How the Connectivity of Methoxy Substituents Influences the

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C: Energy Conversion and Storage; Energy and Charge Transport

How the Connectivity of Methoxy Substituents Influences the Photovoltaic Properties of Dissymmetric Core Materials: A Theoretical Study on FDT Tian Liu, Kuangshi Sun, Rongxing He, Zemin Zhang, Wei Shen, and Ming Li J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 05 Apr 2018 Downloaded from http://pubs.acs.org on April 5, 2018

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The Journal of Physical Chemistry

How the Connectivity of Methoxy Substituents Influences the Photovoltaic Properties of Dissymmetric Core Materials: A Theoretical Study on FDT Tian Liu, Kuangshi Sun, Rongxing He, Zemin Zhang, Wei Shen and Ming Li* Key Laboratory of Luminescence and Real-Time Analytical Chemistry (Southwest University), Ministry of Education, College of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, China

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ABSTRACT: There are a few reports that the optoelectronic properties of the methoxyanilinebased hole transporting materials are intimately correlated with the positions of –OMe substituents. For digging this phenomenon deeply, we theoretically design five new hole transporting materials based on 2′,7′-bis(bis(4-methoxyphenyl)amino)spiro[cyclopenta[2,1b:3,4-b′]dithiophene-4,9′-fluorene] (FDT), just a promising one, through altering the positions of –OMe substituents. Then the electronic structures, optical properties, and hole transporting properties are investigated at the molecular level via density functional theory and Marcus theory coupled with Einstein relation. The calculated results reveal that the derivatives with o-OMe or m-OMe substituent exhibit lower HOMO levels, favoring higher open-circuit voltages. Most importantly, benifited from more order and compact intermolecular stacking, the derivatives with o-OMe substituent (F1, F3) as HTMs exhibit relatively decent hole mobilities (F1: 6.29×10-2 cm2 V-1 s-1；F3: 2.49×10-3 cm2 V-1 s-1), which are two or three orders of magnitude higher than that of FDT. Quantum chemistry calculation and crystal packing arrangement simulation indicate that –OMe substituents at different positions show disparate orientations and thus affect the molecular stacking. Our work reiterates the importance of molecular configuration for the materials properties and provides ones who are engaged in upgrading the performances of hole transporting materials a new train of thought and tactics with ease and economy.


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1. Introduction Because of their straightforward synthesis procedure, inexpensive production cost and remarkably high power conversion efficiency (PCE), organic-inorganic halide perovskite solar cells (PSCs) have aroused considerable attention1-5. One of the most important constituents of PSCs is the solid-state hole transporting material (HTM)6-8, whose development contributes PSCs to show a striking speed of evolution through enhancing the open-circuit voltage, suppressing the charge recombination, and maintaining the long-term stability. There are various kinds of new HTMs having been synthesized and applied in PSCs9. Among them, methoxyaniline-based organic small molecules with three dimensional structure have been confirmed as the promising hole conductors for the well-behaved perovskite devices. For

instance,

2,2',7,7'-tetrakis(N,N'-p-dimethoxy-phenylamino)-9,9'-spirobifluorene

(Spiro-

OMeTAD)10, 4,4'-spirobi[cyclopenta[2,1-b:3,4-b']dithiophene] derivative (Spiro-CPDT)11, and 3,4-ethylene-dioxythiophene (H101)12 are employed in the devices with efficiencies in the range of 10~20%. To date, the amorphous Spiro-OMeTAD is the most prevalent HTM for PSCs, which has been widely exploited due to excellent film-forming performance, desirable molecular stability, and weak absorption in the visible light region13-15. However, the arduous synthesis approach, complicated purification procedure, and low hole mobility dramatically refrain from its large-scale commercial applications16. Hence, it is of significant priority to engineer the suitable and high-performance HTM as an appealing alternative for Spiro-OMeTAD. A large turnout of researchers devoted to exploring the molecular structure-property relationship for the advancement of HTMs in PSCs17-21. Hammett et al. firstly exported that two opposite electronic effects depend on the relative position of methoxy group (−OMe) substituents


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on the aromatic ring, electron-donating effect for para (p) position and electron-withdrawing effect for meta (m) position22. Aside from such electronic effects, the steric hindrance caused by ortho (o) substitution also has a profound impact on the photovoltaic performances of HTMs. The discovery has stimulated a series of successful explorations. For instance, Seok et al.23 pointed out that changing the substitution position of the −OMe substituents in Spiro-OMeTAD is a simple and feasible avenue to fine-tune the optoelectronic properties. In addition, the device employing the ortho-substituted derivatives showed better performances than those using the para- and meta-substituted derivatives. The seminal conclusion has been consolidated by Li and coworkers24, whose work demonstrated the hole mobilities from those adopting the orthosubstitution or the mixed ortho- and para-substitution derivatives are approximately two or three orders of magnitude higher. Nevertheless, the clear physical picture standing behind this phenomenon is absent. We infer that there may exist an underlying effect for the intermolecular packing patterns of methoxyaniline-based HTMs which are associated with the –OMe substituents of different positions adopting the disparate spatial orientations. Moreover, altering the connectivity of –OMe substituents has an impact on the electronic structures of molecules. To confirm our surmise, an inspection is imperative over the individual molecule and its solid assemble at the molecular level. However, every molecule investigated in the past has a symmetric core (i.e., the two halves of the core are identical) and thus possess four bis(methoxyphenyl)amine substituents; it is difficult to grasp the structure-property relationship when four of them are involved. Therefore, we need a new illustrative example which is easy to understand. Very recently, a novel spiro-type HTM (FDT) has been investigated for PSCs application with an impressive PCE of 20.2%25. The photoelectric device that uses FDT as HTM compares


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favorably with those using Spiro-OMeTAD. Moreover, in contrast to traditional HTM SpiroOMeTAD, FDT bears many distinct merits including facile fabrication, excellent dissolved ability in environmentally friendly solvent, and high photovoltaic performance. In addition, unlike above-mentioned HTMs, FDT has a dissymmetrical core incorporating fluorene unit and dithiophene unit, which renders the number of bis(methoxyphenyl)amine substituents of a molecule decreasing to two. It dramatically reduces the difficulty of analysis and enables us studying the effect of the connectivity of methoxy substituents on the molecular packing. From the crystallographic analysis in the experimental literature, the existence of dithiophene aroused extremely short S⋯S contacts between the neighboring columns, which facilitates FDT to interact with the perovskite in a dual-channel fashion26-27. Given that the way -OMe substituents distributed can exert a great impact on the photophysical properties of methoxyaniline-based HTMs, it is mystical and fascinating how the positions of –OMe substituents at phenyl units affect the special interaction (S⋯S contacts), the packing motif, hole transporting capability and photovoltaic performance of FDT. Therefore, investigating it could fill the blank of exploring in great depth the effect of altering the connectivity of –OMe substituents, and has the practical significance of tapping the potential of such promising candidate for HTMs. Herein, we gradually changed the positions of -OMe substituents on the phenyl rings from para to ortho and meta to design five derivatives of FDT, which is depicted and termed F1, F2, F3, F4, and F5 (Fig. 1 and Fig. S1). And then, based on the above fact that altering the connectivity of –OMe substituents can impose great influence on the optoelectronic properties of HTMs, we intend to investigate what effect the different positions of –OMe substituents of FDT would have on their electronic structures, optical absorptions, and hole transporting properties. In the present work, the mysteries of S⋯S contacts and steric effects in FDT and its derivatives


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were distinctly demonstrated at the molecular level by using the density functional theory (DFT) and Monte Carlo simulation method combined with Marcus charge transfer theory28 and Einstein relation29. It is expected that the result can function as a rational strategy in designing excellent HTMs for high performance PSCs.

2. Computational details A series of hybrid functionals containing BMK, B3LYP, B3P86, PBE0 were employed to optimize the ground-state structure of FDT molecule in dichloromethane solvent. The results show that the molecule calculated at DFT level with BMK30 functional and standard basis set 631G(d, p)31 can present the most accurate description of the neutral state (see Table S1). The excitation energies and corresponding oscillator strengths were predicted via the time-dependent density functional theory (TD-DFT) at B1LYP32/6-31G(d, p) level, then the absorption spectra were simulated. The TD-DFT calculation performed in chlorobenzene solvent because the experimental UV-Vis spectra were measured in it. The solvent effect was also taken into consideration through the conductor-like polarizable continuum model (C-PCM)26. The optimized structures show no imaginary frequency, which means the optimized structures are in minimum-energy points. All the calculations were implemented using Gaussian 09 software package33. Considering that there is van der waals interaction between adjacent molecules within the context of organic crystals, we utilized the thermally activated hopping and diffusion model to describe the hole transporting behavior34. In terms of semiclassical Marcus theory, the charge hopping rate (k) can be expressed as35:


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k =

4π h

2

V

2

 λ  exp −  4π λ k BT  4 k BT  1

where kB represents the Boltzmann constant, h is the Planck constant, T denotes the temperature in Kelvin, V is the electronic coupling, and λ signals the reorganization energy, respectively. A negligible reorganization energy and a sizable electronic coupling, in turn, ultimately achieving a decent carrier mobility. The electronic coupling denotes the strength of interaction between adjacent molecules in the crystal structure, which determined by the relative arrangement of the molecule in the solid state. The Marcus-Hush model was carried out to derive electronic coupling (V), which can be described as:

V =

J RP − S RP ( H RR + H PP ) 2 2 1 − S RP

where JRP denotes charge transfer integrals, SRP is spatial overlap, and HRR and HPP are site energies regarding monomer 1 and monomer 2. Only the internal reorganization energy was considered in this work. The internal reorganization energy (λ) was calculated from the adiabatic potential energy surface approach, as exhibited in the below equation36-37:

λ = λ0 + λ+ − = ( E0* − E0 ) + ( E+* − − E+ − ) where E0* and E±*, respectively, signify the total energies of the neutral molecule in the optimized charged geometry and the charged molecule in the optimized neutral geometry. While E0 and E±, respectively, denote the total energies of neutral and charged species in their optimized geometries.


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Supposing that without the applied electric field, charge carrier presents a Brownian motion, the carrier mobility (µ) can be evaluated from the diffusion coefficient (D) with the Einstein equation38.

µ =

eD k BT

where e denotes the charge and D is the diffusion coefficient, which can be approximately calculated by39:

D =

1 2d

∑

ri 2 k i P i

i

where i represents a definite transfer pathway and ri denotes the charge hopping centroid-tocentroid distance, ki is the charge hopping rate in the ith pathway, d is 3 owing to the charge carrier diffusion direct to three dimensions in all hopping pathways, and Pi is the relative probability for charge hopping for all pathways to the ith pathway, which can be obtained by:

Pi =

ki

∑

ki

i

The polymorph module in the Materials Studio (MS)40 was carried out to perform crystal structures prediction for all investigated molecules41. The PBE functional was utilized to derive the electrostatic potential charges of all atoms, while a single molecular structure was optimized by means of DMol3 module. Sokolov et al.42 have validated that the Dreiding force field for molecular crystal prediction is considerably reliable and feasible. As a consequence, the crystal structure prediction was performed employing this force field. The polymorph predictor calculations were only applicable to the five most possible space groups, P21/C, P1, P212121,


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P21, and P1ത43. According to the total energies of obtained crystal structures, we selected those with the lowest energies. And then, all the possible charge transfer pathways were defined within the stable crystal structure. The intermolecular electronic couplings of dimers were calculated with the PW9144 functional and Slater-type triple-ζ plus polarization (TZP) basis set in ADF45 package46. Finally, the First principles calculations combined with Marcus theory were implemented to obtain the hole mobilities of all molecules.

3. Results and discussion 3.1 Geometry and frontier molecular orbital As far as an individual molecule is concerned, the dihedral angle can reflect important structure information. It could dramatically influence the conjugation extent of hole transporting materials. The specific information of dihedral angles between OMe-phenyl groups and the fluorene ring is displayed in Table 1 and Fig. S2. Apparently, the stretch of the m-OMe substituent in space repels the ortho-hydrogen atom in adjacent phenyl ring, and even much more intensive is the repulsion when the –OMe substituent is at the ortho-position. This repulsion makes the two OMe-phenyls connected in the same nitrogen atom prone to be perpendicular to each other, so the dihedral angle is in the following order: F1>F2>FDT or F3>F4>FDT, showing considerable changes with varing the positions of -OMe substituents. Universally, the smaller dihedral angle contributes to the more delocalized π-conjugation, ensuring the lower energy gap in the course of electron excitation. Herein, the spectral absorption of the o-OMe and m-OMe derivatives may blue shift due to bigger torsion, which implies little absorption in the visible light region. As a result, these derivatives will not contend with the light absorber for solar radiation, which is beneficial for the light absorber. Besides, a bigger energy gap is usually


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accompanied with a deeper highest occupied molecular orbital (HOMO) level. Accordingly, FDT will afford higher open-circuit voltages in PSCs if the connectivity of –OMe substituents is changed. The frontier molecular orbitals (FMOs) including the HOMO and the lowest unoccupied molecular orbital (LUMO), can be served as the key factors to illustrate the carrier-transport properties47. Specifically, the wide delocalization of HOMO can attenuate the intra-molecular reorganization and enhance the electronic coupling of adjacent molecules, and thus greatly contributes to the favorable hole transporting behaviors. The HOMOs and the LUMOs for all investigated molecules are depicted in Fig. 2. The pictorial representation of one electron HOMO and LUMO wave functions of all molecules bear significant similarities at first glance and are of apparent π character, that is to say, altering the connectivity of –OMe substituents do not change the wave function spatial distribution in the molecular backbone obviously. What is more, the HOMOs of all molecules are delocalized on the whole molecular skeleton, which means that the positively charged polarons of the fully charge-separated states could be well delocalized across the conjugated backbones in these molecules. On the contrary, the LUMOs of all molecules nearly spread over the conjugated fluorine-dithiophene core. As previously mentioned, the more delocalized HOMOs than LUMOs render these materials have desirable hole-transport properties. To large extent, the open-circuit voltage of a photoelectric device has a dependence on the difference between the HOMO level of the HTM and the quasi-Fermi level of TiO248. In general, a suitable HOMO level of HTM, which is higher than the top of the valence band of the frequently used CH3NH3PbI3 (-5.43 eV), is required to yield a high open-circuit voltage, and hence an impressive PCE for PSC device. It signifies the desirable hole injection ability from the perovskites to HTMs for PSCs. In present work, the calculated HOMO level for FDT of -5.14 eV


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at BMK/6-31G(d, p) level, which coincides well with the experimental value of -5.16 eV25. This evidently justifies the feasibility of our predicting method. The HOMO levels of all investigated molecules are shown in Fig. 3 and Table S2. As expected, the HOMO levels of the FDT derivatives are observed substantially varying with the -OMe positions. For instance, compared with FDT, the m-OMe derivatives (such as F2, F4) possess the lower HOMO levels. This is mainly due to the electron-withdrawing effect of meta substitution, as reported by Hammett and coworkers22. The more sophisticated case comes to the o-OMe derivatives (such as F1, F3). Contrary to the meta substitution, substitution at the ortho affords an electron-releasing capability by resonance stabilization, which is expected to increase the FMO energies. Nevertheless, steric hindrance arising from the repulsion between o-OMe and ortho-hydrogen atoms of phenyl impairs the conjugation of the molecule, allowing the HOMO level decreased. The slightly lower HOMO levels of o-OMe derivatives relative to that of FDT are compromises between these two effects. The electronic and configuration effect being entangled with and in checks and balances end up leading the HOMO levels of all newly designed molecules to be deeper than that of FDT, ensuring that these materials can each provide higher open-circuit voltage. In addition, the HOMO levels of HTMs also reflect the ability of hole extraction from perovskites49-50. The molecular HOMO levels are well matched with the valence bands of frequently used material CH3NH3PbI3 (-5.43 eV)51, which guarantees the effective hole extraction from the perovskites. Meanwhile, the crucial element to avoid the backflow of electrons is the rational LUMO of HTM, which can be evaluated by the equation, ELUMO = EHOMO + Eg. It is worth noticing that the calculated LUMO levels of all materials are higher than the conduction bands of CH3NH3PbI3 (−3.93 eV), indicating the sufficient capability of hindering the flow of electrons from perovskite to metal electrodes. In this context, changing the


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position of –OMe substituents could contribute FDT to more suitable energy level alignment and achieving a higher open-circuit voltage. 3.2 Absorption spectrum and the Stokes shift We sought to explore the impact of altering the connectivity of -OMe substituents on the optical properties of FDT. To this end, TD-DFT theory calculations were performed on the basis of the ground-state structures optimized in chlorobenzene at B1LYP/6-31G(d, p) level, attaining the UV-vis absorption and corresponding optical data. The absorption spectra of all studied molecules are depicted in Fig. 4, while the specific spectroscopic parameters and Stokes shifts are given in Table 2. As shown in Fig. 4, all investigated materials share common spectra comprising of a strong absorption band at long wavelength and a weak absorption band at short wavelength. Furthermore, because all the maximum absorbance arose from the high energy transition HOMO→LUMO+1, the absorption hardly takes place within the visible region. From the perspective of spectroscopic data in Table 2, the maximum absorption wavelengths (λabs) are 390.77nm, 378.03nm, 382.87nm, 363.99nm, 377.19nm, 372.62nm for FDT, F1, F2, F3, F4, F5, respectively. It follows the sequence of FDT > F2 > F1 > F4 > F5 > F3. In this respect, the λabs of new designed molecules, relative to that of FDT, distinctly indicates the existence of a hypochromatic shift trend. Hence, the o-Me derivatives (such as F1, F3), m-OMe derivatives (such as F2, F4) and the both mixed derivatives (such as F5) as HTMs will impose little influence on the light harvesting of perovskites, and are expected to be more preferable for PSCs. The molecular rationale behind this phenomenon could be attributed to the decreased conjugation of molecules as the position changes, which corroborates the previous geometric and FMO analysis.


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Stokes shift is a physical quantity which reflects the geometrical difference between the ground state and the excited state, and thus provides a figure of merit for molecular structure flexibility12. It can be evaluated by the difference between the maximum absorption and emission wavelength, the employed expression is Stokes shift = λem - λabs. As is listed in Table 2, the Stokes shifts follow this trend of F3 > F1 > F5 > FDT > F2 > F4. It was reported that the substantial molecular flexibility may be beneficial for the optimal infiltration in the pore of the semiconductor in PSCs52. As far as this important property is concerned, the o-OMe derivatives (such as F1, F3) and the mixed m-OMe and o-OMe derivatives (such as F5) obtain larger Stokes shifts, guaranteeing greater flexibility, and thereby become the suitable candidates to upgrade the pore-filling performance of PSCs. The reason, we infer, is altering the connectivity of –OMe substituents decreases the conjugation and thereof the molecular rigidity of FDT. In this regard, altering the connectivity of –OMe substituents could facilitate the optical properties of FDT for high performance PSCs. 3.3 Electron density difference and exciton binding energy In order to gain a deep insight into the property of intra-molecular excitation, the charge density difference (CDD) analysis between the excited state and the ground state for all molecules were performed at TD-B1LYP/6-31G(d, p) level. It proved to be capable of providing the important information about spatial location of the excitons and the probability of these excitons escaping from the Coulomb attraction53. The corresponding information, including electron transfer distance (L), the amount of transferred charge (∆e) and CDD map are presented in Table S3. In CDD map, the cyan represents the decrease of electron density while the purple represents the increase of electron density. Accordingly, all molecules are of similar charge transfer character: the electrons are transferred from the bis(methoxyphenyl)amine moieties to


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the central cyclopentadithiophene (CPDT) unit upon S0→S1 transition. The regions where the electron densities increase are exactly the places the LUMOs distribute within the charge transfer (CT) process. Besides, the relative data is analyzed to fully interpret the circumstance of charge transfer. In terms of conjugated small molecules, the more charge involved in the intramolecular charge transfer means the more charge separation; the longer electron transfer distance favors the separation of the exciton at the interface. The order of electron transfer distance is F1 (3.79Å) > F5 (3.65Å) > F3 (3.62Å) > F2 (3.53Å) > F4 (3.48Å) > FDT (3.44Å). It is remarkable that the oOMe (F1, F3), m-OMe (F2, F4) as well as the both mixed (F5) derivatives obtain relatively considerable values. However, there is only a trifle discrepancy in the transferred charges between all molecules. These values are 0.97|e-|, 0.97|e-|, 0.97|e-|, 0.97|e-|, 0.96|e-|, 0.96|e-| for FDT, F1, F2, F3, F4 and F5, respectively. To evaluate their excitation properties thoroughly, we should introduce one more physical parameter. The exciton binding energy (Eb) is the crucial parameter to access the properties concerning electron-hole pair escape from the Coulomb well, which should be small enough so that excitons could overcome the Coulomb attraction between hole and electron into separation. It can be estimated by the energy difference between the neutral singlet exciton and the two free charge carriers54. The formula can be expressed as Eb = Eg - Ex = ∆EH-L - E1, where Eg is the electronic energy gap and can be replaced by the energy gap (∆EH-L) and Ex is the optical band gap (Table S4) and generally viewed as the first singlet excitation energy (E1)55. In this work, E1 is calculated based on the optimized geometry of the S1 state by TD-DFT at B1LYP/6-31G(d, p) level, while ∆EH-L is evaluated from the S0 state optimized geometry. Table 3 lists the values of E1 and Eb. The calculated exciton binding energies are 0.47 eV, 0.53 eV, 0.38 eV, 0.56 eV, 0.31eV and 0.38 eV for FDT, F1, F2, F3, F4 and F5, respectively, and in the trend of F3 > F1 >


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FDT> F5= F2> F4. It can be derived that the m-OMe derivatives (F2, F4) and the mixed m-OMe and o-OMe derivative (F5) display lower values, potentially contributing to the electron-hole dissociation. These findings confirm that altering the connectivity of –OMe substituents is roughly conducive to the formation and separation of excitons in FDT. 3.4 Reorganization energy and hole mobility Entering the mobility formula, two physical quantities play pivotal roles in determing the hole mobility. One of them is reorganization energy (ߣ), which comprise of two contributors: the internal-sphere part (ߣ௜௡ ) and the external-sphere part (ߣ௘௫ ). ߣ௜௡ is the total energy associated with geometry changes of the donor and the acceptor monomer during the transfer process. The ߣ௘௫ involves the energy that occurs to the surrounding polarization. For most organic crystals, the ߣ௘௫ is much more small to ignore, consequently, only the internal reorganization energy will be considered herein56. The ߣ௜௡ is usually predicted in two ways: (1) the adiabatic potential energy surfaces method; (2) the normal-mode analysis method. Herein we arbitrarily invoke the former because Bredas et al57. previously exported that the two methods yield similar results. The reorganization energies are calculated at BMK/6-31G(d, p) level and their tendency is demonstrated in Fig. 5. Remarkably, the hole reorganization energy is much lower than electron reorganization energy, and thereby the hole transporting ability is more preferable than the electron transports ability, ensuring these materials used as HTMs more efficiently than as electron transporting materials. It is worth noticing that all these derivatives have larger ߣ than FDT. In addition, the mixed o-OMe and m-OMe derivatives (F5) display the largest ߣ among all molecules. This is partly because altering the connectivity of –OMe substituents from para to ortho or meta decreases the molecular conjugation and rigidity. From eq 1, it is evident that


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lower reorganization energy is in favor of charge transport, thus the derivatives with m-OMe substituent will constrain the hole transporting properties of FDT. Importantly, the p-OMe substitution and o-OMe substitution are preferable to obtain a lower ߣ. Accordingly, we infer that the para and ortho substitution can promote highly efficient hole hopping in the neighbor list of molecules. Apart from the above-mentioned factor ߣ, another one influences the hole mobility is the electron coupling of adjacent molecules. The physical quantity depends on both the weighted overlap between the orbitals participating in the charge transfer reaction and the intermolecular orientation within a given macroscopic assembly56. It means that the crystal structure information is indispensable to evaluate the electron couplings of adjacent monomers accurately. Like many small molecular organic semiconductors, FDT can self-assemble into the ordered and regular crystalline solid. One can certainly assume that F1~F5 each can also pack into a periodical supermolecular morphology. Herein, computational protocol verified by Li et al.58 represents a powerful tool that allows us to achieve the precise structural prediction of molecular crystals and inspect, at the molecular level, the physical rationale behind the transport properties. Following this protocol, the molecular crystal structures are predicted; the corresponding lattice parameters are collected in the Table S5. From crystal structure prediction, dimers extracted as the hopping pathways for charge transport are defined and the main hopping pathways are exhibited in Fig. 6. The predicted crystal structure of FDT shared consistent space group of P1 with the experimental one, and the lattice parameters also do so25. This validates that the calculation method is reasonable. Among all of the compounds of interest, only the dimers in the unit cells of F1 or F5 exhibit a compact stacking pattern, a phenomenon which could be understood in


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terms of molecular configuration. With the Fig. S3 in supporting information, one can see that when the –OMe substituent, of phenyl groups residing individual molecule, located in one side of the plane of the fluorene core, the molecule tends to adopt π-π stacking mode within the packing configuration. This is the case for molecules F1, F3, and F5, while vice versa for F2, F4, and FDT. Each of the first three molecules enjoys a more favorable packing mode within the unit cell, and the slided π-π stacking of F3 might be due to the O⋯O short contact, which is short to be 4.37 Å. As for the latter three molecules, the shared orientation of the -OMe substituent within the individual molecule may incur a steric hindrance, which prevents the corresponding dimers from being closely stacked. Generally, to pack into a compact solid assembly is difficult for the molecules whose -OMe substituent adopting the orientation toward the plane of fluorene core. It is expected that F1, F3, and F5 exhibit higher hole mobility than F2, F4, and the experimental molecule of FDT. In the case of molecular assembly, the geometrical configurations of all studied materials are mainly characterized by three packing modes: face-to-face, edge-to-face, and edge-to-edge packing. Among the main hopping pathways, the pathway 4 of FDT, pathway 5 of F1, pathway 5 of F2, pathway 5 of F3, pathway 2 of F4 and pathway 5 of F5 belong to face-to-face stacking, respectively (Fig. S4). In general, the face-to-face stacking favors the optimal orbital overlap, thus the sizable interaction may be achieved on those pathways of each molecule. The surmise is confirmed by the sequence of electronic couplings (V, eV) displayed in Table 4, which sets out all the main parameters including centroid to centroid distance of the correlated dimer (ri, Å), charge hopping rate (k, s−1), and hole mobility (µ, cm2 V−1 s−1) of the main hopping pathways. We recall that the electronic coupling of adjacent molecules exponentially decreases with the centroid to centroid distances; however, both the pathway 5 of F2 and the pathway 5 of F3 are


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not the shortest ones for hole hopping. This can be attributed to the packing modes of the shortest pathways (i.e., the pathway 4 of F2 and the pathway 4 of F3) are edge-to-face stacking, the worst packing geometry for charge transfer. The circumstance implies the greater importance of molecular orientation than mutual distance within the context of this class of molecules. Gathering all the charge hopping pathways, the hole mobilities of all molecules based on their crystal structures are derived. Tabulated in Table 4, the hole mobility is in the sequence of F1 (6.29×10-2 cm2 V−1 s−1) > F3 (2.49×10-3 cm2 V−1 s−1) > F5 (2.04×10-3 cm2 V−1 s−1) >F2 (1.99×10-3 cm2 V−1 s−1) > F4 (2.42×10-4 cm2 V−1 s−1)>FDT (5.17×10-5 cm2 V−1 s−1). Because of the more desirable orientation of -OMe substituents, F1, F3, and F5 with decent stacking configuration possess the higher hole mobilities in this six molecules as we predicted above. Evidently, the magnitude of mobility has a correlation with the S⋯S short contact. As shown in Table S6, the short S⋯S contacts render the molecules of F1, F3, and F5 to bear the relatively high hole mobilities; in the case of F2 and F4, the S⋯S contact is disrupted because of too long distance between the two sulfur atoms, and thus the slow hole transfer. It is also worth to note that FDT exhibits the smallest hole mobility among all the investigated molecules in spite of the smallest hole reorganization energy. That is to say, the packing motif plays a central role in the charge transport mechainism. And the connectivity of –OMe substituents turns out to be nonnegligible determinants for the transport performances of materials. Hence the above result indicates that FDT derivatives are promising HTM candidates for the fabrication of highly efficient PSCs.

4. Conclusion


18

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In summary, five new HTMs are designed based on FDT backbone by altering the connectivity of –OMe substituents, whose influences on the frontier molecular orbitals, optical properties, excitation properties, and charge transport properties are theoretically investigated by means of DFT, TDDFT, and Marcus theory. Benefited from accurate quantum chemistry calculation, these properties of molecules have been demonstrated clearly at the molecular level. We can see that, on the one hand, the joint efforts of electronic effect and molecular configuration lead to the change of molecular electronic structures because of the positions changed. This renders the derivatives of FDT possess more appropriate energy alignment and better optical properties. Specially, FDT derivatives as HTMs could contribute to higher open circuit voltage and pore-fill performance. On the other hand, –OMe substituents have better spatial orientation after their positions were changed, and thus the corresponding crystal structures stack toward more regular and tight packing. The much higher mobilities are obtained finally in spite of higher reorganization energies, an abnormal case which highlights the role of molecular geometry and reflects the great potential of altering the substitution position on improving the material properties. Our work sheds some light on the structure-property relationship in this class of molecules and points out the importance of a simple and unremarkable pathway to upgrade the photovoltaic performance of materials. Although so many researches validate that diverse functionalization is a critical pathway to promote the photovoltaic performances of materials, functionalization site is a not less important aspect than the species of functional groups. We hope that this research could stimulate the further advance in PSCs field. ASSOCIATED CONTENT


19


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Supporting Information Electron density difference plots, S⋯S contact distances, information of crystal structures and molecular structures, and additional data and figures. Corresponding Author *E-mail: [email protected] ACKNOWLEDGMENT We acknowledge generous financial support from Natural Science Foundation of China (91741105,

21173169),

Chongqing

Municipal

Natural

Science

Foundation

(cstc2013jcyjA90015). Project supported by Program for Innovation Team Building at Institutions of Higher Education in Chongqing (CXTDX201601011). REFERENCES 1.

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Table 1 Dihedral angles between OMe-phenyl groups and the fluorene ring for all molecules. Compounds

ߙ (degree)

ߚ (degree)

Average(degree)

FDT

66.47

68.29

67.38

F1

62.71

77.10

69.90

F2

61.66

75.27

68.50

F3

68.70

71.60

70.15

F4

67.07

69.6

68.34

F5

60.50

79.75

70.13


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Table 2 Optical properties of the simulated molecules calculated at the B1LYP/6-31G(d, p) method for six molecules. The absorption maxima λabs and the emission wavelengths λem based on s0 and s1 states along with the Stokes shifts. Absorption

Emission

Compounds

λabs (nm)

Assignments

λem (nm)

Stokes shift (nm)

FDT

390.77

H→L (99.34%)

493.26

102.49

F1

378.03

H→L (99.22%)

494.21

116.18

F2

382.87

H→L (99.24%)

480.22

97.35

F3

363.99

H→L (99.33%)

492.87

128.88

F4

377.19

H→L (99.06%)

429.48

52.29

F5

372.62

H→L (99.12%)

480.48

107.86


30

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Table 3 Calculated the first singlet excitation energy (E1, eV) and exciton binding energy (Eb, eV) at the B1LYP/6-31G (d, p) level. Compounds

E1(eV)

Eb(eV)

FDT

2.98

0.47

F1

3.03

0.53

F2

3.14

0.38

F3

3.10

0.56

F4

3.26

0.31

F5

3.23

0.38


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Table 4 The centroid to centroid distances (ri, Å), electronic coupling (V, eV), hole hopping rate (k, s-1), and hole mobility (µ, cm2 V-1 s-1) of main hopping pathways selected based on the all molecular crystalline structures. Compounds FDT

F1

F2

F3

F4

F5

Pathways 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6

ri 14.61 16.43 10.96 10.95 12.47 9.28 16.10 18.28 18.21 6.02 14.47 10.59 19.54 9.71 21.02 11.81 12.82 9.74 11.78 14.10 8.16 16.21 22.72 16.21 11.11 11.09 11.35 12.49 12.90 11.23 17.48 9.03 16.57 17.26 8.62 7.81 17.26

V 8.59×10-6 -4.00×10-5 3.50×10-4 -6.78×10-4 2.46×10-5 -3.94×10-3 1.30×10-3 4.60×10-3 2.15×10-4 5.77×10-2 -3.13×10-3 -2.60×10-3 1.15×10-3 1.60×10-3 -4.19×10-3 1.60×10-4 -4.19×10-3 -4.36×10-3 -4.65×10-3 -3.65×10-3 8.63×10-4 5.49×10-3 -8.41×10-5 5.48×10-3 -9.93×10-4 2.52×10-3 1.61×10-3 2.40×10-3 -2.48×10-4 -9.93×10-4 -2.32×10-5 -2.86×10-3 -1.22×10-3 6.97×10-3 1.77×10-3 -7.59×10-3 6.98×10-3

k 2.03×105 4.41×106 3.38×108 1.27×109 1.67×106 2.63×1010 2.88×109 3.59×1010 7.80×107 5.64×1012 7.02×109 4.82×109 9.41×108 8.00×108 1.26×1010 1.83×107 1.26×1010 1.27×1010 1.44×1010 8.88×109 4.97×108 2.01×1010 4.72×106 2.00×1010 4.56×108 2.93×109 1.20×109 2.66×109 2.85×107 4.56×108 2.50×105 3.00×109 5.44×108 1.78×1010 1.15×109 2.12×1010 1.79×1010

µ 5.17×10-5

6.29×10-2

1.99×10-3

2.49×10-3

2.42×10-4

2.04×10-3


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Figure 1 Chemical structures of all studied molecules. r1 and r2 represent the methoxyphenyl rings with red and blue, respectively. Figure 2 Illustration of the frontier molecular orbitals for all investigated molecules at BMK/631G(d, p) level. Figure 3 Energy level diagram for the FDT and FDT derivatives calculated at the BMK/6-31G(d, p) Level. Figure 4 Calculated absorption spectra of the six molecules using the TD-DFT at B1LYP/631G(d, p) level. Figure 5 Calculated reorganization energies of all studied molecules. Figure 6 Main hole hopping pathways selected based on the experimental crystals structure of FDT and predicted crystal structures for FDT derivatives.


33


Page 34 of 40

Figure 1 Chemical structures of all studied molecules. r1 and r2 represent the methoxyphenyl rings with red and blue, respectively.


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Figure 2 Illustration of the frontier molecular orbitals for all investigated molecules at BMK/631G(d, p) level.


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Figure 3 Energy level diagram for the FDT and FDT derivatives calculated at the BMK/6-31G(d, p) Level.


36

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Figure 4 Calculated absorption spectra of the six molecules using the TD-DFT at B1LYP/631G(d, p) level.


37


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Figure 5 Calculated reorganization energies of all studied molecules.


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Figure 6 Main hole hopping pathways selected based on the experimental crystals structure of FDT and predicted crystal structures for FDT derivatives.


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TOC graphic


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How the Connectivity of Methoxy Substituents Influences the

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