In Situ Analysis Reveals the Role of 2D Perovskite in Preventing Thermal-Induced Degradation in 2D/3D Perovskite Interfaces

Engineering 2D/3D perovskite interfaces is a common route to realizing efficient and stable perovskite solar cells. Whereas 2D perovskite’s main function in trap passivation has been identified and is confirmed here, little is known about its 2D/3D interface properties under thermal stress, despite being one of the main factors that induces device instability. In this work, we monitor the response of two typical 2D/3D interfaces under a thermal cycle by in situ X-ray scattering. We reveal that upon heating, the 2D crystalline structure undergoes a dynamical transformation into a mixed 2D/3D phase, keeping the 3D bulk underneath intact. The observed 3D bulk degradation into lead iodide is blocked, revealing the paramount role of 2D perovskite in engineering stable device interfaces.


Synthesis of 2-thiophene methyl ammonium idodide (2-TMAI)
2-TMAI was synthesized according to the reported procedure. 1 An aqueous solution of HI (1.1 eq) was added dropwise to a stirred 1.0 mol/L ethanol solution of thiophenemethylamine (1.0 eq) at 0°C. The mixture was allowed to gradually reach room temperature and then it was poured into an excess of diethyl ether (Et2O). The formed precipitate was collected and washed thoroughly with Et2O. The salts were recrystallized from EtOH-Et2O mixtures, providing with crystalline solids.

Preparation of 2D/3D perovskite films and device fabrication
FTO-coated glass (Nippon sheet glass) was chemically etched using zinc powder and HCl solution, followed by cleaning using Hellmanex, water, acetone, and 2-propanol. A 30 nm thick compact TiO2 layer as electron transporting layer was deposited by spray pyrolysis using a titanium diisopropoxide bis(acetylacetonate) solution (Sigma-Aldrich) diluted in 2-propanol (1:15 by volume fraction) at 450 °C. On top of the compact layer, a 100 nm thick mesoporous layer of TiO2 was deposited by spin coating TiO2 paste (GreatCellSolar, 30NR-D) diluted in ethanol (1:6.3 by mass fraction) at 5000 rpm for 20 s followed by heating at 125 °C for 10 min and sintering at 500 °C for 20 min. A thin layer of passivating tin oxide of ≈20 nm was spin-coated by using tin (IV) chloride (Acros) solution (12 µL diluted in 988 µL water) at 3000 rpm for 30 s, followed by annealing at 100 °C for 10 min and 190 °C for 1 h. The prepared substrates were treated with UVozone for 15 min before perovskite deposition.

Device characterization
The current density-voltage (J-V) curves were measured under 1 sun illumination (AM 1.5G) by xenon lamp solar simulator (450 W, Oriel Sol3A, AAA class). The light intensity was calibrated to 1 sun by using a Si reference equipped with an IR-cutoff (KG5) filter. An external voltage bias was applied and the current responses were measured at the same time using a digital source meter (Keithley 2400). An active area of 0.0804 cm 2 was determined by a metal mask. The J-V curves were scanned with the rate of 50 mV s -1 without any preconditioning, such as light soaking or prebiasing for a long time. External quantum efficiency (EQE) spectra were recorded using IQE200B (Oriel).

In situ GIWAXS
In situ GIWAXS measurements were performed at the XRD2 beamline at the Brazilian Synchrotron Light Laboratory (LNLS). The energy of the X-ray was 7 keV and the scattering signal was collected using a Pilatus 300K detector with integration time of 3s and 30s between each measurement. The incidence angle of the X-ray beam relative to the film surface was set at 3°. The modified films were measured under nitrogen atmosphere. Each GIWAXS image was azimuthally integrated to obtain 1D X-ray diffraction patterns and plotted as 2D intensity maps with respect to time (abscissa) and the 2theta (ordinate). The intensity was normalized with the storage ring current.

Photoluminescence and UV-Vis Absorption
Photoluminescence spectra of the perovskite films were measured and recorded using Fluorolog3-22 spectrofluorometer. The emission was measured upon excitation at 450 nm. The absorption spectra of the perovskite thin films were measured in an ultraviolet, visible, nearinfrared spectrophotometer (PerkinElmer Lambda 950S).

Time-resolved Photoluminesncence
Time-resolved photoluminescence (TrPL) decays were acquired on a time-correlated singlephoton counting (TCSPC) FL900 spectrometer from Edinburgh Analytical Instruments with a Hamamatsu MCP-PMT R3809U-50, and a PicoQuant LHD-DC-440 pulsed laser diode at lexc = 440 nm (pulsewidth < 80 ps; F = 9.7 nJ cm -2 ). The instrument response was recorded using Ludox samples. At least 1,000 counts in the peak channel were accumulated for the lifetime determination. The emission decays were analyzed using exponential functions. The films used for TrPL measurement were prepared under the same condition as GIWAXS experiment.

Calculation of XRD Peak Position
The interplanar distance in a 2D perovskite can be calculated by the formula: 2 d = E + n*T Where, E is the length of the spacer and T the thickness of one perovskite layer and n the number of layers. For n=1 and n=2 we have, respectively: 2 d (n=1) = E + T Equation (1) d (n=2) = E + 2T Equation (2) As E has not been reported for TMA + yet, we used the n=1 and n=2 synthetized perovskites to estimate d by Bragg's law and calculated E and T using the equations (1) and (2). We found T 6 equals 0.62 nm, which is very consistent with value reported in the literature and with the size of the inorganic octahedron. 3,4 We found that the length of the TMA spacer in the 2D perovskites is 0.82 nm consistent with the length of the benzylammonium (0.91 nm) which have similar structure.
The tables 1 and 2 shows the experimental, theorical and deviation between then for n=1 and n=2 perovskites, respectively. Using these two parameters (T and E), equation 1 and Bragg' law, we estimated the XRD peak positions for n = 3 to n = 10 for different diffraction orders. Table S3 shows the calculated values.