Conductivities and carrier mobilities of layered 2D perovskites depend on the crystallographic direction. Generally, the conductivity and mobility are found to be significantly improved when measured along the planes of the inorganic layers. TEM and X-ray crystallographic techniques help to understand the device crystalline feature needed for improved device performance. Thus, a comparative structural study of 3D and 2D HPs has been given in this section.

The specific interactions between an electron beam with high energy and a thin electron-transparent sample are captured as the contrast of TEM images [already discussed in Figure 2 of earlier section]. High-resolution (resolution ∼ atomic scale) imaging can be achieved in the STEM mode, with few detectors to generate the images (e.g., BF: bright field, ADF: annular dark field, and HAADF: high-angle annular dark field). The crystallographic information is extracted from the electron diffraction patterns (EDPs) of the area of interest in the specimen understudy. Due to the modification of Bragg conditions in thin films, EDPs suffer from 2D imprecise crystallographic data. Energy-dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS) yield the site-specific chemical composition information. EELS allows us to gather evidence about a range of sample-related aspects, such as thickness, chemical surrounding, bonding, and bandgap. (Virdi et al., 2016; Yu et al., 2017). On the other hand, novel EDS detectors allow efficient collection of specimen compositional data even at the atomistic scale. In a recent review, the beam sensitivity of HPs that guides the TEM characterization has been thoroughly discussed (Chen and Gao, 2020). For beam-sensitive HP materials, leading beam-damage mechanisms involve phonon-mediated heating, displacement of atoms and defect formation, that is, knock-on damage, and damage by inelastic scattering and rupture of chemical bonds, that is, radiolysis (Egerton et al., 2004). Several approaches such as optimizing the accelerating voltage and low-dose imaging with improved cameras can be employed to overcome degradation in beam-sensitive materials. The photon-induced dynamics and polarization of MA⁺ ions can affect the physical properties and device performance of MAPbI₃ HPs. Thus, the photon-induced dynamics of MA⁺ is a major challenge for direct imaging of organic cations in TEM. Using an iterative cross-correlation–built algorithm on the filtered images, Zhang D. et al. (2018) successfully observed the ordered nanoscale domains with off-centered MA⁺ cations with differing orientations in MAPbBr₃ crystals. The MA⁺ cations are found to exhibit configurations along the normal and parallel relative to the projection direction, forming in-plane and out-of-plane electric dipoles, respectively. Accordingly, the structural models and the corresponding simulated projected potential describe that the local deviations in the MA⁺ orientation are responsible for ferroelectricity and/or polarization in hybrid HPs. The crystal network of hybrid HPs is much ‘‘softer’’ than that of conventional oxide perovskites. As a result, there is a high probability of local vibrations in structures, modification in structures, and symmetry of the crystals. Thus, to visualize the pristine structure of a CsPbBr₃ perovskite prepared via a catalyst-free solution-phase method (Yu et al., 2016), Yu et al. employed high-resolution TEM (HRTEM) in addition to small-dose in-line holography. The specimen under observation exhibited an atomically layered 2D nanosheet morphology. Employing focal series, they showed the existence of dual phases of CsPbBr₃ cubic and orthorhombic HPs within the nanosheets. TEM can also identify the misalignment between the successive grains and even describe the boundary plane. Twin boundaries in MAPbI₃ HP were identified by Rothmann et al. (2017) using TEM. These twin boundaries in specimens can originate during growth, phase transition, or even under mechanical stress. As HPs exhibit extreme chemical sensitivity, etching using a chemical solution is not a viable method for observing twin boundaries. Thus, electron back-scattered diffraction (EBSD) and TEM are essential tools for revealing twinning in HPs. Based on low-dose TEM imaging and selected area EDPs (SAEDPs), Rothmann et al. showed the presence of twinning in tetragonal MAPbI₃, where these boundaries appeared in the form of parallel striped bands of few hundred nanometer width within MAPbI₃ grains, although the device performance upon incorporation of the material was not clearly described by them. However, another recent study has pointed out that twin boundaries can be very effective in restricting photocarriers (Li M. et al., 2018; Li W. et al., 2018). The other report (Bertolotti et al., 2017) prompted flexibility of HP nanocrystals (NCs) upon halide changes and temperature variation, which has been investigated following high-resolution synchrotron X-ray total scattering data. Variations in the local structure of these NCs were found to exhibit orthorhombic tilting of PbX₆ octahedra within locally ordered subdomains. These subdomains are linked by a 2D/3D network of twin boundaries through which the coherent arrangement of the Pb⁺ over the whole NC is preserved. Thus, we believe that controlling the properties of twinning (size, orientations, etc.) could be a viable approach for achieving and modifying the characteristics of newly designed HPs.

The grain-boundary chemistry in PSCs intensely influences carrier recombination and potential transport of ionic and molecular species, thus affecting the optoelectronic properties and chemical stability of HPs. The grain-boundary chemistry of HPs depends on the HP bulk elemental composition, growth conditions, the structure and the kinetics of the boundary, and careful chemical modifications. In a contemporary study (Zong et al., 2018), triblock copolymer (Pluronic 123P) functionalized MAPbI₃ HPs were found to result in water-resistant HPs. The copolymer establishes the interaction with HP grains via its hydrophilic tails, while its hydrophobic core is placed toward the center of the copolymer wetting film. As a consequence, a water-resistant HP with higher stability was obtained.

Scanning probe microscopy, including AFM, Kelvin probe force microscopy (KPFM), and conductive AFM (C-AFM), has been broadly applied to explore the local physical, electromagnetic, or molecular topographies at a microscopic scale. AFM and KPFM are two different modes of microscopic technique that operate under constant or variable bias conditions. Specifically, KPFM captures the contact potential difference (CPD) between the conductive tip and the sample (Chen Z. et al., 2019). Under an alternative voltage, the CPD is measured by using a vibrating capacitor with the Kelvin method (Benstetter et al., 2017). Instead of the current, the sensitivity of KPFM is improved by recording the electrostatic force interaction between the tip and the sample. By accurate adjustment of the tip work function on the reference sample, the other materials’ surface work function can be determined (Shao et al., 2016). In high-resolution C-AFM (Yang et al., 2009; Shin et al., 2013), the current–voltage characteristics of the sample surface could be easily captured by the mapping of the topography. A large current flow points toward high conductivity of the sample and a strategic character of the charge transport (Bristowe et al., 2015; Wang et al., 2016; Si et al., 2017; Lee et al., 2018; Xu et al., 2018; Eichhorn et al., 2018). Thus, by C-AFM, the conductivity difference among dissimilar perovskite films can be captured and their transport characteristics (at grain boundaries and in grain interiors) can be distinguished with the nanoscale resolution. Even for phase-pure perovskites where compositional engineering is used to overcome difficulties in obtaining high-quality samples, ambient instability degrades the device performance. Lee et al., reported the fabrication of phase-pure formamidinium-lead tri-iodide (FAPbI₃) perovskite films with excellent optoelectronic quality and stability. They showed that incorporation of 1.67 mol% of 2D PEA lead iodide into the precursor solution allows the formation of phase-pure FA perovskites with an enhanced PL lifetime (Lee et al., 2018). Figure 4 describes the band alignment and local conductivity with 2D perovskites, as described by Lee et al.(2018). Figure 4A describes the device schematic comprising the polycrystalline film of a 3D perovskite with grain boundaries containing 2D perovskites, while Figure 4B describes the layer by layer Tauc plots and UV photoelectron spectroscopic analysis of the device band structure. The bare C-AFM images of FAPbI₃ and with films of 2D perovskite on ITO glass coated with SnO₂ are shown in Figure 4 (C, E) and (D, F), respectively. A 100- mV bias voltage was applied to get the images under the room light condition for image (C, D) and low light intensity condition for image (E, F), as provided by the setup. The topology of each film is depicted in the inset of the corresponding images. For (C, D) and (E, F), the corresponding scale bars are shown at the left and right sides, respectively. It describes that under ambient light conditions, the flow of current in the perovskite film with 2D perovskite was higher at/near the grain boundaries while a fairly uniform flow of current was noted in the bare FAPbI₃ film. With light illumination, the flow of current was further enhanced at/near the grain boundaries with the 2D perovskite, whereas the current flow the in bare FAPbI₃ film was uniformly increased, which indicates that separation of charge and collection of photo-generated electrons are more favorable at the grain boundaries with the 2D perovskite. On the other hand, the 2D (PEA)₂PbI₄ perovskite with aromatic rings and longer alkyl chains is projected to be more resilient to moisture. It guards the defective grain boundaries of the 3D perovskite, resulting in the significantly improved moisture stability of the film. Regardless of the improved stability, degraded electronic properties of the film are expected due to the poor charge carrier mobility of the 2D perovskite. Interestingly, a report by Wang et al. highlighted a prominent conducting feature, observed at the layer edges between the insulating bulk regions of the (BA)₂PbI₄ 2D perovskite single crystal. In the bulk region, the electron–hole pairs are tightly bonded. At the layer edges, additional edge state (ES) electrons are located, which control the current detected by the C-AFM measurement (Wang et al., 2019). These observations of the metal-like conducting feature at the layer edge of the 2D perovskite offer a different dimension for improving the performance of the next-generation optoelectronics and innovative nanoelectronics. Piezo-response force microscopy (PFM) is an AFM-based method depending on the inverse piezoelectric effect, analyzing the local electromechanical properties of piezoelectric samples. Remarkably, PFM has been widely employed to study the possible ferroelectricity in the MAPbI₃ perovskite (Fan et al., 2015). The theoretical calculations suggested the presence of a ferroelectric domain in the perovskite samples that would result in the separation of photoexcited electron–hole pairs making an internal junction. It allows the reduction of recombination through the segregation of charge carriers. PFM also ascertains the ferroelectric features in other 2D layered perovskite materials (Sha et al., 2019; Chen and Gao, 2020) which are useful as sensors in flexible devices, soft robotics, and biomedical devices, a great influence toward next-generation devices. However, the ferroelectricity of MAPbI₃ is still under debate (Wang et al., 2017).

Here, we would like to mention that other than conventional microscopic techniques, microwave photoconductivity imaging (MIM) on MAPbI₃ has been proven to be an efficient technique for quantitative estimation of photoconductivity. A recent report highlighted the nanoscale photoconductivity imaging of two MAPbI₃ thin films with different efficiencies by light-stimulated microwave impedance microscopy (Chu et al., 2017). Here, unlike the C-AFM technique, direct contact between the MIM tip and the perovskite thin films is not required. The effective capacitive coupling between the MIM tip and the perovskite thin films at GHz frequencies overcomes the difficulties of nanoscale electrical properties on samples with a capping layer which is insulating in nature.

The two most general schemes practiced to enhance the PCE of 2D and q-2D HP-based SCs are (i) optimizing the layer numbers (n) to get adequate absorption of light and charge generation and (ii) refining the crystallographic alignment for better charge transport to the electrode. Although the layer number can be easily modified by tuning the composition of 2D and q-2D HPs in precursors, the controlled orientational growth remains a big challenge. Noninvasive spectroscopic approaches are the easiest for probing these distinctive characteristics of the 2D perovskite specimens compared to conventional microscopic techniques, as here we do not need any sample preparation. Here, we will discuss a general scaling law that was proposed by Blancon et al. (2018) to determine the binding energy of Wannier–Mott excitons in perovskite QWs of arbitrary layer thickness. Figure 5 describes the absorption and PL, that is, the optical spectroscopic feature of the 2D and q-2D HP crystals, having n = one to five for inorganic RP perovskite QWs following the reference (Blancon et al., 2018). It specifically highlights (A) scaling of experimentally obtained optical bandgap and (B) PL spectra for the material. This understanding provides direct insight about the photophysics of RP perovskite materials utilized in practical devices.

Generally, in 3D HPs, excited charges diffuse quicker than the radiative recombination rate. Thus, most of them are confined at the grain-boundary defects. However, in 2D HPs, free charges and excitons coexist in a small regime, enhancing the radiative combination relative to the bulk specimen. Due to the surface and interface defects present in the sample, the nonradiative recombination still controls the energy transfer phenomena and the PL in 2D HPs remains comparatively low. It is evident from these results that layered HPs might not be a good choice for LEDs when one considers high brightness and effective quantum efficiency (EQE). However, owing to their excellent stability and processing ability, they are considered to be the finest candidates for practical LEDs (Chiba et al., 2018; Yang et al., 2018; Xu et al., 2019).

Metal HPs also show the necessary optical–electrical characteristics for PV devices, such as long carrier diffusion lengths, high carrier mobility, and strong and broad optical absorption, all of which support the notable PCE of the device. In general, 2D flakes of MAPbI₃ are a widely accepted active material for photodetectors for its large COA, high mobility of the carrier, and smaller exciton binding energy (<26 meV, room temperature). 2D RP perovskites are also known to exhibit excellent photodetection performance. In a recent report, Zhou et al. (2016) explained that the RP perovskite film displayed the layer number–dependent response spectrum, which is compatible with their absorption spectrum. As the perovskite films are configured into photodetectors, a better photodetection property was detected in the film with the higher n value. Here, the polycrystalline nature of 2D RP perovskites might be the cause behind the observed lower photoresponse. In recent times, the photodetection ability of 2D RP perovskites was observed to be considerably upgraded by the replacement of the long direct chain n-BA⁺ with branched i-BA⁺ using a method of hot-casting processing (Dong et al., 2019).

The functionality of the devices is mostly dependent on the grain structure of polycrystalline 2D perovskites, but in situ, chemically specific characterization of 2D perovskite grains is currently limited. Here, the polarized ultralow-frequency Raman microspectroscopy is found to be a simplistic yet influential tool for identifying relative grain orientations in 2D perovskite thin films (Toda et al., 2020). Even from temperature-dependent PL spectra of 2D perovskites and following the generalized Elliot formula, the evidence of phonon-assisted sidebands is explained by Feldstein et al. (2020). They determined the spectral linewidth of the energetically lowest excitonic transitions in view of the scattering channels, focusing on the emission and absorption of phonons (low-energy acoustic and higher energy optical) (Feldstein et al., 2020). The low-energy, heat-carrying acoustic phonons play the central role in the management of heat in 2D RP–supported devices, which remained unexplored for a long time. A recent study by Guo et al. (2018) discusses the generation and transmission of coherent longitudinal acoustic phonons along the 2D RP cross-plane direction, resulting in distinct characterizations of lower bandgap refractive indices. They have demonstrated a significant drop in group velocity and propagation length of acoustic phonons in 2D RPs compared to the 3D MAPbI₃ counterpart (Guo et al., 2018). The observed vibrational feature is a consequence of the large acoustic impedance disparity between the successive layers of perovskite and large organic cations.

Pump-probe spectroscopy offers a feasible way to identify the carrier dynamics in the 2D perovskite materials (Giovanni et al., 2018). Transient absorption microscopy (TAM) (Williams et al., 2019) is one of the techniques employed to probe the diffusion of the carrier and two-body recombination processes. Figure 6, as reproduced following Williams et al. (2019), describes the schematic of the concentration distribution of QWs that yields a gradient in excitation frequencies and enables a unidirectional flow of energy as a cascade, obtained by TAM. Figure 6A describes the concentration gradient of QWs throughout a film of layered perovskite. According to the idea, the glass edge of the perovskite film is highly populated with the smallest QWs and the air edge is mostly dense with the thickest QWs. Figure 6B schematically highlights the diagram describing the down-hill energy transfer cascade. It is observed that the excitonic resonances of the QWs decrease with an increase in thickness and the direction of electron transfer is the same as that of energy transfer as compared to the holes which are moving toward the states of higher energy. For applications such as microcavity lasers, boosting of the two-body recombination processes is desirable. Here, a methodical comparison of the layered films of (PEA)₂ (MA) _n−1 [Pb _n I_3n+1] with phase-pure single crystals discloses that diffusion is blocked by grain boundaries in the films, stimulating two-body recombination. The energy transfer rules the sub-200 ps timescale, and the energy levels of the quantum wells are organized such that hole transfer may take place from the air to glass sides of the film at future times. The high-performance nanolasers made up of perovskites have attained low threshold, high-quality factor, and tunable wavelengths under optical excitation (Guo et al., 2018; Zhang H. et al., 2018; Liu et al., 2020). Their compact volume could entrap the light field in a tiny region and improve the light–matter interactions. Another interesting report established that QWs of colloidal lead halide perovskites can produce fully decoupled multi-QW (MQW) superlattices (having intralayer local exciton) with ultrathin organic quantum barriers (Jagielski et al., 2020), same as the addition of monolayer hybrid BN (h-BN). The obtained result demonstrates that photonic sources have emission of narrowband, high quantum yield, improved light outcoupling, and wavelength tenability useful for nano-antennas (near field) and LEDs (far field). Even theoretically, it was proposed (Li et al., 2020) that the use of superlattice structures is a smart approach (such as mixing of cations and partial substitution of halogens by superhalogens) for expanding the family of perovskites and obtaining excellent optoelectronic materials with improved stability.

The VB of 2D perovskites predominantly consists of halide p-orbitals hybridized with metal s-orbitals, and a CB is dominated by metal p-orbitals. In Pb HPs, the corresponding orbitals are 5p of I and 6s and 6p of Pb (Even et al., 2012; Even et al., 2013; Gebhardt et al., 2017). In 2D HPs, the existence of Pb and I prompts a large spin-orbit coupling (SOC) which in the presence of structural inversion asymmetry boosts the spin-degeneracy of the CB and VB (Kepenekian et al., 2015; Kepenekian and Even, 2017). It seems that 2D perovskites self-assemble into regular QW structures that break the symmetry of the 3D system. It amplifies the Rashba effect and escalates their potential for opto-spintronic applications, as observed by transient spectroscopic measurements (Zhai et al., 2017). The rate of charge carrier recombination and spin-coherence lifetimes of 2D RP perovskite single crystals PEA₂PbI_4/(MAPbI₃) _n−1 (with n = 1, 2, 3, 4) have been reported by Chen et al. (2018). Charge carrier recombination rates are observed to be fastest for n = 1 due to the large exciton binding energy and the slowest for n = 2. Spin-coherence times at ambient temperature also demonstrate a non-monotonic layer thickness dependency with a growing spin-coherence lifetime, which increases for n = 1 to n = 4, followed by a reduction in lifetime for n = 4 to ∞. For n = 4, the longest coherence lifetime of ∼7 ps was detected. Their observations are dependent on two contributions: (a) Rashba-splitting increasing the spin-coherence lifetime for the n = ∞ to the smaller layered systems and (b) phonon-scattering increases for smaller layers, reducing the spin-coherence lifetime. The interplay between these two contributions modifies the layer thickness dependency. Here, they also measured the carrier dynamics by the exciton bleach kinetics and the spin-coherence dynamics by employing circularly polarized pump and probe pulses.

From the above discussion, we can extract that for device application, a stable structure is the most feasible. Figure 7 describes that the dimensional reduction and modified/engineered surface needed for stable 2D HP device functionality. Remarkably, a report by Grancini et al. (2017) introduced a stable perovskite device by engineering a (HOOC(CH₂)₄NH₃)₂PbI₄/CH₃NH₃PbI₃ 2D/3D perovskite junction. The unique gradually organized 2D/3D multidimensional interface produces up to 12.9% efficiency in carbon-based architecture and ∼14.6% in standard mesoporous SCs for 1 year. Their innovative, low-cost, and stable architecture has supported the timely commercialization of PSCs. Thus, we propose that a detailed microstructural analysis of 2D HPs is the much-needed approach to engineer new materials for energy research application, especially SCs. In this review, we have tried to sum up the characteristics of 2D HPs reported/proposed to have stable architecture feasible for energy research application.