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C: Energy Conversion and Storage; Energy and Charge Transport

Nano Assembly of Dipolar Imidazoanthraquinone Derivatives Leading to Enhanced Hole Mobility Qamar Tabrez Siddiqui, Prabhjyot Bhui, Mohammad Muneer, Chandrakumar R. S. Kuttay, Sangita Bose, and Neeraj Agarwal J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07224 • Publication Date (Web): 22 Oct 2018 Downloaded from http://pubs.acs.org on October 23, 2018

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The Journal of Physical Chemistry

Nano Assembly of Dipolar Imidazoanthraquinone Derivatives Leading to Enhanced Hole Mobility Qamar T. Siddiqui,a,b Prabhjyot Bhui,c Mohammad Muneer,b K. R. S. Chandrakumar,d Sangita Bosec* and Neeraj Agarwala* aSchool

of Chemical Sciences, UM-DAE, Centre for Excellence in Basic Sciences, Kalina, Santacruz

(E), Mumbai, 400098, India bDepartment cSchool

of Chemistry, Aligarh Muslim University, Aligarh, India

of Physical Sciences, UM-DAE, Centre for Excellence in Basic Sciences, Kalina, Santacruz

(E), Mumbai, 400098, India dTheoretical

Chemistry Section, Bhabha Atomic Research Centre, Mumbai, 400085, India.

ABSTRACT: Imidazoanthraquinone based high dipolar molecules (AQ01 and AQ02) were synthesized and characterized. Photophysical properties in various solvents suggested the polar nature of the ground state for these materials. In addition, fluorescence quenching experiments with the commonly used electron donor poly-3-hexylthiophene (P3HT) in bulk heterojunction (BHJ) solar cells ascertained the electron acceptor properties of these molecules. Theoretical calculations based on density functional theory (DFT) gave insight on the dipolar nature, H-bonding and  interaction in different types of supramolecular assemblies of AQ01 and AQ02. Calculations predicted that  interaction via antiparallel orientation and H-bonding with OH--OH interaction is favored energetically in AQ01. Morphological studies on thermally evaporated thin films indicated interconnected nano-assemblies in AQ01 while random aggregates in AQ02. Charge transport properties of these molecules were estimated

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in for AQ01/AQ02 and their blends with P3HT. Hole mobility in AQ01 based device was found to be as high as 2.4 x 10-4 cm2/Vs. Favorable morphology in AQ01 thin film correlates well with the observed high hole mobility. Our results indicate that the dipolar molecule AQ01 has potential to be used as a nonfullerene based electron acceptor in BHJ solar cell devices.

INTRODUCTION

Non-ionic dipolar organic molecules have been studied for their applications in several emerging areas such as non-linear optics, bulk heterojunction (BHJ) solar cells etc.1-7 Physical properties, such as dielectric constant, morphology in thin film etc., of materials used as active layers in solid state devices are crucial in charge carrier transport.8-13 In solid state devices of polar molecules, low carrier mobilities were attributed to increased energetic disorder caused by high dipole moment and thus preventing the hopping of charges. Following this, small polar molecules were considered as unsuitable for transport based devices.14, 15 About a decade later, very recently, Würthner et al. showed successful applications of dipolar molecules in organic photovoltaic (OPV) devices having high hole mobilities.16-18 The high carrier mobilities were explained based on the formation of favorable morphology which nullified the effect of the dipolar nature of single molecules. Work of Würthner, revived the research on organic molecules with high dipole moment for solid state applications and thus motivated researchers to synthesize new polar materials for device applications.19-21 The design of synthesized polar materials should be such that it promotes charge separation and has promising thin film morphological properties such that it is beneficial for charge transport in devices. A few organic molecules (1-4 in Chart 1) having high dipole moment with respect to Dimethyl sulfoxide (DMSO,  = 4.1 Debye, D) were reported. Molecules 1,2 have both strong electron pushing and pulling groups while in 3 all six electron withdrawing fluorine atoms are in a facial arrangement.9, 22, 23 Positioning of electron donating/withdrawing groups is responsible, to some extent,


2

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for high dipole moment.20 Polymer of 4 showed the effect of high dipole moment on better charge separation.9

C4H9 C4H9 N S

NC

CN

NC S NC

CN

CN HN

N

NH

C7H15

OR

F F

F F

F F

C7H15

S

13.1 D

12.7 D

6.2 D

1

2

3

S S

F

S

OR

ROOC 3.8 D 4

Me N Me

S O

HN N

O

8.0 D AQ01

O

HN N

O

3.8 D AQ02

Chart 1: Compounds 1-4 denote high dipole moments from reference.9,19,23,24 AQ01 and AQ02 synthesized in this work are shown with their calculated dipole moments.

Device performance of same materials varies from device to device due to differences in morphologies obtained in different methods employed in device fabrications. Hence, it is important to understand the factors affecting the morphology, packing structure and charge transport property in thin films. Morphology of organic materials can be altered by suitably choosing the core molecule and placing the substituents which induce H-bonding,  stacking or other inter molecular interactions.24-28 NonFullerene molecules as electron acceptor have been used recently in bulk heterojunction solar cells as they provide economical alternative to 1-[3-(Methoxycarbonyl)propyl]-1-phenyl-[6.6]C61 (PCBM).29-35 Material having donor (D) and acceptor (A) moieties are referred as D-A molecule. Properties of D-A based material can be tuned by varying the nature of donor or acceptor moiety. Self-assembly formation is favourable in D-A system by non-bonding intra- and intermolecular interactions. Such assembly formation favours improvement in carrier mobility.24 Owing to potential of dipolar materials in solid state ACS Paragon Plus Environment

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devices, as demonstrated by Würthner, we designed and synthesized small dipolar molecules to study their thin film properties. In this work, we present the new imidazoanthraquinone D-A based dipolar small organic molecules, their morphological and charge transport properties. Imidazoanthraquinone core is chosen due to its (i) ease of synthesis, (ii) tunability of its dipolar nature by substituting various electron donating groups at one end and (iii) possibility of H-bonding between carbonyl oxygen and hydrogen of NH. AQ01 and AQ02 (shown in Chart 1) were designed and synthesized with substituents having different electron donating capabilities and extended  conjugation. These molecules showed interesting photophysical properties indicating charge transfer and polar nature of ground state. In thin film, AQ01 self-assembled to form wire like structure while AQ02 showed flower like pattern. DFT calculations provided insight on the stronger  interaction and H-bonding in AQ01 which help in attaining long wire structures. Also, AQ01 was found to have high hole mobility ~ 10-4 cm2 V-1s-1. Thus, this work further substantiates the existing theory that high dipolar molecules can be used in solid state devices provided they have favourable morphology.16-21, 36

RESULTS AND DISCUSSIONS Synthesis: Imidazoanthraquinone derivatives AQ01 & AQ02 having electron donating substituents of different strength were synthesized by condensation of 1,2-diaminoanthraquinone and arylaldehyde.37, 38 After purification these compounds were characterized by 1H-NMR, 13C-NMR, and mass spectrometry (See Figure S1-S4). The solubility of these compounds were found to be moderate in common polar organic solvents. In 1H-NMR, a broad singlet was obtained at ~11 ppm which is assigned for NH protons. The broad nature of NH signal suggests the intermolecular H-bonding between imidazolyl NH and CO of anthraquinone. Photophysical studies: Electronic properties of donor-acceptor compounds AQ01 and AQ02 were studied by UV-visible absorption spectroscopy in solution and thin films. Absorption spectra of AQ01 and AQ02 are shown in Figure 1 and photophysical properties are summarized in Table 1. In acetonitrile, ACS Paragon Plus Environment

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a broad band, for AQ01, at ~390-575 nm and for AQ02 at ~360 -480 nm was observed which is assigned to n* transition.39, 40 An intense high energy peak at ~300-320 nm is assigned for * transition. The extinction coefficient and broad nature of lower energy absorption band shows the features of internal charge transfer. Typically, electronic properties of donor-acceptor materials vary as a function of the solvent polarity. Absorption and emission spectra of AQ01 and AQ02 were recorded in several polar and non-polar solvents and are shown in supporting information (Figure S5 & S6). Generally, shift is expected in absorption and emission spectra with variable solvent polarity. Extent of stabilization of ground and excited state leads to change in energy gap. Bathochomic shift are quite common when net change in dipole moment of excited and ground state is positive. Hypsochromic shift is expected when dipole moment of ground state is more than excited state. By and large, for AQ01 and AQ02 a blue shift in the absorption maxima of low energy transition was observed with increasing polar nature of solvents. Absorption spectra in different solvents showed blue shift from 484 nm in Toluene to 472 nm in Methanol. This negative solvatochromism indicates that dipole moment of ground state is higher than excited state. Emission spectra of AQ01 and AQ02 were recorded in solution and a well resolved peak, though low intensity, was observed. Emission maxima is found to be blue shifted in medium polarity solvents. The Lippert-Mataga (L-M) equations were used to study the solvent polarity behavior on absorption and emission properties of these compounds.41-43 Generally, L-M equations provide information about the dipole moments in ground and excited state when molecules are assumed as point dipoles.44, 45 LippertMataga equation generally deviate from its general theory when there is interaction such as hydrogen bonding between fluorophores and solvent or charge transfer state. It was observed that the L-M equation could not be fitted linearly for AQ01 and AQ02, indicating strong charge transfer.46 These photophysical studies further indicate the dipolar nature of these molecules in ground state. Thin film absorption spectra of both these compounds on quartz is presented in Figures 1(a)-(b). Broad absorption and bathochromic shift was observed in thin film which again indicates strong aggregation of AQ01 and AQ02 in solid. Aggregation of AQ01 and AQ02 is later confirmed from morphological studies. It is worth to note that AQ01 showed large bathochromic shift (~40 nm) than AQ02 (~25 nm) in thin film absorption spectra 5 ACS Paragon Plus Environment


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indicating pronounced aggregation due to strong inter-molecular interactions. It has been reported in literature that significant red shift is found in more ordered films where morphology leads to an enhanced conjugation.47 No change in the absorption spectra was observed with change in thickness from 30 to 80 nm. Both these compounds were found to non-emissive in thin films. (see Figure S18-S19).

Figure 1. Absorption spectra of (a) AQ01 (0.9 x 10-5 molL-1 in acetonitrile and in thin film of thickness 30 nm), (b) AQ02 (1.2 x 10-5 molL-1 in acetonitrile and in thin film of thickness 30 nm) and (c) Energy level diagram showing the position of HOMO-LUMO of AQ01, AQ02, P3HT, PCBM and CuPC. ACS Paragon Plus Environment

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Table 1: Photophysical and electrochemical properties of AQ01 and AQ02

aIn

Comp.

a abs

nm (log )

b abs

AQ01

320 (4.6), 477 (4.3)

516

561

-6.62

-3.72

AQ02

304 (4.5), 416 (4.23)

442

567

-6.67

-3.66

(nm)

a em

(nm)

cE HOMO

(eV)

dE LUMO (eV)

CH3CN; bin thin film; cEHOMO = -(E[OX vs Fc/Fc+] + 5.1) eV, dELUMO = -(E[red vs Fc/Fc+] + 5.1) eV.

In BHJ solar cells, photoinduced charge transfer from the electron donor to acceptor is required to eliminate the photoluminescence from donor material.48-50 To explore the possibility of these compounds as electron acceptor fluorescence quenching experiment was performed (See Figure S9). In this, we studied the emission properties of P3HT (electron donor) in blends with AQ01 and AQ02 (electron acceptor) and were compared with neat film of P3HT and P3HT:PCBM blend. A decrease in emission intensity of P3HT at 570 nm was observed in presence of AQ01 and AQ02. This quenching was comparable with P3HT:PCBM blend which indicates the fast electron transfer from P3HT to AQ01 and AQ02.

Cyclic voltammetric studies: Cyclic voltammetric studies were carried out to get the insight on redox behavior of AQ01 and AQ02. The cyclic voltammograms of AQ01 and AQ02 are shown in supporting information (Figure S10). Both these materials show irreversible oxidation peak at ˜1.9 V which is assigned to oxidation of imidazole moiety. Irreversible reduction peaks at ˜-1.0 and -1.7 V which are assigned to carbonyl of anthraquinone.51 The HOMO and LUMO energy levels were estimated, using oxidation and reduction potentials (vs Fc/Fc+)52 and are found to be -6.6 eV and -3.7 eV, respectively. Schematic diagram of energy levels of AQ01 and AQ02 are shown in Figure 1 (c) along with commonly used electron donors and acceptors for BHJ solar cells. This shows the better alignment of AQ01 and AQ02 with P3HT as electron donor as compare to PCBM. ACS Paragon Plus Environment

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Theoretical studies: Dipolar nature of the AQ01 and AQ02 as observed from photophysical studies was also investigated in detail by theoretical calculations at the molecular level.53-55 The geometry of the imidazoanthraquinone derivatives was initially optimized without any symmetry constraints and the equilibrium structures are shown in Figure S11. The dimer complexes are obtained by two different ways, viz., (i) intermolecular hydrogen bonding and (ii) intermolecular  stacking interactions. The optimized geometry of AQ01 and AQ02 in their monomer and dimer forms (hydrogen bonded as well as the  stacking) as obtained by the DFT based BP exchange-correlation function, are shown in Figure S12. We identified the hydrogen bonding interactions between (i) C-O of anthraquinone and C-H of both the monomers, represented as OH---OH as well as (ii) the anthraquinone C-O and C-H from the substituents mediated by imidazole NH bond, represented as OH---NH. The hydrogen bond length in case of OH--OH and OH--NH cases for AQ01 is 2.340 and 2.821 Å, respectively whereas for the case of AQ02, 2.656 and 2.488 Å, respectively. The binding strength as well as dipole moments for different types of assemblies are summarized in Table 2. The  electrons present in the imidazoanthraquinone derivatives are effectively delocalized all over the system. Hence these molecules can be considered as potential candidates for supra-molecular self-assembly via  stacking interactions. Accordingly, we modelled the intermolecular complexes of AQ01 and AQ02 and obtained the dimer models, as shown in Figure 2. Two different types of structures were obtained viz., (a) parallel and (b) anti modes, based on the orientation of the molecules. In case of parallel mode, the molecules are exactly super imposed on top of each other (head-head and tail-tail) and for the latter case, the anti-mode, the molecules are allowed to interact in such a way that the molecular orientation of each monomer is opposite to the other monomer (head-tail and tail-head). The interaction energy of the dimer for both the cases are listed in Table 2 and it is quite evident that the assemblies formed by the  interaction mode is much stronger (by ~30 kcal mol-1) than that of hydrogen bonded assemblies. It is primarily due to the presence of the strongly polarizable keto as well as the imidazole ACS Paragon Plus Environment

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groups. In case of AQ01, the dimer formed by the anti-orientation is more favored by 4 kcal mol-1 than the parallel orientation. However, in case of AQ02, the strength almost remains to be the same for both cases. It is worth to note that the calculated binding strength is almost comparable to that of co-ordination complex formed by NH3 and BH3.56 The minimum distance between the hetero atoms (Carbon/Nitrogen/Oxygen) of the monomers is around 3.32 Å and hydrogen-hydrogen distance is around 2.31 Å. These geometrical parameters are the typical signatures of  interaction. Table 2: The binding strength (kcal/mol) as well as dipole moments (in Debye) of the various assembly patterns of AQ01 and AQ02 System

Dipole Moment (D) Interaction Energy (kcal mol-1)

Hydrogen Bonding AQ01 (Monomer)

7.987

--

Dimer via OH-OH:

15.786

OH--OH: - 6.03

Dimer via NH-OH:

12.770

OH--NH: -3.96

AQ02 (Monomer)

3.774

--

Dimer via OH-OH:

8.618

OH--OH: -5.42

Dimer via NH-OH

7.989

OH--NH: -6.09

 Stacking interactions AQ01 (Monomer)

7.987

---

Dimer in parallel mode 13.405

-29.76

Dimer in anti mode

7.347

-34.05

AQ02 (Monomer)

3.774

---

Dimer in parallel mode 6.682

-33.54

Dimer in anti mode

-33.44

2.071


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The dipole moment of the dimer assemblies formed by AQ01 and AQ02 through hydrogen bonding as well as  interactions were also investigated. It can be noticed from Table 2 that the calculated dipole moments of AQ01 and AQ02 are large and it can also be seen that the dipole moment of monomer of AQ01 (~8.0 D) is almost twice that of AQ02 (~3.8 D). One of the striking features about these molecules is that there is substantial enhancement of their dipole moments in hydrogen bonded dimers (by a factor of two from their respective monomers). In addition, the enhancement of the dipole moment of the dimers was more pronounced when the H-bonding is through the OH--OH as compared to the OH--NH (Table 2) in both the molecules. The dipole moment of AQ01 dimer was found to be as high as ~15.8 D when H-bonding is through OH--OH. The observed trend for the dipole moment of the hydrogen bonded dimers, the dipole moment of the dimers formed through the  interactions depend on the orientation of each monomer during formation of the complexes. For instance, in case of the parallel orientation the dipole moment is observed to be approximately doubled for both AQ01 and AQ02. On the other hand, the dipole moment is drastically reduced for the case of the complexes formed by the antiorientation. The least dipole moment was observed for the case of AQ02 (~ 2.07 D) with anti-orientation. It may be noted that although the binding strength do not differ with respect to orientation, the dipole moment was observed to be affected drastically.


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Figure 2: The optimized geometry of the dimer complexes of AQ01 and AQ02 through π- π stacking interactions as obtained by the DFT based BP exchange-correlation function.

Morphology: Favourable morphology of organic layers in devices plays a crucial role as it influences the carrier mobilities.24-28, 57-60 It has been observed that the formation of nano-assemblies of organics lead to improved device performance. In particular, formation of intercalated pathways have led to efficient charge carrier delivery to the electrodes.61, 62 Small organic molecules are known to aggregate or selfassemble in thin films forming nanostructures, however, there are very few reports available to predict the morphology (computational methods) and control the self-organization in thin films.63 AQ01 and AQ02 have differences in their structures due to the presence of the two different functional groups on them. Inter-molecular bonding in both is also expected to be different due to difference in their electronic nature as shown by DFT calculations. AQ01 and AQ02 both favor hydrogen bonding between the NH group of imidazolyl and CO of anthraquinone (as seen in NMR studies). Thus, it necessitates to study the morphology of thin films of these molecules. The microstructures of the solid-state films of AQ01 and AQ02 were studied using field emission scanning electron microscopy (FESEM). Films were made by ACS Paragon Plus Environment

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both spin coating and vacuum thermal evaporation on Si substrates. Films of different thickness (30 nm and 80 nm) for both AQ01 and AQ02 were deposited for SEM studies. Thin films of these compounds were studied by x-ray diffraction (XRD), which showed they were amorphous in nature (see Figure S14). Figure 3 shows the SEM images of thermally evaporated films (~ 30 nm). Interestingly, thermally evaporated films of both the samples showed completely different morphologies. Large area SEM images of AQ01 shows the formation of long nanowires (Figure 3a). We believe that the inter-molecular hydrogen bonding aids in the formation of these nanowires which are several microns in length (Figure 3a). On the contrary, SEM images of vacuum thermally evaporated films of AQ02 show formation of small nanowires, few of which together form nano-flowers (Figure 3b). These nano-flowers were found to be dispersed overall in the films. For the 30 nm thick AQ01 film, the average length of the nano-wires is about 5-10 m while the average size of the nano-flowers for a 30 nm thick film of AQ02 is about 500 nm. SEM images of films of 30 and 80 nm are shown in the supporting information (Figure S13). In both the compounds, with increasing thickness the density of the nanowires increased. Self-assembly of crosslinked nanowires were observed for AQ01 forming longer inter connected pathways. With increasing film thickness, the size of the individual nano-flowers in AQ02 increase slightly. Thus, these results from FESEM demonstrate that by changing the aryl substituents on imidazoanthraquinone produce different microstructure of the films which is expected to influence the charge transport properties as it has been observed earlier that inter-connecting network (as in AQ01) results in improved charge transport in solid state devices47, 64.


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Figure 3: (a)-(b) SEM images of evaporated films (thickness ~30 nm) of AQ01 and AQ02, respectively showing the formation of nanostructures.

Carrier mobility determination: The carrier mobilities are one of the most important parameters required to establish the suitability of any material for device applications.65-67 As discussed earlier, there are reports that suggests that molecules with high dipole moments cannot have high carrier mobilities.3 Molecules reported in this work have high dipole moment. Therefore, it becomes important to measure the carrier mobilities in solid state devices based on these molecules. We determined the carrier mobilities in AQ01 and AQ02 based devices where a blend was made with the electron donor P3HT (1:1 by weight). It was determined using the space-charge limited current (SCLC) model where the current density (J) of the device is proportional to the square of the voltage (V).68 In this model, the conduction of carriers (holes or electrons) in the device is limited by space charge effects and not by the injection of carriers at the contacts. In low electric fields, Mott–Gurney law gives, 𝐽 =

3 𝜀𝜇 2 2𝐿3/2

𝑉, where,  is the permittivity of

the active organic layer, L is the thickness of the active layer in the device and  is the carrier mobility.68 In this model, since it is assumed that there is only one type of carrier hence hole-only and electron-only devices need to be fabricated to determine the electron and the hole mobilities, separately. The structure of the hole-only device was ITO / PEDOT:PSS / AQ01 or AQ02 or their blend with P3HT / MoO3 / Ag ACS Paragon Plus Environment

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(as shown in Figure 4(a)) and the configuration of the electron-only device was ITO / ZnO / AQ01 or AQ02 or their blend with P3HT / LiF / Al (see Figure S15). The thickness of the active layer in both the devices was ~ 70 -75 nm. The mobilities were estimated by plotting √J vs V (voltage V = Vapp – Vbi, where Vapp is the applied voltage and Vbi is the offset voltage) at low electric fields and fitting it with the Mott-Gurney Law.69 Figures 4(b) and 4(c) show the plot of √J vs V for AQ01+P3HT blend and AQ01 respectively.

Figure 4: (a) Schematic representation of hole only device with energy band diagram (b) – (c) √J vs V plots for one of the hole only AQ01 based devices. The symbols denote the data and the lines denote the straight line fit to the data in the space charge limited region Interestingly, the hole mobilities are about two to three orders of magnitude higher than their electron mobilities for AQ01. The hole mobility of (6.4± 4.0) x 10-4 cm2/Vs was achieved for a 75 nm thick AQ01 sample with its highest value of 2.4 x 10-4 cm2/Vs being observed in one of the devices. As the mobility was measured on the thermally evaporated films which had protruding features, the uncertainty in obtaining the thickness leads to the error bar in the estimation of mobility as in the space charge limited region mobility is related to the thickness of the films (See Mott Gurney Law). Also, it is worth to note that hole mobility for AQ01 in neat sample and in blend with P3HT is roughly similar. For ACS Paragon Plus Environment

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AQ02, the electron and hole mobilities are comparable and found in the range of 9.6 x 10-6 cm2/Vs to 1.4 x 10--6 cm2/Vs (as shown in Figure S15 of the supporting information). Hole and electron mobilities of AQ01 are comparable (~10-4 cm2/Vs and ~ 10-6 cm2/Vs respectively) to literature reported non-fullerene molecules which were used as electron acceptors in OPV.35 The physical origin of this mismatch has been largely attributed to the presence of extrinsic trap states.70 A more specific reason for the mismatch can be understood through detailed quantum mechanical calculations.71 It was shown that even with large mismatch between hole and electron mobility, such materials can be used in OPV showing high short circuit current density (JSC) values.35 Thus, AQ01 can be used as an efficient electron acceptor in inverted type solar cell devices anticipating high short circuit current density values. An inverted solar cell was fabricated with AQ01 as an electron acceptor and P3HT as a donor. The device geometry was ITO/ZnO/(AQ01 + P3HT ) blend/MoO3/Ag. The ITO was cleaned using the same way as described earlier. ZnO, MoO3 and Ag were thermally evaporated in the vacuum chamber under the base pressure of 10-6 mbar. The blend was prepared by keeping the ratio of 1:2 (1mg AQ02+1mg P3HT in 2 ml of chloroform). The blend was then spin coated at the speed of 5000 rpm for 40 s yielding a thickness of 70 nm. The thickness of ZnO, MoO3 and Ag were 30 nm, 25 nm, 120 nm respectively. The current density (J) vs Voltage (V) was measured in presence and absence of light (Figure S14). The short circuit current density (Jsc) was ~4 mA/cm2. The average device gave a fill factor (FF) of 0.32 while the best one gave FF of 0.39 (see Figure S16). Further optimization of the device and measuring it with a calibrated solar simulator will give the optimal efficiency of the device.

CONCLUSION In conclusion, two imidazoanthraquinone derivatives (AQ01 and AQ02) were synthesized and characterized. These compounds showed charge transfer and aggregation properties. Results from powder XRD indicates their amorphous nature. Morphological studies indicate that AQ01 forms interconnected nano-assemblies while AQ02 show random structure. Hole mobility of AQ01 is found to be as high as ACS Paragon Plus Environment

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2.4 x 10-4 cm2/Vs. High ground state dipole moment found for AQ01 is believed to be one of the reason for the major difference in the morphologies of both these materials. DFT calculation of these molecules further provide information on their supramolecular assemblies. AQ01 showed stronger  interaction and H-bonding leading to supramolecular assemblies. Morphology of AQ01 correlates well with the high hole mobility in AQ01. These compounds showed fluorescence quenching (of P3HT) when used in 1:1 blend with P3HT which shows their potential as non-fullerene electron acceptor. Theoretical results clearly demonstrated remarkable reduction of dipole moment in its assembly structure favoured through  interaction in anti-mode. In addition, it has also been found that the assembly structures through Hbonding interaction assists pronounced enhancement of dipole moment. These results imply that to obtain desirable dipole moment and morphology, materials should follow the aggregation assembly in antistacking mode. It is gratifying to note that these results are in corroboration with Würthner’s model on charge transport properties in solid state devices. High dipole moment of AQ01 is believed to be helpful in exciton dissociation at interface which makes them potential candidate for BHJ solar cells. Detailed studies of these acceptors to correlate the structure–property relationship is underway which may help in scrutinize the non-fullerene acceptors for photovoltaics.

EXPERIMENTAL SECTION General Methods. All reagents were purchased from commercial sources. Organic solvents of analytical or spectroscopic grade were dried and freshly distilled by standard procedures if anhydrous solvents were required. The NMR spectra were recorded with a Bruker 500 MHz spectrometer. The ESI-HRMS were recorded on Agilent 6520- QToF. Cyclic voltammetry was performed on CH Instruments 620D electrochemical analyzer. Typically, a three-electrode cell was employed with a glassy carbon working electrode, a Ag/AgCl (nonaqueous) reference electrode, and a Pt wire counter electrode. The measurements were performed at room temperature in anhydrous acetonitrile with tetrabutylammonium hexafluorophosphate ACS Paragon Plus Environment

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solution (0.1 M) as the supporting electrolyte at a scan rate of 100 mV/s. The oxidation potential of ferrocene/ferrocenium was used as internal reference. The absorption and fluorescence data were acquired with ca. ~6 × 10–6 M solutions. The UV/Vis spectra were recorded with a Shimadzu 1800, and the fluorescence studies were performed on Horiba FluroMax 4 spectrofluorometer. The solid films of the compounds were prepared with a spin coater (Holmarc HO-TH-05). For spin coating, solutions with concentration of ~3 mM were prepared in chloroform which were spin coated on the substrates at 5000 RPM for 40 secs. Vacuum deposition was carried out at a base vacuum of 2 x 10-6 mbar. For the case of closed shell systems, the RHF-types of Kohn-Sham methods have been applied. Herein, the def2-TZVP basis set has been employed for all atoms and the energy calculations as well as geometry optimization has been performed at the level of density functional theory (DFT) using the BP86 exchange-correlation functional.17,18 All the theoretical calculations have been performed with the use of ORCA system of programs.54 The grid based DFT has been used as present in ORCA, that employs a typical grid quadrature to compute the integrals. During the SCF procedure, the grid consists of 96 radial shells with 36 and 72 angular points. The charge on each atom has been obtained by the Mulliken population analysis. Device fabrication: ITO coated glass substrates (22 x 12 mm, 15-25Ω/sq, Sigma Aldrich) were etched into desired pattern with the help of Zn powder and 10% HCl such that each substrate could have four separate devices (See schematic of the device in Fig. 4a). Substrate cleaning was done in three simple steps. It was first cleaned with soap solution and rinsed with distilled water. Further, it was sonicated with propanol for 10 mins followed by cleaning with trichloroethylene (TCE) vapors. Thereafter, the substrates underwent an oxygen plasma treatment by keeping them under UV light for 30 mins. For the hole-only device, The PEDOT:PSS layer was spin coated at 8000 rpm for 40 s which was then annealed at 120 0C for 15 mins. After growing the active layer by thermal evaporation (thickness ~70 nm and deposition rate ~ 2 nm/min), 25 nm of MoO3 and 120 nm of Ag were evaporated through shadow masks in vacuum forming devices with active areas between 6-12 mm2. For the electron-only devices, 25 nm of ZnO was thermally evaporated on patterned ITO substrates followed by evaporation of the active layer and LiF and Al. The thicknesses of LiF and Al were kept at 1 nm and 160 nm respectively. The current (I) - Voltage ACS Paragon Plus Environment

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(V) characteristics of the devices were measured at room temperature using a 2400 Keithley source meter. Home built solar simulator having tungsten lamp with output power of 65 mW/cm2 was used for the characterization of solar cell devices. Thickness of each of the layers in the device as well as films of AQ01 and AQ02 grown for morphological characterization by SEM was independently measured and calibrated by making their thin films using shadow masks and measuring their thickness by surface profilometry and cross-sectional SEM. Synthesis of AQ01: 1,2-Diaminoanthraquinone (200 mg, 0.839 mmol) and 4-(dimethylamino)benzaldehyde (150 mg, 1 mmol) were refluxed in 10 mL of nitrobenzene at 160 0C for 18 hours. The reaction mixture is cooled to room temperature and hexane was added to get the precipitate which is then filtered and washed with hexane to get solid compound. The crude mixture was purified by column chromatography using Hexane and Ethyl acetate (70:30) as eluent to get dark violet-blue powder. Yield: 180 mg (58%). 1H NMR (CDCl3, 500 MHz, δ ppm): 11.13 (s, 1H), 8.36 (d, 1H, J = 8.5 Hz); 8.30 (d, 1H, J = 8.5 Hz); 8.20 (d, 1H, J = 8.3 Hz); 8.04 (q, 3H); 7.82 (m, 2H); 6.81 (d, 2H, J = 8.8 Hz); 3.10 (s, 6H); 13C

NMR (CDCl3, 500 MHz, δ ppm): 185.20; 182.53; 157.66; 152.16; 150.12; 134.15; 133.48; 133.26;

128.30; 127.41; 126.32; 124.28; 121.86; 117.116; 115.24; 111.69; 39.96; 29.59; MALDI-TOF: m/z [M]+ calcd. C23H17N3O2, 367.40; found: 368.08; ESI-HRMS: calcd. C23H17N3O2, 367.1320; found: 368.1390 (M+1). Synthesis of AQ02: 1, 2-diaminoanthraquinone (200 mg, 0.839 mmol) and benzo[b]thiophene-2carbaldehyde (165 mg, 1 mmol) were refluxed in 10 mL of nitrobenzene at 160 0C 20 hours. The reaction mixture was cooled to room temperature and hexane was added to get the precipitate. Solid was filtered and washed with hexane several times to get crude solid compound which was further purified by column chromatography using hexane and ethyl acetate (70:30) as eluent to afford AQ02 as brown powder. Yield: 210 mg (55%); 1H NMR (CDCl3, 500 MHz, δ ppm): 11.37 (s, 1H), 8.38 (d, 1H, J = 6.3 Hz); 8.33 (d, 1H, J = 8.2 Hz); 8.28 (d, 1H, J = 8.4 Hz); 8.17 (d, 1H, J = 8.4 Hz); 8.10 (s, 1H); 7.95 (s, 2H); 7.85(m, 2H);


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7.48(m, 2H); MALDI-TOF: m/z [M]+ calcd. C23H12N2O2S, 380.42; found: 381.19; ESI-HRMS: calcd. C23H12N2O2S, 380.0619; found: 381.0690 (M+1). Supporting information: Spectra related to NMR, mass, absorption and emission studies, cyclic voltammograms and figures related to DFT studies, thermal properties, XRD, hole and electron only devices etc. are included. AUTHOR INFORMATION Corresponding Author *([email protected] and [email protected]) Author Contributions All authors have given approval to the final version of the manuscript. Funding Sources No funding is to declare.

ACKNOWLEDGMENT We thank Swati Dixit for assistance in cyclic voltammeteric studies. We thank Tata Institute of Fundamental Research, Mumbai for NMR, MALDI-TOF. We also thank the technical team of FESEM at TIFR, Colaba. Central Drug Research Institute, Lucknow is thanked for HRMS.

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