Recent advances in perovskites-based optoelectronics

2.1.1 3D perovskites

In order to prepare 3D perovskites, there are two synthetic approaches that can be utilized, “bottom-up” and “top-down” [39,40,41]. The first approach is more commonly used, mainly by adjusting the composition at the molecular or atomic level, and synthesizing the final product through solution routes such as sol–gel chemical solution deposition and hydro/solvothermal synthesis. This approach has low costs and does not damage the sidewalls of the structure. However, because particles are distributed irregularly, it is difficult to obtain a regular size distribution. The top-down approach mainly uses focused ion beam milling or photolithography methods to carve away the bulk ferroelectric material and create coherently and continuously ordered nanostructures. The advantage of this process is that the size and shape of the synthesized nanostructures can be controlled precisely, and the drawback is the synthesis speed, and it is not suitable for volume patterned nanostructures [42]. The preparation methods of 3D perovskite and 3D hybrid perovskite are briefly investigated below.

A new tin-based 3D perovskite {en}FASnI₃ (en = ethylenediammonium, FA = formamidinium) (Figure 2a) was proposed and prepared by Ke et al. By introducing a cation, the stability of the tin-based perovskite in the atmospheric environment can be significantly improved, as shown in Figure 2b, and the optoelectronic properties of the absorber prepared by the material are also optimized [43]. More recently, in order to obtain perovskite semiconductor devices that can work stably at above room temperature, an A-site cation [CH₃PH₃]⁺(methylphosphonium, MP) is employed to fabricate a lead-free 3D ABX₃ organic–inorganic halide perovskite semiconductor MPSnBr₃. Attributed to the large volume and heavy mass of MP cation, MPSnBr₃ has a high Curie temperature [44]. At above room temperature, this material exhibits obvious ferromagnetic properties and the direct band gap of it is 2.62 eV. In addition, MPSnBr₃, as a multiaxial molecular ferroelectric, has 12 ferroelectric polar axes, much higher than other materials of the same type [45]. Due to this special feature, the polarization in each grain can be switched more easily between multiple directions to achieve an excellent ferroelectric performance [46]. Furthermore, Körbel et al. have screened the periodic table of the elements for hybrid organic–inorganic halide perovskites via high-throughput density-functional theory calculations [47]. By extracting the band gap energy and the effective masses, they have found that MPSnI₃ is one of the most promising compounds for PVs. It has a smaller band gap (1.18 eV) than MPSnBr₃, indicating more efficient light harvesting. In another study by Ozório et al., a density functional theory investigation of the MPSnI₃ perovskite phases was reported [48]. By calculating the enthalpy of formation, they found that MPSnI₃ has a higher thermodynamic stability at 0 K compared with the FASnI₃ phase. However, due to the weaker structural cohesion, the MPSnI₃ structures can be more affected by moisture and oxygen-rich environment. Currently, the research on MPSnI₃ is only in the theoretical calculation, and its preparation and experiment still need to be further studied. Without organic ligand and toxicity elements, a facile antisolvent method at room temperature is introduced by Wu et al. to synthesize novel lead-free CsAgCl₂ perovskite microcrystals, which exhibit excellent air, thermal, and light stability [49]. Cao et al. designed a 3D star-shaped polymer (polyhedral oligomeric silsesquioxane-poly(trifluoroethyl methacrylate)-b-poly(methyl methacrylate) (PPP)) as a novel modulator to regulate perovskite film crystallization, as shown in Figure 2c [50]. The core of the star-shaped PPP can offer the PPP-based perovskite 3D structure stability. Some researchers found that cation engineering in 3D perovskite absorbers can lead to reduced degradation. As a consequence, two-dimension (2D) Ruddlesden–Popper phase layered perovskites are explored to improve device stability. Take (RNH₃)₂(A) _n−1BX _3n+1 as an example, RNH₃ are large alkylammonium cations. Wang et al. prepared a kind of 2D–3D heterostructured perovskites by introducing n-butylammonium cations into FA_0.83Cs_0.17Pb(I _y Br_1−y )₃ 3D perovskites, as shown in Figure 2d. The use of this material can improve the performance and stability of PSCs [51]. Inspired by the 2D-layered perovskites where the A site molecule is substituted by large alkyl ammonium cations, Fan et al. innovatively introduced a controlled amount of one-dimension (1D) 2-(1H-pyrazol-1-yl)pyridine (PZPY) into the 3D perovskite precursor system to synthesize heterostructural 1D–3D perovskite for preparing 1D–3D perovskite solar cells, as shown in Figure 2e, which possess thermodynamic self-healing ability, high efficiency, and long-term stability [52].

Figure 2

(a) Crystal structure of {en}FASnI₃. (b) Under constant conditions, an aging test of {en}FASnI₃-based unencapsulated solar cells with and without 10% en. Reproduced with permission from ref. [43]. Copyright 2017, American Association for the Advancement of Science. (c) Structural formula of PPP polymer for fabricating PPP-based 3D structure perovskite, reproduced with permission from ref. [50]. Copyright 2021, American Association for the Advancement of Science. (d) 2D–3D schematic diagram of heterostructured butylammonium-caesium-formamidinium lead halide perovskites, reproduced with permission from ref. [51]. Copyright 2017, Springer Nature. (e) Schematic diagram of the heterostructural 1D–3D hybrid perovskite film by introducing PZPY into lead halide 3D perovskites, reproduced with permission from ref. [52]. Copyright 2018, Wiley-VCH.

2.1.2 Low-dimension perovskites

Compared with 3D perovskites, low-dimension perovskites exhibit unique electrical and optical characteristics such as quantum confinements, anisotropic geometry and large surface to volume ratio, which make them promising candidates for next generation electronic and optoelectronic applications. Therefore, most efforts are made to investigate the synthesis of low-dimensional perovskites with high-quality, including 2D nanoplatelets (NPLs), 1D nanowires (NWs), and zero-dimension (0D) QDs.

For 2D materials, the mechanical exfoliation method is an easy-to-operate preparation method, but it can only be used for the preparation of layered perovskites, there is no way to control the morphology of the product, and the yield is low. In order to overcome these shortcomings, solution-phase growth and chemical vapor deposition (CVD) are more used in the preparation of 2D perovskites [5]. In 2014, Ha et al. presented for the first time a synthesis method for perovskite family NPLs with thickness from several atomic layers to several hundred nanometers, which has sizes from 5 to 30 μm and an electron diffusion length exceeding 200 nm. The corresponding schematic of the synthesis setup using a home-built vapor-transport system is shown in Figure 3a [53]. To crystallize MAPbI₃ rapidly at room temperature without the participation of strongly coordinating aprotic solvents, a solvent system with a low-boiling point and low viscosity was introduced by Noel et al. By using this kind of solvent, pinhole-free films with uniform coverage and compactness can be manufactured, which exhibits good photoelectric properties [54]. Another method of anti-solvent-extraction technology can also be employed to prepare uniform pinhole-free CsPbI₂Br film, which possesses good stability, even in a high-temperature pressure environment and continuous light infiltration under the conditions of maximum power point tracking. Figure 3b shows the solvent engineering procedure for preparing the uniform and dense perovskite film [55]. With the need for the actual application, the large-scale synthesis of high-quality perovskite NPLs is desired. Tong et al. proposed a versatile, polar-solvent-free, single-step approach without a polar solution, which can synthesize CsPbX₃ (X = Br or I) NPLs on a large scale, and the NPL thickness can be tuned by direct ultrasonication to the related precursors [56].

Figure 3

(a) The schematic of the synthesis setup for 2D perovskite nanoplate using a home-built vapor-transport system, reproduced with permission from ref. [53]. Copyright 2014, Wiley-VCH. (b) The solvent engineering procedure for preparing the uniform and dense inorganic CsPbI₂Br perovskite film, reproduced with permission from ref. [55]. Copyright 2018, Elsevier. (c) A clear crystal structure transformation in the synthesis process of the all-inorganic cesium lead bromide halide, reproduced with permission from ref. [57]. Copyright 2018, Wiley-VCH. TEM images of CsPbBr₃ PNCs synthesized at (d) 120°C (NPLs) and (e) 185°C (nanocubes), reproduced with permission from ref. [60]. Copyright 2021, American Chemical Society. (f) Schematic of 2D/3D perovskite film formed via the dimensionally graded perovskite formation method, reproduced with permission from ref. [61]. Copyright 2021, Springer Nature. (g) Schematic reaction process of monolithic CH₃NH₃PbI₃ grain from the precursor to monolithic perovskite grains, reproduced with permission from ref. [62]. Copyright 2019, The Royal Society of Chemistry.

With the continuous deepening of perovskite research, researchers are pursuing high purity and high-stable 2D perovskite [57,58,59]. Duan et al. proposed a multistep solution-processing strategy to synthesize CsPbBr₃ perovskite films with high purity. The phase conversion of perovskite can be achieved by adjusting the number of deposition cycles of a CsBr, which is conducive to the formation of monolayer and vertical-aligned grains. During the synthesis process, a clear crystal structure transformation can be observed, shown in Figure 3c [57]. Studies have shown that retarding the crystallization rate of perovskite can improve the stability and efficiency of the perovskite-based device [58,59,60,61]. A method of hydrogen bonding formed by adding poly(vinyl alcohol) (PVA) to the FASnI₃ perovskite precursor solution was introduced to retard the crystalline rate of perovskite by controlling the growth of FASnI₃ perovskite, thereby reducing the trap density of the resulting FASnI₃-PVA perovskite film and improving its compactness. Meanwhile, it exhibited striking long-term device stability [59]. Another method of a one-pot hot injection is employed by controlling the reaction temperature to synthesize CsPbBr₃ NPLs, then neighboring NPLs make facet-to-facet contact and then fuse into larger 2D NPLs (2–5 times) without defects, which is used to fabricate photoconversion device with long-term performance stability. Figure 3d and e shows two different dimensional CsPbBr₃ perovskite nanocrystals (PNCs) synthesized at different temperatures [60]. In 2021, Yang et al. proposed a multifunctional 2D perovskite passivation approach named the dimensionally graded perovskite formation approach to prepare low-photovoltage-loss PSCs and enhance the stability of the solar-cell device. The schematic of 2D/3D perovskite film formed via this approach is shown in Figure 3f [61]. To further grow large-grain and less-defect perovskite films, Yang et al. introduced ammonium benzenesulfonate (ABS) into MAPbI₃ precursor, as shown in Figure 3g. During the entire reaction process, the presence of the ABS can slow down the crystallization process and improve the film quality. In addition, positive and negative charged defects are also effectively passivated for the zwitterion-structured ABS which remains in the perovskite [62]. Chu et al. combined large cation ethylammonium with PEA₂(CsPbBr₃)₂PbBr₄ (PEA = phenylethylammonium) perovskite. Through decreasing the Pb-Br orbit coupling and increasing the band gap for blue emission, efficient and spectra stable blue perovskite LEDs were fabricated successfully [63].

The synthesis method of 1D perovskite can be divided into solution‐phase synthesis, vapor-phase synthesis as well as combined solution-phase and vapor-phase synthesis methods. For example, Yang et al. reported a simple and easily scaled synthesis method at room temperature without any organic solvent and expensive alkyl halide to prepare 1D organic–inorganic hybrid perovskite micro-belt (AD) Pb₂Cl₅ (AD = acridine), and the process is shown in Figure 4a. The obtained low-dimension perovskite has high water stability and luminescent properties, and exhibits optical properties such as up-conversion fluorescence, polarized photoemission, and optical waveguide performances with a low loss coefficient during propagation. It has great potential to be applied in optical communication micro-devices [64]. Lately, Hu et al. synthesized a new 1D chiral hybrid perovskite material through a solution process, which was determined to be a previously unknown low-dimensional hybrid perovskite (R)-(−)-1-cyclohexylethylammonium/(S)-(+)-1 cyclohexylethylammonium) PbI₃ shown in Figure 4b [65]. The exploration of this type of material will help to understand some phenomena such as photo-galvanic effects, electric field, and chiral enantiomer-dependent Rashba-Dresselhaus splitting. In addition, water-stable 1D hybrid lead-free tin and lead halide perovskites were synthesized by a solution process. The SnCl₂·2H₂O and 1,8-diamino octane (2:0.5) were added to a mixed solution of hydrogen iodide and hypophosphorous acid (H₃PO₂) in a volume ratio of 5:8 to prepare (DAO)Sn₂I₆ (DAO, 1,8-octyldiammonium), which is water stable for more than 15 h [66]. Latest, a one-pot solution process anisotropic growth was introduced to synthesize super long monocrystalline CsPbBr₃ perovskite wires, which exhibit well-defined morphology and a high aspect ratio over 10⁵ [67]. The optical microscope photograph for in situ monitoring wire growth is shown in Figure 4c. In contrast, 1D perovskites prepared from the vapor-phase synthesis method have less defect density and higher crystallinity. In 2017, Chen et al. first reported the direct epitaxial growth of CsPbBr₃ NWs and microwires with controlled crystallographic orientations on both p-mica and m-mica, which was carried out in a home-built CVD reactor, the related A schematic illustration is shown in Figure 4d [68]. The wires typically have a width of ∼1 μm and a length of tens of micrometers. Very recently, a non-catalytic CVD growth method was introduced by Hossain et al. to prepare all-inorganic CsPbX₃ perovskite NWs [69]. Besides, mixed preparation methods, such as solution–vapor growth, solution–vapor–solid growth were studied to make high-quality perovskite NWs, which enabled the growth of anisotropic perovskites [70]. Figure 4e shows a schematic of the process to synthesize hybrid perovskite NWs.

Figure 4

(a) The facile synthesis process of organic–inorganic hybrid perovskite-AD crystals in aqueous solution at room temperature, reproduced with permission from ref. [64]. Copyright 2019, The Royal Society of Chemistry. (b) A diagram of the structure of the 1D perovskite, reproduced with permission from ref. [65]. Copyright 2020, American Association for the Advancement of Science. (c) Optical microscope photograph for in situ monitoring wire growth, reproduced with permission from ref. [67]. Copyright 2021, Wiley-VCH. (d) A schematic illustration of the epitaxial growth of CsPbBr₃ NWs on mica, reproduced with permission from ref. [68]. Copyright 2017, American Chemical Society. (e) Schematic of the solution process to fabricate PbI₂ microwires and the vapor phase conversion process to transfer PbI₂ into hybrid perovskite NWs, reproduced with permission from ref. [70]. Copyright 2016, The Royal Society of Chemistry.

The solid-state reaction method is a traditional method for synthesizing perovskite nanopowders, but in order to obtain homogenized colloidal nanocrystals (NCs), the liquid-phase synthesis method is widely used. Among them, the hot injection (HI) method and the ligand-assisted reprecipitation (LARP) method are the two most developed 0D perovskite fabrication methods [71]. In 2015, Protesescu et al. proposed a HI method that can synthesize the solution-processed monodisperse CsPbX₃ (X = Cl, Br, I, mixed Cl/Br, or Br/I systems) NC [72]. NCs are also known as nanocrystal QDs. This research shifts the focus from hybrid organic–inorganic lead halides MAPbX₃ to the previously unresearched all-inorganic cesium lead halide perovskites (CsPbX₃) colloidal nanomaterials. However, the unpurified QDs obtained by this method will be transformed into the orthorhombic phase within a few days. In 2016, Swarnkar et al. improved the preparation method reported by Protesescu et al. and stabilized the CsPbI₃ QDs in the cubic phase in an ambient environment by purifying the QDs [73]. The prepared a-CsPbI₃ QDs are phase-stable for months in ambient air and even at cryogenic temperatures. Lately, Yang et al. presented a modified HI method of one-step synthesis strategy for CH₃NH₃PbBr₃ PQDs. The colloidal solution obtained by this method has bright green emission, and the particle size of the PQDs is less than 10 nm [74]. These strategies can improve the stability or optical properties of PQDs to a certain extent. Yet, the synthesis of PQDs by HI requires a high temperature, inert environment, and some other conditions [75].

LARP is another popular synthesis method because it can be carried out in an atmospheric environment through an ordinary mixing process without heating equipment or inert gas protection. Guo and co-authors employed 3-aminopropyltrimethoxysilane as ligands to prepare large-scale stable CsPbX₃ PQDs at room temperature [76]. Figure 5a is the preparation diagram of CsPbBr₃ PQDs solid powder. In order to further overcome the poor stability of 0D perovskites, Guo and co-authors proposed thioacetamide-ligand-mediated synthesis to prepare novel CsPbBr₃-CsPbBr₃ homostructured NCs through a facile room-temperature reprecipitation method [77]. Because of consuming a large amount of toluene and N, N-dimethylformide, LAPP might lead to low stability, a mixture of morphologies [78]. For the sake of improving the quality and properties of 0D perovskite, many other methods were explored for the synthesis of high-quality and high-performance perovskite with few defects. For instance, an atomic exchange method was introduced by Chiba et al. to prepare red PQDs, which converted the original green CsPbBr₃ QDs into red QDs through the halide-anion-containing alkyl ammonium and aryl ammonium salts, as shown in Figure 5b [79]. The material exhibits a strong red shift and higher PLQY from green emission to a deep-red emission at 649 nm. Another approach of comprehensive defect suppression in perovskite NCs, that is, an ideal one-dopant alloying strategy was designed by Kim et al., which can produce monodisperse colloidal perovskite NCs with smaller sizes and fewer surface defects [80]. As is well-known, the synthesis of large-scale complex NCs is difficult. In 2021, a microfluidic system, as shown in Figure 5c, was applied by Bao et al. for a simple, continuous, and stable synthesis of large-scale CsPbBr₃/Cs₄PbBr₆ complex NCs with a high PLQY of up to 86.9% [81].

Figure 5

(a) Preparation diagram of CsPbBr₃ PQDs solid powder, reproduced with permission from ref. [76]. Copyright 2021, Elsevier. (b) Schematic diagram of anion exchange based on long alkyl ammonium and aryl ammonium, reproduced with permission from ref. [79]. Copyright 2018, Springer Nature. (c) Schematic diagram of the microfluidic system for producing CsPbBr₃/Cs₄PbBr₆ PNCs, reproduced with permission from ref. [81]. Copyright 2021, Elsevier.

2.2.1 Optical properties of perovskites

Perovskite is a direct band gap semiconductor, which exhibits strong absorption of up to 10⁵ cm⁻¹ from UV to visible light. Thus, an extremely thin perovskite film can achieve complete light absorption, which is beneficial to carrier collection and further design of device structure. In addition, the perovskite band gap is within 1.4–3.0 eV, and the light absorption and emission can just cover the entire visible light region, that is why perovskite has broad application prospects. Figure 6 shows the corresponding spectra of different perovskite materials in the visible range [72], in which the spectral range of organic perovskite MAPbX₃ is 390–790 nm, and the spectral range of inorganic perovskite CsPbX₃ is about 400–710 nm. According to the corresponding calculation, the band gap of the perovskite is in the range of 1.4–3.0 eV.

Figure 6

Spectral absorption and emission ranges of different perovskite materials in the visible light region, reproduced with permission from ref. [82]. Copyright 2017, Wiley-VCH.

As the composition changes, the perovskite absorption and fluorescence spectra can be continuously adjusted. For instance, Protesescu et al. developed a series of CsPbX₃ (X = Cl, Br, I) perovskite nanoparticles (NPs) with different components and realized the continuously adjustable fluorescence spectrum from 410 to 710 nm in the visible light range (Figure 7) [72]. Figure 7a shows the tunable luminescence color of the perovskite solutions under the excitation light of 365 nm. Figure 7b and c shows the adjustable optical absorption and emission spectra of CsPbX₃ in the entire visible spectrum by adjusting their composition and particle size. According to Figure 7b, it can be observed that the half-width of the perovskite PL peak is very narrow, between 12 and 42 nm, which fully proves that the material has high monochromaticity. The optical performance adjustment between the same type of perovskite can be achieved through ion exchange, and this can also be achieved between different types of perovskites. Wang et al. prepared FA_0.33Cs_0.67PbBr_3−x I _x (0 ≤ x ≤ 3) organic–inorganic hybrid perovskite through the solution method and adjusted its PL performance by changing the proportion of halogen ions [83].

Figure 7

(a) The luminescence color of the perovskite solution under UV light (λ = 365 nm). (b) The PL fluorescence peak position change of the corresponding component (except for the CsPbCl₃ excitation wavelength of 350 nm, the rest are all 400 nm). (c) The absorption spectrum of the corresponding component. Reproduced with permission from ref. [72]. Copyright 2015, American Chemical Society.

Compared with 3D materials, low-dimensional semiconductors such as QDs and 2D NWs have different optical properties. Under the quantum confinement effect, QD-perovskites and 2D nano-perovskites have variable PL peak positions and absorption edges and have better light stability than 3D-structured perovskites. In addition, PQDs have the advantage of defect tolerance. Although the system has a large number of inherent structural defects, its optical properties still have unique advantages. In recent years, there have been many research reports on PQDs. Including all-inorganic materials and organic–inorganic hybrid materials, almost all types of PQDs have been reported. Furthermore, some experts have discovered that a well-designed hybrid 2D/3D perovskite is a combination of ideal optoelectronic properties and moisture resistance stability, which can cover the advantages of 2D and 3D perovskites.

For perovskite nanomaterials, because of the existence of quantum effects, the optical properties of the perovskite nanomaterials will change to a certain extent with the thickness of the perovskite nanomaterials. Taking MAPbI₃ as an example, Liu et al. synthesized MAPbI₃ nanosheet structures with different thicknesses by solution method and tested PL of samples with different thicknesses [84]. The results show that as the nanosheets increase from a single layer to 10 layers, the PL peak position has a significant redshift from 724 to 755 nm. From a single layer to a bulk phase, the band gap shift can reach 100 meV, which indicates that the thickness of the perovskite nanosheet has a great relationship with its optical properties.

2.2.2 Electrical properties of perovskites

Perovskite has strong carrier transport capacity and higher carrier mobility comparable to GaAs and Si inorganic materials. The carrier mobility of polycrystalline thin-film perovskite is 1–30 cm² V⁻¹ s⁻¹. Because of high purity, few grain boundaries, and low defect density, mobility can reach more than 200 cm² V⁻¹ s⁻¹ in the perovskite single-crystal material [85].

Yang et al. observed that the ion conductivity in MAPbI₃ was significantly higher than the electronic conductivity, indicating the importance of ion transport in perovskite [86]. They tested the electrical data obtained by pure phase doping with a small amount of MAPbI₃. Based on a variety of characterization methods, Senocrate et al. finally determined that the conductive ion in MAPbI₃ is I⁻, that is, the ion conductance in perovskite is mainly contributed by halogen ions [87]. Due to the increase in I⁻ vacancies caused by Na⁺ doping, both the ion conductivity and the electronic conductivity increased by more than one order of magnitude after doping, so it can be inferred that I⁻ plays an important role in ion conductivity. In addition, the conductivity of perovskite is not only related to temperature and I₂ partial pressure but also related to light. Kim et al. found that under light conditions, compared with the dark state, the electronic conductance and ion conductance of perovskite increased significantly, and the ion conductance increased by three orders of magnitude [88]. That is because light effectively increases the chemical potential of I⁻, making it more prone to migration. As the light intensity increases, ion conductance and electronic conductance will continue to increase.

3.4.1 Optical neuromorphic devices based on perovskite materials

Generally, optical neuromorphic devices based on perovskite materials can be mainly divided into two categories: two-terminal memristors and three-terminal transistors. Therefore, we will investigate the research progress of two-terminal and three-terminal optical neuromorphic devices based on perovskite materials in this chapter.

In 1971, when Leon Chua studied the relationship between charge, current, voltage, and magnetic flux, he first proposed the concept of the fourth type of passive device memristor [136]. Due to a lack of empirical evidence, the relevant theory has not attracted attention in the past few decades, until General Electric Research (GER) laboratory Hickmott first reported resistance switch characteristics of the aluminum/alumina/aluminum structure in the 1960s [137]. In the report and study before 2008, most memristors have the mega-ohm resistor sandwich structure using the same metal as a top and bottom electrode. The research on the intermediate function layer focused on the complex perovskite metal oxide [138]. However, the enthusiasm of the researchers for the materials has waned due to complex processing and compatibility issues. In recent years, with the appearance of perovskite halides with simple process and strong compatibility, coupled with their excellent optical properties, the research of perovskite-based photo-memristor has been put on the schedule.

A photonic flash memory based on all-inorganic PQDs was demonstrated by Wang et al. [10] The optical programmable and electrical erasable properties of the implement are based on the heterostructure formed between the CsPbBr₃ QDs and the semiconductor layer (Pentacene/PMMA/CsPbBr₃). Figure 15a is a schematic diagram of synapse structure and artificial synaptic device based on CsPbBr₃ QDs. In addition, this device not only successfully simulates the characteristics of photon enhancement and electrical habit but also simulates the phenomena of spike-rate-dependent plasticity, long-term plasticity, and short-term plasticity in synapses by optical modulation. Figure 15b shows the excitatory postsynaptic current (EPSC) of CsPbBr₃-based synaptic devices, which simulates the phenomenon of memory decline, confirming that stronger memories can be retained in the brain as long-term memories. Figure 15c displays the reliable potentiation-depression functions of CsPbBr₃-based synaptic devices under the photonic pulse (365 nm, 0.153 mW cm⁻², 1 s duration with 10 s interval) and electric pulse (−20 V, 10 ms duration with 1 s interval). Yang et al. demonstrated an ITO substrate-based inorganic perovskite photonic artificial synapse with a two-terminal construction, which more closely resembles the structure of biological synapses [11]. Figure 15d is a typical human brain learning and memory model. The new memory will be temporarily encoded into the hippocampus as short-term memory. Figure 15e confirms the dependence of current on light intensity and illumination time in human visual memory simulated by artificial optical synapses. Moreover, the extra charge transfer absorption of TAPC (an organic matter) doped molybdenum oxide membrane gives the device a transparent appearance. The device responds to light by emitting UV light vertically from the substrate, while performing dual-mode operation under UV and red-light irradiation. This device not only achieves high transparency but also high flexibility. These features help to improve the integration of 3D stacked memristors.

Figure 15

(a) Schematic diagram of synapse structure and artificial synaptic device. (b) Presynaptic application of 365 nm photon pulses to achieve EPSCs under light pulses of different intensities. The device is read at 0.2 V. (c) Reliable potentiation-depression functions of CsPbBr₃-based synaptic devices under photonic pulses and negative electric pulses. Reproduced with permission from ref. [10]. Copyright 2018, Wiley-VCH. (d) Learning and memory models of short-term and long-term memory in the human brain. (e) The letter “U” composed of 3 × 3 pixels and coded according to different light intensities and light times. Reproduced with permission from ref. [11]. Copyright 2021, Wiley-VCH. (f) Schematic diagram of the synaptic cleft connecting the axon of the pre-neuron and the dendrites of the posterior neuron (top) as well as the schematic diagram of the electrical and chemical signals in the synaptic cleft (bottom). (g) Statistical analysis of V _SET and V _RESET for the OHP synaptic devices. Schematics of the suggested mechanism in the absence (h) and presence (i) of light for the perovskite-based photonic memristor. Reproduced with permission from ref. [139]. Copyright 2019, Wiley-VCH.

In biological nerves, synapses are the interconnecting parts of the axons of the pre-neurons and the dendrites of the posterior neurons, and can be used as transmission channels for converting chemical signals into electrical signals (Figure 15f). To build a device with basic synaptic functions such as short-term potentiation, long-term potentiation (LTP), and long-term depression, Ham et al. demonstrated a photonic memristor founded on double-ended organic lead halide perovskite (OHP) with the structure of Ag/CH₃NH₃PbI₃/ITO [139]. Statistical analysis of the OHP synaptic device is shown in Figure 15g, from which it can be seen that the V_SET and V_RESET are about 0.29 ± 0.03 and –0.31 ± 0.11 V, respectively. In addition, the authors investigated the corresponding conductive switching mechanism without (Figure 15h) and with (Figure 15i) light, and the MNIST pattern recognition based on the perovskite-based photonic memristor in detail. Not only that, the threshold of LTP was further reduced when light was exposed to the OHP synaptic device. This increases the number of available intermediate states and reveals the potential mechanism of light influence on the device. Lin et al. proposed to extract organic–inorganic hybrid perovskite material and use it as a resistive layer to prepare non-volatile memory, and the device structure was ITO/PEDOT: PSS/organic–inorganic hybrid perovskite/Cu [140]. The device exhibits excellent electrical bistability and a nonvolatile rewritable memory effect. The new special structure enables it to have light response behavior. Using electric field and light as input sources, the device is proved to have the function of electronic memory and optical induction logic circuit.

CsPbBr₃ QDs were applied to memristors and an asymmetric electrode structure of memristors was prepared [141]. This device can be used to create non-volatile memristor functions with photon modulation and integrated synaptic plasticity tailoring behavior. The device can be programmed and erased by the input bias voltage. In addition, the device exhibits synaptic plasticity and sensitivity to low UV light. Yang et al. proposed to apply the ITO/PMMA/PQDs: PMMA/PMMA/Ag structure based on CH₃NH₃PbBr₃ perovskite quantum dots (PQDs) to nonvolatile memory [74]. The preparation process of QD colloidal solution uses the centrifugal method, which makes the perovskite particle size greatly reduced. The PL peak moves from 552 nm to 535 nm, and the full-width half peak of the PL spectrum decreases from 25 nm to 18 nm, resulting in the quantum confinement effect. Meanwhile, the switching current ratio of the device is greater than 10³ with good reproducibility, reliability, and flexibility. The preparation and synthesis method of QDs are also simple and easy.

An all-inorganic non-volatile memory cell was fabricated by using Cs₄PbBr₆ thin film by Cai et al., which was prepared by the low-temperature synthesis method [142]. The device structure was Au/Cs₄PbBr₆/PEDOT:PSS/Pt, and Cs₄PbBr₆ was used as an insulating layer. This novel structure enables the device to have the light response behavior reflected by the resistance state change. The formation and annihilation of the bromide ion vacancy filaments lead to the logical “OR” function of the device, which can be observed by applying bias voltage and illumination as input signals. Zhang et al. used all-inorganic materials to prepare non-volatile resistive random-access memory (RRAM) with the structure of Au/PMMA/PMMA:CsPbBr₃ NCs/PMMA/ITO. The fabricated device exhibits a change in the formless bipolar high and low resistance ratio from 10⁶ to a stable 10 [143]. In the high resistance state and high resistance ratio, the device shows a fast optical response to the light wavelength from 365 nm to 500 nm, that is, the resistance value changes. At a low resistance switching ratio, the starting voltage of the resistance switch can be controlled by illumination. The work shows that the adjustable resistance switch is realized by light irradiation, and the light response is embodied by the change in the resistance state.

When it comes to three-terminal neuromorphic devices, a synaptic transistor with ionic-liquid-gated SrFeO _x (SFO) films by exploiting the continuous topotactic phase change reported by GE’s group is a good example [144]. Ionic-liquid-gated SFO films play an important role in the structure. The structure shown in Figure 16a is responsible for insulation, while conductivity is due to the filling of oxygen atoms (Figure 16b). Because of the good reversibility of the device, the resistive transition is realized by the transformation between the insulating brownmillerite-SFO phase and the conductive perovskite-SFO phase. LTP and long-term depression (LTD) were successfully simulated by applying pulses of different polarities. Symmetric and asymmetric spike-timing-dependent plasticity (STDP) have also been successfully implemented. Besides, the neural network constructed by the device is able to achieve high recognition accuracy.

Figure 16

The schematic crystal structures of brownmillerite SrFeO_2.5 (a) and perovskite SrFeO_3−δ (b) thin films in the SrTiO₃ substrate. Reproduced with permission from ref. [144]. Copyright 2019, Wiley-VCH. (c) The proposed multi-gate transistor as artificial synapses. Reproduced with permission from ref. [145]. Copyright 2020, Wiley-VCH. (d) Schematic of emulating a biological synapse by IGZO/perovskite NPs/IGZO TFT. Reproduced with permission from ref. [146]. Copyright 2021, American Chemical Society.

Periyal with his team fabricated the IGZO/ITO/CsPbBr₃/PMMA device with a type-II heterostructure, which is formed by spin-coating CsPbBr₃ QDs (≈ 60 nm) on sputtered IGZO (≈60 nm) thin films [145]. Figure 16c is the proposed multi-gate transistor as an artificial synapse. The device makes photoelectric programming and decoupling optical absorption and charge transport possible. Under dark conditions, the devices exhibit typical N-type enhanced mode operation. While being illuminated, the device behaves in depletion mode. By applying a positive (negative) voltage, holes (carrier electrons) move toward the IGZO-CsPbBr₃ interface, resulting in an increase (decrease) in conductivity. The conductance of the device can be adjusted flexibly by the pulse of different parameters, and various synaptic functions can be simulated. Different learning rules can be realized by adjusting the pulse waveform. This is because the wavelength, intensity, and pulse width of optical stimulation all affect the conductance of the device and the weight of the stimulated synapses. Electrical perturbation results in short-term potentiation and, while optical perturbation results in LTP.

Duan et al. further fabricated a photo-synaptic thin-film transistor (TFT) based on the IGZO/perovskite (CsPbBr₃) NP composite active layer (Figure 16d) [146]. In aspects of absorbability, perovskite NPs had a broad peak located at ∼520 nm. And for perovskite NPs, an emission peak located at 515 nm can be observed in the PL spectrum. Besides, it is reported that more oxygen vacancies ionized in oxide semiconductors can promote the persistent photoconductivity effects and the negative bias illumination stress stability. The device successfully simulates the typical functions of biological synapses. By adjusting the number of pulses, the device can behave as short-term plasticity and long-term plasticity. Also, paired-pulse facilitation (PPF) phenomena that the second pulse takes much less time than the first to reach the same current can be observed, which is consistent with the law of learning and relearning in the human brain. Compared with pure IGZO devices, the composite TFTs have higher performance and lower power consumption, which is a significant step forward.

3.4.2 Computation of perovskite photonic neural morphology

Inspired by dopamine promoting synaptic behavior, Ham et al. designed and fabricated a photonic synapse based on double-ended OHP, in which synaptic plasticity was altered by electrical impulses and light exposure [139]. When the light was applied to the device, the threshold of long-term enhancement was lowered and synaptic weight was further modulated. These factors allow for higher-order tuning of synaptic plasticity, which can accelerate learning at lower power levels in neuro-inspired hardware architectures.

Sun et al. demonstrated the plasticity of photoelectric synapses based on 2D lead-free perovskite ((PEA)₂SnI₄) and demonstrated several basic synaptic functions of the photoelectric synapses [12]. The intensity of synaptic connections can be effectively modulated by varying the duration, irradiance, and wavelength of the light spikes. In addition, the electrical and optical properties of 2D perovskites can be rapidly modulated through chemical engineering such as composition control, increasing the complexity and freedom required for neuromorphic calculations.

To construct photoelectric synaptic devices, Yin et al. applied the hybrid structure formed by OHP (MAPbI₃) and Si NPs to transistors [13]. The device is very sensitive to light stimulation and can simulate various functions of biological synapses under low-energy consumption light stimulation. Among them, the tunability of EPSC is used to simulate visual learning and memory processes in different emotional states, which is of great significance for the development of silicon-based neuromorphic computation. Hao et al. combined perovskite CsPbBr₃ QDs with organic semiconductor materials and applied them in transistors as photonic synapses to realize functions. Synaptic responses include tunable synaptic integration behaviors that implement the “AND” and “OR” optical logic functions [147]. By lighting an array of synapses with different densities of light to change the weight of the synapses, the team successfully simulated an artificial vision system. Ma et al. fabricated photoelectric synapses using all-inorganic perovskite nanosheets and investigated the plasticity of electronic and photonic synapses, respectively [148]. The device has excellent optical response properties and successfully simulates multifunctional synaptic functions of the nervous system, including pair impulse facilitation, short-term plasticity, long-term plasticity, a transition from short-term to long-term memory, and learning and experiential behavior. In addition, the photoelectric synapse exhibits a unique memory recall function that can extract historical photoelectric information, a detail previously overlooked in this area of research.

A novel neuromorphic optoelectronic device based on a vertical van der Waals heterojunction phototransistor of colloidal 0D-CsPbBr₃-QDs/2D-MoS₂ heterojunction channel was proposed by Cheng et al. [149]. The device is photoresponsive and exhibits classical outstanding features such as excitatory postsynaptic current, pairwise impulse facilitation, dynamic time filtering, and phototunable synaptic plasticity. In a simple synaptic network, using synaptic plasticity, the efficiency of tunable photoelectronic Pavlovian associative learning and photoelectronic hybrid neuron encoding behavior were successfully realized by the photoelectric collaboration method.

By growing PQDs directly from the graphene lattice, Pradhan et al. prepared G-PQDs superstructure materials, which compensated for the weak charge transfer performance of PQDs [29]. The synaptic function was successfully realized in the photoelectric transistor. The experiment proved that after machine learning, the photon synapse successfully realized the facial recognition function through neural network calculation.

In Table 2, we have a summary of the optical memristive devices based on perovskites. From what has been discussed above, it can be seen that photonic memristor and neural morphology calculation based on perovskites have made great progress and remarkable achievements. Then, there is still a long way to go to satisfy the application of all-optical computing and neuro network. Therefore, great efforts will be made to promote the rapid development of perovskites in photonic memristive devices to meet the exponential growth of data storage and processing as well as brain-inspired synaptic and artificial intelligence.

For a three-terminal transistor, in Feng’s work, a vertical ITO transistor based on the sodium alginate (SA)-based biopolymer electrolyte surface is fabricated [150]. The device less than 10 nm breaks through the ultra-short channel technology for the first time, and can be used to simulate the pain-perceptual nociceptors and the sensitization-regulated nociceptors. Therefore, it has a non-negligible prospect in the bionic robot with the requirements of regulating pain threshold, pain perception, sensitization behavior, and so on. For example, the pain threshold can be adjusted by adjusting the length of the transistor channel (channel layer thickness).

Van der Waals heterostructures have so good interfacial charge transfer characteristics that they are able to solve the problem of insufficient light absorption of single semiconductor materials. Therefore, Cheng’s team developed a novel photoelectrically modulated neuromorphic device based on an ion-coupling gate-tunable vertical 0D-CsPbBr₃-QDs/2D-MoS₂ hybrid-dimensional van der Waals heterojunction architecture [151]. The Boolean logic operation of “AND” and “OR” is completed by adjusting the presynaptic stimulus. In addition to the simulation of typical synaptic functions (such as STDP and PPF), the device can also simulate dendritic structures in biological synapses and achieve dendritic integration behaviors. The appearance of such devices has made great contributions to the development of the intelligent cognitive system and photoelectric neural computing.

Similarly, Xie et al. fabricated 0D-CsPbBr₃-QDs/2D-MoS₂ mixed-dimensional heterojunction transistors capable of simulating biological visual adaptation [152]. Different frequencies and intensities of photoelectric cooperative stimulation have a key impact on the adaptive precision, sensitivity, inactivation, and desensitization of the device. For photoelectric neural devices, the exploration of devices of this kind is of great significance to the construction of artificial vision system and the manufacture of bionic robots.