New Non-Fullerene Acceptor with Extended Conjugation of Cyclopenta [2,1-b:3,4-b’] Dithiophene for Organic Solar Cells

Herein, we design and characterize 9-heterocyclic ring non-fullerene acceptors (NFAs) with the extended backbone of indacenodithiophene by cyclopenta [2,1-b:3,4-b’] dithiophene (CPDT). The planar conjugated CPDT donor enhances absorption by reducing vibronic transition and charge transport. Developed NFAs with different end groups shows maximum absorption at approximately 790–850 nm in film. Because of the electronegative nature of the end-group, the corresponding acceptors showed deeper LUMO energy levels and red-shifted ultraviolet absorption. We investigate the crystallinity, film morphology, surface energy, and electronic as well as photovoltaic performance. The organic photovoltaic cells using novel NFAs with the halogen end groups fluorine or chlorine demonstrate better charge collection and faster exciton dissociation than photovoltaic cells using NFAs with methyl or lacking a substituent. Photovoltaic devices constructed from m-Me-ITIC with various end groups deliver power conversion efficiencies of 3.6–11.8%.

In this paper, we utilize extended conjugation: the conjugation of indacenodithiophene is extended by substituting the outward thienothiophene moieties with cyclopenta [2,1b:3,4-b'] dithiophene (CPDT), which enables construction 9-heterocyclic ring acceptor, meta-methyl-ITIC (m-Me-ITIC). The symmetric and planar conjugated CPDT electron rich moiety enhances absorption because of reduced vibronic transition and enhances the charge transport. Additionally, extended conjugate bridging units improved the absorption in the long-wavelength region. Meta-alkoxy phenyl side chains provide good solubility in common solvents, and different end groups lead to distinct LUMO energy levels with good morphology in the film state [28]. Blend with PBDB-T polymer donor, the m-Me-ITIC device delivers an open-circuit voltage (Voc) of 0.78 V, and short-circuit current density (Jsc) of 23.7 mA·cm -2 , fill factor (FF) of 64%, and PCE of 11.8%. n-BuLi (5.4 mL, 13.5 mmol) was dropped into THF solution of 1-bromo-3-((2-ethylhexyl) oxy)benzene (6.0 g, 13.1 mmol) at −78 • C and string for 1.5 h. Compound 1 (2 g, 2.2 mmol) was added to the mixture, and after stirred at 25 • C overnight, the solvent was removed. The crude product was added to 100 mL THF with 10 g amberlyst ® 15(H) and stirred for 1 h at 70 • C after cooling down to temperature and removing the solvent, crude purified with silica gel chromatography (hexane/dichloromethane 9:1) to give compound 2 (1.9 g, 65%). 1

Result and Discussion
The synthetic scheme of m-Me-ITIC series was presented in Scheme 1. We treated compound 1 with freshly prepared (3-((2-ethylhexyl)oxy)phenyl)lithium-yielded diol, then intramolecular Friedel-Crafts cyclization to was carried out to furnish compound 2 (IT). This IT was reacted with POCl 3 and dimethylformamide to yield carbaldehyde, which was then capped with various IC groups by the Knoevenagel condensation reaction to generate the desired compound. Each of the new acceptors was characterized by NMR, and mass spectrometry (Figures S1-S6 in Supporting Information). The resulting acceptors showed good solubility at room temperature in common solvent.
Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) was carried out to investigate the thermal stability of the m-Me-ITIC acceptors (Figures S7 and S8 in Supporting Information). All four acceptors have decomposition temperatures (Td) > 330 • C (5% weight loss), indicating m-Me-ITIC series have good thermal stability. Differential scanning calorimetry revealed that m-Me-ITIC-Cl undergoes an endothermic melting transition at 144 • C. This thermal transition implies that m-Me-ITIC-Cl has a greater tendency to crystallize than the other derivatives because of end group differences. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) was carried out to investigate the thermal stability of the m-Me-ITIC acceptors (Figures S7 and S8 in Supporting Information). All four acceptors have decomposition temperatures (Td) > 330 °C (5% weight loss), indicating m-Me-ITIC series have good thermal stability. Differential scanning calorimetry revealed that m-Me-ITIC-Cl undergoes an endothermic melting transition at 144 °C. This thermal transition implies that m-Me-ITIC-Cl has a greater tendency to crystallize than the other derivatives because of end group differences.
The ultraviolet-visible absorption spectra and corresponding data of the m-Me-ITIC series in chloroform and thin film are shown in Figure 1b,c and Table 1. The entire series exhibited large red-shifts > 100 nm compared with ITIC-ethylhexyl oxy (OEH), which was composed of thienothiophene instead of CPDT due to the extended conjugation. Furthermore, measurement of ultraviolet-visible spectra in solution revealed that λmax exhibited greater red-shifts with increasing electron acceptor ability: m-Me-ITIC-Me (λmax: 756 nm) < m-ITIC (760 nm) < m-ITIC-F (775 nm) < m-ITIC-Cl (790 nm). In thin film, a broader spectrum and redshifted maximum absorption were observed for m-Me-ITIC acceptors; the absorption maximum of m-Me-ITIC-Cl was shifted by nearly 16 nm compared with m-Me-ITIC-F, suggesting that stronger π-π interactions occur in thin film. We conducted our electrochemical study of the m-Me-ITIC series using cyclic voltammetry for energy level measurement (Figs. S9 in Supporting Information). Owing to the electronegative nature of INIC group, the LUMO energy levels of the corresponding NFAs decreased from  The ultraviolet-visible absorption spectra and corresponding data of the m-Me-ITIC series in chloroform and thin film are shown in Figure 1b,c and Table 1. The entire series exhibited large red-shifts > 100 nm compared with ITIC-ethylhexyl oxy (OEH), which was composed of thienothiophene instead of CPDT due to the extended conjugation. Furthermore, measurement of ultraviolet-visible spectra in solution revealed that λ max exhibited greater red-shifts with increasing electron acceptor ability: m-Me-ITIC-Me (λ max : 756 nm) < m-ITIC (760 nm) < m-ITIC-F (775 nm) < m-ITIC-Cl (790 nm). In thin film, a broader spectrum and redshifted maximum absorption were observed for m-Me-ITIC acceptors; the absorption maximum of m-Me-ITIC-Cl was shifted by nearly 16 nm compared with m-Me-ITIC-F, suggesting that stronger π-π interactions occur in thin film. We conducted our electrochemical study of the m-Me-ITIC series using cyclic voltammetry for energy level measurement ( Figure S9    To demonstrate the photovoltaic performances of m-Me-ITIC acceptors, we fabricated organic photovoltaic (OPV) devices by blending m-Me-ITIC acceptors with PBDB-T polymer donor using an inverted structure with glass/tin-doped indium oxide/ZnO/BHJ/ MoO x /Ag. Current density-voltage (J-V) characteristics of the optimized devices with the m-Me-ITIC acceptors was measured under simulated AM1.5 G with an intensity of 100 mW/cm 2 , the results are shown in Figure 2a and the device data are summarized in Table 2. To verify the measurement accuracy, we crosschecked the external quantum efficiency spectra with the J-V characteristics (Figure 2b and Table 2). We summarize the device optimization data (e.g., thermal annealing, donor/acceptor blending ratio, and processing additive of each OPV device) (Tables S1-S6 in Supporting   To demonstrate the photovoltaic performances of m-Me-ITIC acceptors, we fabricated organic photovoltaic (OPV) devices by blending m-Me-ITIC acceptors with PBDB-T polymer donor using an inverted structure with glass/tin-doped indium oxide/ZnO/BHJ/MoOx/Ag. Current density-voltage (J-V) characteristics of the optimized devices with the m-Me-ITIC acceptors was measured under simulated AM1.5 G with an intensity of 100 mW/cm 2 , the results are shown in Figure 2a and the device data are summarized in Table 2. To verify the measurement accuracy, we crosschecked the external quantum efficiency spectra with the J-V characteristics (Figure 2b and Table 2). We summarize the device optimization data (e.g., thermal annealing, donor/acceptor blending ratio, and processing additive of each OPV device) (Tables S1-S6 in Supporting Information).  To further understand the collection behaviors and charge dissociation of the four fabricated OPV cells, the photocurrent density (J ph ) on the effective voltage (V eff ) was investigated, the results are presented in Figure 2c. V eff was determined using the following equation: V e f f = V 0 − V a , where V 0 indicates the voltage when illuminated with a current density (J L ) identical to the dark current density (J D ), and V a indicates the applied voltage. J ph was determined using the following equation: J ph = J L − J D [52,53]. Each of the OPV cells was biased from −2.0 to 1.0 V to demonstrate the exciton dissociation process; the saturation current density (J sat ) was verified for a V eff of 2.5 V. The J sat values measured for m-Me-ITIC, m-Me-ITIC-Me, m-Me-ITIC-F, and m-Me-ITIC-Cl were 14.12, 9.20, 24.00, and 22.40 mA·cm −2 . We divided the normalized J ph by J sat to calculate P(E,T) to demonstrate the exciton dissociation as well as charge collection properties. The P(E,T) values calculated were 79.00%, 71.24%, 96.84%, and 94.08%, respectively, indicating that OPV cells using novel NFAs with the halogen end groups fluorine or chlorine demonstrate better charge collection and faster exciton dissociation than OPV cells using the other two NFAs [54]. To investigate the charge transport behaviors, we analyzed the J sc value of each cell by varying the irradiated light intensity (P light ) from 0.93 to 100 mW·cm −2 , as shown in Figure 2d. The relationship between J sc and P light can be described as J sc ∝ P α light , where α is an exponential factor. A value for α close to unity indicates weak bimolecular recombination. As the value of α decreases, severe bimolecular recombination occurs [55]. The α-values of m-Me-ITIC-F blend and m-Me-ITIC-Cl blend was calculated as 0.9659 and 0.9673, which is higher than the α-values of m-Me-ITIC blend and m-Me-ITIC-Me blend (0.9519 and 0.9526, respectively). Because bimolecular recombination induces partial charge carrier losses [56], we presume that minimizing bimolecular recombination will help to increase the J sc value and fill factor of the OPV devices based on NFAs with halogen end groups (m-Me-ITIC-F and m-Me-ITIC-Cl).
To study the charge transport properties of the OSCs further, the devices' electron and hole mobilities were evaluated by the space charge limited current (SCLC) method ( Figure S10 and Table S7  Atomic force microscopy (AFM) was employed to investigate the morphologies of the m-Me-ITIC-, m-Me-ITIC-Me-, m-Me-ITIC-F-, and m-Me-ITIC-Cl-based BHJ films, the detail was shown in Figure 3. The root-mean-square surface roughness (RMS) values were higher for all BHJ blend films than for the novel NFA films. The RMS values of m-Me-ITIC and m-Me-ITIC-Me blend were <1 nm (0.97 and 0.83 nm, respectively), while the values of m-Me-ITIC-F and m-Me-ITIC-Cl blend were close to 1.5 nm. The exciton dissociation, charge transport, and charge collection ability of OPVs can be determined by BHJ film morphology. Favorable phase separation between donor and acceptor in m-Me-ITIC-F blend and m-Me-ITIC-Cl blend may have produced a slightly rougher surface, increasing the efficiency of these OPV cells [57,58]. These results consistently show the effects of the halogen end group on the novel NFAs. detail was shown in Figure 3. The root-mean-square surface roughness (RMS) values were higher for all BHJ blend films than for the novel NFA films. The RMS values of m-Me-ITIC and m-Me-ITIC-Me blend were <1 nm (0.97 and 0.83 nm, respectively), while the values of m-Me-ITIC-F and m-Me-ITIC-Cl blend were close to 1.5 nm. The exciton dissociation, charge transport, and charge collection ability of OPVs can be determined by BHJ film morphology. Favorable phase separation between donor and acceptor in m-Me-ITIC-F blend and m-Me-ITIC-Cl blend may have produced a slightly rougher surface, increasing the efficiency of these OPV cells [57,58]. These results consistently show the effects of the halogen end group on the novel NFAs. To confirm the crystallinity of the fabricated BHJ films based on novel NFAs, X-ray diffraction was performed. Figure 4a shows the X-ray diffraction patterns of neat acceptor films. 2θ diffraction peaks are located at 5.22°, 4.20°, 3.54°, and 3.39°, respectively; these values correspond to d-spacings of 16 Although the peaks of the other BHJ films remained visible, lamellar distances were shorter for m-Me-ITIC-F blend and m-Me-ITIC-Cl blend than m-Me-ITIC blend. This might be caused by strong intermolecular interactions through the highly electronegative halogen atoms used as end groups, which could induce greater molecular polarization [59]. This might be consistent evidence for highly efficient OPV cells based on m-Me-ITIC-F and m-Me-ITIC-Cl. To confirm the crystallinity of the fabricated BHJ films based on novel NFAs, X-ray diffraction was performed. Figure 4a shows the X-ray diffraction patterns of neat acceptor films. 2θ diffraction peaks are located at 5.

Conclusions
We designed and synthesized a 9-heterocyclic ring acceptor, m-Me-ITIC, with different end groups by substituting the outward thienothiophene moieties with CPDT at indacenodithiophene. The

Conclusions
We designed and synthesized a 9-heterocyclic ring acceptor, m-Me-ITIC, with different end groups by substituting the outward thienothiophene moieties with CPDT at indacenodithiophene. The developed m-Me-ITIC, m-Me-ITIC-Me, m-Me-ITIC-F, and m-Me-ITIC-Cl showed long-wavelength absorption at approximately 756-790 nm in solution because of the symmetric, planar, and extended conjugated CPDT donor. The m-Me-ITIC-based acceptors were developed into polymer solar cells that contained benzodithiophene-based donor polymers. The devices optimized for efficiency demonstrated PCEs of 5.9%, 3.6%, 11.8%, and 10.8% using m-Me-ITIC, m-Me-ITIC-Me, m-Me-ITIC-F, and m-Me-ITIC-Cl, respectively. Analyses of the charge dissociation and collection behaviors of the four fabricated OPV cells found that the new NFAs with halogen end groups showed salient bimolecular recombination dynamics. The BHJ film morphology for exciton dissociation, charge transport, and charge collection ability of the OPVs showed favorable phase separation in m-Me-ITIC-F blend and m-Me-ITIC-Cl blend. The shorter lamellar distances of these two materials, caused by strong intermolecular interactions through the highly electronegative halogen atoms, could induce greater molecular polarization. This may also be consistent evidence for highly efficient OPV cells based on m-Me-ITIC-F and m-Me-ITIC-Cl.