Mixed Cs and FA Cations Slow Electron-Hole Recombination in FAPbI3 Perovskites by Time-Domain Ab Initio Study: Lattice Contraction versus Octahedral Tilting

A moderate intensity ligand works best: a theoretical study on passivation effects of pyridine-based molecules for perovskite solar cells

Improving battery stability while maintaining high photoelectric conversion efficiency remains the bottleneck in the current development of perovskite solar cells (PSCs). Three π-conjugated pyridine-based molecules, pyridine (Py), bipyridine (Bpy), and terpyridine (Tpy), were adopted to passivate the PSCs in recent experiments (J. Chen, S.-G. Kim, X. Ren, H. S. Jung and N.-G. Park, J. Mater. Chem. A, 2019, 7, 4977-4987; J. Zhang, J. Duan, Q. Zhang, Q. Guo, F. Yan, X. Yang, Y. Duan and Q. Tang, Chem. Eng. J., 2022, 431, 134230), in which Bpy works best in terms of photovoltaic properties and moisture tolerance. In this work, based on density functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) simulations, we demonstrate that Bpy displays the least structural fluctuation when adsorbed on the perovskite surface, enlarges the bandgap suppressing electron-hole recombination, and exhibits remarkable shielding effects against moisture. The appropriate anchoring ability of Bpy retains robust binding strength and preferable charge transfer ability compared to Py at the interfaces between the passivation molecules (PMs) and MAPbI₃. In contrast, although Tpy possesses the strongest charge-transfer capability, it introduces midgap states owing to intense electronegativity, providing additional pathways for nonradiative charge relaxation. Besides, Tpy triggers rapid diffusions of water and larger atomic fluctuations, destroying the structures of the perovskite through the removal of lead atoms. Our computational results not only rationalize the experimental observations but also provide valuable guidance at the atomic level to design novel PMs that endow PSCs with outstanding photovoltaic performance as well as stability against moisture.

Great Influence of Organic Cation Motion on Charge Carrier Dynamics in Metal Halide Perovskite Unraveled by Unsupervised Machine Learning

Unsupervised machine learning combined with time-dependent density functional theory reveals the significant influence of organic cation on the charge carrier lifetime of FAPbI₃ (FA = HC(NH₂)₂⁺) by analyzing their mutual information (MI) between the geometric features and the nonadiabatic coupling (NAC) and bandgap. Analysis of MI values demonstrates that the NAC and bandgap are dominated by the orientation and shape of the inorganic octahedron because iodine and lead atoms are composed of the band edge states. Counterintuitively, the correlated motion promotes the contribution of the FA cation to the NAC; in particular, one type of FA rotation even supersedes the influence of the velocities of the lead and iodine atoms due to the enhanced hydrogen bond interaction. Our study demonstrates the importance of the correlated motion on the excited-state lifetimes of FAPbI₃, which provides a guidance for optimizing the optoelectronic properties of metal halide perovskites.

Collective Motion Improves the Stability and Charge Carrier Lifetime of Metal Halide Perovskites: A Phonon-Resolved Nonadiabatic Molecular Dynamics Study

By implementing a novel algorithm that realizes the constraints of certain normal modes of interest and using nonadiabatic molecular dynamics for the CsPbBr₃, we explicitly demonstrate for the first time that the collective motion between the Cs atom and inorganic octahedra facilitates to delay the nonradiative recombination of negative and positive charges. The Cs atoms can instantaneously respond to the motion of Pb and Br atoms during normal molecular dynamics, maintain the perovskite structure, and homogenize the structural distortion caused by thermal fluctuations, thus decreasing nonadiabatic coupling and charge recombination. In contrast, the perovskite becomes unstable because geometry distortion is strongly localized when the normal modes of Cs atoms are constrained, which increases the nonadiabatic coupling and accelerates charge recombination. The study emphasizes the important role of correlated motion on the stability and charge-phonon dynamics in metal halide perovskites.

Correlated organic-inorganic motion enhances stability and charge carrier lifetime in mixed halide perovskites

Organic cations are believed to have little influence on the charge carrier lifetime in hybrid organic-inorganic perovskites. Experiments defy this expectation. We consider formamidinium lead iodide (FAPbI₃) doping with and without Br as two prototypical systems, and perform ab initio time-domain nonadiabatic (NA) molecular dynamics simulations to investigate nonradiative electron-hole recombination. The simulations demonstrate that correlated organic-inorganic motion stabilizes the lattice and inhibits nonradiative charge recombination in FAPbI₃ upon Br doping. Br doping suppresses the rotation of FA and the vibrations of both organic and inorganic components, and leads to hole localization and the extent of localization is enhanced upon thermal impact, notably reducing the NA coupling by decreasing the overlap between the electron and hole wave functions. Doping also slightly increases the bandgap for further decreasing NA coupling and enhances the open-circuit voltage of perovskite solar cells. The small NA coupling and large bandgap beat the slow coherence loss, delaying electron-hole recombination and extending the charge carrier lifetime to 1.5 ns in Br-doped FAPbI₃, which is on the order of 1.1 ns in pristine FAPbI₃. The obtained time scales are in good agreement with experiments. Multiple phonon modes, including those of both the inorganic and organic components, couple to the electronic subsystem and accommodate the excess electronic energy lost during nonradiative charge recombination. The study reveals the unexpected atomistic mechanisms for the reduction of electron-hole recombination upon Br doping, rationalizes the experiments, and advances our understanding of the excited-state dynamics of perovskite solar cells.

Rational Unraveling of Alkali Metal Concentration-Dependent Photovoltaic Performance of Halide Perovskites: Octahedron Distortion vs Surface Reconstruction

Adding alkali metal in organic-inorganic halide perovskites effectively improves its photovoltaic performance, while excessive alkali metal incorporation would produce a detrimental effect. Through density functional theory and nonadiabatic molecular dynamics simulations, we demonstrate how and why the photogenerated carrier lifetime mutates with the increase of alkali metal concentration. A small amount of Rb doping in the lattice introduces a slight distortion of the octahedron, reducing the overlap of frontier orbitals and decreasing the nonadiabatic coupling, effectively enhancing the photogenerated carrier lifetime. In contrast, excessive Rb will introduce defect states, resulting in the low carrier lifetime by a factor of 2-3 orders of magnitude. Strikingly, the surface formamidinium (FA) cations exhibit unexpected responsibility for the carrier dynamics since its high-frequency thermal vibration strongly leads to the ultrafast hole trapping and carrier recombination. Our results provide new insight into the concentration-dependent photovoltaic performance of alkali metal cations in organic-inorganic halide perovskites.

Novel design strategies for perovskite materials with improved stability and suitable band gaps

The instability of organometallic halide perovskites is deemed a key hindrance hampering their commercial utilization in solar cell research. In the current work, we investigate and compare the dynamics properties of both free NH₄⁺ and that immobilized in a NH₄⁺-H₂O-H₂O-H₂O-H₂O-NH₄⁺ network in a one-dimensional (1D) Pb-I skeleton. The simulations show that both the space occupancy and the hydrogen bonding formation ability of the A-site groups significantly influence the transition of the 1D NH₄PbI₃ perovskite materials to two-dimensional/three-dimensional (2D/3D) hybrid structures. Based on these observations, two possible pathways enhancing the structural stability of the perovskite materials are proposed. To narrow the big band gap introduced by the quantum confinement effect in the low-dimensional structures, large metal complexes are introduced as A-site groups considering that metal ions are involved in the formation of both conduction and valence bands of the perovskites. In this way, the band gap of the 1D perovskite materials is narrowed and the structural stability is enhanced accordingly. In addition, by optimizing the ratio of NH₄⁺ to CH₆N₃⁺ (GUA⁺) groups, novel 2D/3D hybrid perovskite materials of (NH₄)_1-x(GUA)_xPbI₃ with improved stability and narrower band gaps are suggested as well. These structural design ideas hopefully illuminate the development of innovative and stable perovskite materials.

Benign Effects of Twin Boundaries on Charge Carrier Lifetime in Metal Halide Perovskites by a Time-Domain Study

Experiments show that two-dimensional twin boundaries (TBs) defects are benign to the excited-state lifetime of metal halide perovskites and solar cells performance. However, the mechanism remains unclear. By performing nonadiabatic (NA) molecular dynamics simulations on FAPbI₃ (FA= HC(NH₂)₂⁺), we demonstrate that TBs increase the bandgap without introducing midgap states, promote charge separation by localizing electrons and holes that reduce NA coupling and accelerate the loss of coherence, slowing nonradiative electron-hole recombination by a factor of 2.3 compared to pristine FAPbI₃, which occurs within sub-10 ns and agrees well with the experiment. Raising the temperature shortens the coherence time and reduces the NA coupling by increasing the charge localization due to the enhanced distortions of inorganic Pb-I lattice, making the recombination even slower. Our study rationalizes the positive influence of TBs and temperature on perovskite charge dynamics and emphasizes the roles played by the charge localization and quantum coherence.

Elimination of Charge Recombination Centers in Metal Halide Perovskites by Strain

Metal halide perovskites exhibit enhanced photoluminescence and long-lived carriers in experiments under strain. Using ab initio nonadiabatic molecular dynamics, we demonstrate that compressive and tensile strain can eliminate charge recombination centers created by defect states, by shifting traps from bandgap into bands. A compressive strain enhances coupling of Pb-s and I-p orbitals, pushes the valence band (VB) up in energy, and moves the trap state due to iodine interstitial (I_i) into the VB. The strain distorts the system and breaks the I-dimer responsible for the I_i trap. A tensile strain increases Pb-Pb distance, weakens overlap of Pb-p orbitals, and pushes the conduction band (CB) down in energy. The trap state created by replacement of iodine with methylammonium (MA_I) is moved into the CB. Application of strain to the defective systems not only eliminates midgap traps but also creates moderate disorder that reduces overlap of electron and hole wave functions, activates phonon modes accelerating coherence loss within the electronic subsystem, and extends carrier lifetimes even beyond those in pristine MAPbI₃. Our investigation rationalizes the high performance of perovskites solar cells under strain and reveals how strain passivates I_i and MA_I defects in MAPbI₃, providing a new nonchemical strategy for defect control and engineering.

Two-Dimensional Perovskite Capping Layer Simultaneously Improves the Charge Carriers' Lifetime and Stability of MAPbI3 Perovskite: A Time-Domain Ab Initio Study

Two- (2D) and three-dimensional (3D) heterostructured perovskites show enhanced stability and an extended charge lifetime compared to those of the 3D component. The mystery remains unexplored for both phenomena in the class of the typical type-I heterojunction. By using time-domain density functional theory combined with nonadiabatic (NA) molecular dynamics simulations for the MA₃Bi₂I₉/MAPbI₃ (MA = CH₃NH₃⁺) junction, we demonstrate that the formation of I-Pb chemical bonds at the junction suppresses the atomic motions. The inhibited charge recombination in the junction is ascribed to the increased band gap, reduced NA coupling, and shortened coherence time. By localizing the hole wave function, the NA coupling is decreased by about a factor of 1.4. The presence of multiple phonon modes, particularly the Bi-I vibrations, accelerates decoherence about twice as fast as that in the pristine MAPbI₃. As a result, the 2D capping layer reduces the recombination in MAPbI₃ by more than a factor of 2, decreasing charge and energy losses. The strategy can be applied to optimize the performance of other 2D/3D heterostructured perovskite solar cells.

MAI Termination Favors Efficient Hole Extraction and Slow Charge Recombination at the MAPbI3/CuSCN Heterojunction

Photoinduced charge separation is the key step determining the efficiency of photon-to-electron conversion in solar cells, while charge carrier lifetimes govern the overall solar cell performance. Experiments report that copper(I) thiocyanate (CuSCN) is a very promising hole extraction layer for perovskite solar cells. Using nonadiabatic molecular dynamics combined with ab initio time-domain density functional theory, we show that termination of CH₃NH₃PbI₃ (MAPbI₃) at MAPbI₃/CuSCN heterojunctions has a strong influence on both charge separation and recombination. Both processes are favored by MAI termination, compared to PbI₂ termination. Because the MAPbI₃ valence band originates from iodine orbitals while the conduction band arises from Pb orbitals, MAI termination places holes close to CuSCN, favoring extraction, and creates an MAI barrier for recombination of electrons in MAPbI₃ and holes in CuSCN. The opposite is true for PbI₂ termination. The origin of these effects is attributed solely to the properties of the MAPbI₃ surfaces, and therefore, the conclusions should apply to other hole-transporting materials and can be generalized to other perovskites. Importantly, the simulations show that the injected hole remains hot for several hundreds of femtoseconds, allowing it to escape the interfacial region and prevent formation of bound excitons. This study suggests that metal halide perovskites should be treated with an organic precursor, such as MAI, prior to the formation of their interfaces with hole-transporting materials. The reported results advance the fundamental understanding of the highly unusual properties of metal halide perovskites and provide specific guidelines for optimizing the performance of perovskite solar cells and other devices.

Rapid Decoherence Induced by Light Expansion Suppresses Charge Recombination in Mixed Cation Perovskites: Time-Domain ab Initio Analysis

Using time-domain density functional theory combined with nonadiabatic molecular dynamics, we have investigated the effect of light-induced lattice expansion on the nonradiative electron-hole recombination in the mixed-cation perovskite FA_0.75MA_0.25PbI₃. We demonstrate that charge carrier lifetime extends by a factor of 1.5 within 1% lattice expansion; the bandgap grows only by 0.04 eV; the electron-phonon coupling increases by 13%; and the decoherence time shortens by 37%. The small bandgap change has negligible influence on recombination times. Lattice expansion enhances atomic fluctuations that lead to the enhancement of electron-phonon coupling and acceleration of decoherence. By creating several high-frequency phonon modes, the lattice expansion shortens the decoherence time further. As a result, rapid decoherence beats an enhanced electron-phonon coupling, playing the dominant role in suppressing the nonradiative electron-hole recombination. The light-induced lattice expansion or strain effects provide a rational route to improve the perovskite photovoltaic and photoelectronic device performance.

Hot carrier relaxation in Cs2TiI y Br6-y (y = 0, 2 and 6) by a time-domain ab initio study

Cs2TiI y Br6-y is a potential light absorption material for all-inorganic lead free perovskite solar cells due to its suitable and tunable bandgap, high optical absorption coefficient and high environmental stability. However, solar cells fabricated based on Cs2TiI y Br6-y do not perform well, and the reasons for their low efficiency are still unclear. Herein, hot carrier relaxation processes in Cs2TiI y Br6-y (y = 0, 2 and 6) were investigated by a time-domain density functional theory combined with the non-adiabatic molecular dynamics method. It was found that the relaxation time of the hot carriers in Cs2TiI y Br6-y ranges from 2-3 ps, which indicates that the hot carriers within 10 nm from the Cs2TiI y Br6-y /TiO2 interface can be effectively extracted before their energy is lost completely. The carrier-phonon non-adiabatic coupling (NAC) analyses demonstrate that the longer hot electron relaxation time in Cs2TiI2Br4 compared with that in Cs2TiBr6 and Cs2TiI6 originates from its weaker NAC strength. Furthermore, the electron-phonon interaction analyses indicate that the relaxation of hot electrons mainly comes from the coupling between the electrons distributed on the Ti-X bonds and the Ti-X vibrations, and that of hot holes can be attributed to the coupling between the electrons distributed on the X atoms and the distortions of [TiI y Br6-y ]2-. The simulation results indicate that Cs2TiI2Br4 should be better than Cs2TiBr6 and Cs2TiI6 to act as a light absorption layer based on the hot carrier energy loss, and the hot electron relaxation time in Cs2TiI y Br6-y can be adjusted by tuning the proportion of the I element.

Arranging strategies for A-site cations: impact on the stability and carrier migration of hybrid perovskite materials

An essential understanding is obtained from the cation engineering process into the expected excited-state properties of hybrid perovskite materials.

Anharmonicity Extends Carrier Lifetimes in Lead Halide Perovskites at Elevated Temperatures

Lead halide perovskites constitute a very promising class of materials for a broad range of solar and optoelectronic applications. Perovskites exhibit many unusual properties, and recent experiments demonstrate an unusual temperature dependence of charge carrier lifetimes. Focusing on the all-inorganic CsPbBr₃, and using a combination of ab initio nonadiabatic molecular dynamics and time-domain density functional theory, we demonstrate that the unconventional behavior arises because of a highly anharmonic nature of atomic motions in perovskites. As temperature increases, perovskite structure undergoes a notable deformation, reflected in tilting of octahedral units, and experiences large-scale anharmonic movements away from the equilibrium geometry. As a result, the electronic energy gap increases, and phonon-induced loss of coherence within the electronic subsystem accelerates. These two factors slow down nonradiative electron-hole recombination, which constitutes the main limitation on efficiencies of perovskite solar, optical, and electronic devices. The increase of charge carrier lifetimes with temperature is particularly beneficial in applications, because materials heat up, for instance, from sunlight during solar energy harvesting. The behavior of the all-inorganic halide perovskite investigated here is different from that of hybrid organic-inorganic perovskites, which exhibit additional disorder associated with reorientations of the asymmetric organic cations. The reported simulations generate an in-depth understanding of the unusual properties of inorganic perovskites, relevant for photocatalytic, photovoltaic, electronic, and optical applications.

The effect of the magnitude and direction of the dipoles of organic cations on the electronic structure of hybrid halide perovskites

We present ab initio calculations (DFT and SOC-G₀W₀) of the optoelectronic properties of different hybrid-halide perovskites, namely X-PbI₃ (X = methylamonimum, formamidinium, guanidinium, hydrazinium, and hydroxylammonium). These calculations shed new light on how the substitution of different organic cations in the material influences its optoelectronic properties. Our simulations show a significant modification of the lattice parameter and band gap of the material upon cation substitution. These modifications are not only due to steric effects but also due to electrostatic interactions between the organic and inorganic parts of the material. In addition to this, we demonstrate how the relative orientations of neighboring cations in the material modify the local electrostatic potential of the system and its fundamental band gap. This change in the band gap is accompanied by the formation of localized and spatially separated electronic states. These localized states modify the carrier mobility in the materials and can be a reason for the formation and recombination of the charge carriers in these very promising materials.

Doping-Induced Rapid Decoherence Suppresses Charge Recombination in Mono/Divalent Cation Mixed Perovskites from Nonadiabatic Molecular Dynamics Simulation

Experiment shows that solar cells based on FA_0.75Cs_0.25Pb_0.5Sn_0.5I₃ carry a lower charge recombination rate and higher power conversion efficiency than those of FAPbI₃ despite the fact that the former has a smaller band gap. However, the underlying mechanism remains unclear. Using nonadiabatic (NA) molecular dynamics, we demonstrate that low-frequency vibrations drive electron-hole recombination in pristine FAPbI₃ occurring in about 1 ns, showing excellent agreement with experiment. Cs/Sn substitution to FA/Pb not only narrows its band gap by 0.3 eV but also delocalizes the electron wave function significantly, leading to enhancement of NA coupling. Importantly, doping accelerates quantum decoherence caused by increased atomic fluctuations. As a result, rapid decoherence prevails a small band gap and strong NA coupling, slowing charge recombination and extending the charge carriers' lifetime to several nanoseconds. Our study reveals the importance of quantum coherence on quantum dynamics in perovskite materials and suggests a rational strategy to design high-performance perovskite solar cells.