An <i>n</i> -Type All-Fused-Ring Molecule with Narrow Bandgap

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE29 Mar 2022An n-Type All-Fused-Ring Molecule with Narrow Bandgap Yingjian Yu, Yingze Zhang, Junhui Miao, Jun Liu and Lixiang Wang Yingjian Yu State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022 Jilin University of Science and Technology of China, Hefei, 230026 Anhui , Yingze Zhang State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022 Jilin University of Science and Technology of China, Hefei, 230026 Anhui , Junhui Miao *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022 Jilin , Jun Liu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022 Jilin University of Science and Technology of China, Hefei, 230026 Anhui and Lixiang Wang State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022 Jilin University of Science and Technology of China, Hefei, 230026 Anhui https://doi.org/10.31635/ccschem.022.202101752 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail All-fused-ring π-conjugated molecules have received considerable attention because of their unique electronic structures, low conformation disorder, and excellent optoelectronic properties. Most all-fused-ring molecules are p-type organic semiconductors and possess medium bandgaps. In this work, we design and synthesize an all-fused-ring molecule (FM1) with an n-type property and narrow bandgap, which is a 10-fused-ring system composed of one electron-deficient benzotriazole core, two electron-rich thienopyrrole bridging units, and two electron-deficient malononitrile-functionalized end-cappers. FM1 exhibits low-lying highest occupied molecular orbit/lowest unoccupied molecular orbit energy levels of −5.77 eV/−3.89 eV, high electron mobility of 6.0 × 10−4 cm2 V−1 s−1, an optical bandgap of 1.50 eV, and a maximum absorption wavelength of 769 nm. Because of the all-fused-ring skeleton, FM1 shows superior photostability and chemical stability. We use FM1 as an electron acceptor and successfully construct organic solar cell (OSC) devices with a decent power conversion efficiency (PCE) of 10.8%. Most importantly, the intrinsic stability of FM1 leads to its excellent OSC device stability. After irradiation with simulated solar light for 16 h, while control of the OSC device of the state-of-the-art small molecule electron acceptor shows a 46% decrease of PCE, the FM1's unencapsulated OSC device exhibits only a 9% decrease of PCE. Download figure Download PowerPoint Introduction π-Conjugated all-fused-ring molecules have received considerable attention in both experimental and theoretical studies because of their unique electronic structure, low conformation disorder and excellent optoelectronic properties.1,2 Moreover, carefully designed all-fused-ring molecules exhibit excellent device performance in organic optoelectronic devices, such as organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), and organic solar cells (OSCs).3–8 Most all-fused-ring molecules are p-type organic semiconductors and show medium bandgaps.9–12 In contrast, there are very few reports on n-type all-fused-ring molecules with narrow bandgaps.13–15 Narrow bandgap organic compounds possess strong absorption and/or emission in the near-infrared region and are required for solar-energy-related applications, bioimaging, and medical applications.16–20 Generally, organic molecules with narrow bandgaps can be designed by introducting electron donating–accepting (D–A) structures and quinoid structures.21,22n-Type organic semiconductors are essential materials in OFETs and OSCs.23,24 The general strategy to design n-type organic semiconductors is to introduce strong electron-withdrawing groups, such as the imide, amide, cyano, and boron–nitrogen coordination bonds (B←N), to π-conjugated skeletons.25 However, the development of n-type organic semiconductors lags far behind that of p-type counterparts in view of their material diversity and device performance.26,27 It is challenging to design and synthesize n-type all-fused-ring molecules with narrow bandgaps. State-of-the-art OSCs use blends of polymer electron donors (p-type) and small-molecule electron acceptors (n-type) as photoactive layers. OSCs have become a promising photovoltaic technology because of their advantages of flexibility, light weight, and semitransparency.28–35 Efficiency, stability, and cost are the three key issues for practical application of OSCs. In the past 3 years, because of the invention of A-DA′D-A type small molecule-electron acceptors, the power conversion efficiency (PCE) of single-junction OSCs has been boosted to over 18%.36–40 Therefore, in the near future, great attention should be paid to the long-term stability of OSCs.41–46 A critical aspect of OSC device stability is the inadequate intrinsic stability of small-molecule electron acceptors.47–53 For the chemical structures of typical nonfullerene electron acceptors, a fused-ring electron-rich core and two strong electron-accepting terminals are covalently linked by two vinyl units.54–56 The instability of the small-molecule electron acceptors comes from the exocyclic vinyl linkers, which are vulnerable.57 Under illumination, the vinyl linkers will undergo photoisomerization or photooxidation.58,59 In the basic condition, the vinyl linkers may be broken, and the adjacent carbonyl groups may be attacked.60 Several approaches have been reported to deal with the vulnerable property of vinyl linkers in small-molecule electron acceptors. Li et al.61 and Liu et al.62 have used carbon–carbon single bonds and ring-locked vinyl units to replace traditional vinyl units, respectively. Recently, Zhu et al.63 have reported an all-fused-ring strategy to design a small-molecule electron acceptor with enhanced stability. However, in spite of the much enhanced stability, the OSC device efficiency of these small-molecule electron acceptors are unstatisfactory, for example, PCE < 10%. In this work, we report an all-fused-ring molecule with an n-type property and narrow bandgap that can be used as an electron acceptor in OSCs. Figure 1a shows the chemical structure of the molecule, 2,2′-(8-(2-butyloctyl)-16,17-dioctyl-16,17-dihydro-5H-indeno[1″,2″:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2-g]indeno[1′,2′:4,5]thieno[3,2-b][1,2,3]triazolo[4,5-e]indole-5,11(8H)-diylidene)dimalononitrile (FM1). FM1 is a 10-fused-ring system composed of one electron-deficient benzotriazole core, two electron-rich thienopyrrole bridging units, and two electron-deficient malononitrile-functionalized terminals. The electron-deficient core and terminals lead to the n-type property, and the intramolecular electron D–A characteristic results in the narrow bandgap of FM1. Because of the all-fused-ring skeleton, FM1 shows superior photostability and chemical stability. Most importantly, the intrinsic stability of FM1 leads to its excellent OSC device stability together with high device efficiency. Figure 1 | (a) Chemical structure of the all-fused-ring molecule FM1. (b) Synthetic route of FM1. Reagents and conditions: (i) ethane-1,2-diol, 4-methylbenzenesulfonic acid, benzene, 115 °C; (ii) n-butyllithium, tetrahydrofuran (THF), −78 °C; then trimethyltin chloride, −78 °C to r.t.; (iii) trifluoromethanesulfonic acid, fuming nitric acid, 50 °C; (iv) 3, tetrakis(triphenylphosphine)palladium, copper(I) iodide, toluene, 115 °C; (v) diluted hydrochloric acid, THF, 70 °C; (vi) triphenylphosphine, 1,2-dichlorobenzene, 180 °C, then 1-bromooctane, potassium carbonate, potassium Iodide, N,N-dimethylformamide, 100 °C; and (vii) malononitrile, titanium tetrachloride, pyridine, chlorobenzene, 50 °C. Download figure Download PowerPoint Experimental Methods Material synthesis FM1 was synthesized according to the routes in Figure 1b (for details, see the Supporting Information Figure S1). All chemicals were purchased from commercial sources and used without further purification unless otherwise mentioned. Theoretical calculation All theoretical calculations were performed with the Gaussian 09 program. The geometrical structure of FM1 was optimized by using density functional theory (DFT) calculations (b3lyp/6-31g(d,p)). Time-dependent DFT (TD-DFT) calculations at the b3lyp/6-31g(d,p) level of theory were performed to elucidate the absorption spectrum of FM1. TD-DFT calculations for the S0→Sn transitions using the same functional and basis set were then performed based on the optimized structure at the ground state. Material and device stability tests The photostability, thermal stability, and chemical stability of the materials were tested by monitoring the UV–vis absorption spectra of the material solutions/films under different conditions. The thermal stability of materials was also studied by thermogravimetric analysis (TGA). For the device photostability test, the optimal OSC devices were exposed to the XES-40S2-CE Class Solar Simulator (SAN-EI Electric Co., Ltd., Osaka, Japan) with 100 mW cm−2 AM 1.5 G simulated solar light illumination for different times. OSC device fabrication and measurement The OSC devices were fabricated with the structure of ITO/ZnO/active layer/MoO3/Al. The detailed fabrication process is shown in Supporting Information. The current density–voltage (J–V) curves of the OSC devices were measured using a computer-controlled Keithley 2400 SourceMeter under 100 mW cm−2 AM 1.5 G simulated solar light illumination. The external quantum efficiency (EQE) spectra were measured using a Solar Cell Spectral Response Measurement System QE-R3011 (Enlitech Co., Ltd., Taiwan, China). The light intensity at each wavelength was calibrated using a calibrated monosilicon diode. Results and Discussion Synthesis and characterizations The synthetic route of FM1 is illustrated in Figure 1b. Starting from 2-bromo-8H-indeno[2,1-b]thiophen-8-one ( 1), the carbonyl was initially protected by glycol. Then the compound was treated with n-butyllithium and subsequent trimethyltin chloride to afford 3. The nitrification of 4,7-dibromo-2-(2-butyloctyl)-2H-benzo[d][1,2,3]triazole ( 4) gave 5. Subsequent Stille coupling of 5 and 3 afforded 6. 7 was synthesized by the deprotection of 6 under acid conditions. The all-fused-ring molecule 8 was prepared by the double intramolecular Cadogan reductive cyclization of 7 in the presence of triphenylphosphine, followed by the addition of 1-bromooctane under alkaline conditions. Finally, FM1 was synthesized by the Knoevenagel condensation reaction of 8 with malononitrile. The chemical structure of FM1 was fully characterized using 1H NMR, 13C NMR ( Supporting Information Figure S30–S36), matrix-assisted laser desorption ionization time-of-flight mass spectroscopy, and elemental analysis. FM1 is readily dissolved in common organic solvents, such as chloroform (CF), chlorobenzene (CB), and o-dichlorobenzene (o-DCB). According to differential scanning calorimetry measurement, FM1 exhibited a distinct melting/crystallization peak with the enthalpy change of 25.7/16.4 J g−1 ( Supporting Information Figure S7), indicating high crystallinity of FM1. The molecular configuration of the all-fused-ring molecule was investigated by the single crystal structure of the model molecule FM1-nC8. The molecules FM1-nC8 and FM1 have the same skeletal structures but different alkyl chains on the benzotriazole cores. The chemical structure of FM1-nC8 is shown in Supporting Information Figure S2. We prepared the single crystal of FM1-nC8 by the slow diffusion of acetonitrile into FM1-nC8 solution in CF (CCDC: 2125425). As shown in Supporting Information Figure S3a, FM1-nC8 exhibits a banana-shaped molecular skeleton. From the side view and left view in Supporting Information Figures S3b and S3c, FM1-nC8 shows a slightly twisted structure with the dihedral angle between two terminal benzene rings of 12.0°. As shown in Supporting Information Figure S4, one FM1-nC8 molecule overlays on top of another FM1-nC8 molecule through the benzotriazole cores or the terminal groups. FM1-nC8 molecules exhibit a fishbone-type stacking arrangement with an average π–π distance of 3.38 Å ( Supporting Information Figure S5), which is beneficial to electron transport. Optical and electrochemical properties As shown in Figure 2a, FM1 in CF solution shows the maximum absorption wavelength at 715 nm. According to the TD-DFT calculation results on the electronic transitions of FM1 ( Supporting Information Figure S9), the main absorption band of FM1 was assigned to the transitions of highest occupied molecular orbit (HOMO)−2 → lowest unoccupied molecular orbit (LUMO), HOMO−1 → LUMO, HOMO−1 → LUMO+1, HOMO → LUMO, and HOMO → LUMO+1 ( Supporting Information Figure S10). From in solution to in thin film, the maximum absorption wavelength of FM1 redshifted by 54 nm, and the absorption spectrum became much broader, indicating the presence of strong intermolecular interaction in the solid state. In thin film, FM1 exhibited two absorption peaks at 769/696 nm with the maximum extinction coefficient of 1.04 × 105 cm−1, suggesting strong sunlight-harvesting capability. According to the onset absorption wavelength in thin film, the optical bandgap (Egopt) of FM1 is estimated to be 1.50 eV. In CF solution, FM1 showed strong fluorescence with the emission peak at 751 nm ( Supporting Information Figure S19) and fluorescence quantum efficiency (ΦF) of 1.7%. The small Stokes shift of FM1 (670 cm−1, 36 nm) was attributed to the rigid all-fused-ring skeleton. In thin film, the emission peak of FM1 redshifted to 803 nm together with the ΦF of 1.4%. Figure 2 | (a) UV–vis absorption spectra of FM1 in chloroform solution and in thin film. (b) Cyclic voltammogram of FM1. (c) Kohn-sham LUMOs (left)/HOMOs (right) based on DFT calculations at the B31LYP/6-31G(d,p) level for the model molecule of FM1 with all the long alkyl chains being replaced by methyl groups for simplification. Download figure Download PowerPoint The LUMO/HOMO energy levels of FM1 were estimated by cyclic voltammetry (CV). Figure 2b shows the cyclic voltammogram of FM1. Based on the onset potential of the oxidation/reduction waves, the LUMO/HOMO energy levels of FM1 were estimated to be −3.89 and −5.77 eV, respectively. The LUMO and HOMO energy levels of FM1 are fairly comparable to those of high-performance n-type organic semiconductors ( Supporting Information Figure S11). Figure 2c displays the frontier molecular orbitals of the FM1 skeleton, which were obtained by DFT calculations at the B3LYP/6-31G(d,p) level ( Supporting Information Figure S8). The LUMO was well delocalized on the entire molecular skeleton, and the HOMO was mainly distributed on the core unit. This delocalized LUMO was favorable for electron transporting and photoinduced electron transfer. The electron mobility of FM1 was evaluated by the space-charge-limited current (SCLC) method with the electron-only devices (see Supporting Information). The electron mobility of FM1 was 6.0 × 10−4 cm2 V−1 s−1 ( Supporting Information Figure S16). The low-lying LUMO/HOMO energy levels and high electron mobility indicated the n-type property of FM1. Photostability, chemical stability, and thermal stability We studied the photostability, chemical stability, and thermal stability of the all-fused-ring molecule FM1. For comparison, we selected the state-of-the-art small-molecule electron acceptor in OSC devices, 2,2′-((2Z,2′Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2″,3″:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2-g]thieno[2′,3′:4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (Y6, see Figure 3a) as the control compound. Y6 exhibited high OSC device efficiency (PCE = ca. 18%) but suffered from insufficient stability because of the exocyclic vinyl linker. The photostability and chemical stability were studied by monitoring the UV–vis absorption spectra of the solutions of FM1 and Y6. Figure 3b shows the decay of maximum absorbance of the THF solutions of FM1 and Y6 during irradiation under 100 mW cm−2 AM 1.5 G simulated solar light without UV filter. After illumination for 60 min, the absorbance of FM1 solution decreased by 23% while that of Y6 solution decreased by 42% ( Supporting Information Figure S12). We also studied the photostability of FM1 film by a similar method, and the result is shown in Supporting Information Figure S14. These results suggest that FM1 has better photostability than that of Y6 because of its all-fused-ring skeleton. We investigated the chemical stability of the two molecules by treating them in the diluted THF solution (10–5 mol L−1) with 100 equiv ethanolamine for different times. Y6 exhibited completely different absorption spectra with significantly reduced long-wavelentth absorbance after the 12 h of treatment in the alkaline condition ( Supporting Information Figure S13b). As shown in Figure 3c, after 100 equiv ethanolamine was added to the THF solution of Y6 for a basic condition, the maximum absorbance decreased quickly over 2 h and become zero after 12 h. This is due to the breaking of the vinyl linkers or carbonyl groups by nucleophilic attack of electron-rich ethanolamine, which led to degradation of Y6. In sharp contrast, FM1 exhibited excellent chemical resistance against the alkaline condition, and its absorption spectrum remained unchanged after the coexistence of ethanolamine in solution for a long period ( Supporting Information Figure S13a). The maximum absorbance of FM1 solution remained constant during the 12 h of ethanolamine treatment. The substantially enhanced chemical stability of FM1 is due to its all-fused ring skeleton and the absence of the vulnerable vinyl linker. The thermal stabilities of FM1 and Y6 were tested by measuring the thermal decomposition temperature by TGA and monitoring the UV–vis absorption spectra of the films ( Supporting Information Figure S15). The thermal decomposition temperature at 5% weight loss of FM1 was 346 °C ( Supporting Information Figure S6), which is 38 °C higher than that of Y6, indicating that FM1 improved thermal stability. Figure 3 | (a) Chemical structure of the state-of-the-art small molecule electron acceptor containing vinyl linkers, Y6. (b) The absorption decays of the THF solution of FM1 and Y6 at the corresponding maximum absorptions after irradiation under 100 mW cm−2 AM 1.5 G simulated solar light. The inset shows the photos of the two solutions before and after irradiation for 60 min. (c) The absorption decays of the THF solution of FM1 and Y6 at the corresponding maximum absorptions after adding ethanolamine. The inset shows the photos of the two solutions after addition of ethanolamine for 12 h. Download figure Download PowerPoint Photovoltaic properties To investigate the photovoltaic performance of FM1 as an electron acceptor, we selected D18 as a polymer electron donor and fabricated OSC devices with the inverted configuration of indium tin oxide (ITO)/ZnO/D18:FM1/MoO3/Al. The device structure and the chemical structure of D18 are shown in Figures 4a and 4b. The detailed device fabrication process is described in the Supporting Information. The active layer of the optimized device was spin-coated from its CF solution with the donor:acceptor weight ratio of 1:1.7, followed by thermal annealing at 180 °C for 10 min ( Supporting Information Tables S1–S3). We also prepared the control device without the thermal annealing treatment. The J–V curves of the devices with and without thermal annealing under AM 1.5 G illumination (100 mW cm−2) are shown in Figure 4c. The device based on the as-cast active layer showed a PCE of 6.04%, with an open-circuit voltage (VOC) of 0.84 V, a short-circuit current density (JSC) of 14.57 mA cm−2, and a fill factor (FF) of 0.49. In comparison, the device with thermal annealing showed a PCE of 10.82% with a VOC of 0.88 V, a JSC of 18.96 mA cm−2 and an FF of 0.65. This device performance proves that all-fused-ring electron acceptors have the potential to achieve high PCE in OSC devices. Compared with the as-cast device, the thermally annealed device exhibited higher JSC and FF, which are attributed to the improved active layer morphology (vide infra). Figure 4d shows the EQE spectra of the two devices. The two devices exhibited a wide range of photoresponses from 310 to 850 nm. The EQE response of the thermally annealed device is stronger than that of the as-cast device. The integrated photocurrents from the EQE spectra are consistent with the JSC values obtained from the J–V scanning. Figure 4 | (a) The inverted OSC device structure based on D18:FM1 active layer. (b) Chemical structure of the polymer donor D18. (c) J–V curves under AM 1.5 G illumination (100 mW cm−2). (d) EQE spectra of the OSC devices based on D18:FM1 active layer with or without thermal annealing. Download figure Download PowerPoint Active layer morphology The D18:FM1 active layer morphology of the two OSC devices was investigated by two-dimensional grazing incidence wide-angle X-ray scattering (2D-GIWAXS), the photoluminescence (PL) spectra ( Supporting Information Figure S20), and atomic force microscopy (AFM). Figures 5a–5d show the 2D-GIWAXS patterns of the neat films of FM1 and D18 as well as as-cast and thermally annealed blend films of D18:FM1 and D18:Y6. The corresponding out-of-plane and in-plane linecuts are shown in Figures 5e and 5f, respectively. For both D18 and FM1, the π-stacking peak at only in the out-of-plane ( Supporting Information indicating that both D18 and FM1 in thin For the blend film, in the in-plane the peak at is attributed to the peak at is assigned to FM1, and the peak at in the out-of-plane is attributed to the of D18 and FM1. between the linecuts of the as-cast blend film and the thermally annealed blend film is that the peak at in the is much than that in the This much improved crystallinity of FM1 in the blend film after thermal annealing ( Supporting Information Figure The improved crystallinity of FM1 should to the improved enhanced electron and of the OSC device. The of the two D18:FM1 active are shown in Supporting Information Figures and the two active exhibit The of the thermally annealed active layer nm) is slightly higher than that of the as-cast active layer This is attributed to the improved crystallinity of FM1 after thermal annealing and is consistent with the 2D-GIWAXS Figure 5 | 2D-GIWAXS patterns of (a) FM1, (b) (c) as-cast D18:FM1 , and (d) thermally annealed D18:FM1 blend linecuts of the corresponding patterns in the out-of-plane and the in-plane Download figure Download PowerPoint and We evaluated the and electron of the D18:FM1 active using the method with the J–V curves of the device and the electron-only device, (see the Supporting Information Figure The were estimated to be × 10−4 cm2 V−1 × cm2 V−1 s−1 for the as-cast active layer and × cm2 V−1 × 10−4 cm2 V−1 s−1 for the thermally annealed active layer. The thermally annealed active layer exhibited higher and than those of the as-cast active layer. We attributed this to the improved crystallinity of FM1 after thermal annealing. The high and were consistent with the enhanced JSC and FF of the thermally annealed OSC device. In to investigate the and of the OSC devices, we measured the density the voltage and the of JSC on the light density of the two devices. The results are shown in Supporting Information Figure According to the of the devices, the of the devices with and without thermal annealing are and respectively. According to the JSC of the devices, is in the thermally annealed device than in the as-cast device (see the Supporting Information). The and well with the improved PCE of the thermally annealed OSC device. stability The inadequate intrinsic stability of small-molecule electron acceptors containing vinyl linkers is for OSC device stability. Moreover, in OSC devices may with these small electron acceptors, which leads to device For example, a may the of small electron acceptors containing vinyl linkers is with UV The superior intrinsic stability of FM1 that FM1 should exhibit improved OSC device stability. To we the OSC device stability of FM1 and Y6 under AM 1.5 G simulated solar light illumination (100 mW cm−2, without UV The device structure was or and the two devices were Figure 6 shows the of PCE on the illumination of the two devices. After illumination for 16 h, the device of FM1 of its PCE while the control device of Y6 only of its PCE ( Supporting Information Figure and Tables and This result proves the excellent photostability of the device. Figure 6 | PCE of the inverted OSC devices based on D18:FM1 and active after illumination under 100 mW cm−2 AM 1.5 G simulated solar light for different times. The with average values were obtained from devices. Download figure Download PowerPoint To the for the in photostability of the devices based on D18:FM1 and we tested the degradation of the active films under UV irradiation and as well as the morphology of the films under simulated irradiation for different times. The degradation of active films under light or was by their absorption and the film morphology after light was investigated by Supporting Information Figure shows the UV–vis absorption spectra of active films of D18:FM1 and on under UV irradiation for different times. The film exhibited stability with only very small after 16 h of In contrast, films exhibited different absorption spectra with significantly reduced absorbance to the absorption of after 16 h of This result shows that in the presence of the the degradation of Y6 under UV irradiation is significantly higher than that of FM1. Supporting Information Figure shows the UV–vis absorption spectra of blend films of D18:FM1 and with thermal treatment at °C for different times. The absorption intensity of