Metal-Doped Lead Halide Perovskites: Synthesis, Properties, and

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Review

Metal-doped lead halide perovskites: synthesis, properties, and optoelectronic applications Yang Zhou, Jie Chen, Osman M. Bakr, and Hong-Tao Sun Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b02989 • Publication Date (Web): 27 Aug 2018 Downloaded from http://pubs.acs.org on August 28, 2018

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Chemistry of Materials

Metal-doped lead halide perovskites: synthesis, properties, and optoelectronic applications Yang Zhou,‡, 1, 2 Jie Chen,‡, 2 Osman M. Bakr,*, 2 and Hong-Tao Sun*, 1 1. College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China 2. KAUST Catalysis Center (KCC), Division of Physical Science and Engineering (PSE), King Abdullah University of Science and Technology (KAUST), Thuwal 23955, Saudi Arabia ABSTRACT: Doping of lead halide perovskites (LHPs) with the targeted impurities has emerged as an additional lever, a dimension beyond structural perfection and compositional distinction, for the alteration of many properties of halide perovskites. The past several years has seen an explosive increase in our knowledge of doped halide perovskites, which exhibit distinct optical and electronic properties with respect to undoped counterparts and improve performance of perovskite optoelectronic devices. However, there are still a series of fundamental scientific issues unresolved in the domain of doped perovskites. In this review, we present a critical overview of recent advances in the synthesis, property, and functional applications of metal-doped halide perovskites. We lay a particular focus on three-dimensional LHPs, and discuss the influence of doped metal ions on the properties of these perovskites, including main group metal cations, transition metal cations and rare earth (RE) metal cations. We thoroughly summarize the synthesis methods used, doping-induced variation in optoelectronic properties, and the benefit of doping engineering for optimization of device performance. We highlight the milestone achievements in this field and emphasize new properties arising from dopants in halide perovskites. We also address controversies encountered during the development of doped perovskites and examine the remaining challenges in this exciting field of science. Finally, we present our perspectives for further investigation of this star material by doping engineering.

1. INTRODUCTION The term perovskite, named after the mineralogist Lev Perovski, have attracted widespread attention due to the enormousness of compounds in perovskite structure and their diverse physical properties. Historically, many oxide perovskites with attractive properties such as BaTiO3 having excellent ferroelectric response and LaxCa1-xMnO3 having colossal magnetoresistance were discovered, leading to a wide range of functional applications.1-4 Similar to oxide perovskites, halide perovskites have been studied for more than sixty years,5-12 and recently they have become a generation of star material after the initial application in photovoltaic cells by Kojima et al. in 2009.13 Since the first report of solid-state planar perovskite solar cell by Snaith et al. in 2013,14 halide perovskites were well on their way to deliver high photovoltaic performance, and now single-junction solar cells that used organicinorganic hybrid halide perovskite as the light absorbing layer exhibit a record efficiency of 23.3%,15 which is almost comparable to those of conventional photovoltaics such as crystalline silicon, CdTe, GaAs and CuInGaSe based devices. Beyond photovoltaics application, perovskite light-emitting diodes (LEDs) have achieved high external quantum efficiencies over 8% in visible and near-infrared (NIR) regions;16-18 perovskite lasers have shown ultralow threshold (220 nJ cm-2) with high quality factors;19 and perovskite photodetectors and X-ray detectors have

shown great sensitivity with low noise.20, 21 All of these applications are connected with the outstanding optoelectronic properties and low-cost solution processability of halide perovskites.22-30 Halide perovskite has a three-dimensional structural framework with a chemical formula of ABX3, where A is a monovalent organic cation (e.g., methylammonium (MA), formamidinium (FA)) or an inorganic cation (Cs+), B is a metal cation (e.g., Pb2+, Sn2+), and X is a halide anion (I-, Br-, Cl-). Halide perovskites feature immense degree of compositional tunability that can be achieved by mixing halide ions or cations, which leads to tunable bandgap, improved material stability, and enhanced photoluminescence quantum yields (PLQYs) of the resultant systems.24, 31-33 Based on compositional engineering, a series of halide perovskites such as MAPbI3-xBrx, MAPbBr3-xClx, MA0.15FA0.85Pb(I0.85Br0.15)3 and Cs0.17FA0.83Pb(I0.6Br0.4)3 have been developed and used for single-junction or tandem solar cells.31, 34-39 It is noteworthy that the single-junction perovskite solar cells with the highest efficiency is based on mixed composition of MA/FA cations and I/Br halides.38 Although compositional engineering has resulted in greatly improved performance of perovskite devices in terms of stability and efficiency, perovskite optoelectronic materials still suffer from several drawbacks that need to be circumvented before the technology achieves longterm application and hence becomes industrially relevant.

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Chief among these serious problems include intrinsic crystal phase transition in the device operation regime and poor stability against moisture, high temperature and light.40-42 Moreover, the photoelectric performance of perovskite devices is still needed to be improved and desired properties for targeted applications should be further optimized.

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review has yet presented a comprehensive overview of critical issues in the field of doped perovskites and related devices. We also emphasize that in essential doping of foreign ions causes the changes of compositions of the resultant perovskites, that is, it can also be viewed as a branch of compositional engineering. Owing to the vast number of existing literature focusing on the halide, Pb/Sn or Cs/MA/FA mixing for tuning the properties of perovskites, here we will not discuss this and the readers are referred to other related papers.63-70

Doping, a process of intentionally introducing heteroatoms into a target lattice, is a general and effective approach to modulating the fundamental properties of semiconductors for microelectronics and optoelectronics, and, it does not change much of the host crystal structure and basic characteristics.43 Doping engineering has fascinated scientists since the very beginning of semiconductor science in the 1940s. In marked contrast with composition engineering, doping engineering merely relies on the incorporation of a small amount of impurities into the matrices, which causes the changes in stability and/or optoelectronic properties; of particular interest is the impartment of exotic properties to semiconductors. The arguably most stunning example that originates from the elaborate control of dopants is doped silicon, which forms the foundation of present-day microelectronics.

In this review, we present a comprehensive overview of recent advances in the synthesis, property, and functional applications of metal-doped halide perovskites. The literatures have been covered until August 2018. We pay a particular attention to three-dimensional LHPs, and elaborate the effect of doped metal ions on the properties of perovskites, in the order of main group metal cations, transition metal cations and RE metal cations. We thoroughly summarize the synthesis methods used, dopinginduced changes in optoelectronic properties, and the benefit of doping engineering for optimization of device performance. The aim of this review is to critically update the researchers of a wide variety of backgrounds about the recent milestone achievements in this field and the focus is put on new properties arising from dopants in halide perovskites. We also address controversies encountered during the development of doped perovskites, examine the remaining challenges in this field, and finally present our perspectives for further investigation of a series of unresolved scientific issues.

Having this remarkable history in mind and considering the fact that the halide perovskites are in a blossoming status where many of their inherent properties are being revisited and well known from the perspective of both fundamental science and practical application, researchers in this domain gradually recognized that the targeted introduction of impurities into halide perovskites works as an additional lever, a dimension beyond structural perfection and compositional distinction, for the alteration of many properties of a parent compound. This strategy could potentially lead to relevant discoveries for optimized and even novel optoelectronic properties that are beneficial for technological applications of halide perovskites. With this aim, over the past three years, many kinds of metal ions, including Bi3+, Rb+, K+, Mn2+, and RE ions (e.g., Ce3+, Tb3+, Yb3+), have been doped into halide perovskites, and a broad range of exotic properties have been imparted to this prominent material.44-54 For instance, doped halide perovskites exhibit distinct optical and electronic properties with respect to undoped counterparts, with key attributes including enhanced stability, improved PLQYs, new emission characteristics, reduced defect state density or passivated grain boundaries, thus leading to excellent optoelectronic performance of devices that were constructed by using the doped perovskites as active layers.51, 52, 55-57 Despite the achieved progress, there are still a series of fundamental scientific issues unresolved in the context of doped perovskites and related devices. For example, in many cases it remains a mystery whether dopants are introduced into the matrices of LHPs or merely segregate at the surface; and how doped metal ions regulate perovskite’s properties. Although recent reviews have addressed the state-of-the-art progress regarding the structural versatility and optoelectronic properties, synthesis strategies, and device application of halide perovskites,58-62 to the best of our knowledge, no

2. METAL DOPING PEROVSKITE

IN

LEAD

HALIDE

The metallic elements, including main group metal cations, transition metal cations and RE metal cations that have been doped in halide perovskites are marked in the periodic table (Figure 1). The orange, blue, green and yellow colors denote the metal dopants for optoelectronic performance control, crystal growth control, structural stability control and light conversion in halide perovskites, respectively. The metals are differentiated in color by their main functions based on recent advances. In particular, the metallic element, such as Bi, Rb, Sr and Mn, which are marked by two colors, have multiple functions as dopants in halide perovskites. In this section, we first discuss main-group metal doping, and then shift to transition metal and RE doping in LHPs. Since there remains large difference in the synthesis methods for different doped perovskites in the form of polycrystalline films, single crystals, or nanocrystals (NCs), we first describe the synthesis strategies used when discussing different dopants in targeted perovskites, and then discuss the doping-induced alteration of perovskite’s properties and the benefit of doping engineering for improved performance of resultant optoelectronic devices.

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Figure 1. Doped metal elements and their main functions in tailoring lead halide perovskites. 2.1. Main-Group Metal Doping

crystalline films. The photophysical properties of the Bi doped MAPbI3 are shown in Figure 2. With increasing the concentration of Bi dopants, the intensity of the host PL band monotonously decreases, along with the occurrence of an ultrawide emission band peaked at ca. 1140 nm (Figure 2a, b). This suggests that the telecom emission in doped films originates from Bi related active centers (BRACs). Upon Bi incorporation, although the optical absorption edge of MAPbI3 shows a slight red shift compared to the pristine one, the semiconductor preserves a direct bandgap feature (Figure 2c). However, the dynamic process has been obviously changed. As shown in Figure 2d, the decay kinetics of the pristine perovskites at 782 nm is slightly biexponential with a fast component of about 4.7 ns and a slower one of about 39.1 ns. The fast decay became dominant and faster with the increasing of Bi concentration, implying that Bi incorporation greatly alter the deexcitation channels of the carriers. Interestingly, the NIR emission of Bi-doped films can be efficiently photoexcited in a broad spectral range up to ca. 780 nm, corresponding to the bandgap energy of the MAPbI3 (Figure 2e). These phenomena demonstrate that the BRACs can be photoexcited via energy transfer from the perovskite. It is noted that using different lead precursor (PbI2 or Pb(CH3CO2)2) does not change this NIR emission, and, the photophysical properties of Bi-doped perovskites can be tuned by simply adjusting the halide anions (Figure 2f, g).45

2.1.1. Bi Doping. Bismuth is one of the most thoroughly investigated main-group elements. It has been known as ‘the wonder metal’ due to the easy involvement of its p orbital electrons in chemical combinations.71 Bi exhibits several remarkable features as follows: (i) Bi has electronic configuration of (Xe)4f145d106s26p3 with outer 6s and 6p electrons as valence electrons. Therefore, Bi has a variety of oxidation states such as 0, +1, +2, +3, and +5. The electrons in the 6p, 6s, or 5d orbitals are quite sensitive to the coordination environments. (ii) Bi has strong spin-orbit coupling effect. (iii) Bi shows a profound propensity to form clusters in a wide range of systems, such as molten Lewis acids, molecular crystals etc. All these properties allow Bi to act as an optically active center in various host materials. In particular, Bi activated NIR luminescent materials have huge potential for the applications in telecommunications.72 Although metal halide perovskite provides widely tunable optical band gaps by varied halide combinations, it is still difficult to endow them with luminesce at wavelengths longer than 1000 nm, thus seriously limiting their applications in telecommunications.73, 74 Interestingly, Pb2+ and Bi3+ have a similar ionic radius for VI coordination (Pb2+: 1.19 Å and Bi3+: 1.03 Å), which gives rise to the possibility for Bi to dope in the crystal structure of halide perovskites for manipulating of the optical property. Sun et al. first reported that Bi doped LHPs showed ultrawide NIR PL in the range of 850-1600 nm.44 The Bi doped MAPbI3 films were fabricated by spin-coating the precursor solutions, which were prepared by mixing MAI, PbI2 and BiI3 in DMF. Then, the samples were transferred to a hot plate and annealed at 100 °C to obtain uniform poly-

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Figure 2. Photophysical properties of the pristine and Bi-doped MAPbI3 films. a, b) PL spectra of the pristine and Bi-doped MAPbI3 films under the excitation of 517 nm. c) Absorption spectra and d) Decay curves of the pristine and Bi-doped MAPbI3 films. e) Typical PL excitation spectrum of the 0.01% Bi-doped MAPbI3 film monitored at 1150 nm. f) PL spectra of the Pb(CH3CO2)2-derived and PbI2-derived films under the excitation of 407 nm. g) PL spectra of Bi-doped MAPbBr2.4Cl0.6 (Br0.8Cl0.2), MAPbBr3 (Br), MAPbI2.4Br0.6 (I0.8Br0.2) and MAPbI3 (I) films under the excitation of 407 nm light. a-e) and g) Reprinted with permission from ref. 44. Copyright 2016 American Chemical Society. f) Reprinted with permission from ref. 45. Copyright 2017 Royal Society of Chemistry.

The authors further studied the mechanism of NIR luminescence caused by BRACs. Bi element exists as +3 in the perovskite lattice as proved by the ESR and XPS spectra (Figure 3a, b). In addition, it was found that the introduction of Bi into perovskite lattices results in the shift of binding energies of the 4f7/2 and 4f5/2 levels of Pb to lower values, implying that the BRACs have interactions with Pb or [PbI6]- octahedral (Figure 3c). Because of the comparable ionic radii of Bi3+ and Pb2+, a feasible structure was proposed that the Bi-doping-induced optically active center is connected with the distorted [PbI6]- units coupled with spatially localized bipolarons (Figure 3d). This optically active center bears the low-energy electronic transition between the vibronic band and ground level, resulting in the ultrawide NIR emission (Figure 3e).75 This concept was further verified by the PDS spectra. As shown in Figure 3f, the Urbach energy is increased from 14 to 17 meV as the Bi/Pb ratio increases from 0 to 0.05%, which is ascribed to the increased concentration of structural defects induced by Bi doping. Considering that the increased concentration of structural defects is accompanied by the appearance of NIR PL, the results strongly indicates that the NIR PL is induced by Bi doping in perovskite films. Mosconi et al. confirmed that Bi3+ induced deep trap in MAPbI3 via first-principles simulation.76 The electron capture by Bi3+ facilitates a deep transition which is placed at 1.00 eV above the valence band. Meanwhile, Bi3+ introduces a partially localized unoccupied state below the conduction band edge. These results coincide with the PL measurements.

temperature. Recently, over 7% NIR PL quantum yield was achieved at room temperature upon the incorporation of the Bi3+ into CsPbI3 NCs following the modified hot-injection method.77 Compared to MAPbI3, using CsPbI3 as a host material can effectively prevent strong quenching effect that is induced by organic group methylammonium. However, Bi doping did not improve the stability of CsPbI3 NCs, and one-day exposure to air can destroy the samples. Perovskite single crystals have attracted tremendous research attention in the field of optoelectronic device because of their high carrier mobility, long carrier lifetime and ultralow trap state density compared with the multicrystalline films.78-81 A large-area (≈1300 mm2) imaging array composed of 729-pixel sensor array has been achieved based on MAPbBr3 single crystal.82 On the other hand, Single crystals are ideal models for understanding the intrinsic characteristics of semiconductors due to the negligible influence from grain boundary and morphology variations. Bi3+ ions doping in LHPs can be considered as heterovalent doping, which should have significant effects on the intrinsic characteristics of the hosts. Recently, there have been several efforts to introducing Bi3+ ions in the perovskite single crystals in an attempt to

Although Bi3+ doping has successfully induced ultrawide NIR PL, the internal QEs of the telecomemission of Bi-doped MAPbI3 films are still less than 0.04% at room

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Figure 3. a) ESR spectra of the pristine and Bi-doped MAPbI3 films. XPS spectra of b) Bi(4f5/2, 4f7/2) and c) Pb(4f5/2, 4f7/2)for the 0.25% and 0.5% Bi-doped films. d) Three-dimensional schematic illustration and e) Schematic energy diagram of the feasible structure of Bi-doped MAPbI3. f) PDS spectra of pristine, 0.01% and 0.05% Bi-doped Pb(CH3CO2)2-derived films. Inset shows the enlarged PDS spectra from 1.4 to 1.57 eV. a-e) Reprinted with permission from ref. 44. Copyright 2016 American Chemical Society. f) Reprinted with permission from ref. 45. Copyright 2017 Royal Society of Chemistry.

optimize their electronic and optoelectronic properties. Abdelhady et al. reported Bi3+-doped MAPbBr3 single crystal for the first time.83 A series of MAPbBr3 single crystals with different Bi3+ doping concentrations were prepared through inverse temperature crystallization (ITC) technique.84 Typically, Bi doped MAPbBr3 precursor solutions in DMF were prepared by dissolving PbBr2, MABr and BiBr3 in specific Bi concentrations (0-10%) at room temperature. Then, the solutions were placed in vials, which were kept steady in an oil bath between 90 and 100 °C for single crystal growth. Using ITC method, the practical Bi content in crystals was found to be much lower than the nominal amount in precursor solution (Figure 4a). No impurities were found even with the highest doping ratio, meaning a single cubic phase of the doped samples (Figure 4b). With the increase of Bi3+ ions concentration, an increase in conductivity of as much as 4 orders of magnitude was observed with ~10−8 Ω−1 cm−1 for the undoped crystal and ∼10−4 Ω−1 cm−1 for the Bi-doped crystals (Figure 4c). Meanwhile, the free carrier concentration in Bidoped crystals increases to ∼1011 - 1012 cm−3, which is much higher than the ∼109 cm−3 in undoped crystals (Figure 4d). Recently, other single crystals were also studied with Bi doping. Zhang et al. successfully grown Bi-doped MAPbCl3 single crystals through ITC.85 With an increase in Bi doped content, the conductivity increased from 2.85×10-8 to 2.19×10-6 Ω−1cm−1. Bi-doped CsPbBr3 single crystals were reported by Miao et al.86 The conductivity of Bi-doped CsPbBr3 (6.75×10-7 Ω−1cm−1) was obviously higher than that of pure CsPbBr3 (2.47×10-8 Ω−1cm−1).

Figure 4. a) ICP measurement for Bi/Pb atomic ratio as a function of feed solution atomic ratio. b) Powder XRD of ground MAPbBr3 crystals with various Bi% doping. c) Conductivity and d) majority charge concentration of the different crystals as a function of Bi/Pb atomic ratio in MAPbBr3 single crystals. Reprinted with permission from ref. 83. Copyright 2016 American Chemical Society.

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Figure 5. a) Photographs showing MAPbBr3 crystals with various Bi incorporation levels. b) Steady-state absorption spectra of MAPbBr3 crystals with various Bi%. Inset: corresponding Tauc plots. c) Bandgap alignment of MAPbBr3 crystals with various Bi%. Reprinted with permission from ref. 83. Copyright 2016 American Chemical Society.

However, Snaith et al. found that the band gap values of Bi-doped MAPbBr3 single crystals were almost constant in the range 2.383 to 2.389 eV by using a spectroscopic ellipsometry measurement (Figure 6a).90 But the absorption tail increases with the presence of Bi3+, which results in the Urbach energy increasing from 19 meV to 50-55 meV (Figure 6b). Based on these observations, they proposed that Bi3+ doping introduces defect states in the crystals, which facilitate unfavorable nonradiative recombination. The similar conclusion was verified by Yamada and Mosconic groups.76, 91

It is noted that the color of the crystal was readily tuned by controlling the concentration of Bi3+ doping in MAPbBr3 (Figure 5a). The normalized absorbance spectra of Bi-doped crystals show a red-shift proportional to the Bi content (Figure 5b). The band edge shifts from 570 nm for the undoped crystal to 680 nm in the crystal with the highest Bi content. Meanwhile, the conduction band moved downward considerably, meaning that the doping type of halide perovskite was changed from p- to n-type through Bi3+ doping (Figure 5c). The authors proposed that two reasons may lead to the band gap narrowing (BGN). One is that the density of states is changed by the interaction of the electrons and positive impurities.87, 88 Another is that Bi has more covalent bonding strength with Br due to its higher electronegativity than Pb.89

Inspired by the work of Bi doped halide perovskites, recently researchers further explored the use of other trivalent metal ions as dopants to tune the properties of halide perovskites. By adding a small amount of Al3+ in perovskite precursor solution, Snaith et al. found reduced microstrain in the MAPbI3 crystal with the presence of Al3+ during the film crystallization process, which results in a reduction in the density of crystal defects.92 As shown in Figure 7a, the authors calculated the half-width at halfmaximum (HWHM) height distribution of films with different Al3+ doping concentrations. The HWHM reduced from 33 nm to 16 nm with 0.15 mol% Al3+ doping, which means that a more uniform and flat film was formed. Liu et al. reported that doping Al3+ into CsPbBr3 NCs can afford stable blue photoluminescence due to the extended band gap and quantum confinement effect of elongated shape through replacing Pb2+ with Al3+.93 Owing to the similar bond dissociation energy of Al-Br and Pb-Br, Al3+ was easily bound to the perovskite host lattice and create dimeric form of Al2Br6 (Figure 7b). In addition, In-doped LHPs exhibited a good crystal quality with multiple ordered crystal orientations, showing efficient charge transport and thus improved device performance.94 Zhang et al. used Sb3+ incorporated into MAPbI3, which leads to

Figure 6. a) Absorption spectra for pristine and 5% Bi doping MAPbBr3 single crystals. b) Combined absorption spectra from ellipsometry data (top part) and the UV−vis transmission data (bottom part). Reprinted with permission from ref. 90. Copyright 2018 American Chemical Society.

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n-doping of the material, increasing the electron density in conduction band and elevating the quasi-Fermi energy level. Therefore, the built-in potential is enlarged, exhibiting increment of electron transportation.95

Figure 8. a) Tolerance factor of APbI3 perovskite with the oxidation-stable A = Li, Na, K, Rb, Cs; and MA, FA. b) Schematic representation of the polymorphs of perovskites based on Cs, and Rb, and phase transition of the perovskite depending on temperature. a) Reprinted with permission from ref. 47. Copyright 2016 American Association for the Advancement of Science. b) Reprinted with permission from ref. 98. Copyright 2017 John Wiley and Sons.

Figure 7. a) Schematic diagram of the proposed perovskite polycrystalline thin film growth and influence of the Al3+ doping. b) Schematics showing the Al bound to host lattice constituents in cluster form. a) Reprinted with permission from ref. 92. Copyright 2016 Royal Society of Chemistry. b) Reprinted with permission from ref. 93. Copyright 2017 John Wiley and Sons.

Saliba et al. first reported that embedding Rb+ into a photoactive perovskite phase formed multiple A-cation compositions (RbCsMAFA).47 In contrary to the MAFAbased film, the addition of alkali metal ions into precursor solution enforced a film crystallization that starts with a photoactive perovskite phase instead of an inhomogeneous nonperovskite phase, indicating the high phase stability of the RbCsMAFA-based film (Figure 9a, b). Meanwhile, the introduction of alkali metal ions does not change the valence band position, and, the band energy alignment of both films with respect to the Fermi level is the same (Figure 9c).99 To further understand the influence of Rb+ ions incorporation, Duong et al. investigated the interplay of Rb+ ions with the excess PbI2 on the formation of MAFA-based perovskite phases and film morphology through X-ray diffraction (XRD) and cathodoluminescence (CL).48 As shown in Figure 9d, the yellow nonperovskite and PbI2 phases are suppressed upon moderate Rb+ ions doping. It is clear from the SEM images

2.1.2. Alkali Metal Doping. Among the alkali metal, Cs is the only one that acts as A site in perovskite APbX3 because of its large ionic radii, even though the “black” αphase CsPbI3 can only be maintained at over 300°C (Figure 8).96 RbPbX3 with a smaller Rb as A site can never be of black perovskite phase even being heated up to the melting point (Figure 8b).97 Instead, Rb has been shown as a functional dopant in the halide perovskites.

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that the distribution density of PbI2 crystals reduced significantly with the addition of Rb+ ions (Figure 9e-g). All results show that Rb ions doping helps to suppress forming of the impurity phases, contributing to increased lifetime and more efficient charge extraction without obvious changing in band gap. Using this same strategy, the optoelectronic performances of Rb+ ions doping have been explored in different kinds of halide perovskites, 103 including (FAxMA1-x)Pb(IxBr1-x)3,100-102 FAPbI3,98, 104, 105 106 107 MAPbI3, FAPbBr3, and CsPbBr3.

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Especially, Abdi-jalebi et al. demonstrated that the substantial mitigation of both non-radiative losses and photoinduced ion migration in perovskite films and interfaces were effectively restrained by K+ ions.110 The external photoluminescence quantum efficiency (PLQE) shows an obvious increase from 8% to 66%, and, the internal PLQE exceeds 95% for K+ ions assisted samples (Figure 10a). Figure 10b shows that the fast non-radiative decay component was effectively suppressed through doping moderate K+ ions. Meanwhile, the carrier mobility remains approximately constant at a high value of around 42 cm2V-1s-1 (Figure 10c). Interestingly, compared to the counterpart, the high values of PLQE for K+ ions assisted films were maintained under continuous illumination, suggesting that the photoinduced migration processes were inhibited (Figure 10d). The K+ doped sample also has greatly promoted photostability even in a wide range of bandgaps (1.7-1.9 eV) (Figure 10e).

Figure 9. Absorption and PL spectra of MAFA (black) and RbCsMAFA (red) films without a) and with b) annealed (at 100 ℃ for 1 hour). The inset images in a) show the fluorescence microscopy measurements of films. c) Valence band spectra of the MAFA and RbCsMAFA perovskite materials in red and black, respectively, recorded with a photon energy of 4000 eV. The inset shows a schematic drawing of the energy level diagram of these two samples. d) XRD of A-mixed perovskite films on FTO/cp-TiO2/ms-TiO2 substrates with different amounts of excess PbI2 and RbI doping concentration. e-g) SEM images of perovskite film with 15% excess PbI2 and 0%, 5% and 10% RbI doping. The green masks represent the location of PbI2 phase while the red mask represent the Rb – rich phase. The scale bar is 1 µm. a,b) Reprinted with permission from ref. 47. Copyright 2016 American Association for the Advancement of Science. c) Reprinted with permission from ref. 99. Copyright 2017 American Chemical Society. d-g) Reprinted with permission from ref. 48. Copyright 2016 Elsevier.

Figure 10. a) PLQE of K-assistant (Cs,FA,MA)Pb(I0.85Br0.15)3 perovskite thin flms with increasing fraction of K concentration (x= [K]/([A] + [K], A= (Cs, FA, MA)), measured under illumination with a 532-nm laser at an excitation intensity equivalent to approximately 1 sun (60 mW cm−2) after 300 s of illumination. b) Time-resolved photoluminescence decays of the K-assistant perovskite films, with excitation at 400 nm and a pulse fluence of 0.5 μJ cm−2 (excitation density of approximately 1016 cm−3). c) Maximum photoconductance for each of the x contents, extracted from TRMC measurements with an excitation density of approximately 1014 cm−3. d) PLQE time course for (Cs,FA,MA)Pb(I0.85Br0.15)3 films with different K concentrations illuminated with a 532-nm laser at an excitation intensity equivalent to approximately 1 sun (60 mW cm−2) in an ambient atmosphere. e) PL from (Cs,FA,MA)Pb(I1−yBry)3 with different K concentrations. Re-

In addition to Rb, other alkali metals as dopants were also found to be beneficial to perovskites. For instance, Na doping increased the hole concentration and mobility;108 Li doping promoted electron injection in devices.109

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However, Hu et al. raised doubts on the actual location of alkali metal ions in the mixed A-cation perovskite.100 (FA0.83MA0.17)Pb(I0.83Br0.17)3 was prepared and added a defined amount of alkali metal ions (Cs+ or Rb+). With increasing the concentration of Cs+ ions, the XRD peak of the pristine shifts to larger diffraction angles, meaning a shrinkage of the perovskite lattice. By contrast, the peak shifts to lower angles with increasing doping concentration of Rb+ ions, which indicates an expansion of the perovskite lattice (Figure 12a). They proposed that Rb+ ions can cause a Br-deficiency in the mixed-halide perovskite structure through formation of the Br-rich side phases, such that an effective expansion of the I-rich perovskite structure appears (Figure 12b). Almost at the same time, Kubicki et al. reported that Rb+ ions were not incorporated into the mixed A-cation perovskite lattice, which was very different from Cs+ ions.112 Rb+ ions doped halide perovskite will separate into a mixture of Rb-rich phases (e.g. RbPbI3 mixed cesium−rubidium lead iodides, mixture of rubidium halides, various rubidium lead bromides, depending on the exact composition) (Figure 12c). All of these Rb-rich phases potentially act as a passivation layer for the perovskite material. In addition, the authors demonstrated that K+ ions did not incorporate into the 3D perovskite lattices either.113 Using 39K MAS NMR at 21.1 T, they analyzed atomic-level characterization of the Kcontaining phases upon KI doping of LHPs, and the results showed that the formation of a mixture of K-rich phases and unreacted KI (Figure 12d). It is noteworthy that the different synthetic route (solid-state or solutionbased) did not induce the qualitative differences. Their results correspond to the study by Abdi-jalebi et al. who confirmed the formation of K-rich phase at the grain boundaries of the perovskite as well as at the interface of the perovskite/substrate.110 They believed that the K-rich phase inhibited halide migration and suppressed additional non-radiative decay aroused from interstitial halides, showing a greatly passivation property. Besides studying alkali metal doping into perovskite polycrystalline films, alkali metal doped perovskites in the nano-scale have also been explored. Amgar et al. demonstrated that Rb ions were successfully introduced into CsPbX3 (X=Cl, Br) NCs.107 Compared to the original samples, Rb-doped NCs exhibit high PLQYs. Recently, Liu et al. reported that CsPbCl3 NCs showed a blue shift of the absorption edge by the introduction of K ions, which may occupy the lattice sites of Cs ions.114 It is worth noting that the PLQYs at 408 nm was enhanced from 3.2% to 10.3% with K-doped. These works may provide a way to optimize the optical properties of perovskite NCs.

printed with permission from ref. 110. Copyright 2018 Springer Nature.

Generally, mixed-cation perovskites with organic cations as the main part and small amounts of alkali metal ions doped on the A-site have excellent device performances and high stability. The underlying fundamentals of such interesting phenomena were investigated by Ghosh et al. from the aspects of local structures and dynamics of mixed A-cation compositions.111 As the smaller Rb cations are introduced into the perovskite lattice, the N-H stretching peaks become intense and red-shifted, indicating a strengthened hydrogen bonding (Figure 11a). This bonding effect locked the octahedral rotation and impeded the motion or tumbling of the organic cations in the cages. The time constant of organic cation tumbling motion is more than doubled to almost 5 ps relative to 2 ps for undoped samples (Figure 11b). It indicates that the lattice dynamics are significantly reduced, which results in a reduced electronic disorder and increased phase stability.

Figure 11. a) Vibrational spectra of FA+ cations in FAPbI3 and mixed A cation, FA0.9Rb0.1PbI3, in the region of the N−H stretching frequency. Increase in intensity and red-shift of the peaks are evident for the mixed A-site cation compound. b) Vector autocorrelation function of FA+ cation showing the probability of the cation remaining in its initial orientation over time. Inset shows molecular vector of FA+ ion by an arrow. Reprinted with permission from ref. 111. Copyright 2017 American Chemical Society.

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2.1.3. Alkaline-Earth Metal Doping. Three alkaline-earth metals, Ca, Sr, and Ba, are proposed to be able to substitute Pb in the perovskite structure in terms of the classical notion of Goldschmidt’s rules and quantum mechanical principles.115 The band gap of MACaI3, MASrI3, and MABaI3 are estimated to be 2.95, 3.6, and 3.3 eV, respectively, which are not efficient light absorbers for photovoltaic applications.116 However, slight doping alkalineearth metal ions into halide perovskites has remarkable impact on their morphological, optical, and electronic behaviors. For instance, the crystallinity, absorption behavior, and charge separation efficiency of perovskite were obviously enhanced with 3.0 mol% Ba2+ doping.117 Even at 10 mol% Ba doping concentration, Ba-doped perovskite solar cell kept around 10% PCE without significant reduction.118 Navas et al. found that small amounts of Sr2+ doping into MAPbI3 nanoparticles did not modify the normal tetragonal phase. Whereas the band gap value was decreased of up to 4.5% due to the electronic doping of Sr2+.119 Sessolo et al. reported contrary behavior that the position of the valence band maximum and Fermi level in the bandgap does not change with doping of Sr2+ in MAPbI3 thin films.120 This result indicated that there was no electronic doping effect of Sr2+ in the perovskite films (Figure 13a). Interestingly, the charge carrier lifetime of Sr2+ containing perovskites is more than 40 µs at low charge carrier concentrations (≈ 1014 cm-3), which is the longest observed among this family of perovskites (Figure 13b).121, 122 It strongly implies that Sr2+ is capable of substantially retarding the recombination within the film. With the addition of Sr2+ ions, the morphology of the perovskite changes drastically (Figure 13c-e). The counterpart MAPbI3 film exhibits dense packed crystals with an average domain size of ≈ 100-200 nm. However, with the increase of Sr2+ ions doping concentration, the surface shows a smaller grain size (20-100 nm) and a more compact morphology with low roughness (32.3 nm). The change of grain size may be caused by the different electronegativity of Sr (0.95 Pauling scale) and Pb (2.33 Pauling scale).115 The Sr/Pb molar ratios at the surface for 2% and 5% Sr-doped are 0.33 and 0.87, respectively. Such values show that the film surface are strongly enriched with Sr2+ compared to the bulk of the film (Figure 13f). In addition, the presence of an increased amount of oxygen and the appearance of the C 1s signal mean the formation of Sr-compositions, such as SrO (5.7 eV) and SrCO3 (4.3 eV) (Figure 13g, h). These Sr-compositions initially decrease the work function, which in turn provides improved electron collection at this interface (Figure 13i). Consequently, the device fill factor (FF) increased from 78% for the pure perovskite to 85% for the 2% Sr2+ doped sample.

Figure 12. a) XRD patterns of multiple-cation mixed-halide perovskite films on glass/FTO/compact TiO2 substrates upon addition of different amounts of RbI and CsI to the perovskite precursor solution. b) Schematic illustration of the effect of RbI and CsI addition on the multiple-cation mixedhalide perovskite structure. c) 11.7 T Solid-state 87Rb echodetected MAS (20 kHz, 298 K) spectra of various (Cs/Rb/MA/FA)Pb(Br/I)3 systems. d) 39K solid-state NMR spectra at 21.1 T, 20 kHz MAS, and 298 K of reference (blue) and perovskite (black) compositions. a, b) Reprinted with permission from ref. 100. Copyright 2017 American Chemical Society. c) Reprinted with permission from ref. 112. Copyright 2017 American Chemical Society. d) Reprinted with permission from ref. 113. Copyright 2018 American Chemical Society.

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Figure 13. a) Valence band spectra (the inset shows the semilog representation of the valence region close to EF of the pure and Sr2+-substituted MAPbI3 thin flms on PEDOT:PSS). b) Normalized TRMC traces for the pure and Sr2+-containing MAPbI3 thin flms recorded at excitation wavelengths of 650 nm with a ﬂuence of 6 × 109 photons cm−2 per pulse. SEM images of the surface for c) pure MAPbI3 thin flms and for samples with d) 2% and e) 5% Sr2+ concentration. XPS spectra of the f) Sr 3p, g) O 1s, and h) C 1s regions for the same set of samples. i) UPS photoemission cutoff of the pure and Sr2+-substituted MAPbI3. Reprinted with permission from ref. 120. Copyright 2016 John Wiley and Sons.

Independent to the Navas work, by adding small amounts of Sr2+ into precursor solutions, Shai et al. found that the incorporation of Sr into the MAPbI3-xClx crystal lattice can significantly increase stability due to the suppression of unsaturated Pb.123 Meanwhile, the hysteretic thermogravimetric analysis (TGA) phenomenon of the Srdoped samples exhibit higher thermal stability compared to the undoped ones. By using SrCl2 as a dopant, Zhang et al. controlled the crystallization kinetics of MAPbI3 perovskite and prepared the film with fewer defects, which was believed to promote the photovoltaic performance.124 The optimized Sr-doped film can enhance the photovoltaic performance of the derived hole-conductor-free device to 15.9%, outperforming the value (13.0%) of the counterpart sample. Recently, Sr-doped CsPbI2Br films were investigated by Lau et al.125 They showed that doping of Sr leads to a surface that is enriched with Sr, providing a passivation effect facilitating a higher effective lifetime and therefore a better Voc and FF in the device.

Mn doping in II-VI materials (e.g. ZnS, CdS, ZnSe, and CdSe) where Mn doping can generate long-lifetime sensitized dopant luminescence and create a magnetically coupled exciton state.126-129 These properties mainly result from the strong exchange interaction between the charge carriers of the host material and d electrons of the Mn dopant, which allows excitation energy transfer or creates new coupled electronic states between the exciton and dopant.130, 131 Compared to group II-VI semiconductors, the high tolerance for trap states in lead halide perovskite creates much more chance for promoting the excition energy transfer to Mn d-states and consequently Mn d-d emission.132

2.2. Transition Metal Doping. Transition metal doping has been widely explored as a way to impart novel optical, magnetic, and electronic properties to traditional II-VI semiconductors. A representative study to this regard is

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Figure 14. a) Band positions of CsPbX3 and Mn d-states. b) Digital image showing Mn-doped CsPbCl3 NCs. c) The optical properties of undoped and Mn-doped CsPbCl3 NCs. d) Decay dynamics of Mn d-emission from Mn-doped CsPbCl3 nanoplatelets with emission at 586 nm after excitation with a microsecond flash lamp at 300 nm. Inset compares the decay of Mn d-emission for samples having different Mn content. Spectra taken from several single particles comparing the e) normalized exciton and f) Mn luminescence of each one (dot) with the ensemble measurement of Mn-doped CsPb(Cl/Br)3 NCs (line). g, h) Comparison of XRD data of nanoplatelets having different Mn contents with reference data for cubic and tetragonal phase of bulk CsPbCl3. i) EPR data showing hyperfine splitting of Mn2+ ions incorporated in the lattice of CsPbCl3 nanoplatelets. b) Reprinted with permission from ref. 133. Copyright 2017 John Wiley and Sons. c) Reprinted with permission from ref. 52. Copyright 2016 American Chemical Society. d, g-i) Reprinted with permission from ref. 134. Copyright 2017 American Chemical Society. e, f) Reprinted with permission from ref 51. Copyright 2016 American Chemical Society.

nescence, due to the spin-forbidden nature of the 4T1— 6 A1 transition.134 Interestingly, adjusting the Mn-doping concentration from 0.2% to 2% does not have a significant influence on the lifetime of the Mn d-electron emission. The normalized exciton and Mn luminescence spectra of different single particles were demonstrated (Figure 14e, f).51 The positions of the exciton luminescence do not overlap from particle to particle due to the size heterogeneity in the ensemble and quantum confinement effect, but the Mn luminescence position from different particles do not vary. These results with single exponential decay of Mn luminescence imply the relatively uniform local lattice environment at the Mn doping sites in CsPbCl3 host lattice. Mir et al. measured XRD and electron paramagnetic resonance (EPR) of CsPbCl3 with different Mn doping concentrations.134 As shown in Figure 14g, no impurity peak is observed in Mn: CsPbCl3 samples.

Among all kinds of all-inorganic perovskites, CsPbCl3 has the most appropriate band gap for the exciton energy transfer for Mn d-d transition (Figure 14a). Almost at the same time, Dong and Klimov groups reported that doping CsPbCl3 NCs with Mn2+ can be accomplished via a hotinjection method.51, 52 Mn- and Pb-precursors were dissolved together with ligands at high temperatures, and then cesium oleate was injected rapidly to form Mndoped NCs. Upon UV excitation, Mn-doped CsPbCl3 (Mn: CsPbCl3) NCs showed bright yellow emission due to the Mn d-d transition (Figure 14b);133 while undoped CsPbCl3 NCs exhibited narrow band-edge emission at 402 nm. The PL excitation (PLE) spectrum for the dopant emission peak (586 nm) almost coincides with the absorption spectrum by which it suggests that the dopant emission is sensitized by the CsPbCl3 host (Figure 14c). Figure 14d exhibits a very long lifetime (1.6 ms) of the 586 nm lumi-

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The XRD peak at 22° monotonically shifted toward higher 2θ values with an increasing in Mn doping (Figure 14h). This shift indicates that Mn is introduced in the lattice of CsPbCl3 where Mn2+ ions occupy Pb2+ ion positions. Their EPR studies exhibit the sharp hyperfine splitting without much interference, which further corroborates the relative uniformity of the doping sites without forming a phase-segregated state (Figure 14i). Using the same strategy, many different approaches have been explored for doping Mn2+ in lead halide perovskite NCs, and they can be classified in two main categories: one-pot synthesis and postsynthetic anion exchange (Table 1). One-pot synthesis focused on the optimization of reaction conditions including additives assistance (e.g. alkylamine hydrochloride (RNH3Cl), HCl acid, and SiCl4),133, 135, 136 controlling the reaction temperature or Pbto-Mn molar feed ratio,134, 137, 138 and changing precursors.139, 140 Using these methods, high quality Mndoped lead halide perovskite NCs were synthesized rapidly with strong sensitized dopant emission. On the other hand, postsynthetic anion exchange focuses on replacing Pb in preformed octahedron halide structure of PbX64-.141145 It requires a long reaction time compared to one-pot synthesis, but the band edge emission spectra of the final products can be finely tuned by varying the composition of halide anions while retaining the existence of Mn2+ emission, showing a broad range of tunable luminescence.

Figure 15. a) PL spectra of Mn: CsPbCl3−xBrx NCs taken during progressive Br− anion exchange using PbBr2, starting with CsPbCl3 NCs. b) Absolute PLQYs for Mn: CsPbBr3 and Mn: CsPbI3 NCs doped with different nominal Mn2+ contents ranging from 0 to 60 mol%. c) Temperature-dependent PL intensities for excitonic luminescence of Mn:CsPbBr3 (4.3 mol %) and pure CsPbBr3 NCs via three heating/cooling cycles at 100, 150, and 200 °C, respectively. a) Reprinted with permission from ref. 52. Copyright 2016 American Chemical Society. b, c) Reprinted with permission from ref. 147. Copyright 2017 American Chemical Society.

It is noteworthy that the intensities of both the bandedge and dopant emission have dramatic changes upon halide anion exchange (Figure 15a). For Mn: CsPbCl3 system, the first small amounts of Br+ increase the PLQY of both emissions. With increasing Br+ addition, the intensity of the band-edge peak became stronger, while the intensity of the Mn2+ sensitized emission slowly declined. This phenomenon indicates that the exciton energy transfer to dopant states decreased with narrowing of host band gap by Br insertion.146 Therefore, it is readily comprehensible that energy transfer cannot be shown in Mn: CsPbBr3 and Mn: CsPbI3 systems, yet, the Mn2+ ions were demonstrated to have a significant impact on the exciton emissions of CsPbBr3 and CsPbI3. The PLQY for excitonic luminescence of Mn: CsPbBr3 and Mn: CsPbI3 NCs increased with appropriate Mn dopant concentrations (Figure 15b). More importantly, the Mn-doped sample exhibited much better thermal and air stability compared to the undoped one.147 As shown in Figure 15c, the room temperature PL intensity of Mn: CsPbBr3 retain about 120% of their original intensity after three heating and cooling cycles at 100, 150, and 200 ℃. On the contrary, the pure counterpart had obvious fluorescence quenching at high temperature.

Figure 16. a) Calculated three-dimensional stacking diagram of a Mn: CsPbBr3 crystal when Pb2+ ion was substituted by Mn2+ ion with an actual concentration of 2.08 mol % by using first-principle calculations based on DFT. b) Models of relaxed α-CsPbI3 and CsPb0.88Mn0.12I3, and respective DFT projected densities of states calculated at the HSE06 (α = 0.43) level by including spin-orbit. a) Reprinted with permission from ref. 147. Copyright 2017 American Chemical Society. b) Reprinted with permission from ref. 148. Copyright 2017 American Chemical Society.

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Table 1. Reaction conditions for synthesis of different Mn-doped lead halide perovskite NCs. Temperature

PLQY

% doping

Shape/phase

200

0.2

cube/ orthorhombic

26-60

one-pot

200

0.2

cube/ orthorhombic

31

Mn: CsPbCl3

one-pot

185

9.6

cube/cubic

27

52

Mn: CsPbCl3

one-pot (RNH3Cl assistance)

180

1.3

cube/cubic

27

133

Mn: CsPbCl3

one-pot

R.T.

2

platelet/

20

134

Mn: CsPbCl3

one-pot (HCl assistance)

R.T.

—

cube/

39

135

Mn: CsPbCl3

one-pot (SiCl4 assistance)

R.T.

—

cube/cubic

—

136

Mn: CsPbCl3

one-pot

210

46

cube/tetragonal

17

137

Mn: CsPbCl3

one-pot

210

2.4

cube/cubic

62

138

Mn: CsPb(Cl/Br)3

one-pot

R.T.

~30

cube/cubic

41.6

139

Mn:MAPb(Cl/Br)3

one-pot

R.T.

90

cube/cubic

—

140

Mn: CsPb(Cl/Br)3

postsynthetic anion exchange

R.T.

2-12

cube/cubic

5-65

141

Mn: CsPb(Cl/Br)3


R.T.

5.7

cube/ monoclinic

56

142

Mn: CsPbCl3


150

—

cube/tetragonal

—

143

Sample

Mode of synthesis

Mn: CsPbCl3

one-pot

Mn: CsPb(Cl/Br)3

(°C)

(%)

Ref.

51

nm for Zn-doped NCs. Remarkably, the doped NCs still maintain the narrow PL line width (fwhm≈ 80 meV), high PLQYs (> 60%), and sharp excitonic absorption transitions.

To understand these phenomena, Zou et al. analyzed the formation energy for Mn-doped CsPbX3 NCs by firstprinciple calculations.147 It was revealed that the enhanced formation energy owing to the substitution of Pb2+ with smaller Mn2+ in the lattices of CsPbX3 can lead to lattice contractions. As shown in Figure 16a, the bond length of the first, second nearest neighboring and normal Pb-Br bonds are 3.382 Å, 3.034 Å, and 3.003 Å, respectively. The bond length of Mn-Br (2.666 Å) is much shorter than those of Pb-Br bonds. The great difference in bond lengths induce lattice distortions which may virtually stabilize the crystal structure of CsPbX3. Akkerman et al. found similar result in Mn-doped CsPbI3 system.148 As exemplified by the DFT calculations, the cell contraction is due to the shortening of the metal-I bonds, which decreased from 3.14 Å for Pb-I to 2.97 Å for Mn-I. The authors focus on the effects of the partial replacement of Pb with Mn on the electronic structure as well. The projected density of states of α-CsPbI3 and CsPb0.88Mn0.12I3 supercells are compared at the HSE06 (exchange fraction α = 0.43) level by including spin-orbit corrections (Figure 16b). The substitution of Pb2+ with Mn2+ introduces negligible variation of the electronic structures at the band edges, indicating no obvious changes in the PL properties of Mn-doped systems compared to the undoped one.

Figure 17. Tunable photoluminescence of CsPbBr3 NCs upon reaction with divalent cation bromide salts. Photographs of colloidal suspensions under UV illumination of perovskite NCs after reaction of CsPbBr3 NCs with different concentrations of a) CdBr2 and b) ZnBr2. c) Absorption and PL spectra of parent CsPbBr3 NCs and product NCs obtained after reaction with different concentrations of CdBr2 and ZnBr2. Reprinted with permission from ref. 149. Copyright 2017 American Chemical Society.

With the deepening of the research on Mn-doped perovskite NCs, researchers further explored the use of other transition metal ions as dopants to tune the properties of halide perovskites. Stam et al. reported that Cd2+ or Zn2+ ions were introduced into CsPbBr3 NCs to tailor optical properties upon postsynthetic cation exchange strategy.149 As shown in Figure 17, the additional Cd2+ or Zn2+ ions leads to a blue-shift of the absorption and PL spectra. The position of the PL maximum can be tuned between 452 and 512 nm for Cd-doped NCs and between 462 and 512

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Figure 18. Absorption and PL spectra of a) undoped and Ni-doped CsPbCl3 NCs and b) undoped and Ni-doped CsPb(Cl,Br)3 NCs. Low-magnification TEM images of c) undoped NCs and d) 11.9% Ni-doped. Insets of c) and d) show the histograms of edge lengths of corresponding CsPbCl3 NCs. e) k3-weighted Pb LIII-edge EXAFS and f) corresponding Fourier transforms (FTs) of undoped and 11.9% Ni-doped NCs. g) Stability regions of different compounds against Cs and Pb chemical potentials. h) Band structure and DOS of Ni-doped CsPbCl3 without any defect. The horizontal dotted line represents the Fermi level. Reprinted with permission from ref. 150. Copyright 2018 American Chemical Society.

Very recently, Sun et al. reported that CsPbClxBr3-x (x=2, 2.4, 3) NCs with near-unity PLQYs through the substitution of Ni for Pb.150 Different from Mn2+- and Bi3+-doped CsPbX3, NCs, Ni-doped do not exhibit any dopant-related PL (Figure 18a, b). As shown in Figure 18c, d, the average side length of NCs decreases from 10.6 to 8.3 nm with increasing of Ni concentration, showing high uniform size distribution. To understand the Ni-doping-induced giant enhancement of band-edge emission from NCs, X-ray absorption fine structure (EXAFS) spectra at the Pb LIIIedge was carried out for undoped and 11.9 Ni2+ doped samples (Figure 18e, f). It is clearly to find that Ni-doped NCs exhibit a stronger amplitude in R space compared to the undoped counterpart, implying improved order of the local coordination environment of Pb. According to the fitting results of EXAFS spectra, Ni doping can effectively suppress structural defects (e.g., halide vacancies) of NCs, leading to a more ordered perovskite lattice. The experimental results are also verified by DFT calculations. The authors calculated the available equilibrium chemical potential region for CsPbCl3 and found that the formation energy of Pb vacancies (VPb) is much larger than those of Cl and Cs vacancies (VCl, VCs), meaning VCl and VCs as the dominant defects (Figure 18g). Interestingly, Ni doping obviously increases the formation energy of VCl, VCs, and VPb, which can result in improved crystal quality of CsPbCl3 NCs. In addition, it is noted that the band gap of Nidoped CsPbCl3 without defects is almost unchanged compared with undoped counterpart, suggesting that Ni doping hardly influence the state of a self-trapped hole (Figure 18h). All results showed that the use of transition

metal doping engineering provides a new approach to target perovskite NCs with desirable performance, which can be utilized to fabricate functional devices.

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Figure 19. a) TEM images, b) XRD patterns, c) absorption and d) PL spectra of CsPbCl3 NCs doped with different RE ions. e) PLQYs for excitonic emissions from CsPbCl3 NC host (red line) and for the overall emissions from both NC host and RE ions. f) Energy level diagram of Yb3+ ions doped CsPbCl3 NCs and the possible quantum cutting mechanisms. Reprinted with permission from ref. 53. Copyright 2017 American Chemical Society.

contraction owing to that larger Pb2+ was substituted by smaller RE ions, which was also verified by the DFT calculation. Interestingly, lattice contraction enhances the binding energy between anions and cations, leading to the increase of band gap energy and thus blue shift of the PL for CsPbCl3 (Figure 19c, d). Meanwhile, new peaks at around 400-1200 nm appeared upon RE doping, as shown in Figure 19d. Typically, the emissions for RE ions are from the intrinsic electronic transitions, they are 4f-5d transition for Ce3+, 4H5/2-6HJ (J=2/5, 2/7, 2/9, 2/11) for Sm3+, 5 D0-7FJ for Eu3+, 5D4-7FJ (J=3-6) for Tb3+, 4G5/2-6HJ (J=15/2, 13/2, 11/2) for Dy3+, 2H11/2-4I15/2/4S3/2-4I15/2 for Er3+, and 2F5/22 F7/2 for Yb3+ ions. Intriguingly, the doping of RE ions not only forms new luminescence bands but also increases the PLQY of excitonic emission of CsPbCl3 NCs (Figure 19e). The authors believed that the increase of PLQY was resulted mainly from the modification of the defects in the materials. This similar PLQY enhancement also happened to the Ce3+ doped CsPbBr3 NCs.57 In addition, it is worth noting that the NIR emission band associated with Yb3+ doping exhibits a high PLQY of 143%, which be

2.3. RE Doping. RE ions have been widely used as dopants in oxides and II-VI semiconductors to control their optical and electronic properties. RE ions doping has distinct feature that it exhibits various kinds of energy transitions, which lead to abundant PL emissions covering from UV to intermediate infrared range.151, 152 Due to these features, RE-doped LHPs have attracted much attention in the field of luminescence control and device applications. Pan et al. reported the doping of various RE ions (Ce3+, Sm3+, Eu3+, Tb3+, Dy3+, Er3+, and Yb3+ , a gradual change in the decrease of ionic radius) into the lattices of CsPbCl3 NCs by hot-injection method.53 Figure 19a shows the transmission electron microscopy (TEM) images of CsPbCl3 with different RE ions doping. All samples demonstrate cubic morphology with good uniformity. The average size gradually decreases as the atomic number of RE ions increases. The XRD patterns indicate that both doped and undoped NCs have the same tetragonal phase, and the lattice constants for the (101) diffraction planes decrease from 3.94 to 3.87 Å with the increase of atomic number of RE ions from Ce3+ to Yb3+ ions (Figure 19b). These phenomena can be attributed to the lattice

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the emission of Yb3+. The emission intensity of Yb3+ at 988 nm is about twice of the intensity decrease of Ce3+ around 488 nm, showing an efficient quantum cutting emission process. The dynamics of undoped and RE-doped perovskite NCs are shown in Figure 20c. Compared to Yb3+ doped sample, Yb3+, Ce3+ codoped leads the lifetime of CsPbCl1.5Br1.5 host decrease to 2.0 ns. Meanwhile, the lifetime of Yb3+ ions at 988 nm increases about 5%. Figure 20d reveals the mechanism of quantum cutting process for Yb3+, Ce3+ codoped sample. In addition to the energy transfer from perovskite host to Yb3+ ions, part of electrons on the conduction band firstly relax to the 5d state of Ce3+ ions and then transfer to the Yb3+ ions. The codoping 2% Ce3+ NCs achieved the great PLQY of 146%, which was the highest value for the practical measurement in quantum cutting materials.154-157

caused by the quantum cutting of excitonic transition of CsPbCl3 host (Figure 19f).

Figure 20. Absorption (left), visible emission spectra (middle), and near-infrared emission spectra (right, excited by 365 nm light) of a) various Yb3+ ions doped perovskite NCs and b) CsPbCl1.5Br1.5 perovskite NCs codoping with different RE ions. c) Dynamics of pristine and doped CsPbCl1.5Br1.5 NCs monitored at 455 nm and Dynamics of Yb3+ ions emissions in doped CsPbCl1.5Br1.5 NCs monitored at 988 nm. d) Schematic diagram of energy transfer mechanism in the Yb3+, Ce3+ codoped CsPbCl1.5Br1.5 NCs. Reprinted with permission from ref. 153. Copyright 2017 John Wiley and Sons.

RE-doped perovskite quantum cutting materials were systematically studied by Song group.153 Using YbCl3 and PbBr2 mixture as precursor, Yb3+ doped CsPbClxBr3-x NCs were prepared by a modified hot-injection method. After the introduction of Cl- and Yb3+ in the CsPbBr3 host, the PL peak gradual shift from 520 nm to 450 nm. The PL intensity of the perovskite excitonic emission was decreased owing to the partial substitution of Br by Cl element. In the NIR region, an emission around 988 nm can be observed for Yb3+ doped CsPbClxBr3-x, and its intensity enhances significantly with the increasing doping concentration of Yb3+ ions. The PLQY reached the highest value of 94% when 7.2% Yb3+ doping into CsPbBr1.5Cl1.5 NCs. The trend of PL changing indicates the successful introduction of Yb3+ ions in CsPbClxBr3-x, showing an efficient energy transfer from the host to Yb3+ ions (Figure 20a). Another RE ion (Ce3+ of Er3+) was further introduced into the Yb3+ doped CsPbBr1.5Cl1.5 NCs. Figure 20b exhibits the UV-vis absorption and PL spectra of CsPbCl1.5Br1.5, Yb3+: CsPbCl1.5Br1.5, Yb3+, Er3+: CsPbCl1.5Br1.5 and Yb3+, Ce3+: CsPbCl1.5Br1.5 NCs. The introduction of another RE ion has an enormous influence on the emission features of Yb3+ ions, while it has little change in the bandgap and the excitonic emission. As the concentration of Er3+ increases, the emission of 4I13/2-4I15/2 at 1540 nm for Er3+ enhances obviously, but the emission intensity for Yb3+ decreases, due to the energy transfer from 2F7/2 level of Yb3+ ions to 4I13/2 level of Er3+ ions. In contrast, codoping with Ce3+ largely enhances

Figure 21. Fs-TA spectra (excitation 320 nm) taken at several representative probe delays (left panel) and decay-associated spectra (DAS) (right panel) for the a) pristine and b) Cedoped CsPbBr3 NCs. c) Schematic illustration of the involved photophysical processes and mechanisms. d) Trend comparison of the average PL lifetimes and the PLQY results versus the dopant concentration in terms of CeBr3 ratio. Reprinted with permission from ref. 57. Copyright 2018 American Chemical Society.

Recently, Yao et al. reported that doping of Ce3+ ions into CsPbBr3 NCs enhance the intensity of excitonic emission significantly.57 The PLQY increases from 41% to 89% as increasing the doping amount of Ce3+ ions to 2.88%. To understand the mechanism for the Ce3+-doping-induced PLQY improvement in CsPbBr3 NCs, fs-TA measurement

17



was carried out for 2.88% Ce3+ doped and undoped samples. The τ1, τ2, and τ3 refer to the processes of intraband hot-exciton relaxation, exciton trapping to the band gap trap states, and exciton recombination, respectively (Figure 21a-c). The former two processes were accelerated for the doped sample relative to the undoped one because of the modification of doping-induced state. Specifically, doping of Ce3+ ions leads to the increase of the lowest excitonic state density, which facilitates the coupling between the higher-lying excitonic states and the lowest excitonic state (τ1 process) and the coupling between the lowest excitonic state and the band gap trap states (τ2 process). The average PL lifetimes decreases with the increases of Ce3+ doping concentration, which corresponds to the above analysis (Figure 21d). Remarkably, the PLQY reached up to 89%, a factor of 2 increase as compared to the undoped ones, indicating the excellent potential in device applications.

Page 18 of 33

to a relatively large radius (2.53 Å) of the FA cation that makes the tolerance factor of FAPbI3 to be greater than 1.0. Partial incorporation of metal ions, such as Rb, in FAPbI3 renders a more favorable tolerance factor, which effectively suppresses the phase transition of FAPbI3.98 However, it is difficult to confirm whether Rb enters the lattice or not.100, 111, 112 Recently, Hu et al. found that FAPbI3 was stabilized at room temperature through Bi doping.55 No obvious change in color was observed for Bi-doped FAPbI3 film even after 15 days (Figure 22a), while the fresh FAPbI3 film degraded completely (Figure 22b, c). With an optimal Bi content, the devices achieved a substantially improved PCE up to 17.78%, which is much higher than that of the reference sample (Figure 22d). Importantly, the device based on Bi-doped FAPbI3 also showed longterm stability in ambient air (Figure 22e). Using the same strategy, they fabricated full inorganic devices based on the Bi-doped CsPbI3, which achieved high PCE of 13.21% and good stability that 68% of the initial PCE was maintained over 168 h operation under ambient conditions without encapsulation.158 The authors proposed that slight distortion of cubic structure caused by partial substitution of Pb with Bi might be helpful for the structural stability. In addition, metal doping has been proved to be of great benefit in improving the quality of perovskite active layer. Alkali and alkaline-earth metal ions effectively suppress the Pb-rich impurity phases in the film, which contributes to decreased defect density, improved lifetime, more efficient charge extraction, and consequently improved device performance.47, 110, 120, 124

3. OPTOELECTRONIC APPLICATIONS According to the above description, the influence of doped metal ions on the properties of LHPs, in the order of main group metal cations, transition metal cations and RE metal cations, have been discussed. Undoubtedly, dopant engineering endows LHPs with wide variety of unique properties including enhanced stability, improved PLQYs, new emission features, and high quality thin films with reduced defect state density and passivated grain boundaries, thus leading to excellent optoelectronic performance of the corresponding devices. In this section, we first summarize metal doping functions in LHPs-based solar cells, and then shift to LEDs and other devices. Particularly, we focus on the device stability, and discuss the doping-induced stability enhancement strategies in detail. In addition, hysteresis phenomenon as non-negligible parameter in perovskite devices, is discussed as well.

The morphology and crystallinity of the perovskite films have also been improved by metal doping. Liu et al. reported high-performance inorganic perovskite solar cells upon Mn2+ doped into the interstices of the CsPbI2Br lattice (Figure 23a). CsPbI2Br film growth process with less grain boundaries can be controlled by doping Mn2+ ions (Figure 23b-e).56 It is well known that the grain boundaries in the perovskite film induce charge recombination and cause shallow states that will hinder carrier diffusion, which greatly jeopardize the thin film device performance.159, 160 Remarkably, the average size of grains of CsPbI2Br film doped with Mn2+ was enlarged to 1200 nm. As shown in Figure 23f, small amounts of Mn induce slight structure change by inserting into the interstices of the CsPbI2Br lattice. Meanwhile, excess amount of Mn2+ ions are situated around the as-formed crystal. Such a distorted structure and Mn2+-enriched environment effectively prevents nucleation and decreases growth rate, leading to the formation of high crystalline and uniform

3.1. Solar Cells. Solar cells are semiconductor devices that can convert solar energy into electrical energy. The operation of solar cells involves three processes: (i) charge carrier generation and separation by incident light, (ii) carrier transport, and (iii) carrier collection. In general, the important parameters for evaluating solar cells include open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF), and power conversion efficiency (PCE). All the performance parameters of the above metal-doped perovskite solar cells are summarized in Table 2. The solar cells with doped metals have exhibited better performances as compared to the pristine samples. In addition, stability and hysteresis are also discussed as indispensable parameters in device performance. The instability of halide perovskite based devices is mainly due to the intrinsic phase transition and poor quality of the perovskite active layers. For example, despite that perovskite FAPbI3 with a lower band gap (1.48 eV) has an extended light absorption range compared to MA-based or Cs-based perovskite, the black perovskite phase (α-FAPbI3) tends to form a nonperovskite yellow phase (δ-FAPbI3) at room temperature. This is attributed

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Figure 22. a) Photographs of pristine and Bi-doped FAPbI3 films freshly coated and exposed to 50% RH environment for 15 days under dark conditions. Change in absorbance with time of b) pristine and c) Bi-doped FAPbI3 films. d) Current density–voltage characteristics of the best cells based on pristine and Bi-doped FAPbI3 films. e) Photographs of Bi-doped FAPbI3 solar cell while exposed in controlled ambient conditions for 0 h, 500 h, and 1000 h and the comparison of stability of pristine and Bi-doped FAPbI3 solar cells. Reprinted with permission from ref. 55. Copyright 2017 Royal Society of Chemistry.

Figure 23. a) Schematic structure of the device and illustration of the Mn2+ doping modes: interstitial and substituting. SEM iamges of b) pristine CsPbBrI2 and with c) 0.5%, d) 1% and e) 2% MnCl2. f) Schematic diagram for MnCl2-driven crystalline grain growth with surface passivation. Reprinted with permission from ref. 56. Copyright 2018 American Chemical Society.

active layers. As a result, the solar cell achieved stabilized efficiency as high as 13.47%, which is nearly 13% higher

than the reference device. Almost at the same time, Liang et al. reported that Mn-doped CsPbIBr2 perovskite cell

19



Page 20 of 33

Figure 24. a) J-V curves of perovskite solar cells based on (FAPbI3)0.875(CsPbBr3)0.125 doped with alkali metal ions. Capacitance−frequency (C−V) plots of b) FTO/TiO2/ (FAPbI3)0.875(CsPbBr3)0.125/spiro-MeOTAD/Au and c) FTO/(FAPbI3)0.875(CsPbBr3)0.125/Au under one sun illumination. Dark current−voltage curve of d) pristine (without KI) and e) 10 μmol KI doped (FAPbI3)0.875(CsPbBr3)0.125 with the structure of FTO/Perovskite/Au. Reprinted with permission from ref 161. Copyright 2018 American Chemical Society.

reached the highest PCE of 7.36%, which performance is 19.9% higher than the undoped thin film.162

cantly suppress the formation of Frenkel defect because K+ energetically prefers in the interstitial site.165

Hysteresis phenomenon is non-negligible in perovskite solar cells. Hysteresis is that scanning the J-V curve of perovskite solar cells from positive to negative gives different trace as compared to that scanning from negative to positive. It indicates the extraction of powerconversion efficiency from the maximum power point of the J-V curve ambiguously. The hysteretic J-V behavior presents a challenge for measuring the accurate PCE of the solar cells.163, 164 Fortunately, addition of small amounts of Rb+ or K+ ions can effectively eliminate the hysteresis phenomenon.50, 101, 102, 110, 161 Hamill et al. doped Rb+ in (MA0.17FA0.83)Pb(I0.83Br0.17)3.102 It was found that the doped sample had less impurity phases and slightly larger grains, which allowed for a fast redistribution of the ions and avoiding unfavorable trapping of ionic charge, thus promoting charge carrier mobility and suppressing hysteresis. Recently, Son et al. studied effect of alkali metal ions on the hysteresis of (FAPbI3)0.875(CsPbBr3)0.125 perovskite solar cells.161 When increasing ionic radius of alkali metal ions form Li+ to K+, hysteresis tends to decrease and disappears at K+, and it appears again upon further increase in ionic radius from K+ to Cs+ (Figure 24a). With or without selective contacts (TiO2, Spiro), capacitancefrequency behavior shows similar behavior, which indicates the I-V hysteresis is related to bulk defect rather than interface (Figure 24b, c). As shown in Figure 24d, e, VTFL (onset voltage of trap filled limit) decreases from 0.788 V for the pristine perovskite to 0.617 V for the Kdoped, which results in the trap density decreased from 1.367×1016 cm-3 to 0.846×1016 cm-3. Through theoretical calculation, the authors considered that K+ ions signifi-

3.2. LEDs. According to the detailed balance in the Shockley-Quieisser formulation, an efficient solar cell material should also be a good light emitter.73 Thus, lead halide perovskite materials have been widely explored in the field of LEDs. The important performance parameters in perovskite LEDs include EQE, luminance, and current efficiency (CE). Table 3 summarizes the above metaldoped perovskites based LED performances. Typically, the EL in the range of 1000-1600 nm, which covered all of the telecommunication windows, was achieved in a device containing a MAPbI3 layer with 0.005% Bi (Figure 25a).44 The LEDs with Li-doped perovskite as an electron transportation layer exhibited a low turn-on voltage due to the enhancement in transport properties (Figure 25b).109 High-quality Mn2+ and Ce3+ ions doped NCs with much better thermal and air stability and optical performance can be used to fabricate LEDs with higher EQE and CE compared to those undoped samples (Figure 25c, d).57, 147 In addition, owing to the good stability and larger Stokes shift (＞200 nm), Mndoped CsPbCl3 as color conversion materials were fabricated onto ultraviolet LED chips (Figure 25e, f).

3.3. Other Devices. Some works related to device applications were also worth noting. Bakr and Mohammed et al. clearly demonstrated that charge transfer at the interface of NCs can be promoted by Bi3+ ions doping.46 They found that doping increased the energy difference between states of the molecular acceptor and the donor moieties, subsequently facilitating the interfacial charge transfer (Figure 26a). This finding could be helpful for engineering interfacial charge transfer in perovskite NCs

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Figure 25. a) EL spectrum from the Bi-doped MAPbI3 LED at an applied voltage of 6 V. b) I−V characteristics of LEDs without (black) and with (red) a doped perovskite electron transportation layer. c) EQE of devices based pristine and Mn-doped CsPbBr3 NCs. d) EQE of devices based pristine and Ce-doped CsPbBr3 NCs. e, f) Diagram of light-emitting devices by coating the composites of Mn-doped perovskite NCs onto ultraviolet LED chips. a) Reprinted with permission from ref. 44. Copyright 2016 American Chemical Society. b) Reprinted with permission from ref. 109. Copyright 2017 American Chemical Society. c) Reprinted with permission from ref. 147. Copyright 2017 American Chemical Society. d) Reprinted with permission from ref. 57. Copyright 2018 American Chemical Society. e) Reprinted with permission from ref. 137. Copyright 2017 American Chemical Society. f) Reprinted with permission from ref. 139. Copyright 2017 American Chemical Society.

solar cells. RE and Mn2+ ions doped perovskite NCs are employed in the solar cells as the light conversion materials by converting UV light to NIR and visible range, respectively. Song et al. reported that the Yb3+ and Ce3+ codoped perovskite NCs were self-assembled on the surface of the commercial silicon solar cells to enhance the device performance, exhibiting an extraordinary PCE enhancement from 18.1% to 21.5% (Figure 26b, c).153 Wang et al. reported Mn: CsPbCl3 NCs-assisted perovskite solar cells for the first time.166 They found that external quantum efficiency in the UV region was significantly increased, leading to an increased short-circuit current (3.77%) and PCE (3.34%). Meanwhile, the photostability was improved because of the elimination of UV rays. Using the similar principle, Meinardi et al. demonstrated the suitability of Mn: CsPbCl3 QDs as nearly zero reabsorption emitters for LSC,167 as shown in Figure 26d, e. The LSC behaved closely to an ideal device, in which all portions of the illuminated area contribute equally to the total optical power (Figure 26f). Recently, Nafradi et al. developed a magnetic photovoltaic material upon doping Mn2+ ions into MAPbI3 perovskite.168 The material has both ferromagnetism (Tc= 25 K) and high efficiency of photoelectron generation. The Mn-doped sample exhibits an obvious ESR signal and a ferromagnetic order at specific temperature in dark, which results in a great shift of the resonant field and the broadening of the linewidth (Figure 26g). Interestingly, the magnetism of Mn-doped MAPbI3 is photosensitive. 25% of the initial spin-susceptibility disappears when the sample is exposed to light illumination (0.8 μW cm-2) at T= 5 K (Figure 26h). The light-induced

magnetization melting causes ferromagnetic moment reversal rapidly, which may provide opportunities for the development of a new generation of magneto-optical data storage devices (Figure 26i).

21



Page 22 of 33

Figure 26. a) Band gap offset of undoped and Bi-doped CsPbBr3 NCs with respect to the LUMO of tetracyanoethylene. b) Schematic diagram of perovskite flm to enhance the PCE of crystalline-silicon solar cells. c) I–V curves of the best silicon solar cells coated with different thickness of perovskite flm with scan speed 0.1 V s−1. d) Schematic representation of a LSC made of a polymer matrix comprising Mn doped CsPbCl3 NCs. e) Photograph of an LSC comprising Mn-doped CsPbCl3 NCs (LSC dimensions: 25 cm × 20 cm × 0.5 cm). f) Relative optical output power measured from c-Si PVs coupled to one perimeter edge of the NC-LSC as a function of the device area illuminated by a calibrated solar simulator (1.5 AM Global, circles). g) ESR linewidth (red dots) and resonant field (blue dots, offset by a reference value B0) as a function of temperature recorded at 9.4 GHz. Inset: SQUID magnetometry of Mn-doped MAPbI3. h) ESR spectra at 157 GHz and 5 K of pristine MAPbI3 (green line–no signal), of Mn-doped MAPbI3 in dark (blue line) coming from the FM phase and its reduction (red line) on visible light illumination. The difference between light-off and light-on ESR signals is shown in orange. i) Schematic illustration of writing a magnetic bit. a) Reprinted with permission from ref. 46. Copyright 2017 American Chemical Society. b, c) Reprinted with permission from ref. 153. Copyright 2017 John Wiley and Sons. d-f) Reprinted with permission from ref. 167. Copyright 2017 American Chemical Society. g-i) Reprinted with permission from ref. 168. Copyright 2016 Springer Nature.

Table 2. Summary of metal-doped perovskite solar cell performance. The performances of the devices using undoped perovskte were also added for comparison. Doped metal

Voc [V]

Jsc [mA cm-2]

FF [%]

PCE [%]

__

1.01

20.50

62.86

13.02

Bi

1.03

23.65

72.99

17.78

__

0.89

16.02

56.59

8.07

Bi

0.97

18.76

72.59

13.21

__

1.182

23.30

75.00

20.60

Rb

1.180

22.80

81.00

21.80

MAFAPbI3-xBrx

Rb

1.125

23.24

71.90

18.80

48

MAPbI3

__

0.99

22.90

75.00

15.60

105

Sample

FAPbI3

CsPbI3

CsMAFAPbI3-xBrx

22


Ref. 55

158

47

Page 23 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


FAPbI3 FA0.83MA0.17Pb(I0.83Br0.17)3 FAPbI3

FAMAPbI3-xBrx

MAPbI3

(FA0.85MA0.15Pb(I0.85Br0.15)3

CsPbI2Br

(Cs,MA,FA)Pb(I0.85Br0.15)3

(Cs,MA,FA)Pb(I0.4Br0.6)3

MAPbI3

MAPbI3

MAPbI3-xClx

MAPbI3

CsPbI2Br

MAPbI3-xClx

Rb

1.02

22.80

~80.00

18.80

__

0.99

22.10

62.00

13.56

Rb

1.07

23.93

67.00

17.16

Rb

__

__

__

20.35

__

1.089

21.10

64.90

14.90

Rb

1.03

23.80

65.90

16.20

__

1.158

21.50

68.50

17.10

Rb

1.168

22.50

74.70

19.60

__

1.06

20.97

70.00

15.56

Na

1.10

21.16

78.00

18.16

K

1.10

20.88

78.00

17.81

__

~1.08

~22.50

~75.00

~19.00

K

1.167

22.99

76.00

20.32

__

1.18

10.00

70.00

8.20

K

1.18

11.60

73.00

10.00

__

1.05

22.60

73.00

17.30

K

1.17

23.20

79.00

21.50

__

1.12

15.30

72.00

12.30

K

1.23

17.90

79.00

17.50

__

1.08

21.92

73.00

17.30

K

1.11

23.33

74.00

19.27

__

~1.00

~15.00

77.00

~12.00

Sr

~0.95

~17.50

85.00

~15.00

__

1.07

21.94

71.00

16.70

Sr

1.11

21.08

71.00

16.30

__

0.94

19.48

69.00

12.60

Sr

1.05

20.18

75.00

15.90

__

0.962

13.40

59.80

7.70

Sr

1.04

14.90

71.50

11.10

__

0.95

19.20

65.00

11.80

Mg

0.89

18.70

64.80

10.80

Ca

0.96

18.40

67.80

12.00

Sr

0.93

18.60

65.00

11.20

23


98

102

103

49

50

169

110

165

120

123

124

125

117


MAPbI3-xClx

CsPbI2Br

CsPbIBr2

MAPbI3

MAPbI3

MAPbI3

MAPbIxCl3-x

MAPbI3

Page 24 of 33

Ba

0.99

20.40

69.60

14.00

__

1.02

19.90

67.50

13.70

Ba

0.98

19.60

67.80

13.00

__

1.115

14.15

75.30

11.88

Mn

1.172

14.37

80.00

13.47

__

__

__

__

6.14

Mn

__

__

__

7.36

__

1.04

21.30

77.20

17.10

Al

1.10

22.40

77.60

19.10

__

1.10

21.13

74.00

17.11

Zn

1.14

22.17

72.00

18.25

__

1.00

17.50

66.20

11.70

Cd

1.00

18.70

67.30

12.80

Zn

1.05

19.20

68.60

13.76

__

0.91

19.07

72.00

12.61

In

1.03

22.08

78.00

17.55

__

0.89

21.53

65.20

13.10

Sb

0.985

21.82

69.20

15.60

118

56

162

92

170

171

94

95

Table 3. Summary of metal-doped perovskite LED performance. The performances of the devices using undoped perovskite were also added for comparsion. Sample

Doped metal

Peak EL [nm]

EQE [%]

CE [cd A-1]

Lumin [cd m-2]

Ref.

MAPbI3

Bi

1100

__

__

__

44

__

560

1.47

4.98

6421

Rb

532

7.17

24.22

66353

__

512-515

0.81

3.71

7493

Mn

512-515

1.49

6.40

9717

__

~510

1.60

12.50

~2000

Ce

~510

4.40

14.20

~3500

FAPbBr3

CsPbBr3

CsPbBr3

24


106

147

57

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At present, doped perovskites are primarily used for the construction of solar cells and LEDs. Given the attractive properties imparted by dopants, doped perovskites should have broader device applications such as photodetectors, single photon sources and potentially magnetically-controlled optical functionalities.

4. CONCLUSION AND PERSPECTIVE In this review, the comprehensive advances of doped perovskites are thoroughly discussed and summarized in terms of synthesis methods, doping-induced property changes, and the improved performance of corresponding devices, with a particular emphasis on some of the milestone achievements achieved so far. Various dopants, including main group metals (e.g., Bi, Rb, K, Sr, Al), transition metals (e.g., Mn, Zn, Cd), and RE metals (e.g., Ce, Yb, Er), have been successfully doped into halide perovskite polycrystalline films, single crystals, and NCs, giving rise to a wide variety of exotic properties different from mother compounds. The unique properties achieved through dopant engineering include enhanced stability and high quality thin films with enlarged grain size, as well as reduced defect state density and passivated grain boundaries, thus leading to excellent optoelectronic performance of the corresponding devices including solar cells and LEDs. Undoubtedly, dopant engineering has emerged as one powerful strategy to tailoring the properties of halide perovskites, making this prominent material even more attractive for practical applications.

We conclude that the area of doped halide perovskites is only at its infancy. There is much needed urgency for the development of theory-guided screening methods of doped species. Such a development will afford dopinginduced alteration of properties and focus the efforts of researchers on the most rational dopants (out of the multitude of available dopant species), as well as ensuring predictability and reproducibility with continuous efforts in this direction. We believe that doped halide perovskites will play a prominent role in shaping the development of perovskite technologies.

AUTHOR INFORMATION Corresponding Author

Despite the significant progress mentioned above, our understanding of the details related to doped halide perovskites is rather limited, and a series of scientific issues in terms of synthesis or doping methods, structureproperty relationships, and intrinsic mechanisms for optoelectronic properties remain unresolved. The relatively large tolerance factor of the ABX3 perovskite structure renders the compound to be naturally suitable for a variety of different chemical compositions. Although diverse ions have been doped into lead halide perovskite polycrystalline films, single crystals, and NCs, the exact structural information related to doped ions, including but not limited to, the exact crystallographic sites occupied by dopants and the distribution of doped ions in LHPs, remains elusive in most cases. Great efforts on the characterization of materials’ structures – particularly the local environment of dopants – are needed to obtain a rational understanding of doping-induced property changes. Such understanding will pave the way for targeted optimization of properties of lead halide perovskites along with theoryguided screening of doped species.

*Osman M. Bakr. E-mail: [email protected] * Hong-Tao Sun. E-mail: [email protected]

Author Contributions ‡These authors contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (Grant Nos. 11874275 and 11574225), KAUST, and Jiangsu Specially Appointed Professor program (Grant No. SR10900214). Y. Zhou acknowledges the financial support of China Scholarship Council (CSC).

So far the doping-induced optimization of properties have been limited to improved structural stability or enhanced PLQYs. However, the rational design of such doped perovskite systems, which is still a grand challenge, will offer pathways to suppress ion migration (currently an outstanding deficiency of perovskite devices) and magnetism, which may usher perovskites into the realm of spintronics. While 3D halide perovskites have been the prime focus of dopant engineering approaches, lower-dimensional perovskites, which are bulk quantum materials, could be the next frontier for property tuning with dopants. In particular, two-dimensional halide perovskites offer a rich space to study the interaction of electrically-, optically-, or magnetically-active dopants with quantum confinement effects.

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Metal-Doped Lead Halide Perovskites: Synthesis, Properties, and

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