**3.5 Layer orientation**

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

#### **Figure 9.**

*The generalized concept of the Q.W. morphology in both PEA-based and BA-based spin-cast films. Reprinted with permission from Ref. [87].*

the obtained XRD spectra corresponding to in-plane and out-of-plane orientations. In contrast, (111) and (222) reflections were noticed for n = 3 and n = 4 quasi 2D-perovskite films (110) and (220) analogously for n = ∞, which implies the vertical growth orientation (n = 3, 4), increasing towards 3D-perovskite; The preferential layer alignment vanished because few BA cations doped in 3D-perovskite and no influence on orientations **(Figure 9)** [65]. From (SEM, GIWAXS) it was concluded that the hot-cast films grew along certain orientations, confirmed by the most remarkable reflections of (111) and (202) planes. The inorganic crystal plates <(MA)n − 1PbnI3n + 1 > 2− are perpendicular to the substrate, forming continuous charge transfer channels favorable to charge transport for optoelectronic applications [66].
