Lead-Free Perovskite Single Crystals: A Brief Review

Abstract

Lead-free perovskites have received remarkable attention because of their nontoxicity, low-cost fabrication, and spectacular properties including controlled bandgap, long diffusion length of charge carrier, large absorption coefficient, and high photoluminescence quantum yield. Compared with the widely investigated polycrystals, single crystals have advantages of lower trap densities, longer diffusion length of carrier, and extended absorption spectrum due to the lack of grain boundaries, which facilitates their potential in different fields including photodetectors, solar cells, X-ray detectors, light-emitting diodes, and so on. Therefore, numerous research focusing on the novel properties, preparation methods, and remarkable progress in applications of lead-free perovskite single crystals (LFPSCs) has been extensively studied. In this review, the current advancements of LFPSCs are briefly summarized, including the synthesis approaches, compositional and interfacial engineering, and stability of several representative systems of LFPSCs as well as the reported practical applications. Finally, the critical challenges which limit the performance of LFPSCs, and their inspiring prospects for further developments are also discussed.

1. Introduction

As a striking material, lead halide perovskites (APbX₃) have made unprecedented progress in various fields, such as photodetectors, solar cells, X-ray detectors, light emitting diodes, lasers, transistors, and so on [1,2,3,4,5,6]. The merits of low-cost solution processing and remarkable optoelectronic properties, including tunable bandgap, long carrier lifetime and carrier diffusion length, large absorption coefficient, give lead halide perovskites great potential in the photovoltaic power generation field [7,8,9]. Single-junction perovskite solar cells have realized a certified power conversion efficiency (PCE) of 25.5%, which is comparable to that of silicon-based solar cells [10]. In terms of light emitting, perovskites exhibit a narrow full width at half maximum, high photoluminescence quantum yield (PLQY), and wide color gamut [11]. Meanwhile, photodetectors, transistors, and lasers are also developed rapidly.

However, the severe toxicity and chronic degrading of lead (Pb), the aqueous solubility may cause the contamination of ground water, and the poor stability when exposed to oxygen, heat, moisture and UV light, has retarded the expanded applications of lead halide perovskites [12,13,14]. Although numerous nontoxic elements have been reported as dopants, the residual Pb may still present environmental risk. Hence, the development of low-toxic lead-free perovskites is of great significant to replace the classic APbX₃ [15]. Meanwhile, in comparison with polycrystalline perovskites and low-dimension perovskites, perovskite single crystals (PSCs) show excellent optoelectronic properties due to their continuous and unbroken crystal lattices [16], the absence of grain boundaries leads to lower trap densities, longer length for carrier diffusion, and extended absorption spectrum [17,18,19]. Therefore, research of LFPSCs has promoted the enhancement of perovskite materials, and the current high-quality LFPSCs play critical roles in abundant optoelectronic devices. LFPSCs materials are a series of compounds with a general chemical formula of A_xB_yX_z (x, y, z is up to the structural dimensionality), where A represents an organic or inorganic cation such as MA (CH₃NH₃), FA (HC(NH₂)₂, Cs, Rb, B represents a metal cation (Sn/Sb/Bi/Pd/In/Ti/Pt/Au Cu/Ag), and X represents halide anion (Cl/Br/I). In the metal halide octahedra, B-cation stay at the center of the octahedral, and six X-anions are situated at the six corners, which can grow three-dimensional (3D), two-dimensional (2D), one-dimensional (1D), or zero-dimensional (0D) crystal structures [20,21]. The dimensionality of the perovskite crystal structures mainly depends on the size of the cations and should also fulfill the requirement of Goldschmidt tolerance factor (t),

$$\mathrm{t}=\frac{r_A+r_X}{\sqrt{2}\times \left(r_B+r_X\right)}$$

where, r_A, r_B, and r_X are the ionic radius of the A-site, B-site, and halide site, respectively. LFPSCs can be classified into four categories based on their crystal structure and the valency of the B cation: (i) divalent metal cation perovskites: ABX₃ (B is +2 oxidation state, B = Sn/Ge/Yb) or layered perovskites A₂A′_n−1B_nX_3n+1; (A′ = long chained organic cations that do not fit in the [BX₆]⁴⁻ cavity); (ii) trivalent metal cation perovskites: A₃B_2×9 (B is +3 oxidation state, B = Sb/Bi); (iii) tetravalent metal cation perovskites: A₂BX₆ (B is +4 oxidation state) (B = Sn/Ge/Pd/Pt); and (iv) double perovskites: A₂BB′X₆ (B is +1 while B′ is +3 oxidation states) (B is Au/Ag/ACu; B′ is In/Bi/Sb) [12,22].

Though a number of efforts have been devoted to develop various LFPCs, a lack of comprehensive understanding towards the synthesis methods, properties, and the device performance still restrict their practical application. To promote the development of LFPSCs, we mainly focus on summarizing the up-to-date advancement of several representative PSCs and their applications in photodetectors, photovoltaics solar cells, X-ray detectors, light-emitting diodes (LEDs), and other devices [23]. To be specific, various systems including replacement of Pb-based perovskites, perovskite-like derivate and double perovskites are presented. Strategies for development and stabilization of LFPSCs lattice, improvement of optoelectronic performance based on fabrication process, compositional, structural, and interfacial engineering are also discussed. In the end, we provide the challenges and further prospects of LFPSC.

2. Various Systems of Pb-Free Single Crystal

In general, lead halide perovskites possess a universal chemical formula of APbX₃, where A represents an organic/inorganic cation including Cs⁺, methylammonium (MA), formamidinium (FA) or their mixture, and X represents a halide anion which consists of Cl⁻, Br⁻, I⁻, or their mixture. In terms of structure, Pb²⁺ cations are separated by six neighbor X-site anions to bulid Pb-X octahedrons, which corner-share with each other to constitute the main frame and A⁺ intercalates the voids [24]. The replacement of Pb²⁺ with lead-free ions results in both the deformation in nanoscale structure and the conversion of properties because of the differences in chemical valence and ion size [23]. Therefore, LFPSCs exhibit plenty of novelty and diversity, we discussed several typical LFPSCs in this section and summarized in

Table 1. Summary of structural parameters and synthesis methods of representative LFPSCs.

Ion	Perovskite	Bandgap (eV)	Crystal System	Dimension	Synthesis	Ref.
Sn2+	CsSnI3	1.31	Orthorhombic	3D	Bridgeman Method	[26]
Sn2+	α-FASnI3	N/A	Cubic	3D	N/A	[27]
Sn2+	β-FASnI3	N/A	Tetragonal	3D	N/A	[27]
Sn2+	γ-FASnI3	N/A	Tetragonal	3D	N/A	[27]
Sn2+	MASnI3	1.21	Cubic	3D	Cooling-induced crystallization method	[28]
Sn2+	PEA2SnBr4	2.6	Monoclinic	2D	Cooling-induced crystallization method	[29]
Bi3+	Rb3Bi2I9	2.1	Monoclinic	2D	Bridgeman Method	[30]
Bi3+	Cs3Bi2I9	1.9/2.06	Hexagonal	0D	Bridgeman Method	[30,31]
Sb3+	Rb3Sb2I9	2.03	Monoclinic	2D	Bridgeman Method	[31]
Sb3+	Cs3Sb2I9	1.89	Hexagonal	2D	Bridgeman Method	[31,32]
N/A	Cs2AgBiBr6	2.1	Cubic	3D	Inverse temperature crystallization method	[40]
		2.25			Cooling-induced crystallization method	[42]
N/A	Cs2AgInCl6	3.2	Cubic	3D	Cooling-induced crystallization method	[46]

, along with the schematic diagrams of synthesis methods as shown in Figure 1.

2.1. Sn Based Halide Perovskites

Attributed to the same valence and similar properties with Pb²⁺, Sn²⁺ is seen as a crucial candidate to form lead-free perovskites without the sacrifice of the excellent performance. The first Sn-based halide perovskite single crystals were synthesized in 1974 for CsSnX₃ [25]. In 2012, Chung et al. prepared CsSnI₃single crystals using a modified vertical Bridgman technique with refined crystal structure and optical properties [26]. Due to the crystal structure transformation from α-phase to γ-phase during the preparation of CsSnI₃, the crystal configuration of CsSnI₃ is not cubic symmetry, but the stable γ-phase CsSnI₃ SC (direct bandgap =1.3 eV) at room temperature, showing a p-type semiconductor behavior with a carrier concentration and a hole mobility of ≈10¹⁷ cm⁻³ and ≈585 cm² V⁻¹ s⁻¹, respectively. A similar result has been reported in FASnI₃ perovskite single crystals by Kahmann et al. [27]. Yao et al. developed a novel process called local temperature reduction induced crystallization to prepare 110 μm-thick MASnI₃ single crystal wafer which shows good crystallinity and great orientation as well as gives a bandgap of 1.21 eV. In addition, MASnI₃ wafer displays an extended absorption spectrum and red-shifted photoluminescence (PL) peak compared with MAPbI₃ SC [28]. The introduction of appropriate dopants is considered as an effective way to improve performance, Zhang et al. successfully synthesized the Bi-doped two-dimension(2D) LFPSCs of PEA₂Sn_1-xBi_xBr_4+x which exhibited unique crystal structure and layered surface morphology with the undoped one, controllable PL behaviors were obtained by controlling addition of Bi dopants at the same time [29].

2.2. Bi/Sb Based Halide Perovskites

Trivalent ions such as Bi³⁺ and Sb³⁺ are also considered as the alternatives for LFPSCs. Generally, B³⁺ can form a perovskite-like derivate—A₃B₂X₉, with 0D or 2D crystal structures. The initial exploration started by Lehner and co-workers in 2015 [30]. They found A and X atoms are closest-packed while B atoms occupy 2/3 voids of the octahedral X₆, and the crystal structures of A₃B₂X₉ can be classified into two typical types: cubic close-packing and hexagonal close-packing of A and X atoms. Changes of A cation lead to significant differences in structural configurations and properties. To be specific, Cs₃Bi₂I₉ single crystal tends to form a 0D configuration attributed to the isolated [Bi₂X₉] structures, which is resulted from the face-sharing [BiX₆] octahedron. By contrast, K₃Bi₂I₉ and Rb₃Bi₂I₉ tend to generate layer-like 2D structures. Another popular trivalent ion is Sb³⁺. McCall and co-workers synthesized Cs₃Bi₂I₉ and Rb₃Bi₂I₉ SCs by the Bridgman method and characterized their crystal structures by SCs X-ray diffraction, showing [SbI₆] octahedrons and isolating alkali ions for both of them. For optoelectronic properties, they display broad PL emission from 1.75 to 2.05 eV with two peaks located at 1.96 and 1.92 eV, respectively [31]. Besides that, these scholars have made a further investigation on other derivatives, after optimizing the synthesis of single crystal, they implemented the photo-response to observe visible laser emission of Cs₃Bi₂I₉ for the first time. All SCs showed ambipolar response to Am α-particles irradiation with spectra for both electron and hole collection configurations, and it is worth noting that Cs₃Bi₂I₉ and Cs₃Sb₂I₉ showed a superb potential for radiation detection [32].

2.3. Other Metals Based Perovskites

Researchers have also developed some other metal-based PSCs, such as from indium, copper. In the work of Zhou et al., they employed a slow-cooling crystal growth approach by blending CsBr and InBr₃ in HBr at 130 °C for 0.5 h [33]. After cooling down to room temperature, 0D Cs₂InBr₅:H₂O was obtained with a size of around 2 mm. It shows a 0D orthorhombic crystal structure where the [InBr₅O]₄⁻ octahedrons were separated by two Cs⁺-cations. It also displays a bright red luminescence peak at 695 nm with a PLQY of 33% under excitation of 365 nm. Other 0D (C₄H₁₄N₂)₂InBr₁₀ PSCs were synthesized within several minutes by adding InBr₃ solution (dissolved in HBr acid) into a mixture of diethylamine and HBr at 0 °C [34]. The In-Br polyhedrons were separated by the (C₄H₁₄N₂)²⁺-cations to produce a 0-D perovskite-like structure. It exhibits an abrupt absorption from 350 to 600 nm and a broad band emission from 500 nm and near-infrared region attributes to the structural distortion of [InBr₆]^3- octahedral units leading to the formation of self-trapped exciton (STE) states, confirmed also by computational study.

Lin and his co-workers prepared perovskite-like 1D CsCu₂I₃ by anti-solvent vapor-assisted crystallization method where they dissolved equimolar CuI and CsI (DMF:DMSO = 4:1) at 60 °C with an atmosphere of methanol and kept for a few days [35]. A colorless CsCu₂I₃ SCs with orthorhombic crystal structure was developed, showing a broad band white light emission spectrum and PLQY of 15.7% due to the recombination by STE states. To obtain high quality 0D PSCs of (MA)₄Cu₂Br₆, DMF (solvent of MABr and CuBr solution) was slowly evaporated at 50 °C for 2 days [36]. Due to the STE states induced photo-generated excitons relax, showing a bright green emission (peaked at 524 nm) with a high PLQY of 93% and an ultra-long PL life time up to 120 μs.

2.4. Halide Double Perovskites

Apart from single B-site ions-based lead-free perovskite, double perovskites with a formula of A₂B′B″X₆ have been investigated due to their excellent performance [37,38,39]. Pan et al. used a solution-process approach to obtain double perovskite Cs₂AgBiBr₆ single crystals, where centers of the metal bromide octahedron are occupied by alternate Bi³⁺ and Ag⁺. They proposed the presence of cations disorder during the growth process, resulting in the destroyed symmetry of double perovskite. Thermal annealing and surface treatment could eliminate these defects and improve the crystal resistivity effectively [40]. After that, numerous researchers have accomplished research on Cs₂AgBiBr₆ [41,42,43,44], in the work of Zhang et al., the resistivity of the Cs₂AgBiBr₆ was larger than 10¹⁰ Ω cm, the Fermi level was estimated to be 0.788 eV above the valence band and the two near bandgap energies were 1.917 eV and 2.054 eV, respectively [41]. Keshavarz and co-workers employed alkali substitution to tune the structures and properties of Cs₂AgBiBr₆ double perovskites. The fundamental lifetime of carrier recombination at room temperature attained a three-fold increase with the band gap remaining unchanged [44]. Furthermore, Yin et al. synthesized Cs₂AgIn_xFe_1−xCl₆ (0 < x < 1) perovskite SCs employing a simple hydrothermal method, which exhibited a broadband absorbance from 450 to 800 nm and a huge enhancement of PLQY [45]. Luo et al. prepared high quality Cs₂AgInCl₆ SCs by space-confined hydrothermal method, achieving an ultra-low trap density of (8.6 ± 1.9) × 10⁸ cm⁻³ and mobility of 3.31 cm²·V⁻¹·s⁻¹, respectively. In addition, they proposed that oxygen or oxygen-containing functional groups could alter the superficial composition and physical properties [46].

3. Applications

LFPSCs possess numerous fascinating optoelectronic properties in practical applications, as shown in Figure 1. Even if there is still a certain gap between lead-free and lead-based PSCs, several applications of LFPSCs have attracted attention recently. Herein, the reported achievements of applications using LFPSCs, such as photodetectors, solar cells, X-ray detectors, light emitting diodes, and other applications (Figure 2), are discussed.

3.1. Photodetectors

Photodetectors capture optical signals and convert them into electrical signals instantaneously, which have been widely employed in abundant fields. The key factors of excellent photodetectors can be summarized as fast responding speed, high photocurrent intensity, and low detectivity. The research of Dou et al. was the pioneer in applications of photodetectors using perovskite materials [5]. Tang and co-workers firstly fabricated LFPSC based UV photodetector with Cs₂AgInCl₆, under the continuous altering of 365 nm monochromatic illumination, the photodetectors showed photocurrent and dark current with a bias of 5 V. The photocurrent was increased from 5 × 10⁻⁹ A in vacuum to 8 × 10⁻⁹ A in air, attributing to the oxygen-induced enhancement of surface conductance. The response time is 0.97 ms in vacuum, 2.11 ms in air. Meanwhile, a high on/off ratio (~500) and a high detectivity (~10¹² Jones) were obtained [46]. Li et al. fabricated photodetectors with Cs₃Bi₂I₉ single crystal and polycrystal with a unified structure of Au/Cs₃Bi₂I₉/Au, the calculated trap density of single crystal was 5.7 × 10¹² cm⁻³, which is much lower that of polycrystal (1.5 × 10¹⁵ cm⁻³), the carrier mobility of single crystal was estimated as 1.7 × 10⁻² cm² s⁻¹ V⁻¹, which is 3.8 × 10⁴ folds higher than that for polycrystal (4.4 × 10⁻⁷ cm² s⁻¹ V⁻¹). In addition, the Cs₃Bi₂I₉ single crystal photodetector exhibited a high photo-response ON-OFF ratio as 11,000 and outstanding stability [47]. Similarly, Dang et al. assembled Cs₂AgBiBr₆ PSC-based photodetectors, where the photodetection performance was investigated with various electrodes including Ag, Au, and Al under different wavelength illumination [48]. At 5 V bias, the Cs₂AgBiBr₆ SC device displayed a responsivity of 0.9 mA W⁻¹ in air and 0.92 mA W⁻¹ in a vacuum under 400 nm illumination. The detectivity was estimated to be 1.38 × 10⁹ and 2.66 × 10⁹ Jones, respectively, the on/off ratio was estimated as 42 and 153, respectively. All results suggest that Cs₂AgBiBr₆ SC-based photodetector with Ag electrodes exhibit excellent photoresponse. Liu et al. reported a blue light photodetector with a structure of Si/SiO₂/Cs₃Sb₂Br₉/Au, the device possessed low dark current (2.4 × 10⁻¹² A) and impressive photocurrent (3.1 × 10⁻⁸ A) at a bias of 6 V under dark condition and illuminated by 480 nm light, the response and recovery time was 0.2 ms and 3 ms respectively [49]. Compared with other lead-free perovskite-based photodetectors, Zheng et al. fabricated a nanoflake photodetector demonstrating a response speed of 24/48 ms [50].

Table 2. Photodetection parameters of LFPSCs based devices.

LEPSC	Responsivity	Detectivity (Jones)	ON-OFF Ratio	Ref.
Cs2AgInCl6	0.013 A W−1	9.60 × 1011	NA	[46]
Cs3Bi2I9	7.2 × 10−3 A W−1	1.0 × 1011	NA	[47]
Cs2AgBiBr6	0.92 A W−1	2.66 × 109	153	[48]
Cs2AgBiBr6	0.9 mA W−1	1.38 × 109	42	[48]
Cs3Bi2I9	2.29 A W−1	3.77 × 1012	NA	[49]

summarizes some relevant works published on this topic.

3.2. Solar Cells

Since Kojima et al. firstly applied perovskite materials in solar cells with a PCE of 3.81% in 2009 [51], perovskite solar cells have been expected to be an alternative to solve the urgent problems of energy shortage and environmental pollution. Although numerous significant achievements of high-performance lead-based perovskite solar cells have been reported in recent years [4,52,53,54], including research on single crystals [55,56], poor stability and high toxicity are their urgent concerns [57]. For LFPSCs, the toxicity is suppressed by the replacement of lead, and the absence of moisture-sensitive grain boundaries leads to favorable stability, meanwhile extra properties including low trap density, dense structure, and low ion migration are obtained [58]. Several strategies employing single crystals have been proven to benefit the efficiency and stability, nevertheless, there is rarely a report on LFPSC film-based solar cells, and the photovoltaic parameters of some relevant works are summarized in

Table 3. Photovoltaic Parameters of LFPSCs based solar cells devices.

LEPSCs	Voc (V)	Jsc (mA/cm2)	FF	PCE (%)	Ref.
FASnI3	0.63	21.60	74.7	10.17	[62]
FASnI3	0.628	22.23	74.2	10.37	[63]
FASnI3+1%EDAI2	0.58	21.3	72	8.9	[64]
FASnI3+5%PHCl	0.76	23.5	64	11.4	[65]
CsSnI3	0.86	23.2	65	12.96	[66]
(FA)0.75(MA)0.25SnI3+10%SnF2	0.61	21.2	62.7	8.12	[67]
AVA2FAn−1SnnI3n+1	0.61	21.0	68	8.71	[68]
PEAxFA1−xSnI3+NH4SC	0.94	17.4	75	12.4	[69]

. He et al. fabricated the device using synthesized FASnI₃ single crystals as precursors, which possessed high purity, low defect density, and excellent stability in the air [59]. The authors demonstrated that re-dissolved single crystals forming solution effectively prevents the oxidation of Sn²⁺ by reducing impurities and moisture. The single crystal precursors-based films showed smooth morphology and exhibited larger and more uniform grains than conventional films. The PCE of device was 8.9% and 5.5% for spin-coated solar cells and large-scale printed cells, respectively. In addition, FASnI₃ single crystal precursors-based devices retained a higher percentage of initial PCE than conventional devices. The precise controlling of crystallization to obtain near-single-crystalline film is also a viable approach to achieve higher performance. For instance, Li et al. proposed the annealing of FASnI₃ assisted by phenylethyl ammonium chloride enabled to form pure-phase ordered 2D perovskite crystals with excellent vertical orientation, and the fabricated solar cells exhibited a champion PCE of 9.1% after 1500 h of storage under dark condition, with short-circuit current density (J_sc) of 22.06 mA cm⁻², open-circuit voltage (V_oc) of 0.59 V, and fill factor (FF) of 69% [60]. Shao et al. successfully deposited near-single-crystalline FASnI₃ with the orthorhombic a-axis in the out-of-plane direction by mixing a trace amount of layered 2D tin perovskite. The corresponding devices achieved a high PCE of 9.0% [61].

3.3. X-ray Detectors

X-ray detection plays an important role for scientific study, medical diagnosis, and industrial inspection [12,70]. LFPSCs can be good candidates for X-ray detection because of some unique properties including large X-ray attenuation coefficients; a suitable bandgap (1.5 to 5.0 eV); highly crystalline with lower trap density; large bulk resistivity with less ion migration; high sensitivity and stability; and low toxicity [71,72]. The detection of X-ray has been of great importance due to the wide applications of X-ray in various fields, and some relevant works are summarized in

Table 4. X-ray detection parameters of LFPSCs based devices.

LEPSCs	μτ Product (cm2V−1)	Sensitivity (μC·Gyair−1·cm−2)	Detection limit (nGyairs−1)	Ref.
Cs2AgBiBr6	6.3 × 10−3	316.8	59.7	[40]
Cs2AgBiBr6	5.95 × 10−3	1974	226.2	[78]
MA3Bi2I9	NA	1947	83	[79]
Cs3Bi2I9	7.97 × 10−4	1652.3	130	[81]
(BA)2CsAgBiBr7	1.21 × 10−3	4.2	NA	[80]
(H2MDAP)BiI5	NA	1.0	NA	[82]

on this topic. [73,74,75,76]. Liu et al. developed MA₃Bi₂I₉ SCs, showing large X-ray absorptivity, large bulk resistivity of around 4.7 × 10¹⁰ ohm-cm, high density of ~4.1 g/cm³, and high ion migration activation energy of 0.83 eV [77]. MA₃Bi₂I₉ SCs based X-ray detector exhibited fast response of 266 ms, high sensitivity of 872 mC/Gy/cm², detection limit of 31 nGy/s, and good stability under ambient atmosphere. Tang et al. fabricated Cs₂AgBiBr₆ SC X-ray detector s with a vertical Au/Cs₂AgBiBr₆/Au structure and exposed the devices to X-ray to investigate its performance. The gain factor, which could be regarded as the charge collection efficiency, was about 0.14 for a dose rate of 60–138.7 μGy_air s⁻¹, similar to that of photodiode structure MAPbBr₃ X-ray detectors (0.16). It was worth noting that when increasing the dose rate, the gain factor gradually decreased due to the enhanced carrier filling into shallower traps under higher radiation. Benefitting from low ionization energy, the detectors achieved complete absorption and a sensitivity of 105 μC Gy_air⁻¹ cm⁻² at a driving voltage of 50 V, which was four times higher than that of commercial α-Se detectors [40]. Based on these studies, they optimized the growth process of Cs₂AgBiBr₆ employing precisely controlled cooling synthesis to obtain superior single crystals exhibiting a high resistivity with narrow distribution range from 6.10 × 10⁹ to 3.31 × 10¹⁰ Ω cm, by comparison, the resistivity of reference ranged from 6.04 × 10⁷ to 5.61 × 10⁹ Ω cm. The as-prepared X-ray detectors with optimum SCs had a sensitivity of 1974 μC Gy_air⁻¹ cm⁻² under 50 V [78]. Liu et al. fabricated a highly sensitive and stable X-ray detector with 0D MA₃Bi₂I₉ SCs for the first time. A precursor refinement strategy was adopted to synthesize high-quality LFPSCs, consisting of face-sharing (BiI₆)³⁻ octahedrons where the voids between the layers were filled with the MA⁺, in which the formed (Bi₂I₉) ³⁻ are spatially isolated by two MA⁺, resulting in a 0D crystal structure. Attributed to such a special 0D structure, the electron-trap density of MA₃Bi₂I₉ single crystals was calculated to be 1.2 × 10¹⁰ cm⁻³ and the hole-trap density was 7.5 × 10¹⁰ cm⁻³, the resistivity was measured as 3.74 × 10¹⁰ Ω cm. Therefore, the corresponding detector showed a high sensitivity (1947 μC Gy_air⁻¹ cm⁻²) and fast response speed (23.3/31.4 ms) [79]. Other than 3D LFPSCs, 2D PEA-Cs₂AgBiBr₆ and (BA)₂CsAgBiBr₇ SCs based X-ray detectors also demonstrated high sensitivity of 288.8 mC/Gyair/cm² and 4.2 mC/Gyair/cm², respectively [80]. PEA-Cs₂AgBiBr₆ SCs displayed sensitivity of 18.1 mC/Gyair/cm² which was approximately twice higher than that of pristine Cs₂AgBiBr₆ SCs. Benefitting from the higher formation energy encountered in these SCs, they displayed enhanced photostability due to the low defect density and low defect migration.

3.4. Light-Emitting Diodes

Another important optoelectronic application of perovskites is light-emitting diodes (LED) because of their PLQY, tunable band gap, and facile solution preparation. For LFPSCs, low-dimensional halide perovskites have attracted remarkable attention for their spectacular photoluminescence properties and chemical stability, also, the doping strategies have been widely adopted to enable or balance multiple emission centers [83,84,85]. In 2019, a (C₈NH₁₂)₄Bi_0.57Sb_0.43Br₇·H₂O SCs was synthesized and showed ultra-broadband emission spectrum between 400 and 850 nm, with a PLQY value increased from 0.7% ((C₈NH₁₂)₄BiBr₇·H₂O) to 4.5% [83]. Li et al. synthesized (C₈NH₁₂)₆InBr₉·H₂O single crystals exhibiting weak broadband red emission and a PLQY value of 8.85% because of the strong exciton-phonon interaction induced STEs. Doping Sb³⁺ at In³⁺ sites can effectively optimize the band gap structure and enhance ultra-broadband red emission, the PLQY value was increased up to 23.36% via controlling the Sb doping level [84].

3.5. Humidity Sensor and Field-Effect Transistors

Besides the above-mentioned applications, Pb-free PSCs were also employed into other promising applications. Zhou et al. prepared a Pb-free 0D Cs₂InBr₅·H₂O PSC with a broad red luminescence centered at 695 nm and a high PLQY up to 33%, resulting from the deformations of charge carriers via STE states [33]. It exhibits different emission in a moisture-containing condition with good structural- and photo-stability. A PL humidity sensor was fabricated based on switchable dual emission corresponding to the hydrated and dehydrated states, showing good recyclability and fast response time. This pioneering work establishes a foothold for the utilization of Pb-free perovskite in humidity detection and also demonstrates the advantages of exploring novel applications for these Pb-free perovskite materials.

Luo et al. fabricated a field effect transistor using solution-processed Cs₂SnI₆ nanobelts under ambient conditions using a SiO₂ coated silicon wafer substrate and pre-patterned Au as the metal electrode [46]. P-type Cs₂SnI₆ film-based devices possess high I_ON/I_OFF ratio under photoexcitation with hole mobility and hole concentration of 20.2 cm²/V/s, 9.1 × 10¹⁸ cm⁻³, respectively.

4. Challenges and Prospects

In general, LFPSCs have drawn extensive interests due to their environmentally friendly features without involving toxic lead, and excellent optoelectronic properties resulted from the absence of grain boundaries. In the past few years, enormous efforts have been devoted to various explorations of LFPSCs such as optimizing synthesis and doping strategies to produce high-quality single crystals with attractive properties including long carrier lifetimes and carrier diffusion lengths, tunable bandgap, large absorption coefficient, low trap density. Even though several considerable achievements of applications have been reported, the further development of LFPSCs is still facing unsolved problems which may impede the expansion of applications. The current challenges and prospects of LFPSCs are summarized as follows:

(1) The performance of LFPSCs extremely depends on the synthesis methods, hence the exploration of controllable and reliable synthesis to yield stable and high-quality single crystals is necessary to be focused on, with economic and environmental factors under consideration. Moreover, the precise control of thickness and size could be effective approaches to enhance the performance of different devices. For these LFPSCs with low dimensionality crystal structure and wide bandgap, they usually show limited charge transportation and collection and narrow absorption range of the visible range. It is worthy to develop increased dimensional materials to gain a full understanding of the fundamentals and applications of these LFPSCs.

(2) The doping strategies, employed to improve properties, are universal in the research of LFPSCs, but the specific enhancement mechanism has not been thoroughly studied. The intensive study of dopants contributes to obtaining tunable and enhanced single crystals. Most LFPSCs possess wide bandgap which exhibited a narrow range absorption of the solar spectrum. It is important to design more materials with favorable bandgaps and study the structure-property correlation systematically by tuning their bandgaps through halide exchange. For practical applications, large-area fabrication is as important as low-cost, high responsivity, and long-term stability for devices. Preparing high-quality large-scale LFPSCs film with controlled thickness and size for optoelectronic applications remains an unconquered challenge. Moreover, as does the improvement in the optimization and integration of perovskite devices into practical device application.

(3) The reproducibility of LFPSCs based devices remains a huge challenge. Almost all the reported devices are fabricated based on lab-scale. They demonstrated batch-to-batch variations, and most of their results cannot be reproduced. The properties of the synthesized materials highly depended on the operator and condition for processing, storage. There are no standardized protocols for synthesis, characterization, and testing to follow.

(4) The large thickness of LFPSC along the direction of carrier transportation may lead to low current density, while the fabricating technologies of devices also suppress the PCE of solar cells. The existence of migration of ions and vacancies could be a major factor for the decrement of the device performance by retarding the charge carrier transportation. The ion migration can also affect the crystal structure due to the activation energy dependence on the crystal. In addition, various channels based on thermal energy and local polarization have been identified as fast and slow ion migration pathways.

(5) Stability of LEFPSs is another important issue. The degradation of perovskite structures happened when exposed to humidity, oxygen, heating, and UV light illumination as we observed in case of Pb based perovskites. The degradation mechanisms are still unclear. Therefore, huge efforts should be devoted to improve crystal stability as well as maintain their excellent photophysical and chemical properties.

These issues have raised challenges for facilitating the applications of LFPSCs, nevertheless, considering the remarkable optoelectronic properties and stability, we believe LFPSCs have a bright future in optoelectronic applications.