Perovskite Symmetry Research Articles

Highly efficient tunable mid-infrared luminescence achieved by crystal field modulation of CsPb1-xHoxBr3 halide perovskite AlF3-based fluoride glass

Mid-infrared (MIR) light sources have gained significant importance across various applications in spectroscopy, sensing, astronomy, communications and medical surgery. The diverse spectral characteristics of rare earth ions, particularly lanthanide ions, stemming from their distinctive 4f intershell transitions, offer a multitude of potential transitions spanning the UV-visible-infrared spectrum. Despite recent advancements in MIR gain luminescence, the investigation of tunable MIR luminescence mechanisms remains a major technical challenge. Herein, an effective mechanism to modulate the local crystal field of rare earth ions by altering its crystal structure has been revealed, resulting in tunable broad-spectrum emission in the MIR luminescence range of 2800-3000 nm and multi-peak emission in the near-infrared band of Ho3+. Notably, the local crystal field of Ho3+ is adjusted by manipulating the lattice symmetry of CsPb1-xHoxBr3 perovskite through the incorporation of fluoride glass reticulation to control the crystal size of the perovskite and thereby modify the lattice symmetry of CsPb1-xHoxBr3 perovskite. The energy level transition of Ho3+ is influenced by adjusting the crystal field asymmetry, resulting in the splitting of the 5I6 energy level depending on the crystal field. This cleavage affects the transitions from the 5I5 level to 5I6 at 1480 nm and from 5I6 to 5I7 at 2880 nm. As 5I6 acts as the common upper level for the two emission peaks, the infrared peaks at 1480 nm and 2880 nm widen and develop into a dual-peak emission phenomenon. The infrared luminescence produced aligns closely with the distinctive infrared absorption peaks of carbon dioxide, leading to the development of a convenient, high-precision device for monitoring of CO2 concentration in hydrogen energy in real time. These findings are anticipated to pave the way for extensive utilization of novel tunable MIR luminescence.

Evidence of a Large Refrigerant Capacity in Nb-Modified La1.4Sr1.6Mn2−xNbxO7 (0.0 ≤ x ≤ 0.15) Layered Perovskites

In this work, evidence of isothermal magnetic entropy change (∆SM) over a broad temperature region is presented in a series of La1.4Sr1.6Mn2−xNbxO7 Ruddlesden–Popper compounds with niobium modification (Nb) (0.0 ≤ x ≤ 0.15) at the manganese (Mn) site. The ceramic samples were obtained through a solid-state sintering method in optimized conditions. All compounds predominantly possessed Ruddlesden–Popper phase while a few additional reflections were resolved in Nb-doped compounds which indicates the separation of structural phases. These peaks are assigned to a separate layered perovskite and single perovskite with tetragonal symmetry and hexagonal symmetry, respectively. The microstructure of the pure sample reveals uniform grain morphology but in Nb-doped specimens chiefly three types of grains were found. It was assumed that the inter-connected large particles were of R-P phase which is dominant in both parent and x = 0.05 compounds, while the hexagonal and polygonal morphology of grains in higher concentrations of dopants directly corroborates with the symmetry of single perovskite and additional layered perovskite phases, respectively. The parent compound exhibits a single ∆SM curve, whereas all Nb-substituted samples display bifurcated ∆SM curves. This indicated two transition regions with multiple magnetic components, attributed to distinct structural phases. The highest ∆SM values obtained for components corresponding to the R-P phase are 2.32 Jkg−1k−1, 0.75 Jkg−1k−1, 0.58 Jkg−1k−1 and 0.43 Jkg−1k−1 and for the second component located around room temperature are 0.0 Jkg−1k−1, 0.2 Jkg−1k−1, 0.28 Jkg−1k−1 and 0.35 Jkg−1k−1 for x = 0.0, 0.05, 0.10 and 0.15 compositions, respectively, at 2.5 T. Due to the collective participation of both components the ∆SM was expanded through a broad temperature range upon Nb doping.

Temperature-dependent excited states for detecting reversible phase transitions in 2D lead(II) iodide perovskites.

Significant interest exists in water-tolerant 2D lead iodide perovskites owing to their stability and proven potential in photovoltaic and photonic applications. These materials have solid-state phase transitions that are accessible below 100 °C. Here, the study witnesses the multiple phase transitions of the last members of a series of organic-inorganic hybrid materials, [(CnH2n+1NH3)2PbI4], with even n as n = 14, 16, and 18, once again. By employing temperature-dependent steady-state photoluminescence (PL) and temperature-dependent time-resolved photoluminescence (TRPL) spectroscopy in the temperature range of -18 to +90 °C and at -196 °C, we explore the thermal responses of these materials. The investigation reveals reversible phase transitions occurring between room temperature (RT) and elevated temperatures, impacting the optical properties and emitting colors of the perovskite compounds. The longer the alkyl chain, the higher the phase transition temperature, attributed to increased conformational disorder and enhanced perovskite symmetry. The decay constants for all compounds are very close in value, which confirms the underlying excited-state dynamics, pointing to contributions primarily from inorganic components across different phases. We anticipate that our results on the detection of phase transitions in 2D perovskites will not only motivate the use of these techniques for detecting phase transitions but also would help to understand their excited states in more details to selectively use them for solar cell and next-generation display technologies.

Influence of Ni substitution on the magnetic properties of PrCoO3: A comprehensive study

The influence of nickel (Ni) ions on the structural and magnetic properties of PrCo1-xNixO3 perovskites was investigated. The samples were prepared using a sol-gel chemical auto-combustion route. The Rietveld refined X-ray diffraction patterns show that all samples were single-phase polycrystalline with orthorhombic (Pbnm) perovskite symmetry. Fourier transform infrared spectroscopy (FTIR) showed that with increasing Ni dopant content, the spin state changed from a low to an intermediate spin state (LS→IS), leading to an increase in the Co3+ population in the IS state, which improved the stability of the IS state. X-ray photoelectron spectroscopy confirmed the presence of mixed valence states (Co3+/Co4+) and oxygen vacancies. Ni-substituted PrCoO3 compounds exhibited mixed magnetic phases, which were separated by a theoretical model. The susceptibility curves were fitted with the Curie–Weiss law, and the negative value of the Curie–Weiss temperature indicated a strong predominance of antiferromagnetic (AFM) interactions over ferromagnetic (FM) ordering. However, when the Ni content in the host lattice increased, the FM coupling via double exchange became stronger. Thus, saturated FM ordering was achieved with an optimum doping level of Ni ions.

Growth, Structural, and Electrical Properties of Spin Coated LaNiO3 Conducting Oxide

LaNiO3 (LNO) perovskite oxide thin film nanostructure has been prepared using spin coating technique for its homogenous growth, and structural and electrical properties on quartz substrate. X-ray diffraction (XRD) confirmed standard cubic perovskite symmetry. Surface morphological measurements were carried out using atomic force microscope (AFM), which shows uniform, smooth, and crack free surface with RMS roughness of 7.08 nm. This oxide based conducting layer can act as an excellent conducting bottom/top electrode providing brilliant seeding or buffer layer for subsequent ferroelectric/ferromagnetic layers. This incorporates that it can act as an excellent conducting bottom electrode. I-V characteristics of LNO demonstrated a small shift in current with the application of low perpendicular magnetic field of 0.58T. p-type conduction. Average hole concentration of 3.66 x 10+17 cm-2 was estimated from the Hall effect measurements. The magnetoresistance (MR) of the sample was found to be +48.26%.

Visible-Light-Driven AO7 Photocatalytic Degradation and Toxicity Removal at Bi-Doped SrTiO3.

In this study, Bi-doped SrTiO3 perovskites (Sr1−xBixTiO3, x = 0, 0.03, 0.05, 0.07 and 0.1) were synthesized using the solid-state method, characterized, and tested as photocatalysts in the degradation of the azo dye acid orange 7 (AO7) under visible light. The perovskites were successfully synthesized, and XRD data showed a predominant, well-crystallized phase, belonging to the cubic perovskite symmetry. For the doped samples, a minority phase, identified as bismuth titanate, was detected. All doped samples exhibited improved photocatalytic activity under visible light, on the degradation of AO7 (10 mg L−1), when compared with the undoped SrTiO3, with an increase in relative Abs484 nm decay from 3.7% to ≥67.8% after 1 h, for a powder suspension of 0.2 g L−1. The best photocatalytic activity was exhibited by the Sr0.95Bi0.05TiO3 perovskite. Reusability studies showed no significant loss in photocatalytic activity under visible light. The final solutions showed no toxicity towards D. magna, proving the efficiency of Sr0.95Bi0.05TiO3 as a visible-light-driven photocatalyst to degrade both the AO7 dye as well as its toxic by-products. A degradation mechanism is proposed.

Synergistic Regulation Effect of Nitrate and Calcium Ions for Highly Luminescent and Robust α-CsPbI3 Perovskite.

The α-CsPbI3 nanocrystals (NCs) easily transform into yellow non-perovskite, accompanying with declining photoelectric properties that restricting their practical applications in diverse fields. Herein, the highly luminescent and robust α-CsPbI3 NCs is achieved through engineering the lattice symmetry of perovskite, enabled by the synergistic effect of NO3- ion passivation and Ca2+ ion doping. The introduced NO3- ions enhance the phase-change energy barrier and the surface steric hindrance, thus promoting the formation of α-CsPbI3 NCs with hyper-symmetric crystal structure, while the Ca2+ ion doping contributes to improving their lattice symmetry by significant regulation of the tolerance factor. As a result, the obtained α-CsPbI3 NCs display an outstanding photoluminescence quantum yield of 96.6%, together with the reduced defect state density and eminent conductivity. Most importantly, the as-engineered α-CsPbI3 NCs exhibit excellent stability under ambient conditions for 9months and UV illumination for 32 h. It displays brilliant thermal stability, maintaining luminous intensity for 15 min under 140°C, and performing desired durability and reversibility, evidenced by 160°C cyclic test and 120°C reversibility test. Given enhanced robustness, the as-engineered α-CsPbI3 NCs based light-emitting-diode devices are constructed, exhibiting a power efficiency of 105.3lmW-1 and the excellent working stability for 18 h.

Machine learning accelerated search for new double perovskite oxide photocatalysis

Double perovskite oxide <i>A</i><sub>2</sub><i>BB'</i>O<sub>6</sub> has better stability and wider bandgap range than <i>AB</i>O<sub>3</sub>-type oxide, and exhibits great prospects in photocatalytic overall water splitting. However, owing to the diversity of crystal structure and constituents of perovskite oxide, rapidly and accurately searching for <i>A</i><sub>2</sub><i>BB'</i>O<sub>6</sub> for photocatalyst is still a big challenge, both experimentally and theoretically. In this work, in order to screen out suitable double perovskite oxide photocatalysts, a multi-step framework combined with machine learning technique and first-principles calculations is proposed. Nearly 8000 candidates with proper bandgaps for water splitting are screened out from among more than 50000 <i>A</i><sub>2</sub><i>BB'</i>O<sub>6</sub>-type double perovskite oxides. Statistical analysis of the results shows that double perovskite oxides with d<sup>10</sup> metal ions at <i>B/B</i><i><i>'</i></i> sites are more likely to have good absorption of visible light, and the structural symmetry of double perovskite also has influence on the bandgap value. Furthermore, first-principles calculations demonstrate that Sr<sub>2</sub>GaSbO<sub>6</sub>, Sr<sub>2</sub>InSbO<sub>6</sub> and K<sub>2</sub>NbTaO<sub>6</sub> are non-toxic photocatalyst candidates with proper band edges for overall water splitting.

Trojans That Flip the Black Phase: Impurity-Driven Stabilization and Spontaneous Strain Suppression in γ-CsPbI3 Perovskite

The technological progress and widespread adoption of all-organic CsPbI3 perovskite devices is hampered by its thermodynamic instability at room temperature. Because of its inherent tolerance toward deep trap formation, there has been no shortage to exploring which dopants can improve the phase stability. While the relative size of the dopant is important, an assessment of the literature suggests that its relative size and impact on crystal volume do not always reveal what will beneficially shift the phase transition temperature. In this perspective, we analyze the changes in crystal symmetry of CsPbI3 perovskite as it transforms from a thermodynamically stable high-temperature cubic (α) structure into its distorted low-temperature tetragonal (β) and unstable orthorhombic (γ) perovskite structures. Quantified assessment of the symmetry-adapted strains which are introduced due to changes in temperature and composition show that the stability of γ-CsPbI3 is best rationalized from the point of view of crystal symmetry. In particular, improved thermal-phase stability is directly traced to the suppression of spontaneous strain formation and increased crystal symmetry at room temperature.

Size-dependent structural and electrical properties of lead-free BCST ceramics prepared from high-energy ball milled nanopowders

Recently developed (Ba0.88Ca0.12)(Ti0.94Sn0.06)O3 (BCST) ceramic composition gained popularity as a cleaner substitute for the lead-based Pb(Zr,Ti)O3 (PZT) after its electrical properties were found to be comparable to those of some soft-end PZTs. These properties can be further enhanced by optimizing the microstructure and phase-symmetry of the ceramic system. In the present work, BCST nanopowders have been synthesized via solid-state reaction method followed by high-energy ball milling. Initial particle size has been controlled by varying the milling speed from 100−400 rpm at fixed milling time. It has been observed that a mere change in initial particle size strongly affects the microstructural, structural and electrical properties. A monotonous decline in average particle size (∼97 %) with an enormous rise in the average grain size (∼260 %) with milling speed is observed. The increase of milling speed causes the evolution of perovskite crystal symmetry of the ceramics from orthorhombic to tetragonal, with the bulk density acquiring a peak value for BCST ceramic prepared from 175-rpm milled powder. As a consequence of relatively high density and favorable phase-structure, the piezoelectric properties of this ceramic sample exhibit a significant enhancement of about ∼70 %.

Physical properties and sinterability of pure and iron-doped bismuth sodium titanate ceramics

Pure (BNT) and iron-doped bismuth sodium titanate (Fe-BNT) ceramics were produced according to the formula Bi0.5Na0.5Ti1−xFexO3−0.5x, where x = 0 to 0.1. The addition of Fe2O3 enables decreasing the sintering temperature to 900 °C in comparison with 1075 °C for pure BNT, whilst also achieving lower porosities and greater densities. This is attributed to oxygen vacancy generation arising from substitution of Fe3+ onto the Ti4+ site of the BNT perovskite structure, and the resulting increase in mass transport that this enables during sintering. X-ray diffraction (XRD) analysis of Fe-BNT samples shows single-phase BNT with no secondary phases for all studied Fe contents, confirming complete solid solution of Fe. Rietveld refinement of XRD data revealed a pseudocubic perovskite symmetry (Pm-3m), and unit cell lengths increased with increasing Fe content. Scanning electron microscopy (SEM) showed that average grain size increases with increasing Fe content from an average grain size of ~ 0.5 μm in (x = 0) pure BNT to ~ 5 μm in (x = 0.1) Fe-doped BNT. Increasing Fe content also led to decreasing porosity, with relative density increasing to a maximum > 97% of its theoretical value at x = 0.07 to 0.1. The addition of Fe to BNT ceramics significantly affects electrical properties, reducing the remnant polarization, coercive field, strain and desirable ferroelectric properties compared with those of pure densified BNT. At room temperature, a high relative permittivity (ɛ′) of 1050 (x = 0.07) at an applied frequency of 1 kHz and a lower loss factor (tanδ) of 0.006 (x = 0.1) at an applied frequency of 300 kHz were observed by comparison with pure BNT ceramics.

Investigation of Structural and Magnetic Properties of GdFe0.5Cr0.5O3 Perovskite Prepared by Solid-State Route

GdFe0.5Cr0.5O3 compound was synthesized through solid-state reaction route, based on stoichiometric mixture of Fe2O3, Cr2O3 and Gd2O3. The X-ray diffraction data showed that GdFe0.5Cr0.5O3 crystallizes in an orthorhombic distorted perovskite structure. High-resolution electron microscopy showed a well-ordered material and confirms the orthorhombic perovskite symmetry. Magnetic susceptibility data reveal a Curie-Weiss behavior in the temperature range 250 − 340 K and the onset of antiferromagnetic interactions with a Neel temperature of 241 K.

Investigation of proton conductivity in Sc and Yb co-doped barium zirconate ceramics

The oxygen deficient BaZr0.85Sc0.15−xYbxO3-δ (x = 0, 0.05, 0.10) ceramics were synthesized via conventional solid state reaction route. The Rietveld analysis of the x-ray diffraction profile proved that all the prepared compositions presents single phase cubic perovskite symmetry and space group. The actual oxygen occupancy of the materials has been derived from the Rietveld study. Thermo-gravimetric study of the pre-hydrated samples revealed a considerable mass loss, indicating more than 90% of the oxygen vacancies have been successfully filled by protonic defects upon hydration. FESEM images of the fractured surfaces of sintered ceramics showed dense microstructures with sub-micron sized grains and well resolved grain boundaries. The Nyquist plots differentiated the bulk, grain boundary and electrode response at lower temperatures (≤300 °C). The bulk conductivity of the Sc and Yb co-doped barium zirconate was higher compared to that of the respective Sc-doped perovskite oxide. The activation energy of bulk conductivity and total conductivity decreased with the increase in Yb content. The total conductivity of (4.01 × 10−3 Scm−1) at 600 °C has been achieved for BaZr0.85Sc0.10Yb0.05O3, suggesting the composition suitable for SOFC applications.

Magnetic and transport properties driven by Sr substitution in polycrystalline Pr1-xSrxCoO3 (0.1 ≤ x ≤ 0.5) cobaltites

The effect of strontium (Sr) doping on the structural, magnetic, electrical, and thermal properties of Pr1-xSrxCoO3 (0.1 ≤ x ≤ 0.5) has been studied. The samples were synthesized by using the conventional solid-state reaction method. The Rietveld refinement of X-ray diffraction patterns confirms the single-phase composition with orthorhombic (Pbnm) perovskite symmetry. The magnetization measurements revealed the paramagnetic to ferromagnetic transition and the transition temperature (Tc) increased with increasing Sr doping. The effective magnetic moments determined by the Curie-Weiss law show an increase in the Sr concentration. The temperature dependence of electrical resistivity suppressed with increasing the Sr content. Moreover, all the compounds other than x = 0.5 show the semiconducting nature. All semiconductor compositions (x = 0.1, 0.2, 0.3, and 0.4) in the high temperature region can be explained within the framework of the small polaron hopping model and the variable range hopping model, whereas the metallic composition (x = 0.5) is explained by electron-electron, electron-phonon, and electron-spin fluctuation scattering processes. The Seebeck coefficient (S) for all the samples except x= 0.5 is found to be positive, thereby confirming the applicability of the small polaron hopping model in the high-temperature region. The sample with x = 0.5 exhibits a crossover in S from positive to negative values and again attains a positive value.

Spin-orbital model of stoichiometric LaMnO3 with tetragonal distortions

The model developed for LaMnO$_3$ addresses the spin-orbital order by superexchange and Jahn-Teller orbital interactions in the cubic (perovskite) symmetry up to now whereas real crystal structure is strongly deformed. We identify and explain three \textit{a priori} important physical effects arising from tetragonal deformation: (i) the splitting of $e_g$ orbitals $\propto E_z$, (ii) the directional renormalization of $d-p$ hybridization $t_{pd}$, and (iii) the directional renormalization of charge excitation energies. Using the example of LaMnO$_3$ crystal we evaluate their magnitude. It is found that the major effects of deformation are enhanced amplitude of $x^2-y^2$ orbitals induced in the orbital order by $E_z\simeq 300$ meV and anisotropic $t_{pd}\simeq 2.0$ (2.35) eV along the $ab$ ($c$) cubic axis, in very good agreement with the Harrison's law. We show that the tetragonal model analyzed within mean field approximation provides a surprisingly consistent picture of the ground state. Excellent agreement with the experimental data is obtained simultaneously for: (i) $e_g$ orbital mixing angle, (ii)~spin exchange constants, and (iii) the temperatures of spin and orbital phase transition.

Microwave dielectric properties of Sr0.7Ce0.2TiO3–Sr(Mg1/3Nb2/3)O3 ceramics

xSr0.7Ce0.2TiO3–(1 − x)Sr(Mg1/3Nb2/3)O3 ceramics, referred to xSCT–(1 − x)SMN, were successfully produced by conventional solid-state sintered technology. The compounds, belonging to perovskites with a secondary phase of CeO2, can be detected even with x down to 0.1 of SCT composition. The overall trend for grain growth illustrates the increase with increasing SCT doping level. The Raman peak at 825 cm−1 splits into two peaks and causes red shift phenomenon. XPS spectra indicate that Ti and Nb ions exist respectively in tetravalence and pentavalence, and Ce ions exist in trivalence and tetravalence. Dielectrics constant (e r ) of SCT–SMN ceramics gradually increases with increasing theoretical dielectric polarizabilities. A wider width of the 825 cm−1 for FWHM of A1g mode Raman peaks suggests to a lower Q × f value. The increasing tolerance factor in agreement with temperature coefficient of resonant frequency (τ f ), denotes that the rise of perovskite symmetry. The 0.1SCT–0.9SMN ceramic sintered at 1450 °C for 4 h illustrates excellent microwave dielectric properties with e r ~ 35.4, Q × f ~ 11282 GHz and τ f ~ 1.7 ppm/°C. Activation energies of 0.1SCT–0.9SMN ceramic at 100, 300 and 500 V, are ~0.436, 0.427 and 0.331 eV, respectively, indicative of a decreased trend with external electric field.

Microscopic origin of the mobility enhancement at a spinel/perovskite oxide heterointerface revealed by photoemission spectroscopy

The spinel/perovskite heterointerface $\gamma$-Al$_2$O$_3$/SrTiO$_3$ hosts a two-dimensional electron system (2DES) with electron mobilities exceeding those in its all-perovskite counterpart LaAlO$_3$/SrTiO$_3$ by more than an order of magnitude despite the abundance of oxygen vacancies which act as electron donors as well as scattering sites. By means of resonant soft x-ray photoemission spectroscopy and \textit{ab initio} calculations we reveal the presence of a sharply localized type of oxygen vacancies at the very interface due to the local breaking of the perovskite symmetry. We explain the extraordinarily high mobilities by reduced scattering resulting from the preferential formation of interfacial oxygen vacancies and spatial separation of the resulting 2DES in deeper SrTiO$_3$ layers. Our findings comply with transport studies and pave the way towards defect engineering at interfaces of oxides with different crystal structures.

Investigation of structural and some physical properties of Cr substituted polycrystalline Eu0.5Sr0.5Mn1−xCrxO3 (0 ≤ x ≤ 0.1) manganites

The polycrystalline samples with nominal composition Eu0.5Sr0.5Mn1−x Cr x O3 (0 ≤ x ≤ 0.1) were prepared by the conventional solid state reaction method and characterized by X-ray diffraction, scanning electron microscopy and electrical resistivity behavior without and with magnetic field. The structural parameters obtained by using Rietveld refinement of X-ray diffraction data showed that all samples crystallize with orthorhombic perovskite type symmetry with Pbnm space group. The scanning electron micrograph images reveal that the increase in Cr substitution hinders grain growth and grain connectivity. The temperature dependence of electrical resistivity show the semiconducting nature of these compounds and support the small polaron hoping model and variable range hopping conduction model. The calculated hopping distance and activation energy decreased as rate of Cr content increased whereas density of states at Fermi level increased. A large negative magnetoresistance is also present in the sample at the lowest temperature of measurements.

Structural, electrical and magnetic phase evolution of Cr substituted GdMn1−xCrxO3 (0 ⩽ x ⩽ 0.2) manganites

A series of GdMn1−xCrxO3 (0⩽x⩽0.2) have been synthesized by the conventional solid state reaction method mainly to understand the response of Cr substitution on structural, microstructural, magnetic and electrical transport properties. The X-ray diffraction shows that all samples crystallize with orthorhombic perovskite type symmetry with Pbnm space group. The room temperature Raman spectroscopy shows the reduction of Raman shift as doping concentration increase that related to anti-symmetric Jahn Teller stretching mode and symmetric-stretching mode. The temperature dependence of electrical resistivity show the semiconducting nature of these compounds and support the small polaron hoping model (SPH) and variable range hopping conduction model (VRH). The calculated hopping distance and activation energy increased as rate of Cr content increased whereas density of state at Fermi level decreased. The temperature dependence of magnetization presents that under the zero field cooling (ZFC) mode as doping concentration increases the net magnetization reverses its nature below irreversibility temperature. The magnetization results show that all samples show weak ferromagnetic nature that can further be improved with increasing Cr ions concentration.

Superior Photovoltaic Properties of Lead Halide Perovskites: Insights from First-Principles Theory

Organic–inorganic methylammonium lead halide perovskites have recently emerged as promising solar photovoltaic absorbers. In this Feature Article, we review our theoretical understanding of the superior photovoltaic properties, such as the extremely high optical absorption coefficient and very long carrier diffusion length of CH3NH3PbI3 perovskites through first-principles theory. We elucidate that the superior photovoltaic properties are attributed to the combination of direct band gap p–p transitions enabled by the Pb lone-pair s orbitals and perovskite symmetry, high iconicity, large lattice constant, and strong antibonding coupling between Pb lone-pair s and I p orbitals. We show that CH3NH3PbI3 exhibits intrinsic ambipolar self-doping behavior with conductivities tunable from p-type to n-type via controlling the growth conditions. We show that the p-type conductivity can be further improved by incorporating some group IA, IB, or VIA elements at I-rich/Pb-poor growth conditions. However, the n-type co...