Unravelling the Self-Assembly of Diketopyrrolopyrrole-Based

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Unravelling the self-assembly of diketopyrrolopyrrole (DPP)-based photovoltaic molecules YuXin Qian, Bin Tu, Ke Gao, Tianxiang Liang, Xu-Hui Zhu, Bo Liu, Wubiao Duan, Xiaobin Peng, Qiaojun Fang, Yanfang Geng, and Qingdao Zeng Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01798 • Publication Date (Web): 17 Sep 2018 Downloaded from http://pubs.acs.org on September 17, 2018

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Unravelling the self-assembly of diketopyrrolopyrrole (DPP)-based photovoltaic molecules Yuxin Qian,1, 2‡ Bin Tu,2‡ Ke Gao,3 Tianxiang Liang,3 Xuhui Zhu,3 Bo Liu,1 Wubiao Duan,1* Xiaobin Peng,3* Qiaojun Fang,2 Yanfang Geng,2* Qingdao Zeng2* 1

Department of Chemistry, School of Science, Beijing Jiaotong University, Beijing 100044, P.R.

China 2

CAS Key Laboratory of Standardization and Measurement for Nanotechnology, CAS Center

for Excellence in Nanoscience National Center for Nanoscience and Technology (NCNST) , Beijing 100190, P.R. China 3

State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer

Optoelectronic Materials and Devices South China University of Technology, Guangzhou 510640, P.R. China ‡

These authors contributed equally to this work.

ACS Paragon Plus Environment

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ABSTRACT: The nanostructure of bulk heterojunction in organic solar cell dominating the electron transport process plays an important role on improving the device efficiency. However, there is still great need for further understanding the local nanostructures from the view of molecular design because of the complex alignment in the solid film. In this work, four kinds of photovoltaic materials containing diketopyrrolopyrrole (DPP) unit combined with other different building blocks were selected and their self-assembled structures on solid surface were studied by scanning tunneling microscopy (STM) technique in combination with theory calculations. The results reveal these DPP-based photovoltaic molecules self-assembled into different nanostructures, which strongly depend on the chemical structure, in particular the backbones and alkyl side chains. The planarities of backbones are affected both by molecule-substrate interaction and steric hindrance induced by the substituted thiophene or benzo[b]thiophene units on DPP and porphyrin building blocks. The substituted branched alkyl side chains are out of the plane, which are influenced by the alignments of molecular backbones. In addition, the solution concentration also shows large effect on the self-assembled nanostructures. This systematic research on the self-assembled structures of DPP-based semiconductors on surface would provide guidance for designing material and controlling morphology of donor/acceptor heterojunction system.

KEYWORDS: diketopyrrolopyrroles, self-assembly, scanning tunneling microscopy


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INTRODUCTION Solar energy has become a key point in energy and material research field as an inexhaustible and clean energy sources. Bulk heterojunction (BHJ) organic solar cells (OSCs) offer great opportunity as renewable energy sources due to their attractive features such as low-cost lager area fabrication, light weight and good mechanical flexibility.1-6 With the emergence of various new donors and acceptors, great progresses have been achieved in developing OSCs s with a prominent increase of power conversion efficiencies (PCEs) approaching to the commercial request.7-11 In addition to the device optimization, the morphology is a currently requested parameter needed to be controlled in the reported works because the morphology of active layer is very closely related to the device efficiency.12-14 The usually used methods such as selecting appropriate solvent, thermal annealing temperature, or solvent additives have been developed to tune the morphology because these external factors strongly affect the molecular segregation, packing as well as orientation.15 In the nanoscale region, donor and acceptor molecules spontaneously organized into the heterojunction systems. The self-assembly systems finally reach dynamic equilibrium as a consequence of the intermolecular interactions. The molecular orientation affect electron transport process, for instance, the face-on orientation due to the π-π stacking interaction is beneficial to the charge transport and separation.16-18 However, the relationship between intermolecular interaction and morphology still needs further understanding at the molecular scale, and the adjustment of molecular packing based on intermolecular interactions is still a challenge in particular. Diketopyrrolopyrrole (DPP) as an important moiety in photovoltaic materials was selected due to the planar conjugated structure and systemic researches.19


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Since Nguyen et al. developed DPP(TBFu)2, 3,6-bis(5-(benzofuran-2-yl)thiophen-2-yl)-2,5bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione) in 2009, DPP(TBFu)2 has been widely investigated in the field of OSCs.20-27 Our co-workers Peng et al. designed a linearly conjugated molecule DT-DPP(AAnAT)2, in which 9,10-anthracenyl (An) and 3-alkylthiophene (T) are symmetrically connected onto the DT-DPP core via acetylene (A) linkage.28 DT-DPP(AAnAT)2 showed potential application in OSCs due to the strong electron withdrawing ability, intense absorption in the visible range and solution-processed ability. Then, they designed a class of DPPEZnP-based small molecules with different substituents on the mesoposition of ZnP units and obtained high PCE over 12%.29-32 Thereinto, DPPEZnP-TEH (5,15-bis(2,5-bis(2ethylhexyl)-3,6-dithienyl-2-yl-2,5-dihydro-pyrrolo[3,4-c]pyrrole-1,4-dione-5’-yl-ethynyl)-10,20bis(5-(2-ethylhexyl)thienyl)porphyrin zinc(II)) contains a porphyrin linked two DPP units by ethynylene bridges,29-32 and another DPPEZnP-based small molecule 5,15-bis(2,5-bis(2ethylhexyl)-3,6-dithienyl-2-yl-2,5-dihydro-pyrrolo[3,4-c]-pyrrole-1,4-dione-5’-yl-ethynyl)10,20-bis(5-(2-butyloctyl)-benzo[b]thiophen-2-yl)-porphyrin zinc(II) (DPPEZnP-BzTBO) was obtained after replacing TEH with slightly electron-deficient BzTBO unit onto the meso positions of the porphyrin core.29-32 The relationship between chemical structure and molecular packing even morphology needs to be further understood from molecular level. Among numerous techniques charactering molecular alignment, scanning tunneling microscopy (STM) can provide molecular orientation and packing information at the submolecular resolutions.33-36 We have always been committed to understanding the intra- and intermolecular interactions in order to bridging the gap between molecular design and morphology adjustment at the molecular level.37-39 In this work, four DPP-based small molecules DPP(TBFu)2, DT-DPP(AAnAT)2, DPPEZnP-TEH, and DPPEZnP-BzTBO were chosen and


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their self-assemblies structures were studied with the aid of STM characterization and theoretical calculations. The results indicate that these four molecules showed different nanostructures induced by the different intermolecular and intramolecular interactions. The backbone and side chains play an important role on tuning the molecular packing on surface. Although the selfassembled structures are assisted on the substrate, the assemblies and tailored properties of these architectures can provide helpful information on the molecular interactions and alignments in OSCs . S

O

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DT-DPP(AAnAT)2

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DPPEZnP-BzTBO

Scheme 1. Chemical structures of DPP(TBFu)2, DT-DPP(AAnAT)2, DPPEZnP-TEH and DPPEZnP-BzTBO without hydrogen atoms. EXPERIMENTAL SECTION Materials. The compounds DPP(TBFu)2, DT-DPP(AAnAT)2, DPPEZnP-TEH, and DPPEZnPBzTBO were synthesized according to the reported method.28-32 Solvents including 1phenyloctane (PO) and tetrahydrofuran (THF) were purchased from J&K Company, and used without further purification. The highly oriented pyrolytic graphite (HOPG, grade ZYB) was obtained from NT-MDT, Russia and used as the substrate.


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Sample preparation. DPP(TBFu)2 and DT-DPP(AAnAT)2 was dissolved in 1-phenyloctane, while porphyrin molecules (DPPEZnP-TEH and DPPEZnP-BzTBO) were dissolved in tetrahydrofuran (THF) solution. The concentration of DPPEZnP-TEH was denoted by C3. Then dilutions of C3 solution were carried out two times to obtain solutions C2 and C1 (C3 = 2 × C2 = 3 × C1). The concentrations of all the solutions were controlled to be less than 10-4 mol·L−1. And then, a droplet (∼0.5 µL) of solution was first deposited on the freshly cleaved HOPG surface. Note that a droplet (∼0.2 µL) of 1-phenyloctane was dropped onto the preprepared DPPEZnPTEH and DPPEZnP-BzTBO surface. The deposition of 1-phenyloctane HOPG surface before measurement would make the molecular STM image clearer because the immersed STM tip into non-volatile 1-phenyloctane solution would reduce the noise. All the STM investigations were performed at the liquid-solid interfaces. STM investigation. An STM investigation was performed at the liquid-solid interface in constant current mode (Nanoscope IIIa SPM system, Veeco USA) under ambient conditions at the liquid/solid interface. The tips were newly mechanically formed Pt/Ir (80/20) wires, and the specific tunneling conditions are described in the corresponding figure notes, which are the best combination of high resolution image can be obtained after several experiments. All the STM images provided in this paper were raw data and they were calibrated by referring the underlying graphite lattice. The experimental parameters of unit cells in the recorded STM images were obtained using NanoScope analysis software, version 5.31R1. Computational details. All the DFT calculations were performed by DMol3code. Periodic boundary conditions (PBCs) were used to describe the 2D periodic structure on graphite in this study. Perdew and Wang parameterization of the local-exchange-correlation energy was applied in the local-spin density approximation to describe exchange and correlation.40 The all electron


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spin-unrestricted Kohn-Sham wave functions were expanded in a local atomic orbital basis. In such double-numerical basis set, polarization was described. All calculations were all-electron and performed with the extra-fine mesh. A self-consistent field procedure was performed with a convergence criterion of 10-5 au on energy and electron density. Combined with the experimental data, we have optimized the parameters of unit cells and the geometry of the adsorbates in the unit cell. When the energy and density convergence criterion was reached, the optimized parameters and the interaction energy between the adsorbates could be obtained. To evaluate the interaction between the adsorbates and HOPG, we have designed the model system. In our study, because adsorption of the adsorbates on graphite and graphene can be considered as very similar, we have performed our calculations on infinite grapheme monolayers using PBCs. In the super lattice, the graphene layers were separated by 40 Å in the normal direction and represented by orthorhombic unit cells containing two carbon atoms. When modeling the adsorbates on graphene, we used graphene supercells and sampled the Brillouin zone using a 1×1×1 k-point mesh. The interaction energy Einter of adsorbates with graphite is given by Einter = Etot(adsorbates/graphene) - Etot(isolated adsorbates in vacuum) - Etot(graphene).

RESULTS AND DISCUSSION Self-Assembly of DPP(TBFu)2 on the HOPG Surface The self-assembly structure of molecule DPP(TBFu)2 at the 1-phenyloctane/HOPG interface is shown in Figure 1. DPP(TBFu)2 molecules self-assembled into a regular linear structure in the long range. The width (W) and length (L) of a bright spot marked with red rectangle are measured to be 0.8 ± 0.1 nm and 0.5 ± 0.1 nm, respectively. The distance between two ended benzofuran units is calculated to be 2.1 nm after molecular optimization. Hence one can see that


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the DPP(TBFu)2 might be non-planar on surface, resulting in different electronic density of states in STM images. After careful analysis on the STM image, the conclusion is that the DPP(TBFu)2 molecule might align along the red arrow direction. The bright spot marked with red circle might be assigned to be one DPP(TBFu)2.

Figure 1. (a) High-resolution STM image of DPP(TBFu)2 self-assembly (Iset = 233.9 pA; Vbias = 699.9 mV). (b) Proposed molecular model of DPP(TBFu)2 based on the DFT calculations with one molecule highlighted in yellow. (c) The side review of one DPP(TBFu)2 molecule. (d) Proposed molecular model of DPP(TBFu)2 without alkyl chains. In combination with STM image, the parameters of unit cells measured to be a = 1.7 ± 0.1 nm, b = 1.0 ± 0.1 nm, α = 88 ± 1° and the geometry of DPP(TBFu)2 in the unit cell was optimized using DMol3code based on density functional theory (DFT). Figure 1(b) is the proposed molecular and self-assembly model, which is consistent with the STM observations. As the side view of DPP(TBFu)2 in Figure 1(c), the alkyl chains substituted on the DPP core are tilted away from the substrate due to the close packed of molecular backbone. The central DPP unit and the


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end benzofuran groups are not coplanar. In order to clearly see the intermolecular interactions between the molecular skeleton, the alkyl chains were omitted as displayed in Figure 1(d). Along the black arrow direction, adjacent two DPP(TBFu)2 molecules might interact with each other through hydrogen bonding C-H···O, where the O and H atoms come from the benzofuran unit. Along the red arrow direction, adjacent two DPP(TBFu)2 molecules might interact with each other through π - π stacking interaction and hydrogen bonding between benzofuran and thiophene units. Because of the intermolecular interactions, DPP(TBFu)2 molecules aligned closely, resulting in the tilted alkyl chains. Self-Assembly of DT-DPP (AAnAT) 2 on the HOPG Surface Unlike DPP(TBFu)2, DT-DPP(AAnAT)2 molecule contains anthracenyl and 3-alkylthiophene connected by acetylene linkage. Figure 2(a) shows the high-resolution STM image of DTDPP(AAnAT)2 self-assembly, in which DT-DPP(AAnAT)2 molecules arrange closely with each other leading to a formation of 2D network. The width W1 and W2 are measured to be 3.0 ± 0.1 nm and 3.8 ± 0.1 nm, respectively. The length of DT-DPP(AAnAT)2 backbone is about 3.6 nm, which is obtained from molecular model software. Therefore, the bright protrusion marked with red circle might be assigned one DT-DPP(AAnAT)2 molecule. The long alkyl chain dodecyl substituted on end-group thiophene ring might locate in the dark region, while the short alkyl chain ethylhexyl attached on DPP unit might title upwards and out of the surface because there is no enough space for their adsorption on surface. The unit cell parameters were measured to be a = 3.5 ± 0.1 nm, b =1.4 ± 0.1 nm, α = 90 ± 1°. On the basis of the STM observations, the corresponding molecular model optimized by the DFT method is shown in Figure 2(b). Figure 2(c) display the vertical and side view of DTDPP(AAnAT)2 molecule on surface. It can be seen that two anthracenyl units along the


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backbone of DT-DPP(AAnAT)2 are tilted relative to the central DPP core. The ethylhexyl chains floated out of the surface because the close-packed molecule. There is almost no intermolecular interaction between the backbones of DT-DPP(AAnAT)2 molecule as shown in Figure 2(d), in which the alkyl chains were omitted. Therefore, two adjacent DT-DPP(AAnAT)2 molecules interacted with each other mainly via van der Waals interaction between the side alkyl chains of molecules, as marked with red circle in Figure 2(e). These close packed side alkane chains with distances in the range of 0.32 nm ~ 0.38 nm are not planar to the surface and these two alkane chains are also not parallel with each other.

Figure 2. (a) High-resolution STM image of DT-DPP(AAnAT)2 self-assemblies. Scanning conditions: Iset = 129.1 pA; Vbias = 729.8 mV. (b) Proposed molecular model of DTDPP(AAnAT)2 based on the DFT calculations with one molecule highlighted in yellow. (c) The vertical and side views of one DT-DPP(AAnAT)2 molecule. One unit molecular model without (d) and with (e) alkyl chains.


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In the self-assembly of DPP(TBFu)2, the interaction between molecules is calculated to be 27.202 kcal·mol-1, which is smaller than that (-157.379 kcal·mol-1) in the system of DTDPP(AAnAT)2. It can be seen that the van der Waals force between DT-DPP(AAnAT)2 molecules is enough strong to stabilize the molecule. The interaction between DPP(TBFu)2 and substrate is -318.197 kcal·mol-1, while the interaction between DT-DPP(AAnAT)2 and substrate is -388.631 kcal·mol-1. Due to the tilted anthracenyl and 3-alkylthiophene part, the interaction between molecule and substrate is not largely different between these two molecules. As a whole, the total energy of DT-DPP(AAnAT)2 self-assembled structure is larger than that of DPP(TBFu)2. However the looser packing of DT-DPP(AAnAT)2 in comparison with DPP(TBFu)2 might lead to unstable self-assembled structure on surface, which is in accordance with the more negative total energy per unit area of DPP(TBFu)2 than DT-DPP(AAnAT)2 system, as shown in Table 2. Self-Assembly of DPPEZnP-TEH on the HOPG Surface Under STM investigation, the molecular self-assemblies of DPPEZnP-TEH molecules can be clearly observed at the 1-phenyloctane/HOPG interface. DPPEZnP-TEH molecule takes porphyrin as the center with four side chains around it, the whole molecule looks like an ‘X’ type. In the STM image, each bright spot might be assigned to one DPPEZnP-TEH molecule. Because the structure and length of the side chain are different, the shape of X is not symmetrical. Figure S1 shows the dependence of DPPEZNP-TEH self-assemblies on the solution concentrations. Two kinds of different structures DPPEZNP-TEH (I) and DPPEZNP-TEH (II) respectively corresponding to the concentration of C1 (1.0 × 10-4 mol·L-1) and C2 (1.5 × 10-4 mol·L-1) were observed. In domain (I), the nanoarray of DPPEZNP-TEH assembled with same orientation, while DPPEZNP-TEH assembled with interlaced orientation in domain (II).


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Figure 3. (a) High-resolution STM image of DPPEZnP-TEH (I) self-assemblies. Scanning conditions: Iset = 134.9 pA; Vbias = 745.9 mV. (c) High-resolution STM image of DPPEZnP-TEH (II) self-assemblies. Scanning conditions: Iset = 159.5 pA; Vbias = 825.9 mV. (b) and (d) correspond to the schematics molecular models of assemblies in STM image (a) and (c), respectively. The high-resolution STM image of the DPPEZNP-TEH (I) and DPPEZNP-TEH (II) is shown in Figure 3(a) and 3(c), respectively. The measured unit cell is superimposed on the molecular model with a = 2.2 ± 0.1 nm, b =2.5 ± 0.1 nm, α = 60 ± 1° and a = 2.8 ± 0.1 nm, b = 5.2 ± 0.1 nm, α = 86 ± 1°, respectively. Figure 3(b) and 3(d) shows the corresponding molecular model calculated by the DFT method on the basis of the STM observations. With increasing the concentration, the configurations of DPPEZnP-TEH (I) and DPPEZnP-TEH (II) are slightly different identified from the vertical and side views as shown in Figures 4(a) and 4(c). The closed arrangements induce both changes of side chain arrangement and molecular configuration. Comparing these two molecular configurations, it is not difficult to find that the thiophene linked to porphyrin has rotated relative


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to the π-system. In addition, the branched chain connected to DPP in DPPEZnP-TEH (II) is more vertical compared with the arrangement in DPPEZnP-TEH (I). In order to further understand the interactions between the molecular backbones, the branched chain in the model is omitted as shown in Figure 4(b) and 4(d). There is almost no interaction between the main chains. The intermolecular interaction is mainly from the van der Waals force between alkyl chains above the surface. The molecular backbone in DPPEZnP-TEH (II) is more planar than that in DPPEZnP-TEH (I), which leads to larger interaction between molecules and substrate. As shown by Table 2, the interaction between DPPEZnP-TEH (II) and substrate (856.416 kcal·mol-1·Å-2) is much greater than that (-493.324 kcal·mol-1·Å-2) between DPPEZnPTEH (I) and substrate. The total energy per unit area of DPPEZnP-TEH (I) is calculated to be 1.152 kcal·mol-1·Å-2, which is more negative than that of the self-assembled structure DPPEZnP-TEH (II) (-0.651 kcal·mol-1·Å-2). With the increase of concentration, it underwent a structural transformation, mainly coming from the relative rotation of DPPEZnP-TEH molecules. Therefore, DPPEZnP-TEH assembled into 2D well-ordered monolayers mainly via the interactions between the molecular backbone and the substrate. Additionally, the relative arrangement of molecules is affected by the side chain including the van der Waals interactions and the effect of space occupation.


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Figure 4. (a) and (c) The vertical and side view of DPPEZnP-TEH in the self-assemblies (I) and (II), respectively. (b) and (d) Proposed molecular model of self-assemblies (I) and (II) without alkyl side chains. As shown in Figure 5, with an increased DPPEZNP-TEH concentration of C3 (3.0 × 10-4 mol·L-1) in the system, domain (I) and (II) of DPPEZNP-TEH disappeared, and only irregular networks of DPPEZNP-TEH occurred. In order to simplify the analysis, ellipse is used to represent a DPPEZNP-TEH molecule. After careful analysis, it can be found that trimeric DPPEZNP-TEH molecules (blue ellipses) and tetrameric DPPEZNP-TEH molecules (red ellipses) are arranged along the white arrow, and an irregular hexagonal cavity appears in the middle. The schematic illumination of such arrangement is shown in Figure S2. However, in some region such alignment is irregular. As shown in Figure 5(b), there are consecutive two and even more trimeric arrangements that disrupt the arrangement between trimeric and tetrameric. In addition, there are still some defects occupied by rotated molecules marked with green ellipses. Due to both the changes molecular orientation and lack of individual molecules, some irregular cavity structures are formed.


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Figure 5. Self-assembly structures of DPPEZnP-TEH networks under the concentration of C3 at different scales (a) (Iset = 305.8 pA, Vbias = 741.8 mV) and (b) (Iset = 230.4 pA, Vbias = 721.9 mV). Though the arrangement is disordered, the arrangements within the trimeric and tetrameric arrangements are very similar with that of DPPEZnP-TEH (II) as shown in Figure 3(c). Along the yellow arrow, the arrangement of the two closed-packed molecules is similar with the arrangement of Figure 3(a). The distance of L1 and L2 are estimated to be 1.8 ± 0.1 nm and 1.6 ± 0.1 nm, respectively. Therefore, the self-assembled alignment of DPPEZnP-TEH at increased concentration is closer than that obtained at lower concentrations. The closer-packed would induce the alkyl side chain perpendicular to the surface. The distance W1 between two DPPEZnP-TEH molecules across corner of hexagonal cavity along the white arrow is estimated to be 2.9 ± 0.1 nm, which is smaller than the width W2 (3.4 ± 0.1 nm) between two molecules on the outer sides of trimeric. It is possible that the irregular hexagonal cavity is filled with alkyl chains coming from six DPPEZnP-TEH molecules. Self-Assembly of DPPEZnP-BzTBO on the HOPG Surface DPPEZnP-BzTBO molecule has different structure with DPPEZnP-THE, such as a longer alkyl chain in BzTBO than THE, a larger π-system of benzo[b]thiophene than thiophene. Therefore it is possible that DPPEZnP-BzTBO molecule adsorb more stably on surface than DPPEZnP-TEH molecule. After depositing the DPPEZnP-BzTBO solution in tetrahydrofuran


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(THF) on a freshly cleaved HOPG substrate, STM scanning were performed at the 1phenyloctane/HOPG interface. From the high-resolution self-assembled STM image, a wellordered linear structure was fabricated on the surface, as shown in Figure 6(a). DPPEZnPBzTBO molecules arrange in rows and connect closely with each other to form the 2D network. Similar with DPPEZnP-TEH molecule, elliptic bright spots were detected. The width (W) and length (L) of the bright spot are measured to be 1.0 ± 0.1 nm and 2.1 ± 0.1 nm, respectively. The width is in accordance with the size of central porphyrin unit, while the length is much shorter than that of DPPEZnP-BzTBO molecule. In comparison with the DPPEZnP-TEH molecule, the elliptic bright spot might be due to its nonplanar structure on HOPG surface. The unit cell superimposed on STM image was measured to be a = 2.2 ± 0.1 nm, b = 1.9 ± 0.1 nm, α = 83 ± 1°. The parameters of the unit cell can be seen from Table 1.

Figure 6. The self-assembly of DPPEZnP-BzTBO molecule. (a) High-resolution STM image of DPPEZnP-BzTBO self-assemblies. Scanning conditions: Iset = 129.1 pA; Vbias = 729.8 mV. (b) Proposed molecular model of DPPEZnP-BzTBO based on the DFT calculations. (c) Vertical


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view of molecule DPPEZnP-BzTBO. (d) Side view of molecule DPPEZnP-BzTBO without alkyl chains. (e) Proposed molecular model of DPPEZnP-BzTBO, in which the alkyl chains were omitted. Figure 6(b) shows the corresponding molecular model calculated by the DFT method on the basis of STM observations. One DPPEZnP-BzTBO molecule is highlighted with yellow color. From the proposed molecular model, it can be seen that DPPEZnP-BzTBO molecule connects closely with each other. In order to investigate the detailed information about the intermolecular or intramolecular interactions, the molecular vertical and side views are given in Figures 6(c) and 6(d). Along the backbone DPP-ZnP-DPP, the molecule is non-planar, and two BzT units are perpendicular to the DPP-ZnP-DPP skeleton. As shown in Figure 6(e), two adjacent DPPEZnPBzTBO molecules interact with each other through part π-π stacking interaction. Therefore, the substituted alkyl chains play an important role on stabilizing the 2D structure on HOPG surface. With the increase of concentration, the assembly of the DPPEZnP-BzTBO system did not undergo a structural transformation. Consequently, the interactions between molecule-molecule and molecule-substrate make the assemblies of DPPEZnP-BzTBO stable existing on the HOPG surface. Table 1. Experimental (Expt.) and calculated (Calcd.) cell parameters for the 2D self-assemblies. Systems DPP(TBFu)2 DT-DPP(AAnAT)2 DPPEZnP-TEH(I) DPPEZnP-TEH(II)

Expt. Cal. Expt. Cal. Expt. Cal. Expt.

Unit cell parameters a (nm) b (nm) 1.7 ± 0.1 1.0 ± 0.1 1.70 0.90 3.5 ± 0.1 1.4 ± 0.1 3.60 1.40 2.2 ± 0.1 2.5 ± 0.1 2.30 2.50 2.8 ± 0.1 5.2 ± 0.1

α (o) 88 ± 1 89.0 90 ± 1 90.0 60 ± 1 60.0 86 ± 1


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DPPEZnP-BzTBO

Cal. Expt. Cal.

2.80 2.2 ± 0.1 2.30

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5.20 1.9 ± 0.1 2.00

85.0 83 ± 1 83.0

Table 2. Total energy and energy per unit area for the observed self-assemblies.

systems

Interactions between adsorbates (kcal·mol-1)

DPP(TBFu)2 DT-DPP(AAnAT)2 DPPEZnP-TEH(I) DPPEZnP-TEH(II) DPPEZnP-BzTBO

-27.202 -157.379 -70.281 -86.382 -98.770

Interactions between adsorbates and substrate (kcal·mol-1) -318.197 -388.631 -493.324 -856.416 -532.223

Total energy (kcal·mol-1)

Total energy per unit area (kcal·mol-1·Å2 )

-345.399 -546.010 -563.605 -942.798 -621.993

-2.161 -1.065 -1.152 -0.651 -1.364

In all these self-assemblies, whether DT-DPP is in the middle or on both sides, DT-DPP adopts a nonplanar geometry allowing the thiophene out of plane in order to form π-π stacking with neighboring molecules.41 The branched alkyl chains on DPP are orientated upwards, which affect the degree of planarity of DT-DPP. By contrast, the molecules with longer alkyl chains tend to be fully planarzed.42 We speculate that the warped alkyl chains might act as an anchor for the growth of 3D crystal structure.43 During the design of photovoltaic materials, some branched chains are usually introduced into molecules in order to enhance the solubility. Compared with linear alkyl chain, branched chain weakens the intermolecular interaction, resulting in increased disorder of molecular alignment, which is an important factor affecting the electron transport of molecules. Therefore, design of appropriate side chains is important to balance the trade-off between molecular solubility and electron transport. The obtained self-assembled structure would give us more inspiration about the design of molecular backbones and branched chains. In


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order to better understand the interaction between donor and receptor molecules in bulk heterojunction films, two and more components would be considered in the next work. CONCLUSIONS In summary, we have made a systematic research on the self-assemblies of four DT-DPPbased photovoltaic molecules on solid surface in combination with DFT calculations and STM technique. Because of the competitive effect between intermolecular and intramolecular interactions, different assembled structures were obtained. Due to the presence of benzofuran unit, adjacent DPP(TBFu)2 molecules interact with each other through hydrogen bonding controlling the self-assembled structure. The closely-packed DT-DPP(AAnAT)2 molecule is mainly attributed to the tilted long alkyl chains. In the case of DPPEZnP analogues, intermolecular steric effect of porphyrin induces the nonplanar molecular geometry. The surface self-assembly modification can be obtained by co-adsorption with guest molecules and is of great benefit to the control of 2D crystal engineering. In addition, the assembled images are expected to enhance the understandings of interactions in solar cells, and has a guiding effect on the design of photovoltaic materials. ASSOCIATED CONTENT Supporting Information. The dependence of DPPEZNP-TEH self-assemblies on the solution concentrations. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected];


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* E-mail: [email protected]; * E-mail: [email protected]; * E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Basic Research Program of China (2016YFA0200700), the National Natural Science Foundation of China (Nos. 21472029, 21773041, 51473053 and 51773065) and the National Key Research and Development Program of China (2017YFA0206602). REFERENCES 1. Hoppe, H.; Sariciftci, N. S., Organic solar cells: An overview. J. Mater. Res. 2004, 19, (7), 1924-1945. 2. Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y., High-efficiency solution processable polymer photovoltaic cells by self-organization of polymer blends. Nat. Mater. 2005, 4, (11), 864-868. 3. Guenes, S.; Neugebauer, H.; Sariciftci, N. S., Conjugated Polymer-Based Organic Solar Cells. Chem. Rev. 2007, 107, (4), 1324-1338. 4. Thompson, B. C.; Fréchet, J. M. J., Polymer–Fullerene Composite Solar Cells. Angew. Chem. Int. Ed. 2008, 47, (1), 58-77. 5. Cheng, Y. J.; Yang, S. H.; Hsu, C. S., Synthesis of conjugated polymers for organic solar cell applications. Chem. Rev. 2009, 109, (11), 5868-5923. 6. Li, G.; Zhu, R.; Yang, Y., Polymer solar cells. Nat. Photonics 2012, 6, (3), 153-161.


20

Page 21 of 25 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

Langmuir

7. Li, Y.; Guo, Q.; Li, Z.; Pei, J.; Tian, W., Solution processable D–A small molecules for bulk-heterojunction solar cells. Energ. Environ. Sci. 2010, 3, (10), 1427-1436. 8. Anthony, J. E., Small-Molecule, Nonfullerene Acceptors for Polymer Bulk Heterojunction Organic Photovoltaics. Chem. Mater. 2011, 23, (3), 583-590. 9. Walker, B.; Kim, C.; Nguyen, T. Q., Small Molecule Solution-Processed Bulk Heterojunction Solar Cells. Chem. Mater. 2017, 23, (3), 470-482. 10. Lin, Y.; Li, Y.; Zhan, X., Small molecule semiconductors for high-efficiency organic photovoltaics. Chem. Soc. Rev. 2012, 41, (11), 4245-4272. 11. Mishra, A.; Bäuerle, P., Small molecule organic semiconductors on the move: promises for future solar energy technology. Angew. Chem. Int. Ed. 2012, 51, (9), 2020-2067. 12. Yang, X.; Loos, J.; Veenstra, S. C.; Verhees, W. J. H.; Wienk, M. M.; Kroon, J. M.; Michels, M. A. J.; Janssen, R. A. J., Nanoscale Morphology of High-Performance Polymer Solar Cells. Nano Lett. 2005, 5, (4), 579-583. 13. Hoppe, H.; Sariciftci, N. S., Morphology of polymer/fullerene bulk heterojunction solar cells. J. Mater. Chem. 2005, 16, (1), 45-61. 14. And, X. Y.; Joachim Loos, Toward High-Performance Polymer Solar Cells: The Importance of Morphology Control. Macromolecules 2007, 40, (5), 1353-1362. 15. Dang, M. T.; Hirsch, L.; Wantz, G.; Wuest, J. D., Controlling the Morphology and Performance of Bulk Heterojunctions in Solar Cells. Lessons Learned from the Benchmark Poly(3-hexylthiophene):[6,6]-Phenyl-C61-butyric Acid Methyl Ester System. Chem. Rev. 2013, 113, (5), 3734-3765. 16. Walker, B.; Tamayo, A. B.; Dang, X. D.; Zalar, P.; Seo, J. H.; Garcia, A.; Tantiwiwat, M.; Nguyen, T. Q., Flexible Organic Solar Cells: Nanoscale Phase Separation and High Photovoltaic


21

Langmuir 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

Page 22 of 25

Efficiency in Solution‐Processed, Small‐Molecule Bulk Heterojunction Solar Cells (Adv. Funct. Mater. 19/2009). Adv. Funct. Mater. 2009, 19, (19), 3063-3069. 17. Sun, Y.; Welch, G. C.; Wei, L. L.; Takacs, C. J.; Bazan, G. C.; Heeger, A. J., Solutionprocessed small-molecule solar cells with 6.7% efficiency. Nat. Mater. 2012, 11, (1), 44-48. 18. Ameri, T.; Khoram, P.; Min, J.; Brabec, C. J., Organic ternary solar cells: a review. Adv. Mater. 2013, 25, (31), 4245-4266. 19. Li, Y.; Sonar, P.; Murphy, L.; Hong, W., High mobility diketopyrrolopyrrole (DPP)-based organic semiconductor materials for organic thin film transistors and photovoltaics. Energ. Environ. Sci. 2013, 6, (6), 1684-1710. 20. Walker, B.; Tamayo, A. B.; Dang, X. D.; Zalar, P.; Seo, J. H.; Garcia, A.; Tantiwiwat, M.; Nguyen, T. Q., Flexible Organic Solar Cells: Nanoscale Phase Separation and High Photovoltaic Efficiency in Solution‐Processed, Small‐Molecule Bulk Heterojunction Solar Cells. Adv. Funct. Mater. 2009, 19, (19), 3063-3069. 21. Walker, B.; Tamayo, A.; Duong, D. T.; Dang, X. D.; Kim, C.; Granstrom, J.; Nguyen, T. Q., A Systematic Approach to Solvent Selection Based on Cohesive Energy Densities in a Molecular Bulk Heterojunction System. Adv. Energy Mater. 2011, 1, (2), 221-229. 22. Zhang, Y.; Dang, X. D.; Kim, C.; Nguyen, T. Q., Effect of Charge Recombination on the Fill Factor of Small Molecule Bulk Heterojunction Solar Cells. Adv. Energy Mater. 2011, 1, (4), 610617. 23. Liu, J.; Zhang, Y.; Phan, H.; Sharenko, A.; Moonsin, P.; Walker, B.; Promarak, V.; Nguyen, T. Q., Effects of Stereoisomerism on the Crystallization Behavior and Optoelectrical Properties of Conjugated Molecules. Adv. Mater. 2013, 25, (27), 3645-3650. 24. Fernández, D.; Viterisi, A.; Ryan, J. W.; Gispert-Guirado, F.; Vidal, S.; Filippone, S.; Martín,


22

Page 23 of 25 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

Langmuir

N.; Palomares, E., Small molecule BHJ solar cells based on DPP(TBFu)2 and diphenylmethanofullerenes

(DPM):

linking

morphology,

transport,

recombination

and

crystallinity. Nanoscale 2014, 6, (11), 5871-5878. 25. Sun, K.; Xiao, Z.; Hanssen, E.; Klein, M. G.; Dam, H.; Pfaff, M.; Gerthsen, D.; Wong, W. H.; Jones, D., The role of solvent vapor annealing in highly efficient air-processed small molecule solar cells. J. Mater. Chem. A 2014, 2, (24), 9048-9054. 26. Jeon, I.; Sakai, R.; Nakagawa, T.; Setoguchi, H.; Matsuo, Y., Stability of diketopyrrolopyrrole small-molecule inverted organic solar cells. Org. Electron. 2016, 35, 193198. 27. Rahmanudin, A.; Jeanbourquin, X. A.; Hänni, S.; Sekar, A.; Ripaud, E.; Yao, L.; Sivula, K., Morphology stabilization strategies for small-molecule bulk heterojunction photovoltaics. J. Mater. Chem. A 2017, 5, 17517-17524. 28. Wang, L. P.; Xia, Y.; Luo, G. P.; Zhang, C. H.; Liu, Q.; Tan, W. Y.; Zhu, X. H.; Wu, H. B.; Peng, J.; Cao, Y., A Solution‐Processable Dithienyldiketopyrrolopyrrole Dye Molecule with Acetylene as a π‐Linkage for Organic Solar Cells. Asian J. Org. Chem. 2015, 4, (5), 470-476. 29. Gao, K.; Li, L.; Lai, T.; Xiao, L.; Huang, Y.; Huang, F.; Peng, J.; Cao, Y.; Liu, F.; Russell, T. P., Deep absorbing porphyrin small molecule for high-performance organic solar cells with very low energy losses. J. Am. Chem. Soc. 2015, 137, (23), 7282-7285. 30. Gao, K.; Miao, J.; Xiao, L.; Deng, W.; Kan, Y.; Liang, T.; Wang, C.; Huang, F.; Peng, J.; Cao, Y., Multi‐Length‐Scale Morphologies Driven by Mixed Additives in Porphyrin‐Based Organic Photovoltaics. Adv. Mater. 2016, 28, (23), 4727-4733. 31. Xiao, B.; Zhao, Y.; Tang, A.; Wang, H.; Yang, J.; Zhou, E., PTB7-Th based organic solar cell with a high Voc of 1.05 V by modulating the LUMO energy level of benzotriazole-


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Page 24 of 25

containing non-fullerene acceptor. Sci. Bull. 2017, 62, (18), 1275-1282. 32. Liang, T.; Xiao, L.; Gao, K.; Xu, W.; Peng, X.; Cao, Y., Modifying the Chemical Structure of a Porphyrin Small Molecule with Benzothiophene Groups for the Reproducible Fabrication of High Performance Solar Cells. ACS Appl. Mater. Interface 2017, 9, (8), 7131-7138. 33. Reddy, J. S.; Anand, V. G., Aromatic expanded isophlorins: stable 30pi annulene analogues with diverse structural features. J. Am. Chem. Soc. 2009, 131, (42), 15433-15439. 34. Otsuki, J., STM studies on porphyrins. Coordin. Chem. Rev. 2010, 254, (19), 2311-2341. 35. Yuan, Q.; Xing, Y.; Borguet, E., An STM study of the pH dependent redox activity of a twodimensional hydrogen bonding porphyrin network at an electrochemical interface. J. Am. Chem. Soc. 2010, 132, (14), 5054-5060. 36. Akimenko, S. S.; Gorbunov, V. A.; Myshlyavtsev, A. V., Modeling of Monocarboxyphenyl Substituted Porphyrin Adsorption on Au(111) . Procedia Engineering 2015, 113, 108-112. 37. Xu, J.; Liu, W.; Geng, Y.; Deng, K.; Zhan, C.; Zeng, Q., An STM/STS study of site-selective adsorption of C70 molecules onto arc-shaped BODIPY molecular-networks. Nanoscale 2017, 9, (7), 2558-2564. 38. Geng, Y.; Li, P.; Xue, J.; Luo, D.; Zhang, J.; Shu, L.; Deng, K.; Xie, J.; Zeng, Q., Specific distribution of orientated C70-fullerene triggered by solvent-tuned macrocycle adlayer. Nano Research 2017, 10, (3), 991-1000. 39. Dai, H.; Wang, S.; Hisaki, I.; Nakagawa, S.; Ikenaka, N.; Deng, K.; Xiao, X.; Zeng, Q., OnSurface Self-Assembly of a C3 -Symmetric Ï -Conjugated Molecule Family Studied by STM: Two-Dimensional Nanoporous Frameworks. Chem. Asian J. 2017, 12, (19), 2558-2564. 40. Perdew, J. P.; Wang, Y., Accurate and simple analytic representation of the electron-gas correlation energy. Phys. Rev. B: Condens. Matter. 1992, 45, (23), 13244-13249.


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Langmuir

41. Fu, C.; Beldon, P.; Perepichka, D. F., H bonding Control of Supramolecular Ordering of Diketopyrrolopyrroles. Chem. Mater. 2017, 29, (7), 2979-2987. 42.

Fu,

C.;

Bélangergariépy,

F.;

Perepichka,

D.

F., Supramolecular

ordering

of

difuryldiketopyrrolopyrrole: the effect of alkyl chains and inter-ring twisting. CrystEngComm 2016, 18, (23), 4285-4289. 43. Fu, C.; Lin, H.; Macleod, J. M.; Krayev, A.; Rosei, F.; Perepichka, D. F., Unravelling the Self-Assembly of Hydrogen Bonded NDI Semiconductors in 2D and 3D. Chem. Mater. 2016, 28, (3), 951-961.

TOC:

O O

N

N

N S

S S

S

N

O

O

The diketopyrrolopyrrole (DPP)-based photovoltaic molecules with DPP in the middle or on both side show closely packed nanostructures strongly depending on the intermolecular and intramolecular interaction induced by the nonplanar backbone and branched or long alkyl side chains.


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Unravelling the Self-Assembly of Diketopyrrolopyrrole-Based

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