Planar D- π -A Conﬁgured Dimethoxy Vinylbenzene Based Small Organic Molecule for Solution-Processed Bulk Heterojunction Organic Solar Cells

: A new and e ﬀ ective planar D- π -A conﬁgured small organic molecule (SOM) of 2-5-(3,5-dimethoxystyryl)thiophen-2-yl)methylene)-1H-indene-1,3(2H)-dione, abbreviated as DVB-T-ID, was synthesized using 1,3-indanedione acceptor and dimethoxy vinylbenzene donor units, connected through a thiophene π -spacer. The presence of a dimethoxy vinylbenzene unit and π -spacer in DVB-T-ID signiﬁcantly improved the absorption behavior by displaying maximum absorbance at ~515 nm, and the reasonable band gap was estimated as ~2.06 eV. The electronic properties revealed that DVB-T-ID SOMs exhibited promising HOMO ( − 5.32 eV) and LUMO ( − 3.26 eV). The synthesized DVB-T-ID SOM was utilized as donor material for fabricating solution-processed bulk heterojunction organic solar cells (BHJ-OSCs) and showed a reasonable power conversion e ﬃ ciency (PCE) of ~3.1% with DVB-T-ID:PC 61 BM (1:2, w / w ) active layer. The outcome of this work clearly reﬂects that synthesized DVB-T-ID based on 1,3-indanedione units is a promising absorber (donor) material for BHJ-OSCs. compared with the intensity of PL emission in the DVB-T-ID thin film, a strong quenching occurs in the DVB-T-ID:PC 61 BM blend thin films, confirming a good exciton dissociation and a fast charge transfer at the interface of DVB-T-ID:PC 61 BM [36]. The improved charge transportation in the DVB-T-ID:PC 61 BM blend thin film might impart to generate a high photocurrent. Hence, DVB-T-ID:PC 61 BM blend thin films are promising for fabricating efficient BHJ-OSCs.


Introduction
In light of searching for renewable energy resources, solar energy is one of the most promising sustainable energy sources to fulfil global energy demand [1][2][3][4][5]. In the last few years, promising solar cells named solution-processed bulk heterojunction organic solar cells (BHJ-OSCs) have allured extensive research interest, owing to their marvelous advantages, such as their high power conversion efficiency (PCE) over conventional solar cells [6]. Recently, small organic molecules (SOMs) substituting strong electron donor (D) and acceptor (A) groups have been largely employed in BHJ-OSCs. Progressively, SOMs with a variety of D-A molecular networks are witnessed for the incessant upgradation of PCEs, rising from 1% to over 14% within two decades [7]. In BHJ-OSCs, the bulk phases considerably reduce the photoexcitation distance to generate a large amount of charges at separating interfaces, and also enhance the charge dissociation probability at the donor-acceptor interface [8,9]. SOMs and organic chromophores have shown fabulous semiconducting natures in

Synthesis of 1, 3 Indandione (1)
Intermediate 1 was prepared in a similar fashion as reported in our previous work [13]. In a round bottom Schlenk flask, benzoyl chloride (0.913 g, 6.496 mmol), malonyl chloride (0.915 g, 6.496 mmol) and dry AlCl 3 (2.598 g, 19.490 mmol) were dissolved in anhydrous nitrobenzene (50 mL) and stirred at 80 • C for 8 h under argon atmosphere. After refluxing and cooling, the mixture was poured into an aqueous solution of Na 2 CO 3 (120 mL). The organic layer was extracted with dichloromethane (4 × 50 mL), acidified with aqueous HCl (80 mL), separated by diethyl ether (2 × 80 mL) and later dried over Na 2 SO 4 . Applying flash column chromatography, the organic crude product was purified using a mixture of diethyl ether:hexane (1:5, v/v) as eluent, followed by drying in a vacuum oven at 70 • C. The recrystallization of the obtained product was carried out in diethyl ether: EtOH (1:5, v/v) and finally collected as a pale yellow solid product (yield 67.09%, 0.637 g). 1 HNMR (500 MHz, CDCl 3

Device Fabrication
For the fabrication of BHJ-OSCs, ITO (12-16 Ω/sq, Samsung-electronics, Suwon, South Korea) glass substrates were cleaned by sonication with detergent, acetone, DI water and isopropyl alcohol for 10 min and dried in an oven. Firstly, a compact titanium dioxide (c-TiO 2 ) layer was deposited on the ITO substrate using a spin coater at~3000 rpm for~30 s, subsequently annealed at~150 • C in a vacuum oven for 10 min and subjected to 450 • C in a furnace for 30 min. The blended solutions for the photoactive layers were prepared by mixing DVB-T-ID and PC 61 BM in different weight percentages (DVB-T-ID:PC 61 BM, 1:1, 1:2, and 1:3, w/w) in chlorobenzene and stirred at 60 • C for 1 h to obtain the solutions. Thereafter, these solutions were spin coated onto a c-TiO 2 layer with a scan rate of~2000 rpm for~40 s and quickly subjected to annealing in a vacuum oven at~70 • C for 10 min. A top electrode of gold (Au,~100 nm thickness) was deposited over the DVB-T-ID:PC 61 BM/c-TiO 2 /ITO by controlled thermal evaporation to complete the device configuration of Au/DVB-T-ID:PC 61 BM/c-TiO 2 /ITO for solution-processed BHJ-OSC.

Characterizations
FTIR spectroscopy was executed by an FTIR-4100 (JASCO, Tokyo, Japan) spectrometer to investigate the structural features of SOMs. An FT-NMR spectrophotometer (JEOL, Tokyo, Japan) was applied for the proton/C 13 nuclear magnetic resonance ( 1 HNMR (500 MHz) and 13 CNMR (125 MHz)) spectra and measured the chemical shift (δ) in ppm, using tetramethylsilane (TMS) as an internal standard in the reference solvents of CDCl 3 and (CD 3 ) 2 SO. A XEVO TQ-S spectrophotometer was used to determine the mass spectra (MS) of synthesized SOMs. The UV-Vis (V-670 (JASCO, Tokyo, Japan)) and photoluminescence (PL, FP-6500 fluorometer, excitation wavelength: 480 nm) spectroscopies were utilized for investigating the optical behavior of SOMs. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were measured by a TA instrument (DSC-2910, Q-50) at the fixed scan rate of 10 • C/min under N 2 gas. The electrochemical cyclic voltammogram (CV) was evaluated by a WPG 100 Potentiostat/Galvanostat (WonaTech) with three electrode electrochemical systems, including a working SOM-coated glassy carbon electrode, saturated calomel reference electrode (SCE) and a counter electrode composed of platinum wire, using 0.1 M TBAPF 6 in acetonitrile as the supporting electrolyte at a scan rate of 150 mV/s. The surface morphologies of the blended thin films were analyzed by AFM in tapping and 3D mode, using an AFM Nanoscope instrument. To extract the photovoltaic parameters, the current density-voltage (J-V) characteristics of the solar cells were tested under 1 sun (AM 1.5 G, 100 mW/cm 2 ) using the light source of a metal halide lamp (Phillips, 1000 W, Newyork City, NY, USA). The radiation power of the light was fixed by using a Si photodiode as a reference, calibrated at NREL (Golden, CO, USA).

Thermal Properties of DVB-T-ID SOM
The thermal behavior and stability of DVB-T-ID were investigated by measuring the TGA and DSC, as shown in Figure 1. The TGA plot displays a high decomposition temperature (T d ) at~330 • C with 5% weight loss, indicating a high thermal stability of DVB-T-ID SOMs. In support, the DSC thermogram ( Figure 1b) shows a sharp melting temperature (T m ) at~188 • C and an exothermic crystalline transition (T c ) occurs at~370 • C. Strong T m and T c peaks are attributed to a liquid crystal (LC) transition phase and a good intermolecular cohesion in D-π-A, generally due to self-organizational behavior in a solid state, resulting in π-π stacking and possible phase transition [31,32].
surface morphologies of the blended thin films were analyzed by AFM in tapping and 3D mode, using an AFM Nanoscope instrument. To extract the photovoltaic parameters, the current density-voltage (J-V) characteristics of the solar cells were tested under 1 sun (AM 1.5 G, 100 mW/cm 2 ) using the light source of a metal halide lamp (Phillips, 1000 W, Newyork City, NY, USA). The radiation power of the light was fixed by using a Si photodiode as a reference, calibrated at NREL (Golden, CO, USA).

Thermal Properties of DVB-T-ID SOM
The thermal behavior and stability of DVB-T-ID were investigated by measuring the TGA and DSC, as shown in Figure 1. The TGA plot displays a high decomposition temperature (Td) at ~330 °C with 5% weight loss, indicating a high thermal stability of DVB-T-ID SOMs. In support, the DSC thermogram ( Figure 1b) shows a sharp melting temperature (Tm) at ~188 °C and an exothermic crystalline transition (Tc) occurs at ~370 °C. Strong Tm and Tc peaks are attributed to a liquid crystal (LC) transition phase and a good intermolecular cohesion in D-π-A, generally due to self-organizational behavior in a solid state, resulting in π-π stacking and possible phase transition [31,32].

Optical Properties of DVB-T-ID SOM
The solubility test reveals that the synthesized DVB-T-ID SOM presents excellent solubility in most common laboratory solvents like chlorobenzene, tetrahydrofuran (THF) and chloroform. UV-Vis spectroscopy has been performed to explain the absorption behavior of DVB-T-ID SOMs, as depicted in Figure 2a. In chloroform, the prominent absorption band, at ~483 nm, and weak absorption, at ~315 nm, are detected by DVB-T-ID, as summarized in Table 1. These absorption bands basically represent π-π * (weak absorption) and n-π * (strong absorption) transitions, which might be beneficial for the charge transfer process [33]. As compared to the solution sample, the significant red shift with the absorption band at ~515 nm is observed in the DVB-T-ID solid thin film, suggesting the presence of strong intramolecular interaction, backbone planarity and π-π stacking in a solid state [34,35]. Using the absorption onset value of the maximum absorption peak in Figure 2a, the optical energy band gap (Eg opt ) of the DVB-T-ID thin film is estimated as ~2.06 eV by considering an equation of Eg opt = 1240/λonset. To understand the charge carriers and luminescence properties of the DVB-T-ID SOM, the PL spectroscopies ( Figure 2b) are studied in a chloroform solution and in thin films. The DVB-T-ID SOM displays a strong emission peak at ~560 and ~620 nm in chloroform solution and thin film, respectively. The red shift in the thin film is related to the inter-chromophoric aggregation or planarization of organic molecules.

Optical Properties of DVB-T-ID SOM
The solubility test reveals that the synthesized DVB-T-ID SOM presents excellent solubility in most common laboratory solvents like chlorobenzene, tetrahydrofuran (THF) and chloroform. UV-Vis spectroscopy has been performed to explain the absorption behavior of DVB-T-ID SOMs, as depicted in Figure 2a. In chloroform, the prominent absorption band, at~483 nm, and weak absorption, at~315 nm, are detected by DVB-T-ID, as summarized in Table 1. These absorption bands basically represent π-π * (weak absorption) and n-π * (strong absorption) transitions, which might be beneficial for the charge transfer process [33]. As compared to the solution sample, the significant red shift with the absorption band at~515 nm is observed in the DVB-T-ID solid thin film, suggesting the presence of strong intramolecular interaction, backbone planarity and π-π stacking in a solid state [34,35]. Using the absorption onset value of the maximum absorption peak in Figure 2a The DVB-T-ID SOM displays a strong emission peak at~560 and~620 nm in chloroform solution and thin film, respectively. The red shift in the thin film is related to the inter-chromophoric aggregation or planarization of organic molecules.   The donor-acceptor (DVB-T-ID:PC61BM) thin film was further characterized by UV-Vis absorption spectra to investigate the outcome of photon harvesting. Figure 3a depicts the UV-Vis spectra of DVB-T-ID:PC61BM thin films with different blend ratios of 1:1, 1:2 and 1:3, w/w. All blended thin films exhibit a broad absorption which covers the visible region of ~400 to ~600 nm, suggesting DVB-T-ID as a competent light-harvesting OSM for BHJ-OSCs. The DVB-T-ID:PC61BM (1:2, w/w) thin film displays the highest absorption intensity, which might be associated with the non-aggregated, uniform and smooth surface of the thin film. Furthermore, the PL spectra of DVB-T-ID:PC61BM blend thin films are measured to investigate the charge transfer behavior from donor to acceptor, as shown in Figure 3b. As compared with the intensity of PL emission in the DVB-T-ID thin film, a strong quenching occurs in the DVB-T-ID:PC61BM blend thin films, confirming a good exciton dissociation and a fast charge transfer at the interface of DVB-T-ID:PC61BM [36]. The improved charge transportation in the DVB-T-ID:PC61BM blend thin film might impart to generate a high photocurrent. Hence, DVB-T-ID:PC61BM blend thin films are promising for fabricating efficient BHJ-OSCs.   The donor-acceptor (DVB-T-ID:PC61BM) thin film was further characterized by UV-Vis absorption spectra to investigate the outcome of photon harvesting. Figure 3a depicts the UV-Vis spectra of DVB-T-ID:PC61BM thin films with different blend ratios of 1:1, 1:2 and 1:3, w/w. All blended thin films exhibit a broad absorption which covers the visible region of ~400 to ~600 nm, suggesting DVB-T-ID as a competent light-harvesting OSM for BHJ-OSCs. The DVB-T-ID:PC61BM (1:2, w/w) thin film displays the highest absorption intensity, which might be associated with the non-aggregated, uniform and smooth surface of the thin film. Furthermore, the PL spectra of DVB-T-ID:PC61BM blend thin films are measured to investigate the charge transfer behavior from donor to acceptor, as shown in Figure

Electrochemical and Photovoltaic Properties of DVB-T-ID OSM
The electronic properties, in terms of HOMO and LUMO energy levels, were studied by performing cyclic voltammetry (CV) of a DVB-T-ID thin film in freshly prepared 0.1 M TBAPF 6 in acetonitrile at a scan rate of 150 mV/s, using Fc/Fc + as an external reference. From Figure 4, the DVB-T-ID thin film records an oxidation onset potential value of +0.30 V. For the calculation of HOMO and LUMO energy levels, the following equations are employed [13]: where E ox and E 1/2 are the onset oxidation potential of the donor material and the redox potential of Fc/Fc + versus Ag/AgCl, respectively. Using the above equations, the DVB-T-ID SOM possesses an impressive HOMO of~−5.32 eV and LUMO of~−3.26 eV. The energy offset between the LUMO and HOMO of the donor DVB-T-ID SOM with a PC 61 BM acceptor is sufficiently high and provides enough of a driving force for charge dissociation.

Electrochemical and Photovoltaic Properties of DVB-T-ID OSM
The electronic properties, in terms of HOMO and LUMO energy levels, were studied by performing cyclic voltammetry (CV) of a DVB-T-ID thin film in freshly prepared 0.1 M TBAPF6 in acetonitrile at a scan rate of 150 mV/s, using Fc/Fc + as an external reference. From Figure 4, the DVB-T-ID thin film records an oxidation onset potential value of +0.30 V. For the calculation of HOMO and LUMO energy levels, the following equations are employed [13]:     . It is notable that the fabricated BHJ-OSC exhibits a relatively high FF, without the addition of additives or promotors in the active layer. Generally, the FF in organic solar cells (OSCs) relies on the surface properties of semiconducting materials, like charge mobility and the impact of morphology of the active layer [37,38]. Herein, a considerable advancement in the morphology of the active layer of DVB-T-ID:PC 61 BM (1:2, w/w) at the nanoscale level delivers a high FF, as discussed in AFM studies.
of the PC61BM (~−4.37 eV). It is notable that the fabricated BHJ-OSC exhibits a relatively high FF, without the addition of additives or promotors in the active layer. Generally, the FF in organic solar cells (OSCs) relies on the surface properties of semiconducting materials, like charge mobility and the impact of morphology of the active layer [37,38]. Herein, a considerable advancement in the morphology of the active layer of DVB-T-ID:PC61BM (1:2, w/w) at the nanoscale level delivers a high FF, as discussed in AFM studies. To explore the morphological behavior of the blend films, AFM measurements of DVB-T-ID:PC61BM thin films in tapping mode were performed. Figure 6 presents the AFM images of DVB-T-ID:PC61BM thin films with different blend ratios. With a blend ratio of 1:2, w/w, the DVB-T-ID:PC61BM thin film displays smooth and uniform morphology, exhibiting small grains at a nanoscale level, as shown in Figure 6c. In addition, the height and 3D images of the DVB-T-ID:PC61BM (1:2, w/w) thin film again confirm the smooth surface, which might be a consequence of the well-mixed donor-acceptor in blend form, leading to favorable interpenetrating networks with an appropriate degree of phase separation and a high charge carrier extraction [39]. A low root-mean-square roughness (Rrms) value of ~4.37 nm was estimated for the DVB-T-ID:PC61BM (1:2, w/w) thin film, whereas other blend ratios presented high Rrms values. The blend film roughness is basically controlled with the amount of the PC61BM acceptor in the blend film. Herein, further increments of the PC61BM acceptor (1:3, w/w) lead to the formation of large aggregates. Large aggregates in a DVB-T-ID:PC61BM (1:3, w/w) thin film might prevent an immediate charge separation, which is also supported by PL quenching in (1:3, w/w) and thus results in poor photovoltaic performances. Moreover, a high FF of BHJ-OSCs might due to good surface smoothness, uniform grain sizes and low surface roughness [40], which might result in a fast charge extraction. Therefore, the optimal active layer of DVB-T-ID:PC61BM (1:2, w/w) evidences the good miscibility of DVB-T-ID and PC61BM, which might be helpful in forming a bi-continuous network structure through quick phase segregation. This phenomenon promotes the proper charge carrier collection for BHJ solar cell devices, resulting in a high photocurrent and a high PCE. To explore the morphological behavior of the blend films, AFM measurements of DVB-T-ID:PC 61 BM thin films in tapping mode were performed. Figure 6 presents the AFM images of DVB-T-ID:PC 61 BM thin films with different blend ratios. With a blend ratio of 1:2, w/w, the DVB-T-ID:PC 61 BM thin film displays smooth and uniform morphology, exhibiting small grains at a nanoscale level, as shown in Figure 6c. In addition, the height and 3D images of the DVB-T-ID:PC 61 BM (1:2, w/w) thin film again confirm the smooth surface, which might be a consequence of the well-mixed donor-acceptor in blend form, leading to favorable interpenetrating networks with an appropriate degree of phase separation and a high charge carrier extraction [39]. A low root-mean-square roughness (R rms ) value of~4.37 nm was estimated for the DVB-T-ID:PC 61 BM (1:2, w/w) thin film, whereas other blend ratios presented high R rms values. The blend film roughness is basically controlled with the amount of the PC 61 BM acceptor in the blend film. Herein, further increments of the PC 61 BM acceptor (1:3, w/w) lead to the formation of large aggregates. Large aggregates in a DVB-T-ID:PC 61 BM (1:3, w/w) thin film might prevent an immediate charge separation, which is also supported by PL quenching in (1:3, w/w) and thus results in poor photovoltaic performances. Moreover, a high FF of BHJ-OSCs might due to good surface smoothness, uniform grain sizes and low surface roughness [40], which might result in a fast charge extraction. Therefore, the optimal active layer of DVB-T-ID:PC 61 BM (1:2, w/w) evidences the good miscibility of DVB-T-ID and PC 61 BM, which might be helpful in forming a bi-continuous network structure through quick phase segregation. This phenomenon promotes the proper charge carrier collection for BHJ solar cell devices, resulting in a high photocurrent and a high PCE.

Conclusions
New and effective planar D-π-A (DVB-T-ID) SOMs based on 1,3-indanedione acceptor and dimethoxy vinylbenzene donor units are synthesized and used as donor or absorber materials for BHJ-OSCs. DVB-T-ID SOMs show a good solubility in most of the common solvents and demonstrate promising photophysical properties. The moderate HOMO and LUMO energy levels of~5.32 eV and~3.26 eV were obtained for DVB-T-ID SOMs. As a donor material, BHJ-OSC fabricated with DVB-T-ID:PC 61 BM (1:2, w/w) achieved a moderate PCE of~3.1% and a J SC of~10.63 mA/cm 2 , along with an FF of~0.48. In this work, the enhanced PCE and photocurrent density might be associated with good miscibility and intermolecular charge transfer between the DVB-T-ID and PC 61 BM moieties.