Impact of Organic Spacers on the Carrier Dynamics in 2D Hybrid Lead

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Impact of Organic Spacers on the Carrier Dynamics in 2D Hybrid Lead-Halide Perovskites Shou-Feng Zhang, Xiankai Chen, Ai-Min Ren, Hong Li, and Jean-Luc Brédas ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b01888 • Publication Date (Web): 21 Nov 2018 Downloaded from http://pubs.acs.org on November 21, 2018

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ACS Energy Letters

Impact of Organic Spacers on the Carrier Dynamics in 2D Hybrid Lead-Halide Perovskites

Shou-Feng Zhang,12 Xian-Kai Chen,3 Ai-Min Ren,2* Hong Li,3* and Jean-Luc Bredas3* 1Department

of Electronic Engineering, Guangxi University of Science and Technology, Liuzhou 545006, China

2Laboratory

of Theoretical and Computational Chemistry Institute of Theoretical Chemistry Jilin University Changchun 130023, China

3School

of Chemistry and Biochemistry and Center for Organic Photonics and Electronics Georgia Institute of Technology Atlanta, Georgia 30332-0400

* Corresponding authors: [email protected]; [email protected]; [email protected] 1 ACS Paragon Plus Environment

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Abstract We have carried out non-adiabatic molecular dynamics simulations combined with time-dependent density functional theory calculations to compare the properties of the two-dimensional (2D) (BA)2(MA)Pb2I7 and three-dimensional (3D) MAPbI3 (where MA = methylammonium and BA = butylammonium) materials. We evaluate the different impacts that the 2D-confined spacer layer of butylammonium cations and the 3D-confined methylammonium cations have on the charge carrier dynamics in the two systems. Our results indicate that while both the MA+ and BA+ cations play important roles in determining the carrier dynamics, the BA+ cations exhibit stronger non-adiabatic couplings with the 2D perovskite framework. The consequence is a faster hot-carrier decay rate in 2D (BA)2(MA)Pb2I7 than in 3D MAPbI3. Thus, tuning of the functional groups of the organic spacer cations in order to reduce the vibronic couplings between the cations and the Pb-I framework can offer the opportunity to slow down the hot-carrier relaxations and increase the carrier lifetimes in 2D lead-halide perovskites.

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Two-dimensional (2D) hybrid organic-inorganic (lead-halide) perovskites (HOIPs) have garnered much attention given their ambient stability and potential in light-emitting diode (LED)

1-4

and photovoltaic (PV)

5-9

applications. 2D HOIPs can

be viewed as multi-quantum-well (QW) structures with the semiconducting perovskites slabs [(CH3NH3)n-1PbnX3n+1]2- (where n is an integer denoting the number of [PbX6]4− octahedron layers in each slab) acting as potential wells, and the 2D organic cation spacers inserted in between the slabs acting as potential barriers. Such superlattice structures consisting of both organic and inorganic components provide the opportunity to tune the electronic and optical properties via not only the thickness of the quantum wells but also the chemical nature of the organic cation spacers.10-14

To date, reasonably high efficiency has been achieved in 2D HIOP solar cells. A power conversion efficiency (PCE) over 12%

has been reported for 2D

Ruddlesten-Popper (RP) perovskite analogs (RNH3)2(CH3NH3)n−1PbnX3n+1, where n ≥ 3, the organic cation [RNH3]+ is butylammonium (BA)(R=CH3(CH2)3), and X=I.8 More recently, a higher PCE of 14.1% was reached in solar cell devices using a thicker perovskite layer (n=5) and phenylethylammonium (PEA) as the organic spacer.15 Although this efficiency remains significantly below the record 23.2% PCE reported for three-dimensional (3D) HOIP-based solar cells16, their highly tunable electronic and optical properties associated with the 2D hybrid organic/inorganic superlattice structures, offer great potential for various optoelectronic applications.



The three-dimensional (3D) HOIPs have a general formula APbX3 (with A being usually an organic cation, such as CH3NH3 (MA), and X=I, Br, or Cl) and display outstanding (photovoltaic17-21 and other22-24) optoelectronic performance. The reason is a combination of beneficial properties that include suitable bandgaps, small effective masses, small exciton binding energies, low defect densities, and long charge carrier diffusion lengths. However, the 3D HOIP solar cells generally suffer from severe device instability due to their sensitivity to environmental factors, such as moisture, UV light, and heat.25-26

In both 3D and 2D HOIPs, the role of the organic cations (for instance, MA+ in 3D MAPbI3 and MA+ and BA+ in the 2D (BA)2(MA)n−1PbnI3n+1 RP-phase) in determining their structural and electronic properties has been of great fundamental and applications-related interest. Although it is well-established that in both 3D and 2D cases the valence and conduction band edges are dominated by contributions from the inorganic framework consisting of corner-sharing [PbX6]4− octahedra,27-29 the roles of the MA+ and BA+ cations cannot be ignored, especially since the interactions between the organic cations and the inorganic framework can impact these properties as a result of electron-phonon couplings, which is an intrinsic feature of lead-halide perovskites.30-32 Earlier computational and experimental studies on 3D perovskites have indeed highlighted that both the orientations and vibrations of the organic cations can significantly modulate the structural and electronic properties.33-35 In addition, the cation vibrations can strongly impact the dynamics of the processes 4 ACS Paragon Plus Environment

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ACS Energy Letters

associated with charge-carrier transport and energy transfer, which are critical to LED and PV performance. For instance, studies of the hot-carrier dynamics in 3D perovskites have pointed out the role that the vibrational interactions between the organic cations and the inorganic framework play in the slow-down of the hot-carrier relaxations to the band-edge states.36-41

Because of the dimensional change and the presence of the cation spacers, the excitons in 2D perovskites are much more strongly bound than in the 3D case (~380 meV

when

n=1

and

~220

meV

when

n˃

2

for

the

2D

RP-phase

(BA)2(MA)n−1PbnI3n+1)5 and the charge-carrier and exciton-relaxation dynamics become more complex. Recent experimental studies based on time-resolved photoluminescence spectroscopy have revealed that the exciton recombinations in 2D RP-phase perovskites are much slower in large quantum well systems (~nanoseconds for n=3) than in thinner ones (~ 250 ps for n=1).42 Time-domain ab initio dynamics analyses have suggested that it is the enhanced elastic electron-phonon scattering in the former systems that causes rapid loss of electronic coherence and slows down charge recombinations.43 While experimental studies using femtosecond pump/probe nonlinear spectroscopy indicate an ultrafast intraband hot-carrier relaxation of 421 meV in less than 150 fs in 2D (C6H5C2H4NH3)2PbI4,44 a fundamental understanding of the hot-carrier relaxation mechanism in 2D perovskites, especially of the impact of the quantum confinement and nature of the organic cations on the relaxation dynamics, is still missing. 5 ACS Paragon Plus Environment


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Here, we use a computational approach combining ab initio molecular dynamics (AIMD),

non-adiabatic

molecular

dynamics

(NAMD),

and

time-dependent

density-functional theory (TDDFT)(more details, see section S1 in Supporting Information), to compare the hot-electron relaxation dynamics in a 2D RP-phase crystalline perovskite with a low n value (n=2, (BA)2(MA)Pb2I7) to that in the 3D MAPbI3 perovskite (see Figure 1). We chose the n=2 lattice structure since it is the smallest

quantum-well

structure

containing

both

butylammonium

and

methylammonium cations while preserving the pseudo-cubic cage structure found in the 3D perovskites. Our objective is to gain a comprehensive understanding of the effect of quantum confinement in a thin 2D multi-quantum-well structure as well as of the impact of different organic cations, on the carrier dynamics in 2D versus 3D lead-halide perovskites. The geometry optimizations, electronic-structure calculations, and ab initio molecular dynamics (AIMD) simulations were performed with the QuantumEspresso package (version 6.1)45; the PYXAID package46-47 was used to perform the time-domain NAMD process (more details see Methodology section in Supporting Information). The spin-orbital coupling (SOC) and many-body effects are not included considering their extremely high computational cost. Recent state-of-the-art theoretical calculations indicate that SOC can accelerate the hot-carrier relaxations in MAPbI348, and we can expect that 2D (BA)2(MA)Pb2I7 shares the same trend; however, our main conclusions should not be affected since they derived by 6 ACS Paragon Plus Environment

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comparing the two systems at the same level of theory. Notably, the excitonic effects in thin 2D perovskites are also much stronger than in 3D systems. However, the proper evaluation of the excitonic effects requires a time-dependent version of Bethe-Salpeter theory, which is still under development49.

Figure 1. Structures of the (BA)2(MA)Pb2I7 unit cell and MAPbI3 super cell: (a) and (c) as optimized at the DFT/PBE level; (b) and (d) as taken from the molecular dynamics trajectories at 300 K.



Impact of the different cations on geometrical structures and band edge states For the Ruddlesden-Popper phase 2D (BA)2(MA)Pb2I7 perovskite, we consider the noncentrosymmetric orthorhombic Cc2m crystal structure as reported in Ref. 10;10 each unit cell contains two QW structures with each QW consisting of two layers of corner-sharing [PbI6]4− octahedra and two layers of BA+cations, as shown in Figure 1 (a). In order to be able to directly compare the 2D (BA)2(MA)Pb2I7 system with its 3D counterpart (MAPbI3), we chose a 2  2  2 supercell of the pseudocubic MAPbI3 crystal (see Figure 1(c)),50 so that the unit cells have an equal number of inorganic slabs.

To understand the role of the BA+ and MA+ cations in the structural evolution of the 2D (BA)2(MA)Pb2I7 and 3D MAPbI3 systems at room temperature, we first compare their geometrical structures with a series of structures taken from the AIMD trajectories after thermal equilibrium at 300K (see Figure 1(b) and (d) for representative structures). Some characteristic features can be observed: (1) While each [PbI6]4−octahedron in 3D MAPbI3 is connected to six neighboring octahedra via sharing of iodine anions at the six corners, each octahedron in 2D (BA)2(MA)Pb2I7 has a Pb-I dangling bond with the I- ion pointing towards the cationic BA+. This suggests that there can be strong interactions with the BA+ spacer, susceptible to induce larger deformations in the [PbI6]4− octahedra due to the smaller steric hindrance present in the 2D systems. (2) The small MA+ cations in both systems display free rotational movements (in the 8 ACS Paragon Plus Environment

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course of the MD simulations) due to the large voids inside each pseudo-cubic cage. (3) The BA+ cations in 2D (BA)2(MA)Pb2I7 are more spatially confined and display a much smaller degree of rotation of the alkyl chains than the MA+ cations. This is due to the BA+ cations having longer alkyl chains and packing in such a way that each amine group points to the 2D [Pb2I7]3- framework, with each molecular dipole oriented nearly anti-parallel to its neighbor within the 2D organic spacer layer.

The contributions of the BA+ and MA+ cations to the structural fluctuations of the [Pb2I7]3- framework in 2D (BA)2(MA)Pb2I7 have been further quantified by carrying out ab initio MD simulations and using various constraints: (1) We consider that the BA+ and MA+ cations are both fully active and hence all the vibrations of the organic cations are taken into account; we denote this system as BA+(a)-MA+(a). (2) Only the vibrations of the BA+ cations are involved while the MA+cations are frozen, hence the notation BA+(a)-MA+(f). (3) Only the vibrations of the MA+ cations are involved while the BA+ cations are frozen, BA+(f)-MA+(a). Representative AIMD snapshots of the three series of simulations, which reflect the geometric variations occurring after thermal equilibrium at 300 K, are shown in Figure S1 in the Supporting Information. The motion of the [Pb2I7] 3- framework can be characterized by considering the bond-length fluctuations of the two Pb-I bonds r1 and r2 (see Figure 2(a)), with r1 representing Pb1-I1 where the I1 atom is at the edge of the framework and interacts mainly with a BA+ cation, and r2 representing the Pb1-I2 bond where I2 lies in the center layer of the [Pb2I7] 3- framework (it is bound to 9 ACS Paragon Plus Environment


both Pb1 and Pb2 ions), and interacts mainly with the MA+ cation. Thus, the differences in the r1(Pb1-I1) and r2(Pb1-I2) fluctuations originate in two factors: the local octahedron structural effect and the interactions with the organic cations. It is useful to point out that the central r2(Pb1-I2) bond is structurally similar to the Pb-I bonds in 3D perovskites and its dynamical evolution can be expected to be representative of that in the 3D system.

Figure 2b shows the statistical maps of the r1 and r2 fluctuations in the course of the MD simulations under the three constraints described above. When the BA+ and MA+ cations are both active (constraint (1)), r1 displays a much larger range of variations as well as a longer average bond length than r2 (~0.7 Å for r1 fluctuations versus ~0.5 Å for r2; average bond length of 3.53 Å for r1 vs. 3.18 Å for r2). This is consistent with the structural features indicated above, since Pb1-I1 is a dangling bond at the edge of the [Pb2I7] 3- framework while Pb1-I2 is at the layer center.

When the BA+ spacers are frozen while the MA+ cation remains active (constraint (2)), the change in r1 is limited to a much narrower range (~0.45 Å) than in the fully active condition (1), with the average bond length also reducing to 3.23 Å. In comparison to constraint (1), this decrease indicates that the vibrations of the BA+ spacer strongly impact the [Pb2I7] 3- framework structure, especially the Pb-I dangling bonds at the edge of the framework. In contrast, while the r2 fluctuations and average bond lengths also decrease in case (2), their reductions are much smaller than those 10 ACS Paragon Plus Environment

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for r1. Thus, these results indicate that the BA+ vibrations have a much smaller effect on the inner Pb-I bonds than on the edge bonds.

When switching the active cation from MA+ to BA+ (constraint (3)), the average value of r2 (3.10 Å) only slightly increases, a further indication that the BA+ vibrations have negligible effect on the central Pb-I bonds. In addition, under this constraint, the edge Pb-I bonds stretch even further, with the average bond length increasing to ~3.61 Å but fluctuating in a much narrower range of ~0.35 Å. Overall, the three MD simulations performed with different constraints highlight that the interactions with the more bulky BA+ cations induce much larger fluctuations of the Pb-I bonds at the edge of the 2D [Pb2I7]3- framework than those in 3D perovskites. We now turn to a discussion of the impact of these structural differences on the electronic structure.



Figure 2 (a) Structure of the (BA)2(MA)Pb2I7 (BA: CH3(CH2)3NH3+; MA: CH3NH3+) repeat unit illustrating the Pb1-I1 and Pb1-I2 bonds used to trace the structural fluctuations in the [Pb2I7]3- framework. (b) Statistical maps of the Pb-I bond length changes with r2 plotted as a function of r1. The solid circles in red, blue, and green refer to the mean values of r2 and r1 for the BA+(a)-MA+(a), BA+(a)-MA+(f), and BA+(f)-MA+(a) simulations, respectively.

We now examine the electronic structures of the 2D (BA)2(MA)Pb2I7 and 3D MAPbI3 12 ACS Paragon Plus Environment

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systems at 0 K (see Table 1) and their evolutions at room temperature (T = 300K) in order to understand the impact of the organic cations on the electronic properties (note that a detailed methodology discussion regarding the band gaps calculated at the PBE level is given in Section S3 of the SI). The impact of the organic cations is averaged by following the evolution of the instantaneous band gaps over each frame of the ab initio MD trajectory (all cations being active) after the systems reach thermal equilibrium, see Figure 3(a). The band gap variations are much larger in 2D (BA)2(MA)Pb2I7 than 3D MAPbI3. The (BA)2(MA)Pb2I7 gap fluctuates over a range of 0.59 eV, from 1.86 to 2.45 eV, see Figure 3(b), and averages at 2.20 eV; the MAPbI3 gap varies over a much narrower range of 0.38 eV, from 1.78 to 2.16 eV, and averages at 1.95 eV. These results point out that: (i) The average band gaps for both the 2D and 3D structures are slightly larger at room temperature vs. the 0 K structure. (ii) The band gap thermal fluctuations are higher in the 2D structure. Since the valence and conduction band edges in both systems are dominated by contributions from I-5p and Pb-6s/6p orbitals, respectively (see Figures S2-S5 and the discussion on the projected density of states, PDOS), the larger band gap fluctuations in the 2D system correlates with the larger variations in the Pb-I bonds at the edges of the 2D Pb-I framework, which are those interacting directly with the BA+ cations. (A more detailed discussion on the impact of the organic cations on the fluctuations of the valence and conduction band edges is given in Section S4 of the SI).



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Table 1. Comparison of the experimental (Egexp) and calculated band gaps (Egcalc) for (BA)2(MA)Pb2I7 and MAPbI3 at 0K. All data are in eV. The values calculated for MAPbI3 with other theoretical methods are also listed as for the sake of comparison. Egexp(optical)

Egcalc

others

(BA)2(MA)Pb2I7

2.1710

1.98

-

MAPbI3

1.6951a

1.81

1.28 (SOC-GW)51 1.74 (PBE)52 2.16 (HSE06)53

(aat 330 K)

Figure 3. (a) Time evolution of the band gaps in the course of the ab initio MD simulations at 300 K for (BA)2(MA)Pb2I7 and MAPbI3; (b) average band gaps and corresponding fluctuation ranges at 300 K under thermal equilibrium.


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The impact of the different cations on the hot-carrier relaxations After having clarified the major difference in the impact that the BA+ and MA+ cations have on the crystal structures and band edge states, we now compare their influence on the hot-carrier relaxation dynamics in the conduction bands of 2D (BA)2(MA)Pb2I7 and MAPbI3. The hot-carrier dynamics corresponds to the relaxation of a carrier initially created via photon excitation or charge injection; it involves energy transfer to the crystal lattice, mainly driven by carrier-phonon interactions.

Note that the CBMs for (BA)2(MA)Pb2I7 and MAPbI3 (which appear in both systems at the Γ-point in the 0 K geometries) are nearly degenerate with a few other levels (see Figure S2(b) and (d)). Since non-adiabatic transitions among nearly degenerate states can cause severe time-evolution population fluctuations, in order to better characterize the non-adiabatic transition process, we redefine the population of the CBM (conduction-band minimum) state as a sum over the nearly degenerate lowest CB levels: for 2D (BA)2(MA)Pb2I7, the population for the lowest CB level (denoted as CBM0) corresponds to a sum of the populations over the CBM, CBM+1, CBM+2, and CBM+3 levels because the |CBM+1> and |CBM+2> levels fluctuate and switch energies during the NAMD process conducted at 300 K; for 3D MAPbI3, the redefined CBM0 population is a sum over the CBM and CBM+1 levels.

To study the non-adiabatic hot-electron cooling process, we consider that the electron initially populates levels that are well above the lowest, nearly degenerate CB levels 15 ACS Paragon Plus Environment


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(CBM0); these are the CBM+2, CBM+3, and CBM+4 levels for MAPbI3 and CBM+4, CBM+5, and CBM+6 levels for (BA)2(MA)Pb2I7. The average excess energies(∆Ee) calculated based on the NAMD trajectories (given in Table S3) are in the range of ~0.4 eV for MAPbI3 and ~0.7 eV for (BA)2(MA)Pb2I7. Figure 4 depicts the electronic population decay of the hot electrons starting from the various excited sates in (BA)2(MA)Pb2I7 and MAPbI3. In both systems, a higher excess energy generally results in a longer hot-carrier lifetime, which is consistent with the experimental data reported for 3D colloidal perovskite nanocrystals.36

The population decay curve is fitted using a Gaussian plus exponential function: 𝑓(𝑡) = 𝑎𝑒

-

𝑡 2

( ) + (1 ― 𝑎)𝑒 ― ( ) 𝑡

𝜏1

𝜏2

; the relaxation time τe is calculated as 𝜏𝑒 = 𝑎𝜏1

+(1 ― 𝑎)𝜏2; these procedures have been used in nonadiabatic molecular dynamics investigations of lead-halide perovskites and quantum dots systems.41,

54-55

For the

excited states with the lowest excess energy, the relaxation time τe is estimated to be 345 fs and 122 fs for MAPbI3 and (BA)2(MA)Pb2I7, respectively. The calculated values are in very good agreement with the experimental data of 280 fs56 and ~100-150 fs4-5, 44, respectively. In the case of MAPbI3, except for the initial excitation into the CBM+2 state (which has the lowest excess energy ΔEe= 0.20 eV), the population in the initially excited higher energy states CBM+3 and CBM+4 (ΔEe= 0.36 and 0.37 eV) can remain as large as 20% after 1.5 ps. In (BA)2(MA)Pb2I7, the populations of all three states (CBM+4 to CBM+6) decay much more rapidly and approach zero after 1.5 ps. The fitting parameters as well as the relaxation times for 16 ACS Paragon Plus Environment

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other excited states are listed in Table S3 in the SI.

Figure 4. Fitted time-evolution of the populations for hot electrons and average non-adiabatic couplings between all related electronic states during the NAMD simulations for (BA)2(MA)Pb2I7 ((a) and (c)) and MAPbI3 ((b) and (d)).

In order to characterize the difference in the hot-electron relaxation processes in 2D versus 3D perovskites, we have evaluated the average nonadiabatic couplings (NACs) between all related states, as this is one of the most important factors dominating the nonadiabatic transitions. The averaged nonadiabatic couplings for the two materials are presented in Figure 4(c) and (d). We note that the NACs in the valence bands are generally stronger than those in the conduction bands due to their larger density of 17 ACS Paragon Plus Environment


states (DOS); this can lead to faster hot-carrier relaxations in the hole transport channel vs. the electron transport channel. Detailed time-evolution studies of hot-hole populatiosn are described in the SI. Overall, the stronger NACs are localized on the sub-diagonal lines, which refer to the couplings between nearest-neighbor states. Also, a larger energy difference between two neighboring states results in a smaller NAC since the non-adiabatic coupling is inversely proportional to such an energy difference.

Compared to the 3D MAPbI3, the 2D (BA)2(MA)Pb2I7 shows stronger nonadiabatic couplings over a wide range of conduction states. This trend can be explained by the interactions between the 2D [Pb2I7]3- framework and the organic cations, in particular the BA+ organic spacers, which can cause severe structural changes and lead to larger wavefunction overlaps for (BA)2(MA)Pb2I7. This is consistent with the trends in the fluctuations of the instantaneous band gaps, which we discussed earlier. This behavior originates in the phonon-assisted nonadiabatic transitions due to thermal disorder under the breakdown of the Born-Oppenheimer approximation.

To gain further insight into the effect of the organic cations on the hot-electron relaxations in both (BA)2(MA)Pb2I7 and MAPbI3, we have evaluated the influence spectra, using the Fourier transform of the unnormalized autocorrelation function 𝐶𝑖𝑗 (𝑡) = 〈𝛿𝐸𝑖𝑗(𝑡′)𝛿𝐸𝑖𝑗(𝑡 ― 𝑡′)〉for the time-evolution energy gap between a pair of excited states i and j: 18 ACS Paragon Plus Environment

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| [ ]|

𝐼𝑖𝑗(𝜔) ~ 𝐹𝑇

𝐶𝑖𝑗(𝑡)

2

𝐶𝑖𝑗(0)

For a given non-adiabatic transition between states i and j, the intensity of the influence spectrum is directly related to the strength of the electron-phonon coupling, and thus illustrates the contributions from the different vibronic modes (see Figure 5). For MAPbI3, the most intense peak is around 120 cm-1, which can be identified as a vibration frequency of the [PbI3]-inorganic framework, especially the Pb-I stretching mode, according to Raman characterizations.57 A few less intense peaks in the higher frequency region in the 280-500 cm-1 range, can be attributed to the coupled vibronic modes between the MA+ organic cations and the [PbI3]-inorganic framework.58

In 2D (BA)2(MA)Pb2I7, the BA+ organic cation spacer induces high-frequency vibronic modes (up to 1200 cm-1) in the hot-electron relaxations. In the low frequency regime, besides the Pb-I stretching mode of the inorganic framework around 120 cm-1, another dominant peak is observed at ~350 cm-1, which corresponds to the coupling of BA+ with the inorganic framework. It is worth pointing out that, in contrast to 3D MAPbI3 where the hot-electron relaxations are dominated by the Pb-I stretching mode, the coupled modes between the BA+ cation and the inorganic framework contribute more strongly to the hot-electron relaxations in 2D (BA)2(MA)Pb2I7. This is also in line with the larger structural changes at the edges than at the center of the inorganic framework, as discussed above. These strong vibronic modes and their broader frequency range induce larger wavefunction overlaps; consequently, the hot-electron decay rates are much faster in the 2D (BA)2(MA)Pb2I7 perovskite than in 19 ACS Paragon Plus Environment


the 3D MAPbI3. It is worth pointing out that the coupled vibronic modes between the BA+ organic spacer cations and the Pb-I framework could lead to larger electron-phonon couplings and thus induce stronger exciton localizations in the 2D perovskites vs. their 3D counterparts; this can contribute to the increased exciton binding energies measured in the 2D systems.

Figure 5. Influence spectra of the hot-electron relaxation dynamics starting from various excited states in (a) (BA)2(MA)Pb2I7 and (b) MAPbI3.

At last, we further distinguish the impact of the two organic cations MA+ and BA+ in the hot-electron relaxations in 2D (BA)2(MA)Pb2I7, by analyzing the NACs and the influence spectra under the artificial constraints (2) [BA+(a)-MA+(f)] and (3) [BA+(f)-MA+(a)], see Figure 6 and Table S4. A first observation is that constraining the motions of one cationic species in the 2D perovskite results in a significant reduction of the NAC values (see Table S4), which is consistent with the recent report that freezing the MA cations can greatly limit the NACs in 3D perovskites.37 Moreover, the NACs under constraint (3) where the BA+ organic spacers are frozen, 20 ACS Paragon Plus Environment

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display a greater reduction than under constraint (2), especially for the valence band states close to the VBM and the conduction band states with energies higher than CBM+5 (see Figure 6(a) and (b)). The influence spectra in Figure 6(c) and (d) further clarify the contributions of the different vibronic modes: When the BA+ motions are frozen and the MA+ cations remain active, the main vibronic mode is centered around 120 cm-1 (Pb-I stretching) with a less-intense broadening up to 750 cm-1, which is similar to the situation in 3D MAPbI3. On the other hand, when the BA+ cations are active and the MA+ cations frozen, the first main vibronic mode shifts to 350 cm-1 and is associated with the vibrational coupling between the organic spacers and the 2D Pb-I framework. (We note that the origin of the second peak centered around 600 cm-1 is related to the frozen MA+ cations).



Figure 6. Averaged non-adiabatic couplings and influence spectra from NAMD simulations under constraints (2) [BA+(a)-MA+(f)] ((a) and (c)) and (3) [BA+(f)-MA+(a)] ((b) and (d)).

In the present work, using a computational approach combining non-adiabatic molecular-dynamics simulations and time-dependent DFT calculations, we have distinguished the impact of butylammonium and methylammonium organic cations on the charge carrier dynamics in the 2D (BA)2(MA)Pb2I7 and 3D MAPbI3 perovskites. Our results reveal faster hot-electron relaxations in 2D (BA)2(MA)Pb2I7 than in 3D MAPbI3, which is fully consistent with recent experimental data. The differences can be explained in terms of the larger non-adiabatic couplings that occur in 2D (BA)2(MA)Pb2I7, as contributions from coupled vibronic modes between the BA+ 22 ACS Paragon Plus Environment

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organic spacer cations and the Pb-I framework provide for stronger electron-phonon couplings. Thus, the opportunity arises to tune the structure of the organic spacer cations via molecular engineering and thereby to modulate the performance of various opto-electronic devices based on 2D perovskites for applications such as LEDs and PVs. For instance, monoammonium cations with rigid and conjugated moieties, such as phenyl and pyrene groups, have been found to increase the crystal rigidity and reduce the electron-phonon coupling in 2D perovskites, and thus can improve the out-of-plane electrical conductivity and the photoluminescence quantum yield.59-60 Recent experimental and theoretical studies have also revealed improved material stability and interesting electronic structures of 2D perovskites using diammonium cations as spacer layers.61-63 Thus, further studies of the excitonic and optoelectronic properties of a broader range of organic spacer molecules are warranted. Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:XXXX Methodology, representative snapshots and average Pb-I bond lengths in the AIMD simulations, band gaps calculated at the PBE level, band edge fluctuations in the AIMD simulations, excess energies and fitting parameters for hot-carrier relaxation as well as non-adiabatic couplings for different constraint models. The material is available free of charge via the Internet at http://pubs.acs.org.



Acknowledgements This work was financially supported by the Natural Science Foundation of China (Nos. 21473071, 21173099 and 20973078), the Major State Basis Research Development Program (grant 2013CB834801), the Georgia Institute of Technology, the Georgia Research Alliance, the Vasser-Woolley Foundation and the Office of Naval Research (Award No. N00014-17-1-2208). Z.S.F acknowledges financial support by the start-up funds provided by Guangxi University of Science and Technology (Nos.03180030 and 03180031).

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Impact of Organic Spacers on the Carrier Dynamics in 2D Hybrid Lead

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