The three-dimensional (3D) lead halide perovskites (LHPs) are enjoying rapid developments as novel opto-electronic materials since their emergence in photovoltaic applications in 2009. LHPs belong to the larger material family of metal halide perovskites (MHPs), which exhibit very similar structure with CaTiO₃ and could be generally represented by the formula of ABX₃. X represents the halide anion forming octahedrons, which could be Cl⁻, Br⁻, and I⁻ [1]. B represents cation locating in the center of octahedron, which could be Pb²⁺, Sn²⁺, and Ge²⁺ in the more generalized cases of MHPs [2,3,4] but restricted to be Pb²⁺ for LHPs. A represents the cation locating in the vacancy of neighboring octahedron cages, which could be either organic or inorganics: the inorganic examples include Cs⁺ [5], while the organic examples include the methylammonium [6,7], formamidinium [8,9], and recently discovered methylhydrazinium [10,11]. It is expected that inorganic counterparts generally maintain better chemical stability than their organic counterparts [12]. When specified to the 3D LHPs, the materials have advantageous properties from more than one aspect, and people are eager to utilize them as working materials in various opto-electronic devices. In addition to solar cells [1,13], the materials are also utilized in lasers [7,14], light emitting diodes (LEDs) [15,16,17], photodetectors [18,19,20], modulators [21,22,23], and so on. Moreover, the materials, especially those with bromides, are found to exhibit outstanding nonlinear properties [10,24,25,26,27], thus demonstrating promising potential in applications such as imaging, optical limiting, and frequency conversion. Currently, people are incorporating the LHPs into waveguides and micro optical cavities, so as to further miniaturize the LHP-based devices. These efforts are leading to the emergence of integrated photonic/optical systems centering LHPs, which will play a significant role in next-generation opto-electronic applications, resulting in devices not only frequently used in the field of scientific research but also that of consumer electronics. In Figure 1, the crystalline structure of LHP is presented in the center, which is surrounded by examples of potential applications for the LHP-based devices.

In addition to the prosperity of LHP material, novel opto-electronic materials including the atomically thin two-dimensional materials (2D) and one-dimensional (1D) materials are also rapidly growing. These materials, each with their unique properties in certain aspects, are also at the center of attention in the research community. LHPs, along with other materials, are all building blocks to construct devices. To some extent, people are free to choose from this “material bank” to fabricate devices and achieve better performances. In most cases, it is quite rigorous that a single species of material could cover all the application demands and introduce no impediments at the same time. On the other hand, it is more practical to combine different materials into the device and make them collaborate such that the advantages from the participants could be utilized, and the shortages could be made up to some extent. The LHP could be fabricated by solution-based methods and conveniently incorporated with semiconductors, metals, and polymers, making it very practical to construct LHP hybrid devices. By now, people have developed LHP hybrid devices covering lasers, photodetectors, modulators, and so on. The materials in collaboration with LHP include novel materials (for instance, graphene and 2D MoS₂, etc.) and traditional materials (for instance, silicon). In these devices, the advantageous properties of participating materials are exploited, and the device performances have been improved substantially as compared to the devices made from single species of material. It is beneficial to retrospect these valuable works, which could light up more inspiration and enlightenment for future development in LHP hybrid devices.

2. The Properties of the LHP Materials

2.1. Optical and Electrical Properties of LHP

The LHPs are considered as outstanding candidates for next-generation opto-electronic applications, mostly due to the fact that the materials demonstrate outstanding optical and electrical properties in various aspects, as shown in Figure 2. The LHPs are direct band-gap semiconductors, and the band-gap could be conveniently tuned by chemical composition (for instance, changing the halogen elements) and thus enabled to cover a wavelength range from visible to near-infrared [28,29]. The LHPs have large absorption coefficients. Taking CH₃NH₃PbI₃ (MAPbI₃) as an example, its absorption coefficient is found to be in the level of ~10⁵ cm⁻¹ in the visible range, being much larger than GaAs [30]. A large absorption coefficient, as well as a widely tunable bandgap, will contribute to a high-performance in photodetectors. LHPs also have high luminescence quantum yields. For solution-processed MAPbI_3-xCl_x crystalline films as an example, a high photoluminescence (PL) quantum efficiency up to 70% has been reported [31]. MAPbI₃ films have been found to demonstrate an outstanding gain of 3200 ± 830 cm⁻¹, which is comparable to conventional light-emitting materials [32]. The high luminescence quantum yield along with the tunable band-gap, makes LHPs ideal candidates for applications including lasers and LEDs.

Figure 2. Optical and electrical properties of LHPs. (a) Representative photoluminescence (PL) spectra for CsPbX₃Y_3−z, where X and Y represents different halide elements, demonstrating a wide wavelength tunability for LHP. Reprinted with permission from [33]. Copyright 2015 American Chemistry Society (b) The absorption coefficient (α) of CH₃NH₃PbI₃ (MAPbI₃), CsSnI₃ and GaAs. Reproduced with permission from [30].; published by Wiley, 2014. (c) The photoluminescence quantum efficiency (PLQE) of MAPbI_3-xCl_x films. The inset shows the normalized PL intensity (integrated from 1.71 to 1.57 eV) and the transient absorption (TA) signal versus time for device and film samples. Reprinted with permission from [31] Copyright 2014 American Chemistry Society. (d) Absorption coefficient of MAPbI₃ perovskite films, for which the absorption onset is at 1.55 eV. Reproduced with permission from [32].; published by Wiley, 2015. (e,f) mobility and diffusion of MAPbI₃. In both panels, the black dots represent experimental data, and the regions marked by different colors from left to right correspond to orthorhombic, tetragonal, and cubic phase, respectively. The dashed lines in (e) is T^−3/2 dependence prediction for band-like transport, and in (f) is an eye-guide. Reproduced with permission from [34]; published by Wiley, 2015.

LHPs are found to have high carrier mobility, long diffusion length, and low trap-related recombination rates [35,36,37]. These properties govern carriers’ transport dynamics which are important in applications such as photodetection. Carrier mobility and diffusion length are closely related to each other, and these two parameters show similar dependence on factors including crystalline quality and chemical composition [35]. The crystalline quality seems to be a dominant factor, and the mobility/diffusion length is much larger for single-crystal samples than its film counterparts. For instance, the hole mobility has been found to be ~164 cm²/(Vs) in solution-processed single-crystalline characterized using space charge limit current (SCLC) technique [38,39], which goes to ~0.4 cm²/(Vs) in poly-crystalline films [40]. The carrier diffusion length in MAPbI₃ single crystals could be more than 175 μm [39], which could be reduced to 0.1 μm in MAPbI₃ films [41]. Chemical composition tuning also enables the alteration of mobility and diffusion length. For instance, the diffusion length for MAPbI_3−xCl_x is more than 1 μm [40], while in CH(NH₂)₂PbI₃ (FAPbI₃) films the effective diffusion length for electrons and holes are 177 ± 20 nm and 813 ± 72 nm, respectively [42]. When the film processing condition and procedures are optimized, the diffusion length could be further elongated [43,44]. The dependence mentioned above could be finally attributed to intrinsic factors such as Fröhlich interactions and extrinsic factors such as charge-carrier scattering by grain boundaries and dopants [45].

2.2. Intrinsic Instability of LHP

Although LHPs have novel properties leading to outstanding working performances in opto-electronic industry, the massive usage of the materials in practical occasions are mainly hindered by the severe instability of LHPs. As shown in Figure 3, moisture, oxygen, elevated temperature, and light radiation are found to induce material degradation quite easily [46,47,48]. As for moisture and oxygen which is generally not necessary for device function, the corresponding degradation could be generally dealt with by insulation. For instance, it is required that the preparation of LHP–based solar cells should be in a relatively low humidity level of <1% atmosphere [49]. By careful encapsulation or introduction of hydrophobic contacts [50], device lifespan could be improved considerably. Unlike moisture and oxygen, heat is likely to accompany the device operation in some applications. The degree of heat-induced degradation is found to be dependent on chemical composition. For instance, while MAPbI₃ will degrade at 85 ℃ 

[51], CsPbI₃ could endure much higher temperature, showing stability up to 300 ℃