Structural and Functional Diversity in Lead-Free
Halide Perovskite Materials
Weihua Ning and Feng Gao
The self-archived postprint version of this journal article is available at Linköping University Institutional Repository (DiVA):
http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-159290
N.B.: When citing this work, cite the original publication.
Ning, W., Gao, F., (2019), Structural and Functional Diversity in Lead-Free Halide Perovskite Materials, Advanced Materials, 31(22), 1900326. https://doi.org/10.1002/adma.201900326
Original publication available at:
https://doi.org/10.1002/adma.201900326 Copyright: Wiley (12 months)
1
Structural and Functional Diversity in Lead-Free Halide Perovskite Materials
Weihua Ning, Feng Gao*
Dr. W. Ning, Prof. F. Gao
Department of Physics, Chemistry, and Biology (IFM), Linköping University, Linköping SE-581 83, Sweden
E-mail: feng.gao@liu.se Dr. W. Ning
Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Nanjing Tech University, 30 South Puzhu Road, Nanjing, 211816, P. R. China
Keywords: lead-free perovskites, halide double perovskites, structure-property relationships, optoelectronic devices, multifunctional perovskites
Abstract
Lead halide perovskites have emerged as promising semiconducting materials for different applications owing to their superior optoelectronic properties. Although the community hold different views towards the toxic lead in these high-performance perovskites, it is certainly preferred to replace lead with non-toxic, or at least less-toxic, elements while maintaining the superior properties. Here, we present the design rules for lead-free perovskite materials with structural dimensions from three dimension (3D) to zero dimension (0D). We review recent progress on lead-free halide perovskites, and summarize the relationships between the structures and fundamental properties, in particular optical, electric, magnetic-related properties. 3D perovskites, especially A2B+B3+X6-type double perovskites, demonstrate very promising
optoelectronic prospects; while low-dimensional perovskites show rich structural diversity, resulting in abundant properties for optical, electric, magnetic and multifunctional applications. Furthermore, based on these structure-property relationships, strategies on multifunctional perovskite design is proposed. We also highlight the challenges and future directions on lead-free perovskite applications, with emphasis on materials development and device fabrications. The research on lead-free halide perovskites at Linköping University benefits from inspirational discussions with Prof. Olle Inganäs. We would also like to take this opportunity to congratulate his great achievements on a range of different areas throughout his scientific career.
2 1. Introduction
Lead halide perovskites, APbX3 (A = Cs+, methyl ammonium, formamidinium, and other
monovalent organic cations; X = Cl, Br, I), have recently taken a dominant position within the portfolio of compounds, due to their promising chemical and physical properties.[1–4] Strong
interest in lead halide perovskites originated from their superior optoelectronic properties, including long carrier diffusion length, high absorption coefficient, excellent charge transport, and high photoluminescence (PL) efficiencies.[4–8] These properties have resulted in
high-performance optoelectronic devices, including solar cells and light-emitting diodes (LEDs). Very recently, the interest is also extended to ferroelectric, dielectric, piezoelectric, ferromagnetic, and multiferroic properties,[4,9] mainly due to the structural diversity of these
materials. Despite the intensive development of lead halide perovskites, the toxicity of Pb could present a bottleneck for further development. To remedy the lead toxicity issue, the community has now taken increasing efforts to develop new lead-free perovskite materials. Most of these lead-free perovskites should actually be termed as perovskite derivatives, as the strict definition of perovskites should be confined in three dimensionally connected BX6 octahedrons. However,
for simplicity, we simply term these perovskite derivatives also as perovskites, consistent with the common definition in the community.
We note that there have been a range of different lead-free perovskite materials reported, some of which show decent device performance. However, the structure-property relationships in these lead-free perovskites are largely unclear. We present the design rules for the diverse lead-free perovskite materials, and demonstrate how these different lead-free perovskite structures develop from the parent perovskite ABX3. We show the crystal structures of different
lead-free perovskite types, systematically summarize the relationships between structures and optical, electric, magnetic-related properties, and correlate their properties with the performance of the resulting devices. In addition, it is worth noting that lead-free perovskite materials also provide us unique opportunities to achieve multifunctional materials through rational structure design. Finally, we also discuss the challenges and future directions of lead-free perovskite materials for practical technology applications.
2. Design Rules for Lead-free Perovskite Materials
The general chemical formula of the perovskites can be expressed as ABX3, which usually
crystallize in cubic Pm-3m space group. We take ASnX3 as an example. In this case, the cation
3
the anion X is a halide ion, such as Cl, Br, I; Sn and X ions form the SnX6 octahedra which
assembles into a three-dimensional (3D) network by sharing the octahedra corners, with A cations occupying the cubo-octahedral cavities. Based on the parent 3D ABX3 perovskites, we
present the design rules for new lead-free perovskite materials (Figure 1). The key factors affecting the 3D structure include the ionic radius, ionic valence, and the coordination type of A, B, X ions (Table 1).
i) The ionic radii of A, B and X ions. With the assumptions of sphere ions and close packing configurations, the effects of ionic radii on the perovskite structures can be quantified by two factors, i.e. Goldsschmidt’s Tolerance Factor (T.F.)[10] and the Octahedral factor (O.F.). Both
structure factors can be calculated from the ionic radii of A, B, X ions (RA, RB, and RX,
respectively). The T.F. is calculated by (RA + RB)/[√2(RB + RX)], and the O.F. is calculated by
RB/RX. A T.F. between 0.813-1.107 (ideally 1) combined with an O.F. between 0.44-0.90 is
required to maintain the 3D structure.[11] The symmetry of the 3D structures is determined by
the T.F. value, with a T.F. value close to 1 resulting in a high symmetry structure. For example, in the case of ASnI3, we take the Shannon ionic radius RSn2+ of 1.18 Å and RI- of 2.20 Å,[12] and
obtain an O.F. of 0.54, which lies between 0.44 and 0.90. With a T.F. of 1, the most suitable RA
is 2.58 Å to maintain the high symmetry 3D structure of ASnI3. When the T.F. value deviates
from 1, the high symmetry structure will change to lower-symmetry structures. For a T.F. value significantly away from 1, the 3D structures will be tortured, warped, and finally destroyed, resulting low-dimensional (low-D) perovskites. These two factors are quite important for designing new lead-free perovskite materials with different structural dimensionality.
ii) The ionic valences of B cations. Like all other materials, the perovskites must be electrically neutral. In 3D ABX3 perovskites, the cation B must be bivalent (+2), since the anion X is
negatively monovalent (-1) and the cation A is monovalent (+1). The typical examples of divalent B cations include group 14 metal ions (e.g. Ge2+, Sn2+ etc.), divalent rare-earth ions
(e.g. Yb2+, Eu2+ etc.), and alkaline-earth ions (e.g. Sr2+, Ca2+ etc.).[13–15] Note that two B2+ cations
can be replaced by a combination of B+ and B3+ cations, or by a combination of B4+ and a
vacancy, as is the case for 3D lead-free double perovskites Cs2Au+Au3+I6 and Cs2Pd4+◊Br6 (◊
stands for a vacancy).[16,17] Four B2+ cations can also be replaced by B3+, B5+ cations and two
vacancies, forming the vacancy ordered perovskites, such as Cs4Sb3+Sb5+Cl12.[18]
iii) The coordination types of B cations. The B cations incorporated into the perovskite framework must satisfy their coordination tendencies, which can greatly influence their crystal structures. For example, in 3D lead-free perovskite CsSnI3, the Sn2+ cations prefer octahedral
4
coordination with X- anions, so that Sn and X can form BX6 octahedra easily, resulting in highly
symmetric 3D structures.[19] On the contrary, in Cu+-based double perovskite Cs2Cu+Bi3+Br6,
Cu+ cations energetically favor 4-fold coordination and form the CuX4 tetrahedra, instead of
6-fold coordination as required for CuX6 octahedra.[20] The coordination tendency explains why
Cu+-based double perovskite materials (such as Cs2Cu+Bi3+Br6, Cs2Cu+In3+Cl6 etc.) are
unstable.
According to the above three key factors, we present the design rules for lead-free perovskites. We start with the classical 3D AB2+X3 perovskites and derive a wide range of
lead-free perovskites, using the B-site, A-site, and A+B-site substitution strategies. Note that we do not include the X-site substitution in this paper. Although X-site substitution can affect the properties of perovskites (e.g. bandgap, stability, etc.), it does not result in much difference of the structure. We now discuss the different substitution strategies in detail. i) B-site substitution. All the three factors, ionic radius, ionic valence and coordination type, can be possibly changed after B-site substitution, providing wide structural flexibility. As a result, eight different types of perovskite structures, including 3D AB2+X
3, 2D A2B2+X4, 2D A3B3+2X9, 0D A3B3+2X9, 3D
A2B+B3+X6, 2D A4B2+B3+2X12, 3D A2B4+X6, 3D A4B3+B5+X12, have been derived. The structure
dimensions, which strongly relate to their fundamental properties, range from 3D to 0D. ii) A-site substitution. Both of the ionic radius and the ionic valence can be varied with the A-A-site substitution. Although A2+ or A3+ can also result in interesting properties (e.g. strong
emission),[21,22] we only consider A+ substitution in this paper (except for metal-free perovskites,
Section 5), as A2+ or A3+ always lead to irregular structures.[23] The ionic radii of A can
significantly affect the T.F., as determined by (RA + RB)/[√2(RB + RX)]. Through increasing the
ionic radius of A ions, the structure dimensionality can possibly reduce. For example, the classic 3D AB2+X3-type structures can be sliced into a 2D form with a general formula of A2B2+X4
with the large size A incorporation. iii) A+B-site substitution. Combining the two strategies above, plenty of novel low-D lead-free perovskite structures (e.g. 2D A2B2+X4, 1D AB2+X3, 2D
A4B+B3+X8, 0D A3B3+2X9, 1D A2B3+X5, 0D AB5+X6 and 3D metal-free A2+NH4+X3) can be
developed. These different structures provide rich optical, electric, magnetic-related properties for lead-free perovskites.
3. Three-Dimensional Lead-free Perovskites
3D lead-based perovskites have shown great success for a range of optoelectronic applications. With the motivation to replace lead with less-toxic (or non-toxic) elements, it is therefore natural that the community prefers to maintain the 3D structure, aiming to keep the excellent
5
optoelectronic properties. As a result, the design and synthesis of 3D highly symmetric lead-free perovskites are one of the most important issues in the research of lead-lead-free perovskites. In order to maintain a stable 3D AB2+X3 perovskite structure, the ionic radii, ionic valences and
coordination types of A, B, X ions must meet the above design rules. As shown in Figure 1, there are mainly four types of 3D (including Quasi-3D vacancy-ordered perovskites) lead-free perovskites, AB2+X3, A2B+B3+X6, A2B4+X6, A4B3+B5+X12. Their structure information as well
as structure-related properties will be discussed in detail in this section. 3.1. AB2+X3-type Perovskites
The most straightforward approach to develop 3D lead-free perovskites is to replace lead (Pb) with another group 14 elements, e.g. tin (Sn) and germanium (Ge). Due to the relativistic effects, group 14 ions (Pb2+, Sn2+, Ge2+) show similar radii, ionic valences and coordination types
(Table 1), resulting in similar crystal structures and phase-transition behaviors. For example, both CsPbI3 and CsSnI3 have two different crystal phases at room temperature. One is yellow
in color (yellow phase), showing a one-dimensional double-chain structure; while the other one is black in color (black phase), showing a 3D perovskite structure.[19,24] 3D black phases show
superior optoelectronic properties to the 1D yellow phase. In order to find strategies to control the black phase formation and suppress the yellow phase formation, we now discuss the phase transitions of AB2+X
3 lead-free perovskites, with CsSnI3 as an example.
Similar to CsPbI3,CsSnI3 was reported to have three metastable black-phase structures at
different temperatures.[19,25] As shown in Figure 2, the black-phase shows two-step phase
transitions, from cubic (α-CsSnI3, Fm-3m, >440 K) to tetragonal (β-CsSnI3, P4/mbm, 362-440
K) and then to orthorhombic (γ-CsSnI3, Pnma, <362 K) during the cooling process.[19]
The α-CsSnI3 forms the ideal 3D perovskite structure with cubic Pm-3m space group, a =
6.205 Å, the same as α-CsPbI3 (space group Pm-3m, a = 6.201 Å). All Cs, Sn, and I atoms
reside on the special positions of the space group. The Sn2+ center sits in an ideal octahedral
geometry with six I− anions, resulting in stereochemically inactive 6s2 lone pair electrons. The
SnI6 octahedra form a 3D cubic framework via corner sharing. The Cs+ counter cations reside
at 12-coordinate interstices within the network made by eight SnI6 octahedra (Figure 2a). When
cooled, the cubic phase undergoes successive displacive transitions to lower symmetry with no bond breaking (Figure 2b,c). The Pm-3m symmetry is lowered to tetragonal P4/mbm at 380 K (β-CsSnI3) and orthorhombic Pnma at 300 K (γ-CsSnI3). Both the β-phase and γ-phase result
6
undergoes a reconstructive phase transition to the δ-phase when exposed to air at room temperature for 1 h. On the contrary, the δ-phase can then be transformed to the α-phase upon heated above 425 K under inert atmosphere. The phase-transition information is of great help for guiding us to control the black phases (α, β, γ-phases), which show better optoelectronic properties than the yellow phase (δ-phase). In addition, the black phases CsSnI3 are a rare case
of a material that combines high electrical conductivity and strong photoluminescence, which is highly desired for photovoltaics,[26] radiation detectors, LEDs, and other applications.[27]
In addition to Sn-based 3D perovskites, the structure of Ge-based 3D perovskites has also been systematically studied.[28] As shown in Figure 3a, CsGeI3 consists of 3D frameworks,
assembled through single Ge-I-Ge bridges to form corner-sharing GeI6 octahedra. Unlike
CsSnI3, the octahedra adopt a trigonally distorted CaTiO3 structure, resulting in two Ge-I bond
lengths of 2.753 Å and 3.256 Å in GeI6 octahedra (Figure 3a). Particularly, CsGeI3 crystallizes
in the polar trigonal R3m space group. The trigonal distortion causes loss of the 4-fold symmetry axes present in the aristotype cubic perovskite modification Pm-3m. This structural feature indicates that CsGeI3 is a potential candidate for ferroelectric, dielectric, piezoelectric
applications as well as nonlinear optics. In addition, the 3D CsGeI3 shows a direct bandgap of
1.6 eV, which is similar to the 3D lead perovskites MA(FA, Cs)PbI3, indicating that it is
potentially interesting for optoelectronic applications.
However, the reduced inert electron pair effects from Pb2+→Sn2+→Ge2+ lead to decreasing
stability of the divalent oxidation states. It results in poor stability of Sn2+/Ge2+-based
perovskites, limiting their practical applications.[29] Therefore it is necessary to explore
alternative lead-free perovskite candidates, which are more stable and efficient for device applications.
3.2. A2B+B3+X6-type Double Perovskites
By replacing two B2+ with a combination of B+ and B3+, we can obtain A2B+B3+X6 halide double
perovskites. The halide double perovskites offer immense opportunities in terms of combinatorial chemistry, since a wide range of different elements are available for B+ and B3+
ions. Through first-principles calculations, 11 optimal double perovskites with intrinsic thermodynamic stability, suitable band gaps, small carrier effective masses, and low excitons binding energies, have been identified by Zhang and coworkers.[30] These promising properties
7
Karunadasa and coworkers synthesized the first halide-based double perovskite Cs2AgBiBr6.[31] It crystalizes in cubic space group Fm-3m (No. 225) at room temperature (RT,
25 oC), and shows a typical double perovskite structure with alternating AgBr6 and BiBr6
octahedra. The counter ion Cs+ occupies the centre of four AgBr6 and BiBr6 octahedra units.
Figure 3b displays a perfect 3D packing structure of Cs2AgBiBr6, where only two different
bonds exist in the crystaldue to its high symmetry. The high symmetry cubic structure of Cs2AgBiBr6 results in promising optical and physical properties.[32]
However, the bandgap (~1.95 eV) of Cs2AgBiBr6 is too large, and the indirect bandgap
results in strong non-radiative recombination, limiting their optoelectronic applications. In order to address this issue, impurity doping has been employed to achieve visible light response in Cs2AgBiBr6.[33,34] After doping, the crystal structure shows slight changes, with the bandgap
decreasing from 1.95 eV (indirect) to 1.45 eV (direct), similar to that of (CH3NH3)PbI3.
Compared with (CH3NH3)PbI3, doped Cs2AgBiBr6 shows significantly higher heat, light and
moisture stability, indicating their potential in environmentally friendly perovskite applications.[31]
In addition to Cs2AgBiBr6, another double perovskite Cs2AgInCl6 has also attracted
significant attention, mainly due to its direct bandgap and reversible photosensitivity.[35]
Cs2AgInCl6 crystalizes in cubic space group Fm-3m, and shows a similar packing structure as
Cs2AgBiBr6 by replacing the Bi3+, Br- ions with In3+, Cl-, respectively (Figure 3b).[36]
Cs2AgInCl6 single crystal (SC) reveals a low trap density of 8.6 × 108 cm–3, which is even lower
than the value reported in the best lead halide perovskites.[37] Materials with low trap densities
are always preferred for optoelectronic applications. The optoelectronic properties of Cs2AgInCl6 have very recently been demonstrated to be significantly enhanced through rational
alloying, representing promising candidates for light emission applications.[38]
The B3+ also can be replaced by trivalent lanthanide metal ions, forming the
lanthanide-based double perovskites. Due to their typical f-f transitions of lanthanide cations, the lanthanide-based double perovskites (e.g. Cs2NaTbCl6, Cs2NaEuCl6,Cs2LiLaCl6 etc.) have
been demonstrated as promising emitters.[39,40] The crystal structures of Cs2NaTbCl6 and
Cs2NaEuCl6 have been studied as early as 1970.[41,42] Both Cs2NaTbCl6 and Cs2NaEuCl6
crystalize in cubic space group Fm-3m, with a classic 3D double perovskite structure (Figure 3b). As expected, Cs2NaTbCl6/Cs2NaEuCl6 exhibit strong green/red photoluminescence and
high X-ray scintillant yield, indicating their great potential in future radiation detections and medical imaging.
8
A special case of the double perovskites is mixed-valence compounds, e.g. Cs2Au+Au3+I6,
where the monovalent and trivalent cations are the same element. Cs2Au+Au3+I6 has been
explored as potential superconductors[43] and potential lead-free photovoltaic materials.[16] It
crystallizes in a distorted tetragonal mixed-valence perovskite structure with space group I4/mmm,[44] and the Au cations undergo charge disproportionation,[45] with Au+ and Au3+
oxidation states in a rocksalt ordering.[46] Due to the difference in coordination types for Au+
and Au3+, the primary bonding around Au+ and Au3+ can perhaps more accurately be described
as square planar for Au3+ and linear for Au+, with long Au···I contact nominally completing
octahedral coordination. Cs2Au+Au3+I6 has a direct-band-gap feature, and optical simulation
predicts that a very thin layer of active materials is sufficient to achieve a high photoconversion efficiency.[16]
3.3. A2B4+X6-type Vacancy-ordered Double Perovskites
Following the design rules in Figure 1, two B2+ in AB2+X
3 perovskites also can be substituted
by one B4+ and a vacancy, forming vacancy-ordered double perovskites, A
2B4+X6 (e.g.,
A2B4+◊X6, where ◊ is a vacancy). They are a family of perovskite derivatives composed of a
face-centered lattice of nearly isolated BX6 units with A-site cations occupying the
cuboctahedral voids. Figure 3d shows a distorted quasi-three-dimensional (quasi-3D, 0D for strictly speaking) structure of A2B4+X6. In A2B4+X6, half of the octahedral B atoms are missing,
creating discrete BX6 octahedra in vacancy-ordered double perovskites A2B4+X6. Although
every other BX6 octahedron is removed, the close-packed anionic lattice similar to 3D AB2+X3
perovskites is retained, resulting in similarities to 3D AB2+X3. Additionally, the
vacancy-ordered structure type undergoes cooperative octahedral tilting and rotations in symmetry-lowering phase transitions upon cooling, making them a potential candidate for dielectric-related applications.[47,48]
Kanatzidis and co-workers reported the crystal structure of the most-studied vacancy-ordered double perovskites, Cs2SnI6. It crystallizes in the cubic Fm-3m space group, which is
the same as conventional double perovskite A2B+B3+X6.[25,49] Cs2SnI6 is ambipolar (i.e. it can
be doped as n-type or p-type), and exhibits excellent carrier mobility after doped as an n-type semiconducting material. In addition, it has a relatively optimum energy band gap (Eg) of ca. 1.3 eV and high absorption coefficient of over 105 cm-1 for energies above 1.7 eV, which shows
great potential as light absorbers.[49–51] Despite the presence of isolated octahedral units, the
closely packed iodide lattice provides significant room-temperature carrier mobility of Cs2SnI6,
9
respectively).[19,49] Although Cs2SnI6 shows promising fundamental properties for potential
optoelectronic applications, efficient devices have not been demonstrated yet, mainly due to its metallic conductivity. The high density of iodine vacancies and impurities such as CsI are the main sources for the high conductivity in Cs2SnI6. Therefore, structure engineering strategies
to reduce the conductivity of Cs2SnI6 are the current research focus for this material.
Several other A2B4+X6-type vacancy-ordered double perovskites have also been investigated.
By replacing the Sn4+ with Pd4+, Snaith and coworkers reported a new type of lead-free
vacancy-ordered double perovskite, Cs2PdBr6, with an analogous structure to Cs2SnI6.[17]
Cs2PdBr6 exhibits very promising properties, including long-lived photoluminescence, suitable
bandgap of 1.6 eV, dispersive electronic bands, and outstanding stability toward light, humidity, and heat, indicating its great potential in optoelectronics.[17,52] Very recently, Ti-based
vacancy-ordered double perovskites Cs2TiX6 have been reported by Zeng and coworkers.[53] These
materialspossess a combination of several desirable attributes, including suitable bandgaps (1.38-1.78 eV), excellent optical absorption, benign defect properties, and high stability, reflecting wide application prospects for Cs2TiX6. In addition, incorporating semimetal ion Te4+
into A2B4+X6-type perovskites, a new class of A2Te4+X6 perovskites with tunable band gap
(1.42-2.02 eV), a low trap density (∼1010 cm-3), and a high mobility (∼65 cm2 V-1 s-1), were
reported.[54] Especially, the MA2TeBr6 single crystal with a band gap of 2.00 eV possesses a
long carrier lifetime of ∼6 μs and corresponding carrier diffusion lengths of ∼38 μm, which are ideal characteristics for a material for photodetectors and tandem solar cells.
A unique feature of vacancy-ordered double perovskites is that they have isolated BX6
octahedra, resulting in quantum confinement effect, which is potentially beneficial for the photoluminescence of the materials. The photoluminescence of vacancy-ordered double perovskites can be further enhanced by suitable impurity doping (e.g, Mn2+, Bi3+ etc.), which
has demonstrated to be an effective way to control the physical properties and even to induce new functions for vacancy-ordered perovskite.[55] These results indicate that A2B4+X6-type
vacancy-ordered double perovskites with impurity doping can be promising candidates for luminescent applications. However, the rules about how to select the right doping ions is still unclear at present. Further work on A2B4+X6-type vacancy-ordered double perovskites through
impurity doping is required to explore the full potential of this type of materials. 3.4 A4B3+B5+X12-type Vacancy-ordered Perovskites
10
By replacing four B2+ with a combination of B3+, B5+and two vacancies, we can obtain a new
type of vacancy-ordered halide perovskites, A4B3+B5+X12. The first perovskite in this category
was Cs4Sb3+Sb5+Cl12, reported as early as in 1963.[56] Five years later, Jacobson et al. reported
the real structure of Rb4Sb3+Sb5+X12, and confirmed that it was a mixed-valence (Sb3 +and Sb5+,
rather than Sb4+) perovskite.[18] Actually, the report of the cesium salt Cs2Sb4+Cl6 could be
traced back to 1901,[57] when it was incorrectly believed to be isomorphous with Cs2Pb4+Cl6.
A4B3+B5+X12-type perovskite Rb4Sb3+Sb5+Br12 crystallizes in tetragonal space group I41/amd
(D4hl9), forming quasi-3D vacancy-ordered perovskite structures. As shown in Figure 3d, both
the [Sb3+Br6]3- and the [Sb5+Br6]- octahedrons are distorted from Oh symmetry and possess D2d
symmetry. These distortions indicate considerable interactions between the [Sb3+Br6]3-,
[Sb5+Br6]- and the Rb+ ions, which possibly bring special properties for these materials. For
example, these types of perovskites are abnormally dark in color, indicating a strong absorption of visible light.[56] The strong absorption was due to the electron transfer from the [Sb3+X6]
3-octahedron to the [Sb5+X
6]- octahedron via van der Waals contacts through the halogens and/or
the cations. Such strong absorption as well as their high symmetry structures indicate that these materials have wide optoelectronic applications. However, there are few reports on this type of perovskites till now, and the fundamental properties (photophysics, charge transport etc.) as well as the potential device applications remain unclear. We believe that A4B3+B5+X12-type
vacancy ordered perovskites deserve further investigations. 4. Low-dimensional Lead-free Perovskites
In addition to 3D perovskites, low dimensional perovskites also attract wide attention in recent years, due to their excellent stability and structure diversity, resulting in rich properties for multifunctional devices. As we mentioned in Section 2, the ionic radii, ionic valences and coordination types of A, B, X are the three key factors affecting the 3D ABX3 structure.
Significant changes in these three key factors will result in low-D perovskites, e.g. by A, B, or X ions replacement (affecting the ionic radii), or by heterovalence ions substitution of B ions (synergistically affecting the three factors). In this section, we will review the recent progress on low-D lead-free perovskite derivatives as well as their derivative paths from the 3D AB2+X3
-type perovskites. They are mainly about the following low-D perovskite -types (ranging from 2D to 0D): 2D A2B2+X4, 2D A4B+B3+X8, 2D Cs4B2+B3+2X12, 2D A3B3+2X9, 1D AB2+X3, 1D
A2B3+X5, 0D A3B3+2X9 as well as 0D AB5+X6 perovskites.
11
By replacing the cation A with larger organic cation (>2.6 Å), such as butylammonium (BA: C4H9NH3+) and phenylethylammonium (PEA: C8H9NH3+), the 3D lead-free perovskites
AB2+X3 can be sliced into a 2D structure with a general formula of A2B2+X4 (A = PEA, BA,
etc.). The size of cation A must be bigger than 2.6 Å, and contain terminal functional groups (usually, NH3+) that can ionically interact with the anionic inorganic layers. In 1996, layered
lead-free Sn(Ge)-based perovskites, (C4H9NH3)2Sn(Ge)I4, were reported to investigate their
crystal structures and optical properties, but the photoelectric behavior of these compounds remains unclear.[58] (C4H9NH3)2SnI4 and (C4H9NH3)2GeI4 crystalize in orthorhombic space
group Pbca and Pcmm, consisting of single-layer-thick perovskite sheets of distorted corner-sharing SnI6 (GeI6) octahedra separated by n-butylammonium cation bilayers (Figure 4a). The
relatively weak van der Waals force between the alkyl chains separating the layers provides a highly ordered 2D structure. Recent investigations demonstrate that the 2D A2B2+X4-type
perovskites exhibit highly enhanced moisture stability compared with their 3D counterparts, due to the protective organic ligand layer made of PEA or BA.[58–61]
In addition, the group 14 metal ions (Sn2+, Ge2+) can also be substituted by transition metal
ions (e.g. Cu2+, Fe2+ etc.), forming transition metal-based 2D A
2B2+X4-type perovskites. These
2D layered perovskites have similar structures for a range of divalent metal ions, such as Cu2+,
Fe2+, Cr2+, Zn2+ etc.[62–68] Among them, we take the magnetic ion Fe2+-based perovskite
(FEA)2Fe2+Cl4 as an example. At room temperature (FEA)2Fe2+Cl4 crystallizes in orthorhombic
space group Pbca, which is the same as Sn2+-based 2D perovskite (C
4H9NH3)2SnI4. Figure 4b
shows the crystal structures of (FEA)2Fe2+Cl4 in the view (along b-axis) parallel to the inorganic
layers. The planar layers of corner-sharing FeCl6 slightly distorted the octahedra separated by
the PEA cations. This is much different from A2BX4 (B = Sn2+, Ge2+) perovskites, where the
corner-sharing SnX6 or GeX6 are regular octahedral coordination without any distortion. As a
result of distorted octahedra FeCl6, transition metal-based perovskites are not possible to form
2D perovskites with multilayers. Instead of π-π stacking, the adjacent benzene rings show edge to face (C-H…π H-bonds) in (FEA)2Fe2+Cl4. There is a cascade of phase transitions that are
electronically and thermally induced, involving concerted tilting and rotation of the FeCl6
octahedra accompanied by localization of the organic amines in different orientations. Kurmoo and coworkers systematically studied their phase transitions as well as their physical properties at different phases, showing promising multiferroicity with coexistence of ferroelasticity and canted antiferromagnetism.[69]
12
In the transition metal-based A2B2+X4-type perovskites, most of them contain Jahn-Teller
metal ions (configurations low-spin d7, d9 or high-spin d4) perovskites, featuring distorted BX6
that enables great possibilities for the interactions between A cations. These interactions are likely to induce structural phase transitions, which could be useful for memory and sensor applications. Moreover, most of the transition metal ions incorporated in A2B2+X4-type
perovskites are magnetic, e.g. Fe2+. Fe2+-based perovskite materials are usually ferromagnetic,
which provide a wide platform for memories and spintronics.[70,71] It is worth noting that Fe2+
-based perovskites can also be ferroelectric by rational A-site substitution, resulting in significant multiferroics (Sub-section 6.3).
4.2. 2D A4B+B3+X8-type Double Perovskites
By replacing the A cation with larger organic cation (>2.6 Å), the 3D A2B+B3+X6-type double
perovskites can be sliced into 2D A4B+B3+X8-type double perovskites. Very recently,
Karunadasa and co-workers reported the first 2D halide double perovskite (BA)4AgBiBr8.[72]
(BA)4AgBiBr8 crystalizes in monoclinic space group C12/m1, featuring metal-halide sheets of
monolayer thickness, where the ordered double perovskite lattice is partitioned by organic cations (Figure 4c). Unlike 3D cubic double perovskite Cs2AgBiBr6, the inorganic sheets in
(BA)4AgBiBr8 are heavily distorted, particularly on the Ag site. The AgBr6 octahedra shows a
tetragonal distortion with unusually short bonds between Ag and the axial terminal bromides (Ag-Brax = 2.6156 Å), and long bonds between Ag and the bridging equatorial bromides
(Ag-Breq = 3.0608 Å). For the organic moiety, an order to disorder phase transition happens in the
temperature range 293-100 K by comparing their temperature dependent crystal structures. The phase transition feature indicates that (BA)4AgBiBr8 can be exploited as dielectric switches and
sensors. In addition, previous electronic structure calculations indicate that the indirect bandgap of Cs2AgBiBr6 turns into direct when the infinitely thick inorganic frame is reduced to
monolayer. As a result, compared with the indirect bandgap Cs2AgBiBr6, 2D monolayer
(BA)4AgBiBr8 is more promising for light-emitting and laser diodes. Unfortunately, the PL
quantum yield (PLQY) of (BA)4AgBiBr8 is quite low, and structure engineering as well as
alloying are required to enhance their PLQY in the future. 2D A4B+B3+X8-type double
perovskites substantially expand on the substitutional flexibility of 3D halide double perovskites to encompass greater compositional and electronic diversity. Hence, we would expect more research on the design, synthesis as well as fundamental properties of 2D A4B+B3+X8-type double perovskites.
13
By replacing B2+ with trivalent B3+, the charge balance of AB2+X3 perovskites is broken, leading
to lattice reconstruction to a lower energy structure A3B3+2X9. A 2D A3B3+2X9-type perovskite
derivative Cs3Sb2I9 has been intensively investigated by Mitzi and coworkers.[73] Cs3Sb2I9 is
known to have two different structures, the 0D dimer form (space group P63/mmc, no. 194,
Figure 4h) and the 2D corrugated layers of polyanions form (P-3m1, no.164, Figure 4d). The 0D dimer Cs3Sb2I9 film can be prepared by solution process (spin coating) directly, while the
high-quality 2D Cs3Sb2I9 thin film needs to be fabricated by annealing the 0D Cs3Sb2I9 film in
SbI3 vapor. The 2D Cs3Sb2I9 film exhibits a 2.05 eV bandgap and enhanced stability in ambient
air, providing a decent platform for optoelectronic applications.
A further A-site substitution of Cs3Sb2I9 results in a new 2D A3B3+2X9-type perovskite
derivative NH4Sb2I9.[74] NH4Sb2I9 crystallizes in monoclinic space group P121/ m1, and
presents a similar 2D layered structure as Cs3Sb2I9 by replacing the Cs+ with homo-valent ion
NH4+. The single crystal of NH4Sb2I9 exhibits a low optical band gap of 1.92 eV. The carrier
mobilities of the NH4Sb2I9 single crystal are high, with a hole mobility of 4.8 cm2 V-1 s-1 and an
electron mobility of 12.3 cm2 V-1 s-1, comparable to the commonly studied lead-based
perovskites. These superior physical properties make them promisingly for optoelectronic applications.
The A-site also can be replaced by the other organic cations, e.g. MA, forming MA3Sb2X9.
Very recently, it has been demonstrated that Cl-incorporated MA3Sb2ClxI9−x (x=0-9, 2D)show
better absorption, higher carrier mobilities as well as higher efficiency in the solar cells than 0D MA3Sb2I9.[75] Theoretical studies indicate that the inclusion of methylammonium chloride
into the precursor solutions can suppress the formation of the undesired 0D MA3Sb2I9
dimer-phase. This result provides a good example, showing that physical properties are strongly related to their structure dimensions. High-dimensional structures are more favorable for photovoltaic applications than low-dimensional structures in the view of lower bandgap, smaller exciton banding energy, longer exciton diffusion length and so on.
4.4. 2D A4B2+B3+2X12-type Perovskites
By introducing the additional B2+ into 2D double-layered A3B3+2X9-type perovskites, the charge
imbalance leads to lattice construction, resulting in 2D triple-layered A4B2+B3+2X12-type
perovskites. Recently, Solis-Ibarra and coworkers introduced Cu2+ into 2D large bandgap
perovskite Cs3Sb2Cl9 (above 3.0 eV), obtained the first 2D A4B2+B3+2X12-type perovskite
14
Figure 4e, it shows a ⟨111⟩-oriented triple-layered (n = 3) perovskite of alternating, corner sharing CuCl6 and SbCl6 octahedra with cesium atoms occupying the voids in the framework.
Both the CuCl6 and SbCl6 octahedra are significantly distorted. Most notably, the Cu-Cl bond
lengths are 2.299 and 2.808 Å for the equatorial and axial bonds, respectively. This disparity is characteristic of a Jahn-Teller distorted Cu2+ ion.
Cs4CuSb2Cl12 shows semiconducting properties with a direct bandgap of 1.0 eV and
conductivity one order of magnitude larger than that of MAPbI3. In addition, Cs4CuSb2Cl12 is
highly stable to light, temperature and moisture, and can be conveniently obtained by simple solution methods in the gram scale. These promising physical and chemical properties of Cs4CuSb2Cl12 indicate that this perovskite type might have wide applications in optoelectronic,
dielectric and magnetic areas.[77] However, due to their low stability in common solvents, it is
quite challenging to prepare Cs4CuSb2Cl12 films. New film fabrication methods are required to
fabricate high-quality films based on this promising perovskite. In addition, since the development of this type of perovskites is at a very early stage with only one report up to now, we expect further developments where other first row transition metal ions (in addition to Cu2+)
are also explored in the near future. 4.5. 1D AB2+X3-type Perovskites
As we mentioned in Sub-section 3.1, the yellow phase (δ-phase)CsSnI3 is the most well-known
1D AB2+X3-type perovskites, which show a 1D double-chain structure (Figure 2c). However,
these 1D AB2+X3-type perovskites show inferior optoelectronic properties to 3D black phases
(α, β, γ-phases) AB2+X3-type perovskites. Based on their isolate packing structure of anion and
cation moieties, it might be possible to incorporate additional functions beyond optoelectronics into this type of perovskites by structure engineering.
By introducing potential order-to-disorder (OTD) A cations and transition metal ions B2+
into AB2+X
3-type perovskites, a series of new functional 1D AB2+X3-type perovskites can be
achieved. The potential OTD A cations are usually flexible alkyl amines, saturated naphthene-based amines, saturated heterocycle-naphthene-based amines, rigid DABCO (DABCO= N-methyl-N'-diazabicyclo[2.2.2]octonium) as well as their derivatives. Following this strategy, a new 1D AB2+X3-type perovskite (pyrrolidinium)MnCl3 with promising ferroelectric features and high
photoluminescence has been developed.[78] (Pyrrolidinium)MnCl3 crystalizes in orthorhombic
nonpolar Cmcm at room temperature, consisting of linear chains of face-sharing MnCl6
15
exhibiting a mm2 symmetry. After cooling to 273 K, the disordered pyrrolidinium becomes ordered, together with the crystal structure changing to orthorhombic polar Cmc21. The
non-polar to non-polar phase transition indicates that (pyrrolidinium)MnCl3 is potentially ferroelectric.
Further experiments confirm excellent ferroelectricity with a saturation polarization of 5.5 μC/cm2 as well as intense red luminescence with high quantum yield of 56%. Since the
functional groups are A-site in this case, ferroelectric properties can be improved effectively through right structure modification on A cations. For example, by replacing pyrrolidinium with 3-pyrrolinium, a new isostructural 1D AB2+X3-type perovskite (3-pyrrolinium)MnCl3 was
developed, with the spontaneous electronic polarization improved to 6.2 µC/cm2, and high
fatigue resistance and emission efficiency of 28%.[79]
We emphasize that the properties of potential OTD A cations are the key factor affecting the ferroelectric, dielectric and piezoelectric properties of this type of perovskites. In addition, the potential OTD A cations can maintain the intrinsic properties originated from inorganic chains or clusters. For example, trimethylchloromethyl ammonium (TMCM) has been employed to replace the pyrrolidinium/3-pyrrolinium, achieving a new isostructural perovskite TMCM-MnCl3, which also shows excellent ferroelectric and photoluminescence properties.[80] For
suitable B-site substitution, by replacing the Mn2+ with Cd2+ (Mn2+ and Cd2+ have the same
ionic radii and coordination types), another isostructural 1D perovskite TMCM-CdBr3 with
similar ferroelectric features is achieved.[81] These facts indicate the possibility to design
multifunctional lead-free perovskites, such as ferroelectric photovoltaics, ferroelectric LEDs, and multiferroics (Section 7).
4.6. 1D A2B3+X5-type Perovskites
By increasing the ionic radii of A and replacing the B2+ with trivalent B3+ at the same time, the
large cation A and charge imbalance in ABX3 lead to a lattice reconstruction, resulting in low
energy 1D A2B3+X5-type perovskites. Recently, Luo and co-workers synthesized a
one-dimensional lead-free perovskite (C6H13N)2BiI5,[82] which belongs to the monoclinic system
with a space group of P21/n. The crystal structure of (C6H13N)2BiI5 forms a typical chainlike
architecture, including zigzag chains of a corner-sharing distorted BiI6 octahedron interleaved
by the organic cation C6H13N+ (Figure 4g). The 1D zigzag chain maintains its semiconducting
properties and directional carrier transport behavior, which may result in better device performance than the isolated 0D structure. Organic cations (C6H13N+) are located inside the
zigzag chains of cavities and bonded to the BiI6 octahedron through NH···I hydrogen bonds. In
16
systematically studied. 1D perovskite (C6H13N)2BiI5 shows a direct bandgap of 2.02 eV, high
moisture stability, long room-temperature PL lifetime, and high photoconductivity, which encourage further exploration of 1D lead-free perovskites for optoelectronic devices. A challenge with (C6H13N)2BiI5 is that the large insulating organic cations limit the transport
along a-axis and c-axis, although it demonstrates preferable carrier transport along b-axis. Therefore, introducing planar and electron-rich cation A (tetrathiafulvalene free radical cation TTF•+), or polymeric A (polyethylenimine cation PEI+) to enhance their transport might be
promising for future 1D lead-free perovskites (Section 7). 4.7. 0D A3B23+X9-type Perovskites
Similar to 2D A3B3+2X9-type perovskites, the 0D A3B3+2X9-type perovskites also can be
achieved by replacing B2+ with trivalent B3+. In this case, the B3+ breaks the charge balance of
AB2+X3, and destroys the 3D (BX3)- frame into isolated (B3+2X9)3- clusters and dissociated A+ .
There is quite low energy barrier between the two heterogeneous phases, and hence both phases can be achieved by using different preparation methods at room temperature.[83] As we
discussed in Sub-section 4.3, through suitable X-site substitution, the two phases can be transformed reversibly between 0D and 2D.[75] Recently, a large amount of 0D A
3B23+X9-type
perovskite materials have been reported as promising candidates for solar cells, light emitting diodes, photodetectors and dielectric-related applications. Kaskel and co-workers systematically investigated the single crystal structure of 0D MA3Bi2I9 perovskite for the first
time.[84] The MA3Bi2I9 crystallizes in hexagonal space group P63/mm (194), where every two
Bi3+ ions were connected with each other via three equivalent symmetrical I- ions and every
three terminal I- ions were situated on the other sides of the mirror planes. The absorption
coefficient of MA3Bi2I9 was estimated to around 1×105 cm-1 at 450 nm with a direct bandgap
estimated to be around 2.1eV.[85] The exciton binding energy is calculated to be around 70 meV
from the bandgap and the PL spectra.[86]
By A-site substitution, another homogeneous 0D perovskite, Cs3Bi2I9, has been
experimentally and computationally investigated.[87] Based on the calculated band structures,
Cs3Bi2I9 is an indirect bandgap semiconductor, and the absorption edge locates at
approximately 2 eV. By comparing the bandgaps between MA3Bi2I9 and Cs3Bi2I9, the bandgap
slightly decreases from 2.1 to 2.0 eV after Cs+-substitution, indicating that small ionic radius
A-site substitution might be a good strategy to decrease the bandgap of 0D A3B2X9 perovskites.
17
By increasing the ionic radii of A and replacing the B2+ with B5+ at the same time, we can also
obtain another unique 0D AB5+X6-type perovskites. The structure of the most studied 0D
AB5+X6-type perovskite, (C2H5)4NSb5+Br6, was reported in 1971.[88] (C2H5)4NSb5+Br6
crystallizes in the tetragonal space group I41md with four molecules in a unit cell. The structure
consists of slightly distorted octahedral Sb5+Br6 and (C2H5)4N+, which are two-fold disordered
about intersecting mirror planes. The close Br…Br van der Waals contact can be observed along the a-axis, which has potential contribution for absorption. Indeed, this perovskite shows dark red-brown in colour, indicating a strong absorption of visible light. Troshin and coworkers attempted to fabricate solar cells based on (C2H5)4NSb5+Br6, but failed.[89] The biggest
challenge of this perovskite is the super-low solubility in common organic solvents. In order to increase the solubility of these 0D AB5+X6-type perovskites, they used the large size N-EtPy+
to replace (C2H5)4N+, and obtained another 0D AB5+X6-type perovskite (N-EtPy)[Sb5+Br6].[89]
Due to the large irregular N-EtPy+ cation contained in the perovskite structure,
(N-EtPy)[Sb5+Br
6] crystallizes in lower symmetry monoclinic space group, P21/n. In the crystal
structure, the anionic moiety is represented by the SbBr6 mononuclear units with the typical
Sb-Br covalent bonds. The packing structure of (N-EtPy)[Sb5+Br
6] is rather remarkable due to the
presence of short Br···Br contacts between the neighboring SbBr6 octahedra, which are in some
cases significantly shorter (3.31-3.45 Å) than the sum of the van der Waals radii of two Br atoms (3.6 Å). This means the existence a strong acting force between the neighboring SbBr6
octahedra, indicating similar properties as the 3D structure. Because of the quasi-3D structure feature and Sb5+ incorporation, (N-EtPy)[Sb5+Br6] shows a bandgap of 1.65 eV, which is
considerably smaller than that of 2D Sb3+-based perovskite Cs3Sb2I9 (2.05 eV) and lead halide
perovskite MAPbBr3 (2.30 eV). Based on these excellent photophysical properties of
(N-EtPy)[Sb5+Br6], a power conversion efficiency (PCE) of around 4% was achieved in
(N-EtPy)[Sb5+Br6]-based solar cells, comparable to that of MAPbBr3. Since the organic cation
N-EtPy+ is too large and insulating, future work on the N-EtPy+ substitution is a good strategy to
further enhance the PCE. 5. Metal-free Perovskites
A unique case of lead-free perovskites is metal-free perovskites, where both A and B sites are organics. Different from all the lead-free perovskites we previously discussed, in metal-free perovskites, B-site is monovalent (e.g. NH4+) and A-site is divalent (A = C6H14N22+, C4H12N22+
and their derivatives, etc.). The construction of the (NH4+)X6 octahedra is via H-bond or
18
cavity between the (NH4+)X6 octahedra, forming the 3D structure. The cation A2+ also provides
a wide chemical diversity, structural flexibility, and additional functionalities which are difficult to achieve in inorganic perovskites. The first metal-free perovskite reported in literature was C4H12N2·NH4Cl3·H2O, consisting of a metal-free 3D corner-sharing network of
(NH4)Cl6 octahedra.[90] The octahedra are held together with charge-assisted hydrogen bonding,
with the divalent piperazinium cations and water molecules in the octahedra cavities.Later on, it was discovered that water is not necessary to maintain the 3D structure.[91]
Based on the A-site substitution strategy, Xiong and co-workers developed a series of 3D metal-free perovskites using different A2+ organic cations (Figure 5a). Among them,
MDABCO-NH4I3 (MDABCO = N-methyl-N'-diazabicyclo[2.2.2]octonium) has been
considered as the most promising metal-free perovskites for dielectric-related applications.[92]
As shown in Figure 5b, MDABCO-NH4I3 shows a simple 3D structure, and crystallizes in
trigonal R3 (polar space group) at room temperature, with unit cell dimensions a = b = c = 7.259 Å and α = β = γ = 84.767°. In the cage-like unit cell, the MDABCO cation locates at the center; NH4+ groups locate at vertexes; and I- ions are at the centers of the edges. By comparing the
distance between N and nearby Br, the (NH4)I6 octahedron is obviously distorted. After heating
to 448K, MDABCO-NH4I3 transforms into the cubic P432 (nonpolar space group), with unit
cell dimensions a= 7.516 Å. In the high temperature phase (HT-phase), MDABCO becomes disordered and the (NH4)I6 octahedron becomes regular (Figure 5c). This significant
order-to-disorder phase transition (polar phase to nonpolar phase transition) in MDABCO-NH4I3
indicates that this material is potentially ferroelectric. Indeed, experiments revealed a saturated polarization as high as 22 mC/cm2 for MDABCO-NH4I3, comparable to that of BaTiO3.[92]
3D A2+(NH4+)X3-type metal-free perovskites, which combine the outstanding ferroelectric
properties and advantages of organic materials, hold great potential for the next generation of microelectromechanical systems, flexible devices, wearable devices, and bionics. This is a very interesting yet currently underexplored area, requiring further investigations. A potential challenge facing these metal-free perovskites is the poor stability, as the 3D frame is constructed by H-bonds between NH4+ and X-.
6. Optoelectronic, Dielectric, Magnetic Properties and Applications
Current interest in halide lead-free perovskites are mainly focused on optoelectronic devices, including solar cells, LEDs, photodetectors, field-effect transistors (FETs), etc. In fact, promising dielectric-related and magnetic properties also can be exploited in halide lead-free
19
perovskites by structure engineering. In this section, we review the optical, electric, magnetic-related properties of these new lead-free perovskites as well as their applications.
6.1. Optoelectronic Properties and Applications
As we discussed in Section 3-4, a wide range of lead-free perovskites demonstrate attractive properties for optoelectronic applications. However, the development of optoelectronic devices based on lead-free perovskites currently lags behind the materials development. In this sub-section, we will discuss state-of-the-art lead-free perovskite optoelectronic devices, focusing on solar cells, LEDs, photodetectors, and FETs. We aim to highlight the challenges and propose approaches for future development.
3D Sn-based perovskite solar cells show reasonable PCEs, but suffer from poor stability. In 2014, 3D MASnI3 was reported as the first lead-free solar cells based on a traditional
mesoscopic device structure with a PCE of 6.4%.[93] Sn-based perovskites are prone to
self-doping in ambient air, resulting in poor stability and poor reproducibility. In order to improve the stability of MASnI3, all-inorganic perovskite solar cells based on CsSnI3 were demonstrated,
with a decent efficiency of 3.56% and enhanced device stability.[26] In spite of these
development, the poor stability of Sn-based perovskite devices is intrinsic, as a result of easy oxidation of Sn2+. Efficient antioxidant strategies might be required to in situ encapsulate the
perovskite grains for enhanced stability.[94]
3D lead-free double perovskites A2B+B3+X6 (e.g. Cs2AgBiBr6) demonstrate superior
material stability, but the photovoltaic performance based on double perovskites remains low. There are two main reasons for the low efficiencies of double perovskite solar cells: i) it is challenging to obtain high-quality films; ii) most of the reported double perovskites show a large bandgap.[95–97] Among others, we have recently made some progresses in the development
of double perovskite solar cells. We have been able to fabricate high-quality Cs2AgBiBr6 films,
by preparing the precursor solutions from Cs2AgBiBr6 single crystals.[97] As shown in Figure
6, the films are high-crystal-quality grains with diameters equal to the film thickness, thus minimizing the grain boundary length and the carrier recombination. Through carrier dynamics study, our high-quality double perovskite films show long electron-hole diffusion lengths greater than 100 nm, enabling the fabrication of planar structure double perovskite solar cells. The PCE of our devices was 1.2%, mainly limited by the large indirect bandgap (~1.95 eV) of Cs2AgBiBr6. In order to develop double perovskites into efficient solar cells, it is necessary to
20
AgBi2I7, which does not belong to any type of 3D lead-free perovskites discussed in Section
3, also shows interesting photovoltaic properties.[98] AgBi2I7 is a unique 3D structure perovskite,
which is presented by incorporation of monovalent silver cations into iodo-bismuthates instead of alkali metal cations. Each Ag+ and Bi3+ in the unit cell displays a coordination polyhedron
of iodides in the form of an AgI6 octahedron and a BiI8 hexahedron. Since BiI8 hexahedra are
connected with AgI6 octahedron via corner sharing, there are no metal vacancies in the AgBi2I7
crystal structure. Sargent and co-workers initiated the investigations of cubic AgBi2I7, which
shows a bandgap of 1.87 eV and a PCE of 1.22%. Since then, a range of different Ag-Bi-I analogues have been reported as absorber materials. For example, both Ag2BiI5- and AgBiI4
-based solar cells achieved PCEs over 2.0%;[99,100] Ag3BiI6-based solar cell devices even
demonstrated a high PCE of 4.3%.[101] Future investigations on fundamental properties and
controlled synthesis of pure phases of Ag-Bi-I perovskites are required to further improve the efficiency of solar cells based on this unique type of perovskites.
In addition to 3D perovskites, low-D lead-free perovskites, including A3B3+2X9 and A2B2+X4,
have been explored as potential light absorbers for photovoltaics. For example, Johansson and co-workers fabricated the first Bi-based perovskite solar cells successfully using 0D A3B23+X9
-type perovskites (e.g. MA3Bi2I9, Cs3Bi2I9, and MA3Bi2I9).[85] The best device only gave a PCE
of 1.09%, due to high exciton binding energies and low charge carrier mobilities in these 0D perovskites. Another 0D perovskite, (N-EtPy)[SbBr6], has also been developed for photovoltaic
applications. The external quantum efficiencies of (N-EtPy)[SbBr6]-based solar cells were as
high as 80%.[89] Short Br···Br contacts between the neighboring [SbBr6]- octahedra might help
to assist carrier transport in films. This result is particularly exciting, implying that 0D perovskites do not necessarily result in poor charge separation. Detailed mechanistic understanding (e.g. through photophysics investigations) is required to understand charge generation and recombination in different 0D perovskites, so that the community can rationally design new 0D perovskites for highly efficient devices.
In addition to photovoltaics, lead-free perovskites have also attracted attention for applications in another light to electrical technology, i.e. photodetectors. Up to now, there are mainly four types of lead-free perovskite detectors reported for photodetectors, including AB2+X3-type, A2B+B3+X6-type, A2B3+X5-type and A3B3+2X9-type. Fan and coworkers reported
a high performance photodetector based on 3D CH3NH3SnI3 (AB2+X3-type) with 1.1 mWcm-2
intensity, 0.47 A/W responsivity and specific detectivity of 8.80 ×1010 Jones.[102] Tang and
21
perovskite Cs2AgBiB6 (A2B+B3+X6-type) single crystals.[103] Their photoelectric process and
photoconductive gain are demonstrated with the Au/Cs2AgBiBr6 SC/Au device structure under
X-ray radiation (Figure 7). Through thermal annealing and surface treatment, the authors were able to largely eliminate Ag+/Bi3+ disordering and improve the crystal resistivity. As a result,
they achieved a detector with a minimum detectable dose rate as low as 59.7 nGyair s-1,
comparable to the record of 0.036 μGyair s-1 using CH3NH3PbBr3 single crystals. In addition,
high-efficiency and air-stable photodetectors based on Cs2AgBiB6 films were also
demonstrated,[104] with a high responsivity of 7.01 A/W, an on/off photocurrent ratio of 2.16 ×
104, a specific detectivity of 5.66 × 1011 Jones, an external quantum efficiency of 21.46%, and
a response speed of 956/995 µs. The unencapsulated photodetectors demonstrated remarkable operational stability over the aging test (36 h, 35-45% humidity).
As for low-dimensional lead-free perovskites, 0D MA3Sb2I9 (A3B3+2X9-type) single crystals
have been reported as a sensitive photodetector.[105] MA3Sb2I9 crystals show a low trap-state
density of ~1010 cm-3, high carrier mobility of 12.8 cm2 V-1 s-1 and long carrier diffusion length
reaching 3.0 μm. As a result, the responsivity of the photodetectors reached 40 A/W for monochromatic light (460 nm) and response time reached 1 ms, corresponding to both high gain and gain-bandwidth products.
Compared with light-to-electricity applications, the applications of lead-free perovskites for electricity-to-light application (i.e. LEDs) is at a very early stage. In 2016, Chao and coworkers reported CsSnI3-based infrared LEDs with a maximum radiance of 40 W sr-1 m-2 and a
maximum EQE of 3.8% at 4.5V.[106] However, CsSnI3 is unstable in air, and can be
spontaneously oxidized to air-stable vacancy-ordered double perovskite Cs2SnI6.[51] The PLQY
of Cs2SnI6 is as low as 0.48%, which is far from what is required for an efficient light-emitting
material.[107] Impurity doping has been demonstrated to be an effective approach to manipulate
the physical properties and even to induce new functions for metal halide perovskites. In order to enhance the PLQY of Cs2SnI6, Tang and co-workers doped vacancy-ordered double
perovskite Cs2SnCl6 using Bi. The resulting Cs2SnCl6: Bi compound exhibits highly efficient
deep-blue emission at 455 nm, with a Stokes shift of 106 nm and a high PLQY close to 80%. In a subsequent work, the same group used a similar strategy and achieved Na-alloying double perovskite Cs2AgInCl6 for efficient and stable warm-white emission.[108] The white emission
originates from self-trapped excitons (STEs), which are induced by the Jahn-Teller distortion of AgCl6 octahedron in the excited state. Alloying Na+ into double perovskite Cs2AgInCl6 led
22
by 0.04% Bi doping. These results can stimulate further research on single-emitter-based white emissive phosphors and diodes for next-generation lighting and display technologies.
The development of lead-free perovskite for transistors has actually been active for a long time. Low-D lead-free perovskite PEA2SnI4 (2D A2B2+X4-type) promises both the superior
carrier mobility and the solution processability. The high carrier mobility of PEA2SnI4 has been
utilized as the semiconducting channel in effect transistor (FET) devices, with a field-effect mobilities of 0.6 cm2 V-1 s-1 and current modulation greater than 104.[109] By using A-site
substitution, Mitzi and co-workers further improved the value to 1.4 cm2 V-1 s-1 based on a
PEA2SnI4 perovskite derivative.[110] This high value has been revisited recently by both film
optimization to reduce trap density and device engineering to improve carrier injection, with impressive enhancement to 15 cm2 V-1 s-1.[111] Materials structure engineering on both the
organic and inorganic moieties of the perovskites, as well as device engineering on film/interlayers optimizations are expected to further improve device performance for low-cost lead-free FETs.
6.2. Ferroelectric/dielectric/piezoelectric Properties and Applications
Although the research into halide perovskites has mainly focused on their optoelectronic properties, these perovskites potentially demonstrate interesting ferroelectric/dielectric/piezoelectric and magnetic properties (Sub-section 6.3), which are the focus of many other perovskites (like oxide perovskites). For example, the earliest perovskite CaTiO3 is a ferroelectric, which exhibits the photorefractive effect and promising piezoelectric
features.[112] Ferroelectric materials are a type of special dielectrics and a subgroup of
pyroelectrics and piezoelectrics.[113] Properties of ferroelectric materials are directly linked to
the emergence and motions of electric polarization in the structures. At the vicinity of the structure transition temperature (Tc), various physical properties (e.g. heat behaviors, dielectric, nonlinear optics, thermocurrent) exhibit anomalies. These anomalies are recorded as electric hysteresis loops and visualized as electric domain patterns, being direct proofs of ferroelectricity. Ferroelectric materials show excellent properties in dielectricity, second harmonic generation, piezoelectricity, pyroelectricity,[114,115] which can be used as ferroelectric
random access memories, ferroelectric field-effect transistors, infrared detectors, piezoelectric sensors, nonlinear optical devices, fast displays in electronic equipment, capacitors, and so on. For lead-free halide perovskites, ferroelectricity can be usually obtained by replacing the A-site with potential OTD phase transition cations (BA+, PEA+, H2DABCO2+ as well as their
23
derivatives), although displacive ferroelectricity might also be possible. Usually at least one phase transition will be observed in temperature dependent crystal structures in ferroelectric materials. The phase transition can result from different possibilities, including paraelectric-ferroelectric, ferroelectric-paraelectric, ferroelectric-paraelectric-ferroelectric, paraelectric-paraelectric transitions. Only paraelectric-ferroelectric and ferroelectric-paraelectric transitions can contribute to ferroelectricity or antiferroelectricity.
A recent work demonstrated ferroelectricity in 1D AB2+X3-type perovskite, where A is
large-sized monovalent organic cation and B is a transition metal (e.g. Mn2+, Cd2+).[80,81] As
shown in Figure 8b, lead-free perovskite TMCM-Mn2+Cl3 shows a dielectric anomaly,
corresponding to the OTD phase transition on the TMCM+ cation, with a Tc of 406 K (16 K
higher than BaTiO3). The ferroelectric, dielectric, piezoelectric properties of TMCM-Mn2+Cl3
have been studied in detail, with a saturate polarization (Ps) of 4.0 mC/cm2 and an excellent
piezoelectric response (d33 = 185 pC/N) that is close to that of traditional inorganic BaTiO3 (d33
= 190 pC/N). (Figure 8) Considering that the ferroelectric, dielectric, piezoelectric properties mainly origin from cation A, Xiong and co-workers kept the same cation TMCM+, and replaced
Mn2+ with Cd2+ (similar ion radius and coordination type as Mn2+), achieving a new lead free
perovskite ferroelectric TMCM-Cd2+Br
3.[81] As expected, the structure as well as ferroelectric
and piezoelectric properties of TMCM-Cd2+Br
3 are almost the same as TMCM-Mn2+Cl3.
Recently, Luo et al. introduced the large-sized N-methylpyrrolidinium cation into A3B3+2X9
-type perovskites, achieving new 0D ferroelectric perovskites, (N-methylpyrrolidinium)3Sb2X9.
These perovskites show large ferroelectric polarizations of 5.2-7.6 μC/cm2 as well as
pronounced semiconducting performance.[116] In addition, the widely tunable bandgaps enable
superior visible-light-induced photocurrents (∼10 nA/cm2) in (N-methylpyrrolidinium)3Sb2X9
crystals. These properties make them interesting for potential applications in environmentally benign multifunctional devices (e.g. ferroelectric photovoltaics etc.).
Another example of ferroelectric halide perovskites is metal-free perovskites, as we discussed in Section 5. Metal-free perovskites have attracted increasing attention, due to their mechanical flexibility, low weight, environmentally friendly processing, and low processing temperatures. In this new effort, Xiong and co-workers developed a new strategy to create 23 metal-free perovskites.[92] Among these metal-free perovskites, MDABCO-NH4I3 shows a high
Tc of 448 K and a large Ps of 22 mC/cm2, comparable to those of BaTiO3 (390 K, 26 mC/cm2).
These promising features make metal-free perovskites attractive for applications in flexible devices, soft robotics, biomedical devices, and other areas.
