Comparison of the performances of 2D perovskites-based photodetectors.
Compared to their conventional three-dimensional (3D) counterparts, two-dimensional (2D) halide perovskites have attracted more interests recently in a variety of areas related to optoelectronics because of their unique structural characteristics and enhanced performances. In general, there are two distinct types of 2D halide perovskites. One represents those perovskites with an intrinsic layered crystal structure (i.e. MX6 layers, M = metal and X = Cl, Br, I), the other defines the perovskites with a 2D nanostructured morphology such as nanoplatelets and nanosheets. Recent studies have shown that 2D halide perovskites hold promising potential for the development of new-generation photodetectors, mainly arising from their highly efficient photoluminescence and absorbance, color tunability in the visible-light range and relatively high stability. In this chapter, we present the summary and highlights of latest researches on these two types of 2D halide perovskites for developing photodetectors, with an emphasis on synthesis methods, structural characterization, optoelectronic properties, and theoretical analysis and simulations. We also discuss the current challenging issues and future perspective. We hope this chapter would add new elements for understanding halide perovskite-based 2D materials and for developing their more efficient optoelectronic devices.
- halide perovskite
- two-dimensional layered structure
- and optoelectronic device
1. Introduction to photodetectors and halide perovskites
Photodetection techniques have demonstrated many important applications in a variety of fields or occasions, including medical X-ray imaging, ubiquitous visible light cameras, near-infrared medical imaging and short-wave infrared surveillance and machine vision [1, 2, 3, 4, 5]. Current photodetection relies primarily on conventional semiconductor materials with an appropriate bandgap, capable of transducing photons with different energies into electrical signals for subsequent processing, image reconstruction and storage. Historically, silicon and III–V semiconductors have been most commonly used as photodiodes and phototransistors (Figure 1) [6, 7, 8]. However, these conventional inorganic semiconductors are broadband absorbers. Thus, they produce a broad spectral photoresponse that could complicate the architecture and fabrication of devices and makes it difficult in pure color replication. Since the discovery of electrical conductivity in doped polypyrrole in the 1960s  and myriads of small-molecule and polymeric conductors and semiconductors during the 1970s and 1980s, organic semiconductors have shown promising applications in solar cells, field-effect transistors (FET) and photodetectors [10, 11, 12]. However, engineering organic semiconductors with an optical bandgap below 1 eV remains a challenge, as well as their carrier mobility is normally low. In addition, most organic planar photodetectors suffer from slow response time (≥10 ms), high operation voltage (≥10 V) and lack of stability to UV light (190–350 nm) . This has driven the development of colloidal quantum dot (CQD)-based photodetectors, such as CdS, CdSe and PbS [14, 15, 16], as a possible option to expand the range of optical path difference (OPD). In most CQD-based photodetectors, however, a compromise between carrier extraction and absorption efficiency is required in order to reduce the electrical volume of the photoactive material but without sacrificing absorption. More recently, metal-halide perovskites have emerged as new candidate materials for optoelectronic devices (solar cells in particular) with the conversion efficiency reaching over 20% . It is thus expected that using metal-halide perovskites as the key material component could construct new-generation photodetectors. In general, perovskites are optically tunable from 390 to 790 nm via halide-exchange or quantum confinement effect (Figure 1), so that they possess an appropriate bandgap exhibiting a steep absorption edge and demonstrate remarkable efficiency in photo-generated charge collection [18, 19, 20, 21, 22]. These unique optical properties make halide perovskites play key roles in the recent rapid development of solar cells, which has also spawned the exploration of diverse applications of halide perovskites across the domain of photodetectors. Metal halide perovskite-based photodetectors could have the notable advantages such as ease of processing, tailorable optical property, good compatibility with flexible substrates and synergistic integration with complementary metal oxide semiconductors [23, 24, 25, 26].
Compared to three-dimensional (3D) materials, two-dimensional (2D) materials, defined as crystals with very high aspect ratios and thicknesses corresponding to a few atomic layers, usually exhibit unique properties [27, 28, 29, 30]. Graphene is the first example discovered with a true 2D atomic crystal, which is known by its high chemical stability  and large specific surface area . In particular, graphene has a very high electronic conductivity  and absorbs the incoming radiation almost independent of wavelength , which can perform synergistic functionality in optoelectronic devices. Besides, other newly emerged 2D materials, such as hexagonal boron nitride (h-BN),  transition-metal dichalcogenides (TMDs) , layered metal oxides , and Venes (phosphorene and arsenene) [38, 39] have also attracted broad interests in various fields. Unlike above-mentioned 2D materials, however, 2D perovskites in the form of nanoplatelet (NPL) or nanosheet and layered-structure perovskites are ionic materials. This chemical bonding nature could endow them special properties distinguished from their 3D structures. For example, these 2D perovskites have an increased exciton binding energy, can reduce fluorescence decay time, and enhance absorption cross sections with respect to the bulk as well as having a notable optical nonlinearity [40, 41, 42]. Moreover, energy transfer in the stack of 2D perovskite sheets or platelets is significantly enhanced due to the large contact area between individual platelets. Note that the construction of 3D perovskites needs to match some critical parameters such as Goldschmidt’s Tolerance Factor, ionic radii, charge balance and bonding/coordination preference, in order to produce stable structures. In 2D perovskites, in contrast, a remarkable structural tunability is feasible especially for the interlayer “A” cation length. This character provides a high flexibility for the design and synthesis of desirable 2D perovskite crystal structures with new physicochemical properties.
This chapter aims at summarizing recent advances and discussing current challenges and ongoing efforts in the development of 2D perovskite-based photodetectors. We first briefly overview recent research progress in 2D perovskites before focusing on the optoelectronic properties of 2D layered perovskite structures or perovskite nanoplatelets and nanosheets and photodetectors. We summarize a number of popular methods for the preparation of 2D perovskites, their structural characterizations and fabrication and tests of photodetectors. Also included is a brief discussion on theoretical simulations of electronic structures of halide perovskites for their perspective applications in future optoelectronic devices.
2. Design and preparation of 2D halide perovskites
To obtain desirable optical properties, the design and controlled synthesis of 2D metal-halide perovskites is a critical step. In this section, we focus on summarizing the recent progress in the synthesis of 2D halide perovskites.
2.1. Solution-processed methods
Some 2D halide perovskites can be prepared via solution-processed methods, which represent low-cost, flexible and efficient approach in many cases. For example, Sun et al. recently reported the synthesis of CsPbBr3 nanostructures with different morphologies by adjusting different groups of acids and amines (Figure 2a) . In the presence of hexanoic acid and octylamine, the nanocrystals can be tailored into spherical quantum dots. Nanocubes were generated under the coordination effects of oleic acid and dodecylamine, while nanoplatelets formed with the help of oleic acid and octylamine. The average size for these three types of nanostructures is 4.3, 9 and 100 nm, respectively. Their photoluminescence (PL) decay lifetime was determined to be in the range from several tens to hundreds of nanoseconds . In another approach, Bekenstein et al. prepared colloidal nanoplatelets (NPLs) of CsPbBr3 at elevated temperatures, in which hot Cs-oleate solution was quickly injected into the PbBr2 solution containing octadecene, oleylamine and oleic acid . The shape and thickness of CsPbBr3 NPLs was roughly controllable by adjusting the injection temperature of Cs-oleate solution. Thin NPLs, thicker NPLs, nanocubes and large nanocubes together with nanowires were obtained under 90, 130, 150, 170–200°C, respectively. However, no nanocrystals are generated below 70°C. CsPbBr3 NPLs obtained at 130°C have a lateral dimension of ~20 nm and a thickness of ~3 nm. PL spectra were tuned from 512 nm (5 unit cell thickness) to 405 nm (1 unit cell thickness). For those NPLs with 5, 4, and 3 unit cell thickness, the photoluminescence quantum yield (PLQY) is 84.4 ± 1.8, 44.7 ± 2.6 and 10 ± 0.5%, respectively. The perovskite NPLs can be further assembled into stacked columnar phases by concentrating NPLs solutions and large 2D sheets by diluting samples . Pan et al. studied the effects of organic acid, base and cesium precursors on the morphology of CsPbBr3 perovskite nanocrystals (Figure 2b) . The use of shorter chain amines resulted in the formation of thin nanoplatelets, while using shorter chain carboxylic acids led to large-size nanocubes at high temperatures. Furthermore, using CsOAc as the Cs source can overcome the solubility problem that existed in Cs2CO3 precursor, showing superior performance, which allows the use of low boiling point reactants, such as acid or amine with chain length shorter than eight carbons. These findings have demonstrated the feasibility to tune the shape, size and optoelectronic properties of halide perovskites .
2.2. Vapor-phase methods
Vapor-deposition methods including chemical vapor deposition (CVD), van der Waals (VDW) epitaxy and other related methods are effective approaches to the preparation of high quality 2D nanocrystals especially for conventional semiconductors [46, 47]. These methods have now been attempted for the synthesis of 2D perovskites. Following is a brief summary with some representative examples illustrated.
Ha et al. first reported the preparation of CH3NH3PbX3 nanoplatelets via a gas-solid hetero-phase reaction,  as illustrated in Figure 3a. By means of VDW epitaxy, lead halide nanoplatelets were initially grown on muscovite mica and then were converted into perovskites via gas-solid reaction with CH3NH3X. The resulting nanoplatelets had the lateral dimension in the range of 5–30 μm and the thickness from several atomic layers to several 100 nm. CH3NH3PbI3 nanoplatelets exhibited a large diffusion length of ~200 nm (Figure 3b) . Adding phenyl-C61-butyric acid methyl ester (PCBM) layer onto the CH3NH3PbI3 platelets resulted in the reduction of PL lifetime from 6.8 ± 0.4 to 0.278 ± 0.004 ns. Besides, the electron-diffusion length was estimated to be 210 ± 50 nm,  which can be attributed to the high quality perovskites prepared from vapor-phase method. Wang et al. prepared crystalline 2D MAPbCl3 perovskite thin platelets on muscovite mica using VDW epitaxy method, with the thickness of sub-10 nm and a few tens of micrometers in the lateral dimension (Figure 3c) . Weak VDW interaction and delocalized bonding characters of ionic and metallic crystals play key roles in forming ultrathin and large-scale MAPbCl3 platelets.
A single-step CVD approach was developed by Wang et al. to prepare all-inorganic halide perovskite, CsPbX3 microplatelets . The two types of precursor powders (PbX2 and CsX) were mixed in a stoichiometric ratio and reacted in a home-built CVD system under controlled temperatures . A quality factor up to 2863 was achieved in CsPbX3 microplatelets-based gain medium and whispering gallery mode cavity, while the photodetector based on a sandwiched structure of graphene/CsPbX3 microplatelets/vertically stacked graphene displayed a high photoresponsivity >105 A/W. Similarly, Wang et al. demonstrated a two-step wafer-scale growth of large arrays of CH3NH3PbI3 microplate crystals . The SiO2/Si substrate was functionalized by (octadecyl)trichlorosilane (OTS) forming a hydrophobic surface, followed by a selective growth of PbI2 plates on the hydrophilic region generated by the oxygen-plasma treatment. Finally, perovskite platelets were obtained via immersing PbI2 plates into CH3NH3I vapor (Figure 3d).
2.3. Other synthesis methods
Besides solution-processed and vapor-phase methods, other methods have also been attempted to prepare 2D metal halide perovskites such as self-organization synthesis and mechanochemical approaches. Still, more alternative methods are in progress with the aim to control the morphology and thickness of 2D halide perovskites more effectively.
Liu et al. demonstrated the self-organization process for the transformation of CH3NH3PbBr3 nanodots (NDs) to NPLs . In detail, the CH3NH3PbBr3 NDs were dissolved in a nonpolar solvent and aged in darkness (Figure 3e). The thickness of resulting CH3NH3PbBr3 NPLs is similar to the size of original NDs, but the lateral dimension is about fourfold to the original NDs. Theoretical simulations show that the side-side orientation with the coordination between electrostatic and van der Waals interactions is favored during the self-organization process. The resulting NPLs loaded into a polymer matrix (4-methyl-1-pentene, TPX) displayed cos/sin intensity dependence on the polarizer angles for the PL emission. In addition, this method offers a new way for controlling the shape of perovskite-based nanomaterials with polarized emission . Jodlowski et al. reported the preparation of a bi-dimensional (i.e. 2D) perovskite (Gua2PbI4) using a mechanochemical synthesis approach (Figure 3f) . The precursors of PbI2 and MAI were mixed in a 1:1 molar ratio inside a ball mill under ambient conditions to produce polycrystalline powders. This synthesis process featured solventless environment, swiftness, simplicity, reproducibility and promising potential for exploring the alternative design of various perovskite nanostructures .
3. Structural characteristics and analysis of 2D halide perovskites
3.1. General structural features
In general, there are two kinds of 2D metal halide perovskites. One is the intrinsically layered crystal structure, which are electronically “2D” due to the quantum confinement in the layers composed of MX6 octahedra separated by organic cations. The other is those perovskites with 2D nanostructured morphology but intrinsically have 3D structures.
In the 2D layered perovskites, there is no restricted size confinement for the interlayer cations, as indicated in 3D perovskites. This structure feature allows organic cations such as butylammonium ion (C4H9NH3+) , phenylethylammonium ion (C8H9NH3+) , aromatic biimidazolium dications (C6H8N4+) to access to the interlayer space between inorganic layers . In (NBT)2PbI4, for example, PbI6 octahedra can link together to form a flat layer in the bc-plane. Instead of NBT cation, EDBE cation makes (EDBE)PbI4 be featured as a distorted layer, that is, a <110>-oriented zigzag structure. Increasing the alkyl chain length can narrow the band gap of hybrid perovskites, which is invoked by the distortion of inorganic layers and B-I-B bond angles. Moreover, the geometrical distortion of perovskite layers can also affect the defectivity and charge relaxation dynamics. Tan et al. reported the synthesis of (C4H9NH3)2PbBr4 crystals by a solution-processed method, which have a domain size of several to tens of micrometers and the thickness of several to tens of nanometers . PbBr6 octahedra are connected to each other via corner-sharing bromide atoms forming a 2D perovskite layer in the bc-plane, as shown in Figure 4a. Meanwhile, the space between layers is separated by C4H9NH3+ cations for charge balance. (C4H9NH3)2PbBr4 crystal-based photodetectors assisted with the protection and electric contact of monolayer graphene was reported to exhibit very low dark current (~ 10−10 A), high on/off current ratio (up to 103) and a high responsivity of ~2100 A/W . In addition, the “A” moiety can be further substituted by two types of organic cations, named as Ruddlesden-Popper family. In (BA)2(MA)n−1PbnI3n + 1 (n = 1, 2,3,4, ∞) perovskites, the layers are composed of tilted, corner-sharing PbI6 octahedra, propagating in the ac plane, while the inorganic sheets are disconnected by the organic bilayers. For such perovskites (BA)2(MA)n−1PbnI3n + 1, their band gap is 2.43 (n = 1), 2.17 (n = 2), 2.03 ((n = 3) and 1.91 (n = 4) eV, respectively . If BA+ cation is substituted with aromatic cations, such as PEA+ cation, then resulting (PEA)2(MA)2Pb3I10 (PEA = C6H5(CH2)2NH3+, MA = CH3NH3+) features n = 3 layers with the PEA cations sandwiched between layers and MA+ cations situated in the layers, displaying an optical band gap of 2.06 eV (Figure 4b) .
If the inorganic framework of 3D perovskites are slit along <111> axis, <111> oriented perovskites can be obtained. Leng et al. reported a collaborative solvent- and ligand-assisted re-precipitation method for the synthesis of MA3Bi2X9 (X = Cl, Br, I) quantum dots. (CH3NH3)3Bi2Br9 has a distorted layered structure composed of corner-shared BiBr6 octahedra with Cs+ ions filled between interlayers . Also, other interesting types of layered perovskite structures have been investigated. For instance, the tetragonal phase of CsPb2Br5 exhibits a sandwich structure consisting of [Pb2Br5]− layers and intercalated Cs+ as shown in Figure 4c. In the [Pb2Br5]− layer, one Pb2+ cation coordinates with four Br− forming the elongated pentahedron. All the Pb2+ ions are confined in the center of the layer, while Br− ions are located in both the bottom and top surface of the layer. The structural feature of the CsPb2Br5 crystals is similar to that of layered double hydroxides, the nanoplatelets of which can be steadily prepared by a facile precipitation process . Vargas et al. prepared a 2D inorganic metal halide perovskite single crystal of Cs4CuSb2Cl12 by a solution-processed method (Figure 4d). They found that this perovskite has a direct bandgap of 0.98 eV obtained by DFT calculations and exhibits higher conductivity than that of MAPbI3 (MA = methylammonium). Besides this, perovskite displayed high photo/thermal-stability and also tolerant to humidity, which makes it a promising candidate for optoelectronic applications .
As for the perovskites with a 2D morphology, Levchuk et al. reported the synthesis of CH3NH3PbX3 (X = Br, I) nanoplatelets via a ligand-assisted re-precipitation method, tailoring the thickness of nanoplatelets between 1 and 8 unit cell monolayers . Thanks to the quantum confinement effects, the thickness-dependent PL properties of these nanoplatelets would offer an effective method to tune the emission color of perovskites . Vapor-phase methods have also been employed for the synthesis of 2D perovskites. CVD was used to synthesize 2D CsPbBr3 nanoplatelets directly on the mica substrate via the reaction between PbX2 and CsX (Figure 4e). The growth mechanism of the nanoplatelets suggested that layered mica could facilitate the VDW epitaxial growth of 3D perovskites into ultrathin 2D morphology . Meanwhile, they also employed a two-step CVD procedure for the preparation of MAPbI3 platelets. In this method, PbI2 was firstly deposited on the silicon substrate, followed by the vapor-phase conversion of the PbI2 into MAPbI3 . Zhang et al. reported the preparation of CH3NH3PbI3-aXa (X = I, Br, Cl) nanoplatelets grown on muscovite mica, which showed triangular or hexagonal platelets with nanoscale thickness (10–300 nm) and edge length of several to tens of micrometers (Figure 4f) . For a typical CH3NH3PbI3 triangular nanoplatelet, a lasing threshold of ~37 μJ cm−2 is deduced; while for CH3NH3PbI3-aBra and CH3NH3PbI3-aCla, a higher threshold of ~128 μJ cm−2 is achieved but with a larger cavity quality factor, Q ~ 900. Integrating onto other conductive substrates such as Si, Au and ITO, respectively, the spontaneous emission lifetimes are 2.6, 1.2 and 1.9 ns. These emission times are much shorter than those on mica. However, the threshold of these nanoplatelet lasers remained to be ~40 μJ cm−2, because the exciton avalanche occurs in a much shorter time-scale than that of the carrier transfer/trapping at the surface/interfaces. In the family of the unique perovskite nanoplatelets, they have demonstrated strong optical-pumped room-temperature near-infrared lasing with low thresholds and wide mode-tunability . Liao et al. reported the single crystalline square microdisks (MDs) of CH3NH3PbBr3 prepared by using a one-step solution self-assembly method for the first time . Square MDs of CH3NH3PbBr3 are highly emissive with PLQY = 22% ± 5%. Q value of 430 was obtained at 555 nm presented by a transverse magnetic (TM) mode, which is smaller than the simulated one. This is most likely due to the shape deformation of CH3NH3PbBr3 MDs derivate from perfect squares. Single mode lasing at 557.5 nm was achieved in a 2.0 × 2.0 × 0.6 μm3 square MD with a threshold of 3.6 ± 0.5 μJ cm−2. Built-in whispering-gallery mode (WGM) microresonator in single-crystalline square MDs, ambipolar charge transport and the high carrier mobility make perovskites attractive candidates to realize electrically pumped on-chip coherent light sources .
3.2. High-resolution characterization tools
Grazing incidence wide angle X-ray scattering (GIWAXS) has been widely used to evaluate the lattice expansion in perovskite materials. As shown in Figure 5a, the d spacing of relative peaks is larger than that of the bulk crystals, consistent with the transmission electron microscopy (TEM) measurements of single sheets .
Atomic force microscopy (AFM) measurements are usually employed to detect the thickness and surface of perovskite nanosheets and nanoplatelets. Yang et al. reported that the thickness of 2D (PEA)2PbI4 NSs is measured as 2.0 ± 0.1 nm by AFM (Figure 5b), showing the formation of single-layer nanosheets . According to the crystallographic model, the theoretical interlayer distance should be 1.64 nm. Thus, it is very close to the AFM results, given the fact that some organic molecules adsorbed on the surface of perovskite nanosheets. Besides, a quite smooth surface of the (PEA)2PbI4 nanosheet was observed from the high-magnification height profile (right, Figure 5b).
Transmission electron microscopy (TEM), high-resolution TEM (HRTEM) and high-angle annular dark-field scanning TEM (HAADF-STEM) are all-powerful tools for analysis of 2D perovskite structures. Akkerman et al. reported the face-to-face alignment of CsPbBr3 nanoplatelets with a thickness of 3.0 ± 0.4 nm (corresponding to 5 unit cells) and the average lateral dimensions of 7.9 ± 1.2 nm × 40.9 ± 6.8 nm (Figure 5c). HRTEM images confirmed that CsPbBr3 nanoplatelets are lying flat with respect to the substrate (Figure 5d). And the high crystallographic structure of these nanoplatelets can be well characterized by Fast Fourier Transform (FFT) images (Figure 5e) . In the STEM mode, these nanoplatelets are lying flat or edge-on on the substrate (Figure 5f).
Single crystal X-ray diffraction is another powerful technique to analyze the structure of new materials by collecting diffraction data on single crystals. Recently, Cortecchia et al. found that a significant deformation of the Pb − X bond length and the X − Pb − X bond angle can result in broadened PL spectra for the two perovskites, (NBT)2PbI4  and (EDBE)PbI4  (NBT = n-butylammonium and EDBE = 2,2-(ethylenedioxy)bis(ethylammonium)) .
4. Applications of 2D perovskites as photodetector materials
Responsivity, speed and sensitivity are three main factors for evaluating photodetectors. Responsivity is the ratio of photocurrent/photovoltage to the incident optical power, R = Iout/Pin. Speed is derived from the rise and fall response times, respectively, of a transient photocurrent. Sensitivity measures the ability to detect a weak light signal and is reported as the specific detectivity, D* = R (A·Δf)1/2 Inoise−1. Devices with a high responsivity and a small noise current can achieve a signal-to-noise ratio of 1, which corresponds to the noise-equivalent intensity (i.e. the detection limit) at a lower incident intensity. Specific detectivity is normalized to the device area and the frequency bandwidth of the noise measurement, enabling comparison among different device architectures, areas and measurement bandwidths.
There are three kinds of photodetectors: photoconductors, phtototransistors and photodiodes. Charge carriers in photoconductors are re-circulated through symmetrical contacts, in which an external voltage is applied to read out the photon-derived changes and to achieve gains through multiple carrier re-circulation. In contrast to photoconductors, a built-in potential in a junction of photodiodes is essential for extraction of the photo-generated carriers. In phototransistors, a gate-programmable semiconducting channel is optoelectronically modulated, which might assist or hinder the charge transport while providing gain.
In this section, applications of 2D layered perovskites and perovskite nanoplatelets or nanosheets as the key component in fabrication of photodetectors are summarized and discussed.
4.1. Photodetectors based on 2D layered perovskites
Compared to perovskite film-based photodetectors, photodetectors made of perovskite single crystals exhibit more efficient photoresponsivity. In 2016, Tan et al. reported a solution-processed synthesis of (C4H9NH3)2PbBr4 crystals on SiO2/Si substrates under room temperature and application in the fabrication of photodetectors . Individual 2D perovskite crystal-based photodetectors were constructed along with large-area monolayer single-crystal graphene film used as a protection layer from dissolution into water or acetone, which has improved the stability of this device (Figure 6a). More importantly, water was carefully excluded during the transfer of graphene onto the perovskite crystals through a special “dry” transfer method. Interdigital graphene electrodes were designed to improve the absorption cross section of 2D perovskite crystals (Figure 6b). Finally, the photodetector displayed an ultrahigh photoresponsivity up to 2100 A/W and an On/Off ratio of 103 (Figure 6c and d). Such an efficient photoresponse is most likely attributed to the strong adsorption of 2D perovskite crystals on and enhanced carrier collection by graphene electrodes. This report represents the first time to fabricate a novel photodetector based on 2D individual perovskite crystals combining graphene-based source-drain electrodes, and the results could open a new avenue for the development perovskite crystals based optoelectronic devices .
Zhou et al. reported the synthesis of three different 2D layered structures, (C4H9NH3)2PbI4 (n = 1, one-layered perovskite), (C4H9NH3)2(CH3NH3)Pb2I7 (n = 2, two-layered perovskite) and (C4H9NH3)2(CH3NH3)2Pb3I10 (n = 3, three layered perovskite) . Especially, the photodetector based on three-layered perovskite displayed higher Ilight/Idark ratio on the order of ~103(Figure 6e). However, compared to 3D perovskites, these photodetectors showed much lower responsivities, which can be attributed to their larger optical band gap, small absorption in the UV-vis absorption spectrum as well as the limitation of charge carrier mobility from the insulting butylammonium layers. Among three photodetectors, three-layered photodetector displayed better performance than the others did. The reason might be that it has a narrow band gap and its microstructure is favorable to achieve higher output current.
More recently, the first 2D and mixed-halide anion of bismuth was synthesized by Li et al. via solution-processed method . In this compound, a unique 2D honeycomb-like [Bi2I7Cl2]n3n− layers are isolated by TMP2+ cations (N,N,N′,N′-tetramethylpiperazine). The bandgap of this material is 2.10 eV according to optical measurements. For the electrical conductivities, pellets and single crystal of (TMP)1.5[Bi2I7Cl2] displayed higher conductivity of 1.59 × 10−6 S/cm and 2.37 × 10−6 S/cm, respectively. Under the illumination of 450 W Xenon lamp and external applied voltage of 25 V, the photocurrent increased to 0.68 μA (Figure 6f). Furthermore, no material decomposition was observed after stored in air for 1 week, showing superior stability over commonly used APbI3 .
4.2. Photodetectors using 2D perovskite nanoplatelets or nanosheets
Song et al. first reported the fabrication of atomically thin, 2D CsPbBr3 nanosheets on ITO/PET substrates as a flexible and ultrathin photodetector via centrifugal casting method (Figure 7a) . CsPbBr3 nanosheets were synthesized with a thickness of ~3.3 nm and an edge length of ~1 μm, and their typical shape is tetragonal. In contrast to organometallic halide perovskites, the as-prepared CsPbBr3 nanosheets are well dispersible in several organic solvents. UV–vis absorption spectra indicated a favorable absorption capability with a direct bandgap of ~2.32 eV for the nanosheets.
Linear and symmetrical photocurrent plots further evidenced the uniform dispersion of perovskite nanosheets on the ITO electrode . A high light on/off ratio of >103 under 442 nm laser was demonstrated, showing good light-switching behavior. And a peak value of 0.25 A W−1 was obtained under 5 V at 517 nm, as shown in Figure 7b, comparable to that of commercial Si photodetectors (< 0.2 A W−1). Besides, external quantum efficiency (EQE) was measured to be 53% at 515 nm and 10 V bias voltage (Figure 7c). The rise and decay time of the CsPbBr3photodetector were 19 and 25 μs, respectively. Meanwhile, the photodetectors were resistant to external bending stress as the photocurrents and dark currents remained unchanged at different bending states (Figure 7d). Also, after bending and recovering for >1000 times, the photodetector still showed high light on/off ratio, showing very high stability. In contrast to serious degradation of MAPbBr3 photodetectors, CsPbBr3 nanosheet photodetector almost remained unchanged under ambient environment. These results open up the opportunity for the practical application of 2D perovskite materials used as photodetectors due to their high stability . In another example, Liu et al. reported a combined solution processing and vapor-phase conversion method to synthesize CH3NH3PbI3 nanosheets as thin as a single unit cell (~ 1.3 nm), which were used in fabrication of a field-effect transistor . This device displayed enhanced photocurrent upon illumination as well as high ratio of photocurrent to dark current, attributing to strong light-matter interaction and broad-band light-harvesting capability in a single-unit-cell nanosheet. Under illumination of a 405 nm laser, photoresponsivity can reach 22 A W−1. Besides, the rise and decay time of the photodetector shorter than 20 and 40 ms exhibited a much faster response time than bulk films, showing potential application for optoelectronic switches and photodetectors.
Cyan-emitting CsPbBr3 nanosheets based photodetector was reported by Lv et al. . The source-drain current of CsPbBr3 nanosheet film in the dark was detected to be on the order of 10−12 A due to the existence of long-chain organic ligands on the nanosheet surface. Under illumination of a laser diode at 450 nm, a significant increase in photocurrents was observed. Besides, excellent optical switching and stability of this photodetector were also proved under 1 Hz pulse laser (450 nm) at a fixed light density of 13 mW cm−2 with a bias voltage of 1 V. Through fitting the photocurrent response curve (Figure 7e), the rise time was estimated to be 17.8 ms and the decay times were determined to be 14.7 and 15.2 ms . Li et al. used a combined solution-processed and vapor-phase conversion method for the synthesis of 2D CH3NH3PbI3 nanosheets with a thickness of ~20 nm . Vertical- and planar-type photodetectors based on CH3NH3PbI3nanosheets were tested to evaluate their optoelectronic properties (Figure 7f). With increasing the incident laser power, the photocurrent increased to 600 nA at the power of 98.56 μW. The rise and decay times of the photodetector were measured to be 320 and 330 ms, respectively. Comparable to planar-type photodetector, the vertical-type photodetector displayed higher photoresponsivity, about 36 mA W−1 under excitation power of 50.82 μW, resulting from short transmission distance of the photocarriers between the electrodes in the vertical photodetector and appropriate band energy distribution (Figure 7g). All these results might open a new avenue for the investigation of 2D perovskites including their optical, electronic and optoelectronic properties and for potential application as practical photodetectors.
Other photodetectors based on 2D halide perovskites have also been reported. Table 1 compares the structure and key performance parameters of recently reported photodetectors.
|Device component and structure||Perovskite structure||Rise-decay time (ms)||On/off ratio||Responsivity (A W−1)||Ref.|
|Glass/(C4H9NH3)2PbI4/Au||Nanocrystal||28.4/27.5||1 × 102||3.00|||
|Glass/(C4H9NH3)2(CH3NH3)Pb2I7/Au||Nanocrystal||8.4/7.5||1 × 102||7.31|||
|Glass/(C4H9NH3)2PbI4/Au||Nanocrystal||10.0/7.5||1 × 103||12.78|||
|Si/SiO2/CH3NH3PbI3/Ti/Au||Nanosheet||–||1 × 102||22|||
|Au/CsPbBr3/Au||Nanosheet||14.7/15.2||1 × 102||–|||
5. Theoretical analysis and simulations
First-principles calculations based on density functional theory (DFT) have been used to simulate the electronic structures of many body systems from atomic level quantum mechanics. Recently, perovskite-based materials including lead-free perovskites, 3D perovskites, especially 2D perovskites have been studied using first-principles calculations, exploring their electron/hole effective masses, band gaps, carrier mobility, theoretical absorption spectra and other properties related to their potential applications. Besides DFT calculations, the local density approximation (LDA) and the generalized gradient approximation (GGA) functional have also been used in many systems due to their simplicity. However, these two methods usually underestimate band gaps of solids by 30–100%. Therefore, Green-function approximation (GWA) , time-dependent DFT (TDDFT),  HSE06,  and PBE0 hybrid functionals,  and more recent Delta self-consistent-field method  were mainly employed in theoretical calculations of perovskites. Spin-orbit coupling (SOC) is required to be included in calculations with the presence of heavy atoms to achieve a more accurate prediction of electronic structures.
For lead-free peroskites, first-principles quantum mechanical calculations were used to investigate their thermodynamic stability and photovoltaic-related properties. To this end, a series of design metrics including thermodynamic and crystallographic stability, light absorption, carrier effective mass, dopability, exciton binding and others should be considered . For example, the latest calculations have identified 18 compounds out of ~100 possible candidates as most-likely targets (Figure 8a). Furthermore, the structure-property relationship was also studied and showed that changes in microscopic structures could significantly affect photovoltaic performance. Theoretical computations could serve as a roadmap for facilitating experimental discovery of alternative solar-energy hybrid perovskites. Sn- and Ge-based alternatives were reported to be more stable than Pb-based perovskites. However, they displayed poor performance in solar cells largely because of their intrinsic p-type conductivity and decrease in photocurrent density, and fill factor. Therefore, other factors beyond intrinsic bulk properties should be included in order to evaluate their performance more reliably.
Using DFT the electronic band structure and density of states of some 2D perovskites were computed, which confirmed that these 2D perovskites are indeed direct band gap semiconductors. Their valence band mostly consists of I 5p states with a small amount of Pb 6 s, while their conduction band primarily consists of Pb 6p states. For example, the band gap is 1.99 eV for (BA)2PbI4, 1.78 eV for (BA)2(MA)2Pb3I10 and 0.96 eV for (BA)2(MA)3Pb4I13 (Figure 8b and c), respectively. It is found that the band gap decreases with increasing the number of perovskite layers. Besides, the structural stability of these materials was predicted by DFT, finding that non-centrosymmetric structures in 2D perovskites with a large number of layers are favorable. However, there is still a complex relationship between the perovskite layer thickness and lateral dimension . Akkerman et al. reported the SOC-DFT calculations to investigate the electronic properties of CsPbBr3 NPLs (Figure 8d), which is highly consistent with the experimentally observed bandgap evolution . SOC effects can determine electronic and optical properties of perovskites as well as their quantum confinement behavior. However, deviations become obvious as the quantum wells decreased to ultrasmall dimension. This is mainly attributed to the tunneling of the electron/hole wave functions outside the well boundaries such as the lack of effectively infinite potential barriers at the NPL border. In addition, the effective masses and the exciton both reduced with increasing quantum confinement, suggesting ambipolar transport occurred in CsPbBr3 crystals.
To better understand the self-organization mechanisms of CH3NH3PbBr3 QDs (or nanodots (NDs)) into NPLs, Liu et al. performed theoretical simulations based on the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory . They suggested that during the self-organization process, two or four coalescent QDs might be considered as stable units. Two coalescent QDs attracted each other from bottom and top to form a four-dot alignment under dipole-dipole attraction (Figure 8e). With a combination of VDW and electrostatic driving forces, the self-organization of CH3NH3PbBr3 QDs preferred via side-side orientation to form nanoplatelets. Consequently, the axis size of NPLs is about four-fold that of building block QDs.
Cortecchia et al. have performed ab initio calculations to investigate the structural effects on the defectivity and energetic landscape for (EDBE)PbI4. For the neutral Pb vacancy, formation of I3− trimer is preferred in (EDBE)PbI4 by ~1 eV, which is driven by under-coordinated iodine atoms at the organic/inorganic interface with partial charge stabilized by EDBE2+ . However, iodine atoms per organic cation in (NBT)2PbI4 has a stronger electrostatic interaction, which prevents the vicinal iodine motion from forming I3−. Hybrid DFT calculations including spin-orbit coupling (PBE0-SOC) was also conducted to verify the formation of I3− trimer in (EDBE)PbI4 (Figure 8f). An optical transition at 0.21 eV corresponds to the highest occupied molecular orbital (HOMO)-Lowest unoccupied molecular orbital (LUMO) transition of I3−, leading to the emergence of Stokes-shifted, broadened luminescence according to the relaxation mechanism for trapped charge carriers. With the templating cation assisting charge localization, the increased structural distortions in the PbX6 octahedral coordination in (EDBE)PbI4 lower the energy barrier for self-trapping and thus stabilize the trimer I3−.
In addition, Monte Carlo calculations were used to explain the growth mechanism of 2D sheets of MAPbCl3 on mica . The VDW nucleation and growth model was proposed (Figure 8g) . The competition between the VDW diffusion and cohesive energy of perovskites can in principle increase the thickness of perovskite sheets. This means much thicker layered perovskites could be prepared through surface passivation. Furthermore, the development of two-photon-pumped laser combined theoretical simulations (e.g. two-photon absorption (TPA)) would enable to study certain perovskites towards understanding their nonlinear optical properties. For example, Chen et al. have recently revealed that the TPA process proceeds through a virtual level and then ended in an exciton band state. This observation would lay a solid physic basis for the development of high-performance nonlinear optical perovskite materials .
6. Concluding remarks and outlook
Metal halide perovskites have increasingly attracted research interests in recent years, largely because of their remarkable optical and electronic properties such as high carrier mobility, small exciton binding energy, high photoluminescence quantum yield, narrow bandwidth, long charge diffusion length and broadly tunable bandgap. They can further exhibit unique optical properties and enhanced stability in their 2D form due to quantum confinement effects, which has led to increasing interest in the design and assembly of 2D perovskites for their applications in photodetectors. This chapter mainly focused on the introduction of synthesis methods, structure characterizations for both 2D layered perovskites and 2D perovskites in shape, along with summarizing photodetection application of these two types of 2D perovskites.
Researchers have made tremendous progress in studying 2D perovskite-based photodetection techniques, but some challenges from both basic science and device engineering perspective remain to be overcome. Recent studies have shown the feasibility of preparation of perovskite nanosheets and nanoplatelets, but the control of their morphology and thickness needs to be improved. Due to the quantum confinement effects, the thickness of nanoplatelets or nanosheets is a critical factor for their optical properties. Therefore, the development of improved synthetic methods for better controlling morphology and thickness are indeed required, which has been well achieved in conventional semiconductors and metal nanocrystals. With the development of two-photon-pumped laser for studying CsPbBr3 QDs, the nonlinear optical properties of perovskites have just been taken to the front of the research, such as two-photon absorption (TPA). Although 2D metal halide perovskites have displayed excellent optical properties, water and air stability is still a detrimental problem towards their practical applications. Some recent researches have attempted to solve this issue. For example, organic molecular capping, silica or Al2O3 coatings were used to improve the stability, which has been to some extent successful. However, further efforts still need to find more general ways with broad suitability for 2D perovskites. Meanwhile, another important issue, similar to stability, is the toxicity of Pb element, which is one of the most insidious factors for the environment and human. Albeit with the excellent properties of lead-containing 2D perovskites, it still exposes as a detriment factor for their applications. Many attempts have been made to substitute Pb with nontoxic metal elements, but the resulting perovskites normally do not have the same level performance. For example, Sn-based perovskites have appropriate bandgaps for optoelectronic applications, but they display much lower stability than Pb-containing perovskites due to the oxidation of Sn2+ to Sn4+ under ambient conditions. For Ge- and Bi-based perovskites, they indeed display high stability. However, these two types of perovskites possess a large bandgap, which limits the light absorption in long-wavelength range. Therefore, it is high desirable to design new 2D layered perovskites with appropriate bandgap and large absorption coefficient and high stability.
In terms of photodetection applications, being comparable to conventional colloidal quantum-dot-based photodetectors much improvement is needed for perovskites. For example, perovskite-based photoconductors suffer from slow photoresponse, low sensitivity and large electrical hysteresis. Some recent efforts have partially overcome these issues by using surface passivation, phototransistor gating and fabricating hybrid bilayer photodetectors. Interfacing perovskite layers with other functional nanomaterials is proposed to be an effective way for fabricating perovskite-based optoelectronic devices. In a broader perspective, perovskite-based materials could be tuned and engineered for fabrication of new-type devices that are endowed with new functions such as multi-spectral sensing.
In conclusion, in view of their novel structures and optical properties we would believe that 2D perovskites are on the way becoming promising materials for next-generation optoelectronic devices.
This work was supported by Independent Research Fund Denmark-Nature Sciences (DFF-FNU, to Q.C.) and the Villum Foundation (to Q.C. and Y.T.). Y.T. is grateful for the Villum Foundation supported postdoc fellowship. X.C. is grateful for the support from the Chinese Scholarship Council (PhD scholarship No. 201406170040).