# Tightly Compacted Perovskite Laminates on Flexible Substrates via Hot-Pressing
## 3 Results and Discussion  
The lamination of perovskites is a powerful tool in device architectures and material bonding, and it cannot be achieved by conventional processing methods. In order to demonstrate the degree of mechanical bonding force of CsPbBr\({}_{3}\) perovskites after lamination, mechanical characterization was conducted. The laminated perovskites films were prepared by face-to-face hot-pressing two independently fabricated half-stacks of perovskite film on a flexible polyethylene terephthalate (PET) foil, as displayed in Figure 1a. To check the bonding degree between the two perovskite films, forces were exerted to separate the laminated films in either a horizontal or vertical direction, as shown in Figure 1b,c. The force was divided by the laminated area to calculate the pressure as the degree of bonding and the horizontal and vertical pulling pressures at different temperatures and pressures were summarized. It was found that the perovskite films can only be successfully laminated within the temperature range from 70 \({}^{\circ}\)C to 150 \({}^{\circ}\)C and pressure range from 5 MPa to 10 MPa. For samples laminated at 70 \({}^{\circ}\)C, the horizontal pulling pressures were 83 kPa, 125 kPa, and 241 kPa at imposed pressures of 5 MPa, 8 MPa, and 10 MPa, respectively. When the temperature increased to 90 \({}^{\circ}\)C, the horizontal pulling pressures increased to 114 kPa, 444 kPa, and 636 kPa, respectively. When the temperature reached 150 \({}^{\circ}\)C, the horizontal pulling pressures were 236 kPa, 219 kPa, and 295 kPa at imposed pressures of 5 MPa, 8 MPa, and 10 MPa, respectively. For samples laminated at 90 \({}^{\circ}\)C, the vertical pulling pressures were 13 kPa and 71 kPa at imposed pressures of 8 MPa and 10 MPa, respectively. When the temperature increased to 150 \({}^{\circ}\)C, the vertical pulling pressures were 14 kPa and 36 kPa, respectively. At a fixed temperature, the pulling pressure to separate two films increases as the imposed pressure increases, which means that the degree of bonding increases. The horizontal and vertical pulling pressure reach the optimal point simultaneously at 90 \({}^{\circ}\)C and 10 MPa. It is apparent that the perovskite films can only be laminated when both heat and pressure are applied. Pressure and heat have a synergistic effect on perovskite lamination and cause recrystallization of the perovskite. Room temperature or pressure without heat cannot lead to a successful lamination. However, when the imposed pressure was further increased to 12 MPa, the lamination failed regardless of the heating temperature, probably because the relatively high pressure damaged the recrystallization of the perovskite films. To further prove the successful lamination of perovskites, we characterized the separated perovskite films after lamination (90 \({}^{\circ}\)C, 10 MPa) using a metallurgical microscope and SEM, as well as pristine perovskite films without lamination. As shown in Figure 1d, the result of metallurgical microscopy reveals distinct differences. The metallurgical microscope images are true color luminescence images excited by a mercury short arc lamp (OSRAM HBO 103 W/2). The emission area of films without lamination is complete and uniform, whereas that of separated laminated films becomes incomplete, meaning that the perovskite materials were transferred to the other stack after lamination. This phenomenon was confirmed in the SEM results shown in Figure 1e. The surface of the separated perovskite film became incomplete after lamination, proving that the perovskite on one stack was successfully bonded to the perovskite on the other stack. The laminating method is well reproducible, and laminates fabricated over half a year prior were still tightly compacted, demonstrating that the laminating method is effective long term as well.