Hydrogen Passivated Silicon Grain Boundaries Greatly Reduce

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Hydrogen Passivated Silicon Grain Boundaries Greatly Reduce Charge Recombination for Improved Silicon/Perovskite Tandem Solar Cell Performance: Time-Domain Ab initio Analysis Siyu Wang, Weihai Fang, and Run Long J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 29 Apr 2019 Downloaded from http://pubs.acs.org on April 30, 2019

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The Journal of Physical Chemistry Letters

Hydrogen Passivated Silicon Grain Boundaries Greatly Reduce Charge Recombination for Improved Silicon/Perovskite Tandem Solar Cell Performance: Time-Domain Ab initio Analysis Siyu Wang,1 Wei-Hai Fang,1* Run Long1† 1College

of Chemistry, Key Laboratory of Theoretical & Computational

Photochemistry of Ministry of Education, Beijing Normal University, Beijing, 100875, P. R. China Abstract: By performing nonadiabatic molecular dynamics simulations, we demonstrate that grain boundaries (GBs) can induce the indirect-to-direct transition of silicon band gap. However, missing a silicon atom creates an electron trap state in the GBs. Electron trapping by the silicon vacancy occurs on tens of picoseconds followed by recombination of trapped electron and valence band hole on sub-100 picoseconds, which operates parallel to recombination of free electron and hole on a similar time scale. The recombination is greatly accelerated by two orders of magnitude compared to the GBs without a silicon vacancy. Hydrogen passivation eliminates the trap state and notably delays the charge recombination due to an increased band gap and a shortened coherence time, extending the excited-state lifetime to sub-10 nanoseconds. Our study provides an atomistic description of how the charge recombination in the silicon can efficiently reduce, suggesting a rational route to enhance silicon/perovskite

*

Corresponding author, E-mail: [email protected]

†

Corresponding author, E-mail: [email protected] 1

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The Journal of Physical Chemistry Letters 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

tandem solar cells performance. TOC Graphic

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Hybrid organic-inorganic perovskite solar cells have attracted intense interest because the power conversion efficiency (PCE) rapidly increases from 3.8%1 to 27.3%2 in the past decade, stemming from the excellent electronic and optical properties that include a long electron and hole diffusion length,3 low electron-hole recombination rate,4 and a broad absorption spectrum in the solar light.5 However, the classic perovskite MAPbI3 (MA = CH3N3) still suffers from a relatively large band gap over 1.5 eV and mixing with other cations or anions can decrease the band gap slightly, restricting the adsorption edge in the visible light regime and hindering the further improvement.6 Besides the band gap problem, the stability issue against the humidity7-9 and light-irradiation10-11 constitutes another obstacle for real applications. As the cornerstone of the modern semiconductor industry,12 silicon is extremely stable and has narrow band gap allowing to absorb infrared light that occupies 49% of the entire solar spectrum. The typical PCE of industry silicon solar cells is currently around 20%-22%13-14 and can approach to its theoretical threshold of 26% for real photovoltaic cells.15 Integrating the advantages of perovskites and silicon provides an excellent platform for the silicon/perovskite tandem solar cells innovation that simultaneously increase the stability and range of light harvesting, approaching to 95% of the solar spectrum. The PCE of silicon/perovskite tandem solar cells increases from 23.6%,16 to 25.2%,17 to 30.2%18 within only three years, which has already exceeded the theoretical lab limit of 29.4% of silicon solar cells. Unfortunately, the indirect band gap of crystalline silicon19-20 blocks the further improvement of the PCE of silicon and/or silicon/perovskite solar cells, because the small transition dipole moment, 3



arising due to the mismatch of the momentum between the valance band maximum (VBM) and the conduction band minimum (CBM), is the disadvantage of light harvesting and necessitates emitting a phonon during transition. One of the advantages of crystalline silicon possess high carrier mobility by reducing potential barriers for charge transport. In contrast, amorphous silicon thin film is provided with a direct band gap21 and facilities light absorption, but which is subjected to many unavoidable defects, and therefore inducing notable degradation in devices.22-23 The quality of polycrystalline silicon films lies between the crystalline and amorphous films whose charge mobility is close to the crystalline silicon. It could greatly improve the performance of silicon/perovskite tandem solar cells if one can achieve a direct band gap for polycrystalline silicon through defect engineering. Grain boundaries (GBs)24 are common defects in the polycrystalline and amorphous silicon, which create deeplevel defect states, known as recombination centers, and thus decreasing the charge mobility and the excited-state charge carrier lifetime significantly.25 Importantly, electronic structure calculations have predicted that silicon containing Σ3(110) GB is a direct band gap semiconductor, has no deep traps, and realizes high carrier mobility,26 potentially avoiding charge and energy losses. To this end, silicon/perovskite tandem solar cells based on the GB-engineered direct band gap silicon may provide a rational way to increase the PCE further. However, GBs break perfect crystalline symmetry and easily lose a lattice silicon atom, leading to form a silicon vacancy with a deep defect state,27 and thus accelerating charge dissipation into heat. In order to increase the carrier lifetimes and improve the performance of silicon/perovskite tandem solar cells, one 4


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must need to reduce nonradiative electron-hole recombination because it constitutes a major route for charge and energy losses. Chemical doping is a commonly used method to passivate deep defect traps and has been applied on silicon solar cells since the 1980s.13,

28-33

Among dozens of

methods,34-36 the most effectively and widely used approach is to attach an hydrogenrich plasma SiNx film to the surface37-38 because it is stable for a long time exposure to ultraviolet rays irradiation in sunshine.39 SiNx layer can act as a antireflection coating layer, and after deposition annealing, hydrogen atoms diffuse from the silicon nitride layer into bulk Si as a result of passivating the silicon defects, such as silicon vacancy,4041

and thereby increasing the efficiency notably.42 Recently, Hegmann and coauthors

have reported that the charge carriers in non-hydrogenated amorphous silicon films containing GBs are trapped in band edge states on sub-picosecond, while in the crystalline silicon in the presence of GBs last tens of picoseconds, using ultrafast timeresolved THz spectroscopy.43 GBs passivated with hydrogen show a substantial improvement in open-circuit voltage, short-circuit current, and bulk lifetime of silicon solar cells. The great improvement in solar cells performance suggests that hydrogen passivation could reduce the density of deep defects traps, suppress electron-hole recombination, and extend the excited-state lifetimes.44 The underlined mechanism, however, remains unclear and calls a detailed time-domain investigation of photoexcitation charge dynamics in real time and at the atomistic level. Motivated by the recent experiments studying the influence of GBs and hydrogen passivation on the charge dynamics in silicon, we apply time-dependent density 5



functional theory (TDDFT) and nonadiabatic molecular dynamics (NAMD) to study phonon-induced nonradiative electron-hole recombination in silicon containing GBs, GBs in the presence of silicon vacancy (GB_v) and GB_v with hydrogen passivation (Si_vH). We show that Σ3 (110) GBs are self-healed and avoid mid-gap states with a 1.12 eV direct energy gap obtained from HSE06 functional45 using optimized geometry that falls into the infrared light regime. The obtained nonradiavtive electron-hole recombination for the GBs takes place on 3.9 ns that is even an order of magnitude longer than the MAPbI3 perovskite.46-47 Introduction of a silicon vacancy into the GBs creates a deep electron trap 0.7 eV below the CBM, which makes the excited electron either be trapped within 18 ps and then recombine with the ground state on sub-100 ps, or occur a direct band-to-band recombination on a similar time scale. The acceleration is rationalized by larger NA coupling, arising due to the faster motions of the Si atoms and larger overlap of electron and hole wave functions in the presence of silicon vacancy. The simulated time scales are slightly longer than experimentally reported ones because the samples used experimentally are amorphous and polycrystalline films with high density of defects.43 Importantly, the GB_v passivated with hydrogen cancels the deep defect state and increase the energy gap to 1.42 eV, approaching to the optimal energy gap of 1.34 eV for an ideal single-cell photovoltaic device of the 32% Schockley-Queisser limit,48 which slows electron–hole recombination extends the excited-state lifetime to 5.4 ns. In addition to the increased energy gap, the deacceleration is attributed to the rapid loss of coherence of electronic subsystem, arising from that fast hydrogen atoms introduce higher-frequencies optical modes. Our 6


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study suggests a mechanistic understanding for that silicon vacancy accelerates charge recombination in the GBs and hydrogen passivation reduces it, providing valuable guidelines for the improvement of silicon/perovskite efficiency via rational choice of dopants. NAMD49 simulations are performed with the decoherence-induced surface hopping (DISH) approach50 implemented within TDDFT in the Kohn-Sham representation.51-52 Electrons are light and move fast, and thus are described quantum mechanically. Nuclei are heavy and move slowly, and therefore are treated (semi)classically. The decoherence, known as pure-dephasing in optical response theory,53 is needed to incorporate into the electron-hole recombination simulation because the decoherence time is significantly shorter than the recombination time. The approach has been applied to study photoexcitation dynamics for a variety of systems,46-47, 54-62 including MAPbI3 containing GBs,46-47 interacting with water,54 MAPbBr3 with a localized charge,55 inorganic perovskite quantum dots (QDs),56 Si QDs,57 and other systems.58-62 Geometry relaxation, electronic structure and adiabatic MD calculations are performed by the Vienna ab initio simulation package (VASP).63 A widely used semilocal functional of Perdew-Burke-Ernzerhof (PBE) is adopted to describe the electronic exchange-correlation effect.64 The projector-augmented wave (PAW)65 approach is used to treat the interaction between the ionic cores and the valence electrons. An energy cutoff of 500 eV is used to converge total energy. A (6 × 6 × 4) Monhorst-Pack k-point mesh66 is chosen for geometry optimization and electronic 7



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structure calculations. The NA coupling is obtained at Γ point because the direct band gap of all systems under investigation is at Γ point. After he geometry is relaxed at 0 K, all the systems are heated to 300K by velocity rescaling. Then, 5 ps adiabatic MD trajectories are generated in the microcanonical ensemble with a 1 fs atomic time-step. The first 1000 geometries from the adiabatic MD trajectories are chosen as initial configurations for the NAMD simulations of the nonradiative electron-hole recombination by PYXAID.67-68 The GBs are represented by 144-atom Σ3 (110) GBs (Figure 1a) because it is a direct band gap semiconductor based on previous first-principles calculations.26 The GB_v model is created by removing a silicon atom belonging to the middle boundary, which is labelled by a red dashed circle in Figure 1b. It is known that silicon structure has the tetragonal sp3 hybridization, the absence of a Si atom produces four dangling Si bonds, and therefore four hydrogen atoms are used to passivate the dangling bonds created by the missed silicon atom, denoted by GB_vH (Figure 1c). A comparison between the optimized geometries at 0 K and the snapshots of the corresponding structures from the MD trajectory at 300K, we observed that all geometries remain stable. In order to get microscopic insights of the thermal fluctuations on the geometry stability and nuclear dynamics for the GB, GB_v, and GB_vH systems, we calculate the canonically averaged standard deviations of the positions of Si atom, because nuclear dynamics create the NA coupling, induce loss of quantum coherence, and drive nonradiative electron-hole recombination, also because they. The standard deviations are calculated according to the formula 𝜎𝑖 =

〈(𝑟𝑖 ― 〈𝑟𝑖〉) 〉

8


2

and whose values are

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summarized in Table 1. Where 𝑟𝑖 represents the position of an atom i at time t in the 5 ps MD trajectories. Then, we average the standard deviations of Si atoms belonging to bulk and boundaries in the three systems. Based on the formula, the larger the standard deviations and the bigger fluctuations are responsible for stronger electronphonon coupling. Table 1 shows that the standard deviations in the positions of Si atoms subject to bulk and boundaries increase from GBs, to GB_vH and the GB_v. The results suggest that introduction of Si vacancy decreases the stability of GBs while hydrogen passivation stabilizes the geometry of GB_v. Thus, the atomic fluctuations analyses show that GBs containing Si vacancy occurs notable geometry change, followed by GB_vH and the GBs, leading one to expect stronger NA electron-phonon coupling in the systems containing Si vacancy and faster nonradiative electron-hole recombination.43

Figure 1. Simulation cells showing the optimized geometry at 0 K (top panel) and a snapshot of MD at 300 K (bottom panel) of (a) 144-atom silicon Σ3 (110) GBs, (b) GB with a silicon vacancy (GB_v), and (c) hydrogen passivated GB_v (GB_vH). The 9



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missed silicon atom is highlighted by red dashed circle. Hydrogen atoms are colored in dark blue in panel (c).

Table 1. Standard Deviations (Å) of Atom Position of Bulk and Boundaries Atoms in GB, GB_v and GB_vH systems.

GB

GB_v

GB_vH

Bulk

0.096

0.145

0.11

GB

0.095

0.152

0.145

Because the CBM and VBM constitutes the initial and final states for electronhole recombination across the energy gap, one needs to know how the atomic contributions to the CBM and VBM in the GBs, GB_v, and GB_vH systems. And therefore, we calculated their density of states (DOS) and presented them in Figure 2. The DOS is split into the contributions to the Si atoms subject to bulk and boundaries. Because semilocal PBE functional overestimates electron delocalization effects and thus underestimates the band gap, we scaled the band gap to the values obtained from HSE06 hybrid functional.45,

69-70

The calculated 1.12 eV CBM-VBM energy gap

(Figure 2a and Table 2) of the Si Σ3 (110) GBs agrees well with theoretical calculation at the same level,26 as well as approaches to the measured direct band gap of bulk Si nanocrystal.71 Introduction of a Si vacancy increases the energy gap to 1.23 eV which is separated by an occupied midgap trap state with a 0.66 eV below the CBM and a 0.57 10


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above the VBM (Figure 2b and Table 2). The deep trap state can act as electron recombination center and accelerate charge and energy losses. Hydrogen passivation removes the trap state and increases the band gap to 1.42 eV (Figure 2c and Table 2). This value is very close to the 1.33 eV band gap for an ideal single solar cell of the 32% Schockley-Queisser limit.48 Panel a-c of Figure 2 shows that the boundaries atoms contribute significantly to the VBM, CBM, and the electron trap state, respectively, suggesting that the states supported by GBs play key role to generate NA electronphonon coupling and have significant effect on the charge recombination.

Figure 2. Density of states (DOS) of (a) GB, (b) GB_v and (c) GB_vH. The total DOS is plotted in black line and the contribution to the total DOS arises from the GB atoms is plotted in red line. The DOS are calculated using the PBE functional but the energy gap of each system was scaled to the values obtained from HSE06 functional. Zero energy is set to the Fermi level.

The charge densities for the key electronic states, such as VBM, CBM, and the midgap state, participating into the electron-hole recombination of the three systems are shown in Figure 3. To distinguish the atomic contribution to the charge densities, Figure 3 provides a side view of the supercell. In the case of GB, the VBM is localized 11



on the top layer and third layer (top panel of Figure 3a) while the CBM is delocalized on the entire simulation cell (bottom panel of Figure 3a). Such situation suggests a low overlap between the VBM and CBM and leads to a small NA electron-phonon coupling (Table 2). For the GB_v system, Si vacancy accumulates the charge density around the vacant, particularly for the midgap state (middle panel of Figure 3b). The charge density of VBM increases and primarily localizes around the vacant (top panel of Figure 3b) while the CBM remains largely unchanged. Apparently, localized charge increases the overlap of each pair of states and thus increases the NA couplings (Table 2). It is expected that electron-hole recombination will be accelerated assisted by trap state. For the GB_vH system, hydrogen passivation increases the extent of charge delocalization of the VBM (top panel of Figure 3b) while strengthens the charge localization of the CBM in the left-hand side bulk region of the middle boundary while retains charge densities unchanged in other regions (bottom panel of Figure 3b), compared to the GB system. Consequently, the CBM-VBM overlap increases and the NA coupling grows with respect to the GB system (Table 2).

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Figure 3. Charge densities of the valence band maximum (VBM), the conduction band minimum (CBM), and midgap state of (a) Si GB, (b) GB_v and (c) GB_vH.

By performing Fourier transform (FT) of fluctuations of energy gap between pair of electronic states can obtain the spectral densities, which can characterize the phonon modes that create NA coupling, induce loss of coherence, and lead to electron-hole recombination. Figure 4 shows that the major peak at around 320 cm-1 is observed in the GB system (Figure 4a). A single Si vacancy broadens the major peak and forms a band arising due to symmetry breaking (Figure 4b). Passivated GB_v with hydrogen actives higher-frequency vibrations because hydrogen is light and fast. Focusing on particular phonon modes on each system, Figure 4a shows that the main peak at ~300 cm-1 in the GB system can be contributed to the 2TA acoustical phonons.72 The second major peak at about 520 cm-1 is a diagnostic mode of silicon and can be assigned to Si-Si bond stretching mode.73 The minor park at 1000 cm-1 can be attributed to the high frequency 2TO optical phonons .72 Silicon vacancy increases the amorphization and forms a continuum band between 300 cm-1 and 520 cm-1 for all dynamic process (Figure 4b). The peak at 520 cm-1 shows higher intensity than others modes and is responsible for creating the largest NA electron-phonon coupling and accelerating decoherence. Figure 4c demonstrates that hydrogen passivation increases the magnitude of peak at 1000 cm-1 and 1500 cm-1. The very high-frequency vibrations represent fast motions and accelerate loss of coherence and slow charge recombination.

13



Figure 4. Spectral density obtained from the Fourier transforms (FTs) of fluctuations for the energy gaps of each pair of key electronic states in (a) GB, (b) GB_v, and (c) GB_vH.

Figure 5 shows the evolution of populations of key states related to dynamic processes in the three systems. The data shown in Figure 5 a and c are fitted by exponential function to short-time linear approximation 𝑃(𝑡) = exp ( ―𝑡/𝜏) ≈ 1 ― 𝑡/𝜏, while the curves shown in Figure 5b are fitted to an exponential function 𝑃(𝑡) = 𝐴 exp ( ―𝑡/𝜏) + 𝐶.Where 𝜏 represents the time scale in two functions for each dynamic process. In the second function, constant A reflects the amplitude of each process. The small C term is needed to reflect the fact that the population change is not exponential but rather Gaussian at early time. The fitted times are summarized in Table 2. In the GB system (Figure 5a and Table 2), the electron-hole recombination occurs on 3.9 ns and which even much slower than the recombination time scale for the MAPbI3 perovskite.46-47 In the GB_v system (Figure 5b and Table 2), the excited electron initially gets trapped by the midgap state on 18 ps and then the trapped electron recombines with the hole remaining on the VBM on sub-100 ps. Alternatively, the 14


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excited electron on the CBM can directly recombine with the hole residing on the VBM on 100 ps. In both pathways, the excited-state lifetimes decrease two orders of magnitude compared to the GB system and which are harmful for solar cell operation. The computed timescales are a bit longer than the experiment because the experimental samples are amorphous films or polycrystalline with high defect concentraion.43 In the GB_vH system (Figure 5c and Table 2), the electron-hole recombination across the CBM-VBM energy gap decreases by a factor of 1.4 with respect to the GB system, extending the excite-state lifetime to 5.4 ns. The change trend in the time scales for each process of the three systems can be rationalized according to the energy gap, NA electron-phonon coupling, and decoherence time. The three quantities are summarized in Table 2. In the GB_v system, the excited electron gets trapped rapidly because the NA coupling (1.51 meV) is strong and the CBM-trap energy gap is small (0.66 eV). Then, the recombination can occur either between the trap state and the VBM or between the CBM and VBM bypassing the trap state on similar time scale. The recombination is accelerated by two orders of magnitude compared with the GB_v system, arising due to the reduced energy gap (0.57 vs 1.12 eV and 0.66 vs 1.12 eV) and increased NA coupling (0.79 meV vs 0.23 meV and 0.66 meV vs 0.23 meV) corresponding to trap-VBM and CBM-VBM transitions, respectively. These energy gaps together with the decoherence time (Table 2) also rationalize the different time scales for the three processes occurring in the GB_v system. The decoherence times decreases compared to the GB system because the fastatomic motions (Table 1). Comparing the GB_vH with the GB systems, the higher15



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frequency phonon modes (Figure 4) presented in the latter case shortens the coherence time (2.27 fs vs 2.75 fs) and increases the NA coupling (0.42 meV vs 0.23 meV) simultaneously. They have an opposite effect on the excited-state lifetime. However, the shortened coherence time and the increased energy gap compete successfully with the larger NA coupling, suppressing electron-hole recombination. The long-lived excited state up to 5.4 ns favors remaining the carrier sufficiently “hot” and reducing charge and energy losses. The simulations suggest that a rational choice of dopants can simultaneously stabilize silicon and reduce electron-hole recombination, providing a promising route for design of high performance of Si/perovskite tandem solar cells.

Figure 5. Phonon-induced charge trapping and recombination dynamics in (a) silicon GB, (b) GB_v and (c) GB_vH systems.

Table 2. Energy gap, Average NA coupling, Pure-Dephasing Time and Nonradiative Charge Dynamics Time for Each Process in the GB, GB_v, and GB_vH systems. Energy gap

NA coupling

Dephasing time

Charge

(eV)

(meV)

(fs)

Dynamics time (ps)

GB

1.12

0.23 16


2.75

3900

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GB_v

GB_vH

VBM-trap

0.57

0.79

2.50

83

CBM-trap

0.66

1.51

1.88

18

VBM-CBM

1.23

0.66

2.70

100

1.42

0.42

2.27

5400

In conclusion, phonon-assisted charge trapping and recombination dynamics of silicon GBs, GBs containing a silicon vacancy with and without hydrogen passivation have been explored with special emphasis on the lifetime and recombination pathways, using a combined time-domain density functional theory and nonadiabatic molecular dynamics simulation. Silicon containing specific Σ3 (110) GBs can make energy gap occur an indirect-to-direct transition and improve the light harvesting. Driven primarily by the low-frequency acoustics modes, the electron-hole recombination takes places on 3.9 ns. Missing a single silicon atom creates a midgap state that can trap the excited electron very rapidly on tens of picoseconds. Then the trapped electron recombines with hole remaining on the valence band within sub-100 ps. Alternatively, the excited electron can directly occur band-to-band recombination bypassing the trap state on similar time scale. The simulated time scales show a little longer than experiment arising because the measured data obtained from the amorphous or polycrystalline films in the presence of high defect concentration. The two orders of magnitude acceleration in recombination in both pathways is detrimental to the solar cell performance, which can be attributed to the enhanced NA electron-phonon coupling, generated by a broad spectrum of low-frequencies acoustics modes. Importantly, GB_v passivated with hydrogen stabilizes the geometry, heals the harmful trap state, and increase the band 17



gap to 1.42 eV, which is close to the ideal band gap value for a single solar cell of Schockley-Queisser limit. The fast motions of hydrogen atoms introduce higherfrequencies optical modes, accelerating phonon-induced loss of coherence, slowing the recombination, and extending the excited-state lifetime to 5.4 ns. The long-lived charge state is beneficial for reducing energy losses. Our results encourage the study of these features in silicon to optimize the silicon/perovskite tandem solar cell performance via precisely defect passivating strategies for device fabrication technology.

Acknowledgments The authors thank Prof. Hao Jin for fruitful discussions. The work as supported by the National Science Foundation of China, grant Nos. 21573022, 51861135101, 21688102, 21590801, and 21703222. R. L. is grateful to the Recruitment Program of Global Youth Experts of China, the Fundamental Research Funds for the Central Universities, and the Beijing Normal University Startup Package.

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Reference (1) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050-6051. (2) Oxford PV sets world record for perovskite solar cell. https://www.oxfordpv.com/news/oxford-pvsets-world-record-perovskite-solar-cell (accessed 25 June 2018). (3) Alarousu, E.; El-Zohry, A. M.; Yin, J.; Zhumekenov, A. A.; Yang, C.; Alhabshi, E.; Gereige, I.; AlSaggaf, A.; Malko, A. V.; Bakr, O. M., et al. Ultralong Radiative States in Hybrid Perovskite Crystals: Compositions for Submillimeter Diffusion Lengths. J. Phys. Chem. Lett. 2017, 8, 4386-4390. (4) Tress, W.; Marinova, N.; Inganäs, O.; Nazeeruddin, M. K.; Zakeeruddin, S. M.; Graetzel, M. Predicting the Open-Circuit Voltage of CH3NH3PbI3 Perovskite Solar Cells Using Electroluminescence and Photovoltaic Quantum Efficiency Spectra: the Role of Radiative and Non-Radiative Recombination. Adv. Energy Mater. 2015, 5, 1400812. (5) Ogomi, Y.; Morita, A.; Tsukamoto, S.; Saitho, T.; Fujikawa, N.; Shen, Q.; Toyoda, T.; Yoshino, K.; Pandey, S. S.; Ma, T., et al. CH3NH3SnxPb(1–x)I3 Perovskite Solar Cells Covering up to 1060 nm. J. Phys. Chem. Lett. 2014, 5, 1004-1011. (6) Suarez, B.; Gonzalez-Pedro, V.; Ripolles, T. S.; Sanchez, R. S.; Otero, L.; Mora-Sero, I. Recombination Study of Combined Halides (Cl, Br, I) Perovskite Solar Cells. J. Phys. Chem. Lett. 2014, 5, 1628-1635. (7) 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. (8) Schlipf, J.; Bießmann, L.; Oesinghaus, L.; Berger, E.; Metwalli, E.; Lercher, J. A.; Porcar, L.; Müller-Buschbaum, P. In Situ Monitoring the Uptake of Moisture into Hybrid Perovskite Thin Films. J. Phys. Chem. Lett. 2018, 9, 2015-2021. (9) Song, Z.; Shrestha, N.; Watthage, S. C.; Liyanage, G. K.; Almutawah, Z. S.; Ahangharnejhad, R. H.; Phillips, A. B.; Ellingson, R. J.; Heben, M. J. Impact of Moisture on Photoexcited Charge Carrier Dynamics in Methylammonium Lead Halide Perovskites. J. Phys. Chem. Lett. 2018, 9, 6312-6320. (10) Manshor, N. A.; Wali, Q.; Wong, K. K.; Muzakir, S. K.; Fakharuddin, A.; Schmidt-Mende, L.; Jose, R. Humidity versus photo-stability of metal halide perovskite films in a polymer matrix. Phys. Chem. Chem. Phys. 2016, 18, 21629-21639. (11) Nie, W.; Blancon, J.-C.; Neukirch, A. J.; Appavoo, K.; Tsai, H.; Chhowalla, M.; Alam, M. A.; Sfeir, M. Y.; Katan, C.; Even, J., et al. Light-activated photocurrent degradation and self-healing in perovskite solar cells. Nat. Commun. 2016, 7, 11574. (12) Schmitt, S. W.; Schechtel, F.; Amkreutz, D.; Bashouti, M.; Srivastava, S. K.; Hoffmann, B.; Dieker, C.; Spiecker, E.; Rech, B.; Christiansen, S. H. Nanowire Arrays in Multicrystalline Silicon Thin Films on Glass: A Promising Material for Research and Applications in Nanotechnology. Nano Lett. 2012, 12, 4050-4054. (13) Blakers, A. W.; Green, M. A. 20% efficiency silicon solar cells. Appl. Phys. Lett. 1986, 48, 215217. (14) De Wolf, S.; Descoeudres, A.; Holman Zachary, C.; Ballif, C. High-efficiency Silicon Heterojunction Solar Cells: A Review. Green 2012, 2, 7. (15) Battaglia, C.; Cuevas, A.; De Wolf, S. High-efficiency crystalline silicon solar cells: status and perspectives. Energy Environ. Sci. 2016, 9, 1552-1576. 19



(16) Bush, K. A.; Palmstrom, A. F.; Yu, Z. J.; Boccard, M.; Cheacharoen, R.; Mailoa, J. P.; McMeekin, D. P.; Hoye, R. L. Z.; Bailie, C. D.; Leijtens, T., et al. 23.6%-efficient monolithic perovskite/silicon tandem solar cells with improved stability. Nat. Energy 2017, 2, 17009. (17) Sahli, F.; Werner, J.; Kamino, B. A.; Bräuninger, M.; Monnard, R.; Paviet-Salomon, B.; Barraud, L.; Ding, L.; Diaz Leon, J. J.; Sacchetto, D., et al. Fully textured monolithic perovskite/silicon tandem solar cells with 25.2% power conversion efficiency. Nat. Mater. 2018, 17, 820-826. (18) Netherlands’ ECN reaches 30.2% efficiency for bifacial tandem cell based on perovskite. https://www.pv-magazine.com/2019/03/04/netherlands-ecn-achieves-30-2-efficiency-for-bifacialtandem-cell-based-on-perovskite/ (accessed Mar 05, 2019). (19) Wang, S.; Weil, B. D.; Li, Y.; Wang, K. X.; Garnett, E.; Fan, S.; Cui, Y. Large-Area Free-Standing Ultrathin Single-Crystal Silicon as Processable Materials. Nano Lett. 2013, 13, 4393-4398. (20) Xue, M.; Islam, R.; Meng, A. C.; Lyu, Z.; Lu, C.-Y.; Tae, C.; Braun, M. R.; Zang, K.; McIntyre, P. C.; Kamins, T. I., et al. Contact Selectivity Engineering in a 2 μm Thick Ultrathin c-Si Solar Cell Using Transition-Metal Oxides Achieving an Efficiency of 10.8%. ACS Appl. Mater. Interfaces 2017, 9, 4186341870. (21) Agrawal, B. K.; Agrawal, S. First-principles study of one-dimensional quantum-confined Hpassivated ultrathin Si films. Appl. Phys. Lett. 2000, 77, 3039-3041. (22) Fehr, M.; Simon, P.; Sontheimer, T.; Leendertz, C.; Gorka, B.; Schnegg, A.; Rech, B.; Lips, K. Influence of deep defects on device performance of thin-film polycrystalline silicon solar cells. Appl. Phys. Lett. 2012, 101, 123904. (23) Hu, S. M. Defects in silicon substrates. J. Vac. Sci. Technol. 1977, 14, 17-31. (24) Istratov, A. A.; Hieslmair, H.; Vyvenko, O. F.; Weber, E. R.; Schindler, R. Defect recognition and impurity detection techniques in crystalline silicon for solar cells. Sol. Energy Mater. Sol. Cells 2002, 72, 441-451. (25) Liang, J.; Schiff, E. A.; Guha, S.; Yan, B.; Yang, J. Hole-mobility limit of amorphous silicon solar cells. Appl. Phys. Lett. 2006, 88. (26) Raghunathan, R.; Johlin, E.; Grossman, J. C. Grain Boundary Engineering for Improved Thin Silicon Photovoltaics. Nano Lett. 2014, 14, 4943-4950. (27) Roberson, M. A.; Estreicher, S. K. Vacancy and vacancy-hydrogen complexes in silicon. Phys. Rev. B 1994, 49, 17040-17049. (28) Wang, A.; Zhao, J.; Green, M. A. 24% efficient silicon solar cells. Appl. Phys. Lett. 1990, 57, 602604. (29) Descoeudres, A.; Barraud, L.; De Wolf, S.; Strahm, B.; Lachenal, D.; Guérin, C.; Holman, Z. C.; Zicarelli, F.; Demaurex, B.; Seif, J., et al. Improved amorphous/crystalline silicon interface passivation by hydrogen plasma treatment. Appl. Phys. Lett. 2011, 99, 123506. (30) Geerligs, L. J.; Komatsu, Y.; Röver, I.; Wambach, K.; Yamaga, I.; Saitoh, T. Precipitates and hydrogen passivation at crystal defects in n- and p-type multicrystalline silicon. J. Appl. Phys 2007, 102, 093702. (31) Chan, T.-L.; Ciobanu, C. V.; Chuang, F.-C.; Lu, N.; Wang, C.-Z.; Ho, K.-M. Magic Structures of H-Passivated Silicon Nanowires. Nano Lett. 2006, 6, 277-281. (32) Demichel, O.; Calvo, V.; Besson, A.; Noé, P.; Salem, B.; Pauc, N.; Oehler, F.; Gentile, P.; Magnea, N. Surface Recombination Velocity Measurements of Efficiently Passivated Gold-Catalyzed Silicon Nanowires by a New Optical Method. Nano Lett. 2010, 10, 2323-2329. (33) Fernández-Serra, M. V.; Adessi, C.; Blase, X. Conductance, Surface Traps, and Passivation in 20


Page 20 of 23

Page 21 of 23 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


Doped Silicon Nanowires. Nano Lett. 2006, 6, 2674-2678. (34) Angermann, H.; Rappich, J.; Korte, L.; Sieber, I.; Conrad, E.; Schmidt, M.; Hübener, K.; Polte, J.; Hauschild, J. Wet-chemical passivation of atomically flat and structured silicon substrates for solar cell application. Appl. Surf. Sci. 2008, 254, 3615-3625. (35) Dingemans, G.; Kessels, W. M. M. Status and prospects of Al2O3-based surface passivation schemes for silicon solar cells. J. Vac. Sci. Technol., A 2012, 30, 040802. (36) Mueller, T.; Schwertheim, S.; Fahrner, W. R. Crystalline silicon surface passivation by highfrequency plasma-enhanced chemical-vapor-deposited nanocomposite silicon suboxides for solar cell applications. J. Appl. Phys 2010, 107, 014504. (37) Jan, S.; Mark, K.; Andrés, C. Surface passivation of silicon solar cells using plasma-enhanced chemical-vapour-deposited SiN films and thin thermal SiO2 /plasma SiN stacks. Semicond. Sci. Technol. 2001, 16, 164. (38) Stavola, M.; Jiang, F.; Kleekajai, S.; Wen, L.; Peng, C.; Yelundur, V.; Rohatgi, A.; Hahn, G.; Carnel, L.; Kalejs, J. Hydrogen Passivation of Defects in Crystalline Silicon Solar Cells. MRS Proceedings 2011, 1210, 1210-Q1201-1201. (39) G, A. A. Surface passivation of crystalline silicon solar cells: a review. Prog. Photovotl: Res. Appl. 2000, 8, 473-487. (40) Jiang, F.; Stavola, M.; Rohatgi, A.; Kim, D.; Holt, J.; Atwater, H.; Kalejs, J. Hydrogenation of Si from SiNx(H) films: Characterization of H introduced into the Si. Appl. Phys. Lett. 2003, 83, 931-933. (41) Derycke, V.; Soukiassian, P. G.; Amy, F.; Chabal, Y. J.; D'Angelo M, D.; Enriquez, H. B.; Silly, M. G. Nanochemistry at the atomic scale revealed in hydrogen-induced semiconductor surface metallization. Nat. Mater. 2003, 2, 253-258. (42) Hallam, B.; Sugianto, A.; Mai, L.; Xu, G.; Chan, C.; Abbott, M.; Wenham, S.; Uruena, A.; Aleman, M.; Poortmans, J. In Hydrogen passivation of laser-induced defects for silicon solar cells, 2014 IEEE 40th Photovoltaic Specialist Conference (PVSC), 8-13 June 2014; 2014; pp 2476-2480. (43) Titova, L. V.; Cocker, T. L.; Xu, S.; Baribeau, J.-M.; Wu, X.; Lockwood, D. J.; Hegmann, F. A. Ultrafast carrier dynamics and the role of grain boundaries in polycrystalline silicon thin films grown by molecular beam epitaxy. Semicond. Sci. Technol. 2016, 31, 105017. (44) Adamczyk, K.; Søndenå, R.; Stokkan, G.; Looney, E.; Jensen, M.; Lai, B.; Rinio, M.; Di Sabatino, M. Recombination activity of grain boundaries in high-performance multicrystalline Si during solar cell processing. J. Appl. Phys 2018, 123, 055705. (45) Heyd, J.; Scuseria, G. E.; Ernzerhof, M. Hybrid functionals based on a screened Coulomb potential. J. Chem. Phys. 2003, 118, 8207-8215. (46) Wang, Y.; Fang, W.; Long, R.; Prezhdo, O. V. Symmetry Breaking at MAPbI3 Perovskite Grain Boundaries Suppresses Charge Recombination: Time-Domain Ab Initio Analysis. J. Phys. Chem. Lett. 2019. (47) Long, R.; Liu, J.; Prezhdo, O. V. Unravelling the Effects of Grain Boundary and Chemical Doping on Electron–Hole Recombination in CH3NH3PbI3 Perovskite by Time-Domain Atomistic Simulation. J. Am. Chem. Soc. 2016, 138, 3884-3890. (48) Shockley, W.; Queisser, H. J. Detailed Balance Limit of Efficiency of p‐n Junction Solar Cells. J. Appl. Phys 1961, 32, 510-519. (49) Jasper, A. W.; Nangia, S.; Zhu, C.; Truhlar, D. G. Non-Born−Oppenheimer Molecular Dynamics. Acc. Chem. Res. 2006, 39, 101-108. (50) Jaeger, H. M.; Fischer, S.; Prezhdo, O. V. Decoherence-induced surface hopping. J. Chem. Phys. 21



2012, 137, 22A545. (51) Craig, C. F.; Duncan, W. R.; Prezhdo, O. V. Trajectory Surface Hopping in the Time-Dependent Kohn-Sham Approach for Electron-Nuclear Dynamics. Phys. Rev. Lett. 2005, 95, 163001. (52) Kohn, W.; Sham, L. J. Self-Consistent Equations Including Exchange and Correlation Effects. Phys. Rev. 1965, 140, A1133-A1138. (53) Mukamel, S. Principles of nonlinear optical spectroscopy. Oxford University Press: New York: 1995. (54) Long, R.; Fang, W.; Prezhdo, O. V. Moderate Humidity Delays Electron–Hole Recombination in Hybrid Organic–Inorganic Perovskites: Time-Domain Ab Initio Simulations Rationalize Experiments. J. Phys. Chem. Lett. 2016, 7, 3215-3222. (55) Zhang, Z.; Long, R.; Tokina, M. V.; Prezhdo, O. V. Interplay between Localized and Free Charge Carriers Can Explain Hot Fluorescence in the CH3NH3PbBr3 Perovskite: Time-Domain Ab Initio Analysis. J. Am. Chem. Soc. 2017, 139, 17327-17333. (56) He, J.; Vasenko, A. S.; Long, R.; Prezhdo, O. V. Halide Composition Controls Electron–Hole Recombination in Cesium–Lead Halide Perovskite Quantum Dots: A Time Domain Ab Initio Study. J. Phys. Chem. Lett. 2018, 9, 1872-1879. (57) Hyeon-Deuk, K.; Madrid, A. B.; Prezhdo, O. V. Symmetric band structures and asymmetric ultrafast electron and hole relaxations in silicon and germanium quantum dots: time-domain ab initio simulation. Dalton Trans. 2009, 10069-10077. (58) Zhang, Z.; Fang, W.-H.; Tokina, M. V.; Long, R.; Prezhdo, O. V. Rapid Decoherence Suppresses Charge Recombination in Multi-Layer 2D Halide Perovskites: Time-Domain Ab Initio Analysis. Nano Lett. 2018, 18, 2459-2466. (59) Yang, Y.; Fang, W.-H.; Long, R. Disparity in Photoexcitation Dynamics between Vertical and Lateral MoS2/WSe2 Heterojunctions: Time-Domain Simulation Emphasizes the Importance of Donor– Acceptor Interaction and Band Alignment. J. Phys. Chem. Lett. 2017, 8, 5771-5778. (60) Long, R.; Fang, W.-H.; Prezhdo, O. V. Strong Interaction at the Perovskite/TiO2 Interface Facilitates Ultrafast Photoinduced Charge Separation: A Nonadiabatic Molecular Dynamics Study. J. Phys. Chem. C 2017, 121, 3797-3806. (61) Wei, Y.; Long, R. Grain Boundaries Are Benign and Suppress Nonradiative Electron–Hole Recombination in Monolayer Black Phosphorus: A Time-Domain Ab Initio Study. J. Phys. Chem. Lett. 2018, 9, 3856-3862. (62) Long, R.; Prezhdo, O. V. Quantum Coherence Facilitates Efficient Charge Separation at a MoS2/MoSe2 van der Waals Junction. Nano Lett. 2016, 16, 1996-2003. (63) G. Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169. (64) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865-3868. (65) Blöchl, P. E. Projector augmented-wave method. Phys. Rev. B 1994, 50, 17953-17979. (66) Monkhorst, H. J.; Pack, J. D. Special points for Brillouin-zone integrations. Phys. Rev. B 1976, 13, 5188-5192. (67) Akimov, A. V.; Prezhdo, O. V. The PYXAID Program for Non-Adiabatic Molecular Dynamics in Condensed Matter Systems. J. Chem. Theory Comput. 2013, 9, 4959-4972. (68) Akimov, A. V.; Prezhdo, O. V. Advanced Capabilities of the PYXAID Program: Integration Schemes, Decoherence Effects, Multiexcitonic States, and Field-Matter Interaction. J. Chem. Theory 22


Page 22 of 23

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Comput. 2014, 10, 789-804. (69) Heyd, J.; Peralta, J. E.; Scuseria, G. E.; Martin, R. L. Energy band gaps and lattice parameters evaluated with the Heyd-Scuseria-Ernzerhof screened hybrid functional. J. Chem. Phys. 2005, 123, 174101. (70) Heyd, J.; Scuseria, G. E. Assessment and validation of a screened Coulomb hybrid density functional. J. Chem. Phys. 2004, 120, 7274-7280. (71) Persson, C.; Lindefelt, U. Detailed band structure for 3C-, 2H-, 4H-, 6H-SiC, and Si around the fundamental band gap. Phys. Rev. B 1996, 54, 10257-10260. (72) Iatsunskyi, I.; Nowaczyk, G.; Jurga, S.; Fedorenko, V.; Pavlenko, M.; Smyntyna, V. One and twophonon Raman scattering from nanostructured silicon. Optik 2015, 126, 1650-1655. (73) Momose, M.; Hirasaka, M.; Furukawa, Y. Raman spectroscopic study on boron-doped silicon nanoparticles. Vib. Spectrosc. 2014, 72, 62-65.

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Hydrogen Passivated Silicon Grain Boundaries Greatly Reduce

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