Two Novel, Simple-Structured and Easy-Synthesized Acceptors Based on Fluorene or Carbazole for Non-Fullerene Organic Solar Cells


 Two novel D-π-A-π-D type non-fullerene acceptors (FPTC and CPTC) composed of fluorene or carbazole as acceptor unit, benzene as intermediates and 2-(6-oxo-5,6-dihydro-4H-cyclopenta[c] thiophen-4-ylidene) malononitrile (TC) as terminal groups are synthesized through only two procedures. Also, their electrochemical behavior, photophysical properties and photovoltaic performance are systematically characterized and thoroughly studied. In consequence, the FPTC has better performance than C PTC, and the PCE of this device based on FPTC: PTB7-Th is nearly 1% higher than that of CPTC: PTB7-Th device, reaching up to 1.09% with a V OC of 0.71 V, a J SC of 3.42 mA cm − 2 . The higher PCE of the device based on FPTC is attributed to the fact that this molecular has a wider absorption spectrum and a higher molar extinction coefficient nearly four times than that of CPTC, a higher initial oxidation potential, and a lower onset reduction potential. Also, the higher electron mobility rate and hole mobility rate contribute to the device based on FPTC better performances. Compared with carbazole, the fluorene as the acceptor unit provides potential possibilities for the construction of high-performance organic solar cells.


Introduction
In the past few decades, Organic solar cells (OSCs) have attracted a great deal of attention for their visible advantages such as low-cost manufacturing, simple manufacturing process, material diversity, modi able structure, great exibility, solution processability which has been known as one of the most promising materials to generate electricity in a green and environmentally friendly [1][2][3][4] . It is well known that organic solar cells (OSCs) are composed of electron-sucking acceptors and electron-donators. The electron acceptors based on fullerene derivatives once have Has brought a research upsurge to researchers for isotropic charge transport and favorable nanoscale network forming behaviors which made its power conversion e ciency (PCE) reached more than 11.3% 4 . However, the acceptor of fullerene derivatives still has many defects that make it not widely used in high-e ciency organic solar cells. In particular, the compound with two organic cyclic adducts is randomly distributed on the fullerene cage, which means that the compound has multiple stereoisomers and even the structure is uncertain, resulting in considerable large structure and location disorder, Therefore, this phenomenon inhibits charge transfer leads to their poor photovoltaic performance [5][6][7][8] , In recent years, owning to well-determined molecular structure and weight, tunable molecular energy levels, low-cost synthesis for easier puri cation, better repeatability of device, potential of industrial production for low batch-to-batch variation, increasing number of researchers have invested more efforts to study non-fullerene acceptors(NFAs) which have obviously advantages different from fullerene receptors 3,[9][10][11] . Today, with the continuous improvement of materials, innovation device engineering and morphological optimization, the photovoltaic performance based on non-fullerenes has already surpassed that of fullerenes derivatives, with PCE of more than 18%. In consequence, the design and synthesis of NFAs provides a new opportunity for the commercial application of OSCs.
Small molecule organic solar cells (SM-OSCs) and polymer solar cells (PSCs) are consisted of Nonfullerene organic solar cells. Nowadays, although the performance of the OSCs based small molecule still lags behind that of polymer, the SM-OSCs have their unique advantages such as well-determined and design structure, easier synthesis with higher puri cation, solution-processed, intermolecular arrangements characterized by crystallographic analysis, which attracts increasing attention from researcher 2, 12 . In SM-OSCs, combining electron-rich groups and electron-de cient groups as the e cient electron acceptors is the most common design method in order to obtain e cient intermolecular charge transfer (ICT) and low energy gap, which enable to broaden the absorption spectrum and promote the transition or separation of electrons from excited molecules to acceptors. According to reports, the D-π-Aπ-D A-π-A, A-π-D, D-π-D and D-A-D conjugated structures are commonly applied in small organic molecules. the A-π-A, A-π-D, D-π-D and D-A-D conjugated structures have many inevitable problems such as ladder-type polycyclic compound containing strongly de cient electrons and rich electrons group involves complex synthesis and multi-step puri cation, which immensely increases the cost of material and synthesis. However, D-π-A-π-D type is the most attractive conjugated structures, because its photovoltaic performance modi ed by electron-rich donors, electron-de cient acceptors, and π bridges and its 3D π -conjugated conformations provides high electron cloud density to improve charge delocalization, which helps reduce the energy level gap 2,13 .
In this work, we designed and synthesized two small molecules based on uorene and carbazole as acceptors for bulk heterojunction (BHJ) SM-OSCs. The design of this molecule is based upon the following consideration: ) With the planarization of biphenyl units, uorene is a kind of electron-rich rigid conjugated molecule similar to a spiro ring. which can decrease the oxidation potential, show great thermal photochemical, thermal stability and low lying lowest unoccupied molecular orbital (LUMO), The uorenyl moiety can cause a highly sterically hindered structure that has a huge impact on the solubility of the entire molecule [13][14][15][16][17][18][19] . ) Carbazole has a low-lying Highest Occupied Molecular Orbital (HOMO) level and only one substitution position (N-position) which can be expected the π-π stacking distance between molecules 20-23 . ) 2-(6-oxo-5,6-dihydro-4H-cyclopenta [c] thiophen-4-ylidene) malononitrile (TC) was an excellent end group for acceptor moiety for its strong light-harvesting from the electron delocalization and good charge transport originate from the strong intermolecular S…S interactions. ) Benzene is an electron-rich molecule with a conjugate plane and is a common structural unit used as πbridge in organic optoelectronic materials.
Herein, according to these factors, we synthesized two novel non-linear D-π-A-π-D type small molecule FPTC and CPTC ( Fig. 1) acceptors that bear the same 2-(6-oxo-5,6-dihydro-4H-cyclopenta[c]thiophen-4ylidene) malononitrile(TC) as acceptor moiety, uorene or carbazole as donor moiety and benzene as πbridge for FPTC and CPTC, respectively. The synthesis of the target molecule only passes through two steps and the yield exceeds 80%, which may be provide the possibility of the potential application of large-scale production. Both compounds have great solubility in common organic solvents, such as dichloromethane, chloroform, tetrahydrofuran (THF), and chlorobenzene (CB) at room temperature. To greatly match energy levels and widen complementary absorption spectra, we have treated FPTC and CPTC as a donor component along with poly[(4,8-bis(5-(2-ethylhexyl) thiophen-2-yl)benzo [1,2-b:4,5-b']dithiophene2,6-diyl)-alt-(2-ethylhexyl-3-uorothieno [3,4-b]thiophene-2 carboxylate-4,6-diyl)] (PTB7-Th) as donor to fabricate the inverted bulk-heterojunction organic solar cells (BHJ-OSCs). Also, in this work, we utilized a great deal of method to characterized these SMs structure, optical and electrochemical properties, morphology and photovoltaic parameters, which could deeply understand the structure − property relationships and provide strategy to construct this type compound. Finally, the FPTC has better performance than CPTC, and the PCE of this device based on FPTC: PTB7-Th is nearly 1% higher than that of CPTC: PTB7-Th device, reaching up to 1.09% with a V OC of 0.71 V, a J SC of 3.42 mA cm − 2 . The higher PCE of the device based on CPTC: PTB7-Th is attributed to the fact that this device has a wider absorption spectrum and a molar extinction coe cient nearly four times higher than that of FPTC: PTB7-Th device, a higher initial oxidation potential, and a lower onset reduction potential. Also, the higher electron mobility rate and hole mobility rate contributes to its better performances. By changing the central core, we have a great understand that uorene core is a good strategy for constructing high performance SMAs.
The reaction of intermediates 3 with 2-(6-oxo-5,6-dihydro-4H-cyclopenta[c]thiophen-4ylidene)malononitrile in chloroform (4) at 25 ℃ resulted FPTC in good yield of 84.2%. Similar to the synthesis method of target compound FPTC, the reaction of intermediates 3 with molecule 4 at 25 ℃achieved CPTC in good yield of 81.5%. All molecules structure was characterized by MALDI-TOF-MS, 1 H Nuclear Magnetic Resonance ( 1 H NMR) and 13 C NMR and the synthetic details and characterization data are presented in the Supporting Information.
The new acceptors FPTC and CPTC have excellent solubility in common solvents such as dichloromethane, toluene, chlorobenzene, tetrahydrofuran, chloroform and chlorobenzene at ambient temperature, which means that they have the potential of large scale production via processing solvents in the eld of photovoltaic applications. The thermal stabilities of FPTC and CPTC were characterized by thermogravimetric analysis (TGA), and it shows that the decomposition temperature (T d , corresponding to a 5% weight loss) of FPTC and CPTC is 212℃ and 173℃ under inert atmosphere (Fig. S1), respectively, which proves that thermal decomposition temperature of FPTC is nearly 40 degrees Celsius higher than that of CPTC and it means that FPTC bearing uorene as central core have greater thermal stabilities and more feasible for vacuum-deposition fabrication.

Optical and electrochemical properties
The UV-Vis absorption spectra of the molecule of FPTC and CPTC in chloroform solution and thin lm form are shown in Fig. 2a-2b. Relevant optical parameters have recorded in Table 1. Both of FPTC and CPTC demonstrate a wide and intense absorption that extends from 350 to 650 nm. The broad absorption suggests strong intramolecular charge transfer (ICT) actively from donor to acceptor moieties. In the neat lm, for CPTC and FPTC, both display broader absorption spectra and signi cantly red-shifted absorption spectra than in solution, with the maximum absorption peaks at 515 nm and 517 nm, respectively. Compared to CPTC, FPTC exhibits distinct peaks in the wavelength of 498nm with the highest molar extinction coe cients of 6.64 × 10 4 M -1 cm -1 , whereas CPTC just gave a relatively faint vibronic peak in 486 nm and its highest molar extinction coe cients only have 1.75 × 10 4 M -1 cm -1 , which should be attributed to the fact that FPTC has stronger intermolecular p-p, π-π stacking and mutual conjugation than CPTC. Owing strong intermolecular interactions in the solid state, the absorption peak of FPTC is red shifted around 30 nm from the solution to the solid state. However, the interesting phenomenon is that the maximum absorption peaks of CPTC in lm is 513 nm with the edge absorption wavelength of 643nm, which is red-shifted by 27 nm compared with that in solution, and even 10 nm Contrast to the molar extinction coe cient, we can infer that all of them have a relatively high molar extinction coe cient, indicating that the donor units effectively enhanced the absorption when the uorene and carbazole was used as a central core, respectively. The molar extinction coe cient of FPTC is about 3.8 times that of CPTC which means that FPTC has a better ability to capture photons due to the best planarity of uorene central core and the strongest ICT effect, and nally increase the Jsc of OSCs. Based on the principle of mutual energy level matching and complementary light absorption, we decide PTB7-Th (-5.22/-3.64 eV) as donor and our molecules as acceptor. The energy diagram relative to the vacuum level were shown in Fig.1d. The LUMO-LUMO offset of two acceptors and PTB7-Th are 0.41 -0.49 eV, and all of them exceed 0.3 eV which is enough for e cient exciton dissociation and charge separation at the donor-acceptor interface. What's the most important is that the HOMO energy offset (E HOMO ) between PTB7-Th and the acceptors are 0.48 and 053 eV, respectively, which enable e cient hole transfer from the acceptors the PTB7-Th to donor.

Computational analysis
The theoretical calculations of density functional theory (DFT) was conducted by Gaussian 09 package57 at the B3LYP/631G(d) level to investigate the in uence of molecular structure and electronic distribution on absorption spectra and electrochemical performance 24 . The optimized molecular structures and frontier molecular orbitals of the two acceptors are shown in Figure 3a-b, corresponding detail parameters are summarized in Table 2. In the optimized molecular structures of FPTC and CPTC, both of their structures are symmetrical and the dihedral angles on the left and right sides are basically the same. We record θ as the dihedral angle. The π-bridged phenyl group shows a large dihedral angle with two acceptor moieties or with terminal groups, forming a non-planar structure. This must seriously affect the aggregation state of the molecules. In contrast, the SMAs have large dihedral angle between phenyl bridges and terminal TCs reaching up to 90°. The dihedral angle between the central donor unit carbazole and the adjacent π-bridge phenyl group is approximately 18°, which the dihedral angle between the central uorene and adjacent phenyl bridge is only 16°. It means that the presence of carbazole unit intensify the conjugation of the molecule, but it is not conducive to the coplanarity of the whole molecules due to the angle with the π-bridge and intramolecular steric hindrance, which prevents the transfer of intramolecular charge. In other words, FPTC has a better planar molecular con guration, which, in turn, is conducive to intrinsic ICT interactions, resulting in a narrower band gap, which is consistent with absorption spectrum observations. In Fig. 3, although the SMAs π-electrons in the HOMO are highly delocalized throughout the molecular skeleton, which provides e cient orbital interactions, the π-electrons in the LUMO are insu ciently delocalized at the terminal electron-de cient TC unit. This is mainly due to the large dihedral angle between the acceptor moiety and the phenyl group, which hinders the effective transfer of intramolecular charge, thus affecting the separation of photoelectron. By using DFT calculation, the HOMO/LUMO energy levels of FPTC and CPTC are -4.43/-3.67 and -4.37/-3.65 eV, respectively. Most importantly, although the predicted values of the HOMO/LUMO energy levels and are not very accurate, the predicted value trend is consistent with the trend of the value calculated from the ground state absorption.

Photovoltaic properties
In order to further evaluate the photovoltaic performance of these SMs, the solution-processable BHJ-SM-OSCs devices were fabricated using FPTC and CPTC as the acceptor and PTB7-Th as the donor, which is an effective electron transport material and has an energy level that matches most acceptors and a  Table 2. In contrast, it was found that the FPTCbased devices containing uorene as acceptor unit exhibited more excellent photovoltaic performance than CPTC-based devices whose donor unit is carbazole.
The device with FPTC: PTB7-Th shows a PCE of 0.87% with a Jsc of 3.42mA/cm 2 , a V oc of 0.71 V and an FF of 36.19% without any optimization. Through further optimization of thermal annealing and other conditions, the performance of the device was further improved, and a relatively high PCE of 1.09% with a Jsc of 4.21 mA/cm 2 , a V oc of 0.72 V and an FF of 36.81% were obtained. However, the CPTC based BHJ-SM-OSCs shows slightly inferior PCE of 0.05% with a V oc of 0.49 V, a J sc of 0.42 mA cm -2 and an FF of 0.24. By continuously optimized, its photovoltaic performance has been improved very little with a PCE of 0.12%. In comparison, the photovoltaic performance of FPTC-based devices is generally higher than that of CPTC-based devices. For example, the V oc of FPTC and the PCE is about 47% and 1% higher than that of CPTC, respectively. Particularly, the J sc of FPTC-based devices is 10 times that of CPTC-based devices.
The CPTC-based BHJ-SM-OSC exhibits relatively poor photovoltaic performance whose reason may be that the carbazole unit has a stronger electron-donating capacity than fullerene moiety, which destroys the balance in the molecule to a certain extent, and the carbazole between the π bridge or terminal groups are not greatly conjugated and form a larger dihedral angle. Therefore, in the process of preparing the lm, too much intermolecular aggregation will be inevitably occurred, which is not conducive to intramolecular charge transfer. These results are completely consistent with the results of UV-vis absorption spectra and molecular calculations. In other words, the device containing fullerene group has more potential to be applied to the construction of high-e ciency organic small molecule solar cells.
In order to further evaluate the accuracy of photovoltaic performance measurement, the external quantum e ciency (EQE) spectra of the optimal FPTC: PTB7-Th and CPTC: PTB7-Th BHJ-SM-OSCs are shown in Fig. 4b. Apparently, the EQE curves have a similar trend to that of the J sc and all devices showed a broad solar spectral response ranging from 300 to 800 nm. Compared to CPTC-based device, it could be seen that the EQE value of FPTC-based device are higher with a maximum EQE value of 19% in the whole spectral range, which is around four times of the CPTC device. The lower EQE of PTB7-Th: FPTC suggests that there is an exciton dissociation and charge transfer barrier or a severe charge recombination in the process of photocurrent generation. The J sc of the devices calculated from integration of the EQE spectra with a 1.5 G reference spectrum are 3.58 mA cm −2 and 0.44 mA cm −2 respectively, which is almost consistent with the measured J sc value, and the error is less than 5%. E cient charge transport matters the prosperity of BHJ-OSCs and anticipates the device performance.
Therefore, to further investigate the charge extraction and exciton dissociation properties between the PTB7-Th: FPTC and PTB7-Th: CPTC blend systems, the space charge limited current (SCLC) method was utilized to measure the charge transport properties. The hole-only device utilizes a structure of ITO/PEDOT: PSS/PTB7-Th: acceptors/Au, while the electron-only device adopts an Al/PTB7-Th: acceptors/Al con guration. The results of hole and electron mobilities are shown in Fig. 4c and 4d (Supporting Information) and the detail data are listed in Table 2. The PTB7-Th: FPTC based device shows a hole and electron mobility of 1.69× 10 -4 cm 2 V -1 s -1 and 2.28× 10 -4 cm 2 V -1 s -1 , respectively. However, compared to PTB7-Th: FPTC based device, the PTB7-Th: CPTC based device is a little bit worse with the value of 0.54×10 -4 cm 2 V -1 s -1 and 0.62×10 -4 cm 2 V -1 s -1 , respectively. Obviously, the device containing uorene moiety has higher and balanced charge mobility than their carbazole-containing counterparts, which re ects better device performance.

Morphological analysis
To further investigate the surface morphology of the morphology of PTB7-Th: FPTC and PTB7-Th: CPTC blend lms, the tapping-mode atomic force microscopy (AFM) measurements were utilized and the corresponding height image and phase image were shown in Fig. 5. It is known that the nanoscale morphology of active layer is highly important for high exciton dissociation and effective generation of free charge carriers. As shown in Figure 5a-d, the uorene and carbazole based blend lms exhibited rootmean-square (RMS) roughness of 4.57 nm and 5.42 nm, respectively, indicating that the active uorenecontained layers is relatively smoother than the other. Moreover, in the phase images, the blend thin lm of shows PTB7-Th: FPTC shows relatively large dark domains and sizes of light that correspond to aggregations of carbazole-contained acceptor and PTB7-Th, which means that these domain sizes are not bene cial for effective exciton dissociation in the active layer. On the contrary, the blend lm with FPTC showed slightly better phase separation with uniform interpenetrating network structures, which decreases the geminate recombination and bimolecular recombination, resulting in an improved hole and electron mobilities. Considering both AFM images and RMS roughness values, PTB7-Th: FPTC and PTB7-Th: CPTC blend lms exhibited similar and reasonable morphology, which is consistent with the abovementioned characterization results.

Conclusion
In summary, two D-π-A-π-D small molecules, FPTC and CPTC with the same electron-withdrawing terminal acceptor group and π-conjugated bridge, but different electron-donating groups as the central core ( uorene, carbazole), have been designed, synthesized, and utilized to fabricate e cient solutionprocessable SMOSCs as acceptors materials. Both compounds were synthesized through only two procedures with high yields and all of them have great solubility in common organic solvents. Also, in this work, we utilized a great deal of method to characterized these SMs structure, photovoltaic and photoelectric properties, which could deeply understand the structure − property relationships and provide strategy to construct this type compound. Finally, we nd that this FPTC: PTB7-Th device has a wider absorption spectrum and a molar extinction coe cient nearly four times higher than that of the CPTC: PTB7-Th device, a higher initial oxidation potential, and a lower onset reduction potential. Also, the higher electron mobility rate and hole mobility rate contributes to its better performances than CPTC, and the PCE of this device based on FPTC: PTB7-Th is nearly 1% higher than that of CPTC: PTB7-Th device, reaching up to 1.09% with a V OC of 0.71 V, a J SC of 3.42 mA cm − 2 . Moreover, it's convinced that there are many great features such as low-cost raw material, easy accessible of target compound, broad absorption bands reached near-infrared region, high extinction coe cients, good lm-forming ability, suitable LUMO/HOMO levels, which implies that uorene based small molecules are a potential candidate for photovoltaic applications and continuous development.   The optimum geometry and electron-state-density distributions of HOMO and LUMO of FPTC and CPTC.