Recent advance of high-quality perovskite nanostructure and its application in flexible photodetectors

Flexible photodetectors (PDs) have garnered increasing attention for their potential applications in diverse fields, including weather monitoring, smart robotics, smart textiles, electronic eyes, wearable biomedical monitoring devices, and so on. Notably, perovskite nanostructures have emerged as a promising material for flexible PDs due to their distinctive features, such as a large optical absorption coefficient, tunable band gap, extended photoluminescence decay time, high carrier mobility, low defect density, long exciton diffusion lengths, strong self-trapped effect, good mechanical flexibility, and facile synthesis methods. In this review, we first introduce various synthesis methods for perovskite nanostructures and elucidate their corresponding optical and electrical properties, encompassing quantum dots, nanocrystals, nanowires, nanobelts, nanosheets, single-crystal thin films, polycrystalline thin films, and nanostructured arrays. Furthermore, the working mechanism and key performance parameters of optoelectronic devices are summarized. The review also systematically compiles recent advancements in flexible PDs based on various nanostructured perovskites. Finally, we present the current challenges and prospects for the development of perovskite nanostructures-based flexible PDs.

This review comprehensively surveys the latest progress in fabricating perovskite nanostructures and their utilization in flexible PDs.Specifically, sections 2 and 3 delve into the chemical compositions, synthetic methods, as well as optical/ electrical properties and structure designs of various perovskite nanostructures.In addition, section 4 systematically delineates the working mechanisms and performance parameters of flexible PDs.Notably, recent breakthroughs, applications, and emerging trends within the realm of perovskites nanostructures-based flexible PDs are summarized in section 5. Our review aims to inspire researchers to harness the potential of nanostructured perovskite materials in developing high-performance flexible PDs while also providing fresh insights for the design and advancement of flexible optoelectronic devices based on novel functional materials.

Synthetic methods of perovskite nanostructures
The synthetic procedure plays a pivotal role in elucidating the chemical composition, crystal size, crystal structure, and optoelectronic properties of perovskite nanomaterials [50].Establishing efficient and controllable synthesis strategies is fundamental for designing and manipulating novel perovskite materials, thus advancing our understanding of their intrinsic mechanisms and enhancing performance in optoelectronic devices [51][52][53].In the subsequent sections, we will summarize several primary preparation methods for perovskite nanostructures.

Vapor−liquid−solid methods
The vapor−liquid−solid (VLS) method stands as a widely utilized technique for the precise growth of perovskite NWs, offering control over morphology, positioning, growth density, and chemical composition [54].Successful implementation requires simultaneous consideration of (i) the formation of dispersed, nanoscale catalytic low-melting-point alloy droplets, typically resulting from the interreaction of metal catalysts with perovskites; (ii) the establishment of a vapor phase at specific pressures, which is generally composed of NW materials [55,56].
In 2017, Meyers et al developed a self-catalyzed VLS method for growing lead halide perovskite (MAPbI 3 ) NWs (figure 1(a)).In this process, the liquid Pb catalyst was supersaturated with halogen X through vapor-phase incorporation of both Pb and X.The NWs maintained their original topography, confirming growth orientation along the 〈1̅ 21̅ 0〉 direction [57].In 2020, Meng et al innovatively incorporated molybdenum trioxide (MoO 3 ) as a surface catalyst, effectively enabling surface charge transfer doping of singlecrystalline CsPbBr 3 NWs in a VLS process (figure 1(b)).The hole mobility was significantly increased from 1.5 to 23.3 cm 2 /(Vs) because of the efficient interfacial charge transfer and reduced impurity scattering [58].Subsequently, in 2022, Li and co-workers demonstrated a direct self-catalyzed synthesis of CsPbBr 3 NWs via the VLS process (figure 1(c)).It was noteworthy that the surface energy of substrates was modulated by their surface roughness, which played a crucial role in mediating the self-catalytic growth of CsPbBr 3 NWs [59].In 2023, Meng et al proposed a method for growing Au-seeded CsPbI 3 NWs through the VLS mechanism.As shown in figure 1(d), the Au seeds reacted with Pb vapor to form molten Au-Pb droplets at temperatures as low as 212 °C.This approach facilitated low-temperature growth of CsPbI 3 NWs, reducing bandgap trap states and hindering Shockley-Read-Hall recombination [60].

Wet chemical growth methods
To date, wet chemical growth is a prevalent method for fabricating perovskite nanostructures, prized for its costeffectiveness and simplicity.This encompasses techniques like hot injection, inverse temperature crystallization, coprecipitation, thermal decomposition, and so on [61].
The hot injection process comprises two key steps.Initially, the precursor is dissolved in an organic solvent.Subsequently, the precursor solution is injected into a hightemperature reactor to decompose into perovskite nanomaterials.The perovskite nanomaterials exhibit uniform size, controllable morphology, and excellent dispersibility through hot injection [62].In 2016, Pan et al conducted a systematic investigation into the independent influences of acids and amines on the size and shape of CsPbBr 3 NCs using the hot injection method (figure 2(a)).Their study confirmed the presence of both carboxylate and alkyl ammonium ligands on the NCs surfaces, but the alkyl ammonium ligands exhibited  higher fluidity and were more easily separated from the NCs surfaces during polar solvent washing [63].
Inverse temperature crystallization is a universal method for synthesizing single-crystal perovskites, which is conducted within a closed system.The dynamic conditions for single crystal growth are provided by exploiting the difference in solubility at varying temperatures [64].In 2021, Liu et al introduced an effective additive strategy to achieve 2 inch sized FAMACs single crystals through inverse temperature crystallization (figure 2(b)).The study revealed that a reasonably selected reductant can effectively reduce the oxidation of iodides and the deprotonation of cations.Furthermore, the FAMACs single crystal exhibited an impressive over 5-fold enhancement in carrier lifetime, carrier diffusion distance, charge mobility, and long-term stability [65].
Co-precipitation is another prevalent technique for synthesizing perovskite NCs [66].In 2017, Zhang et al successfully fabricated a mixed-cation perovskite (Cs 0.87 MA 0.13 PbBr 3 ) emission layer through co-precipitation (figure 2(c)), and obtained a light-emitting diode with a remarkable brightness of 91 000 cd m −2 and impressive external quantum efficiency (EQE) of 10.4% through the deposition of a hydrophilic polyvinyl pyrrolidine polymer (PVP) into the ZnO electronic transport layers [67].

Spin-coating methods
The spin-coating method has gained widespread adoption in various applications, particularly in perovskite solar cells and thin-film devices, owing to its distinguished, including low-cost, environment-friendly and the ability to achieve controlled thickness [50,68].In 2014, Heo et al described each stage in a typical spin-coating process.As illustrated in figure 3(a), the MAPbBr 3 precursor was initially deposited onto the bl-TiO 2 /FTO substrate and then subjected to controlled acceleration.This controlled acceleration resulted in the expulsion of an excess solution, while further evaporation led to the crystallization of MAPbBr 3 thin films [69].In 2022, Fradi et al outlined a method for fabricating hightemperature-resistant MAPbI 3(1-x) Br 3x films employing toluene as a moisture absorber.The study demonstrated that appropriately increasing the annealing temperature led to an augmentation in grain size (figure 3(b)) [70].In the same year, Zhu et al introduced trimethylthiourea (3T) into the spin-coating process.As depicted in figure 3(c), the addition of 3T to the FASnI 3 precursor significantly decelerated the transition of the wet film of the FASnI 3 precursor into a rough dark brown upon contact with the anti-solvent, resulting in an orange-brown FASnI 3 film.Besides, the potential efficacy of 3T through hydrogen bonding was thoroughly discussed [71].

Chemical vapor deposition
Chemical vapor deposition (CVD) is a controllable and precise method for fabricating high-quality perovskite nanostructures, typically conducted within a tubular reactor.During the CVD process, perovskite functional materials are grown either epitaxy or non-epitaxy on specific substrates.For epitaxial growth, the functional materials either have a wellmatched lattice or a dangling bond that forms van der Waals interactions with the substrates [72].
In 2018, Du et al synthesized well-defined triprismshaped CsPbBr 3 micro/nanowires on SiO 2 /Si substrates using a CVD method.These structures exhibited a highly smooth surface and fell within an ideal size range (figure 4(a)), and further demonstrated the strong coupling of exciton-photon and polariton by placing the CsPbBr 3 micro/ nanowires in optical microcavities [73].In 2022, Tavakoli et al proposed a two-step chemical reaction for perovskite growth via CVD.Firstly, the metal film was converted to PbI 2 /InI 2 , followed by the formation of perovskite film by the

Ligand-assisted reprecipitation methods
Ligand-assisted reprecipitation (LARP) method represents a cost-effective and efficient strategy for synthesizing perovskite NCs.Typically conducted at temperatures below 60 °C, the LARP method consistently produces perovskite NCs characterized by remarkable size uniformity, robust PL emission, and a high photoluminescence quantum yield (PLQY) [76].Notably, the careful selection of anti-solvents in the LARP process is deemed crucial, given that different antisolvents can induce variations in morphologies and properties of the resultant products [77].
In 2018, Han et al reported the synthesis of FAPbBr 3 NC thin films with a robust PL emission and PLQY of up to 78% using the LARP method.The preparation process was illustrated in figure 5(a).Initially, the FAPbBr 3 precursor in the polar solvent DMF was spin-coated onto ITO glass substrates.Subsequently, the anti-solvent toluene was added dropwise during rapid rotation.The solubility difference between DMF and toluene induced the nucleation process of FAPbBr 3 perovskite NCs [78].In 2023, Chen et al synthesized FASnI 3 NCs with an average diameter of 7.7 nm and a narrow PL emission centered at 825 nm through the LARP process.As depicted in figure 5(b), when compared to the split-ligand reprecipitation (SLRP) method, the perovskite NCs prepared by the LARP approach exhibited superior PL intensity and improved crystallinity.These features are advantageous for efficient charge carriers transport in photovoltaic devices [79].In the same year, Treber et al employed a combination of the LARP method and the hot injection method to synthesize fully inorganic Mn: Cs 2 NaBiCl 6 NCs.The low-temperature synthesis process was shown in figure 5(c).Initially, the perovskite precursors were dissolved in a toluene solvent, followed by the injection of trimethylsilylchloride (TMSCl) to initiate the formation of perovskite NCs under a constant temperature.Moreover, the Mn:Cs 2 NaBiCl 6 NCs exhibited a bright orange PL with a peak at 595 nm, originating from the d-d transition of the Mn 2+ dopant [80].In summary, the LARP method offers a facile pathway for the large-scale production of high-performance perovskite NCs.

Chemical compositions and crystal structures of perovskites
Typical perovskites are characterized by the chemical composition ABX 3 , with A and B representing cations of varying size (A = Cs + , MA + , FA + , etc; B = Ti4 + , Pb 2+ , Sn 2+ , etc), and X generally denotes halide ions (X = Br − , Cl − , I − ) [81,82].The perovskite structure belongs to the isometric system with a space group of Pm3m (No. 221).Their crystals can be envisioned as A and X arranged in a cubic close packing structure, with B atoms occupying the octahedral voids [83].From the perspective of coordination polyhedral, the crystal structure of ABX 3 is composed of a grid framework of [BX 6 ] octahedra connected by co-vertex corners in 3-dimensional space.A cation forms a coordinated [AX 12 ] cuboctahedron with X-halide ions due to a coordination number of 12.The X halide ions are adjacent to two B cations, which can be attributed to a coordination number of 2. Furthermore, the original structure of perovskite can undergo distortions leading to various structures, such as ordered arrangements (A-site-ordered, B-site-ordered, and AB-double-site-ordered), anion-deficient structure (A 2 B 2 X 5 ), anion-rich structure (A 2 B 2 X 7 ), and so on [84,85].
In practice, the types and radii of A, B, and X ions are different.To quantitatively assess the structural stability of perovskites, the 'tolerance factor' (τ) has been introduced, which can be expressed as [86]: It is noteworthy that a perovskite structure is considered stable within a range of μ values spanning from 0.44 to 0.90.This metric provides valuable insights into the structural behavior of perovskites under diverse conditions [89].
In 2022, Yan et al presented a ligand-assisted antisolvent strategy to fabricate stable CsPbBr 3 QDs by replacing conventional oleic acid (OA) ligands with 2-hexyldialkylic acid (DA) ligands, and they achieved a remarkable enhancement in PLQY.Specifically, the PLQY of CsPbBr 3 -DA QDs reached 96%, in contrast to the 84% observed for CsPbBr 3 -OA QDs (figure 6   Simultaneously, Wang's group introduced a modified ligandassisted anion-exchange method for CsPb(I x Br 1−x ) 3 NCs purification.This method involved potassium iodide as an inorganic additive, effectively mitigating the instability in perovskite NCs arising from surface traps.Comparative analysis with the most stable red perovskite LEDs reported revealed a 6-fold increase in EQE for LEDs prepared using this method (figure 6

1D perovskite nanostructures
1D perovskite nanostructures are emerging with many distinguished characteristics, such as a large specific surface area and a single-crystal structure of a specific orientation.This leads to a low defect density and a high absorption coefficient, which are expected to result in highly efficient carrier transport and ultra-high IQE/EQE [98,99].Moreover, 1D nanomaterials tend to exhibit excellent flexibility, laying the cornerstone for the development of flexible optoelectronic fields [100,101].
Recently, all-inorganic perovskites have become an exciting material due to their excellent photovoltaic properties and high stability.In 2023, Dai et al reported an all-inorganic single-crystal CsSnI 3 perovskite NWs with a perfect lattice structure, and the HRTEM demonstrated that the singlecrystal CsSnI 3 NWs were grown along the (002) direction (figure 7(a)).Additionally, the CsSnI 3 NWs exhibited low carrier trap density (≈5 × 10 10 cm −3 ), long carrier lifetime (46.7 ns), and excellent carrier mobility (<600 cm 2 V −1 s −1 ) [102].Taking advantage of the low defect density of perovskite single-crystal NWs, some interesting work was reported.For example, in 2023, Cha et al introduced a strategy for growing CsPbBr 3 NWs at the interface of conventional thin film perovskite devices.This approach led to the passivation of defects at grain boundaries and efficient transmission of charges across interfaces.As depicted in figure 7(b), the CsPbBr 3 NWs exhibited a high aspect ratio with a width of 10 nm and a length exceeding 10 μm.The NWs-modified perovskite thin film evidently displayed a lower defect density and non-radiative recombination.Ultimately, a power conversion efficiency (PCE) of 21.56% was achieved over a large span of 3500 h [103].Meanwhile, Shin et al devised a dual-phase passivation strategy to synthesize all-inorganic CsPb 2 Br 5 /CsPbBr 3 NWs, as illustrated in figure 7(c) (left), a localized CsPb 2 Br 5 layer was synthesized on the CsPbBr 3 NWs surface after a slow reaction at 135 °C for 50 min, the core-shell structure of CsPb 2 Br 5 /CsPbBr 3 NWs effectively reduced trap density and increased the PLQY.The dual-phase NWs were also employed as surface modifiers in thin film perovskite solar cells (PSCs) to enhance charge carrier transfer through defect passivation.The PCSs exhibited a remarkable PCE of 22.87% when using the singlephase CsPb 2 Br 5 /CsPbBr 3 NWs as surface modifiers [104].
It is worth noting that the cross-connection of NWs hinders the transport of carriers by a straightforward solution process.To address this, novel approaches have been proposed to improve the controllability and scalability of perovskite single crystals NWs.For example, Zhang et al reported a strategy for growing vertically aligned and individual CsPbBr 3 NWs using nanoporous anodic aluminum oxide (AAO) templates through a low-temperature solution process.The diameters of the CsPbBr 3 NWs were determined by the AAO pore size, while their length ranged from 1 to 20 μm depending on the precursor amount.The growth mechanism was displayed in figure 7(d): initially, the NWs preferentially nucleated along the (110) plane, which parallels the AAO pore walls and has the lowest solid-solid interfacial energy.The remaining supersaturated solution further promoted the growth of NWs.Ultimately, vertically aligned CsPbBr 3 NWs arrays were formed [105].Additionally, Zhou et al introduced a hot-pressure welding strategy to enhance the ordering of the all-inorganic CsSnI 3 NWs network, as shown in figure 7(e), after initially growing a disordered network of CsSnI 3 NWs through a spin-coating/soaking two-step process, a mica sheet was placed on top of the sample under a specific pressure while maintaining the temperature at 300 °C.This process led to the reduction of micro-interfaces in the single-crystalline NWs.The PDs based on ordered CsSnI 3 NWs showed a high R of 9.9 × 10 −3 A W −1 , a good D * of 7.2 × 10 8 Jones, and an LDR of 37 dB for 473 nm light [106].1D perovskite NWs demonstrate compatibility with bendable substrates, signifying a significant advancement in the development of flexible optoelectronics.In 2022, Li and co-workers presented a method based on temperature gradient-assisted nanoimprinting to prepare perovskite NW/MWs arrays on curved substrates.As shown in figure 7(f), PDMS with a periodic micro-stripe structure was brought into close contact with the curved substrate under pressure.The MAPbBr 3 perovskite precursor was then deposited at the hot end of the PDMS microchannel, resulting in the formation of curved MAPbBr 3 MWs.In addition, the flexible PDs demonstrated excellent optoelectronic properties (R = 414 A W −1 , D * = 1.2 × 10 14 Jones, and EQE > 140 000%) along with superior mechanical stability [107].

2D perovskite nanostructures
2D perovskite nanostructures can be visualized as sheets derived from the slicing of 3D perovskites along specific crystal planes.This process involves the addition of halide ions to facilitate surface metal coordination, along with the insertion of larger cations to serve as spacers between the inorganic layers [108,109].The unique properties of 2D perovskite nanostructures, including metasurface, exceptional light absorption, extended carrier diffusion lengths, short charge transfer distances, broadband STE, and remarkable stability, have garnered significant attention.These structures are typically prepared through wet-chemical approaches and mechanical exfoliation processes [110,111].
To gain a deeper understanding of the growth mechanisms behind 2D perovskite NSs, some researchers have conducted in-depth studies.For instance, in 2023, Yuan et al proposed a method for synthesizing all-inorganic Cs 3 Bi 2 I 9 perovskite NSs at the liquid-air interface by adding an excess of iodine, which induced the growth of large-size perovskite NSs with high crystalline quality due to the pronounced volatility of iodine (figure 8

3D perovskite nanostructures
Recently, significant strides have been achieved in optoelectronic devices utilizing 3D polycrystalline perovskite films.The granularity, crystallinity, uniformity, and thickness of these films can be precisely controlled by adjusting various process parameters.Additionally, the conformal growth of polycrystalline films on flexible polymer substrates has been advanced through interface modification, thereby reducing the non-radiative recombination of carriers stemming from defects at the grain boundaries.The preparation of 3D perovskite polycrystalline films primarily employs low-temperature solution processes and chemical vapor deposition methods.Among them, the one-step spin-coating stands out as the most straightforward, wherein the perovskite precursor swiftly crystallizes directly onto the substrate during rapid rotation [69,[115][116][117][118][119][120].
Currently, the simplest and most common way for preparing perovskite polycrystalline films is spin coating.To gain a comprehensive understanding and control over the formation process of perovskite films from precursors, in 2022, Fradi et al conducted a systematic and meticulous investigation into the influence of varying doping ratios of the monovalent halide anion Br − on the structural properties and stability in MAPbX 3 thin films via spin-coating.As depicted in figure 9(a), a significant diffraction peak was observed at 14.1°, appointed to the (001) lattice plane.With an increase in Br content, this peak gradually shifted to a higher angle, and the sharp diffraction peak indicated enhanced stability and crystallinity in the Br-incorporated perovskite films

Performance parameters and working mechanisms of the flexible PDs
To effectively and fairly evaluate the performance of PDs with different materials and device structures, a set of key parameters have been introduced.In the following part, we will present the specific connotations of the parameters.The spectral responsivity (R( ) l ) reflects the change in the responsivity of the device under the irradiation of different wavelengths and is defined as [123]: According to the distribution of R( ) l at different wavelengths, devices can be divided into narrowband PDs and broadband PDs [124].Besides, PDs can be classified into ultraviolet ones, visible ones, and infrared ones in conformity with the different ranges of the spectral responsivity intensities [125].
4.1.3.Noise equivalent power (NEP) and detectivity (D).NEP is defined as the power of the smallest incident optical signal that can be distinguished from noise.It is expressed by the following equation: where P th is the incident optical power when the signal-tonoise ratio is 1, and the i n and R i correspond to the corresponding noise current and optical responsivity at a specific pulse frequency, respectively.In addition, D is introduced to characterize the ability of the device to detect weak radiation, which can be qualitatively expressed as (5) [126]: Note that D is related to the junction area A ( ) and bandwidth f , (∆ ) therefore D is usually normalized for comparison of different devices.The concept of specific detectivity D* ( ) is introduced, and the specific formula is [127]: Response time (t f and t f ) and bandwidth ðΔf Þ.The response speed is to characterize the strength of the transient response effects of the device, and the rise time t r ( ) and fall time t f ( ) are used to compare the response speed of different devices [128].The corresponding incident light frequency is defined as the upper cutoff frequency f up ( ) when the maximum output current drops to 1 2 / of the initial maximum current, and the difference between the f up and the frequency for normal operation is defined as the bandwidth f .∆ 4.1.5.Linear dynamic range (LDR).The corresponding current range is defined as the LDR when the output current is linearly related to the power of the incident light.It is given as follows [129]: . 9 p d ( ) = 4.1.6.External/internal quantum efficiency (EQE and IQE).EQE is defined as the number ratio of electrons produced to the photons reaching the effective region per unit time of the device; IQE is the number ratio of electrons generated to the photons absorbed that reach the effective region per unit time of the device.They can be written as [130]:

Working mechanisms
The device architectures and working mechanisms vary depending on the application fields and the required key parameters.In this section, we will introduce several major mechanisms related to flexible PDs, which encompass the photoconductive effect, photovoltaic effect, and photogating effect.
4.2.1.Photoconductive effect.The photoconductive device comprises a photosensitive semiconductor and a pair of metal electrodes.Its working mechanism is the photoconductive effect (figure 10(a)).When the incident photon energy surpasses the band gap of the photosensitive materials, a substantial number of electron-hole pairs are generated and subsequently separated under an applied voltage, resulting in an increase in conductivity and output current [131].
The photoconductivity gain (G) is defined by the following equations:   The phototransistor shares a similar structure to field-effect transistors (FET), consisting of photosensitive materials, dielectric materials, and three electrodes: gate, source, and drain.Its working mechanism is based on the photogating effect (figure 10(c)), which takes full advantage of perovskite materials with high absorbance and high mobility.When incident light of a specific wavelength irradiates the device, only one type of carrier contributes to the photocurrents, while the other type of carrier becomes trapped in the photosensitive materials.These trapped carriers efficiently modulate the conductivity of the semiconductors through capacitive coupling [131].
The characteristics of phototransistors can be summarized in the following four aspects: (1) the channel conductance can be easily adjusted by varying the magnitude of the gate voltage under a given bias.(2) Free carriers can be recycled multiple times to obtain high gain.(3) The carrier transit time is long due to the trap effect in the perovskites, resulting in a slow response speed of the phototransistors.(4) There is no built-in electric field in the phototransistor.Therefore, the device can't be operated in self-driving mode.

Flexibility and reliability
Stress, encompassing intrinsic stresses and thermal stresses, plays a significant role in determining the performance of nanostructured perovskites devices [134].Specifically, when the device undergoes bending or stretching, the increase in intrinsic stresses contributes to heightened defects within the functional material, leading to an increase in leakage current.Simultaneously, difference in expansion coefficients between the functional layers and flexible substrates, as well as between the functional materials and the metal electrodes, result in escalating thermal stresses.The imposition of these combined stresses leads to the degradation of crucial parameters such as R, D * , and IQE/EQE.These detrimental effects pose a significant challenge to the optoelectronic performance and the long-term reliability of the PDs.
To ensures the sustained optimal functionality of PDs under conditions of bending, curling, compression, or stretching, it is crucial to carefully select appropriate substrates and engineer the electrode structure.Firstly, the flexible substrates must seamlessly conform to the functional materials, ensuring a precise fit to the substrate surface.Additionally, achieving lattice matching with the functional materials is crucial for optimal performance.It is noteworthy that a variety of flexible substrates, including metal foils, polymer films (PET, PI and PEN), and biodegradable films, have been widely employed in flexible PDs, showcasing a commendable level of flexibility and reliability [135][136][137].Secondly, flexible electrodes serve as a vital link connecting the functional materials within a device.To maintain high conductivity under diverse deformations, metallic materials can be ingeniously designed into various patterns, such as serpentine-honeycomb structure [138].Additionally, a range of nanostructured electrodes with excellent mechanical flexibility and electrical properties, including Ag NWs, graphene, and carbon nanotubes, have been extensively developed in flexible PDs [139][140][141].

Flexible PDs based on perovskite nanostructures
Up to now, numerous research groups have designed and implemented a variety of preparation methods and modification strategies for perovskite nanostructures-based flexible PDs.Various perovskite nanostructures are significant as building blocks for flexible PDs to meet users' expectations, which exhibit favorable characteristics of high absorption coefficients/carrier mobilities/dielectric constants, long carrier lifetimes/diffusion lengths, low exciton binding energies and tunable direct band gaps (from UV to NIR) due to unique electron-matter and light-matter interactions [142][143][144][145].In particular, the 0D nanostructures have high surface-to-volume ratios and size-tunable optoelectronic features, which can be uniformly distributed on substrates with high mechanical flexibility.Moreover, the 1D perovskite NWs have a regular and smooth morphology, exhibiting excellent carrier transport and mechanical flexibility in specific directions.Meanwhile, the 2D perovskite nanostructures have excellent in-plane directional carrier transfer, separation, and collection abilities, which are beneficial to efficient internal/external quantum efficiencies in the application of flexible PDs.In the case of 3D perovskite polycrystalline films, the area, thickness, and grain size of the perovskite films are extremely easy to regulate by the preparation process, and the large-area 3D polycrystalline films offer the possibility of growing patterned and multifunctional optoelectronic devices [146][147][148].
In this section, we delve into flexible PDs constructed from perovskite nanomaterials of diverse dimensions, focusing on the molecular composition, the range of detection wavelengths, as well as mechanical and optoelectronic properties.These aspects are succinctly summarized in table 2. Notably, flexibility and stretchability emerge as pivotal parameters for large-area flexible PDs.[149,150].

0D perovskite nanostructures-based flexible PDs
Zheng et al proposed an ultrahigh durability flexible PDs based on CsPbI 3 perovskite quantum dots (PQDs)/singlewalled carbon nanotube (SWNT) hybrid structure [135] (figures 11(a)(i)), where electron-hole pairs were generated in PQDs, and electrons would be trapped inside the CsPbI 3 PQDs, however, the holes would be transferred to the SWNT channels, leading to a significant optical detection gain.As shown in figures 11(a)(ii), he flexible hybrid devices based on PDMS templates exhibited strong flexibility and robustness under the 450, 535 and 635 nm incident wavelengths with a bending radius of 2 mm.The outstanding consistency and durability of the PD were confirmed in figures 11(a)(iii), and the photocurrent was maintained under 1000 cycles of stretching and bending cycles.In addition, the flexible PDs were successfully attached to the surface of a leaf for detecting the light intensity under sunlight illumination.
Shi and co-workers proposed highly flexible PDs based on CsPbBr 3 NCs through a ligand cross-linking strategy [136].The complex was bonded by the interaction of the F ions in the Perfluorodecyltrichlorosilane (FDTS) ligands with the Pb and Br ions in the CsPbBr 3 NCs.Besides, the hydroxyl groups in the FDTS facilitated the cross-linking of the CsPbBr 3 NCs, resulting in the formation of flexible and elastic CsPbBr 3 NCs devices on the flexible PI substrates.As depicted in figures 11(b)(i), the PDs possessed outstanding mechanical flexibility and detected UV light while maintaining a curved shape.For purified CsPbBr 3 NCs based PDs, the S and R decreased to half of their initial values when the bending radius was 2.5 mm, whereas there was no significant attenuation for the FDTS modified PDs (figure 11(b)(ii)).Likewise, the performance (both S and R) of purified CsPbBr3 PDs decayed to 30% after 5000 bending cycles.In contrast, the performance of the CsPbBr 3 @FDTS PDs retained 70% of their pristine state (figure 11  floated after bending at 60°(figure 11(c)(ii)).As displayed in figure 11(c)(iii), both photocurrent and dark current were maintained at 93% of their initial values after the 1600 bending endurance tests, suggesting good mechanical flexibility and reproducibility.Soft-X ray detectors have attracted serious concern of scholars because of their applications in medical diagnosis, radiological investigations, imaging.Liu et al developed a flexible and printable soft-X ray detector using all-inorganic CsPbBr 3 PQDs as the active material due to its low cost and high sensitivity to x-rays [151].A schematic of the flexible x-ray array PD was illustrated in figure 11(d)(i), in which the x-ray detector achieved a high sensitivity of 1450 μC Gy air −1 cm −2 at a low x-ray dose rate (0.0172 mGy air s −1 ) under 0.1 V. Besides, the configuration based on the PET substrate could be easily bent without destroying the structure of the functional layers.Furthermore, the photocurrent dropped to 75% of the initial value due to the reduction of the effective area at a bending angle of 120°(figure 11(d)(ii)).After 200 bending cycles, the device photocurrent decreased by only 12% (figure 11(d)(iii)), suggesting that all-inorganic PQDs flexible soft-X ray detectors with good stability and durability was promising for further development in the field of imaging technology.

1D perovskite nanostructures-based flexible PDs
Recently, Cao and co-workers developed ferroelectricityassisted MAPbI 3 NWs PDs for the simultaneous realization of flexible and self-powered functions through a simple imprinting process [152].Consequently, the device showed a high D * of 7.3 × 10 12 Jones with an outstanding R of 12.5 mA W −1 under 650 nm illumination.Note that the angle θ between the bending direction and the NWs orientation had a decisive influence on the performance of flexible PDs owing to the highly oriented 1D CH 3 NH 3 PbI 3 NWs.As displayed in figures 12(a)(i), there was no significant change in either the photocurrent or the dark current when θ was 0°.However, the photocurrents gradually decayed as the θ increased (figures 12(a)(ii) and (a)(iii)), which originated from the structural damage of CH 3 NH 3 PbI 3 NWs.Definitely, the response only showed a tiny decline after bending 200 times with the intersection angle of 0°(figure 12(a)(iiii)).In order to overcome the hindrance of optical absorption loss and interlayer defects in vertical heterojunction devices, Wang et al reported a novel lateral CsPbI 3 /CsPbBr 3 NWs heterojunction flexible PDs via in situ transition and stampassisted electrode printing process [153].A photograph of the CsPbI 3 /CsPbBr 3 NWs PD based on PEN substrate as depicted in figure 12(b)(i), which exhibited an obvious interference phenomenon.The photocurrent of the flexible PDs can be stabilized above 90% after 500 bending cycles (figure 12(b)(ii)), and the effect of bending angles (0 to 180°) on the device performance was almost negligible, in other words, the photocurrent can be maintained at more than 95% of the initial state (figure 12(b)(iii)).The results confirmed that lateral perovskite NWs heterojunction has great potential for flexible optoelectronics.
Wu et al proposed the addition of ionic liquid, 1-butyl-3methylimidazolium tetrafluoroborate (BMIMBF 4 ) to passivate defects and form nano-channels favorable for carrier transport in MAPbI 3 NWs [154].Under these, PDs with BMIMBF 4 additives exhibited ultrahigh stability and remarkable performance, maintaining 100% of its initial performance after 5000 h and reaching a maximum R, D * , and LDR of 37.14 A W −1 , 2.06 × 10 13 Jones and 160 dB, respectively.As shown in figure 12(c)(i), MAPbI 3 NWs with BMIMBF 4 additives grown on flexible PET substrates had good mechanical flexibility.Figure 12(c)(ii) showed that the flexible PDs could be switched reversibly at different light intensities from 1.45 × 10 −5 to 1.45 × 10 2 mA cm −2 , indicating its excellent photodetection ability.In addition, the R of the flexible PDs did not decay within 100 bending cycles at a bending radius of 27 mm, and the R remained at 86% of the initial state even after 1000 bending cycles (figure 12(c)(iii)).This result provided a reference for improving the performance of flexible perovskite NWs PDs.NWs PDs with and without the PMMA protective layer suggested significant photoconductive behavior under bending conditions, however, the device with the PMMA protective layer had a higher on/off ratio (figure 12(d)(ii)).Figure 12(d)(iii) demonstrated that the photocurrent of the flexible PDs gradually decreased, but the response speed remained constant as the bending radius varied from 0 to 1 cm −1 .As can be seen from figure 12(d)(iiii), the photocurrent of the CH 3 NH 3 PbI 3−x (SCN) x NWs PDs could be maintained at more than 90% of the initial value when the device was subjected to over 1500 consecutive bending tests.

2D perovskite nanostructures-based flexible PDs
Recently, various 2D perovskite NSs have been widely studied in flexible PDs.For instance, Zheng et al reported a lowtemperature insertion reaction method for the preparation of highly crystalline MAPbI 3 NSs on flexible PET templates.The flexible PDs relying on the as-grown MAPbI 3 NSs could simultaneously achieve a fast response speed of 2.2 ms as well as a high R of 12 A W −1 [156].The structure of the MAPbI 3 NSs flexible PDs with the photoconductance effect was shown in figure 13(a)(i).Note that photogenerated carriers would be separated, transported and collected by external bias when incident light was irradiated onto the MAPbI 3 NSs.A complete bending/recovery process was shown in the inset of figure 13(a)(ii), and both the photocurrent and the dark current of the PDs gradually decreased after 400 bending cycles, which was due to the inherent instability of the organic-inorganic MAPbI 3 perovskite towards oxygen and water.In addition, figure 13(a)(iii) showed that the photocurrent, on/off ratio, and dark current of the MAPbI 3 NSs flexible PDs fluctuated slightly after 400 bending cycles, indicating good bending stability and durability of these flexible PDs.
Jing et al designed an ultrathin MAPbI 3 NSs flexible PDs with ultrahigh R of 5600 A W −1 and ultrafast response time of 3.2 μs/9.2 μs.[157].The structure of the MAPbI 3 NSs device based on a flexible PET substrate was shown in figure 13(b)  (i), where the ultrathin (20 nm) MAPbI 3 NSs were synthesized by the limited growth space provided by two mica sheets.As the flexible PD was gradually bent from flat to a bending radius of 20 mm and then back to flat, the variation of the I-V curves of the device under 514 nm light illumination was shown in figure 13(b)(ii), noting that there was no significant loss of photocurrent, indicating that the bending behavior did not separate the MAPbI 3 NSs from the PET substrates.Furthermore, there was no obvious decay in photocurrent and on/off ratio of the MAPbI 3 NSs PD after 1000 bending cycles (figure 13 Kim et al proposed a chemical modification strategy for the synthesis of 2D perovskite passivation layers by posttreating FAMAPb(Br X I 1−X ) 3 films with organic halide saltphenylethylammonium iodide (PEAI), which could effectively passivate surface halide defects and withstand mechanical strains [159].As shown in figure 13(d)(i), 10 × 10 flexible PD arrays were prepared by placing 2D-3D FAMAPb(Br X I 1−X ) 3 active layers in the middle of two gold electrodes 75 μm apart.As shown in figures 13(d)(ii) and (iii), I-t curves of the flexible PDs with and without PEAI-treated were tested under different bending radii, compared to the devices with PEAI-treated, the untreated devices showed a significant loss of photocurrent at specific bending radii (0.5, 1.0, 3.0 and 5.0 mm).In fact, the on/off of the PEAI-treated device remained at 83.0% of the original value.However, the on/off ratio of the untreated device dropped dramatically to 23.0% at a bending radius of 0.5 mm (figure 13(d)(iiii)).The results demonstrated that PEAI post-treatment was highly effective in improving the mechanical flexibility of perovskite-based devices.

3D perovskite nanostructures-based flexible PDs
Perovskite films are the most widely and systematically studied nanostructures among all flexible PDs because of their low cost and low-temperature fabrication techniques.Wang and co-workers proposed the incorporation of poly (vinyl alcohol) (PVA) microscaffolds into FAPbI 3 films, which not only improved the stability of the organic-inorganic perovskite by blocking water molecules but also enhanced the mechanical flexibility of the FAPbI 3 PDs by self-healing process [160].As shown in figure 14(a)(i), cracks were created in the FAPbI 3 films along the bending direction, PVA accumulated at the grain boundaries where the cracks were created, and then the hydroxyl groups of PVA were activated upon absorption of water, resulting in the self-healing of the FAPbI 3 films in a few minutes.In addition, the whole process of self-healing was further confirmed by performing bending cycle tests and long-term stability tests.The R of the PVAtreated flexible PDs maintained above 70% of the initial value after 1000 bending cycles, which was much higher than the control PDs.In particular, the R of the PVA-treated PDs gradually recovered in an environment with 80% humidity, whereas the control PDs were completely destroyed after 5 h (figure 14 He et al reported the addition of the phenyl-ethyl ammonium (PEA+) into FASnI 3 films for passivating trap/ defect states and releasing crystal strains [162].The FA 0.8 PEA 0.2 SnI 3 PDs had a high R of 0.262 A W −1 , a large D * of 2.3 × 10 11 Jones, and a very fast response speed of 25 μs/42 μs.In order to investigate the mechanical flexibility of FA 0.8 PEA 0.2 SnI 3 PDs on PEN substrates, first, the R of the flexible devices was recorded at different bending angles (from −180°to 180°).It could be observed that the R is constant within an order of magnitude throughout the bending/recovery process (figure 14(c)(i)).At the same time, the mechanical durability test of the flexible PDs was performed for 10 000 bending cycles, and the R of the flexible device started to decrease after 100 consecutive bending cycles.In addition, it could be inferred that the response speed was not affected by the bending behavior, and the rise/fall time was maintained at ∼30 μs for 10 000 bending cycles (figure 14(c) (ii)).Remarkably, the flexible FA 0.8 PEA 0.2 SnI 3 PDs played a role in imaging applications.The flexible PD arrays were prepared with excellent flexibility by depositing electrodes with strip-like structure as shown in figure 14(c)(iii), which consisted of 5 × 5 pixels.The dark current and photocurrents were uniformly distributed in the range of 0.1-1 nA and 60-800 nA, respectively.The results demonstrated the great potential of FA 0.8 PEA 0.2 SnI 3 flexible PDs for high-resolution imaging (figure 14(c)(iiii)).
In order to solve the problem of difficulty in growing CsPbI 3 films on flexible ITO/PET substrates due to the hightemperature crystallization process.Qu et al proposed a strategy to greatly reduce the annealing temperature of CsPbI 3 films using 1D (APP)PbI 3 perovskite as templates and the flexible 1D/3D (APP)PbI 3 /γ-CsPbI 3 heterojunction PDs brought high EQE of 2377% under a low external bias [163].Figure 14(d)(i) showed the setup for bending tests with a 532 nm laser as the excitation light.As shown in figure 14(d) (ii), the R of the flexible PDs gradually diminished as the bending radius decreased, and the R was over 60% of the initial value even at a bending radius of 2.5 mm. Figure 14(d) (iii) indicated that the R remained above 90.2% and 80.3% after 20 000 bending cycles when the bending radii are 6.5 and 4.5 mm, respectively.Figure 14(d)(iiii) summarized the properties of flexible PDs based on various materials reported in recent years.It could be inferred that this device had optimal bending stability, which was attributed to the stable framework structure of the face-connected [PbI 3 ] − chains in 1D/3D (APP) PbI 3 /γ-CsPbI 3 perovskite heterojunctions.

Summary and outlook
In this review, we have thoroughly examined recent breakthroughs in nanostructured perovskites, spanning from various synthetic methods (including vapor-liquid-solid methods, wet chemical growth, spin-coating process, and chemical vapor deposition) to their multiplicate applications in flexible electronics.To date, flexible PDs employing perovskite nanostructures as photosensitive materials with various flexible templates, working principles, and device designs have sprung up like mushrooms.Notably, 0D perovskite nanostructures offer the advantage of facile and uniform distribution over large-area flexible substrates, rendering them suitable for selective light detection.However, it is worth noting that the R may be comparatively lower than those observed in flexible PDs of other dimensionalities.Optoelectronic devices hinging on 1D or 2D perovskite nanostructures exhibit exceptional photoresponsivity owing to their high crystallinity.Specifically, 2D perovskite nanostructures-based PDs boast remarkable mechanical flexibility and can withstand substantial deformations due to their ultrathin characteristics.Nevertheless, their response times typically fall within the range of milliseconds, as they necessitate the formation of hybridized heterojunctions or composites with other semiconductor materials.Notably, 3D perovskite thin films can be grown on a variety of flexible substrates owing to low-temperature solution preparation, allowing not only large-scale production but also integration of multiple functions.So far, plenty of image sensor devices have been successfully demonstrated.
All in all, certain reported flexible perovskite PDs exhibit advantages over conventional semiconductor counterparts in some aspects, confirming their reasonable commercialization potential.Nevertheless, there are several critical considerations for further advancement: (1) long-term stability and thermal stability.Perovskites, particularly organic-inorganic hybridized perovskites, are susceptible to degradation when exposed to air filled with moisture, heat, and oxygen.To propel the continued progress of flexible PDs based on perovskite nanostructures, it is imperative to pioneer advanced encapsulation engineering, interfacial defect passivation, and doping strategies.(2) Ensuring safety and reliability.Lead halide perovskites flexible PDs may pose potential risks when in direct contact with human skin, especially as wearable electronic devices.Therefore, there is an urgent need to find alternative materials from the huge library of lead-free perovskites and low-dimensional perovskites.In fact, not only perovskite materials, but also flexible templates, electrodes, and encapsulation layers should be considered for safety issues.(3) Large-scale and low-cost production.For instance, space-confinement is currently the most widely used method for synthesizing high-quality perovskite NWs, which usually consumes several days due to the slow diffusion of the precursors, hindering the realization of large-scale production.In addition, the growth of 2D perovskite NSs typically involves a CVD process, limiting its prospects due to the inability of flexible polymer templates to withstand high temperatures.Thus, there is a pressing need to explore preparation methods that can be integrated with established industries to facilitate large-scale and cost-effective production of flexible perovskite-based devices.(4) Machanical robustness.The performance parameters still fall short of the anticipated levels when the device undergoes deformation.Existing studies either involved a limited number of bending cycles at large bending radii or lacked comprehensive examinations of the tensile properties for flexible devices [135][136][137][151][152][153][154][155][156][157][158][159][160][161][162][163].This limitation arises from the inevitable formation of cracks in the perovskite functional films during repeated bending processes, leading to accelerated degradation of the perovskite materials and damage to device performance.Furthermore, weak adhesion between electrodes (e.g.metal grids, Ag NWs, graphene, and carbon nanotubes) and the perovskite functional layers, as well as between the perovskite functional layers and flexible substrates (e.g.PET, PI and PEN), results in severe flaking under small bending radii.To align key parameters with those fabricated on rigid substrates, the effective strategy includes the following two segments: (i) interfacial engineering: coating a capping layer or inserting a film that can form chemical bonding between different layers to enhance adhesion.(ii) Chemical engineering: developing new fabrication techniques to reduce the thickness and limit the growth orientation of perovskites, Additionally, designing deformable physical geometries of electrodes and substrates to meet multi-functional applications.Moreover, to facilitate fair comparisons of mechanical durability across different devices, there is a critical need for uniform measurement standards.In conclusion, perovskite nanostructures-based materials have shown promising potential in the field of flexible PDs with great achievements in recent years.However, more efforts are needed to achieve better long-term reliability, which is expected to revolutionize the optoelectronics market in a major way.

Figure 4 .
Figure 4. (a) Illustration depicting the strong coupling of exciton-photon and polariton in cavities, along with the SEM image of CsPbBr 3 NWs prepared by CVD.Reprinted from [73] with permission (Copyright 2018, American Chemical Society).(b) CVD growth process and the corresponding SEM image of Pb-In-based perovskite films.Reprinted from [74] with permission (Copyright 2022, John Wiley and Sons).(c) CVD growth process and the SEM image of the mixed CsPbBr 3 /Cs 4 PbBr 6 perovskite films.Reprinted from [75] with permission (Copyright 2023, John Wiley and Sons).

Figure 5 .
Figure 5. (a) Crystallization sequence for FAPbBr 3 NCs films prepared by LARP method.Reprinted from [78] with permission (Copyright 2018, American Chemical Society).(b) Comparison of PL spectra and XRD patterns of FASnI 3 NCs synthesized via LARP and SLRP methods.Reprinted from [79] with permission (Copyright 2023, AIP Publishing).(c) Schematic diagram and spectral analysis of Mn: Cs 2 NaBiCl 6 NCs via the LARP method under ambient conditions.Reprinted from [80] with permission (Copyright 2023, John Wiley and Sons).
(b))[94].Furthermore, Vasavi Dutt et al introduced a post-treatment method involving benzoic acid for CsPbBr 3 perovskite NCs to enhance PL stability in the green band.This treatment involved passivating surface defects by coordinating of carboxylate groups with ligandsurface Pb atoms.As depicted in figure 6(c), The PL decayed by only 36% over a year[95].Note that various other approaches have also been reported for the growth of 0D perovskite materials.For instance, Wei et al proposed a generalized ternary-solvent-ink strategy to fabricate green CsPbX 3 QD QLEDs, achieving a remarkable EQE of 8.54% (figure6(d)) [96].In 2023, Que et al developed a novel method for passivating defects on the surface of FAPbBr 3 QD films by passivating Pb salts and employing damage-free drying processes.The results showed successful filling of both Pb 2+ and Br − vacancies, and high-quality FAPbBr 3 QD films were obtained by this process.The open-circuit voltage (V oc ) of the PbBr 2 -treated device reached 1.67 V due to fewer defects on the surface of FAPbBr 3 QD films (figure 6(e)) [97].

− 1
(a)), at the same time, the excess iodine played a beneficial role in repairing the iodine vacancy defects during the formation of 2D perovskites NSs [112].Furthermore, Feng et al successfully synthesized Cs 3 Bi 2 I 9 NSs with a thickness of 8 nm by a self-template-oriented process (figure 8(b)).The heterojunction Cs 3 Bi 2 I 9 /CeO 2 −3:1 constructed through simple electrostatic self-assembly exhibited excellent photocatalytic activity for the reduction of CO 2 coupled with oxidation of H 2 O.It also demonstrated a high electron consumption yield of 877.04 μmol g −1[113].In addition, Li and co-workers demonstrated that 2D BA 2 MA n PbnBr 3n+1 perovskite NSs efficiently produced photosensitized singlet oxygen ( 1 O 2 ) (figure8(c)) and catalyzed the ENE and 1,2-addition reactions of olefins with 1 O 2[114].In 2023, Liu et al reported a blue emission from pure brominated perovskites achieved by incorporating the ideal spacer cation L-arginine for in situ formation of low-dimensional NSs.L-arginine played a crucial role in promoting the formation of perovskite NSs due to its strong interaction with the [PbBr 6 ] 4 − octahedral layer.Furthermore, the carboxyl group in L-arginine effectively passivated the uncoordinated Pb 2+ ions.As a result, the blue LED with L-arginine-additive displayed a peak luminance of 2152 cd m −2 and an EQE of 5.4% (figure8(d))[115].
[70].Lead-free perovskites, such as Sn-based halide perovskites, have recently gained the attention of numerous researchers due to their similar properties to lead-based counterparts.Zhu et al introduced trimethylthiourea (3T) into the precursor of FASnI 3 perovskites, leading to a substantial improvement in the morphology of the resulting FASnI 3 thin films.As illustrated in figure 9(b), the precursor solution containing the 3T ligand resulted in smoother and denser FASnI 3 films compared to the control group.Furthermore, solar cells based on FASnI 3 films with the 3T ligand exhibited a notably high open-circuit voltage of 0.92 V [71].

4. 1 .
Performance parameters 4.1.1.On/off ratio ðηÞ.The on/off ratio ( ) h of the device is defined as the ratio of the photocurrent I p ( ) to the dark current I d ( ) under the irradiation of incident light, that is [121]: Among them, I d is mainly derived from noise, which is related to the material nature, material quality, electrode contact, and other factors.In addition, large I p require efficient conversion of photons to electron-hole pairs and fast separation.

4. 1 . 2 .
Responsivity (R).R is an important parameter that characterizes the ability of PDs to convert an optical signal
μ and V represent the average carrier lifetime, carrier transit time, channel length, carrier mobility, and external voltage, respectively[132].Achieving a high gain necessitates a long , lifetime t and a short .transit t Consequently, the gain and response speed of the photoconductive PDs generally involve a trade-off.

4. 2 . 2 .
Photovoltaic effect.The photovoltaic device employs a vertical structure, which is composed of photosensitive materials and top and bottom electrodes.As depicted in figure 10(b), when the incident light irradiates the device, excitons generated in the junction region separate under the built-in electric field.Subsequently, the separated electrons and holes drift towards the two electrodes and generate photocurrent.Photosensitive materials should possess high carrier mobility and lifetime, along with balanced electron and hole mobility.In addition, the photosensitive materials are also required to meet the principle of energy level matching[133].In comparison to photoconductive PDs, photovoltaic PDs offer three key advantages: (1) the existence of junction barriers effectively reduces the probability of electron-hole pairs recombination, thereby suppressing dark current and achieving high D * and LDR.(2) The carriers can be quickly separated under the built-in electric field, resulting in a faster response speed.(3) Photovoltaic PDs can operate without external bias, significantly reducing the power consumption of the devices.However, their R are not as high as those of photoconductive or photogating PDs due to the absence of internal gain.

Figure 10 .
Figure 10.(a) Schematic of the photoconductive effect and its typical I ds -V ds curves.(b) Schematic of the photovoltaic effect and its typical I ds -V ds curves.(c) Schematic of the photogating effect and its typical I ds -V ds curves.Reproduced from [131] with permission (Copyright 2015, Royal Society of Chemistry).
(b)(iii)), demonstrating enhanced flexibility and mechanical stability through cross-linking strategy.The study of flexible PD arrays has also grown rapidly in recent years.Shen et al proposed a broadband flexible PD utilizing CsBr/KBr-assisted CsPbBr 3 PQDs deposited on flexible ITO/PET substrates via a facile one-pot synthesis strategy [137], the flexible PDs showed a high R of 10.1 A W −1 , and a large D * of 9.35 × 10 13 Jones at selfpowered mode.As shown in figure 11(c)(i), the flexible PD possessed good flexibility.Specifically, the photocurrent retained 94% of its initial value while the dark current barely

Figure 11 .
Figure 11.(a) (i) Schematic of the reticulated SWNT/CsPbI 3 PQD PDs.(ii) Photocurrent of the flexible PDs with various bending radii.(iii) Photocurrent testing after 1000 stretching and bending cycles, respectively.Reprinted from [135], with permission (Copyright 2019, Royal Society of Chemistry).(b) (i) Schematic of CsPbBr 3 @FDTS PDs.(ii) On/off ratios (S) and R of the purified CsPbBr 3 PDs and CsPbBr 3 @FDTS PDs in (ii).various bending radius, and (iii) various bending times.Reprinted from [136].CC BY 4.0.(c) (i) Photograph and schematic of flexible CsPbBr 3 PQDs PDs arrays.(ii) I-t curves of flexible PD arrays with bending angles of 0°and 60°, respectively.(iii) I-t curves of flexible PDs arrays after the various bending times.Reprinted from [137].CC BY 4.0.(d) (i) Schematic and photography of the flexible CsPbBr 3 NCs x-ray detector arrays.I-V curves of the flexible device (ii) at various bending angles and (iii) various bending times.Reprinted from [151], with permission (Copyright 2019, John Wiley and Sons).

Figure 12 .
Figure 12.(a) I-t curves of flexible MAPbI 3 NWs PDs when the bend angle θ at (i) 0°, (ii) 45°, and (iii) 90°, respectively.(iiii) Bending cycles at the θ of 0°and the bending angle of 120°.Reprinted from [152] with permission (Copyright 2019, John Wiley and Sons).(b) (i) Photograph of the flexible CsPbI 3 /CsPbBr 3 NWs PDs.(ii) The bending test with different bending cycles and (iii) different bending angles, respectively.Reprinted from [153], with permission (Copyright 2020, John Wiley and Sons).(c) (i) Photograph of the flexible PD based on PET substrates.(ii) I-t curves under various light intensities.(iii) Normalized R before and after different bending cycles.Reprinted from [154].CC BY 4.0.(d) (i) Measurement set up for flexible CH 3 NH 3 PbI 3−x (SCN) x PDs under bending conditions.(ii) I-V curves of the flexible PDs with and without PMMA protective layer.(iii) The photocurrent and the response time for different bending curvature.(iiii) The photocurrent under different bending cycles.Reprinted from [155], with permission (Copyright 2022, John Wiley and Sons).
(b)(iii)).Meanwhile, its R and D * remained quite stable over 1000 bending cycles (figure 13(b) (iiii)), suggesting that the ultrathin MAPbI 3 NSs PDs yielded good mechanical flexibility and bending reliability.2D single crystal perovskite can be applied to detecting circularly polarized light (CPL) owing to the intrinsic chirality.Wang and co-workers proposed a method to synthesize chiral quasi-2D [(R)-β-MPA] 2 MAPb 2 I 7 by incorporating chiral β-methylphenethyl ammonium into the structure [158].The CPL-sensitive flexible PDs based on the above [(R)-β-MPA] 2 MAPb 2 I 7 possessed comprehensive performance.Specifically, the device had a high R of 1.1 A W −1 , a large D * of 2.3 × 10 11 Jones, and an anisotropy factor of 0.2. Figure 13(c)(i) displayed the schematic diagram of the 2D [(R)-β-MPA] 2 MAPb 2 I 7 CPL flexible PDs with a lateral structure.Figure 13(c)(ii) demonstrated that the [(R)-β-MPA] 2 MAPb 2 I 7 flexible PDs possessed ultra-low dark current of 1.8 × 10 −12 A, which originated from the low free carrier density of the 2D layered materials.As shown in figure 13(c)(iii), the photocurrent of the flexible device remained constant at different bending angles (from 0°to 60°).At the same time, the photocurrent and anisotropy factor exhibited less than 10% attenuation after 100 bending cycles (figure 13(c)(iiii)).Revealing remarkable mechanical flexibility and robustness.
(a)(ii)).Recently, Vuong et al presented a strategy to grow MA 3 Bi 2 I 9 films and mixed halide analogs MA 3 Bi 2 I 6 Br 3 and MA 3 Bi 2 I 6 Cl 3 on flexible PET substrates by chemical vapor deposition [161].MA 3 Bi 2 I 6 Cl 3 -integrated PDs obtained a high R of 0.92 A W −1 and a large D * of 2.9 × 10 13 Jones in selfpowered mode.The flexible and self-powered MA 3 Bi 2 I 6 Cl 3 PDs with a vertical structure were shown in figure 14(b)(i), where CuI was used as the hole transport layer while SnO 2 was employed as the electron transport layer.The photocurrents of the flexible PDs were tested under different strains, and the photocurrents maintained 95% of the original values at 5.86% bending strain (figure 14(b)(ii)), and the corresponding parameters of R and D * decreased slightly under various strains (figure 14(b)(iii)).At the same time, the photocurrent maintained 75% of the initial value after 5000 bending cycles at 5% bending strain (figure 14(b)(iiii)), and the corresponding R and D * maintained 85% of the original value (figure 14(b)(iiiii)), showing good mechanical stability and mechanical durability.

Figure 13 .
Figure 13.(a) (i) Schematic diagram of the MAPbI 3 NSs PDs based on flexible PET.(ii) I-t curves of the flexible PDs after different bending cycles.(iii) The photocurrent, on/off ratio, and dark current of the flexible PDs after different bending cycles.Reprinted from [156], with permission (Copyright 2017, Royal Society of Chemistry).(b) (i) Schematic diagram of the ultrathin MAPbI 3 NSs PDs based on flexible PET.(ii) The relationship of I−V curves versus bending radius under 514 nm light illumination.(iii) I−t curves, and (iiii) the R and D * of the flexible PDs under 1000 bending cycles.Reprinted from [157], with permission (Copyright 2020, American Chemical Society).(c) (i) Schematic diagram of the CPL-sensitive flexible PDs based on [(R)-β-MPA] 2 MAPb 2 I 7 .(ii) I-V curves of the flexible PDs.(iii) I-t curves at different bending angles.(iiii) The relationship of photocurrent and anisotropy factor (g Iph ) versus bending cycles.Reprinted from [158], with permission (Copyright 2020, John Wiley and Sons).(d) (i) 3D exploded view of the FAMAPb(Br X I 1−X ) 3 flexible PDs arrays.The relationship of I-t curves versus bending radius of the flexible PDs arrays (ii) without and (iii) with PEAI treatment, respectively.(iiii) The relationship of on/off ratio versus bending radius of the flexible PDs arrays.Reprinted from [159], with permission (Copyright 2022, American Chemical Society).

Figure 14 .
Figure 14.(a) (i) Schematic diagram of the self-healing process for FAPbI 3 films.(ii) Variation of R in bending cycle tests and long-term stability tests.The RH is 5% and 80% for dry and humid environments, respectively.Reprinted from [160], with permission (Copyright 2021, John Wiley and Sons).(b) (i) Schematic diagram of the MA 3 Bi 2 I 6 Cl 3 flexible PDs.(ii) Photocurrent of the flexible PDs under different bending strains.(iii) The R and D * under different bending strains.(iiii) Photocurrent under different bending cycles at 5% strain.(iiiii) The R and D * under different bending cycles.Reprinted from [161], with permission (Copyright 2023, Elsevier).(c) (i) The relationship of R versus bending angles.(ii) The relationship of R and response time versus bending cycles (fixed bending angle of 120°).(iii) The pictures of the flexible FA 0.8 PEA 0.2 SnI 3 PD arrays.(iiii) The dark and photocurrent of the FA 0.8 PEA 0.2 SnI 3 PD arrays.Reprinted from [162], with permission (Copyright 2023, John Wiley and Sons).(d) (i) Schematic diagram of the setup for bending tests.(ii) The relationship of R versus bending radius.(iii) The relationship of R versus bending cycles with different bending radii.(iiii) Summarized flexibility of reported flexible PDs based on various materials.Reprinted from [163], with permission (Copyright 2023, John Wiley and Sons).

Table 1 .
Summary of the synthesis, morphology, and property of perovskite nanostructures.

Table 2 .
Summary of perovskite flexible PDs in different dimensions in terms of the elemental composition, target wavelengths, optoelectronic properties, and mechanical properties.