“…Among the many A 2 BX 6 compounds, [96][97][98][99] A 2 SnI 6 (A = Cs, MA), [100][101][102][103][104][105][106][107][108][109] Cs 2 PdBr 6 , [110,111] and Cs 2 TiBr 6 [112,113] have been investigated for photovoltaic applications. As the [BX 6 ] octahedra in A 2 BX 6 compounds are isolated, the optoelectronic properties of these compounds are rather different from those of 3D ABX 3 (B = Pb, Sn, and Ge) halide perovskites.…”
Section: D a 2 B(iv)x 6 Perovskite Variantsmentioning
serious challenges due to the inclusion of toxic Pb and instability against moisture, heat, and irradiation. In recent years, significant efforts have been paid to develop Pb-free halide perovskite and perovskite derivative absorber materials. However, so far, solar cells based on these materials show significantly inferior performances as compared to their Pb halide perovskite counterparts. Herein, we provide reviews on the fundamental understandings of the photovoltaic properties from Pb halide perovskites to Pb-free metal halide perovskites and perovskite derivatives as well as the experimental results reported in the literature. The results suggest that replacing Pb by nontoxic elements may degrade the superior photovoltaic properties seen in Pb halide perovskites, explaining the fact that all reported solar cells based on Pb-free perovskites or perovskite derivatives show performances significantly lower than Pb halide perovskite solar cells.
Overview: Approaches and Consequences of Pb ReplacementWe first provide an overview of the approaches and consequences of Pb replacement reported in the literature, as shown in Figure 1. In general, there are two categories for Pb replacement: using either homovalent elements such as Sn and Ge or heterovalent elements such as Bi and Sb. The heterovalent replacement category can be divided into two subcategories based on the ways to maintain the charge neutrality: ion-splitting and ordered vacancy. The ion-splitting subcategory can be further divided into mixed anion compounds with the formula of AB(Ch,X) 3 , where Ch represents a chalcogen element, and X represents a halogen element, and mixed cation compounds with a formula of A 2 B(I)B(III)X 6 , which are commonly called double perovskites. The ordered vacancy subcategory can also be divided into the B(III) compounds with a formula of A 3 ◻B(III)X 9 and the B(IV) compounds with a formula of A 2 ◻ B(IV)X 6 (the sign of ◻ indicates vacancy). Figure 1 also summarizes the photovoltaic properties and the stabilities of each group of materials, which provides a clear overview of the consequences of Pb replacement. While the homovalent replacement by Sn or Ge leads to instability, the heterovalent Pb replacement leads to degraded electronic properties mainly due to the reduction on electronic dimensionality. As a result, all the reported Pb-free perovskite and perovskite derivative solar Despite the exciting progress on power conversion efficiencies, the commercialization of the emerging lead (Pb) halide perovskite solar cell technology still faces significant challenges, one of which is the inclusion of toxic Pb. Searching for Pb-free perovskite solar cell absorbers is currently an attractive research direction. The approaches used for and the consequences of Pb replacement are reviewed herein. Reviews on the theoretical understanding of the electronic, optical, and defect properties of Pb and Pb-free halide perovskites and perovskite derivatives are provided, as well as the experimental results available in the literature....
“…Among the many A 2 BX 6 compounds, [96][97][98][99] A 2 SnI 6 (A = Cs, MA), [100][101][102][103][104][105][106][107][108][109] Cs 2 PdBr 6 , [110,111] and Cs 2 TiBr 6 [112,113] have been investigated for photovoltaic applications. As the [BX 6 ] octahedra in A 2 BX 6 compounds are isolated, the optoelectronic properties of these compounds are rather different from those of 3D ABX 3 (B = Pb, Sn, and Ge) halide perovskites.…”
Section: D a 2 B(iv)x 6 Perovskite Variantsmentioning
serious challenges due to the inclusion of toxic Pb and instability against moisture, heat, and irradiation. In recent years, significant efforts have been paid to develop Pb-free halide perovskite and perovskite derivative absorber materials. However, so far, solar cells based on these materials show significantly inferior performances as compared to their Pb halide perovskite counterparts. Herein, we provide reviews on the fundamental understandings of the photovoltaic properties from Pb halide perovskites to Pb-free metal halide perovskites and perovskite derivatives as well as the experimental results reported in the literature. The results suggest that replacing Pb by nontoxic elements may degrade the superior photovoltaic properties seen in Pb halide perovskites, explaining the fact that all reported solar cells based on Pb-free perovskites or perovskite derivatives show performances significantly lower than Pb halide perovskite solar cells.
Overview: Approaches and Consequences of Pb ReplacementWe first provide an overview of the approaches and consequences of Pb replacement reported in the literature, as shown in Figure 1. In general, there are two categories for Pb replacement: using either homovalent elements such as Sn and Ge or heterovalent elements such as Bi and Sb. The heterovalent replacement category can be divided into two subcategories based on the ways to maintain the charge neutrality: ion-splitting and ordered vacancy. The ion-splitting subcategory can be further divided into mixed anion compounds with the formula of AB(Ch,X) 3 , where Ch represents a chalcogen element, and X represents a halogen element, and mixed cation compounds with a formula of A 2 B(I)B(III)X 6 , which are commonly called double perovskites. The ordered vacancy subcategory can also be divided into the B(III) compounds with a formula of A 3 ◻B(III)X 9 and the B(IV) compounds with a formula of A 2 ◻ B(IV)X 6 (the sign of ◻ indicates vacancy). Figure 1 also summarizes the photovoltaic properties and the stabilities of each group of materials, which provides a clear overview of the consequences of Pb replacement. While the homovalent replacement by Sn or Ge leads to instability, the heterovalent Pb replacement leads to degraded electronic properties mainly due to the reduction on electronic dimensionality. As a result, all the reported Pb-free perovskite and perovskite derivative solar Despite the exciting progress on power conversion efficiencies, the commercialization of the emerging lead (Pb) halide perovskite solar cell technology still faces significant challenges, one of which is the inclusion of toxic Pb. Searching for Pb-free perovskite solar cell absorbers is currently an attractive research direction. The approaches used for and the consequences of Pb replacement are reviewed herein. Reviews on the theoretical understanding of the electronic, optical, and defect properties of Pb and Pb-free halide perovskites and perovskite derivatives are provided, as well as the experimental results available in the literature....
“…The measurement shows that Cs 2 TiI 2 Br 4 possesses a direct bandgap of 1.38 eV (Figure 10d,e), which is close to the ideal bandgap (1.34 eV), while Cs 2 TiBr 6 exhibits a larger bandgap (1.8 eV). [174] Despite the significant reduction in bandgap to 1.22 eV when Cl is replaced with Br, the indirect character and the aforementioned energy level mismatch create serious limitations to its applications in solar cells and other optoelectronic devices. Other factors such as nonmatching energy levels of vacancy ordered halide double perovskites, e.g., Cs 2 PdBr 6 [173] with common electron and hole extraction layers need to be addressed.…”
Section: Wwwadvancedsciencenewscommentioning
confidence: 99%
“…Although the diode did not exhibit significant rectification, an increase in conductivity under illumination was observed. Calculations on Cs 2 PdX 6 (X = Cl, Br) by Bhamu et al revealed their p‐type nature and indirect bandgap . Despite the significant reduction in bandgap to 1.22 eV when Cl is replaced with Br, the indirect character and the aforementioned energy level mismatch create serious limitations to its applications in solar cells and other optoelectronic devices.…”
Section: Optoelectronic Properties Of Halide Double Perovskitesmentioning
Although impressive performance has been obtained, PSCs are still far from commercial or real-life availability due to serious issues such as toxicity [15,16] and poor stability to heat, [17] oxygen, [18] moisture, [19,20] electric field, [21] and light. [18,22] The toxic nature of hybrid organic-inorganic lead halide perovskites has been traced to the presence of Pb in its chemical composition. [15,16,[23][24][25] Pb 2+ readily dissolves in water (e.g., rain water) to form a toxic solution capable of causing serious environmental pollution, harmful to human beings and the ecosystem. Besides their sensitivity to moisture, oxygen, light, electric field, or thermal stress, an existing self-degradation pathway [26,27] is also a big issue in hybrid organic-inorganic lead halide perovskites. Mixed-halide and mixed-cation perovskites have been investigated to address these issues. [28][29][30][31][32] The group IV elements, tin (Sn) [23,[33][34][35] and germanium (Ge), [34,36] have been employed as the replacements for Pb. However, the device performance through this approach has fallen short of the Pb-based ones. For example, the PCEs reported for Sn-based perovskite solar cells are usually less than 10%. [23,[33][34][35][37][38][39][40] In addition, the easy oxidation of Sn and Ge from the +2 state to the +4 state due to their high energy 5s and 4s orbitals makes them less promising for application in stable and long-term PSCs. [41] High throughput calculations also demonstrate that these substitutions are likely to compromise the ideal optoelectronic properties of MAPbI 3 . [42,43] Furthermore, low dimensional (e.g., 2D, 1D, and 0D) perovskites have also been used to address the stability issues in PSCs. [44][45][46][47][48] Recently, a stabilized PCE of 21.7% resulting from a 2D/3D bilayer PSC was reported. [49] However, the highest certified PCE in a 2D-only planar PSC is 15.3%, [50] which is far below that of their 3D perovskite-based counterparts. It thus stimulates the interest to develop new classes of materials which can solve the issues of toxicity and stability while still maintaining the fascinating properties of lead-based perovskite materials.Recent theoretical calculations demonstrate that a halide double perovskite structure, A 2 B′B″X 6 , which could be formed through a replacement of two toxic Pb 2+ in the crystal lattice with a pair of nontoxic heterovalent (i.e., monovalent and trivalent) metal cations, is a promising alternative to realize high-performance, lead-free, and stable PSCs. [51,52] Although, spectroscopic limited maximum efficiency (SLME) calculations revealed an efficiency limit less than 8% for the most prominent member www.advancedsciencenews.com double perovskites with a vacancy ordered structure. [88] In particular, the unit cell axis of Cs 2 AgBiBr 6 is given to be ≈11.25 Å, [25] which is two times larger than that of MAPbBr 3 (≈5.92 Å). [89] Both Ag + and Bi 3+ occupy the B-site of the crystal lattice with slightly varied metal-halide bond lengths. The dissimilar bond lengths st...
“…Reproduced with permission . Copyright 2014, American Chemical Society. c,d) Electronic band structure of Cs 2 PdBr 6 and 3D plot of the wave function distribution at the points marked 1, 2, and 3 on the band structure [71b]. Reproduced with permission.…”
Section: Materials and Structuresmentioning
confidence: 99%
“…With an aim of removing the lead content in the materials, a new perovskite system Cs 2 PdBr 6 is also reported . To study the photophysical properties of Cs 2 PdBr 6 , a band structure is calculated by the GGA functional and corrected by the HSE hybrid functional, obtaining an indirect bandgap between Γ point (VBM) and X point (CBM) of 1.56 eV [71b]. However, as Figure c shows, the valence band edge at the X point is only 0.03 eV that is lower than that at the Γ point.…”
Lead halide perovskite (ABX3) has attracted considerable attention due to its applicability as absorber layers in highly efficient photovoltaic cells. With regard to the lead toxicity, double‐metallic lead‐free perovskite, A2BIBIIIX6, in which the neighboring B+ and B3+ sites in the crystal microstructure are alternately occupied by monovalent‐metal and trivalent‐metal cations, is regarded to be a promising alternative to the widely used lead‐based perovskites. This review aims to summarize the recent advances in the new class of A2BIBIIIX6 double‐metallic lead‐free perovskites. In particular, the electronic structure, synthesis, property, and their applications in devices, for example, photovoltaics, photodetectors, and light emitting diodes, is carefully classified and presented. Notably, the theoretical calculations point out that there is much room toward potential applications for this new class of perovskite materials. The present review provides a holonomic conclusion and opens new perspectives toward realizing higher performance of A2BIBIIIX6‐based devices.
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