Lewis Base Passivation of Hybrid Halide Perovskites

Letter Cite This: J. Phys. Chem. Lett. 2018, 9, 1164−1171

pubs.acs.org/JPCL

Lewis Base Passivation of Hybrid Halide Perovskites Slows Electron− Hole Recombination: Time-Domain Ab Initio Analysis Lihong Liu,† Wei-Hai Fang,† Run Long,*,† and Oleg V. Prezhdo‡ †

Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education, College of Chemistry, Beijing Normal University, Beijing 100875, China ‡ Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States

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S Supporting Information *

ABSTRACT: Nonradiative 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 nonadiabatic 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 with thiophene due to the more electronegative nitrogen atom relative to thiophene’s sulfur. Both molecules reduce two-fold the nonadiabatic coupling and electronic coherence time. A broad range of vibrational modes couple to the electronic subsystem, arising from inorganic and 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.

H

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 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 nonradiative 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

ybrid 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 3.81 to 22.1%4,6 within only 6 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 detrapped and take a long time to recombine,16 providing a plausible rationalization for the remarkable performance of perovskite solar cells. Nonradiative 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 © 2018 American Chemical Society

Received: January 17, 2018 Accepted: February 20, 2018 Published: February 20, 2018 1164

DOI: 10.1021/acs.jpclett.8b00177 J. Phys. Chem. Lett. 2018, 9, 1164−1171

Letter

The Journal of Physical Chemistry Letters

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 wave function.

perovskites containing dopants,49 grain boundaries,35 and defects50 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 is 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 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 band gaps61 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. To obtain an accurate band gap with spin−orbit interaction turned on, one needs to either use a hybrid functional or perform a GW calculation.64 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

solar cells and optoelectronic devices require slow nonradiative 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 timeresolved experiments.26,28 We employ real-time time-dependent density functional theory (TDDFT) and nonadiabatic molecular dynamics (NAMD) to investigate the atomistic mechanisms underlying the observed reduction of nonradiative 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 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 2, 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-ofthe-art simulations highlight various factors affecting the nonradiative 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 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,33,36−48 including 1165


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Because phonon quanta available in the system are much smaller than the band gap, even including C−H stretches of the organic molecules, the nonradiative electron−vibrational relaxation is a multiphonon process. The PDOS of the Cl-doped MAPbI3 (001) surface with adsorbed pyridine and thiophene molecules, shown in Figure 2b,c, 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 with 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 with 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 band gaps at the Γ point obtained from the 6 ps MD trajectory for the surface/ pyridine and surface/thiophene systems are 1.788 and 1.818 eV, respectively (Table 1). They reflect thermal effects and are

the microcanonical ensemble with a 1 fs atomic time step. The NA couplings are calculated at the Γ-point because it corresponds to the fundamental band gap 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 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 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. Figure 2a presents the projected density of states (PDOS) of the bare Cl-doped MAPbI3 (001) surface split into contribu-

Table 1. LUMO−HOMO Band Gap, Absolute Value of NA Coupling, Pure-Dephasing Time T2*, and Nonradiative Electron-Hole Recombination Time for the Cl-Doped MAPbI3 (001) Surface with and without the Lewis Base Moleculesa

bare pyridine thiophene

band gap (eV)

NA coupling (meV)

dephasing (fs)

recombination (ns)

1.850 1.788 1.818

1.613 0.765 0.770

6.7 3.5 3.3

1.27 9.22 16.6

a

Data are canonically averaged over of the 6 ps MD trajectories at the Γ point.

most relevant for comparison with experiment. The band gap reduction is larger for pyridine, which is consistent with the stronger influence of pyridine on the CB edge. The small changes in the band gap 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. 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 wave functions, the sensitivity of the wave functions to atomic motions, and the speed of the motions. Because intraband electron and hole relaxation proceeds on a subpicosecond time scale,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 adsorption of the Lewis bases. On the contrary, the unoccupied MOs of the adsorbed molecules are close to the CB edge (Figure 2) and the molecules significantly affect the perovskite LUMO (Figure 3).

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 panels 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.

tions 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 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 wave function through electrostatic interaction. The band gap of the Cl-doped MAPbI3 (001) surface averaged canonically over the 6 ps MD trajectory is 1.840 eV. This amount of energy is deposited into phonon modes during the nonradiative electron−hole recombination. 1166


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under investigation. Known as influence spectra, they characterize the phonon modes that couple to the electronic degrees of freedom during the nonradiative transition. The vibrational modes induce decoherence in the electronic subsystem and accommodate the excess energy lost during the nonradiative 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 ∼30 to >400 cm−1. The higher frequency vibrational motions become important in the presence of pyridine and thiophene and especially pyridine, which interacts strongly with the perovskite (Figure 1b). The peaks at 62 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 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; 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. The decoherence times were computed as the puredephasing times of the optical response theory73 using the second-order cumulant approximation20,33−35,52

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 undercoordinated Pb atom (Figure 1b) localizes the LUMO on pyridine and nearby Pb atoms, reducing the nonadiabatic coupling and accelerating decoherence and hence slowing 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, 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; 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

⎛ 1 Dij(t ) = exp⎜ − 2 ⎝ ℏ

∫0

t

dt ′

∫0

t′

⎞ dt ′′ Cij(t ′′)⎟ ⎠

(1)

Here Cij(t) is the unnormalized autocorrelation function (ACF) of the phonon-induced fluctuation of the energy gap, δEij(t), between electronic states i and j, defined as Cij(t ) = ⟨δEij(t ′)δEij(t − t ′)⟩t′

(2)

The influence spectra shown in Figure 4a are obtained as Fourier transforms of the unnormalized autocorrelation ACF

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 band gap fluctuation squared; the bigger the fluctuation, the faster the dephasing.72 1167


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adsorbed Lewis bases reduce electron−hole recombination by an order of magnitude, rationalizing the experimental observation.26 The recombination is slowed by the adsorbed molecules because the NA coupling and decoherence times are decreased two-fold (Table 1). In general, strong NA coupling and long-lived coherence lead to fast quantum transitions. Weaker coupling and shorter coherence retard the dynamics. The dynamics stops if the coherence time becomes infinitely small, giving rise to the quantum Zeno effect.74,75 Because population transfer requires a buildup of quantum coherence first, suppressing the coherence slows population dynamics. The NA coupling is decreased and the decoherence is accelerated because the Lewis base molecules change localization of the electron wave functions (Figure 3). The dephasing is accelerated because wave functions 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 wave function 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 with 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 the 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. Because of 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 time scales 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, to produce accurate band gaps,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 modeled the recent time-resolved experiments26 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 nonradiative 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-wave functions, 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

shown in the inset 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 the incorporation of decoherence into the NAMD simulation. Adsorption of the Lewis base molecules accelerates decoherence by a factor of 2. Analysis of the second-order cumulant approximation for the optical response function leads one to conclude72 that the fast decoherence can arise from either a rapid decay or a large initial value of the energy gap ACF, shown in the inset in Figure 4b. Because the three ACFs decay on similar time scales, the differences in the puredephasing times are rationalized by the ACF initial values, Cij(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, Cij(0), can be placed in front of the integral. Hence, a larger Cij(0) leads to faster decoherence Dij(t). 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 wave function overlap, which decreases when the LUMOs become more localized in the presence of the molecules (Figure 3). Figure 5 demonstrates the evolution of the excited-state population in the three perovskite systems under consideration.

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 nonradiative decay due to smaller NA coupling and faster loss of coherence (Table 1). The coupling decreases because the electron wave functions become more localized (Figure 3b,c).

The nonradiative 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 1168


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(3) Wehrenfennig, C.; Liu, M.; Snaith, H. J.; Johnston, M. B.; Herz, L. M. Homogeneous Emission Line Broadening in the Organo Lead Halide Perovskite CH3NH3PbI3−XClX. J. Phys. Chem. Lett. 2014, 5, 1300−1306. (4) Jeon, N. J.; Noh, J. H.; Yang, W. S.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I. Compositional Engineering of Perovskite Materials for High-Performance Solar Cells. Nature 2015, 517, 476. (5) Christians, J. A.; Manser, J. S.; Kamat, P. V. Multifaceted Excited State of CH3NH3PbI3. Charge Separation, Recombination, and Trapping. J. Phys. Chem. Lett. 2015, 6, 2086−2095. (6) Pazos-Outón, L. M.; et al. Photon Recycling in Lead Iodide Perovskite Solar Cells. Science 2016, 351, 1430−1433. (7) Yamada, Y.; Yamada, T.; Shimazaki, A.; Wakamiya, A.; Kanemitsu, Y. Interfacial Charge-Carrier Trapping in CH3NH3PbI3Based Heterolayered Structures Revealed by Time-Resolved Photoluminescence Spectroscopy. J. Phys. Chem. Lett. 2016, 7, 1972−1977. (8) Guo, Z.; Wu, X. X.; Zhu, T.; Zhu, X. Y.; Huang, L. B. ElectronPhonon Scattering in Atomically Thin 2D Perovskites. ACS Nano 2016, 10, 9992−9998. (9) Xing, G.; Mathews, N.; Sun, S.; Lim, S. S.; Lam, Y. M.; Grätzel, M.; Mhaisalkar, S.; Sum, T. C. Long-Range Balanced Electron- and Hole-Transport Lengths in Organic-Inorganic CH3NH3PbI3. Science 2013, 342, 344−347. (10) Stewart, R. J.; Grieco, C.; Larsen, A. V.; Doucette, G. S.; Asbury, J. B. Molecular Origins of Defects in Organohalide Perovskites and Their Influence on Charge Carrier Dynamics. J. Phys. Chem. C 2016, 120, 12392−12402. (11) Peng, W. N.; Anand, B.; Liu, L. H.; Sampat, S.; Bearden, B. E.; Malko, A. V.; Chabal, Y. J. Influence of Growth Temperature on Bulk and Surface Defects in Hybrid Lead Halide Perovskite Films. Nanoscale 2016, 8, 1627−1634. (12) Wang, B. H.; Wong, K. Y.; Yang, S. F.; Chen, T. Crystallinity and Defect State Engineering in Organo-Lead Halide Perovskite for High-Efficiency Solar Cells. J. Mater. Chem. A 2016, 4, 3806−3812. (13) Tian, Y. X.; et al. Enhanced Organo-Metal Halide Perovskite Photoluminescence from Nanosized Defect-Free Crystallites and Emitting Sites. J. Phys. Chem. Lett. 2015, 6, 4171−4177. (14) Cui, P.; Fu, P. F.; Wei, D.; Li, M. C.; Song, D. D.; Yue, X. P.; Li, Y. Y.; Zhang, Z. R.; Li, Y. F.; Mbengue, J. M. Reduced Surface Defects of Organometallic Perovskite by Thermal Annealing for Highly Efficient Perovskite Solar Cells. RSC Adv. 2015, 5, 75622−75629. (15) Kim, J.; Lee, S.-H.; Lee, J. H.; Hong, K.-H. The Role of Intrinsic Defects in Methylammonium Lead Iodide Perovskite. J. Phys. Chem. Lett. 2014, 5, 1312−1317. (16) Li, W.; Liu, J.; Bai, F.-Q.; Zhang, H.-X.; Prezhdo, O. V. Hole Trapping by Iodine Interstitial Defects Decreases Free Carrier Losses in Perovskite Solar Cells: A Time-Domain Ab Initio Study. ACS Energy Lett. 2017, 2, 1270−1278. (17) Kanemitsu, Y. Luminescence Spectroscopy of Lead-Halide Perovskites: Materials Properties and Application as Photovoltaic Devices. J. Mater. Chem. C 2017, 5, 3427−3437. (18) Yang, J.; Siempelkamp, B. D.; Liu, D.; Kelly, T. L. Investigation of CH3NH3PbI3 Degradation Rates and Mechanisms in Controlled Humidity Environments Using in Situ Techniques. ACS Nano 2015, 9, 1955−1963. (19) Christians, J. A.; Miranda Herrera, P. A.; Kamat, P. V. Transformation of the Excited State and Photovoltaic Efficiency of CH3NH3PbI3 Perovskite Upon Controlled Exposure to Humidified Air. J. Am. Chem. Soc. 2015, 137, 1530−1538. (20) Long, R.; Prezhdo, O. V. Dopants Control Electron−Hole Recombination at Perovskite−TiO2 Interfaces: Ab Initio TimeDomain Study. ACS Nano 2015, 9, 11143−11155. (21) Liu, J.; Prezhdo, O. V. Chlorine Doping Reduces Electron− Hole Recombination in Lead Iodide Perovskites: Time-Domain Ab Initio Analysis. J. Phys. Chem. Lett. 2015, 6, 4463−4469. (22) Kim, H.-S.; Im, S. H.; Park, N.-G. Organolead Halide Perovskite: New Horizons in Solar Cell Research. J. Phys. Chem. C 2014, 118, 5615−5625.

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 because 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 2, 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 deceleration mechanism. The experimental trends are rationalized by considering chemical bonding, wave function localization, inelastic and elastic electron−vibrational scattering, and electronic coherence. The generated insights can be used to formulate practical guidelines for the reduction of nonradiative 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.

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ASSOCIATED CONTENT

S Supporting Information *

The material is available free of charge via the Internet at The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.8b00177. HOMO and HOMO−1 charge densities of the bare Cldoped MAPbI3 (001) surface and the surface with adsorbed pyridine and thiophene. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Wei-Hai Fang: 0000-0002-1668-465X Run Long: 0000-0003-3912-8899 Oleg V. Prezhdo: 0000-0002-5140-7500 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS L.L., W.-H.F., and R.L. acknowledge the 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).

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