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Role of Chalcogens in the Exciton Relaxation Dynamics of Chalcogenol-Functionalized CdSe QD: A Time-Domain Atomistic Simulation Md Habib,† Moumita Kar,‡ Sougata Pal,*,† and Pranab Sarkar*,‡ †
Department of Chemistry, University of Gour Banga, Malda 732103, India Department of Chemistry, Visva-Bharati University, Santiniketan 731235, India
‡
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S Supporting Information *
ABSTRACT: Colloidal CdSe nanocrystals are often stabilized by organic ligands. The choice of such ligands has tremendous detrimental effects on interparticle charge transfer (CT) dynamics in nanocrystalline thin-film devices. It is evident from the recent experiment that photoexcited hole migrates from CdSe quantum dot (QD) to surface-passivating phenyl chalcogenol ligands (PhEH; E = S, Se, Te) at different time scales. But the backward electron−hole (e−h) recombination at the interface remained unexplored. A deep-level understanding of the mechanism of CT at the interface is therefore required to unravel the key role of chalcogen for the betterment of the device performance. Herein, we have performed timedomain density functional theory calculation along with nonadiabatic molecular dynamics (NAMD) simulation to investigate the photoinduced CT at the CdSe−PhEH interfaces. The simulated time scales for hole transfer (HT) are found to follow the trend PhSH > PhSeH > PhTeH that concur excellently with the experimental observations. We propose that lower electronegativity of the E atom that binds with the CdSe QD facilitates the hole migration. In addition, the delocalized nature of initial donor states, phonon modes, NA coupling, and quantum coherence are the major factors that control the faster HT. Meanwhile, for the first time, we study the linker atom-dependent e−h recombination at such interface. The recombination event is remarkably slower than the HT and occurs at the nanosecond regime. Due to the greater electronegativity of linker atom (E = S), a broad range of phonon vibration and longer-lived quantum coherence expedite the recombination at a higher rate. In contrast, for higher chalcogens with lower electronegativity (Se, Te), the exciton relaxes relatively at a lower rate. We believe our results of atomistic, time-domain methodology provide valuable insight into the exciton relaxation dynamics in CdSe−chalcogenol interface and may be useful for the enhancement of performance of future devices.
1. INTRODUCTION A quantum dot (QD) is defined as a nanoparticle whose electronic motions are confined in all three dimensions. The QDs exhibit size- and shape-dependent optoelectronic properties when their dimensions are in the comparable range of exciton Bohr radius (a few nanometers).1 Surface-passivated colloidal QDs have been the subject of active research because of their several attractive features like size-dependent band gap, high photoluminescence quantum yields, extraordinary photostability, and possibility of various functionalization.2−18 Furthermore, they are found to show promising applications in light-emitting diodes,2 QD-sensitized solar cells,3,5,9,12,19−24 and low-threshold lasers.25 In particular, cadmium chalcogenide QDs are very well known for their photosensitizing activities in solar cells and other solar fuels.7,12 However, proper utilization of colloidal QDs in solar energy harvesting process stems from the capability of generating photoexcited charge carriers (electron and hole) at the interface of the functionalized QDs (typically organic− inorganic interface), followed by capturing them by the individual materials.15,26−28 It has been reported that the © XXXX American Chemical Society
typical time scale for extracting holes from QD’s surface by the noncovalently linked molecular acceptors ranges from microseconds to nanoseconds.6,29−31 If, however, covalent linkage is made between the acceptor molecule and the QD, it drastically reduces the hole extraction time, up to several picoseconds.32−34 Similar is the situation for electron extraction from QD to electron-accepting molecule or metal oxide, for which the required time scale is also in the picosecond order.13,35−37 For cadmium chalcogenide QDs in particular, the hole extraction process has been shown to be very challenging because of the larger effective mass of holes than that of the electrons, and consequently, they become localized at a certain region, thereby causing poor coupling with rest of the system.38,39 It is therefore necessary to adopt some different strategies for transferring and capturing holes in those specified systems.10,40 Charge-transfer dynamics involving QD−TiO241−43 and QD−organic molecule composites16,35 Received: February 11, 2019 Revised: May 16, 2019 Published: May 22, 2019 A
DOI: 10.1021/acs.chemmater.9b00605 Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials
3. RESULTS AND DISCUSSION The present simulation focuses on the photoinduced charge transfer and recombination dynamics at the CdSe−PhEH inorganic−organic interface. Absorption of photon energy hv by CdSe QD leads to e−h separation. Photoexcitation of CdSe induces hole transfer to PhEH through path 1 of Figure 1.
have been studied intensively. However, most of them are mediated by electrons and they occur in ultrafast time scales. Contrarily, the reported charge transfer dynamics mediated by holes is quite scattered in the literature.20,44−46 Extracting electrons and holes with comparable time scale is of foremost importance, especially for QD-based photovoltaics because an accumulation of either charge carrier increases the charge recombination, thereby limiting the device efficiency.21,47,48 In such a view, several strategies have been adopted to control the device performance particularly by modulating the undesired charge recombination process.49−52 To tackle this bottleneck, significant efforts have been made by various research groups.53−58 Recently, Buckley et al.59 have studied the possibility of surface passivation of CdSe QDs by chalcogenol ligands and their influences on the relaxation dynamics of the charge carriers. Their time-resolved photoluminescence studies reveal that the hole transfer occurs from QD to the ligands. The hole decay dynamics occur at the picosecond time regime, i.e., the process is ultrafast. Further, they claimed that PhSeH and PhTeH are more efficient hole acceptors than PhSH. But, the key role of the chalcogen atoms in such CT dynamics of CdSe QD−PhEH remained unexplored. Moreover, the e−h recombination process has hardly been discussed therein. To grasp the full understanding of the underlying processes, further time-domain simulation is required, which can provide valuable guidelines to the experimentalists in material design for solar cell applications. In view of this, we herein performed time-domain density functional theory calculations along with nonadiabatic molecular dynamics (NAMD) simulations on CdSe QD−PhEH composite systems to understand the role of linker atoms on the hole transfer and e−h recombination dynamics.
Figure 1. Schematic diagram of the energy levels involved in the charge separation dynamics at the CdSe−PhEH composite. Absorption of photon hv by CdSe leads to e−h separation. After excitation, CdSe induces hole transfer to PhEH, path 1. Following the charge transfer, weakly bound electron and hole can recombine inside the material (path 2) or at the interface (path 3).
Following the CT event, the electron and hole can recombine at the interface (path 3) or can recombine inside the material (path 2). Herein, we will provide atomistic insights into the event of hole transfer and charge recombination dynamics at the interface. Particularly, our major focus would be to unveil the role of the nature of the passivating ligands in the recombination dynamics. 3.1. Geometry and Electronic Structure. The initial structures of the Cd33Se33 QD ligated with chalcogenol PhEH were first optimized at 0 K, and then, the systems were heated to 300 K during 5 ps using uniform velocity rescaling. The resultant trajectories for the NA electron vibrational dynamics were generated in the microcanonical ensemble. The electronic time step for the NA dynamics was 1.0 fs. Figure 2 (top) shows the geometries of the systems, relaxed at 0 K, whereas the bottom panel is a snapshot from MD simulation at an ambient
2. THEORETICAL METHODOLOGIES The photoinduced charge transfer (CT) and recombination dynamics at the CdSe−PhEH interface were studied by using self-consistent charge density functional tight binding (SCC− DFTB) theory-based nonadiabatic (NA) molecular dynamics (MD) simulation method.60 The method adopts fewest switches surface hopping (FSSH) 61,62 technique to NAMD63,64 in the Kohn−Sham representation of DFT and employs classical path approximation,65,66 thereby reducing the computational cost significantly. Within this NAMD methodology, the electronic motions are guided by quantum mechanical principles, while the nuclear motions are determined by applying (semi)classical treatment. The nuclear trajectories are evolved under the guidance of quantum forces provided by the electronic subsystem, whereas the electronic subsystems are evolved by the direct influence of external classical force, which is generated by the nuclear displacement. Noteworthy, the FSSH61,62 approach was already implemented within PYXAID, the Python extension for Ab Initio Dynamics simulation package,65,66 which, however, has been recently interfaced with the SCC−DFTB method.60 The performance and accuracy of the method have also been tested with a broad range of systems of larger dimensions, and the results corroborate well with the time-resolved experiments performed on similar systems.17,67−73 The methodological details are given in the Supporting Information (SI).
Figure 2. Optimized geometries of CdSe−PhEH systems at 0 K (top) and during molecular dynamics simulation at 300 K (bottom). The white, black, light golden, yellow, brown, and dark brown balls represent H, C, Cd, S, Se, and Te atoms, respectively. B
DOI: 10.1021/acs.chemmater.9b00605 Chem. Mater. XXXX, XXX, XXX−XXX
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Figure 3. (a) Projected densities of states (PDOS) of the interacting CdSe and PhEH subsystems. CdSe has higher DOS profile than PhEH ligand. (b) Charge densities of the donor (left) and acceptor orbital (middle) for the hole transfer process. The vertical arrows connecting parts (a) and (b) relate the key orbital image to energies. The hole donor state for each system is delocalized between the two components, while acceptor state is completely localized on PhEH species. The charge densities of band edge states are solely localized on QD for LUMO (right) and ligand for HOMO (middle) states, respectively, for all systems. These states are considered for the simulation of backward e−h recombination.
Figure 4. Charge densities of the delocalized hole donor orbital(s) for CdSe−PhEH composite systems. HOMO-3 is the hole donor delocalized orbital of CdSe−PhSH composite, while HOMO-1 and HOMO-2 (CdSe−PhSeH), and HOMO-1 and HOMO-2 (CdSe−PhTeH) are the corresponding orbitals. For Se- and Te-containing ligand systems, there remain more than one delocalized states, indicating that some fraction of charge has already been transferred to the ligands containing Se and Te during the photoexcitation. This explains why the hole transfer is faster for Se- and Te- than for S-containing ligand.
temperature of 300 K. Several possible orientations have been considered for covalent attachment of the chalcogenol ligands in the surface of CdSe QD. A comparative study between the systems at 0 and 300 K reveals that thermal fluctuations affect the geometries although the structure of QD changes little as the temperature increases because the QD atoms are coordinated multiple times. At both temperatures, the chalcogenols form covalent bonds with QD. The most stable geometry corresponds to covalent bonding between surface Cd with E (S, S, and Te) of chalcogenol, indicating strong interactions between them. The bond lengths of Cd−E (S, Te) are relatively longer while comparing to that with E = Se. This is also true for geometry at ambient temperature. The chalcogenol ligand makes a tilt angle when it binds with the QD. At room temperature, this angle undergoes fluctuation from the initial geometry, and it is worth mentioning that this angle fluctuation is the highest for tellurol. In Figure 3a, we have shown the projected density of states (PDOS) of the combined systems along with the contribution from CdSe (black line) and the PhEH (red line) under investigation at 0 K. In PDOS, the LUMO of the combined CdSe−PhEH system is due to QD, while the HOMO has contribution from the passivated organic linker. The DOS
contribution from QD is higher than the PhEH ligand. The strength of the donor−acceptor coupling is directly controlled by the extent of mixing between the donor and acceptor orbitals. It is quite obvious that the stronger is the electronic interaction, the more is the mixing between the donor and acceptor states. Figure 3b displays the isosurfaces plots of the charge density distributions for the donor and acceptor states, which are involved in the HT process. The QD states in valence band (VB) are the donor part for the charge separation. It is revealed that, for all of the three systems studied here, the donor states are delocalized over the whole ligated QD but with different extent (Figure 4). Now, for effective charge separation, the charge carriers must overcome the Coulomb attraction, namely, exciton binding energy. The reported exciton binding energies of CdSe QDs range from 0.05 to 0.2 eV.74 The computed thermodynamic driving force for hole transfer ranges from 0.17 to 0.25 eV for the studied chalcogens, which are quite larger than the reported exciton binding energy of the QD. Therefore, one may assume that photoinduced hole transfer from QD to the ligand is feasible for these QD−ligand composites. 3.2. Vibrational Motions Contributing to the Recombination Dynamics. The time evolutions of VB states of C
DOI: 10.1021/acs.chemmater.9b00605 Chem. Mater. XXXX, XXX, XXX−XXX
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greater than Se, and even Te, which shows an immense effect on the NA coupling value, as shown in Table 1. 3.3. Photoinduced HT Dynamics. As already mentioned, the HT from the CdSe to ligand takes place through NA transition between the corresponding donor and acceptor states, as shown in Figures 3 and 4. The time-dependent population decay of the donor states for the HT process is illustrated in Figure 6a, and the corresponding data are presented in Table 1. The dynamics does not follow a purely exponential pathway but with a notable Gaussian component at the initial times. Thus, we fitted the data with a combined Gaussian and exponential function: y = A exp(−t/τexp) + (1 − A) exp(−0.5[t/τgau]2). The time scales were then obtained as the weighted average, τ = Aτexp + (1 − A)τgau. Note that, shorttime quantum dynamics always follows a Gaussian (or cosine) curve because at the initial times, the population is transferred from the initial state to one or few strongly coupled states, following a two-state Rabi oscillation. At a later time, the exponential decaying nature is reached when quantum dynamics involve multiple states. The photoinduced HT from the QD to the PhEH ligand occurs on time scales 39 (E = S), 28 (E = Se), and 21 ps (E = Te), as illustrated in Figure 6a. These simulated times are in excellent agreement (40 ps for PhSH,
Meanwhile, selenol ligand has intermediate HT time than tellurol and thiol because (i) phonon modes involved here is greater than that of S but less than that of Te, (ii) high NA coupling value compared to Te, and (iii) the least dephasing time compared to other two chalcogens. On the other hand, for thiol-passivated QD, the hole decays at a considerably lower rate compared to other ligands. It is primarily due to the fact that (i) there is only one mixing state of initial hole donor, whereas for other partners, there remain more than one mixing states, (ii) lesser extent of phonon involvement in hole transfer, (iii) moderate dephasing time, and (iv) lower coupling value. Finally, the exciton relaxation dynamics of the CdSe−PhEH system was analyzed in terms of quantum chemical parameters. After photoexcitation of the composite, there remains a hole inside the CdSe moiety. Then, the hole migrates to the suitably aligned PhEH systems. Alternatively, one can say that the electron migrates from the PhEH ligands to QD. Hence, it is the charge migration ability that would determine the feasibility of the HT event. From separate quantum chemical calculation, it is found that ionization potential, electron affinity, electronegativity, and global hardness of the PhSH species are the highest among the studied ligands; the details can be found in the SI (Table S1). Presumably, that is why the PhSH species is found to be less prone to charge migration. 3.6. Electron−Hole Recombination. Nonradiative process like e−h recombination is the main source of charge and energy loss in photovoltaic devices. The return of the electron from the QD to the surface-passivated ligand occurs via a transition from the bottom of the conduction band of the QD to the ligand HOMO. We consider the e−h recombination that follows charge separation at the ligand−QD interface and compare the interfacial recombination by changing the chalcogen atom of the ligand. The time evolution of the excited-state populations is presented for the composite systems in Figure 8a. Remarkably, for all of the composites, the time scales for recombination are quite longer and appear to occur at nanosecond time regime. Such slowing down of the recombination event is always desirable to improve the device performance.76,79,80 The NA coupling depends on nuclear velocity that correlates with nuclear fluctuation. Thus, further investigation was done for the thermal fluctuation of the chalcogen atoms in the chalcogenol ligands by calculating canonically averaged standard deviation of atomic position
Table 2. Standard Deviation in Å of the Chalcogen Atom in the Ligand of CdSe−PhEH (E = S, Se, and Te) Systems chalcogen atom
standard deviation (Å)
S Se Te
0.412 0.265 0.351
Te > Se. A lower value of standard deviation reflects weaker NA coupling. For thiol composite, the exciton recombines at a relatively faster time scale followed by tellurol. This feature can be explained on the basis of phonon mode involvement between band edge states and NA coupling between them. For thiol, the electron and hole couple to several phonon vibrations, extending up to nearly 500 cm−1, as illustrated in Figure 8b. But in the case of the other two, such vibration mode participation is of lesser extent. In particular, Te composite involves higher-frequency phonon modes around 180 cm−1, whereas Se mainly couples with low-frequency phonon modes (50 cm−1). Noteworthy, for these composites, only a single phonon peak at 180 cm−1 is responsible for such e−h recombination dynamics. Why the e−h recombination decays at a higher rate for thiol-stabilizing QD interface can be rationalized on the basis of following: (i) relatively longer coherence at the phase (see Figure 9a), (ii) greater extent of phonon mode participation, and (iii) stronger NA coupling (Figure 9b) in band edge facilitates recombination of the exciton. Due to moderate dephasing time (4.65 fs), an involvement of higher-frequency phonon modes, and comparable coupling value with S, the Te-containing composite recombines at an average rate (2.8 ns). The time scale for Secontaining composite show a lower recombination rate than the other chalcogenols. This feature can be explained on the basis that this system sustains for a shorter time in phase (3 fs) at an interface, having the lowest coupling value (see Figure 9b) and low-frequency phonon mode participation than thiol and tellurol ligands.
4. CONCLUSIONS In summary, we have investigated photoinduced CT dynamics at the CdSe−PhEH interface by applying recently developed methodologies combining NAMD with SCC−DFTB. Upon photoexcitation, HT from CdSe to organic chalcogenol ligand occurs in picosecond time scale, which excellently matches
using the equation σi = ⟨( ri ⃗ − ⟨ ri ⃗⟩)2 ⟩ , where ri ⃗ stands for the location of atom i at time t along the 5 ps MD trajectories and the angular bracket represents canonical averaging. The F
DOI: 10.1021/acs.chemmater.9b00605 Chem. Mater. XXXX, XXX, XXX−XXX
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Figure 9. (a) Dephasing functions for backward e−h recombination at the CdSe−PhEH interface. Longer-lived coherence at the phase leads to rapid recombination of the exciton for thiol ligand. (b) Average rms value of NAC for e−h recombination at the CdSe−PhEH interface. For the backward recombination process, the coupling among the states are higher for S ligands and hence the exciton quenches rapidly at the interface. The black, red, and blue lines correspond to the decay curve of S, Se, and Te.
*E-mail:
[email protected] (P.S.).
with recent experimental findings. Moreover, the present simulation rationalizes why the charge transfer phenomenon occurs at the ultrafast time regime that has recently been observed experimentally. The time scale for HT among the linker is found to follow the order PhSH > PhSeH > PhTeH. Alternatively, we proposed that with a decrease in electronegativity of the linker atom, the hole extraction becomes faster. The faster hole injection can be attributed to the following facts: (i) initial donor state is significantly more delocalized, (ii) there is participation of a wide range of phonon modes, (iii) higher NAC value, and (iv) longer-lived quantum coherence. Thus, we conclude that it is only the linker atom that plays a crucial role and is responsible for modulating the hole transfer phenomenon. Furthermore, for the first time, we propose how the linker atom can influence the e−h recombination dynamics, one of the major bottlenecks for future device performance. Our simulations reveal that e−h recombination decays at a higher rate for thiol-stabilizing QD interface, which can be rationalized by the longer coherence at the phase as well as the involvement of broad range of phonon modes. In contrast, for higher chalcogens with lower electronegativity (Se, Te), the exciton relaxes relatively at lower rate. As a consequence, we claim that the linker atom greatly affects the recombination dynamics and slows down the recombination process. Therefore, we prescribe to use less electronegative atom for binding the QD surface to achieve undesirable voltage loss during recombination. The present simulations provide a comprehensive and atomistic description of the CT dynamics at the CdSe QD ligated with chalcogenol hybrid and suggest a protocol to control unwanted recombination to design novel photovoltaic devices.
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ORCID
Sougata Pal: 0000-0002-1514-2728 Pranab Sarkar: 0000-0003-0109-6748 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from DST Nano-Mission Project (SR/NM/NS-1005/2016(G)). M.K. acknowledges CSIR for her Senior Research Fellowship (SRF).
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.9b00605.
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REFERENCES
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Details of the theoretical methods and computational details for calculating vertical electron affinity and ionization potential and their numerical values (Table S1) (PDF)
AUTHOR INFORMATION
Corresponding Authors
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DOI: 10.1021/acs.chemmater.9b00605 Chem. Mater. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.chemmater.9b00605 Chem. Mater. XXXX, XXX, XXX−XXX
