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Lewis Base Passivation of Hybrid Halide Perovskites Slows Electron-Hole Recombination: Time-Domain Ab Initio Analysis Lihong Liu, Weihai Fang, Run Long, and Oleg V. Prezhdo J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b00177 • Publication Date (Web): 20 Feb 2018 Downloaded from http://pubs.acs.org on February 20, 2018

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The Journal of Physical Chemistry Letters

Lewis Base Passivation of Hybrid Halide Perovskites Slows Electron-Hole Recombination: Time-Domain Ab Initio Analysis

Lihong Liu1, Wei-Hai Fang1, Run Long1*, and Oleg V. Prezhdo2 1

Key Laboratory of Theoretical and Computational Photochemistry, Ministry of

Education, College of Chemistry, Beijing Normal University, Beijing 100875, China 2

Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States

Abstract: Non-radiative electron-hole recombination plays a key role in determining photon conversion efficiencies in solar cells. Experiments demonstrate significant reduction in the recombination rate upon passivation of methylammonium lead iodide perovskite with Lewis base molecules. Using non-adiabatic molecular dynamics combined with time-domain density functional theory, we find that the nonradiative charge recombination is decelerated by an order of magnitude upon adsorption of the molecules. Thiophene acts by the traditional passivation mechanism, forcing electron density away from the surface. In contrast, pyridine localizes the electron at the surface, while leaving it energetically near the conduction band edge. This is because pyridine creates a stronger coordinative bond with a lead atom of the perovskite, and has a lower energy unoccupied orbital, compared to thiophene, due to the more electronegative nitrogen atom, relative to thiophene’s sulfur. Both molecules reduce twofold the non-adiabatic coupling and electronic coherence time. A broad range of vibrational modes couple to the electronic subsystem, arising from inorganic and

*

Corresponding author: E-mail: [email protected]

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organic components. The simulations reveal the atomistic mechanisms underlying the enhancement of the excited state lifetime achieved by the perovskite passivation, rationalize the experimental results, and advance our understanding of charge-phonon dynamics in perovskite solar cells.

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Hybrid organic-inorganic halide perovskites, such as CH3NH3PbI3 (MAPbI3), have attracted great attention as a new class of optoelectronic semiconductors.1-8 The power conversion efficiency of the MAPbI3 perovskite solar cells has increased from 3.8%1 to 22.1%4, 6 within only six years of active research, due to excellent electronic and optical properties, including long carrier diffusion lengths9 and high light absorption coefficients.1 Material performance is limited by defects that provide charge scattering and recombination sites. Most often, defects are located at surfaces and crystal grain boundaries that contain under-coordinated atoms. Significant theoretical and experimental efforts are dedicated to studies on defect chemistry in hybrid perovskites.10-14 It has been discovered that the energy of many intrinsic defects, such as halide vacancies, is close to band edges.15 Charges trapped in such shallow states can be easily de-trapped and take a long time to recombine,16 providing a plausible rationalization for the remarkable performance of perovskite solar cells. Non-radiative recombination rates are still relatively high, as inferred from low photoluminescence efficiencies under small excitation densities.17 Perovskites suffer from low stability against humidity,18-19 which increases defect concentration and constitutes a major obstacle to large-scale applications. Replacing some iodine atoms in MAPbI3 with chlorines extends excited state lifetimes, in part due to surface passivation,20-21 triggering experimental22-23 and theoretical24-25 studies of chlorine influence on geometric, electronic and dynamical properties of MAPbI3, and allowing improvements in solar cell performance. Focusing on mixed iodine/chlorine perovskite MAPbI3-xClx, Noel et al. undertook 3 ACS Paragon Plus Environment


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further steps for improving material quality by passivating perovskite crystal surfaces with organic Lewis base molecules.26 By treating perovskite films with pyridine and thiophene, they observed a significantly inhibited nonradiative charge recombination, especially at low photoexcitation levels, leading to power conversion efficiencies of 15.3 and 16.5% for the thiophene and pyridine-treated devices, respectively.26 The authors postulated that the observed decrease in non-radiative decay occurred due to passivation of under-coordinated lead atoms on perovskite crystal surfaces via coordinate bonding to the nitrogen atom in pyridine or the sulfur atom in thiophene. Besides solar cells, the passivation technique can be used with other optoelectronic applications of perovskites, such as lasers and light-emitting diodes.26-27 Both solar cells

and

optoelectronic

devices

require

slow

non-radiative

electron-hole

recombination, because this process constitutes a major pathway for charge and energy losses. Further progress requires a thorough understanding of the mechanism by which Lewis base passivation reduces charge losses. Such understanding can be provided by atomistic ab initio studies, particularly in the time-domain, directly mimicking the time-resolved experiments. 26, 28 In this letter, we employ real-time time-dependent density functional theory (TDDFT) and non-adiabatic molecular dynamics (NAMD) in order to investigate the atomistic mechanisms underlying the observed reduction of non-radiative electron-hole recombination in MAPbI3 achieved by surface passivation with the Lewis base molecules.26 The simulations indicate that pyridine and thiophene achieve similar effects but through different mechanisms. Thiophene acts by removing 4 ACS Paragon Plus Environment

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electrons density from the surface. In contrast, pyridine attracts electron into an unoccupied orbital that appears near the bottom of the perovskite conduction band (CB) and that is localized around the strong coordinative bond formed by the pyridine nitrogen and surface lead atoms. It is essential that no trap states are created, and the electron can easily escape into the CB. Both Lewis bases reduce the NA electron-vibrational coupling and electronic coherence time by a factor of two, enhancing the excited state lifetime by an order of magnitude. A broad spectrum of vibrational modes, arising from the inorganic and organic parts, couple to the electronic subsystem. The state-of-the-art simulations highlight various factors affecting the non-radiative electron-hole recombination, and provide valuable chemical and physical insights that can be used to improve further the performance of perovskite solar cells. The simulations employ the decoherence induced surface hopping (DISH) approach29 implemented30 within real-time time-dependent density functional theory (TDDFT) in the Kohn-Sham framework.31-32 The DISH approach for NAMD incorporates loss of quantum coherence into the quantum-classical approximation by introducing transitions of classical trajectories between quantum states at decoherence events. The lighter and faster electrons are treated quantum mechanically, whereas the heavier and slower nuclei are described classically. Directly related to pure-dephasing of the optical response theory,20,

33-35

decoherence should be included in the

calculation, because it is significantly faster than the electron-hole recombination. We applied this approach to study excitation dynamics in diverse systems,33, 5 ACS Paragon Plus Environment

36-48


including perovskites containing dopants,49 grain boundaries35 and defects,50 in contact with TiO220, 51 and water,34 and exhibiting polarons52 and ordered phases.53 A detailed description of the theoretical approach can be found elsewhere.30, 54 Considering the previous experimental and theoretical studies of the MAPbI3 systems,55-56 we chose the Cl-doped PbI-terminated tetragonal MAPbI3 (001) surface to model electron-hole recombination with and without the adsorbed Lewis base molecules. The periodically repeated 72-atom (1×1) surface is composed of three MAPbI3 layers, with the bottom layer frozen in the bulk configuration. The geometry optimization, adiabatic MD, and NA coupling calculations are carried out using the Vienna ab initio simulation package (VASP). 57 The exchange-correlation interactions are treated using the Perdew-Burke-Ernzerhof (PBE) functional.58 The interaction between the ionic cores and the valence electrons are described by the projector-augmented wave approach.59 The van der Waals interactions are treated by the Grimme DFT-D2 method.60 Spin-orbit interaction is important in MAPbI3 due to in the presence of the heavy elements, Pb and I. However, it has been shown that semilocal PBE without spin-orbit interactions is able to provide good agreement with the experimental bandgaps61 and generate important insights into perovskite electronic structure,62-63 justifying the use of the less expensive methods for exploring novel effects. The errors due to lack of spin-orbit interaction and presence of electron self-interaction cancel in the simple PBE calculation. In order to obtain an accurate bandgap with spin-orbit interaction turned on, one needs to either use a hybrid functional or perform a GW calculation.64 6 ACS Paragon Plus Environment

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It is impossible to perform quantum dynamics calculations on the current system with either hybrid functional or GW, because of large computational efforts. Therefore, we use simple PBE, which gives good agreement with experiment.61 This approach provided good results in our previous studies of hybrid organic-inorganic perovskites.20, 34-35, 51-52 The simulated systems are shown in Figure 1. The periodic images are separated along the surface normal by a vacuum region of 20 Å. The geometry optimization is performed using an 8×8×1 Monhorst-Pack k-point mesh,65 while the electronic structure calculations are carried out with a denser 10×10×1 mesh. After relaxing the system geometries at 0 K, velocity rescaling66-68 is employed to bring the temperature to 300 K. Then, a 6 ps adiabatic MD simulation is performed in the microcanonical ensemble with a 1 fs atomic time-step. The NA couplings are calculated at the Γ-point, since it corresponds to the fundamental bandgap of the three systems. The electron-hole recombination is simulated by the PYXAID code.30, 54 The initial 2000 fs from the 6 ps MD trajectories are used as initial geometries for the NAMD simulations.



Figure 1. Simulation cells showing optimized geometries of (a) the Cl-doped MAPbI3 (001) surface, and the same surface with adsorbed (b) pyridine and (c) thiophene molecules. Both molecules connect to the under-coordinated Pb atom; however, the coordinate (dative) bonding is stronger through the nitrogen atom of pyridine than the sulfur atom of thiophene. This difference has a notable influence on localization of the electron wavefunction.

Figure 1 shows the optimized geometries of the three systems under investigation. The Pb-I inorganic cages and the organic MA groups of the slabs maintain their bulk configurations. Adsorption of pyridine and thiophene creates a small perturbation to the topmost surface layer. The coordinate (dative) bonding between the N atom of pyridine and an under-coordinated Pb atom of the Cl-doped MAPbI3 (001) surface is quite strong. In contrast, the interaction of the S atom of thiophene with the Pb atom of the surface is weaker. The calculated binding energies are -0.892 eV and -0.541 eV for pyridine and thiophene, respectively. In combination with the larger electronegativity of N relative to S, the difference in the bonding strength is responsible for the different effects pyridine and thiophene have on the CB edge of the perovskite surface.


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30

CH3NH3 CH 3NH3

(a)

I Cl Pb

20 10 0

PDOS (1/ev)

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


Bare Pyridine

(b)

20

0.3 0.2 0.1

10

0.0 1.0

0

1.5

2.0

Bare Thiophene

(c)

20

0.3 0.2 0.1

10

0.0 1.0

0

-2

1.5

0

2.0

2

Energy (eV) Figure 2. Partial density of states (PDOS) of (a) the bare Cl-doped MAPbI3 (001) surface split into contributions from CH3NH3, I, Cl, and Pb, (b) the same surface (black line) with adsorbed pyridine (red line), and (c) the same surface (black line) with adsorbed thiophene (blue line). The insets in (b) and (c) show magnified PDOS near the conduction band minimum (CBM). The Fermi level is set to zero. Molecular levels are far from the valence band maximum, but close to the CBM.

Figure 2a presents the projected density of states (PDOS) of the bare Cl-doped MAPbI3 (001) surface split into contributions from CH3NH3, I, Cl and Pb. The data demonstrate that the valence band (VB) and CB edges originate primarily from atomic orbitals of I and Pb, respectively. The energies of the orbitals of the organic 9 ACS Paragon Plus Environment


MA groups reside deep inside the bands. Therefore, these cations have no direct contribution to the NA coupling responsible for the electron-hole recombination. At the same time, they affect the band edge wavefunction through electrostatic interaction. The bandgap of the Cl-doped MAPbI3 (001) surface averaged canonically over the 6ps MD trajectory is 1.840 eV. This amount of energy is deposited into phonon modes during the non-radiative electron-hole recombination. Since phonon quanta available in the system are much smaller than the bandgap, even including C-H stretches of the organic molecules, the non-radiative electron-vibrational relaxation is a multi-phonon process. The PDOS of the Cl-doped MAPbI3 (001) surface with adsorbed pyridine and thiophene molecules, shown in parts (b) and (c) of Figure 2, are separated into the components arising from the surface and the molecules. The insets zoom onto the CB edges. Figure 2b demonstrates that pyridine has states near the CB edge. In comparison, thiophene has very little contribution near the edge. The difference arises due to the larger electronegativity of the N atom of pyridine, compared to the S atom of thiophene. The more electronegative atom lowers the energy of the electronic levels, bringing them closer to the CB edge. The stronger N-Pb coordinative bonding, compared to S-Pb, Figure 1, also contributes to hybridization of perovskite and pyridine orbitals near the CB edge. Neither molecule contributes to the VB edge. Both occupied and unoccupied molecular orbitals (MOs) of pyridine reside lower in pyridine than thiophene. The canonically averaged bandgaps at the Γ point obtained from the 6ps MD trajectory for the surface/pyridine and surface/thiophene systems are 10 ACS Paragon Plus Environment

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1.788 eV and 1.818 eV, respectively, Table 1. They reflect thermal effects and are most relevant for comparison with experiment. The bandgap reduction is larger for pyridine, which is consistent with the stronger influence of pyridine on the CB edge. The small changes in the bandgap should have negligible influence on electron-hole recombination. In comparison, changes in localization of orbitals near the CB edge should have a more significant effect on the recombination.

Table 1 LUMO-HOMO bandgap, absolute value of NA coupling, pure-dephasing time T2*, and non-radiative electron-hole recombination time for the Cl-doped MAPbI3 (001) surface with and without the Lewis base molecules. The data are canonically averaged over of the 6 ps MD trajectories at the Γ point.

Bare

Bandgap (eV) 1.850

NA coupling (meV) 1.613

Dephasing (fs) 6.7

Recombination (ns) 1.27

Pyridine

1.788

0.765

3.5

9.22

Thiophene

1.818

0.770

3.3

16.6

The electron-hole recombination rate depends strongly on the magnitude of the NA coupling between the initial and final states. In turn, the strength of the coupling depends on the overlap of the corresponding wavefunctions, the sensitivity of the wavefunctions to atomic motions, and the speed of the motions. Since intraband electron and hole relaxation proceeds on a sub-picosecond timescale,9,

69-70

we

consider the NA coupling between the highest occupied MO (HOMO) and lowest unoccupied MO (LUMO), which support relaxed hole and electron. The adsorbed molecules have little effect on the orbitals near the VB edge, Figure S1 of the Supporting Information, because their occupied MOs reside deep inside the VB, Figure 2. The HOMO is created by I atomic orbitals and remains unchanged upon 11 ACS Paragon Plus Environment


adsorption of the Lewis bases. On the other hand, the unoccupied MOs of the adsorbed molecules are close to the CB edge, Figure 2, and the molecules affect significantly the perovskite LUMO, Figure 3.

Figure 3. LUMO charge density in (a) the bare Cl-doped MAPbI3 (001) surface, and the surface with adsorbed (b) pyridine and (c) thiophene. The LUMO of the bare surface is formed primarily by Pb atoms. Strong dative N-Pb bonding of pyridine to the under-coordinated Pb atom (Figure 1b) localizes the LUMO on pyridine and nearby Pb atoms, reducing the non-adiabatic coupling, accelerating decoherence, and hence, slowing down charge recombination. Thiophene acts as a traditional surface passivating agent by forcing the LUMO away from the surface, which is known to enhance charge recombination.20, 35

Both Lewis bases localize the LUMO, decreasing its overlap with the HOMO, and hence, reducing the NA coupling and the coherence time, Table 1. At the same time, the localization mechanism varies between pyridine and thiophene. Thiophene forces the LUMO away from the surface, Figure 3c. This is the expected behavior of surface passivating species: Unsaturated chemical bonds and defects encountered on surfaces create charge recombination sites, and surface passivation is designed to eliminate such sites and delocalize charges away from the surface. In contrast,


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pyridine creates a localized surface state, Figure 3b, because the N atoms makes a strong bond with the perovskite, Figure 1b, and is more electronegative, contributing to the PDOS near the CB edge, Figure 2b. It is particularly important that the surface state created upon binding of pyridine to the perovskite is not a deep electron trap: Its energy is at the perovskite CB edge, and therefore, the electron can easily access the CB and travel to an electrode or a photochemical reaction site. At the same time, the localized LUMO reduces the NA coupling with the HOMO, slowing down the charge recombination. Figure 4 shows Fourier transforms (FTs) of the fluctuations of the HOMO-LUMO energy gaps for the three systems under investigation. Known as influence spectra, they characterize the phonon modes that couple to the electronic degrees of freedom during the non-radiative transition. The vibrational modes induce decoherence in the electronic subsystem and accommodate the excess energy lost during the non-radiative transition. A relatively broad range of phonon modes, arising from the both the I(Cl)-Pb inorganic cage and the organic MA cations, participate in the electron-hole recombination in all three systems. The frequencies extend from around 30 cm-1 to above 400 cm-1. The higher frequency vibrational motions become important in the presence of pyridine and thiophene, and especially pyridine that interacts strongly with the perovskite, Figure 1b. The peaks at 62 cm-1 and 100 cm-1 can be assigned to Pb-I bending and stretching modes, respectively.71 The peaks in the 200-400 cm-1 frequency range are attributed to the torsional motions of the MA cations.71 The LUMO and HOMO of the perovskites originate from the atomic 13 ACS Paragon Plus Environment


orbitals of the Pb and I atoms, Figures 2a and 3a. The energy levels of the MA cations reside deep inside the CB and VB, and therefore, molecular orbitals of the MA cations do not contribute directly to the electron and hole states near the band edge, Figure 2a. However, they affect the electron and hole wave functions near the band edge by electrostatic interactions, because the cations are charged and carry dipole moments. The Lewis base molecules introduce additional vibrational modes. The new signal at 300 cm-1 is particularly strong in the pyridine system, which bonds to the perovskite via the N-Pb coordinate bond, Figure 1b. Thiophene also extend the range of active vibrational motions to higher frequencies, although these peaks are less significant than for pyridine, Figure 4a, because the thiophene-perovskite bonding is not as strong.

Figure 4

(a) Fourier transforms (FT) of the unnormalized autocorrelation

functions for the HOMO-LUMO gap fluctuations in the Cl-doped MAPbI3(001) surface with and without the adsorbed pyridine and thiophene molecules. (b) Corresponding pure-dephasing functions. A broad range of phonons couple to the electronic subsystem, resulting in fast loss of quantum coherence. Decoherence accelerates in the presence of pyridine and thiophene molecules due to involvement of higher frequency modes. The inset in (b) shows the unnormalized autocorrelation functions of the three systems. The initial values represent the bandgap fluctuation 14 ACS Paragon Plus Environment

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squared; the bigger the fluctuation, the faster the dephasing.72

The decoherence times were computed as the pure-dephasing times of the optical response theory,73 using the second-order cumulant approximation:20, 33-35, 52

′

() = − ħ ′ ′′ ′′

(1)

Here, () is the unnormalized autocorrelation function (ACF) of the

phonon-induced fluctuation of the energy gap (), between electronic states i and

j, defined as () = 〈 ( ) ( − )〉

(2)

The influence spectra shown in Figure 4a are obtained as Fourier transforms of the unnormalized autocorrelation ACF shown in the insert of Figure 4b. The pure-dephasing functions, Figure 4b, were fit by Gaussians, exp[-0.5(t/τ)2], and the times, τ, are summarized in Table 1. Phonon-induced loss of electronic coherence is fast, sub-10 fs in the present systems, because they contain a broad spectrum of vibrations stemming from both inorganic and organic parts. Many of these vibrations couple to the electronic subsystem, Figure 4a. The decoherence times are much shorter than the electron-hole recombination times reported in the experiment,26 requiring incorporation of decoherence into the NAMD simulation. Adsorption of the Lewis base molecules accelerates decoherence by a factor of two. Analysis of the second-order cumulant approximation for the optical response function leads one to conclude,72 the fast decoherence can arise either from a rapid decay or a large initial value of the energy gap ACF, shown in the insert in Figure 4b. 15 ACS Paragon Plus Environment


Since the three ACF decay on similar time scales, the differences in the pure-dephasing times are rationalized by the ACF initial values, (0), which are equal to the canonically averaged squares of the energy gap fluctuation. The decoherence function is calculated by integrating and exponentiating the unnormalized ACF, Eq. (1), whose initial value, (0), can be placed in front of the integral. Hence, a larger (0)

leads to faster decoherence ( ) . The gap

fluctuations have more significant amplitudes in the systems with the adsorbed molecules due to stronger electron-phonon coupling, as reflected by the taller peaks in the influence spectra, Figure 4a. Decoherence arises from elastic electron-phonon scattering, while inelastic scattering is represented in the NAMD simulation by the NA coupling. The NA matrix elements are smaller for the perovskite surfaces containing pyridine and thiophene than for the bare surface, Table 1, because the NA coupling depends on the wavefunction overlap, which decreases when the LUMOs become more localized in the presence of the molecules, Figure 3.

Figure 5. Electron-hole recombination in the Cl-doped MAPbI3(001) surface with and without the adsorbed Lewis base molecules. Both pyridine and thiophene slow down the non-radiative decay, due to smaller NA coupling and faster loss of coherence, Table 1. The coupling decreases because the electron wavefunctions 16 ACS Paragon Plus Environment

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become more localized, Figure 3b,c.

Figure 5 demonstrates evolution of the excited state population in the three perovskite systems under consideration. The non-radiative decay times, summarized in Table 1, are obtained using the short-time, linear approximation to the exponential decay, f(t) = exp(-t/τ) ≈ 1-t/τ. The charge recombination takes a long time, over 1 ns in all three systems. This is beneficial for minimizing charge and energy losses, and achieving high photon-to-current conversion efficiencies of perovskite solar cells.4, 6, 9 The calculations demonstrate that the adsorbed Lewis bases reduce electron-hole recombination by an order of magnitude, rationalizing the experimental observation.26 The recombination is slowed down by the adsorbed molecules because the NA coupling and decoherence times are decreased twofold, Table 1. Generally, strong NA coupling and long-lived coherence lead to fast quantum transitions. Weaker coupling and shorter coherence retards the dynamics. The dynamics stops if the coherence time becomes infinitely small, giving rise to the quantum Zeno effect.74-75 Since population transfer requires a buildup of quantum coherence first, suppressing the coherence slows down population dynamics. The NA coupling is decreased and the decoherence is accelerated because the Lewis base molecules change localization of the electron wavefunctions, Figure 3. The dephasing is accelerated since wavefunctions localized in different parts of the system evolve independently and are not correlated. Pyridine and thiophene achieve this effect by different mechanisms. Pyridine forms a strong coordinative bond to the



perovskite surface, localizing the wavefunction around this bond. In comparison, thiophene acts by the traditional surface passivation mechanism, in which the charge density is removed from the surface. It is important that the pyridine energy level is still near the perovskite CB edge, Figure 2b.

Even though the pyridine DOS is small

compared to the perovskite DOS, the pyridine state mixes with the perovskite states, such that the LUMO of the combined system becomes delocalized between pyridine and two perovskite surface layers. Such surface state of electron decouples from the hole, leading to reduction of the electron-hole recombination rate. The calculated electron-hole recombination times are shorter than the experimental values,26 most likely because the experimental data account for charge diffusion inside perovskite. Due to computational limitations, the present work employs a small simulation cell, in which the electron and hole are kept artificially close to each other, enhancing the interaction and accelerating the recombination. Nevertheless, the experimental trend is reproduced very well, including both the significant slowing down of the recombination by the Lewis base molecules, and the small differences in the experimental timescales between pyridine and thiophene passivations, with the thiophene system showing a slightly slower recombination.26 The use of a hybrid functional would produce smaller NA coupling76 and better agreement with the experiment.26 However, in order to produce accurate bandgaps,61 hybrid functionals have to be combined with spin-orbit effects,62-63 both of which significantly increase the computational cost. In summary, using NAMD combined with ab initio real-time TDDFT, we 18 ACS Paragon Plus Environment

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modeled the recent time-resolved experiments26 in order to elucidate the atomistic origin of the observed slowing of charge recombination in the MAPbI3 perovskite upon surface passivation with the Lewis base molecules. The non-radiative electron-hole recombination constitutes a major pathway for charge and energy losses in these materials, limiting the photon-to-electron conversion efficiency. The experimental data show that absorption of the Lewis bases extends the excited-state lifetime and increases the efficiency of the perovskite solar cells. Our simulations demonstrate that the Lewis base molecules change localization of the electron wavefunctions, thereby decreasing the NA coupling. Both pyridine and thiophene achieve the same effect, however, by different mechanisms. Thiophene acts according to the general expectation by forcing electron density away from the surface, which usually acts as a charge recombination site. In comparison, pyridine localizes electron near the surface, because it creates a strong coordinative bond with a surface Pb atom, and since its LUMO resides close to the perovskite CB edge, due to large electronegativity of the N atom. The localized state created by pyridine remains very close to the CB edge, avoiding a deep trap and allowing the charge to escape easily into the band. Both molecules decrease the NA electron-phonon coupling and coherence time by a factor of two, enhancing the computed excited state lifetime by an order of magnitude. A broad spectrum of vibrational modes, including those of both inorganic and organic components, couple to the electronic subsystem and induce rapid coherence loss. The insights obtained during the simulations establish the fundamental principles of charge recombination and reveal an unexpected 19 ACS Paragon Plus Environment


deceleration mechanism. The experimental trends are rationalized by considering chemical bonding, wavefunction localization, inelastic and elastic electron-vibrational scattering, and electronic coherence. The generated insights can be used to formulate practical guidelines for reduction of non-radiative charge recombination in perovskites via rational choice surface passivation agents. The study advances our understanding of the key factors influencing and controlling the performance of hybrid organic-inorganic perovskite solar cells.

Acknowledgements The first three authors acknowledge support of the National Science Foundation of China (Grant Nos. 21503017, 21573022, 21520102005, 21421003, and 21590801). R.L. is grateful to the Recruitment Program of Global Youth Experts of China, the Beijing Normal University Startup, and the Fundamental Research Funds for the Central Universities. O.V.P. acknowledges support of the U.S. Department of Energy (Grant No. DE-SC0014429).

Supporting Information Available: HOMO and HOMO-1 charge densities of the bare Cl-doped MAPbI3 (001) surface, and the surface with adsorbed pyridine and thiophene. The material is available free of charge via the Internet at http://pubs.acs.org.


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