Unique Metal Dicorrole Dyes with Excellent Photoelectronic Properties

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Unique Metal Di-Corrole Dyes with Excellent Photoelectronic Properties for Solar Cells: Insight from Density Functional Calculations Chun Zhu, Jinxia Liang, and Zexing Cao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp403793f • Publication Date (Web): 11 Jun 2013 Downloaded from http://pubs.acs.org on June 18, 2013

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The Journal of Physical Chemistry

Unique Metal Di-Corrole Dyes with Excellent Photoelectronic Properties for Solar Cells: Insight from Density Functional Calculations Chun Zhu1,2, Jinxia Liang1 and Zexing Cao1* 1

State Key Laboratory of Physical Chemistry of Solid Surfaces and Fujian Provincial Key

Laboratory of Theoretical and Computational Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 360015, China 2

School of Chemistry and Chemical Engineering, Guizhou University, Guizhou 550025, China

KEYWORDS : Dye-Sensitized Solar Cells (DSCs), Unique Metal Di-Corrole Dyes, Electron Injection Mechanism, Density Functional Calculations. ABSTRACT: A new type of metal di-corrole dyes has been designed and their optical and electronic properties have been characterized by density functional calculations. These novel dicorrole-based sensitizers have a strong light harvesting ability and an excellent charge separation in the excited states. Their optical and electronic properties can be well modulated by incorporating the bridge-conjugated group. Introduction of the electron-withdrawing substituent to the meso position of corrole ring regulates the energy levels of key molecular orbitals in the metal-di-corrole dyes to facilitate the regeneration of the oxidized dyes. Calculations show that

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the metal di-corrole sensitizers are quite promising for fabrication of the high performance dyesensitized solar cells. Based on the first-principles calculations, plausible mechanisms for direct and indirect electron injections from the adsorbed dye to TiO2 have been discussed.

1. Introduction Dye -sensitized solar cells (DSCs), as an inexpensive and environmentally friendly alternative to the conventional silicon-based photoelectric conversion device, have attracted a considerable attention since O’Regan and Grätzel reported their groundbreaking work in 1991.1 Upon absorption of solar light, the dye in DSCs is promoted to an electronically excited state, which injects electrons to the conduction band of semiconductor such as TiO2. The newly-formed oxidized dyes can be rapidly regenerated back to their ground states by the electron donation from a redox mediator, such as I-/I3- redox couple, in the electrolyte solution. Subsequent reduction of the oxidized mediator at the counter-electrode completes the photoelectric conversion process. Over the past decades, huge research efforts have been made to improve the power conversion efficiency (PCE) of DSCs, both theoretically and experimentally,2-27 including seeking new and efficient dyes,2-15,23 optimization of the semiconductor structure,16-19,24 adjustment of the electrolyte composition,20-22,25 as well as the utilization of different photoactive working electrodes and counter electrodes.26 Among these factors accounting for the performance improvement of DSCs, the dyes as the photosensitizers with wide absorption band and high absorption efficiency play an important role. Based on the crucial requirements and the essential characteristics for photosensitizers in DSCs, a large number of different photosensitizers including metal complexes, ruthenium (Ru(II)) complexes, prophyrins, phthalocyanines and


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metal-free organic dyes have been designed and applied to DSCs in the past decades.28 Among the synthesized dyes, the highly efficient light-harvesting ruthenium sensitizers showed quite promising photovoltaic performance,29-31 and the power conversion efficiency is higher than 11% for the heteroleptic ruthenium29,30 dye incorporating an electron-rich hexylthio-terminal chain in combination with iodide/triiodide redox couple.30 The porphyrin dyes are one of the earliest and frequently-studied sensitizers because of their important role in natural photosynthesis. More recently, the porphyrin-based solar cells in conjunction with the cobalt-based redox electrolyte have reached the new PCE record of 12.3% under simulated air mass 1.5 global sunlight.2 Similarly, the corrole27,32 dyes, as the ringcontracted analogues of porphyrins, exhibit novel photophysical properties, lower oxidation potentials, larger Stokes shift, and relatively more intense absorption in the red light region, compared to porphyrins.33 However, to the best of our knowledge, the corrole-based dyes have received little attention in DSCs, and in only a piece of previous work, the corrole-sensitized TiO2 solar cells were found to have relatively low PCEs.34 With the aim of improving the power conversion efficiency of the corrole-based sensitizers the corrole-sensitized TiO2 solar cells and exploring their potential applications to DSCs, a novel type of corrole-based dyes with the unique di-corrole structure serving as the antenna to harvest solar energy have been constructed theoretically, and their optical properties and interactions with TiO2 have been explored. Based on the extensive calculations, effects of the structural modification on spectroscopic properties and charge separation behaviors of these new corrole dyes and their adsorbed TiO2 systems under light irradiation were discussed, and plausible electron injection mechanisms have been suggested.


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2. Computational Details Structural optimizations and subsequent frequency calculations for the isolated dyes were performed using the B3LYP35,36 hybrid functional in combination with the all-electron 6-31G(d) basis set37 for all atoms implemented in Gaussian 09 package.38 Bulk solvent effects were evaluated by using the continuum solvation model of SMD.39 Acetonitrile was considered as solvent in analogy with the general experimental medium.34 Vertical excitation energies were computed by means of time-dependent density functional theory (TD-DFT) with CAMB3LYP40,41 exchange-correlation functional based on the optimized structures by B3LYP.23 The two-dimensional slab model of TiO2 nanostructure was obtained by appropriately cutting the most stable anatase TiO2 (101) surface to model the TiO2 nanoparticle in DSCs. Calculations of the structural and electronic properties for the periodic systems with and without the adsorbed sensitizer were performed by using the plane-wave technique implemented in Vienna ab initio simulation package (VASP).42,43 The generalized gradient approximation (GGA) with the PBE functional44 was employed to describe the exchange-correction potential in all calculations. The projector augmented wave (PAW) method45 was used to describe the electron-ion interaction and the cutoff energy was set to 380 eV. All atomic positions were optimized by the conjugated gradient method with the converging tolerance of 0.02 eV/Å for the force on all atoms. The 2D periodic boundary conditions are considered along the growth direction of the slab TiO2 nanostructure. A vacuum distance >8 Å was set to keep the negligible interaction between layer nanostructures in the adjacent cells. Monkhost pack meshes of Gamma point were used for the geometry optimization. The 2×2×1 and 11×11×1 Г-centered k-point meshes were used in the density of states (DOS) integral for the periodic systems with and without the adsorbed dye,


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respectively. The Brillouin zone was sampled by 44 k-point and 20 k-point by using the linemode to calculate their electronic band structures. Here the electron injection times (τ in fs) from the dye to the semiconductor (TiO2) were estimated by using a model derived from the Newns-Anderson approach,46-49 as follows:

τ = C/∆ where C is 658 (fs meV) in convenient numerical units and ∆ (meV) is the energetic broadening of the donor orbital of the dye upon adsorption.

3. Results and Discussion 3.1. Electronic and optical properties of free metal-corrole dyes 3.1.1 Metal-mono-corrole dyes The structural and electronic properties of metal-corrole dyes with relatively low PCEs34 were calculated at first. Optimized geometries and the selected molecular orbitals were shown in Figure 1 and the predicted UV-vis spectra in the gas phase and in acetonitrile were shown in Figure S1 (Supporting Information). As these spectra and orbitals indicate, although they have strong absorptions in the visible region, their highest-occupied molecular orbital (HOMO) and the lowest-unoccupied molecular orbital (LUMO) are delocalized over the whole dyes, and thus the charge is less separated. Moreover, even if the mono-corrole dyes have been optimized by incorporating different π bridge-conjugated groups (See Figure S2 in Supporting Information) to enhance the molar extinction coefficient and broaden the spectral region of absorption, the charge separation still has not been achieved. Presumably, the charge recombination, the injected


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electron directly moving back into the dye, should be facile in these systems, which may account for the low PCEs of those corrole-based dye DSCs observed experimentally.34 Alternatively, the coexistence of injecting and non-injecting states of dyes are also responsible for the low PCEs as shown in previous studies.50

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Scheme 1. Unique metal di-corrole dyes consisting of the metal di-corrole (donor), different πspace bridge-conjugated groups, and the cyanoacrylate group (acceptor)

Architecture screening of metal di-corrole dyes. As Scheme 1 shows, the phenylamino moiety with substitution of Ga-corrole for hydrogen behaves as the electron donor, and the cyanoacry acid behaves as the electron acceptor. In order to seek the dyes with the remarkable charge separation, the various di-corrole dye isomers arising from the C-N bond couplings of different β-carbon atoms in corrole with the bridging nitrogen were screened, and the selected frontier molecular orbitals are presented in Figure S3 (Supporting Information). Among these isomers, the dyes of 2-8, 2-10 and 11-11, here the numbers referring to the serial numbers of beta-carbon atoms as shown in Scheme 1, show significant charge separation between HOMO and LUMO as displayed in Figure 2. In particular, the predicted UV-vis spectra of these metal di-corrole-based dyes have two quasi-porphyrin characteristic absorption bands in visible region corresponding to the Soret and Q bands, respectively, as shown in Figure 3 and Table S1 (Supporting Information). Furthermore, there are stronger absorptions in acetonitrile solution with respect to


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the gas phase. Accordingly, these newly-designed dyes are most likely to significantly improve the performance of corrole-base sensitizers in DSCs, owing to their excellent photoelectronic properties.

Figure 2. Selected HOMO and LUMO orbitals of 2-8, 2-10 and 11-11 dyes.


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Figure 3. Calculated absorption spectra of 2-8, 2-10 and 11-11 dyes in the gas phase and in acetonitrile solution. Effects of structural modification. Based on these screened architectures, the dyes were further modified by embedding various π bridge-conjugated51,52 groups Bn (n=1-5) of typical thiophene52 as shown in Scheme 1. As a result, the absorptions in visible region of these dyes, taking 2-10-B1, 2-10-B2 and 2-10-B3 dyes as examples, are remarkably enhanced, broadened, and red-shifted as shown in Figure 4 and Table S2 (Supporting Information), compared to their counterparts without the bridge-conjugated group. These dyes with the bridge-conjugated group have two very strong quasi-porphyrin characteristic absorption bands in visible region with peaks around 400 nm and 600 nm, corresponding to the Soret and Q bands, respectively, and such strong adsorptions can be notably enhanced in acetonitrile solution as found in the dyes without the bridge-conjugated group. Obviously, these spectroscopic features are very favorable for the dyes to harvest the solar energy in practical application of DSCs. We note that their frontier


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molecular orbitals of HOMO and LUMO with the π-orbital character dominate the electronic transition, and they are shown in Figure 5 and Figure S4 (Supporting Information).

Figure 4. Calculated absorption spectra of 2-10-B1, 2-10-B2 and 2-10-B3 in the gas phase and in acetonitrile solution.

Figure 5. Optimized structures and selected molecular orbitals of 2-10-B1 in the gas phase and in solution.


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As Figure 5 shows, the HOMO and LUMO orbitals of 2-10-B1 are basically distributed over the two corrole rings and the bridge-anchor moiety, respectively. Such well-localized characteristics of frontier orbitals in the metal di-corrole dye can be ascribed to the delocalization enhancement arising from the ring linkage through the nitrogen atom and its deviation from the bridge–anchor conjugated plane, compared to mono-corrole dyes. Presumably, the HOMOLUMO electronic excitation may lead to highly efficient charge separation in these metal dicorrole dyes, which may facilitate the interfacial electron injection from the dye to TiO2. The molecular orbital composition analyses of HOMO and LUMO in 2-10-Bn (n=1-5) structures reveal that HOMO orbitals are basically composed of Donor (D) and π-Bridge (π), while LUMO orbitals are mainly contributed by π-Bridge (π) and Acceptor (A) as shown in Table 1. Such local Table 1. Selected molecular orbital compositions (%) in 2-10-B1, 2-10-B2, 2-10-B3, 2-10-B4 and 2-10-B5 dyes with the donor(D)-bridge(B)-acceptor(A) structure.

Contributions (%) Dye

MO D

B

A

HOMO

67.02

31.05

1.93

LUMO

6.65

55.94

37.41

HOMO

55.39

43.58

1.03

LUMO

1.29

70.03

28.68

HOMO

60.83

38.18

0.99

LUMO

1.41

68.2

30.39

HOMO

68.56

31.23

0.21

LUMO

6.22

53.29

40.49

HOMO

68.08

31.35

0.57

LUMO

1.35

63.32

35.33

2-10-B1

2-10-B2

2-10-B3

2-10-B4

2-10-B5


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distributions of frontier orbitals account for the charge separation in the excited state of metal dicorrole dyes. Clearly, the excess charges at the acceptor group (the bridge and anchor moiety) of these dyes in the excited states will lead to a strong electronic coupling between dye and TiO2 surface and thus enhance the electron injection efficiency. Figure 6 shows the variation of the total electronic densities between the ground state and the first excited state of 2-10-B1. As Figure 6 indicates, the electron densities are significantly depleted at the two corrole rings of dye while there are strikingly increase at the cyanoacrylic acid and the linker group, resulting in the remarkable charge transfers as the strong electronic transition occurs under the optical excitation.

Figure 6. Calculated electronic transitions and electron density differences between the first excited state and the ground state of 2-10-B1 in acetonitrile (yellow and blue refer to an decrease and an increase of electron density, respectively; isovalue: 0.00002 au).


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Another crucial factor to improve the PCE of DSCs is to enhance short-circuit current density (JSC). JSC is mainly determined by the light harvesting efficiency (LHE) and the electron injection efficiency (Φinject) closely associated with the driving force (∆G inject) of the electron injection. Here LHE and ∆G inject of selected dyes have been computed by the relationship: LHE=1-10-f,53 where f is the absorbance of the dye at the maximum wavelength, and Katoh’ equation,54 respectively. The predicted orbital energies, transition energies, and factors accounting for the performance improvement of DSCs are compiled into Table 2. Table 2. HOMO and LUMO energies (eV), transition energies (λ) and corresponding oscillator strengths (f), the light harvesting efficiency (LHE), and the driving force (∆Ginject) for 2-10, 2-10B1 and C6F5-2-10-B1 dyes

species

EHOMO

ELUMO

λ(nm)

Excited state

f

LHE

∆Ginject (eV)

2-10

-5.58

-2.89

514

231→232

0.2299

0.41

-0.83

2-10-B1

-4.74

-2.67

542

259→261

0.4382

0.64

-1.55

524

259→260

0.5387

453

259→260

0.9506

407

259→261

0.3586

559

419→420

0.5408

0.71

-1.13

529

419→421

0.4350

446

419→421

0.9959

420

419→420

0.6567

C6F5-2-10-B1

-5.09

-2.88


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As Table 2 shows, the dye with the bridge-conjugated group exhibits relatively larger LHE as compared to its counterpart without the bridge-conjugated group, indicating that the introduction of bridge-conjugated group will enhance the light absorption to some extent. It is interestingly to note that the introduction of the bridge-conjugated group can increase ∆G inject of dye significantly. Accordingly, such structural modifications can improve the light harvesting ability of the dye and the electron injection efficient for DSCs. In comparison with the bottom of conduction band of TiO2 (-4.0 eV vs. vacumm),55 the computed LUMO energies of these dyes are all more positive (see Table S2 in Supporting Information), ensuring an effective electron injection from the excited dyes to TiO2. While the computed HOMO energies are slightly more positive than the potential of I-/I-3 redox couple (4.8eV vs. vacuum) as well as CoIII(dbbip)23+/CoII(dbbip)22+ redox couple (-4.86 vs. vacuum).56,57 We note that the modified dye molecules by introducing electron-withdrawing groups58, such as the pentafluoride-phenyl group, onto the meso-positions of corrole framework (C6F5-2-10-B1 in Figure S5 in Supporting Information), not only have larger LHE, but also have more lower HOMO energies, compared to the corresponding unsubstituted dye molecules as shown in Table 2. Accordingly, the fluorine substituents here might facilitate regeneration of the oxidized dyes and thus ensure the high performance of the metal di-corrole dyes for DSCs. 3.2 Electronic and optical properties of the dye-TiO2 system Adsorption of dye on the TiO2 surface. In order to evaluate the performance of these dyes adsorbed on TiO2 substrate for DSCs, the further investigation on the interaction between the novel sensitizer and the TiO2 nanoparticle is highly required. Here the most probably adsorption mode, the bridged-bidentate coordination with the deprotonated form for its cyanoacrylic acid


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archoring group, has been explored according to previous investigations.59,60 Figure 7a shows the optimized structure of the 2-10-B1 adsorbed TiO2 system, where the dye molecule is bidentatively bound to the TiO2 surface through the interactions between its two oxygen atoms with two titanium atoms on the TiO2 (101) surface in a chelating configuration, and the proton is transferred to the oxygen atom on the surface of TiO2 near the adsorbed position. There are no significant variations on the internal geometrical parameters of 2-10-B1 dye, except for the anchoring group before and after adsorption on the surface of TiO2, revealing the overall rigidity of the backbone of the dye. The average distances of Ti−O (2.19 Å) between the carboxylic oxygen atoms and the Ti atoms on the (101) surface of TiO2 in the chelate configuration are comparable to the Ti−O distance (1.973 Å) in the bulk TiO2, showing the strong interaction between the 2-10-B1 dye and the TiO2 surface.

Figure 7. (a) Optimized structure of the 2-10-B1-TiO2 system with the dye 2-10-B1 adsorbed on the TiO2 (101) surface; (b) The total density of states (DOS) of bare TiO2 (on the top) and the total DOS and partial density of states (PDOS) of 2-10-B1-TiO2 system (at the bottom).


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Energy band structure and density of states. To have a further insight into the electronic coupling strength between the dye and TiO2 semiconductor, the density of states (DOS) of the bare TiO2, the total DOS and the 2-10-B1 partial DOS (PDOS) of the dye-TiO2 system are computed and provided in Figure 7b on the basis of the optimized structure. As Figure 8 shows, in comparison with the DOS of bare TiO2, the 2-10-B1 adsorption introduces some new πoccupied levels on the top of the TiO2 valence band, which are corresponding to the linear band structures, illustrating that these new levels are entirely composed of the molecular orbitals of dye. We note that the bare TiO2 model has the broad surface valence bands and conduction bands with a relatively large band gap (2.64 eV by PBE and 3.86 eV by HSE06) and the PDOS of dye has a strong overlap with the valence bands and the higher energy conduction bands of the TiO2 semiconductor over a wide range of energies. The new π-occupied levels from the dye become the top of the valence band for the dye-TiO2 system. Considering that the pure functional GGA-PBE generally underestimates the band gap remarkably, the hybrid functional HSE06, which can predict relatively accurate band gap,61,62 has been considered in calculation. The increment of 1.22 eV is incorporated into the underestimated gaps (see Figure S6 in Supporting Information). Accordingly, the corrected band gap of the dye-TiO2 system is 1.37 eV.


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Figure 8. The band structures and the density of states of the 2-10-B1-TiO2 system (on the top) and the selected occupied molecular orbitals of 2-10-B1 in the ground state (at the bottom). Electron injection mechanism. Further comparison of the partial charge densities at Gamma (G) point of bands of the dye-TiO2 system and the molecular orbitals of the free dye, as shown in Figure 8, shows that the partial charge density of the highest valence band 1184 at G point of the 2-10-B1/TiO2 system is very similar to the HOMO orbital, suggesting that the band mainly


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comes from the contribution of the HOMO of pure 2-10-B1 dye. Similarly, the partial charge densities of bands 1183, 1182, 1181, 1180 and 1179 correspond to the HOMO-1, HOMO-2, HOMO-3, HOMO-4, and HOMO-5 molecular orbitals of the free dye, respectively. Contrary to the valence bands, the lowest unoccupied band 1185 of the 2-10-B1/TiO2 system is basically composed of Ti(3d) of the substrate TiO2 conduction band. Therefore, one of the most possibly injection processes arises from the direct photoexcited electron transfer from HOMO in the ground-state dye into the conduction band if this direct electronic excitation is accessible, and this electron injection is denoted as Path I in Figure 9.

Figure 9. Direct and indirect electron injections from the dye into TiO2. However, significant contributions of dye to higher energy levels of conduction band have also been found. For example, the dye shares the charge densities of 1419, 1431 and 1434 bands by 55%, 68% and 59%, respectively, suggesting that the indirect electron injection from dye to TiO2 is also possible. As Path II in Figure 9 shows, in the indirect injection process, the dye is


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initially promoted to its low-lying excited states under optical excitation, and followed by the decay of the excited dye to its S1 state, the electron of the dye can be injected into the conduction band of TiO2. Here the injection times from the unique metal di-corrole dye to TiO2, as an important parameter of electron transfer efficiency, have been estimated by a simple model derived from Newns-Anderdon approach, which has been successfully used to evaluate the electron injection from the N3 dye to TiO2 in previous studies.49 The predicted injection times are about 13 fs for the metal-di-corrole dye (2-10-B1) in the S1 state, indicating that the novel corrole dye has very high electron transfer efficiency and the indirect injection mechanism (Path II) also plays an important role in DSCs.

4. Conclusion The newly-constructed dyes with the unique di-corrole architecture have strong light harvesting ability in broad visible region and their excited states show excellent charge separation features. Calculations show that the electronic and photoelectronic properties of these metal di-corrole dyes can well be modulated by incorporating different bridge groups. Introduction of electron-withdrawing groups onto the meso-positions of corrole ring can modulate the energy levels of key molecular orbitals of the metal di-corrole dye to better match various redox couples, improving the performance of sensitizer for DSCs. The DSC device fabricated with these dyes is expected to have high efficiency. On the basis of extensive firstprinciples calculations on the dye-TiO2 system, possible mechanisms for direct and indirect electron injections were proposed. In the indirect process, the dye is initially promoted to the excited state with the charge separation under light excitation of relatively short wavelength region, followed by the electron transfer from the excited dye to TiO2, and such electron


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injection process is predicted to be very fast. On the contrary, the direct electron injection from dye to TiO2 may occur under optical excitation in relatively long wavelength region. The present theoretical studies open a new avenue to design the metal di-corrole-based dye for DSCs. ASSOCIATED CONTENT Supporting Information. Predicted electronic spectra, HOMO and LUMO orbitals, and band structures of selected systems. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * Phone: +86−592−2186081 E-mail address: [email protected] ACKNOWLEDGMENT This work was supported by the National Science Foundation of China (Grant No. 21133007), the Ministry of Science and Technology (Grant Nos. 2011CB808504 and 2012CB214900) and Science Foundation of Guizhou Province (No. [2012]2151). REFERENCES (1)

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Unique Metal Dicorrole Dyes with Excellent Photoelectronic Properties

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