**3.4 Device fabrication of 2D-perovskite films**

The finite preparation methods of 2D-perovskite films are different from the multiple preparation methods of 3D-perovskite films. *One-step spin-coating methods* are the most used to prepare 2D-perovskite films [75]. In this method, organics and metal halides are dissolved in solvents, e.g., DMF or (DMSO/GBL(1/1)) on substrates. By adjusting the ratio of the precursors, the dimension of perovskite is changed. 2D-perovskite films in both n-i-p and p-i-n structures are fabricated using one-step spin-coating methods as (PEA)2(MA)n − 1PbnI3n + 1, (BA)2(MA)n − 1PbnI3n + 1 and (PEI)2(MA)n − 1PbnI3n + 1 [65, 76, 77]. The *fast deposition-crystallization procedure* was also introduced to fabricate 2D-perovskite by dropping antisolvent, e.g., chlorobenzene, during the spin-coating process. It is shown, homogeneous nuclei are formed immediately and grow up slowly. Finally, dense and uniform films are obtained without oversize grains that may destroy the morphology [78]. The preferred *hot-cast method* was introduced for (BA)2(MA)3Pb4I13 films on PEDOT:PSS substrate [66]. The FTO/PEDOT:PSS substrate was heated before the precursor solution was spin-coated on it. **Figure 8** shows the photograph of (BA)2(MA)3Pb4I13 films that were prepared on substrates with different hot-cast temperatures from room temperature (R.T.) to 150°C. The films became dark and shiny with lower

#### **Figure 8.**

*(a) The (BA)2(MA)3Pb4I13 films cast from room temperature (R.T.) to 150°C, (b) GIXRD spectra, (c, d) AFM images, and (e, f) SEM images of films prepared by traditional room-temperature-cast method. Reprinted with permission from Ref. [66].*

pinhole density with rising temperature, resulting in high-efficiency devices. The inorganic layers in 2D-perovskite films prepared by the hot-cast method have a preferential orientation vertical to the substrate with excellent crystallinity and few carrier traps, which favors the charge transport.

*Another solution vapor annealing method* was used to prepare high-quality (PEA)2PbBr4 nanosheets at room temperature as an emitting layer for LEDs [79]. Using a precursor solution of DMF comprising PEABr and PbBr2 (2/1) was spincoated on the top of the ITO/PEDOT:PSS substrate and the productive sample was placed face down on the edge of a glass dish without contacting it. In the following step, the dish was transferred into a lidded beaker filled with DMF to form a closed space with DMF vapor at 30°C for several minutes before the DMF vapor diffused under the (PEA)2PbI4 film and contacted with it. The film was removed out into open air rapidly and heated at 100°C for 10 min while turning into purple. By using DMF vapor annealing, the small and compact (PEA)2PbI4 perovskite grains was recrystallized into sized nanosheets equally distributed on the substrate, which has larger grain size, higher crystallinity, and higher P.L. intensity (higher photoluminescence quantum yield (PLQY) due to the quantum confinement), comparing with unprocessed films. The nanosheet-LEDs showed a longer P.L. lifetime (much longer than the 3D-perovskite MAPbI3). They exhibited a small leak of current and low turn-on voltage, the external quantum efficiency (EQE) was 20 times higher than poly-LEDs.

*Sequential deposition method* The sequential deposition is used to prepare a sequential dipping process and sequential spin-coating of 2D-perovskite films [80]. The dipping process was used to fabricate quasi-2D and quasi 3D-perovskite films with spin-coating (IC2H4NH3)2PbI4 layers on mp-TiO2 substrates [81]. It is observed, immersing the films into MAI solution with a specific concentration for different times (1–5 min), transferring the films into cleaning fluid (2 mL isopropanol mixed with 10 mL methylbenzene) to remove the MAI residual. With increasing dipping time, MA+ cations in solution passed through (IC2H4NH3)2PbI4 films and increased inorganic PbI4 layers' thickness. Then, the nanoparticles regularly grew up and interconnected with each other in film morphology (uniform films without

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*Mixed 2D-3D Halide Perovskite Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.97684*

formed, with migrating PEA<sup>+</sup>

**3.5 Layer orientation**

residuals), leading to sharper diffraction peaks. In the previous studies, ITO/NiOx/ mixed-dimensional perovskite/PCBM/bis-C60/Ag devices were prepared using the sequential spin-coating method with mixed-cations perovskite FAxPEA1 − xPbI3 as the active layer [81]. With observing, a PbI2 layer was spin-coated on NiOx substrate, followed by the PEAxFA1 − xI solution loaded on it for 15 s before spin-coating. After thermal treatment, the mixed-cations FAxPEA1 − xPbI3 perovskite films were

3D-perovskite FAPbI3 to form quasi 3D- rather than quasi 2D-perovskite at room temperature. This self-assembly organic shell could prevent perovskite crystals from ambient moisture and passivated the surface defects to enhance the device performance and quasi 3D-perovskite's stability (the transition energy transformed from black phase to yellow phase). In summary, the 2D-perovskite films were commonly fabricated using one-step spin-coating method (simple process and low cost), with small-n members contrary to their 3D-counterparts but increasing "n" showed pre- or post-treatment is necessary such as hot-cast, antisolvent or solution vapor annealing processes for better crystallization. On the other hand, sequential deposition was used for obtaining efficient charge collection and extraction, dense and uniform films (due to the superior PbI2 framework for crystal growth). In sequential spin-coating, the large cations were not likely to enter into the lattice but pack on the surface of 3D-perovskite grains, forming a quasi 3D-structure. For future large-scale fabrication, the solution process was not enough for high-quality. Still, doctor blading, pressure-processing method, and so on were more appropriate as long as high-quality crystal is needed [82]. Melt processing is another innovative technique with an excellent quality. It was implemented in 2017 to fabricate lead iodide based 2D-perovskites using PEA derivatives [83]. Although it had not been used for device fabrication yet, however it is a promising one that it exhibits high phase purity, crystallinity, and potential crystal orientation control under ambient

air, but its disadvantage is the used toxic solvents in processing.

The device performance has been enhanced using the vertical alignment of the inorganic sheets of the 2D-perovskites concerning the substrate. The vertically oriented inorganic slabs provided a direct pathway for charge transport between layers, whereas the bulky organic separators act as electrical insulators hindering out-of-plane conduction of charges [63]. 2D single-layered (n = 1) halide perovskites have shown to align horizontally to the substrate on which they were grown. When "n" was greater than 1, competition arose between horizontal and vertical to the perovskite layers alignment (caused by BA and MA cations). When n = 1–4, it was found that the devices with n ≥ 3 had a better performance than lower "n", as shown in previous studies [84, 85]. It showed that for BA2MA3Pb4I13, the nucleation process and film growth orientation occurred at the liquid-air interface rather than at the liquid–liquid or liquid–substrate interfaces (since surface tension made nucleation and growth at the liquid-air interface more favorable) and that the initial nuclei are oriented in vertical configuration [86]. With increasing "n", the probability of obtaining an n-homogeneous-film decreased, giving rise to a mixture of a 3D-like dominant phase with some 2D-perovskite phases. Finally, films with n = 5–10 tend to align perpendicular to the substrate **(Figure 9**) [87]. This crystal orientation was examined using a scanning electron microscope (SEM), X-ray diffraction (XRD), and grazing incidence wide-angle X-ray scattering (GIWAXS) tests. XRD spectra of (BA)2(MA)n − 1PbnI3n + 1 (n = 1, 2, 3, 4, ∞), with (n = 1) showed (00 k) peaks implying the preferential growth along (110) direction, in (n = 2), (0 k0) reflections occurred in addition to (111) and (222) in

cations to lattice surfaces and grain boundaries of
