Effects of Fluorination on Exciton Binding Energy and Charge

Subscriber access provided by WEBSTER UNIV

C: Energy Conversion and Storage; Energy and Charge Transport

Effects of Fluorination on Exciton Binding Energy and Charge Transport of #-Conjugated Donor Polymers and the ITIC Molecular Acceptor: a Theoretical Study Leandro Benatto, and Marlus Koehler J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b12261 • Publication Date (Web): 22 Feb 2019 Downloaded from http://pubs.acs.org on February 23, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Effects of Fluorination on Exciton Binding Energy and Charge Transport of -Conjugated Donor Polymers and the ITIC Molecular Acceptor: a Theoretical Study

Leandro Benatto,a and Marlus Koehlera

a

Departamento de Física, Universidade Federal do Paraná, C.P 19044, 81531-980, Curitiba-PR, Brazil

*Corresponding Author: Dr Leandro Benatto Email: [email protected]

1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract

The intelligent addition of fluorine atoms in the chemical structure of the conjugated polymers has been a popular approach to improve the efficiency of organic photovoltaic (OPV) devices. Recently this strategy has been extended to non-fullerene acceptor (A) molecules in the best-performing bulk heterojunction (BHJ) devices. Yet many details involved in the role of fluorination to enhance the photovoltaic response of organic semiconductors are still unclear. Here we theoretically investigate the changes in key properties of representative fluorinated oligomers of

polymers commonly used as

donors (D) in BHJ-based OPVs. We then extend our analysis to consider the fluorination of ITIC, a very promising non-fullerene acceptor. We focus on the variation of the exciton binding energy (Eb) with the fluorination of the oligomer (molecule). Our calculations indicate that the fluorine substitution tends to lower the exciton binding energy which can enhance charge generation after light absorption. Considering complexes of two oligomers (molecules), we also investigate the effects of fluorination on charge transport. We found that the intermolecular binding energy is considerably higher for oligomers (molecules) with fluorine atoms. The increased electronic coupling tends to induce a better packing along the π−π direction which can explain the differences observed in the morphology of thin solid films. Calculation of the hole mobility for the oligomers (and electronic coupling for the acceptor molecules) showed higher values with the fluorination. Our results are consistent with the space charge-limited current (SCLC) measurements performed in fluorinated conjugated materials and highlight the main reasons behind the better performance of fluorinated BHJ devices.


Page 2 of 38

Page 3 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. Introduction Organic semiconductors (OS) are under intense research in recent years as potential materials for applications in optoelectronic devices like organic light emitting diodes (OLEDs), organic photovoltaic devices (OPVs) and in organic field effect transistors (OFETs).1,2 In order to improve the performance of those devices, there have been remarkable advances in the processes and new synthesis strategies of OS.3–5 One of those design strategies is the intelligent addition of fluorine in the chemical structure of the chain.6–8 Indeed most best-performing OS have fluorine atoms in its structure.9 In particular, it has been observed that modifications in the design of the organic semiconductors are fundamental to enhance the charge generation of OPVs (that are continuously breaking records of efficiency).10–12 In this field the fluorination of the electron donor material in bulk-heterojunction (BHJ) solar cells can lead to a considerable improvement of the efficiency13 and different physical mechanisms have been proposed to explain this phenomena. For example, it has been suggested that the fluorination increases the electron affinity (EA) of the polymer without adding very large side groups to its chemical structure. 14 The absence of those groups can improve the intermolecular packaging which favors the electronic transport.15,16 In addition, fluorination can increase the ionization potential (IP) of the donor material leading to larger open circuit voltages (VOC) in BHJ devices.17 Other factors commonly invoked to explain the improved performance of OPVs devices fabricated with F-substituted polymers as donors species are (i) higher polymer backbone planarity which enhances the charge carrier mobility,18 and (ii) impacts on the active layer morphology like higher crystallinity,16 enhanced domain purity and reduced domain size.19 Because fluorine is the most electronegative chemical element, there is an increase in the electronic polarization of materials when F atoms chemically substitutes H atoms in specific positions of the chain. As a consequence, the fluorination of the backbone usually tends to increase the electric dipole moment of the polymer. In line with those considerations, Density Functional Theory (DFT) calculations performed on oligomers of some representative polymers showed that the dipole moment of the ground state (μg), the dipole moment of the excited state (μe) and the change between them (Δμge) increase with the chain fluorination.20,21 This effect may have an impact on 3 ACS Paragon Plus Environment


the performance of OPVs since higher values of Δμge have been related to a greater intramolecular charge separation of the electron and hole induced after light absorption.12,22 The associated reduction of the electrostatic interaction would then decrease the exciton binding energy (Eb) which would facilitate the charge dissociation.23,24 The introduction of F along the chain would then improve the charge generation in the active layer of solar cells. Another significant change observed in conjugated polymers with the fluorine substitution is the greater packaging of the chains which may result in increased hole mobility.6 It is believed that the fluorine atom can make noncovalent attractive interactions between the hydrogen or sulfur atoms (F···H or F···S), which may contribute to the chain planarization and therefore the crystallinity.25 Indeed DFT calculations of the potential energy surface as a function of the rotation between chemical groups in oligomers with F indicates that the energy minimum tends to angles closer to 0 o and 180o compared to the oligomers without F.26,17 Despite those advances, many key aspects that link the fluorination of the chain to the physical-chemical processes involved in the OPV performance are still elusive and demand further experimental and theoretical investigations.13 In particular, a systematic theoretical study that investigates the effects of the active layer fluorination on the exciton binding energy are still lacking. This is an important factor that can strongly impact the performance of those photovoltaic devices. Herein we theoretically study (using DFT calculations) the variations in the electronic properties of three pairs of oligomers when there are fluorine substitutions in specific positions of the molecules. Those oligomers are representative of donoracceptor (D-A) copolymers in which the F atoms were inserted in units of benzothiadiazole (an electron-acceptor moiety)20,27,28 or thiophene (an electron-donor moiety)18,29. Polymers with those chemical structures have been applied in OPVs using fullerene as electron acceptor and an increase of the photovoltaic response was obtained after the fluorination of the chain.21,30 Nevertheless the advances in the field are not restricted to the donor species of the BHJ layer. In recent years, high efficiency values were obtained for OPVs using a fluorinated non-fullerene electron acceptor.11,9 Motivated by those results, we extended our study to investigate the influence of fluorination of the ITIC molecule. This 4 ACS Paragon Plus Environment

Page 4 of 38

Page 5 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


material has been extensively explored in the literature of OPVs 31–33 as a highly efficient non-fullerene electron acceptor.34–37 In the case of this molecule, 4 fluorine atoms substitutes 4 hydrogen atoms in two opposite aromatic rings at the ITIC molecule’s edge.8,36,38 This chemical substitution gives rise to the ITIC-4f molecule. We focus our analysis on the variation of the exciton binding energy 39 upon the Fluorine substitution. Yet, in order to further investigate other effects related to F substitution, we also considered complexes of two oligomers or molecules. Using those systems we calculated the complex’s biding energy17 and the transfer integrals (electronic coupling),40 which in combination with the reorganization energy can be applied to estimate ideal values for the charge carrier mobility.41,42 Our calculations revealed that the introduction of fluorine atoms in the chain tends to lower the exciton binding energy of the oligomers and ITIC basically because it increases the electron affinity of the fluorinated chemical species. This effect is related to the fluorine tendency to withdraw electrons from the chain which greatly affects the distribution of charge along the chain. Our results also confirm the relationship suggested in different works between high values of Δμge and lower values of Eb. The results for the complexes also showed that the intermolecular binding energy, which is related to the greater tendency for packing, is considerably higher for the oligomers of polymers with fluorine and for the ITIC-4f molecule. The higher binding energy of the complexes helps to explain the differences observed in the morphology of thin solid films and also produce a greater electronic coupling between the frontier orbitals of fluorinated chains. Those findings explain why the hole (electron) mobility is higher for polymers (acceptor molecule) with fluorine.

2. Materials and methods 2.1 Materials In this contribution we apply DFT calculations to study key properties of three pairs of oligomers representative of polymers in which the F atoms were inserted in units of benzothiadiazole or thiophene. The benzothiadiazole is an electron-acceptor (A) whereas thiophene is an electron-donor (D) moiety in D/A copolymers. 18 As can be 5 ACS Paragon Plus Environment


seen in figure 1, the first pair of oligomers is composed of 4,7-di(thiophen-2yl)benzothiadiazole (DTBT) bonded with two thiophenes (DT) for the case without fluorine and of DTffBT-DT for the case with fluorine.21 The difference of the second polymer pair is a change from group DT to naphtho[2,1-b:3,4-b0]dithiophene (NDT).43 The third pair of oligomers is composed of 4,8-bis((2-ethylhexyl)-oxy)benzo[1,2-b:4,5b′]dithiophene (PBDT) bound with a thiophene (T) for the case without fluorine and PBDT-ffT the case with fluorine.13 Polymers with these chemical structures were applied in OPVs with [6,6]-phenyl-C61-butyric acid methyl ester (PC 71BM) as electron acceptor and an efficiency increase was obtained with the insertion of F. We also investigate the influence of fluorination in a non-fullerene electron acceptor molecules.8 For this analysis we chose the ITIC molecule which has been extensively explored in the literature as highly efficient non-fullerene acceptor. 37,44,45 In this case, 4 fluorine atoms substitutes 4 H atoms in two opposite aromatic rings at the ITIC molecule’s edge, gives rise to the ITIC-4F molecule (Figure 1).9,11


Page 6 of 38

Page 7 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


DTBT-DT

DTffBT-DT

DTBT-NDT

DTffBT-NDT

PBDT-T

PBDT-ffT

ITIC

ITIC-4f

Figure 1 - Chemical structures of materials without and with fluorine.



Page 8 of 38

2.2 Computational Methods The first step was to optimize the ground state geometry of the oligomers and the molecules depicted in Figure 1. We then use two simplifications that provide a great reduction in the computational cost and allows us to increase the level of theory of the calculations. The first simplification is to replace the side chains (C 8H17 and C10H21 in the oligomers and C6H13 in the acceptors, see figure 1) for methyl groups. This has been done in many works26,46,47 and is justified by the fact that the side groups are added to improve solubilization of the main chain, and thus they do not influence the electronic properties of the oligomer. The second simplification is our choice to consider in the calculations oligomers with three mers (n=3) and to obtain the ionization potential, electron affinity and optical gap we perform a linear extrapolation of these magnitudes in relation of the reciprocal of the number of mers (1/n). 22,48 With the extrapolation, a limited number of mers taken into account allowed a comparison with some experimental results. Therefore, significant differences related to the substitution of H by F can be evidenced by DFT calculations in order to get a deeper understanding of the experimental observations.13,49 To simulate the polarization effects associated to the presence of neighboring chains in the solid state, we use the Polarizable Continuum Model (PCM)50 to consider a chain solvated by a continuous medium of its own type. The “solvent” in PCM calculation is characterized by the dielectric constant, the number of solvent molecules per unit volume and radius of the molecules. These values are obtained using the following methods: (i) the dielectric constant (ε) was estimated using the Clausius– Mossotti equation ε−1 4 π ρ = N α, ε +2 3 M A

(1)

where ρ is the density of the material, M the molecular mass, NA is the Avogadro number, and α is the electronic polarizability. (ii) ρNA/M is the reciprocal of the molecular volume. (iii) The radius of the molecule is defined as the radius of a sphere which has equivalent volume of the concerned molecule.51 This approach has been used


Page 9 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


in many theoretical works to simulate the materials in solid. 52,53 The electronic polarizability was taken from equation54 2

μ α=α ' + , 3 ε0 k B T

(2)

where α', μ, ε0, kB and T are the isotropic polarizability (at zero frequency), dipole moment, electrical permittivity of vacuum, Boltzmann constant and temperature, respectively. To obtain ε (for monomers and molecules), we applied the long range corrected (LRC) ωB97XD functional,55 which includes empirical dispersion that minimizes the delocalization error,16,42 in combination with 6-31G(d,p) basis set. The dielectric constants obtained using these procedures are in Table S1. One can see that there is a good agreement between the calculated value and the experimental results available in literature for the ITIC molecule. Since the wb97XD functional includes corrections to describe the influence of long range dispersion forces, it is also quite adequate to optimize the geometry of the complexes involving two oligomers or molecules.56 The exciton binding energy can be estimated via DFT by subtracting the fundamental energy gap (Egap) from the optical gap (Eopt).39,57 The optical gap corresponds to the energy of the lowest electronic transition accessible via absorption of a single photon (obtained from TD-DFT calculations). The fundamental gap is defined as the energy difference between the ionization potential (IP = Etotal(cation) - Etotal (neutral)) and electron affinity (EA = Etotal (neutral) - Etotal (anion)).58 The cation and anion energies from this are derived in the ground state geometry of the neutral molecule, so that the vertical IP and EA are obtained.59 In a recent work that compares a series of DFT functionals, calculations using the M0660 estimated reasonable values for the exciton binding energy (Eb) for 121 small and medium sized molecules. 58 Since Eb is a fundamental parameter for optoelectronic devices, we decided to use the M06 functional61–63 and 6-31G(d,p) basis set for calculations of IP, EA, Eopt and Eb energies (in molecular geometries obtained with ωB97XD/6-31G(d,p)). Table S2 compares the calculated IP energy (in solid with PCM) for each polymer and molecule with the experimental data. One can see that there is a good agreement between theory and



Page 10 of 38

experiment. This was already expected due to the solid state effects introduced by the PCM method.51,57 We will emphasize that the PCM method will be used only for obtaining IP, EA, Eopt and Eb. The change of the dipole moment between the fundamental and excited states for the simulated materials was obtained in gas phase and is given by Δμge = [(μgx - μex)2 + (μgy - μey)2 + (μgz - μez)2]1/2. In order to obtain the excited state dipole moment, we take the geometry of ground state and apply a TD-DFT calculation to optimize the geometry of the first excited state.12 This calculations was done with ωB97XD/6-31G(d,p) combination. To investigate the effect of fluorination in the transport properties of the materials we use the Einstein relation for hopping mobility of holes or electrons: 2

e Dh /e e L K h /e μ h/ e = = , k BT kBT

(3)

where kB is the Boltzmann constant, T is the temperature, e is the elementary charge, D is the diffusion coefficient, L is the separation between sites and Kh/e is the rate constant for charge transport between adjacent molecules.41,42 The subscript h is associated to the quantities related to the transport of holes, whereas the subscript e is associated to quantities related to the transport of electrons. kh/e can be estimated within the framework of the semi-classical Marcus/Hush theory,

k h / e=

−λh / e 4 π2 1 t 2h /e exp h √ 4 π λ h/ e k B T 4 kB T

[

]

,

(4)

where the intramolecular reorganization energy (λh/e) and the charge transfer integral (th/e) are the two key parameters that govern the behavior of the charge transfer rate.2,64,65 It is important to mention that the μh/e calculated using Eqs. (3) and (4) is an ideal mobility, where effects like energetic and positional disorder and the presence of traps are neglected for simplicity. In other words, the values obtained applying this theory represent a theoretical superior limit for μh/e.66 However this approach is useful because it can easily gauge relative variations in the transport properties associated to


Page 11 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


chemical modifications of the molecules. The intramolecular reorganization energies λh/e can be evaluated by67 ±

0

(5)

λ h/ e =( E 0 −E±)+( E±−E0 ),

where E0 (E0– ) is the energy of the cation (anion) calculated with the optimized structure of the neutral molecule; E(E – ) is the energy of the cation (anion) calculated with the optimized cation (anion) structure; E0 (E0– ) is the energy of the neutral molecule calculated in the cationic (anionic) state, and E0 is the energy of the neutral molecule at the ground state.68 We will neglect the outer component of reorganization energy that account the surrounding medium effect, λs, as a first approximation for condensed-state systems.41,69 It was seen in other works that λs is very small compared to the internal component.70,71 It was found that the B3LYP functional and 6-31G(d,p) basis set give reasonable values for the reorganization energy when compared with experimental results.72 Therefore, we used B3LYP/6-31G(d,p) to calculate λh/e,73 which also allows the comparison of our results with calculations performed for similar molecules reported in the literature.74 th/e are obtained by simulating complexes where the optimized structures of these isolated materials (obtained with ωB97XD/6-31G(d,p)) were put into the face-on configuration. A second calculation of energy minimization with the same theory level was then performed to find the complex’s most stable configuration. Due to the high computational cost involved in DFT energy minimization of complexes with a large number of atoms, we performed these calculations considering just one mer for polymers of figure 1. Using this optimized structure, the electronic coupling between HOMO orbitals |th| and between LUMO orbitals |te| are obtained from a fragment orbital analysis.40 This approach implies that the values of the mobility estimated from Eq.(3) are related to the hooping process along π-stacking direction. We will point out that for donor polymers we will calculate the quantities related to the transport of holes (μh, kh, λh and |th|) and for acceptor molecules the quantities related to the transport of electrons (μe, ke, λe and |te|). As an additional bonus, the calculations performed for the complex also give the energy difference between one isolated fragment (molecule) compared to the whole 11 ACS Paragon Plus Environment


Page 12 of 38

structure (complex). This quantity estimates then the binding energy of the complex, which is related to its stability.17 Finally we would like to mention that the choice of different DFT methods was based on guidelines given by other theoretical works in the literature. We then applied the most reliable method to calculate a specific molecular property at the lowest computational cost. In order to systematize our theoretical approach, Table S3 relates the molecular properties studied in this work with the corresponding DFT methods employed to quantified it. The GAMESS76

DFT

simulations

programs.

The

were charge

performed density

with

images

09 75

and

generated

with

Gaussian were

Avogadro77 software and the molecular orbitals images with ChemCraft. 78 The electronic coupling between molecular orbitals (MOs) was obtained with AOMix79 software for MOs analysis.

3. Results and Discussion 3.1 Fluorination of donor oligomers First we will focus our attention to study the changes in the charge density distribution (CDD) along the chain induced by the fluorination of the oligomers. In Figure 2 one can see that the higher CDD variations are concentrated in the vicinities of the chemical moiety where the F atoms are introduced. Due to the high electroaffinity of the fluorine, the carbons of benzothiadiazole in ffBT and of the thiophene in ffT groups are more positively charged compare to the BT and T groups in the molecules without F substitutions. A similar effect was observed with the fluorination of aromatic systems.80 The positive polarization of those rings helps to reduce the energy of the molecular orbitals associated to the π-system in this region. Taking into account the results of Ref.13, the strong positive polarization of the benzothiadiazole can explain the lowering of the frontier orbitals energies induced by the fluorination. Thus the physics behind those changes is essentially electrostatically driven, controlled by Coulomb interactions from local modifications of the charge density. The introduction of the fluorine atoms on the benzothiadiazole tends to locally reduce the energies of the orbitals associated to the BT group. This effect changes the hybridization as well as the 12 ACS Paragon Plus Environment

Page 13 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


orbital overlap of the chemical moieties along the chain. As a result, the LUMO of the oligomer is close to the deepest LUMO of the constituting moieties. On the other hand, there is a reduction in the hybridization magnitude between the occupied orbitals from the BT group and those occupied orbitals from other moieties in the chain 13 which shifts the HOMO level of the fluorinated oligomer to lower values. Indeed a considerable increase of the electron affinity was observed in polymers submitted to fluorination.15,16 Likewise it was also experimentally observed a decrease of the LUMO orbital19 that is also reproduced by the DFT calculations49 (see Figure S1 and Table S4 to S7 of the supplementary materials where there is a clear increase in the ionization potential and in the electron affinity with the introduction of the fluorine atom as substituent along the conjugated backbone). The results in Figure S1 indicates that the fluorine substitution (that tends to withdraw electrons from the chain) is more effective to induce modifications in the frontier orbitals levels when introduced in the electron donating moiety of the chain.

DTBT-DT

DTffBT-DT

DTBT-NDT

DTffBT-NDT

PBDT-T

PBDT-ffT

Figure 2 - Charge density distribution of monomers without and with fluorine. In red (blue) the regions with higher density of negative (positive) charges (isovalues 0.01). The fluorine atoms are indicated by arrows.



A key point to OPV applications is to determine how the fluorination of the chain modifies the exciton biding energy (Eb). As it was detailed in the previous section, Eb can be estimated from the difference between the fundamental gap (Efund) and the optical gap (Eopt) of the molecule so that Eb = Efund - Eopt. Efund is obtained from the difference between the ionization potential and electron affinity (Efund = IP - EA) and Eopt corresponds to the energy of the lowest electronic transition excited by a single photon absorption.57 In this way, the exciton binding energy can be written in the form Eb = (IP - EA) - Eopt. As a consequence, since the fluorination greatly affects the values of IP and EA, it can also significantly change the values of Eb as well. Figure 3 shows the extrapolated Efund and Eopt energies (using the PCM method) for a large number of mers48 (see Table S4 to S7 of the supplementary materials). Those values will be used to determine Eb of the oligomers. The fluorination increases both IP and EA but the variation of the electron affinity is greater. These effects lead to a decrease of the difference between IP and EA energies (Efund) with fluorination.13 Since Eopt is little affected by the introduction of F atoms compared to Efund, the final result is a decrease of the exciton binding energy. The same behavior was obtained for the materials in gas phase (see table S8). The weakening of the exciton binding energy with the fluorine substitution can have a strong influence on the photovoltaic response of those materials. Lower values of Eb are related to a greater intramolecular separation between the hole and the electron upon excitation.20,21 The higher degree of polarization of the excited state can then enhance the values of Δμge. Our results in table 1 confirm this trend since the introduction of F simultaneously reduces Eb with a corresponding increase of Δμge (details of Δμge calculation in Table S9). This result gives further support to recent works that related higher magnitudes of Δμge with an improved charge generation in OPVs.22


Page 14 of 38

Page 15 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3 - Extrapolation results of fundamental gap (Efund) and optical gap (Eopt) energies obtained with PCM for the pair of polymers without and with fluorine. Inset: double arrow represent the exciton binding energy (Eb = Efund – Eopt). Table 1 - Calculation results of extrapolated exciton binding energy (Eb), change in dipole moment (Δμge) for monomers, mean dihedral angles between adjacent chemical moieties (θ) . We consider oligomers with three mers to evaluate θ.

Polymers

Eb (eV)

Δμge

θ (deg)

DTBT-DT

0.63

0.59

28.98

DTffBT-DT

0.58

0.61

26.42

DTBT-NDT

0.60

0.60

31.12

DTffBT-NDT

0.57

0.89

23.79

PBDT-T

0.80

0.53

19.85

PBDT-ffT

0.74

0.76

4.58

Further insights on the mechanisms that influences the reduction of Eb can be obtained from a closer look to the spatial distributions of the HOMO and LUMO orbitals showed in Figure 4. First it is clear in this figure the mesomeric contribution from the F atoms to the frontier molecular orbitals. 13 For the BT-based oligomers, the HOMO is delocalized along the chain while LUMO is more localized around the BT unity. In contrast, both frontier orbitals (HOMO and LUMO) are delocalized along the 15 ACS Paragon Plus Environment


Page 16 of 38

DTBT-DT

DTffBT-DT

DTBT-NDT

DTffBT-NDT

PBDT-T

PBDT-ffT

HOMO

LUMO

HOMO

LUMO

HOMO

LUMO Figure 4 - HOMO and LUMO molecular orbitals of polymers with three mers (isovalues 0.02). The fluorine atoms are indicated by arrows.

chain for the PBDT-T and PBDT-ffT oligomers. The greater overlap between the HOMO and LUMO orbitals in those materials tends to increase Eb compared to the value calculated for the BT based oligomers. In those systems the decrease of the exciton binding energy is associated to a longer average separation of the electron-hole pair81 produced by a lower spatial overlap between the frontiers orbitals. Another factor 16 ACS Paragon Plus Environment

Page 17 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


that contributes to further decrease the exciton binding energy in the BT based oligomers is its higher dielectric constant relative to the other oligomers (see Table S1). The screening produced by an easier medium polarization helps to reduce the electronhole interaction. Although it is extremely difficult to directly measured Eb,82 there are experimental evidences that the fluorination of the chain may decrease the exciton binding energy. For instance, in Ref.30 it was suggested that charges are more efficiently separated and extracted in OPVs with PBDT-ffT in comparison with devices using PBDT-T as donor layers. By the same token, the authors of Ref. 21 observed that the bimolecular recombination is more suppressed in active layers with DTffBT-DT relative to layers with DTBT-DT. Likewise in Ref43 the influence of the polymer fluorination to reduce the exciton binding energy was inferred from the photovoltaic efficiency of the devices. The authors found that the absorption coefficient of DTBT-NDT is 11% higher than the absorption coefficient of DTffBT-NDT. However, the photocurrent (Jsc) of the device based on DTBT-NDT is only 4% greater than the Jsc generated by an equivalent device based on DTffBT-NDT. In addition, a single morphological or structural parameter was not adequate to fully explain the variations of Jsc since no difference in the blend morphology

was

observed

either

by

transmission

electron

microscopy

or

photoluminescence quenching. .This result indicates that there is a non-optical improvement of charge generation for the blend with DTffBT-NDT that compensates the weaker absorption of this polymer. An easier exciton dissociation due to the lower Eb can be a possible mechanism behind the improved charge generation of the DTffBTNDT polymer. Further experimental evidences can be found in Ref43 where it was suggested that the introduction of F would lead to a stronger internal dipole moment that weakens the Coulombic attraction between the e–h pairs after exciton splitting, leading to lower recombination rates. Indeed the oligomer of DTffBT-NDT has the highest Δμge among the donors reported in Table 1. Finally in Ref.14 the enhanced charge separation for the fluorinated polymer (named PBT-nF, n=0-3) was also explained assuming that Eb decreases with the fluorination. Our calculations in Table 1 give theoretical support to those explanations.



Another interesting point is that DFT calculations in the literature showed that the backbone tends to have a more planar conformation in fluorinated polymers.17 This effect was also observed in our results. For instance, the average dihedral angle between adjacent chemical moieties83 in Table 1 is always lower for the oligomers with fluorine substitutions. It was often observed that this property induces a stronger - stacking and higher crystallinity, usually leading to a favorable morphology for photovoltaic applications.17 Yet the reasons behind this planarization are still unclear. Some quantum chemical calculations emphasize the role of hydrogen-fluorine interactions as the main effect to lock the chain conformation into a more planar state.13 Other works called attention to the possibility that S···F and F···F interactions may favor the planarization.25 Here we note that the introduction of the fluorine atom in the conjugated ring tends to slightly shrink the single bonds between carbon atoms that link adjacent chemical moieties of the main chain. This effect tends to increase the backbone rigidity.

3.1.1 Transport Effects We study the influence of fluorination on charge carrier transport using the results calculated for complexes of two monomers, see Figure 5. Those results are summarized in Table 2 which shows the values for the main parameters that contributes to the hole transport. An important result in Table 2 is the stronger binding energy between molecules of the complexes with F substitutions. This effect confirms that fluorinated chains would tend to better pack compared to chains of materials without fluorine.17 Grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements are in agreement with those findings since they showed that chains of DTffBT-DT21 and DTffBT-NDT43 are better packed in π−π direction than DTBT-DT and DTBT-NDT, respectively.


Page 18 of 38

Page 19 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


DTBT-DT

DTffBT-DT

DTBT-NDT

DTffBT-NDT

PBDT-T

PBDT-ffT

Figure 5 - HOMO orbitals of complexes without and with fluorine (isovalues 0.01).

Many effects can contribute to favor stronger intermolecular interactions between the molecules with F. First, the higher planarity of the conjugated backbone induces stronger π-π interactions as mentioned above. Second, the permanent dipole moment of molecules with F is usually higher compared to the dipole of molecules without fluorine, which increases the intermolecular interactions due to dipole-dipole forces. Third, the higher polarizability improves the van der Walls attraction between the fluorinated molecules. In Table S1 one can indeed find that the polarizability of the molecule increases with the addition of fluorine atoms. Finally, the presence of intermolecular F···H or F···S interactions increases the binding forces of the complexes formed by molecules with fluorine.25 As a consequence, the electronic coupling (charge transfer integrals) between the molecules with F is higher. Figure 5 shows the spatial distribution of the HOMO orbital obtained for the final optimized structures of the complexes. The higher values of |th| in Table 2 corresponds to a higher degree of spatial delocalization of the HOMO orbitals in the intermolecular space that separates the fluorinated oligomers.



Page 20 of 38

Table 2 - Results of electronic coupling between HOMO orbitals |th|, intramolecular reorganization energy for holes λh, hole transfer rate kh, calculated hopping mobility of holes μh,cal and experimental hole mobility obtained with space charge-limited current (SCLC) μh,exp.

Polymers

Binding energy (eV)

|th| (meV)

λh (eV)

kh (1/s)

μh,cal μh,exp Ref. of (cm2 V-1 s-1) (cm2 V-1 s-1) μh,exp

DTBT-DT

1.63

118.3

0.28

2.9 x 1012

1.4 x 10-1

5.24 x 10-4

DTffBT-DT

1.80

133.1

0.27

4.2 x 1012

2.0 x 10-1

11.60 x 10-4

DTBT-NDT

2.27

75.3

0.27

1.3 x 1012

6.8 x 10-2

0.96 x 10-5

DTffBT-NDT

2.31

98.2

0.24

3.2 x 1012

1.6 x 10-1

4.49 x 10-5

PBDT-T

1.17

89.6

0.38

5.3 x 1011

2.7 x 10-2

1.50 x 10-4

PBDT-ffT

1.33

111.3

0.36

1.1 x 1012

5.2 x 10-2

3.10 x 10-4

[21] [43] [30]

Table 2 also shows that the intramolecular reorganization energy decrease with the introduction of F atoms in the chain. As discussed above, the introduction of the fluorine atom tends to withdraw electrons from the main chain. This effect introduces a rigidity of the polymeric backbone so that there are smaller conformational variations between the neutral and the charged states of the chain, giving rise to lower reorganization energies.64 A similar effect was observed for calculations in poly(3hexylthiophene) (P3HT) where the capacity of the substituent to enhance the rigidity of the polymer chain was associated to its capacity to withdraw charges from it.64 Using Equations (3) and (4), the lower reorganization energy and the higher values of |th| tends to increase the magnitude of the hole mobility for materials with fluorine (see Table 2). Here it is important to emphasize that the calculated values of μh in Table 2 are a superior theoretical limit for the hole mobility when effects like energetic and positional disorder or the presence of traps (that tend to decrease μh ) are completely neglected. Despite those limitations, the variations of μh upon fluorination (see Table 2) are surprisingly consistent with the changes of the hole mobility measured using space charge-limited currents (SCLC). This result indicate that the enhancement of the hole mobility observed using SCLC are mainly due to improvements in the intrinsic parameters that determine the charge transport (like |th| and λh) induced by the fluorine substitutions.


Page 21 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.2 ITIC Fluorination

The charge generation in organic photovoltaics depends on the exciton dissociation at donor/acceptor heterojunctions. With this in mind, we will study the fluorination of an acceptor molecule in order to make a relative comparison with the effects produced by the fluorination of the donor oligomers. We will focus our attention on the ITIC molecule which is very promising non-fullerene acceptor. 8 We see from Figure 6 a significantly change in CDD with ITIC fluorination. Contrary to what was observed for the oligomers (figure 3), the fluorination of end-capped 2-(3-oxo-2,3dihydroinden-1-ylidene)malononitrile (INCN)84 groups is able to influence the CDD even in regions far away from the fluorine atoms. The variation is more intense in the pentagonal ring of the INCN end-group, where the region close to the carbon linked to oxygen and nitrogen atoms shifts from positively charged (in the ITIC) to negatively charged (in the ITIC-4f ). The fluorination of the ITIC molecule decreases simultaneously the energy of the IP and the EA, the same behavior observed for the donor oligomers. This feature has important consequences for the OPVs operation since the open circuit voltage (Voc) critically depends on the energies of the effective gap across the donor/acceptor heterojunction. Since the electrochemical potential of both donor and acceptor materials have equivalent downward shifts upon fluorination, this process does not tend to significantly change the Voc of the devices with fluorinated heterojunctions.

ITIC

ITIC-4f

Figure 6 - Charge density distribution (CDD). In red (blue) the regions with higher density of negative (positive) charges (isovalues 0.01). The fluorine atoms are indicated by arrows.

The fluorination of ITIC increase IP and EA by 0.04 eV and 0.11 eV, 21 ACS Paragon Plus Environment


respectively (details in Table S4). Since the LUMO shows a higher density in regions around the INCN moiety (see Figure 7), the variation of the CDD near this group may explain the stronger increase of EA relative to the IP. In addition, there is also a small reduction of Eopt with the fluorine substitution. As a result, the exciton binding energy (Eb) decreases by 0.04 eV in the ITIC-4f relative to the ITIC, see Figure 8. Following the same trend observed for the oligomers, Δμge increases for the ITIC-4f. The weakening of Eb might explain the increase of the exciton dissociation probability from 90% to 93% measured in devices using ITIC and ITIC-4f , respectively.11 Recent studies reported polymer/indacenodithiophene blends showing efficient exciton dissociation rates85 despite the low driving forces (defined as the energy difference between the local excited state and the charge transfer state). 86 However the charge dissociation process in those system are considerably slower (15-30 ps) when compared to the dissociation at polymer/fullerene blends (sub-300 fs). 86 Due to the relative slowness of the dissociation process, the molecule at polymer/indacenodithiophene interface might have enough time to relax from the ground state (GS) geometry to the lowest excited state (ES) geometry before charge dissociation. This motivated us to obtain the exciton binding energy for the materials in the ES geometry using the formalism described in section 2. Our results are reported in SI (Tables S10 to S12 and Figure S2). They show that (on average) the Eb values decreased by ~0.05 eV when calculated at the ES geometry, keeping the fluorinated materials with the lowest values. This subtle decrease of Eb can be related to the increase of the chain planarity since the molecular structure tends to assume a quinoid form.22,87 The increase of the molecular planarity raises the electronic delocalization which contributes to decrease the exciton binding energy.22 The results showed in Figure S2 suggest that the delayed time for exciton dissociation in small driven force blends might help to further decrease Eb via relaxation of the molecular structure. Indeed a rough estimate based on the Heisenberg’s principle and using the total energy variation at the two molecular geometries (~ 0.17 and 0.3 eV for molecules and oligomers) indicates that the GS to the ES relaxation would happen on femtosecond time scale (around three orders of magnitude faster then the exciton dissociation process in those systems).86


Page 22 of 38

Page 23 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ITIC

ITIC-4f

HOMO

LUMO Figure 7 - HOMO and LUMO of acceptor molecules without and with fluorine (isovalues 0.03). The fluorine atoms are indicated by arrows.

Figure 8 - Fundamental gap (Efund) and optical gap (Eopt) for acceptor molecules with and without fluorine. Inset: double arrow represent the exciton binding energy (Eb).

It has been found that the high planarity of ITIC and ITIC-4f is due to the O···S intramolecular noncovalent interaction of the central adjacent rings. 88 Another characteristic that contributes to the planarity of those molecules can be observed in Figure 7 that shows the spatial distribution of the frontier orbitals. In this Figure the push-pull character84 of the ITIC and the ITIC-4f molecules is evidenced by the spatial concentration of those orbitals around moieties with donor and acceptor character. One can see in

Figure 7 that the HOMO is mainly located in the central

indacenodithiophene group, whereas the LUMO is mostly located near the INCN endgroup for both acceptors. This distribution of the frontier orbitals is characteristic of 23 ACS Paragon Plus Environment


Page 24 of 38

molecules with an internal charge transfer (ICT)38,89 which is a process that favors planarization.90 In addition the fluorine substitutions in the ITIC molecule are made in positions that do not affect the number of intramolecular noncovalent interactions (in contrast to the case of the oligomers). As a consequence of all those effects, the decrease of the mean dihedral angle (θ) with the fluorination of the ITIC molecule (Table 3) is much smaller compared to averaged decrease for the oligomers.

Table 3 - Calculation results of exciton binding energy (Eb), change in dipole moment (Δμge), mean dihedral angles between adjacent chemical moieties (θ) and mean distance between adjacent chemical moieties (d).

Molecules

Eb (eV)

Δμge

θ (deg)

ITIC

0.51

0.13

1.00

ITIC-4f

0.47

0.15

0.94

3.2.1 Transport Effects In this section we repeat the procedure followed above and study the influence of fluorination on charge carrier transport using the results obtained for complexes of two acceptor molecules, see Figure 9. Those results are summarized in Table 4 which shows the values for the main parameters that contributes to the electron transport. In general

those calculations revealed similar trends to the behavior found for the

oligomers. For example, the introduction of four fluorine atoms in the ITIC-4f increases the binding energy between the two molecules of the complex. The stronger intermolecular noncovalent interaction favors the formation of bigger crystalline domains. This result is in accordance with X-ray diffraction (XRD) measurements performed to investigate the influence of fluorination on the crystalline properties of the acceptors. In those measurements the ITIC-4f films showed more ordered intermolecular structure in comparison with ITIC films.11 Also, the higher binding energy between the fluorinated acceptors explains the improved thermal and illumination stabilities of the non-fullerene OPVs devices.9


Page 25 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ITIC

ITIC-4f

Figure 9 - LUMO orbitals of complexes without and with fluorine (isovalues 0.01).

Table 4 - Complex simulation results of electronic coupling between LUMO orbitals |te|, intramolecular reorganization energy for electrons λe, rate constant for transport of electrons ke, calculated hopping mobility of electrons μe,cal and the experimental electron mobility obtained with space charge-limited current (SCLC) μe,exp.

Molecules

Binding |te| Energy (eV) (meV)

ITIC

1.75

88.4

ITIC-4f

1.99

91.4

a

λe (eV)

ke (1/s)

μe,cal μe,exp Ref. of -1 -1 2 -1 -1 (cm V s ) (cm V s ) μe,exp 2

0.16 (0.15a) 6.92 x 1012 0.16

7.40 x 10

12

3.3 x 10-1

3.58 x 10-4

3.5 x 10

5.05 x 10

-1

[11]

-4

Theoretical result of Ref.32 with the same theory level.

Concerning directly the transport parameters, λe does not change with the addition of fluorine substitutions to the ITIC molecule. This result is related to the fact that the fluorine substitutions in the ITIC molecule do not affect the number of intramolecular noncovalent interactions as can be inferred from the small variation of θ upon fluorination. Comparing the values of λe in Table 4 with similar non-fullerene acceptor molecules based on fused-ring indacenodithiophene, IDIC and IDTBR,74 the results of Table 4 are 0.05eV and 0.01eV smaller, respectively. This result may be related to the lateral substituent in the central region of the ITIC and ITIC-4f molecules that have a larger volume compared to the respective substituents in the IDIC and the IDTBR molecules. It was found for the canonical polymer P3HT that the insertion of lateral substituents with larger volumes tend to decrease the reorganization energy. This effect was attributed to the fact some large substituents can cause steric hindrance in the chain after charge transfer, resulting in a more rigid polymer chain.64 The electronic coupling in Table 4 for both molecules is so strong that violates the approximations that justifies the calculation of the charge transfer rate by application of the hopping model.91 When |te| > λe/2, the condition for the presence of charge25 ACS Paragon Plus Environment


localized states do not hold which eliminates the activation barrier between the initial and final adiabatic states.92 The violation of the hopping model was also observed in a theoretical study with PCBM.93 Yet there are some works in the literature that estimate the charge transfer and the charge carrier mobility using the Marcus/Hush theory even in a regime that violates the condition above74 (for completeness we report those results in Table 4 but it is clear that the use of Marcus/Hush theory to quantify the relative change of the electron mobility between the ITIC and ITIC-4f does not agree with the experimental observations). Here we followed a more caution approach and decided to focus our discussion only on variations of the charge transfer integrals induced by the fluorination of the molecules. From Table 4, we can see that the electronic coupling is slightly larger for the molecule with fluorine, which is justified by the stronger π-π interactions. This result indicates that the electron mobility of electrons is higher in the ITIC-4f. This trend was indeed observed experimentally using SCLC measurements in ITIC and ITIC-4f films, see Table 4. It is important to mention that the electron transport in films of those acceptor molecules are strongly influenced by the preparation conditions, which significantly modify the film's morphology, as seen for ITIC in Ref.44,94. Our result of |te| is a ideal value (or maximum value), simulating a situation of high molecular organization that is desired in the preparation of molecular films.

5. Conclusion DFT calculations were applied to determine the effects of fluorine substitutions on key properties of materials commonly employed in organic solar cells. The general conclusion is that fluorination of both species that form the bulk heterojunction layer tends to improve the performance of the device because (i) it decreases the exciton binding energy on both materials that constitutes de BHJ (donor and acceptor) which helps the charge dissociation. (ii) The fluorination lowers simultaneously the IP and the EA energies of the donor as well as of the acceptor. This effect has a double consequence since it does not change significantly the band-gap of the non-fluorinated analogue species nor introduces Voc losses related to the decrease of the BHJ effective gap. (iii) The F substitution tends to reduce the intramolecular reorganization energy 26 ACS Paragon Plus Environment

Page 26 of 38

Page 27 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(although this variation was not verified for the ITIC acceptor due to the position of fluorine substitutions that do not affect the number of intramolecular noncovalent interactions) and increases the intermolecular interaction between adjacent molecules. This is evidenced by a higher binding energy and a higher electronic coupling (transfer integrals) in complexes formed by two fluorinated oligomers (molecules). Those effects favor the charge transport of the majority carriers (holes in the donor and electrons in the acceptor) which helps to limit the bimolecular recombination and decreases the serial resistance of the BHJ layer. The higher binding energy between fluorinated molecules also contributes to an easier aggregation among them which explains why polymers with F substitutions tends to aggregate even in solution.26 (iv) The fluorination induces a planarization of the oligomers’ backbone (again this effect is very small for the ITIC molecule). The planarization of the chain together with the tendency to aggregate can decisively impact the resulting morphology of the films with a great influence on the effective charge carrier mobility and recombination. They improve the formation of crystalline domains with a high degree of purity and induce a more balanced charge transport. The complexity of the physical-chemical interactions involved in the photovoltaic response of OPVs always limit the attempts to establish a general picture covering all the consequences of fluorine substitutions. However, the majority of the experimental data seems to confirm the advantages of OPV devices based on fluorinated BHJ blends. Indeed many devices formed by donors and acceptors with F substitutions have

superior

photovoltaic

performances

compared

to

their

non-fluorinated

counterparts. Our theoretical analysis suggests the main reasons behind this trend. It also provide subsides to the discovery of new design strategies to further improve the OPV performance. Supporting Information PCM parameters for the calculation of dielectric constant; comparison of calculated ionization potential with experimental results; summary of the DFT methods employed in the calculations; details of the calculation of exciton binding energy in the ground state geometry; details of dipole change calculation; details of the calculation of exciton binding energy in the excited state geometry. 27 ACS Paragon Plus Environment


Acknowledgments "This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001". Research developed with the assistance of CENAPAD-SP (Centro Nacional de Processamento de Alto Desempenho em São Paulo), project UNICAMP / FINEP - MCT.

References (1)

Ostroverkhova, O. Organic Optoelectronic Materials: Mechanisms and Applications. Chem. Rev. 2016, 116, 13279–13412.

(2)

Coropceanu, V.; Cornil, J.; Filho, D. A. da S.; Olivier, Y.; Silbey, R.; Bredas, J. L. Charge Transport in Organic Semiconductors. Chem. Rev. 2007, 107, 926–952.

(3)

Helgesen, M.; Søndergaard, R.; Krebs, F. C. Advanced Materials and Processes for Polymer Solar Cell Devices. J. Mater. Chem. 2010, 20, 36–60.

(4)

Verheyen, L.; Leysen, P.; Van Den Eede, M. P.; Ceunen, W.; Hardeman, T.; Koeckelberghs, G. Advances in the Controlled Polymerization of Conjugated Polymers. Polym. (United Kingdom) 2017, 108, 521–546.

(5)

Yamamoto, N. A. D.; Lavery, L. L.; Nowacki, B. F.; Grova, I. R.; Whiting, G. L.; Krusor, B.; De Azevedo, E. R.; Akcelrud, L.; Arias, A. C.; Roman, L. S. Synthesis and Solar Cell Application of New Alternating Donor-Acceptor Copolymers Based on Variable Units of Fluorene, Thiophene, and Phenylene. J. Phys. Chem. C 2012, 116, 18641–18648.

(6)

Li, W.; Albrecht, S.; Yang, L.; Roland, S.; Tumbleston, J. R.; McAfee, T.; Yan, L.; Kelly, M. A.; Ade, H.; Neher, D.; et al. Mobility-Controlled Performance of Thick Solar Cells Based on Fluorinated Copolymers. J. Am. Chem. Soc. 2014, 136, 15566–15576.

(7)

Zhou, H.; Yang, L.; Stuart, A. C.; Price, S. C.; Liu, S.; You, W. Development of Fluorinated Benzothiadiazole as a Structural Unit for a Polymer Solar Cell of 7% Efficiency. Angew. Chemie - Int. Ed. 2011, 50, 2995–2998.

(8)

Zhang, J.; Tan, H. S.; Guo, X.; Facchetti, A.; Yan, H. Material Insights and Challenges for Non-Fullerene Organic Solar Cells Based on Small Molecular Acceptors. Nat. Energy 2018, 3, 720–731.

(9)

Fan, Q.; Su, W.; Wang, Y.; Guo, B.; Jiang, Y.; Guo, X.; Liu, F.; Russell, T. P.; Zhang, M.; Li, Y. Synergistic Effect of Fluorination on Both Donor and Acceptor


Page 28 of 38

Page 29 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Materials for High Performance Non-Fullerene Polymer Solar Cells with 13 . 5 % Efficiency. Sci. China Chem. 2018, 537, 531–537. (10)

Son, H. J.; Carsten, B.; Jung, I. H.; Yu, L. Overcoming Efficiency Challenges in Organic Solar Cells: Rational Development of Conjugated Polymers. Energy Environ. Sci. 2012, 5, 8158–8170.

(11)

Zhao, W.; Li, S.; Yao, H.; Zhang, S.; Zhang, Y.; Yang, B.; Hou, J. Molecular Optimization Enables over 13% Efficiency in Organic Solar Cells. J. Am. Chem. Soc. 2017, 139, 7148–7151.

(12)

Kan, B.; Li, M.; Zhang, Q.; Liu, F.; Wan, X.; Wang, Y.; Ni, W.; Long, G.; Yang, X.; Feng, H.; et al. A Series of Simple Oligomer-like Small Molecules Based on Oligothiophenes for Solution-Processed Solar Cells with High Efficiency. J. Am. Chem. Soc. 2015, 137, 3886–3893.

(13)

Leclerc, N.; Chávez, P.; Ibraikulov, O. A.; Heiser, T.; Lévêque, P. Impact of Backbone Fluorination on π-Conjugated Polymers in Organic Photovoltaic Devices: A Review. Polymers (Basel). 2016, 8, 11–38.

(14)

Zhang, M.; Guo, X.; Zhang, S.; Hou, J. Synergistic Effect of Fluorination on Molecular Energy Level Modulation in Highly Efficient Photovoltaic Polymers. Adv. Mater. 2014, 26, 1118–1123.

(15)

Wang, Y.; Parkin, S. R.; Gierschner, J.; Watson, M. D. Highly Fluorinated Benzobisbenzothiophenes. Org. Lett. 2008, 10, 3307–3310.

(16)

Park, J. H.; Jung, E. H.; Jung, J. W.; Jo, W. H. A Fluorinated Phenylene Unit as a Building Block for High-Performance n-Type Semiconducting Polymer. Adv. Mater. 2013, 25, 2583–2588.

(17)

Do, K.; Saleem, Q.; Ravva, M. K.; Cruciani, F.; Kan, Z.; Wolf, J.; Hansen, M. R.; Beaujuge, P. M.; Brédas, J. L. Impact of Fluorine Substituents on π-Conjugated Polymer Main-Chain Conformations, Packing, and Electronic Couplings. Adv. Mater. 2016, 28, 8197–8205.

(18)

Fei, Z.; Shahid, M.; Yaacobi-Gross, N.; Rossbauer, S.; Zhong, H.; Watkins, S. E.; Anthopoulos, T. D.; Heeney, M. Thiophene Fluorination to Enhance Photovoltaic Performance in Low Band Gap Donor-Acceptor Polymers. Chem. Commun. (Camb). 2012, 48, 11130–11132.

(19)

Deng, D.; Zhang, Y.; Zhang, J.; Wang, Z.; Zhu, L.; Fang, J.; Xia, B.; Wang, Z.; Lu, K.; Ma, W.; et al. Fluorination-Enabled Optimal Morphology Leads to over 11% Efficiency for Inverted Small-Molecule Organic Solar Cells. Nat. Commun. 2016, 7, 1–9.

(20)

Stuart, A. C.; Tumbleston, J. R.; Zhou, H.; Li, W.; Liu, S.; Ade, H.; You, W. 29 ACS Paragon Plus Environment


Fluorine Substituents Reduce Charge Recombination and Drive Structure and Morphology Development in Polymer Solar Cells. J. Am. Chem. Soc. 2013, 135, 1806–1815. (21)

Jo, J. W.; Bae, S.; Liu, F.; Russell, T. P.; Jo, W. H. Comparison of Two D-A Type Polymers with Each Being Fluorinated on D and A Unit for High Performance Solar Cells. Adv. Funct. Mater. 2015, 25, 120–125.

(22)

Benatto, L.; Marchiori, C. F. N.; da Luz, M. G. E.; Koehler, M. Electronic and Structural Properties of Fluorene–Thiophene Copolymers as Function of the Composition Ratio between the Moieties: A Theoretical Study. Phys. Chem. Chem. Phys. 2018, 20, 20447–20458.

(23)

Carsten, B.; Szarko, J. M.; Lu, L.; Son, H. J.; He, F.; Botros, Y. Y.; Chen, L. X.; Yu, L. Mediating Solar Cell Performance by Controlling the Internal Dipole Change in Organic Photovoltaic Polymers. Macromolecules 2012, 45, 6390– 6395.

(24)

Xu, T.; Lu, L.; Zheng, T.; Szarko, J. M.; Schneider, A.; Chen, L. X.; Yu, L. Tuning the Polarizability in Donor Polymers with a Thiophenesaccharin Unit for Organic Photovoltaic Applications. Adv. Funct. Mater. 2014, 24, 3432–3437.

(25)

Kawashima, K.; Fukuhara, T.; Suda, Y.; Suzuki, Y.; Koganezawa, T.; Yoshida, H.; Ohkita, H.; Osaka, I.; Takimiya, K. Implication of Fluorine Atom on Electronic Properties, Ordering Structures, and Photovoltaic Performance in Naphthobisthiadiazole-Based Semiconducting Polymers. J. Am. Chem. Soc. 2016, 138, 10265–10275.

(26)

Fei, Z.; Boufflet, P.; Wood, S.; Wade, J.; Moriarty, J.; Gann, E.; Ratcliff, E. L.; Mcneill, C. R.; Sirringhaus, H.; Kim, J. S.; et al. Influence of Backbone Fluorination in Regioregular Poly(3-Alkyl-4-Fluoro)Thiophenes. J. Am. Chem. Soc. 2015, 137, 6866–6879.

(27)

Albrecht, S.; Janietz, S.; Schindler, W.; Frisch, J.; Kurpiers, J.; Kniepert, J.; Inal, S.; Pingel, P.; Fostiropoulos, K.; Koch, N.; et al. Fluorinated Copolymer PCPDTBT with Enhanced Open-Circuit Voltage and Reduced Recombination for Highly Efficient Polymer Solar Cells. J. Am. Chem. Soc. 2012, 134, 14932– 14944.

(28)

Tumbleston, J. R.; Collins, B. A.; Yang, L.; Stuart, A. C.; Gann, E.; Ma, W.; You, W.; Ade, H. The Influence of Molecular Orientation on Organic Bulk Heterojunction Solar Cells. Nat. Photonics 2014, 8, 385–391.

(29)

Jo, J. W.; Jung, J. W.; Wang, H. W.; Kim, P.; Russell, T. P.; Jo, W. H. Fluorination of Polythiophene Derivatives for High Performance Organic Photovoltaics.


Page 30 of 38

Page 31 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Chem. Mater. 2014, 26, 4214–4220. (30)

Wolf, J.; Cruciani, F.; El Labban, A.; Beaujuge, P. M. Wide Band-Gap 3,4Difluorothiophene-Based Polymer with 7% Solar Cell Efficiency: An Alternative to P3HT. Chem. Mater. 2015, 27, 4184–4187.

(31)

Pan, Q. Q.; Li, S. B.; Duan, Y. C.; Wu, Y.; Zhang, J.; Geng, Y.; Zhao, L.; Su, Z. M. Exploring What Prompts ITIC to Become a Superior Acceptor in Organic Solar Cell by Combining Molecular Dynamics Simulation with Quantum Chemistry Calculation. Phys. Chem. Chem. Phys. 2017, 19, 31227–31235.

(32)

Han, G.; Guo, Y.; Song, X.; Wang, Y.; Yi, Y. Terminal π-π Stacking Determines Three-Dimensional Molecular Packing and Isotropic Charge Transport in an A-πA Electron Acceptor for Non-Fullerene Organic Solar Cells. J. Mater. Chem. C 2017, 5, 4852–4857.

(33)

Han, G.; Yi, Y.; Shuai, Z. From Molecular Packing Structures to Electronic Processes : Theoretical Simulations for Organic Solar Cells. Adv. Energy Mater. 2018, 8, 1702743–1702759.

(34)

Dai, S.; Zhao, F.; Zhang, Q.; Lau, T.-K.; Li, T.; Liu, K.; Ling, Q.; Wang, C.; Lu, X.; You, W.; et al. Fused Nonacyclic Electron Acceptors for Efficient Polymer Solar Cells. J. Am. Chem. Soc. 2017, 139, 1336–1343.

(35)

Lin, Y.; Zhao, F.; He, Q.; Huo, L.; Wu, Y.; Parker, T. C.; Ma, W.; Sun, Y.; Wang, C.; Zhu, D.; et al. High-Performance Electron Acceptor with Thienyl Side Chains for Organic Photovoltaics. J. Am. Chem. Soc. 2016, 138, 4955–4961.

(36)

Cheng, P.; Li, G.; Zhan, X.; Yang, Y. Next-Generation Organic Photovoltaics Based on Non-Fullerene Acceptors. Nat. Photonics 2018, 12, 131–142.

(37)

Chen, W.; Zhang, Q. Recent Progress in Non-Fullerene Small Molecule Acceptors in Organic Solar Cells (OSCs). J. Mater. Chem. C 2017, 5, 1275–1302.

(38)

Hou, J.; Inganas, O.; Friend, R. H.; Gao, F. Organic Solar Cells Based on NonFullerene Acceptors. Nat. Mater. 2018, 17, 119–128.

(39)

Zhu, L.; Yi, Y.; Wei, Z. Exciton Binding Energies of Non-Fullerene Small Molecule Acceptors: Implication for Exciton Dissociation Driving Forces in Organic Solar Cells. J. Phys. Chem. C 2018, 122, 22309−22316.

(40)

Valeev, E. F.; Coropceanu, V.; Da Silva Filho, D. A.; Salman, S.; Brédas, J. L. Effect of Electronic Polarization on Charge-Transport Parameters in Molecular Organic Semiconductors. J. Am. Chem. Soc. 2006, 128, 9882–9886.

(41)

Hutchison, G. R.; Ratner, M. A.; Marks, T. J. Hopping Transport in Conductive Heterocyclic Oligomers: Reorganization Energies and Substituent Effects. J. Am. 31 ACS Paragon Plus Environment


Chem. Soc. 2005, 127, 2339–2350. (42)

Wang, L.; Nan, G.; Yang, X.; Peng, Q.; Li, Q.; Shuai, Z. Computational Methods for Design of Organic Materials with High Charge Mobility. Chem. Soc. Rev. 2010, 39, 423–434.

(43)

Yang, L.; Tumbleston, J. R.; Zhou, H.; Ade, H.; You, W. Disentangling the Impact of Side Chains and Fluorine Substituents of Conjugated Donor Polymers on the Performance of Photovoltaic Blends. Energy Environ. Sci. 2013, 6, 316–326.

(44)

Upama, M. B.; Elumalai, N. K.; Mahmud, M. A.; Wright, M.; Wang, D.; Xu, C.; Uddin, A. Effect of Annealing Dependent Blend Morphology and Dielectric Properties on the Performance and Stability of Non-Fullerene Organic Solar Cells. Sol. Energy Mater. Sol. Cells 2018, 176, 109–118.

(45)

Wang, Y.; Fan, Q.; Guo, X.; Li, W.; Guo, B.; Su, W.; Ou, X.; Zhang, M. HighPerformance Nonfullerene Polymer Solar Cells Based on a Fluorinated Wide Bandgap Copolymer with a High Open-Circuit Voltage of 1.04 V. J. Mater. Chem. A 2017, 5, 22180–22185.

(46)

Yang, L.; Feng, J. K.; Ren, A. M. Theoretical Studies on the Electronic and Optical Properties of Two Thiophene-Fluorene Based π-Conjugated Copolymers. Polymer (Guildf). 2005, 46, 10970–10981.

(47)

Few, S.; Frost, J. M.; Kirkpatrick, J.; Nelson, J. Influence of Chemical Structure on the Charge Transfer State Spectrum of a Polymer:Fullerene Complex. J. Phys. Chem. C 2014, 118, 8253–8261.

(48)

Zade, S. S.; Bendikov, M. From Oligomers to Polymer: Convergence in the HOMO-LUMO Gaps of Conjugated Oligomers. Org. Lett. 2006, 8, 5243–5246.

(49)

Bronstein, H.; Frost, J. M.; Hadipour, A.; Kim, Y.; Nielsen, C. B.; Ashraf, R. S.; Rand, B. P.; Watkins, S.; McCulloch, I. Effect of Fluorination on the Properties of a Donor-Acceptor Copolymer for Use in Photovoltaic Cells and Transistors. Chem. Mater. 2013, 25, 277–285.

(50)

Barone, V.; Cossi, M.; Tomasi, J. A New Definition of Cavities for the Computation of Solvation Free Energies by the Polarizable Continuum Model. J. Chem. Phys. 1997, 107, 3210–3221.

(51)

Nayak, P. K.; Periasamy, N. Calculation of Electron Affinity, Ionization Potential, Transport Gap, Optical Band Gap and Exciton Binding Energy of Organic Solids Using “solvation” Model and DFT. Org. Electron. physics, Mater. Appl. 2009, 10, 1396–1400.

(52)

Schwenn, P. E.; Burn, P. L.; Powell, B. J. Calculation of Solid State Molecular Ionisation Energies and Electron Affinities for Organic Semiconductors. Org. 32 ACS Paragon Plus Environment

Page 32 of 38

Page 33 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Electron. physics, Mater. Appl. 2011, 12, 394–403. (53)

Kraner, S.; Prampolini, G.; Cuniberti, G. Exciton Binding Energy in Molecular Triads. J. Phys. Chem. C 2017, 121, 17088–17095.

(54)

Talebian, E.; Talebian, M. A General Review on the Derivation of ClausiusMossotti Relation. Optik (Stuttg). 2013, 124, 2324–2326.

(55)

Chai, J.-D.; Head-Gordon, M. Long-Range Corrected Hybrid Density Functionals with Damped Atom–Atom Dispersion Corrections. Phys. Chem. Chem. Phys. 2008, 10, 6615–6620.

(56)

Sini, G.; Schubert, M.; Risko, C.; Roland, S.; Lee, O. P.; Chen, Z.; Richter, T. V.; Dolfen, D.; Coropceanu, V.; Ludwigs, S.; et al. On the Molecular Origin of Charge Separation at the Donor–Acceptor Interface. Adv. Energy Mater. 2018, 8, 1–15.

(57)

Bredas, J.-L. Mind the Gap! Mater. Horiz. 2014, 1, 17–19.

(58)

Lee, J.-C.; Chai, J.-D.; Lin, S.-T. Assessment of Density Functional Methods for Exciton Binding Energies and Related Optoelectronic Properties. RSC Adv. 2015, 5, 101370–101376.

(59)

Tsai, C.-W.; Su, Y.-C.; Li, G.-D.; Chai, J.-D. Assessment of Density Functional Methods with Correct Asymptotic Behavior. Phys. Chem. Chem. Phys. 2013, 15, 8352.

(60)

Zhao, Y.; Truhlar, D. G. The M06 Suite of Density Functionals for Main Group Thermochemistry, Thermochemical Kinetics, Noncovalent Interactions, Excited States, and Transition Elements: Two New Functionals and Systematic Testing of Four M06-Class Functionals and 12 Other Function. Theor. Chem. Acc. 2008, 120, 215–241.

(61)

Risko, C.; McGehee, M. D.; Brédas, J.-L. A Quantum-Chemical Perspective into Low Optical-Gap Polymers for Highly-Efficient Organic Solar Cells. Chem. Sci. 2011, 2, 1200–1218.

(62)

Damas, G.; Marchiori, C. F. N.; Araujo, C. M. G. On the Design of DonorAcceptor Conjugated Polymers for Photocatalytic Hydrogen Evolution Reaction: First-Principles Theory Based Assessment. J. Phys. Chem. C 2018, 122, 26876– 26888.

(63)

Cristina Wouk de Menezes, L.; Jin, Y.; Benatto, L.; Wang, C.; Koehler, M.; Zhang, F.; Stolz Roman, L. Charge Transfer Dynamics and Device Performance of Environmentally Friendly Processed Non-Fullerene Organic Solar Cells. ACS Appl. Energy Mater. 2018, 1, 4776–4785.



(64)

Oliveira, E. F.; Lavarda, F. C. Reorganization Energy for Hole and Electron Transfer of Poly(3-Hexylthiophene) Derivatives. Polym. (United Kingdom) 2016, 99, 105–111.

(65)

Biswas, S.; Pramanik, A.; Pal, S.; Sarkar, P. A Theoretical Perspective on the Photovoltaic Performance of S,N-Heteroacenes: An Even-Odd Effect on the Charge Separation Dynamics. J. Phys. Chem. C 2017, 121, 2574–2587.

(66)

García, G.; Moral, M.; Garzón, A.; Granadino-Roldán, J. M.; Navarro, A.; Fernández-Gómez, M. Poly(Arylenethynyl-Thienoacenes) as Candidates for Organic Semiconducting Materials. A DFT Insight. Org. Electron. physics, Mater. Appl. 2012, 13, 3244–3253.

(67)

Leng, C.; Qin, H.; Si, Y.; Zhao, Y. Theoretical Prediction of the Rate Constants for Exciton Dissociation and Charge Recombination to a Triplet State in Pcpdtbt with Different Fullerene Derivatives. J. Phys. Chem. C 2014, 118, 1843–1855.

(68)

Wang, Q.; Song, P.; Ma, F.; Sun, J.; Yang, Y.; Li, Y. A Rigid Planar Low Band Gap Polymer PTTDPP-DT-DTT for Heterojunction Solar Cell: A Study of Density Functional Theory. Theor. Chem. Acc. 2018, 137, 1–14.

(69)

Thomas, A.; Chitumalla, R. K.; Puyad, A. L.; Mohan, K. V.; Jang, J. Computational Studies of Hole/Electron Transport in Positional Isomers of Linear Oligo-Thienoacenes: Evaluation of Internal Reorganization Energies Using Density Functional Theory. Comput. Theor. Chem. 2016, 1089, 59–67.

(70)

McMahon, D. P.; Troisi, A. Evaluation of the External Reorganization Energy of Polyacenes. J. Phys. Chem. Lett. 2010, 1, 941–946.

(71)

Norton, J. E.; Brédas, J. L. Polarization Energies in Oligoacene Semiconductor Crystals. J. Am. Chem. Soc. 2008, 130, 12377–12384.

(72)

Coropceanu, V.; Malagoli, M.; da Silva Filho, D. A.; Gruhn, N. E.; Bill, T. G.; Brédas, J. L. Hole- and Electron-Vibrational Couplings in Oligoacene Crystals: Intramolecular Contributions. Phys. Rev. Lett. 2002, 89, 275503.

(73)

Yang, H.; Gajdos, F.; Blumberger, J. Intermolecular Charge Transfer Parameters, Electron-Phonon Couplings, and the Validity of Polaron Hopping Models in Organic Semiconducting Crystals: Rubrene, Pentacene, and C60. J. Phys. Chem. C 2017, 121, 7689–7696.

(74)

Wang, Q.; Li, Y.; Song, P.; Su, R.; Ma, F.; Yang, Y. Non-Fullerene AcceptorBased Solar Cells: From Structural Design to Interface Charge Separation and Charge Transport. Polymers (Basel). 2017, 9, 692.

(75)

Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et 34 ACS Paragon Plus Environment

Page 34 of 38

Page 35 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


al. GAUSSIAN 09. Gaussian, Inc., Wallingford, CT 2009. (76)

Allen, J. P. General Atomic and Molecular Electronic Structure System. J. Comput. Chem. 1993, 14, 1347–1363.

(77)

Hanwell, M. D.; Curtis, D. E.; Lonie, D. C.; Vandermeerschd, T.; Zurek, E.; Hutchison, G. R. Avogadro: An Advanced Semantic Chemical Editor, Visualization, and Analysis Platform. J. Cheminform. 2012, 4, 1–17.

(78)

Chemcraft - Graphical Software for Visualization of Quantum Chemistry Computations.

(79)

Gorelsky, S. I. AOMix: Program for Molecular Orbital Analysis. 2015.

(80)

Reichenbächer, K.; Süss, H. I.; Hulliger, J. Fluorine in Crystal Engineering—“the Little Atom That Could.” Chem. Soc. Rev. 2005, 34, 22–30.

(81)

Few, S.; Frost, J. M.; Nelson, J. Models of Charge Pair Generation in Organic Solar Cells. Phys. Chem. Chem. Phys. 2015, 17, 2311–2325.

(82)

Endres, J.; Pelczer, I.; Rand, B. P.; Kahn, A. Determination of Energy Level Alignment within an Energy Cascade Organic Solar Cell. Chem. Mater. 2016, 28, 794–801.

(83)

Lourenco, O. D.; Benatto, L.; Marchiori, C. F. N.; Avila, H. C.; Yamamoto, N. A. D.; Oliveira, C. K.; Da Luz, M. G. E.; Cremona, M.; Koehler, M.; Roman, L. S. Conformational Change on a Bithiophene-Based Copolymer Induced by Additive Treatment: Application in Organic Photovoltaics. J. Phys. Chem. C 2017, 121, 16035–16044.

(84)

Lin, Y.; Wang, J.; Zhang, Z.-G.; Bai, H.; Li, Y.; Zhu, D.; Zhan, X. An Electron Acceptor Challenging Fullerenes for Efficient Polymer Solar Cells. Adv. Mater. 2015, 27, 1170–1174.

(85)

Liu, Y.; Zuo, L.; Shi, X.; Jen, A. K.-Y.; Ginger, D. S. Unexpectedly Slow Yet Efficient Picosecond to Nanosecond Photoinduced Hole-Transfer Occurs in a Polymer/Nonfullerene Acceptor Organic Photovoltaic Blend. ACS Energy Lett. 2018, 3, 2396–2403.

(86)

Qian, D.; Zheng, Z.; Yao, H.; Tress, W.; Hopper, T. R.; Chen, S.; Li, S.; Liu, J.; Chen, S.; Zhang, J.; et al. Design Rules for Minimizing Voltage Losses in HighEfficiency Organic Solar Cells. Nat. Mater. 2018, 17, 703–709.

(87)

Wang, X.; He, G.; Li, Y.; Kuang, Z.; Guo, Q.; Wang, J. L.; Pei, J.; Xia, A. OddEven Effect of Thiophene Chain Lengths on Excited State Properties in Oligo(Thienyl Ethynylene)-Cored Chromophores. J. Phys. Chem. C 2017, 121, 7659–7666. 35 ACS Paragon Plus Environment


(88)

Liu, Y.; Zhang, Z.; Feng, S.; Li, M.; Wu, L.; Hou, R.; Xu, X.; Chen, X.; Bo, Z. Exploiting Noncovalently Conformational Locking as a Design Strategy for High Performance Fused-Ring Electron Acceptor Used in Polymer Solar Cells. J. Am. Chem. Soc. 2017, 139, 3356–3359.

(89)

Yao, H.; Cui, Y.; Yu, R.; Gao, B.; Zhang, H.; Hou, J. Design, Synthesis, and Photovoltaic Characterization of a Small Molecular Acceptor with an UltraNarrow Band Gap. Angew. Chemie - Int. Ed. 2017, 56, 3045–3049.

(90)

Su, Y.; Lan, S.; Wei, K. Organic Photovoltaics In the Last Ten Years, the Highest Efficiency Obtained from Organic. Mater. Today 2012, 15, 554–562.

(91)

Oberhofer, H.; Reuter, K.; Blumberger, J. Charge Transport in Molecular Materials: An Assessment of Computational Methods. Chem. Rev. 2017, 117, 10319–10357.

(92)

Blumberger, J. Recent Advances in the Theory and Molecular Simulation of Biological Electron Transfer Reactions. Chem. Rev. 2015, 115, 11191–11238.

(93)

Gajdos, F.; Oberhofer, H.; Dupuis, M.; Blumberger, J. On the Inapplicability of Electron-Hopping Models for the Organic Semiconductor Phenyl-C61-Butyric Acid Methyl Ester (PCBM). J. Phys. Chem. Lett. 2013, 4, 1012–1017.

(94)

Chen, S.; Lee, S. M.; Xu, J.; Lee, J.; Lee, K.; Hou, T.; Yang, Y.; Jeong, M.; Lee, B.; Cho, Y.; et al. Ultrafast Channel II Process Induced by a 3-D Texture with Enhanced Acceptor Order Ranges for High-Performance Non-Fullerene Polymer Solar Cells. Energy Environ. Sci. 2018, 8, 3477–3494.


Page 36 of 38

Page 37 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



The Journal of Physical Chemistry Page 38 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

ACS Paragon Plus Environment

Effects of Fluorination on Exciton Binding Energy and Charge

Recommend Documents