Section To measure the polarization and impedance of perovskite material, a sandwich structure of 300-nm-thick MAPbI 3 was fabricated between fluorine-doped tin oxide (FTO) and Au as two contacts. After cleaning the FTO glass properly, perovskite layer was deposited by the antisolvent method in a spin-coating process. 28,29 Methylammonium iodide and lead iodide were dissolved in dimethylformamide (DMF) to get a 1.4 M solution. A three-step spin-coating speed was used for 500, 3500, and 5000 rpm for 3, 10, and 30 s, respectively. 600 µl toluene was dropped before 4 s of the third step, resulting in a brownish-colored film that was further annealed at 100 ºC for 10 min, resulting in dark-brown perovskite film. Finally, 50 nm of Au were deposited. For J-V hysteresis characterization, complete solar cells were also fabricated by incorporating a mesoscopic structure as an electron transport layer. For this structure, a 50-nmthick TiO 2 compact layer was spin-coated on FTO glass. Then, a 200-nm TiO 2 porous layer was coated and then annealed at 500 o C for 30 min. A 350-nm perovskite was coated on the porous layer. Subsequently, 2,2,7,7 -tetrakis(n,n-dip-methoxyphenylamine)-9,9 -spirobifluorene (Spiro-MeOTAD; Merck, Germany) was used as a hole transport layer (HTL) by spin-coating a HTL solution, which consists of 80-mg Spiro-MeOTAD, 30-μL bis(trifluoromethane) sulfonimide lithium salt stock solution (500-mg Li-TFSI in 1-mL acetonitrile), and 30-μL 4-tertbutylpyridine (TBP), and 1-mL chlorobenzene solvent, at 4000 rpm for 30 s. Finally, 50-nm Au was deposited by thermal evaporation. The J-V characteristics of the cells, which have a 0.12-cm 2 masked area, were measured using a 2400 SourceMeter (Keithley) under simulated one-sun AM 1.5G illumination (100 mw cm -2 ) (Oriel Sol3A Class AAA Solar Simulator, Newport Corporation). To characterize the J-V hysteresis of PSCs, the cell was measured in forward-scan mode, i.e., from short circuit to forward bias, and reverse-scan mode, i.e., from forward bias to short circuit. Before starting the reverse scan, the cell was biased at 1.1 V for 1 min. To characterize the polarization charge and the possible ferroelectric property of the MAPbI 3 material itself, a Ferroelectric Tester was used and polarization, remnant polarization, and Positive(P)-Up(U)-Negative(N)-Down(D), or PUND, measurements were conducted in a nitrogen glove box. To measure the type of carriers and origin of polarizations, impedance spectroscopy (IS) measurements were done using an electrochemical workstation at various conditions in a nitrogen glove box. The reflection spectra were measured in the ambient environment using a spectrometer equipped with an integrating 2
sphere. To heat up the sample for various measurements, a thermoelectric stage was used connected with a thermoelectric controller with a precision of ±1 K. Figure S1. The J-V curves of a typical cell exhibit similar J sc but less V oc for forward scan (FS) when comparing to the reverse scan (RS), resulting in the commonly observed hysteresis phenomenon, with power conversion efficiency of 15.78% for reverse scans and 10.67% for forward scans. 3
Figure S2. The basic Sawyer Tower test configuration. As the sensing capacitor is charged, its back voltage will change the applied voltage (field) on the device under test (DUT), and therefore, it introduces errors. This is particularly a problem for testing very leaky materials such as hybrid perovskites. Very large sensing capacitors on the order of a faraday must be used to minimize its back voltage ; however, the capacitance value of such a large capacitor cannot be accurately determined, introducing further measurement errors. 4
Figure S3. (a) For a given triangular driving voltage pulse, the polarization charge in timedomain is drawn schematically for an ideal ferroelectric, linear dielectric, and linear conductor, respectively. The measured charge is assumed to be zero at the beginning of the driving pulse. For the ferroelectric that was previously poled to -P F, when the voltage (section 1) is below a critical value, the polarization maintains P F and therefore no charges are measured until the voltage arrives at the critical value. At this instant, the polarization changes from P F to +P F ; correspondingly, a total charge of 2P F is flowed through and is measured. This amount of charge 5
does not change with the voltage because no more current flows until a negative voltage (section 3) arrives at the critical field to change polarization from +P F to P F ; then the total measured charge becomes zero again. For the linear dielectric, because the capacitance is a constant, the measured charge is proportional to the driving voltage. For the conductor, because its current i is proportional to V or t, the time integral of I, or the measured charge Q, is proportional to t 2 and therefore it has a parabolic profile. (b) Their corresponding plots of Q-E (or P-E) loops when converting from the Q-t plots in (a). (c) is their corresponding C-E plots, where C is the differential capacitance (C = dq ). Please note that in (b, c), the loops of ferroelectric and dv conductor were not shifted in the vertical direction to a center. (d) The more realistic P-E loops for a typical ferroelectric, when shifted in the vertical direction to the center. (e) Differential capacitance C vs. E plot for a typical ferroelectric. Figure S4. The polarization loops measured at different triangular waveform pulse frequencies, with similar loop profile. 6
Figure S5. Schematics of atomic crystal structures of MAPbI 3 : (a) tetragonal phase and (b) cubic phase. 7
Figure S6. (a) Schematically shows the conventional positive-up-negative-down (PUND) or double-wave method for remnant polarization measurement with sine-shape driving pulses. Here, the initial negative poling pulse is not drawn. In this method, at the beginning of each pulse, the measured charge is reset to zero by the instrument. After the initial negative poling, the P pulse will measure a total charge P T = 2P F + P D + Q C. 2P F comes from the polarization reversion. Then this measured total charge is reset to zero before starting the U pulse. Subsequently, the measured charge in the U pulse will be P D + Q C because there is no ferroelectric polarization reversion. Here, the measured conduction charge in pulse U is assumed to be the same as that in pulse P. Therefore, subtracting the measured charge in the U pulse from that in the P pulse gives 2P F. (b) Our room-temperature measured results that show a trivial ferroelectric polarization from MAPbI 3 film. 5 s of interval between the pulses were introduced, so that the device can reach to steady state after each pulse. 8
Figure S7. (a) Polarization measured at various temperatures under same electric field pulse using PUND method. (b) Polarization measured under various electric field pulse at room temperature using PUND method. 9
Figure S8. Impedance spectra of MAPbI 3 thin film under dark condition at different temperatures. Figure S9. Temperature dependence of dielectric loss and conductivity at several specific frequencies, suggesting a phase transition between 40 o C 50 o C. 10
Figure S10. The fittings to the measured impedance spectra using the associated model, from which electronic resistance R e, ionic resistance R ion, geometric and chemical capacitance C bulk, and C ion for contact blocking of mobile ions are extracted. 11
Figure S11. Temperature-dependent resistance and capacitance extracted from Fig. S10. Two different regions, corresponding to two crystal phases, can be identified. 12
Figure S12. (a) Tauc plot from absorbance spectra at different temperatures. (b) Measured bandgap from the Tauc plot at different temperatures. 13
Figure S13. (a) Temperature dependent J-V curves of typical cells with both forward and reverse scans. (b) Power conversion efficiency (PCE) extracted from the J-V curves measured at different operation temperatures. 14