A microstructural analysis of 2D halide perovskites: Stability and functionality

Recent observations indicated that the photoelectric conversion properties of perovskite materials are intimately related to the presence of superlattice structures and other unusual nanoscale features in them. The low dimensional or mixed dimensional halide perovskite family are found to be more efficient materials for device application compared to 3-dimensional halide perovskites. The emergence of perovskite solar cell has revolutionized the solar cell industry because of their flexible architecture and rapidly increased efficiency. Tuning the dielectric constant, charge separation are the main objective in designing a photovoltaic device that can be explored using 2-dimensional perovskite family. Thus, revisiting the fundamental properties of perovskite crystals could reveal further possibilities for recognizing these improvements towards device functionality. In this context, this review discusses the material properties of 2-dimensional halide perovskite and related optoelectronic devices aiming particularly for solar cell application.


INTRODUCTION
As witnessed in the past decade, the halide perovskite family has emerged as a high-performance photovoltaic material. This reappearance has involved extensive development of the three-dimensional (3D) perovskites, specifically CH3NH3PbI3.The crystallographic formula ABX3 of halide perovskite (HP) materials comprised of A, organic cation such as methylammonium (MA): CH3NH2 + ) or alkali cation or mixing of both, B the metal cation (Pb +2 or Sn +2 ), and X the halide anions (X = I − , Br − , or Microstructural analysis of 2D halide perovskites 2 Cl − ). HPs are mainly classified into two categories depending on the occupancy of A-site whether an organic-molecular cation or an elemental cation. The catergories are organic-inorganic hybrid HPs and all-inorganic HPs, respectively. In hybrid HPs, the organic cations occupying A-site of the perovskite structure are usually non-symmetric and inclined to rotation at room temperature and above . This extraordinary phenomena has been hardly observed in other materials. As a result, hybrid HPs have the interconnected [BX6] 4 octahedra continued in an ordered manner that form the crystal structure with local disorder located within the [BX6] 4 framework. At room temperature, the most-reviewed hybrid HP, CH3NH3PbI3 or (MAPbI3), the reorientation of polar MA + ion are found in [PbI6] 4 octahedral (between the faces, corners, or edges) having a residence time of ~14 ps (Leguy et al., 2015). At lower temperatures, the rotational dynamics in HPs are slowed down  following their dielectric response. HPs can have different tunable chemical compositions, such as A2BX4, ABX4, A3BX9, well known as Ruddlesden-Popper (RP) phases (Yu et al., 2017). The defects in the crystal, crystallographic alignments, surfaces, grain boundaries, and interfaces in HPs can be customized preferentially. These preferential nano-and micro-structures can be addressed using indepth conventional microscopic characterization methods such as atomic force microscopy (AFM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As an example, consider the case of a perovskite solar cell (PSC). A PSC includes a hybrid organic-inorganic or allinorganic perovskite structured compound as the light-harvesting active layer. The compositional variation of the material and its dependency on phase transition temperature are related to electrical and ionic properties of the active layer (Correa-Baena et al., 2017, Lee et al., 2012.
Thus we can infer that measuring a minute variation of structural property and optimizing the same will be a quality control measure for optoelectronic devices.
The large exciton binding energy due to the reduced dielectric screening and quantum confinement are the most fascinating optoelectronic properties of two-dimensional (2D) materials. 2D HPs with its tuneable photophysical properties and superior chemical stability compared to their 3D counterpart (Ding et al 2018, Zheng et al 2018, Dong et al 2019 have created a great influence on the semiconductor research and technology fields. 2D HPs obey stereochemical rules that determine the growth of the perovskite sheets, (i)corner-sharing, (ii)edge-sharing, and (iii)face-sharing-octahedral connections has been experimentally observed. Here the characteristics for size restriction for A site is determined by the suitability of a cation spacer whether a) its net +ve charge at the perovskite attaching point and substitution amount (b) capacity of hydrogen bonding (c) flexible stereochemical arrangement, (d) ability of space filling (Mao et al 2019). Most cations are found to have corner sharing Microstructural analysis of 2D halide perovskites 3 2D structure but sometime the cation-imposed configuration strain in 2D network that stabilizes edgesharing or face sharing networks (Kamminga et al 2016). In 2D perovskites, extra larger organic cations (L) are inserted as spacers, separating the inorganic metal halide octahedra layers to form quantum well superlattices that is different from conventional 3D HP. Here introducing hydrophobic spacer cations, the ionic lattice of inorganic octahedrons are effectively isolated from the ambient water molecules. The newly introduced additional spacing cations and asymmetric lattice structures provide tunable intrinsic physical features, such as dielectric constant, optical band gap, and exciton binding energy, (Blancon et al., 2017). As a result, the photophysical behaviors like exciton dynamics (Proppe et al., 2019), charge carrier transport , and electron−phonon coupling (Straus et al., 2018) that strongly influence the performance of optoelectronic devices (light emitting diode (LED), solar cell(SC)) can also be modulated accordingly (Zheng et al., 2018). In a recent report, the mechanical properties of (C6H5CH2NH3)2PbCl4 are found to be anisotropic, and the organic components and van der Waals interactions between layers play a significant role in the structural stability of the 2D structure . They found that the substituting the organic parts with rigid and multifunctional organic components with Pb will improved stability and carrier mobility of the PSC absorber layer.
The interaction of electromagnetic fields with photo-and electro-active materials is the heart of energy conversion research. Graphene, hexagonal boron nitride, transition metal dichalcogenides, MXenes etc. other classical 2D materials are being widely studied due to their fascinating properties such as high electrical conductivity, low density, large surface area, tunable electric and magnetic properties (Novoselov et al 2004, Mas-Ballesté et al 2011, Jiang et al 2020. Pristine graphene is a zero-bandgap semimetal. while RGO and GO are a semiconductor and an insulator, respectively. 2D TMDs display semiconducting nature (Rao et al., 2009, Ferrari et al., 2015. MXenes sometimes behaves as metals, semiconductors, superconductors, topological insulators, and, most importantly, half-metallic characteristics (Naguib et al., 2011, Naguib et al., 2012, Anasori et al., 2015. graphene and related two-dimensional (2D) materials propose extraordinary advancement towards in device performance even at the atomic scale. A compromised combination of these 2D materials with silicon chips promotes a massively enhanced potential compatible for on silicon technology. Low cost, simple and inexpensive layered two-dimensional (2D) halide perovskites are a novel class of materials with outstanding layer-dependent tunable optical properties, generation of room temperature stable excitons, and modified exciton binding energy originating from the quantum and dielectric confinement effects. They also offer the improved environmental stability and photostability needed for the light absorbing layer photovoltaic devices. The enormous potential of this thin film SC Microstructural analysis of 2D halide perovskites 4 technology is expected to become a low-cost commercial alternative in the near future to the presently available silicon photovoltaic. This manuscript evolves as follows: introduction to material properties, highlighting 2D perovskite structural aspects. Next, the characterization that identifies exotic material property of 2D HP has been discussed. Finally, the microstructural characteristics of 2D HPs that leads to promising potential applications as high-performance optoelectronic devices especially SC has been pointed out.

STRUCTURAL ASPECTS OF 2D HP: ENGINEERED ENERGY LANDSCAPE
Microscopically, layered HPs contain of mono or few thin atomic layers of HP .
Atomically (morphologically) thin HPs or crystallographically layered HPs are considered as 2D/quasi-2D (q-2D) HPs. Dimensional confinement upon crystallization of HPs using the long-chain organic ligands-templated growth, , Sheng et al., 2018 or selftemplated growth, (Ha et al 2014, Niu et al. 2016) preserving ABX3 formula results morphological 2D/q-2D HPs. Using bulky organic separators and by slicing 3D HPs along ⟨100⟩, ⟨110⟩, and ⟨111⟩ crystal plane three different layered crystallographic 2D/q-2D HPs are obtained. As a consequence, the chemical formulae of 2D HPs modified methodically according to the number of layers and crystal plane orientation. The chemical formula of A2An-1BnX3n+1 specifically belongs to ⟨100⟩, A2AmBmX3m+2 ⟨110⟩, and A2Aq-1 BqX3q+3 ⟨111⟩ oriented HP family respectively. The ⟨100⟩ oriented layered HP family, owing to their high acceptance to many distinct organic and inorganic components, is the most extensively investigated among them. The HP family oriented along ⟨100⟩ direction are allocated further into two sub-categories, RP and Dion-Jacobson (DJ) (Ortiz-Cervantes et al. 2019) type. However, the DJ HPs are not often studied and applied in practical devices, thus less popular in device field . The name of RP HPs derived from their resemblance in crystal with inorganic Sr3TiO7 RP perovskites (Ruddlesden et al. 1958). To emphasis on the comparison in crystal structure of HPs we will focus our discussion to a schematic. Figure 1 (a) describes schematic of the 3D structure and the [001] projections of conventional inorganic CsPbBr3 cubic and orthorhombic perovskite phases and RP phases as labelled in ref (Yu et al., 2017). Here, the material Csn+1PbnBr3n+1 or CsBr(CsPbBr3)n, is found to be composed of "n" layers of CsPbBr3 unit cells separated by an extra CsBr layer. The RP phases are considered as a repeatedly integral extrinsic stacking error, designed by shifting two neighbouring CsPbBr3 units by an in-plane lattice vector (1/2 1/2) compared to each other. Figure 1 For clarity, the general formula for 2D hybrid RP perovskite can be rewritten as (RNH3)2MX4 or (NH3RNH3)MX4, where M is metal cation including Pb +2 , Cu +2 , and Sn +2 , R: prototype organic molecule e.g. phenethylammonium (PEA) or butylammonium (BA) (Mitzi et al 2001). In hybrid RP perovskites, a extraordinary ability to regulate the thickness of the inorganic layers are found, which is a highly needed characteristics for photovoltaic (PV) applications, specifically SCs. As a consequence, RP perovskites are found to be the most used and studied. For the construction of interconnected few layers of corner-sharing M-X6 octahedra in q-2D HPs, the short-chain inorganic or organic cations are required. The value of "n" in the general formulae considered here is the number of layers in 2D and q-2D HPs. For n = 1, we get the 2D HPs. q-2D HPs are obtained for n ≥ 2. The intrinsically insulating large organic layers induce multiple quantum well structures  in between the assemblies of these organic layers with M-X6 octahedron layers. As a result, the 2D and q-2D RP HPs are found to exhibit electronic and quantum confinement even in thin films (Ding et al., 2019, Spanopoulos et al., 2019. A comparison between 2D and 3D network of perovskite and the modified band structure has been described in ref (Straus et al 2018).  (Ishihara et al., 1989, Koh et al., 2017. In 2D perovskites due to a dielectric confinement effect in the layers, the electrons are Microstructural analysis of 2D halide perovskites 6 strongly attracted to the holes. They attribute higher exciton binding energies for 2D compared to their 3D counterpart. For SCs application, the exciton binding energy should be as small as possible. The lead HPs with n ≥ 3, have bandgap, Eb comparable to 3D perovskites the dielectric constant increases largely with n, are able to perform decently as SC absorbers (Pazoki et al., 2018). The bandgap is one of the most crucial parameters in SC applications. It determines the maximum theoretical efficiency for the corresponding single p-n junction device according to the Shockley-Queisser limit (Rühle et al., 2016).
A bandgap of 1.34 eV results in a power conversion efficiency (PCE) of about 33.5%, (Rühle et al., 2016, Slavney et al., 2017 while materials with bandgaps >1.9 eV are found to be the smart choice for tandem SCs. The bandgap for 2D HP is typically determined by the composition and thickness of the inorganic layers. The bulkier halides lessen the bandgap. For n = 1, 2D perovskites with formula (PEA)2PbX4, have bandgaps of about 3.8, 3.0 and 2.5eV for Cl -, Brand Irespectively (Kitazawa et al., 1997). The heavier, less electronegative halides have more delocalized electronic density and it better overlap with that of the metal atoms (Slavney et al., 2017). Therefore, halide substitutions can be a method for systematically engineered the bandgap of 2D hybrid HP by adjusting their composition of the chemical constituents. The procedure allows broad absorption across visible and near-infrared region of wavelengths. The other way of the bandgap and photoluminescence (PL) wavelength modification is by changing the n value. With increase in n , the bandgap becomes smaller, and tends towards the bandgap value of 3D perovskite as n approaches to , and the PL peak shifts asymptotically towards lower energies with increase in n (Stoumpos et al., 2017). Again, the coefficient of absorption (COA) is directly proportional to the strength of electronic dipole transitions. In 2D-perovskites, light absorption is lesser due to a low density of metal/halogen atoms causing a lower absorption crosssection. COA increases rapidly as the n value increases. 2D perovskites with n ≥ 3, are comparable to 3D perovskites, have high enough COA can harvest most of the incoming radiation. Following the trend, a general scaling law was proposed to determine the binding energy of Wannier-Mott excitons in perovskite quantum wells of arbitrary layer thickness . In another approach (Mitzi et al., 1995), modified orientation of the conducting perovskite layers through choice of organic cation has been achieved. Interestingly, they found conducting layered organic-inorganic halide containing <110> oriented perovskite sheet. Here it must be mentioned that surface modification also plays a critical role in device engineering. It was also observed that the layer edge electrons which are not related to the surface charging effect; but associated with the local energy states corresponding to of the edge electronic structure . Therefore, in the next section the focus of Microstructural analysis of 2D halide perovskites 7 discussion is on the characterisation of engineered 2D HP energy landscape and extrapolation of these specific features towards device application.

Crystallographic Characterisation
Conductivities and carrier mobilities of layered 2D perovskite depends 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 technique helps to understand the device crystalline feature needed for improved device performance. A comparative structural study of 3D and 2D HPs has been followed 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 1 of earlier section].
High resolution (resolution ~ atomic scale) imaging, can be achieved in STEM mode, with few detectors to generate the images (example BF: bright field, ADF: annular dark field, and HAADF: high-angle annular dark field). The crystallographic information is extracted from 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) gives the site-specific chemical composition information. EELS allows to gather evidences about a range of sample-related aspects, such as thickness, chemical surrounding, bonding, bandgap etc. (Yu et al., 2017, Virdi et al., 2016.
On the other hand, novel EDS detectors allows efficient collection of specimen compositional data even at the atomistic scale. In a recent review the beam sensitivity of HP that guide the TEM characterization has been thoroughly discussed . 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 describes that the local deviations in the MA + orientation is responsible for ferroelectricity and/or polarization in hybrid HPs.
The crystal network of hybrid HPs is much ''softer'' compared with conventional oxide perovskites. performance upon incorporation of the material was not clearly described by them. However, another recent study has pointed that twin boundaries can be very effective in restricting photocarriers . The other report (Bertolotti et al., 2017), prompts flexibility of HPs nanocrystals (NC) upon halide changes and temperature variation which has been investigated following high-resolution synchrotron X-ray total scattering data. Variations in local structure of these NCs was found to exhibits 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 twining (size, orientations, etc.) could be a viable approach for the achieving and modifying the characteristics newly designed HPs.
The grain-boundary chemistry in PSCs, intensely influence carrier recombination and potential transport of ionic and molecular species, thus affecting the optoelectronic properties and chemical Microstructural analysis of 2D halide perovskites 9 stability of HPs. The grain-boundary chemistry of HPs dependents on the HP bulk elemental composition, growth conditions, the structure and the kinetics of the boundary, and careful chemical modifications. In a contemporary literature (Zong et al., 2018), triblock copolymer (Pluronic P123) functionalized MAPbI3 HP are found to result in water-resistant HPs. The copolymer establish the interaction with HP grains via its hydrophilic tails, while its hydrophobic core is placed towards the center of the copolymer wetting film. As a consequence, a water resistant HP with higher stability was obtained.

Morphological and conductive characterization: Scanning Probe Microscopy
Scanning probe microscopy, including AFM, Kelvin probe force microscopy ( (Sha et al., 2019 useful as sensors in flexible devices, soft robotics and biomedical devices, a great influence towards next generation devices. However, ferroelectricity of MAPbI3 is still under debate .

11
Here would like to mention that, other than conventional microscopic techniques microwave photoconductivity imaging (MIM) on MAPbI3 has been proved to be an efficient technique for quantitative estimation of photoconductivity. A recent report highlights 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, a 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.

Distinctive Spectroscopic Characteristics: An en route to device functionality
The two most general schemes practised to enhance the PCE of 2D and q-2D HPs-based SCs are: i) optimising the layer numbers (n) to get adequate absorption of light and charge generation, ii) refining 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, while the controlled orientational growth remains a big challenge. Non-invasive spectroscopic approaches are the easiest to probe these distinctive characteristics of the 2D perovskite specimens compared to conventional microscopic technique as here we don't need any sample preparation. Here we will discuss, a general scaling law was proposed by Blancon et al., 2018 to determine the binding energy of Wannier-Mott excitons in perovskite quantum wells of arbitrary layer thickness. Figure 4 describes the absorption and PL i.e., optical spectroscopic feature of the 2D and q-2D HP crystals having n =1-5 for inorganic RP perovskite quantum wells following the reference . It specifically highlights (A) scaling of experimentally obtained optical bandgap and (B) photoluminescence spectra for the material. This understanding provides direct insight about the photophysics of RP perovskite materials utilised 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 co-exists in a small regime, enhancing the radiative combination relative to bulk specimen.
Due to the surface and interface defects present in the sample, the non-radiative 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 the good choice for LEDs when one considers high brightness and effective quantum efficiency (EQE). However, owing to their excellent stability and Microstructural analysis of 2D halide perovskites 12 process ability, they are considered to be finest candidates for practical LEDs Chiba et al., 2018;Xu et al., 2019).
Metal HPs also show necessary optical-electrical characteristics for PV devices, such as long carrier diffusion lengths, high carrier mobility, strong and broad optical absorption, all of which support the notable PCE of the device. In general, 2D flakes of MAPbI3 is 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 the excellent photodetection performance. In a recent report by Zhou et al.  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, 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, 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., 2018).
The functionality in the devices is mostly dependant on the grain structure of polycrystalline 2D perovskites, but in-situ, chemically specific characterization of 2D perovskite grains are currently limited. Here the polarized ultra-low-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 generalized 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 technique employed to probe diffusion of carrier and two-body recombination Microstructural analysis of 2D halide perovskites 13 processes. Figure 5, as reproduced following Williams et al. 2019, describes the schematic of concentration distribution of quantum well that yield a gradient in excitation frequencies and enables a unidirectional flow of energy as a cascade, obtained by TAM. Figure 5 (A) describes the concentration gradient of quantum wells (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 thickest QWs. Figure 5 (B) Schematically highlights the diagram describing down-hill energy transfer cascade. It is observed that excitonic resonances of the quantum wells decrease with increase in thickness and the direction of electron transfer is same as that of energy transfer as compared to the holes which are moving towards 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[PbnI3n+1] with phase-pure single crystals disclose that diffusion is blocked by grain boundaries in the films, stimulating two-body recombination. The energy transfer rules the sub-200 ps time scale and the energy levels of the quantum wells are organised 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, 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 establish that quantum wells of colloidal lead halide perovskites can produce fully decoupled multi quantum well (MQW) superlattices (having intralayer local exciton) with ultrathin organic quantum barriers (Jagielski et al., 2020), same as the addition of monolayer hybrid boron nitride (h-BN). The obtained result demonstrates that photonic source have emission of narrowband, high quantum yield, improved light outcoupling, and wavelength tenability useful for for nano-antenna (near field) and LED (far field). Even theoretically it was proposed  that the use of superlattice structures is an 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 improve stability.
VB of 2D perovskites is predominantly consist of halide p-orbitals hybridized with metal sorbitals and a CB is of metal p-orbitals dominated. In lead HPs, the corresponding orbitals are 5p of I and 6s and 6p of Pb (Gebhardt et al., 2017, Even et al., 2013, Even et al., 2012. 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., 2017, Kepenekian et al., 2015. It seems that 2D perovskites self-assemble into regular "quantum-well" structures that break the symmetry of the 3D system. It amplify the Rashba-effect and escalates their Microstructural analysis of 2D halide perovskites 14 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, PEA2PbI4 /(MAPbI3)n−1 (with n = 1, 2, 3, 4) has been reported in Ref (Chen et al., 2017).
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 nonmonotonic layer thickness dependency with a growing spin-coherence lifetime, 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, 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.
There from the above discussion we can extract that for device application a stable structure is the most feasible. Figure 6 describes the dimensional reduction, modified/engineered surface for a stable 2D HP device functionality. Remarkably, a report by G. Grancini et al. (Grancini et al., 2017) introduced a stable perovskite device by engineering (HOOC(CH2)4NH3)2PbI4/CH3NH3PbI3 2D/3D perovskite junction. The unique gradually-organized 2D/3D multi-dimensional interface produces up to 12.9% efficiency in a 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 are the much needed approach to engineer new materials for energy research application specially SC. 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.

CONCLUSION
This review shows that for commercialisation of HP optoelectronic devices, a thorough in-depth characterisation of 2D HP materials are essential. Most unique feature essential for light absorbing layers of solar cell related to this type of material is their optimised dielectric constants and exciton binding energy, compared to conventional semiconductors, and it also include a rotational component connected with relaxation of molecular dipole. Although great efforts have been given to study various 2D HP materials, many challenges still there to utilize these materials in actual practical purposes. But, the fabricated devices using 2D HPs still facing degradation problem. More experimental and 25 Zong, Y., Zhou, Y., Zhang, Y., Li, Z., Zhang, L., Ju, M. G., Chen, M., Pang, S., Zeng, X.