Nuclear Quantum Effects Accelerate Charge Recombination but Boost the Stability of Inorganic Perovskites in Mild Humidity.

Experiments have demonstrated that mild humidity can enhance the stability of the CsPbBr3 perovskite, though the underlying mechanism remains unclear. Utilizing ab initio molecular dynamics, ring polymer molecular dynamics, and non-adiabatic molecular dynamics, our study reveals that nuclear quantum effects (NQEs) play a crucial role in stabilizing the lattice rigidity of the perovskite while simultaneously shortening the charge carrier lifetime. NQEs reduce the extent of geometric disorder and the number of atomic fluctuations, diminish the extent of hole localization, and thereby improve the electron-hole overlap and non-adiabatic coupling. Concurrently, these effects significantly suppress phonon modes and slow decoherence. As a result, these factors collectively accelerate charge recombination by a factor of 1.42 compared to that in scenarios excluding NQEs. The resulting sub-10 ns recombination time scales align remarkably well with experimental findings. This research offers novel insight into how moisture resistance impacts the stability and charge carrier lifetime in all-inorganic perovskites.

Dynamics of interfacial charge transfer between CsPbBr3perovskite nanocrystals and molecular acceptors for photodetection application.

Perovskite nanocrystals (NCs) recently emerged as a suitable candidate for optoelectronic applications because of its simplistic synthesis approach and superior optical properties. For better device performance, the effective absorption of incident photons and the understanding of charge transfer (CT) process are the basic requirements. Herein, we investigate the interfacial charge transfer dynamics of CsPbBr3NCs in the presence of different molecular acceptors; 7,7,8,8-Tetracyanoquinodimethane (TCNQ) and 11,11,12,12 tetracyanonaphtho-2,6-quinodimethane (TCNAQ). The vivid change in CT dynamics at the interfaces of NCs and two different molecular acceptors (TCNQ and TCNAQ) has been observed. The results demonstrate that the ground state complex formation in the presence of TCNQ acts as additional driving force to accelerate the charge transfer between the NCs and molecular acceptor. Moreover, this donor (NCs)-acceptor (TCNQ, TCNAQ) system results in the higher absorption of incident photons. Finally, the photo detector based on CsPbBr3-TCNQ system was fabricated for the first time. The device exhibited a high on-off ratio (104). Furthermore, the CsPbBr3-TCNQ photodetector shows a fast photoresponse times of 180 ms/110 ms (rise/decay time) with a specific detectivity (D*) of 5.2 × 1011Jones. The simple synthesis and outstanding photodetection abilities of this perovskite NCs-molecular acceptor system make them potential candidates for optoelectronic applications.

Spin Coherence and Spin Relaxation in Hybrid Organic-Inorganic Lead and Mixed Lead-Tin Perovskites.

Metal halide perovskites make up a promising class of materials for semiconductor spintronics. Here we report a systematic investigation of coherent spin precession, spin dephasing and spin relaxation of electrons and holes in two hybrid organic-inorganic perovskites MA0.3FA0.7PbI3 and MA0.3FA0.7Pb0.5Sn0.5I3 using time-resolved Faraday rotation spectroscopy. With applied in-plane magnetic fields, we observe robust Larmor spin precession of electrons and holes that persists for hundreds of picoseconds. The spin dephasing and relaxation processes are likely to be sensitive to the defect levels. Temperature-dependent measurements give further insights into the spin relaxation channels. The extracted electron Landé g-factors (3.75 and 4.36) are the biggest among the reported values in inorganic or hybrid perovskites. Both the electron and hole g-factors shift dramatically with temperature, which we propose to originate from thermal lattice vibration effects on the band structure. These results lay the foundation for further design and use of lead- and tin-based perovskites for spintronic applications.

Ultranarrow Line Width Room-Temperature Single-Photon Source from Perovskite Quantum Dot Embedded in Optical Microcavity.

Ultranarrow bandwidth single-photon sources operating at room-temperature are of vital importance for viable optical quantum technologies at scale, including quantum key distribution, cloud-based quantum information processing networks, and quantum metrology. Here we show a room-temperature ultranarrow bandwidth single-photon source generating single-mode photons at a rate of 5 MHz based on an inorganic CsPbI3 perovskite quantum dot embedded in a tunable open-access optical microcavity. When coupled to an optical cavity mode, the quantum dot room-temperature emission becomes single-mode, and the spectrum narrows down to just ∼1 nm. The low numerical aperture of the optical cavities enables efficient collection of high-purity single-mode single-photon emission at room-temperature, offering promising performance for photonic and quantum technology applications. We measure 94% pure single-photon emission in a single-mode under pulsed and continuous-wave (CW) excitation.

Highly Durable Inverted Inorganic Perovskite/Organic Tandem Solar Cells Enabled by Multifunctional Additives.

Inverted perovskite/organic tandem solar cells (P/O TSCs) suffer from poor long-term device stability due to halide segregation in organic-inorganic hybrid wide-bandgap (WBG) perovskites, which hinders their practical deployment. Therefore, developing all-inorganic WBG perovskites for incorporation into P/O TSCs is a promising strategy because of their superior stability under continuous illumination. However, these inorganic WBG perovskites also face some critical issues, including rapid crystallization, phase instability, and large energy loss, etc. To tackle these issues, two multifunctional additives based on 9,10-anthraquinone-2-sulfonic acid (AQS) are developed to regulate the perovskite crystallization by mediating the intermediate phases and suppress the halide segregation through the redox-shuttle effect. By coupling with organic cations having the desirable functional groups and dipole moments, these additives can effectively passivate the defects and adjust the alignment of interface energy levels. Consequently, a record Voc approaching 1.3 V with high power conversion efficiency (PCE) of 18.59% could be achieved in a 1.78 eV bandgap single-junction inverted all-inorganic PSC. More importantly, the P/O TSC derived from this cell demonstrates a T90 lifetime of 1000 h under continuous operation, presenting the most stable P/O TSCs reported so far.

Surface Reaction Induced Compressive Strain for Stable Inorganic Perovskite Solar Cells.

Cesium-based inorganic perovskites have emerged as promising light-harvesting materials for perovskite solar cells (PSCs) due to their promising thermal- and photo-stability. However, obstacles to commercialization remain regarding their phase instability. In this work, we report a facile and effective strategy to regulate the surface compressive strain via in-situ surface reaction to stabilize CsPbI3 perovskite. The use of a chelating ligand with a molecular configuration closely matching the integer multiples of the unit cell lattice parameters of CsPbI3 induces compressive strain at the surface of CsPbI3. The chemical bonding and strain modulation synergistically not only passivate film defects, but also inhibit perovskite phase degradation, thus significantly improving the intrinsic stability of inorganic perovskite. Consequently, enhanced power conversion efficiency (PCE) of 21.0% and 18.6% were respectively achieved in 0.16-cm2 lab-scale devices and 25.3-cm2 solar modules. Further, surface reaction enables PSCs with enhanced thermal and operational stability; these devices retain over 95% of their initial PCE after damp-heat tests (i.e., in 85 ℃ and 85% R.H. air) for 2000 h, and remain 99% of their initial PCE after operating for 2000 h, representing one of the most stable inorganic PSCs reported so far.

Creation of a Field-Induced Co(II) Single-Ion Magnet by Doping into a Zn(II) Diamagnetic Metal-Organic Framework.

The combination of single-ion magnets (SIMs) and metal-organic frameworks (MOFs) is expected to produce new quantum materials. The principal issue to be solved in this regard is the development of new strategies for the synthesis of SIM-MOFs. This work demonstrates a new simple strategy for the synthesis of SIM-MOFs where a diamagnetic MOF is used as the framework into which the SIM sites are doped. 1, 0.5, and 0.2 mol% of the Co(II) ions are doped into the Zn(II) sites of [CH6 N3 ][ZnII (HCOO)3 ]. The doped Co(II) sites in the MOFs perform as SIM with a positive D term of zero-field splitting. The longest magnetic relaxation time is 150 ms (0.2 mol% Co) at 1.8 K under a static field of 0.1 T. Temperature dependency of the relaxation time suggests suppressing magnetic relaxation by reduction of spin-spin interaction upon doping in the rigid framework. Thus, this work represents a proof of concept for the creation of a single-ion doped magnet in the MOF. This simple synthetic strategy will be widely applied for the creation of quantum magnetic materials.

Improving Photoelectric Conversion with Broadband Perovskite Metasurface.

The miniaturization and integration of optoelectronic devices require progressive size reduction of active layers, resulting in less optical absorption and lower quantum efficiency. In this work, we demonstrate that introducing a metasurface made of hybrid organic-inorganic perovskite (HOIP) can significantly enhance broadband absorption and improve photon-to-electron conversion, which roots from exciting Mie resonances together with suppressing optical transmission. On the basis of the HOIP metasurface, a broadband photodetector has been fabricated where photocurrent boosts more than 10 times in the frequency ranging from ultraviolet to visible. The device response time is less than 5.1 µs at wavelengths 380, 532, and 710 nm, and the relevant 3 dB bandwidth is over 0.26 MHz. Moreover, this photodetector has been applied as a signal receiver for transmitting 2D color images in broadband optical communication. These results accentuate the practical applications of HOIP metasurfaces in novel optoelectronic devices for broadband optical communication.

Phase Separation of Br-Doped CsPbI3: A Combined Cluster Expansion, Monte Carlo, and DFT Study.

Cluster expansion, which is a method that describes the concentration-dependent thermodynamic properties of materials while maintaining density functional theory accuracy, was used to predict new (CsPbIxBr1-x) structures. The cluster-expansion method generated 42 new stable (CsPb)xIyBrz (where x = 1 to 3 and y and z = 1 to 8) structures and these were ranked as meta-stable structures based on their enthalpies of formation. Monte Carlo calculations showed that CsPbI0.5Br0.5 composition separates into different phases at 300 K, but changes to a homogeneous phase at 700 K, suggesting that a different phase of CsPbI3 may exist at higher temperatures. Among the 42 predicted structures, randomly selected structures around iodide-rich, 50:50, and bromine-rich sites were studied further by determining their electronic, optical, mechanical, and thermodynamic properties using first-principle density functional theory. The materials possess similar properties as cubic Br-doped CsPbI3 perovskites. The mechanical properties of these compounds revealed that they are ductile in nature and mechanically stable. This work suggests that the introduction of impurities into CsPbI3 perovskite materials, as well as compositional engineering, can alter the electronic and optical properties, making them potential candidates for solar cell applications.

Design Rules for Two-Dimensional Organic Semiconductor-Incorporated Perovskites (OSiP) Gleaned from Thousands of Simulated Structures.

Two-dimensional (2D) halide perovskites are an attractive class of hybrid perovskites that have additional optoelectronic tunability due to their accommodation of relatively large organic ligands. Nevertheless, contemporary ligand design depends on either expensive trial-and-error testing of whether a ligand can be integrated within the lattice or conservative heuristics that unduly limit the scope of ligand chemistries. Here, the structural determinants of stable ligand incorporation within Ruddlesden-Popper (RP) phase perovskites are established by molecular dynamics (MD) simulations of over ten-thousand RP-phase perovskites and training of machine learning classifiers capable of predicting structural stability based solely on generalizable ligand features. The simulation results show near-perfect predictions of positive and negative literature examples, predict trade-offs between several ligand features and stability, and ultimately predict an inexhaustibly large 2D-compatible ligand design-space.

Schottky-Contacted High-Performance GaSb Nanowires Photodetectors Enabled by Lead-Free All-Inorganic Perovskites Decoration.

The surface Fermi level pinning effect promotes the formation of metal-independent Ohmic contacts for the high-speed GaSb nanowires (NWs) electronic devices, however, it limits next-generation optoelectronic devices. In this work, lead-free all-inorganic perovskites with broad bandgaps and low work functions are adopted to decorate the surfaces of GaSb NWs, demonstrating the success in the construction of Schottky-contacts by surface engineering. Benefiting from the expected Schottky barrier, the dark current is reduced to 2 pA, the Ilight /Idark ratio is improved to 103 and the response time is reduced by more than 15 times. Furthermore, a Schottky-contacted parallel array GaSb NWs photodetector is also fabricated by the contact printing technology, showing a higher photocurrent and a low dark current of 15 pA, along with the good infrared photodetection ability for a concealed target. All results guide the construction of Schottky-contacts by surface decorations for next-generation high-performance III-V NWs optoelectronics devices.

Insights Into to the KX (X = Cl, Br, I) Adsorption-Assisted Stabilization of CsPbI2 Br Surface.

Despite the excellent optoelectronic properties, organic-inorganic hybrid perovskite solar cells (PSCs) still present significant challenges in terms of ambient stability. CsPbI2 Br, a member of all-inorganic perovskites, may respond to this challenge because of its inherent high stability against light, moisture, and heat, and therefore has gained tremendous attraction recently. However, the practical application of CsPbI2 Br is still impeded by the notorious phenomenon of photoinduced halide segregation. Herein, by applying first-principles calculations, the stability, electronic structure, defect properties, and ion-diffusion properties of the stoichiometric CsPbI2 Br (110) surface and that with the adsorption of KX (X = Cl, Br, I) are systematically investigated. It is found that the adsorbed KX can serve as an external substitute of the halogen vacancies on the surface, therefore inhibiting halogen segregation and improving the stability of the CsPbI2 Br surface. The KX can also eliminate deep-level defect states caused by antisites, thereby contributing to the promoted optoelectronic properties of CsPbI2 Br. The mechanistic understanding of surface passivation in this work can lay the foundation for the future design of CsPbI2 Br PSCs with optimized optoelectronic performance.

Stabilized High-Membered and Phase-Pure 2D All Inorganic Ruddlesden-Popper Halide Perovskites Nanocrystals as Photocatalysts for the CO2 Reduction Reaction.

In contrast to the 2D organic-inorganic hybrid Ruddlesden-Popper halide perovskites (RPP), a new class of 2D all inorganic RPP (IRPP) has been recently proposed by substituting the organic spacers with an optimal inorganic alternative of cesium cations (Cs+ ). Nevertheless, the synthesis of high-membered 2D IRPPs (n > 1) has been a very challenging task because the Cs+ need to act as both spacers and A-site cations simultaneously. This work presents the successful synthesis of stable phase-pure high-membered 2D IRPPs of Csn+1 Pbn Br3n+1 nanosheets (NSs) with n = 3 and 4 by employing the strategy of using additional strong binding bidentate ligands. The structures of the 2D IRPPs (n = 3 and 4) NSs are confirmed by powder X-ray diffraction and high-resolution aberration-corrected scanning transmission electron microscope measurements. These 2D IRPPs NSs exhibit a strong quantum confinement effect with tunable absorption and emission in the visible light range by varying their n values, attributed to their inherent 2D quantum-well structure. The superior structural and optical stability of the phase-pure high-membered 2D IRPPs make them a promising candidate as photocatalysts in CO2 reduction reactions with outstanding photocatalytic performance and long-term stability.

Photophysical and Photoelectrochemical Properties of CsPbBr3 Films Grown by Electrochemically Assisted Deposition.

Perovskite have had a great impact on the solid-state physics world in the last decade not only achieving great success in photovoltaics but, more recently, also in the implementation of other optoelectronic devices. One of the main obstacles for the adoption of Pb-based perovskite technologies are the high amounts of Pb needed in the conventional preparation methods. Here we present for the first time a detailed analysis of the photophysical and photoelectrochemical properties of CsPbBr3 films directly grown on fluorine-doped tin oxide (FTO) coated glass through a novel technique based in the electrodeposition of PbO2 as CsPbBr3 precursor. This technique allows to save up to 90 % of the Pb used compared to traditional methods and can be scalable compared with the commonly used spin-coating process. The low temperature analysis of their photoluminescence spectra, performed in both steady state and time dependence, revealed a strong interaction between electrons and longitudinal optical (LO) phonons dominant at high temperatures. On the other hand, the electrochemical and photoelectrochemical analysis proves that CsPbBr3 prepared using this new method has state-of-the-art features, showing a p-type behavior under depletion regime. This is also confirmed by photoelectrochemical measurements using p-benzoquinone as target molecule. These results prove that the proposed method can be used to produce excellent CsPbBr3 films, saving much of the lead waste.

Cooperative Nature of Ferroelectricity in Two-Dimensional Hybrid Organic-Inorganic Perovskites.

Two-dimensional (2D) ferroelectric (FE) hybrid organic-inorganic perovskites (HOIPs) are promising for potential applications as miniaturized flexible ferroelectric/piezoelectric devices. Recently, several 2D HOIPs [e.g., Ruddlensden-Popper type HOIP BA2PbCl4 (BA = C6H5CH2NH3+)] were reported to possess room-temperature ferroelectricity. However, the underlying microscopic mechanisms for ferroelectricity in 2D HOIPs remain elusive. Here, by performing first-principles calculations and symmetry mode analysis, we demonstrate that there exists a cooperative coupling between A-site organic molecules and B-site inorganic Pb2+ ions that is essential to the ferroelectricity in 2D BA2PbCl4. The nonpolar ground state of the closely related compounds BA2PbBr4 and BA2PbI4 can also be explained in terms of the weakened cooperative coupling. We further predict that 2D BA2PbF4 displays in-plane ferroelectricity with a higher Curie temperature and larger electric polarization. Our work not only reveals the unusual FE mechanism in 2D HOIPs but also provides a solid theoretical basis for the rational design of 2D multifunctional materials.

Synthesis, Photoluminescence and Vibrational Properties of Aziridinium Lead Halide Perovskites.

Three-dimensional lead halide perovskites are known for their excellent optoelectronic properties, making them suitable for photovoltaic and light-emitting applications. Here, we report for the first time the Raman spectra and photoluminescent (PL) properties of recently discovered three-dimensional aziridinium lead halide perovskites (AZPbX3, X = Cl, Br, I), as well as assignment of vibrational modes. We also report diffuse reflection data, which revealed an extended absorption of light of AZPbX3 compared to the MA and FA counterparts and are beneficial for solar cell application. We demonstrated that this behavior is correlated with the size of the organic cation, i.e., the energy band gap of the cubic lead halide perovskites decreases with the increasing size of the organic cation. All compounds show intense PL, which weakens on heating and shifts toward higher energies. This PL is red shifted compared to the FA and MA counterparts. An analysis of the PL data revealed the small exciton binding energy of AZPbX3 compounds (29-56 meV). Overall, the properties of AZPbX3 are very similar to those of the well-known MAPbX3 and FAPbX3 perovskites, indicating that the aziridinium analogues are also attractive materials for light-emitting and solar cell applications.

Temperature-dependent structural fluctuation and its effect on the electronic structure and charge transport in hybrid perovskite CH3 NH3 PbI3.

The recently discovered hybrid organic-inorganic perovskites have been suggested for high-performance optoelectronic applications. Owing to the mechanical flexibility of these compounds, they demonstrate structural fluctuation at finite temperatures that have been widely discussed with respect to their optical properties. However, the effect of temperature-induced structural fluctuation is not clear until now, with respect to the equally important charge transport properties. In the present study, through ab initio molecular dynamics simulations of cubic-phase CH3 NH3 PbI3 at different temperatures, the temperature-dependent electronic structure and charge carrier transport properties are examined. Compared with the significant structural fluctuation of organic cations, the structural change of the inorganic framework is minor. In addition, because the band edge states at R point are mainly influenced by the anti-bonding character of the Pb-I bond, CH3 NH3 PbI3 demonstrates relatively small deformation potentials as well as low temperature dependence of band gaps (ΔEg ≈ 50 meV from 330 K to 400 K) and electron-phonon coupling strengths, despite the large structural fluctuation of organic cations. Furthermore, the effective mass of the valence band increases with the increase of temperature. The predicted mobilities of CH3 NH3 PbI3 can reach above 75 cm2 V-1  s-1 near room temperature, exhibiting an appropriate optoelectronic potential, while the temperature dependence is steeper than T-1.5 of the traditional semiconductors because of the enhanced effective masses.

Managing Defects Density and Interfacial Strain via Underlayer Engineering for Inverted CsPbI2 Br Perovskite Solar Cells with All-Layer Dopant-Free.

Inorganic perovskite CsPbI2 Br has advantages of excellent thermal stability and reasonable bandgap, which make it suitable for top layer of tandem solar cells. Nevertheless, solution-processed all-inorganic perovskites generally suffer from high-density defects as well as significant tensile strain near underlayer/perovskite interface, both leading to compromised device efficiency and stability. In this work, the defect density as well as interfacial tensile strain in inverted CsPbI2 Br perovskite solar cells (PeSCs) is remarkably reduced by using a bilayer underlayer composed of dopant-free 2,2',7,7'-tetrakis(N,N-dip-methoxyphenylamine)-9,9'-spirobifluorene (Spiro-OMeTAD) and copper phthalocyanine 3,4',4″,4'″-tetrasulfonated acid tetrasodium salt (TS-CuPc) nanoparticles. As compared to control devices with pristine Spiro-OMeTAD, devices based on Spiro-OMeTAD/TS-CuPc exhibit remarkably improved photovoltaic performance and enhanced thermal/humidity stability due to the better perovskite crystallization, improved interfacial passivation, and hole-collection as well as efficient interfacial strain release. As a result, a champion efficiency of 14.85% can be achieved, which is approaching to the best reported for dopant-free and inverted all-inorganic PeSCs. The work thus provides an efficient strategy to simultaneously regulate the defects density and strain issue related to inorganic perovskites.

Design of Low Bandgap CsPb1- x Snx I2 Br Perovskite Solar Cells with Excellent Phase Stability.

Novel all-inorganic Sn-Pb alloyed perovskites are developed aiming for low toxicity, low bandgap, and long-term stability. Among them, CsPb1- x Snx I2 Br is predicted as an ideal perovskite with favorable band gap, but previously is demonstrated unable to convert to perovskite phase by thermal annealing. In this report, a series of CsPb1- x Snx I2 Br perovskites with tunable bandgaps from 1.92 to 1.38 eV are successfully prepared for the first time via low annealing temperature (60 °C). Compared to the pure CsPbI2 Br, these Sn-Pb alloyed perovskites show superior stability. Furthermore, a novel α-phase-stabilization mechanism of the inorganic Sn-Pb alloyed perovskite by reconfiguring the perovskite crystallization process with chloride doping is provided. Simultaneously, a dense protection layer is formed by the coordination reaction between the surface lead dangling bonds and sulfate ion to retard the permeation of external oxygen and moisture, leading to less oxidation of Sn2+ in perovskite film. As a result, the fabricated all-inorganic Sn-Pb perovskite solar cells (PSCs) show a champion power conversion efficiency of 10.39% with improved phase stability and long-term ambient stability against light, heat, and humidity. This work provides a viable strategy in fabricating high-performance narrow-bandgap all-inorganic PSCs.

Engineering Elastic Properties of Isostructural Molecular Perovskite Ferroelectrics via B-Site Substitution.

Managing elastic properties of ABX3 type molecular perovskite ferroelectrics is critical to their future applications since these parameters determine their service durability and reliability in devices. The abundant structural and chemical viability of these compounds offer a convenient way to manipulate their elastic properties through a facile chemical approach. Here, the elastic properties and high-pressure behaviors of two isostructural perovskite ferroelectrics, MDABCO-NH4 I3 and MDABCO-KI3 (MDABCO = N-methyl-N'-diazabicyclo[2.2.2]octonium) is systematically investigated, via the first principles calculations and high-pressure synchrotron X-ray diffraction experiments. It is show that the simple replacement of NH4 + by K+ on the B-site respectively results in up to 48.1%, 52.4%, and 56.3% higher Young's moduli, shear moduli and bulk moduli, which is attributed to the much stronger KI coordination bonding than NH4 …I hydrogen bonding. These findings demonstrate that it is possible to tune elastic properties of molecular perovskite ferroelectrics via simply varying the framework assembling interactions.