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Experiments demonstrate that grain boundaries (GBs) exhibit detrimental effect on carrier lifetimes in MAPbI 3 (MA= CH 3 NH 3 + ). On the basis of the nonadiabatic (NA) molecular dynamics simulations, we demonstrated that NH 4 Cl can simultaneously passivate the common point defects that introduce recombination centers at GBs and accelerate electron-hole recombination but shows small effects in the bulk. The MA interstitial (MA i ) and the substitutional MA to Pb (MA Pb ) in pristine MAPbI 3 leave the band gap and charge recombination rates largely unchanged but create deep electron traps at GBs by separately either distorting inorganic octahedra or creating an I-dimer. Cl - and NH 4 + remove the in-gap states by either restoring the distorted octahedra or destroying the I-dimer. Thus, the band gap recovers to the pristine system, NA coupling decreases, and decoherence accelerates, extending the carrier lifetime even twice longer than MAPbI 3 . This study shows that the negative role of GBs can be removed by dually passivating with NH 4 Cl.

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 MAPbI 3 . Our investigation rationalizes the high performance of perovskites solar cells under strain and reveals how strain passivates I i and MA I defects in MAPbI 3 , providing a new nonchemical strategy for defect control and engineering.

The ordered alignment of polar organic cations in hybrid organic-inorganic perovskites causes a ferroelectric phase, which is believed to be benign for photovoltaic devices. Using a combination of time-domain density functional theory and nonadiabatic molecular dynamics, we study the influence of the interactions of polar MA (CH 3 NH 3 + ) cations and between their inorganic counterparts on the nonradiative electron-hole recombination in the room-temperature ferroelectric MAPbI 3 perovskite. We show that ferroelectric alignment of the polar C-N bonds favors charge separation and reduces nonadiabatic coupling. Symmetry breaking enhances low-frequency collective motions and introduces additional high-frequency vibrations, thus accelerating quantum decoherence. Both factors contribute to suppressing the nonradiative electron-hole recombination, extending the charge carrier lifetimes to several nanoseconds and showing a factor of 3 increase compared to the pristine system. Consequently, ferroelectric engineering provides an excellent route to improve hybrid perovskite device performance as a result of long-range charge separation and slow charge recombination.

Experiments show that excess lead atoms accelerate charge recombination while oxygen passivation can heal the defects and enhance solar cell efficiency. Using ab initio nonadiabatic (NA) molecular dynamics, we demonstrate that an excess lead atom forms a Pb-dimer with a single surface lead atom of CsPbBr 3 (001) surface and creates a deep hole trap. The electron-hole recombination is accelerated to over 10 ps via fast hole trapping or bypassing the hole trap compared to the pristine CsPbBr 3 , occurring on tens of picoseconds. Pb-dimer passivated with oxygen molecules forms Pb-O bonds, breaks the Pb-dimer, and removes the trap state, leading to a decrease in the recombination and extending excited-state lifetime to over 100 ps. The deceleration arises mainly due to the reduced NA coupling and short decoherence time. The study advances our understanding of excited-state dynamics of all-inorganic perovskites in the presence of excess lead and oxygen atmosphere.

Using time domain density functional theory combined with nonadiabatic (NA) molecular dynamics, we show that electron-hole recombination takes subnanoseconds in FAPbI 3 , showing excellent agreement with experiment. Cs doping retards charge recombination by factors of 1.1 and 3.1 due to lattice contraction and octahedral tilting, respectively. Lattice contraction decreases the NA coupling and increases the coherence time arising from the suppressed atomic fluctuations, slightly slowing recombination because the two factors have an opposite influence on quantum transition. In contrast, octahedral tilting simultaneously decreases the NA coupling, thanks to the reduced overlap between Pb and I orbitals, and the coherence time, extending the excited-state lifetime over 1 ns. Our simulations provide a mechanistic understanding for delayed charge losses in the mixed Cs and FA system, suggesting a rational strategy to improve perovskite solar cell performance.

Using nonadiabatic (NA) molecular dynamics combined with time domain density functional theory, we simulate electron-hole recombination in pristine and doped inorganic Pb-free double perovskite Cs 2 TiBr 6 . We show that replacing the titanium and/or bromine with silicon and/or chlorine extends the charge carrier lifetime. Importantly, dopants avoid deep traps despite the fact that they do not change the fundamental band gap of Cs 2 TiBr 6 , and they decrease the NA electron-phonon coupling and accelerate decoherence arising from the reduced overlap of electron and hole wave functions as well as fast phonon modes induced by light dopants, respectively, suppressing electron-hole recombination. More importantly, codoping can reduce the formation energy of silicon and achieve higher doping concentration, potentially increasing the lifetime further. Our study suggests a rational strategy to reduce energy losses by codoping in design of high-performance all-inorganic Pb-free perovskite solar cells.