All-inorganic lead-free perovskites for optoelectronic applications

Abstract

Organic and inorganic hybrid lead halide perovskites have successfully emerged as revolutionary optoelectronic semiconductors for use in various applications. The long-term stability and lead toxicity of hybrid lead halide perovskites have attracted increased attention; therefore, all-inorganic lead-free perovskites have become an alternative perovskite for use in optoelectronic applications. Among them, all-inorganic CsSnI₃ has also been developed. The main issue limiting the optoelectronic performance and stability of Sn-based perovskites is the low chemical stability of Sn²⁺, which is very easy to be oxidized to Sn⁴⁺ under air conditions. Several approaches have been adopted to prevent the oxidation of Sn, thereby improving its performance. However, its chemical stability is still difficult to manage. Other than Sn, other transition metals such as Ge, Bi, and Cu have also been used to substitute Pb and form novel lead-free perovskites. Although such non-Sn lead-free perovskites exhibit much better stability, their photovoltaic performances are lower as compared to those of Pb- or Sn-based perovskites. However, these novel all-inorganic lead-free perovskites exhibit potential in photoluminescence and other optoelectronic applications. Overall, we have reviewed the recent progresses and outlooks regarding the prospects and challenges faced by all-inorganic lead-free perovskites.

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Yongbing Lou

Prof. Yongbing Lou received his BS degree from Nankai University in 1996 and received his MS degree from Nanjing University in Polymer Materials in 1999. After acquiring his PhD in Chemistry from Case Western Reserve University in 2007, he joined the School of Chemistry and Chemical Engineering at Southeast University. His research interests include the preparation of novel nanomaterials for optoelectronic, sensing, and energy-related applications. He has published more than 50 peer-reviewed papers.

Yixin Zhao

Yixin Zhao is a professor at Shanghai Jiao Tong University. He obtained his PhD from Case Western Reserve University in 2010, followed by working as a postdoctoral fellow at Penn State University and National Renewable Energy Laboratory. His current research interests focus on perovskite solar cells, photoelectrochemical catalysis, and environmental remediation. Professor Zhao is on the editorial board for several journals and has coauthored over 110 peer-reviewed publications.

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1. Introduction

Organic–inorganic lead halide perovskites (generally having the formula ABX₃, where A is a monovalent organic cation, B is a lead cation, and X is a halide anion) have emerged as one of the most promising potential optoelectronic materials in the past few years because of their tunable direct bandgap, high absorption coefficient, low exciton binding energy, and high carrier mobility.^1–5 Various applications including solar cells, light-emitting devices, photocatalysis, photodetectors, and lasers have been reported.^6–13 The power conversion efficiency (PCE) of solar cells formed using halide perovskites have been improved from the initially unstable 3.8% in 2009 to the recently certified 23.3%.^14–23 In spite of the considerable progress made in the fabrication of hybrid lead halide perovskite materials, there are still some scientific challenges and technical obstacles for their commercial applications.²⁴ Among them, lead toxicity is a fatal drawback of using hybrid lead halide perovskites. Therefore, it is imperative to develop alternative lead-free perovskites with excellent performance that can be used in optoelectronic applications.

It has been reported that lead's lone pair 6s² electrons that align with the halides to form antibonding coupling play an important role in the excellent photovoltaic properties of lead halide perovskites.^25–27 Therefore, metal elements that have similar electronic structures as Pb to form stable perovskites have been considered as a suitable alternative. Sustained efforts have been devoted toward developing lead-free perovskites that have prominent photovoltaic performance.^28,29 For example, it has been demonstrated that MASnI₃ exhibited narrower bandgap than MAPbI₃.³⁰ The Pb–Sn alloyed perovskite is the main candidate for preparing low-bandgap perovskite solar cells. Noel et al. reported MASnI₃ perovskites as 1.23 eV-based solar cells with PCE of over 6% under 1 illumination.³¹ Bi-based perovskites exhibited superior stability than their Pb analogues,^32–34 indicating that Pb could be successfully substituted with other elements.

Since the organic cation may affect long-term photostability, alternative inorganic cations could further enhance the robustness of perovskite materials and devices.^13,35–43 Therefore, there are still several challenges in developing more stable perovskite photoelectronic devices with higher efficiencies. In this review, we summarized the recent advances in all-inorganic lead-free perovskites and discussed various candidates and approaches to improve their optoelectronic performance.

2. Lead substitution

The Goldschmidt's tolerance factor (t) that depends on the ionic radius of the ions in perovskites is an empirical parameter that can be used to determine the structure of perovskites.
where R_A, R_B, and R_X are the ionic radii of A, B, and X ions, respectively.⁴⁴ A stable perovskite structure can be predicted when t is in the range from 0.75 to 1.0.⁴⁵ The ionic radius of lead is 1.19 Å.⁴⁶ Therefore, lead in perovskites can be substituted by several alternative metal elements with similar ionic radii. Zhang and coworkers theoretically identified 14 lead-free perovskites based on Group 14 elements (Sn, Ge) and 11 nontoxic double perovskites with suitable bandgaps, intrinsic thermodynamic stabilities, small carrier effective masses, and low exciton binding energies by means of first-principles calculations.^47,48 These candidates are shown in Fig. 1.

Fig. 1 Distribution of (a) AM^IVX^VII₃ perovskites and (b) A₂M⁺M³⁺X^VII₆ perovskites with respect to the octahedral factor (μ) and tolerance factor (t). Reprinted with permission from ref. 47 and 48. Copyright 2017 American Chemical Society.

In this review, we summarize several comprehensively studied candidates for substitution with Pb. The most straightforward candidates are Group 14 elements, namely, Sn and Ge, which have electrical structures similar to that of Pb. Then, there are Group 15 elements, namely, Bi and Sb: trivalent Bi³⁺ and Sb³⁺ possess similar electronic configurations. As compared to Pb, they can exhibit more diverse dimensionalities due to the presence of the BiX₆³⁻ (SbX₆³⁻) octahedron, and they are much less toxic than Pb. Recently, Cu and Ti have become new candidates, which are earth-abundant, nontoxic, and biocompatible elements. Table 1 lists the ionic radii and electronic configurations of metal cations for the substitution of Pb.

Table 1 Ionic radii and electronic configurations of metal cations discussed in this review

Metal cations	Ionic radius (Å)	Electronic configuration
Pb^2+	1.19	6s^2
Sn^2+	1.02	5s^2
Ge^2+	0.73	4s^2
Bi^3+	1.03	6s^2
Sb^3+	0.76	5s^2
Sn^4+	0.69	4d^10
Ti^4+	0.53	3p^6
Cu^2+	0.73	3d^9

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2.1 Sn

Sn, a Group 14 element with an electronic structure similar to lead, has been firstly considered as the alternative metal for halide perovskite materials. In addition, Sn-based perovskites exhibit a narrower bandgap than their Pb analogues with low exciton binding energies and longer carrier diffusion lengths.^49,50 Therefore, Sn-based perovskites are regarded as promising light absorbers in optoelectronic applications. Considerable efforts have been devoted toward formulating Sn-based perovskites over the past few years. Snaith and coworkers reported lead-free MASnI₃ perovskite solar cells with PCE of over 6%.³¹ Hao et al. reported that the band structure of MASnI₃ could be tuned by substituting iodide with bromide to cover more visible-light region.³⁰ Ma et al. suggested that the carrier diffusion length of MASnI₃ could exceed 500 nm after doping SnF₂, indicating the potential of MASnI₃-based solar cells.⁵¹ Liu et al. combined hot antisolvent treatment and dimethyl sulfoxide (DMSO) vapor to increase the film coverage and average crystallite size of FA_0.75MA_0.25SnI₃, achieving PCE exceeding 7%.⁵² Qian et al. fabricated 2D perovskite (PEA)₂SnI₄ in flexible photoconductors with excellent photoresponsivity of 16 A W⁻¹ and detectivity of 1.92 × 10¹¹ Jones under 470 nm illumination.⁵³ However, Sn-based perovskites are sensitive toward moisture and oxygen owing to the oxidation process of Sn²⁺ to Sn⁴⁺. In addition, the product of Sn⁴⁺—a p-type dopant—makes the materials metallic via a self-doping process.⁵⁴ This oxidation process results in instabilities and poor performance of optoelectronic devices. Various strategies have been developed to inhibit the oxidation of Sn²⁺, including adding additives in the precursor solution, adjusting the atmosphere, and controlling the morphology of the perovskite crystal.

2.1.1 Adding additives. All-inorganic CsSnI₃ has been studied as a thermoelectric material and has been reported to function as a hole transport material in dye-sensitized solar cells.⁵⁵ It has been reported that excess Sn can facilitate the reduction in Sn vacancies and p-type electrical conductivity.⁵⁶ Therefore, in order to suppress the oxidation of Sn²⁺ in CsSnI₃, additives (e.g., SnF₂ and SnI₂) were added in the precursor solution to reduce Sn vacancies. Kumar et al. investigated the effect of SnF₂ as an additive in CsSnI₃.⁵⁷ The addition of SnF₂ did not result in a significant variation in the CsSnI₃ phase, suggesting that F⁻ did not substitute I⁻ in the perovskite structure due to the much smaller ionic radius of F (1.33 Å) as compared to that of I (2.20 Å). SnF₂ was uniformly mixed with CsSnI₃ to reduce the concentration of Sn vacancies, which form the dominant defects, affecting the p-type carrier density. With regard to 20% SnF₂-CsSnI₃ solar cells, PCE of 2.02% could be achieved with the following photovoltaic parameters: J_sc = 22.7 mA cm⁻², V_oc = 0.2 V, and FF = 0.37. This approach introduced new possibilities for exploiting all-inorganic lead-free perovskites (e.g., CsSnI₃).

In order to further improve the V_oc value of CsSnX₃ materials, the impact of Br⁻ replacing I⁻ was exploited. Sabba et al. demonstrated that the substitution of I with Br is an effective way to improve the V_oc value of CsSnI₃ solar cells.⁵⁸ The crystal structure of CsSnI₃ changing from orthorhombic to cubic for CsSnBr₃ was accompanied with the bandgap changing from 1.27 eV to 1.75 eV by progressively replacing I⁻ with Br⁻ (Fig. 2a). Br-Rich perovskites exhibited low carrier densities and fewer charge recombinations, resulting in high open-circuit voltages. With the increase in Br doping, V_oc increased from 2.30 mV for CsSnI₃ to 190 mV for CsSnBr₃ even without the addition of SnF₂.

Fig. 2 (a) Crystal structure and bandgap of CsSnX₃ (X = I, Br). Reprinted with permission from ref. 58. Copyright 2015 American Chemical Society. (b) Stability of CsSnI₃ with different Sn salt additives. Reprinted with permission from ref. 59. Copyright 2016 Springer Nature. (c) Possible mechanism of hydrazine preventing the oxidation of Sn²⁺. Reprinted with permission from ref. 54. Copyright 2017 American Chemical Society.

The role of SnF₂ on the properties of CsSnBr₃ was also investigated.⁶⁰ The ultraviolet photoelectron spectroscopy (UPS) data reveal that the conduction band minimum (CBM) of CsSnI₃ perovskite increases with doping SnF₂, which would reduce the electron injection barrier between the perovskite and CBM of the TiO₂ layer. Therefore, CsSnBr₃ with SnF₂ doping exhibited fewer recombinations than those in pristine perovskites. Further, the addition of SnF₂ moves the valence band maximum (VBM) of the perovskite closer to the HOMO level of HTM (spiro), thereby decreasing the large loss of voltage at the HTM–perovskite interface. Consequently, perovskite solar cell (PSC) devices based on CsSnBr₃ with 20 mol% SnF₂ exhibited enhanced photovoltaic performance with η = 2.17%, J_sc = 9.1 mA cm⁻², V_oc = 0.42 V, and FF = 0.56 as compared to pristine CsSnBr₃ with η = 0.01%, J_sc = 0.4 mA cm⁻², V_oc = 0.1 V, and FF = 0.33. This result indicates that the performance of CsSnBr₃-based devices can be improved by tuning the band alignment.

Other Sn halide salts such as SnI₂, SnCl₂, and SnBr₂ were also introduced as additives to improve PCE and stability. The introduction of 10% excess SnI₂ in CsSnI₃ preparation increased the device performance from η = 0.75% to η = 1.5%.⁶¹ However, CsPbI₃-based PSC devices are highly sensitive to air and their efficiencies quickly decreased more than 30% after storing in the dark in ambient air for 1 h. Further research revealed that excess SnCl₂ could form a thin layer on the surface of CsSnI₃ crystallites because Cl cannot substitute I in CsSnI₃.⁵⁹ However, the SnCl₂ layer could protect CsSnI₃ by forming a stable hydrate (SnCl₂·2H₂O), resulting in better stability than other Sn salt additives (Fig. 2b). In addition, SnCl₂ n-doped the fullerene layer, allowing the device to function effectively without an electron blocking layer (EBL): this minimized the pinhole density of the film. Therefore, devices using SnCl₂ as additives exhibited the highest η value due to a simple fabricating process with η = 3.26%, J_sc = 11.25 mA cm⁻², V_oc = 0.40 V, and FF = 0.55.

The effect of SnBr₂ as the additive in CsSnBr₃ was investigated via the vapor deposition method using CsBr and excess SnBr₂.⁶² The V_oc value of CsSnBr₃-based devices without SnF₂ was enhanced because excess SnBr₂ reduced the density of the defect states and prevented the oxidation of Sn²⁺ to Sn⁴⁺ states in ambient air. This demonstrated that SnF₂ dopants passivated bulk and grain boundary defects. Therefore, devices based on CsSnBr₃ with 2.5% mol SnF₂ retained 80% of their performance over 50 min, and the films retained the same absorption value over 24 h; however, CsSnI₃ and CsSnI₂Br degraded within a few minutes when exposed to air. This indicated that CsSnBr₃ could be a promising air-stable all-inorganic lead-free perovskite material as compared to CsSnI₃. Besides Sn halide salts, hypophosphorous acid (HPA) was also introduced as an additive in CsSnIBr₂.⁶³ HPA could strongly coordinate with Sn²⁺ and stabilize the CsSnIBr₂ precursor by generating Sn–O–P–O–Sn bond connection, preventing the oxidation of Sn²⁺. The amount of Sn⁴⁺ in CsSnIBr₂ with HPA was much lower than that of the pristine film. Further, the introduction of HPA did not induce phase impurity. In addition, HPA reduced the carrier mobility and charge carrier density of the CsSnIBr₂ film, which inhibited charge recombinations. Therefore, devices based on HPA addition achieved average PCE of 3.0% with J_sc = 17.4 mA cm⁻², V_oc = 0.31 V, and FF = 0.56. Furthermore, devices retained 98% of their performance up to 473 K.

2.1.2 Preparation under reducing vapor atmosphere. Although excess divalent Sn compound additives (e.g., SnF₂) improved device performance, these additives could also induce poor morphologies.⁶⁴ Reducing vapor was employed to suppress the oxidation of Sn²⁺. Hydrazine was used as the vapor to improve the quality of the perovskite film.⁵⁴ Because of its strong reducibility and high volatility, hydrazine could serve as an oxygen scavenger (N₂H₄ + O₂ → N₂ + 2H₂O) to decrease the surrounding oxygen level. More importantly, as the possible mechanism shown in Fig. 2c, hydrazine prevented the oxidation of Sn²⁺ by the following reaction: 2SnI₆²⁻ + N₂H₄ → 2SnI₄²⁻ + N₂ + 4HI. Therefore, the PCEs of CsSnI₃ and CsSnBr₃ devices were improved from an average of ∼0.16% to 1.5% and ∼2.36% to 2.82%, respectively. By combining excess SnI₂ as the additive in CsSnI₃ and a reducing vapor atmosphere, devices with enhanced performances having η = 4.81% could be successfully achieved.⁶⁵

2.1.3 Controlling the morphology. Perovskites nanocrystals (NCs) have attracted tremendous attention due to their versatile surface chemistry, favorable combination of quantum-size effects, and high photoluminescence (PL) yields. Several efforts have been devoted toward fabricating NCs with controllable morphologies (Fig. 3a) to improve their optoelectronic properties and stabilities. Nano-cubic CsSnX₃ (X = Cl, Cl_0.5Br_0.5, Br, Br_0.5I_0.5, I) was synthesized by a typical hot-injection method.⁶⁹ Mixed halide perovskites could be formed by either using different halide sources or through a post-synthesis anion exchange reaction. The optical bandgap could be tuned from the visible to the near-IR region by quantum confinement effects and employing different halides in perovskite crystals. However, NCs degraded and PLQE significantly decreased when they were exposed to ambient air for about 5 min, which is due to the oxidation of Sn²⁺ to Sn⁴⁺.

Fig. 3 (a) Cs₂SnX₆ (X = Cl, Br, I) with different morphologies. Reprinted with permission from ref. 66. Copyright 2016 American Chemical Society. (b) Morphology and stability of CsSnBr₃ nanocages. Reprinted with permission from ref. 67. Copyright 2017 American Chemical Society. (c) Grain coarsened with an increase in temperature. Reproduced with permission from ref. 68. Copyright 2016 John Wiley and Sons.

In order to improve stability, hollow CsSnBr₃ nanocages (Fig. 3b) were synthesized by a hot-injection colloidal approach for further advancing Sn-based perovskites.⁶⁷ Hollow nanocages showed enhanced stability than perovskites with other morphologies under ambient conditions (Fig. 3b). In addition, NCs were treated with perfluorooctanoic acid (PFOA) to passivate the surface of the CsSnBr₃ nanocage, which significantly improved its stability against moisture, oxygen, and light. Uniform CsSnX₃ (X = Cl, Br, I) quantum rods (QRs) were successfully synthesized by a solvothermal method.⁷⁰ These QRs exhibited tunable emission wavelengths ranging from 625 to 709 nm that could be achieved by adjusting the halide composition. The maximum external quantum efficiency (EQE) peaks of these QRs reached over 63% (CsSnCl₃), 65% (CsSnBr₃), and 70% (CsSnI₃) because of the larger surface area of QRs as compared to the bulk materials. Furthermore, the stability of CsSnI₃ was enhanced as compared to MAPbI₃. Further, it was reported that the grain morphology was an important factor affecting the performance of the devices.⁶⁸ Abundant grain boundaries in the thin films resulted in a significant recombination of electrons–holes. The grain size of the CsSnI₃ thin film increased with fewer grain boundaries after high-temperature annealing (Fig. 3c). In addition, NiO_x was introduced as the hole transport material for matching the energy level of CsSnI₃. PCE of 3.31% with V_oc = 0.52 V, J_sc = 10.21 mA cm⁻², and FF = 0.625 could be achieved under optimized conditions.

2.1.4 Sn⁴⁺-based perovskites. In order to further improve stability, chemically stable Sn⁴⁺ was expected to replace Sn²⁺ to form perovskites. Sn in A₂SnX₆ with [SnX₆]²⁻ octahedra is Sn⁴⁺ rather than Sn²⁺, which is the reason for this improved stability. Cs₂SnI₆ was used as an air-stable hole transporter in dye-sensitized solar cells.⁷¹ Stable Cs₂SnI₆ NCs with different morphologies were prepared by a hot-injection method.⁶⁶ These NCs were stable under ambient conditions for 1 week due to the higher oxidation state of Sn. Unfortunately, the fluorescence of Cs₂SnI₆ was quickly quenched once the NCs were removed from the mother solution and PLQEs were still low (0.48% for quantum dots (QDs), 0.11% for nanorods, 0.08% for nanowires, 0.054% for nanobelts, and 0.046% for nanoplatelets). Further, the optical properties of Cs₂SnI_xCl_6−x could be tuned by controlling the ratios of I/Cl.⁷² The bandgap of the crystals with different morphologies could be tuned from ∼1.3 eV to 4.6 eV. I-Rich perovskites with lower Cl doping (Cs₂SnI_5.28Cl_0.72) showed higher PL intensity and exhibited better stability without obvious decomposition under ambient conditions for over 5 months. The doping of Cl further improved the phase stability of Cs₂SnI_xCl_6−x at an increased phase decomposition temperature. Cs₂SnI_6−xBr_x prepared by a two-step solution method was introduced in the solar cells.⁷³ By carefully tuning each step of the process, a series of Cs₂SnI_6−xBr_x with different morphologies were produced. A device with optimized conditions achieved PCE of 2.02% and improved stability.

2.2 Ge

Ge, another Group 14 element, was also considered as a potential candidate for Pb substitution in halide perovskites. The structural and electronic properties of MAGeX₃ (X = Cl, Br, I) were investigated by theoretical calculations.⁷⁴ The results revealed that MAGeI₃-based perovskites exhibited similar hole and electron conductive behaviors, stabilities, and optical properties as those of MAPbI₃. This indicated that Ge was a competitive alternative to Pb for use in perovskites. AGeI₃ (A = MA, FA, Cs) was prepared by Krishnamoorthy et al.⁷⁵Fig. 4a shows the band structure of three perovskites. Among them, CsGeI₃ (Fig. 4b) exhibited the highest thermal stability (up to 350 °C) when compared with the other two Ge-based perovskites.⁷⁶ However, solar cells exhibited poor performance due to the oxidation of Ge²⁺. CsGeX₃ (X = Cl, Br, I) QRs (Fig. 4c) were prepared by a solvothermal method.⁷⁷ The emission wavelength could be tuned from 607 to 696 nm by changing the composition of the different halides (Fig. 4d). PCE of 4.94% for solar cells based on CsGeX₃ could be achieved with V_oc = 0.51 V, FF = 0.51, and J_sc = 18.78 mA cm⁻². The EQE values of the top-performing devices were observed to exceed 66% (for CsGeCl₃), 68% (for CsGeBr₃), and 79% (for CsGeI₃).

Fig. 4 (a) Band structure of CsGeI₃, MAGeI₃, and FAGeI₃. Reproduced with permission from ref. 75. Copyright 2015 The Royal Society of Chemistry. (b) Crystal structure of CsGeI₃. Reproduced with permission from ref. 76. Copyright 2016 The Royal Society of Chemistry. (c) Morphologies of CsGeX₃ (X = Cl, Br, I) QRs and (d) absorption spectra of CsGeX₃ (X = Cl, Br, I). Reproduced with permission from ref. 77. Copyright 2018 The Royal Society of Chemistry.

2.3 Bi

Bi is considered as a potential substitute because its valence electronic shell is isoelectronic to Pb. Furthermore, its ionic radius (1.03 Å) is similar to that of Pb (1.19 Å).⁷⁸ Therefore, Bi has attracted considerable attention for use in halide perovskite optical applications. In the structure of A₃Bi₂X₉, A and X atoms are the closest-packed and Bi atoms occupy two-thirds of the octahedral X₆ voids. Park et al. prepared Bi-based perovskites with the chemical structure A₃Bi₂I₉ (A = MA⁺, Cs⁺), which consisted of (Bi₂I₉)₃ clusters surrounded by cations (Fig. 5a).⁷⁹ Cs₃Bi₂I₉ exhibited five times higher PL intensity when compared with the other two perovskites (MA₃Bi₂I₉Cl_x and MA₃Bi₂I₉). Cs₃Bi₂I₉-based devices exhibited the best performance with J_sc = 2.15 mA cm⁻¹, V_oc = 0.85 V, FF = 0.60, and PCE = 1.09% (Fig. 5b). Furthermore, the light absorption spectra and PCE showed marginal changes after one-month storage (in dry air, humidity less than 10%, and in the dark). It has been demonstrated that Bi-based perovskites exhibited high PLQY and stability against moisture and heat. Such enhanced stabilities could be attributed to their lower-dimensional crystal phase as compared to regular 3D Pb halide perovskites, but the lower dimension also accounts for the lower PV performance. Cs₃Bi₂X₉ NCs were synthesized by a facile room-temperature reaction.⁸⁰ The emission wavelength could be tuned from 400 nm to 560 nm with different halide compositions. The PLQY of Cs₃Bi₂Br₃ was improved from 0.2% to 4.5% by adding oleic acid surfactant that passivated the fast trapping process. Furthermore, Cs₃Bi₂Br₉ NCs exhibited high air stability for over 30 days because the adsorption of water on the surface of Cs₃Bi₂Br₉ formed perovskite hydrates, thereby passivating the perovskite.

Fig. 5 (a) Crystal structure of Cs₃Bi₂I₉ and (b) performance of solar cells fabricated using Bi-based perovskites. Reprinted with permission from ref. 79. Copyright 2015 John Wiley and Sons. (c) Stability of Cs₃Bi₂Br₉ against moisture.³³ (d) Quality of Cs₃Bi₂I₉ and CsBi₃I₁₀ films. Reprinted with permission from ref. 81. Copyright 2016 American Chemical Society.

Lou and coworkers reported Cs₃Bi₂X₉ (X = Cl, Br, I) QDs prepared by the steady binding of octylammonium halide (OA–X) and oleic acid.⁸² Cs₃Bi₂Br₉ exhibited PLQY of up to 22% with an emission peak at 414 nm and a full width at half maximum value of 38 nm, which could remain stable even up to 180 °C. In addition, Cs₃Bi₂Cl₉ and Cs₃Bi₂I₉ could be easily obtained by a fast-reversible anion exchange reaction, exhibiting fluorescence emission wavelengths ranging from 380 nm to 526 nm with PLQYs of 62% and 2.3%, respectively.

Nelson et al. synthesized Cs₃Bi₂Br₉ NCs by a hot-injection approach.⁸³ The growth of Cs₃Bi₂Br₉ was initiated by the formation of Bi⁰ seeds, and the morphologies were controlled by the precursor and seed concentrations. The perovskites exhibited excellent stability with unchanged characteristic absorbance peak at 489 nm exposed to the ambient condition. Cs₃Bi₂X₉ perovskite QDs were reported by Leng et al. using ethanol as the main solvent.³³ The tunable PL peaks of the perovskites were adjusted from 393 nm to 545 nm by means of different halide compositions. Cs₃Bi₂Br₉ QDs achieved PLQY of ∼19.4% with the emission peak at 410 nm and FWHM value of 48 nm. Furthermore, as shown in Fig. 5c, the hydrolytic product BiOBr passivated the surface defects of the perovskite when Cs₃Bi₂Br₉ was exposed to the ambient condition, resulting in good stability. Cs₃Bi₂Br₉ QDs exhibited only 10% relative reduction in PL intensity after storage for 8 h.

Johansson et al. synthesized CsBi₃I₁₀ with a smaller bandgap (1.77 eV) than that of Cs₃Bi₂I₉.⁸¹ As shown in Fig. 5d, CsBi₃I₁₀ exhibited a layered crystal structure, giving more uniform coating, while Cs₃Bi₂I₉ dominated in a different growth direction. The solar cells based on CsBi₃I₁₀ yielded higher photocurrent, which could be attributed to the extended light harvesting originating from the smaller bandgap as compared to that of Cs₃Bi₂I₉. However, the PCE of devices was still low (0.4%). The CsBi₃I₁₀ film was stable in ambient air for 17 h.

2.4 Sb

Trivalent Sb with lone-pair 5s² electrons was considered as another candidate for lead substitution.^84,85 Furthermore, Sb is more than twice as cheap as Sn.⁸⁶ The structure of A₃Sb₂X₉ is the same as A₃Bi₂X₉, where Cs₃Sb₂I₉ also possessed the 0D dimer form and 2D layered form. (NH₄)₃Sb₂I_xBr_9−x (0 ≤ x ≤ 9) was prepared using ethanol as the solvent. The absorption peak could be tuned from 558 nm to 453 nm with an increase in Br. Although the PCE of (NH₄)₃Sb₂I₉-based devices was still low (0.51%), this work provided a nontoxic way to fabricate perovskite solar cells.⁸⁷ The all-inorganic perovskite Cs₃Sb₂I₉ was prepared by a two-step deposition approach.⁸⁸Fig. 6a shows the 2D structure of Cs₃Sb₂I₉. Cs₃Sb₂I₉ exhibited indirect bandgap of 2.05 eV and enhanced stability against ambient conditions as compared to those of MAPbI₃. However, the solar cells based on Cs₃Sb₂I₉ exhibited a V_oc value of 0.25–0.30 eV, which is lower than its organic–inorganic hybrid analogue (CH₃NH₃)₃Sb₂I₉ (0.89 V).⁸⁹

Fig. 6 (a) Structures of Cs₃Sb₃I₉ and Cs₃Sb₂I₉. Reprinted with permission from ref. 88. Copyright 2015 American Chemical Society. (b) Structure of A₂TiX₆. Reprinted with permission from ref. 90. Copyright 2018 American Chemical Society. (c) Schematic illustration for preparing Cs₂TiBr₆ by vapor deposition method. Reprinted with permission from ref. 91. Copyright 2018 Elsevier.

2.5 Ti

Ti, an earth-abundant, nontoxic, and ultrastable element, was discovered as a promising candidate for substitution with Pb.⁹⁰Fig. 6b shows the crystal structure of A₂TiX₆. As compared to APbX₃, A₂TiX₆ has a vacancy-ordered double perovskite structure, similar to that of A₂SnX₆. Padture and coworkers prepared Cs₂TiI_xBr_6−x and its bandgap could be tuned from 1.02 eV to 1.78 eV.⁹⁰ This group also synthesized Cs₂TiBr₆ by a two-step vapor deposition method (Fig. 6c).⁹¹ Cs₂TiBr₆ thin films showed bandgap of 1.82 eV with carrier diffusion lengths for electrons and holes of 121 nm and 103 nm, respectively. C₆₀ facilitated electrons transfer from Cs₂TiBr₆ to TiO₂ and improved the microstructure of Cs₂TiBr₆ thin films. Therefore, solar cells based on Cs₂TiBr₆ achieved PCE of 3.28% with V_oc = 1.02 V, FF = 0.56, and J_sc = 5.69 mA cm⁻². Furthermore, Cs₂TiBr₆ thin films exhibited much higher tolerance against heat, moisture, and light when compared with MAPbI₂Br.

2.6 Cu

Cu, a transition metal with the electronic configuration 3d⁹ (t⁶_2ge³_g), has attracted considerable attention due to the excellent stability of its oxidation state and large absorption coefficient of its compounds.⁹² The organic–inorganic hybrid copper-based perovskite C₆H₄NH₂CuBr₂I was prepared with excellent hydrophobic properties. The XRD patterns remained unchanged even after 4 h of water immersion. However, solar cells based on such perovskites exhibited poor photovoltaic performance with PCE of 0.5%.⁹³ All-inorganic copper-based perovskite Cs₂CuX₄ (X = Cl, Br, Br/I) QDs were firstly synthesized by Lou and coworkers.⁹⁴ PL peaks could be tuned from 385 nm to 468 nm by the anion exchange reaction. Furthermore, the bandgap tuning of such perovskites could be attributed to the particle size by adjusting the ratios of the precursor. These copper-based perovskites exhibited excellent air and photostability when compared with other lead-free perovskites. Cs₂CuBr₄ maintained 92% of the original PL intensity after storage under ambient conditions for 30 days. Even after 5000 min of UV-light irradiation, 34.2% of the initial PL intensity was retained.

2.7 Double perovskites

Recently, it was reported that Pb²⁺ cation could be substituted with one monovalent M⁺ and one trivalent M′³⁺ cation to form 3D double perovskites AMM′X₆ (A = Cs, CH₂NH₃; M = Ag; M′ = Bi, Sb; X = Cl, Br, I) (Fig. 7a).^95–97 Cs₂AgBiBr₆ with indirect bandgap of 1.95 eV was reported by Slavney et al.⁹⁸ The lifetime of the intermediate progress in powders is much shorter than that in single crystals due to the higher number of defects and surface states in the powder, indicating that the short and intermediate progresses originate from the trap and surface state emissions, respectively. The radiative recombination lifetime of Cs₂AgBiBr₆ was higher than that of MAPbI₃ films. Further, Cs₂AgBiBr₆ exhibited better stability against light and moisture than MAPbI₃.

Fig. 7 (a) Schematic illustration of Pb substituted by M³⁺ + M⁺. Reprinted with permission from ref. 97. Copyright 2017 The Royal Society of Chemistry. (b) Photocatalytic CO₂ reduction performance of Cs₂AgBiBr₆. Reprinted with permission from ref. 100. Copyright 2018 John Wiley and Sons. (c) Stability of Cs₂AgInCl₆ under continuous illumination. Reprinted with permission from ref. 102. Copyright 2018 Springer Nature. (d) Responses of CsAgBiBr₆ and MAPbBr₃ with and without tubes, respectively. Reprinted with permission from ref. 43. Copyright 2017 Springer Nature.

McClure et al. synthesized Cs₂AgBiBr₆ and Cs₂AgBiCl₆ from both solid state and solution routes.⁹⁹ The presence of Ag reduced the bandgap and resulted in indirect bandgap. The bandgaps of 2.19 eV and 2.77 eV were determined for Cs₂AgBiBr₆ and Cs₂AgBiCl₆, respectively. The reflectance spectrum and XRD patterns of Cs₂AgBiCl₆ changed marginally when the samples were exposed to both light and air for 4 weeks. Zhou et al. synthesized Cs₂AgBiBr₆ NCs with a cubic shape and high crystallinity via a hot-injection method.¹⁰⁰ These NCs exhibited stability against 55% relative humidity (RH) for 90 days and retained their structure in low-polarity solutions up to 3 weeks. These Cs₂AgBiBr₆ NCs were used in the photochemical conversion of CO₂, achieving total electron consumption of 105 μmol g⁻¹ under AM 1.5G illumination for 6 h (Fig. 7b). Zhou et al. synthesized pure Cs₂AgInCl₆ crystals with dimensions from several micrometers to several millimeters by a hydrothermal method.⁹⁷ The UV-vis measurement results indicated that the direct bandgap of Cs₂AgInCl₆ crystals was 3.23 eV, which was in good agreement with the result obtained from the band structure and optical absorption calculations (3.33 eV). Furthermore, the crystals exhibited no obvious changes when stored under both light and moisture conditions for 26 days. The double perovskite was relatively stable at about 400 °C.

Cs₂AgSbCl₆ and Cs₂AgSbCl₆/TiO₂ heterostructure nanoparticles have been fabricated to investigate the interfacial effect.¹⁰³ The Cs₄Cl₄–TiO₂ interface enhanced the separation of photoinduced carriers, which decreased the bandgap and improved light absorption. Therefore, light absorption exhibited a significant red-shift. Moreover, the interfacial states facilitated the tunneling of the photoinduced electrons from the double perovskite layer to TiO₂ to improve the PCE of devices. Gao et al. prepared smooth and highly crystalline Cs₂AgBiBr₆ film when isopropanol (IPA) was used as the antisolvent.¹⁰¹ The crystallinity increased with increased temperatures up to 300 °C. The fabricated devices based on Cs₂AgBiBr₆ films achieved PCE of 2.23% with V_oc = 1.01 V, J_sc = 3.19 mA cm⁻², and FF = 0.69. In addition, the solar cells exhibited long-term stability when stored in air for over 10 days. Tang and coworkers reported double-perovskite Cs₂AgInCl₆ with efficient and stable white-light emissions.¹⁰² They revealed that the PL efficiency of Cs₂AgInCl₆ could be improved by three orders of magnitude by alloying sodium cations. In addition, Cs₂(Ag_0.60Na_0.40)InCl₆ achieved 86 ± 5% quantum efficiency and worked for over 1000 h (Fig. 7c). Cs₂AgBiBr₆ single crystals were applied in X-ray detectors with low detection limits.⁴³ Such a detector exhibited a low detection limit of 59.7 nGyair s⁻¹, which was comparable to that of detectors based on MAPbBr₃ single crystals (0.036 μGyair s⁻¹). Furthermore, as shown in Fig. 7d, devices based on Cs₂AgBiBr₆ exhibited strong response after passing through a thick copper tube, while MAPbBr₃ control devices showed no response under the same conditions, indicating the potential industrial applications of Cs₂AgBiBr₆-based detectors.

3. Conclusions and outlook

In summary, although hybrid lead halide perovskite materials have made considerable progress in optoelectronic applications, there are still several challenges for commercial applications. The intrinsic toxicity of lead perovskites and instability of the organic components can be overcome by all-inorganic lead-free perovskites. Here, we summarize the recent progresses made in all-inorganic lead-free perovskites that can be used for optoelectronic applications. Several research efforts have been devoted toward substituting lead with different elements. Sn-Based perovskites exhibited narrower direct bandgap as compared to their Pb analogues, but the intrinsic instability of Sn²⁺ induced a serious self-doping problem, which significantly affected the performance of Sn-based perovskites. Bi- and other transition-metal-based lead-free perovskites showed excellent chemical stability, but there are still many challenges. Bi³⁺- and Sb³⁺-based perovskites usually exhibit low-dimension crystal phases, which account for their enhanced stabilities, but this also leads to wider bandgaps and lower PV performances. The main problem in novel double perovskites with Bi and Ag are their indirect bandgap, which is undesirable in photovoltaic applications, but they exhibited promising potential for other optoelectronic applications, particularly white-light PL. Currently, all-inorganic lead-free perovskite-based solar cells have exhibited far lower PV performances as compared to hybrid and all-inorganic lead halide perovskites (Table 2). Therefore, much more experimental and theoretical research efforts need to be devoted toward understanding the electrical and optical properties of such lead-free perovskites. Further, new charge transport materials as well as synthesis technologies need to be developed to match the energy level of lead-free perovskites to improve their photovoltaic performance.

Table 2 Photovoltaic performances of solar cells based on all-inorganic lead-free perovskites

Absorber	PCE (%)	V oc (V)	J sc (mA cm^−2)	FF (%)	Ref.
CsSnI3	4.81	0.38	25.71	49	65
CsSnI3	2.76	0.43	12.3	39	61
CsSnI3	3.31	0.52	10.21	62	68
CsSnI3	3.26	0.40	11.25	56	59
CsSnI2.9Br0.1	1.76	0.22	24.16	33	58
CsSnI3	2.02	0.24	22.70	37	57
CsSnBr3	3.04	0.36	13.96	59	54
CsSnBr3	2.1	0.41	9	58	60
CsSnBr3	0.55	0.45	2.4	55	62
CsSnIBr2	3.0	0.33	16.7	53	63
CsSnCl3	9.66	0.87	19.82	56	70
CsSnBr3	10.46	0.85	21.23	58	70
CsSnI3	12.96	0.86	23.2	65	70
Cs2SnI6	1.47	0.36	6.75	59	73
Cs2SnI5Br1	1.60	0.44	6.57	55	73
Cs2SnI4Br2	2.02	0.56	6.22	57	73
Cs2SnI2Br4	1.08	0.57	3.41	54	73
Cs2SnI1Br5	0.002	0.57	0.01	37	73
CsGeI3	4.94	0.51	18.78	51	77
CsGeBr3	4.92	0.48	19.49	52	77
CsGeCl3	2.57	0.35	18.57	40	77
Cs3Bi2I9	1.09	0.85	2.15	60	79
CsBi3I10	0.40	0.31	3.40	38	81
Cs2TiBr6	3.28	1.02	5.69	56	91
Cs2AgBiBr6	2.23	1.01	3.19	69	101

---

Although the photovoltaic performance of all-inorganic lead-free perovskites still needs to go a long way to be able to compete with that of hybrid lead halide perovskites, such all-inorganic lead-free perovskites are still promising for use in other optoelectronic applications. Therefore, it still requires additional research efforts including both theoretical predictions and experimental optimizations to develop all-inorganic lead-free perovskites with high performance and good stability.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the NSFC (Grant 51861145101, 21777096), Huoyingdong Grant (151046), Shanghai Shuguang Grant (17SG11).

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