Article Cite This: ACS Appl. Energy Mater. 2019, 2, 4730−4736
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Medium-Bandgap (Acceptor′−Donor)2Acceptor-Type SmallMolecule Donors Based on an Asymmetric Thieno[3,2‑c]isochromene Building Block for Organic Solar Cells with High Efficiency and Voltage Wenchao Wang,†,‡ Guangjun Zhang,§ Jiali Guo,‡ Ziqian Gu,‡ Rulin Hao,‡ Zixuan Lin,‡ Yu Qian,‡ Mengbing Zhu,‡ Hao Xia,‡ Wenhong Peng,‡ Xianzhi Liu,†,‡ Qiang Peng,*,§ and Weiguo Zhu*,†,‡ Downloaded via BUFFALO STATE on July 22, 2019 at 07:57:04 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
†
College of Chemistry, Xiangtan University, Xiangtan 411105, China School of Materials Science and Engineering, Jiangsu Collaborative Innovation Center of Photovoltaic Science and Engineering, Jiangsu Engineering Laboratory of Light-Electricity-Heat Energy-Converting Materials and Applications, National Experimental Demonstration Center for Materials Science and Engineering, Changzhou University, Changzhou 213164, China § School of Chemistry, Sichuan University, Chengdu 610064, China ‡
S Supporting Information *
ABSTRACT: Small-molecule donors have a critical effect on the properties of the small-molecule-based organic solar cells (SM-OSCs). In order to develop novel small-molecule donors, two (A′−D)2A-type (A = acceptor; D = donor) small molecules (IDO-TiC-T)2-DFBT and (Rh-TiC-T)2-DFBT are designed and synthesized, in which 5,6-difluoro-2,1,3-benzothiadiazole (DFBT) and novel asymmetry electron-rich thieno[3,2-c] isochromene (TiC) are introduced as the central electron-accepting (A) and the armed electron-donating (D) units, respectively. Also, their photoelectronic properties are studied. A significant effect of the TiC and electron-accepting (A′) end units on these properties is observed. A deep HOMO energy level of −5.45 eV with a medium bandgap of 1.77 eV is exhibited. A significantly improved power conversion efficiency (PCE) of 7.55% is received in the (IDO-TiC-T)2-DFBT/PC71BM-based OSCs with a high open circuit voltage (VOC) of 0.9 V, which is 1.38 times higher than that in the (Rh-TiC-T)2-DFBT/PC71BM-based cells. Our results illuminate that the asymmetrical TiC unit has a great potential to build up small-molecule donors for OSCs with high PCE and VOC. KEYWORDS: thieno[3,2-c]isochromene, small-molecule donor, asymmetric electron-rich unit, fullerene, organic solar cells these SM-OSCs.20−23 It is found that the organic photovoltaic materials in the active layer have a critical influence on device performance. Organic donor materials are one of important organic photovoltaic materials and have been rapidly developed in the recent decade.20−31 Some strategies to construct novel organic donor materials have been demonstrated, i.e., the design of donor−acceptor (D−A) frameworks, electron-rich donor
1. INTRODUCTION Organic solar cells (OSCs) have drawn great attention in recent decades owing to their great potential for flexible, semitransparent, large-area devices.1−5 Many advances have been made for OSCs. Power conversion efficiencies (PCEs) of about 13−14% for single-junction bulk heterojunction (BHJ) architectures6−9 and 15−17% for tandem architectures10,11 have been obtained in polymer-based organic solar cells (POSCs). Also, because of the definite structures, lack of batch difference, and ease of synthesis, the small-molecule-based organic solar cells (SM-OSCs) have also been developed rapidly.12−19 The PCE values of about 11−12% are achieved in © 2019 American Chemical Society
Received: February 15, 2019 Accepted: June 4, 2019 Published: June 4, 2019 4730
DOI: 10.1021/acsaem.9b00303 ACS Appl. Energy Mater. 2019, 2, 4730−4736
Article
ACS Applied Energy Materials Scheme 1. Synthetic Routes for (IDO-TiC-T)2-DFBT and (Rh-TiC-T)2-DFBT
(A′−D)2A-type small-molecule donors (IDO-TiC-T)2-DFBT and (Rh-TiC-T)2-DFBT based on a novel asymmetric electron-rich thieno[3,2-c] isochromene (TiC) used as the armed electron-donating (D) unit. In both SMs, the center is an electron-accepting (A) unit of 5,6-difluoro-2,1,3-benzothiadiazole (DFBT). The π bridge is thiophene, and the terminal is another electron-accepting (A′) unit of a 1,3-indanedione (IDO) or rhodanine (Rh) derivative. In a comparison with DTP, we belive TiC has a slightly reduced electron-donating capability and can make its SMs have a deeper HOMO energy level, as benzene replaces thiophene in the TiC. Furthermore, this type of SM based on the TiC unit should have a decreased self-aggregation owing to its weakening planarity, which is beneficial for morphological control and further improvement of the charge transporting property in its resulting materials.34,35 As expected, excellent solubility in general organic solvents and a low HOMO energy of −5.45 eV are observed for both SMs. However, (IDO-TiC-T)2-DFBT presents better photovoltaic properties than (Rh-TiC-T)2-DFBT in the fullerene-based OSCs. An advanced PCE of 7.55% accompanied by an open circuit voltage (VOC) of 0.9 V is received in the (IDO-TiC-T)2-DFBT/PC71BM-based OSCs, which is 1.38 times higher than that in the (Rh-TiC-T)2-DFBT/PC71BMbased cells. Our results illuminate that this asymmetric TiC
units, electron-deficient acceptor units, and side-chain engineering.24−28 It is found that a few asymmetric electronrich donor units, such as indenothiophene (IDT) and 5Hdithieno[3,2-b:2′,3′-d]pyran (DTP), were introduced to synthesize low-bandgap photovoltaic materials for highefficiency OSCs owing to their strong electron-donating capability, improved solubility, and good charge transport properties.29−35 For example, the Yang group used an asymmetric DTP unit to build D−A copolymers, one of which exhibits a PCE of 8% in bulk-heterojunction solar cells.29 Zheng et al. developed the IDT-based copolymers, which led to their P-OSCs exhibiting 9.14% efficiency and close to 1 V open circuit voltage (VOC).30,31 In a comparion with symmetric units, asymmetric units can further tune energy levels and electron-donating ability well, owing to the electronrich nature of the oxygen atom and the reduced steric hindrance between the donor and acceptor units. However, a high HOMO energy level for these donor materials based on the DTP unit is damaging to the voltage, and the reduced electron-donating ability based on IDT restricts the absorption spectrum. In order to develop high-efficiency organic small-molecule donor materials and to further study the structure−property relationship, in this contribution, we developed two novel 4731
DOI: 10.1021/acsaem.9b00303 ACS Appl. Energy Mater. 2019, 2, 4730−4736
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ACS Applied Energy Materials
Figure 1. Top (a, c) and side (b, d) views of the optimized molecular geometries of (IDO-TiC-T)2-DFBT (a, b) and (Rh-TiC-T)2-DFBT (c, d) by DFT calculations.
Figure 2. Absorption spectra and energy levels of (IDO-TiC-T)2-DFBT and (Rh-TiC-T)2-DFBT.
solvents, such as CH2Cl2, CHCl3, and toluene under room temperature. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were measured and further characterized the molecular thermal stability and crystallinity. The recorded TGA and DSC curves are shown in Figures S1 and S2 in Supporting Information. The decomposition temperatures (Td’s) of 312 and 349 °C are observed for (IDO-TiC-T)2DFBT and (Rh-TiC-T)2-DFBT at 5% weight loss, respectively. This implies that both SMs possess high thermal stability. However, (IDO-TiC-T)2-DFBT exhibits better crystallinity than (Rh-TiC-T)2-DFBT on the basis of the DSC curves. Therefore, changing the terminal electron-accepting unit can further tune molecular crystallinity. Figure 1 and Figure S1 show the optimized molecular geometries and electronic distributions of (IDO-TiC-T)2DFBT and (Rh-TiC-T)2-DFBT obtained by theoretical calculation at the B3LYP/6-31G* level. A planar configuration in its entirely with a little twisting geometry between the thiophene and TiC units is observed for both SMs. Two dihedral angles between thiophene and TiC units are 6.34° at the left and 16.10° at the right for (IDO-TiC-T)2-DFBT. Also, the corresponding dihedral angles are 14.86° at the left and 11.55° at the right for (Rh-TiC-T)2-DFBT. This indicates that
unit has a great potential to build up small-molecule donors for OSCs with high PCE and VOC.
2. EXPERIMENTAL SECTION 3-Methoxythiophene and 1,3-indanedione were purchased from Energy Chemical. 4,7-Bis(2-trimethy-5-thienyl)-5,6-difluoro-2,1,3benzothiadiazole was purchased from Derthon. Other chemicals were used as received, unless otherwise specified. SM-OSCs were fabricated based on (IDO-TiC-T)2-DFBT or (RhTiC-T)2-DFBT as a donor and PC71BM as an acceptor. The typical device structures are ITO/PEDOT:PSS/SM:PC71BM/Ca/Al. The detailed device fabrication and characterization are presented in the Supporting Information.
3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterizations. (IDO-TiC-T)2DFBT and (Rh-TiC-T)2-DFBT were prepared via Stille coupling, bromination, Grignard reaction, and Knoevenagel condensation according to the synthetic routes described in Scheme 1. The detailed synthetic procedures are presented in the Supporting Information. Both molecular structures were characterized and confirmed with 1H and 13C NMR and MALDI-TOF. They exhibit great solubility in common 4732
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ACS Applied Energy Materials Table 1. Optical and Electrochemical Data of (IDO-TiC-T)2-DFBT and (Rh-TiC-T)2-DFBT donor (IDO-TiC-T)2-DFBT (Rh-TiC-T)2-DFBT
λmaxa [nm]
εmaxa [L g−1 cm−1]
λmaxb [nm]
λonsetb [nm]
Egoptc [eV]
EHOMOd [eV]
ELUMOd [eV]
EgCV [eV]
576 575
1.78 × 10 1.34 × 105
580 585
700 700
1.77 1.77
−5.45 −5.45
−3.26 −3.23
2.19 2.23
5
In chloroform solution. bIn neat film. cOptical bandgap, Egopt = 1240/λonset eV in the film. dMeasured with cyclic voltammetry (CV) method, EHOMO = −[(Eox‑onset − Eferrocene) + 4.8] eV, ELUMO = −[(Ered‑onset − Eferrocene) + 4.8] eV.
a
Figure 3. (a) J−V and (b) EQE curves of the (IDO-TiC-T)2-DFBT and (Rh-TiC-T)2-DFBT devices with/without SVA treatment.
Table 2. Optimized Device Properties of the (IDO-TiC-T)2-DFBT/PC71BM- and (Rh-TiC-T)2-DFBT/PC71BM-Based Solar Cells active layera
VOC [V]
JSC [mA/cm2]
FF [%]
(IDO-TiC-T)2-DFBT (IDO-TiC-T)2-DFBTb (Rh-TiC-T)2-DFBT (Rh-TiC-T)2-DFBTb
0.90 0.90 0.87 0.87
10.93 12.63 9.94 11.34
55.6 66.4 45.9 54.8
PCEc [%] 5.47 7.55 3.97 5.41
(5.15) (7.20) (3.48) (5.11)
Rs [Ω cm2]
Rsh [Ω cm2]
13.1 10.4 20.5 14.2
145.3 283.6 102.5 122.3
μhd [cm2 V−1 s−1] 3.68 1.30 6.40 2.37
× × × ×
10−5 10−4 10−5 10−5
a
Using DIO as additive. bSolvent vapor annealing (SVA) by CS2 for 20 s. cData in parentheses indicate the average PCEs of 8 cells. dMeasured by space charge limited current (SCLC) method.
those in their solution, indicating that both SMs have stronger intermolecular π−π stacking in the thin films. The optical bandgap (Egopt) of both SMs is 1.77 eV calculated from the absorption onset in the thin film. Cyclic voltammetry (CV) is used to evaluate the electrochemical properties of (IDO-TiC-T)2-DFBT and (Rh-TiCT)2-DFBT. Figure S4 depicts the measured CV curves. Table 1 lists the HOMO and LUMO energy levels calculated by CV data. We find that there exists an almost identical HOMO energy level of −5.45 eV for both SMs. This indicates that both terminals of IDO and Rh units have almost the same influence on HOMO and LUMO energy levels. 3.3. Photovoltaic Properties. Using (IDO-TiC-T)2DFBT or (Rh-TiC-T)2-DFBT as electron donor (D) and PC71BM as electron acceptor (A) in the active layer, a series of bulk-heterojunction (BHJ) OSCs were fabricated. Various D/ A weight ratios, DIO additive concentrations, and solvents used as vapor annealing (SVA) were selected to optimize device performance. Tables S1−S6 (in Supporting Information) summarize the photovoltaic performances of these devices. The most optimal D/A weight ratio of 1:1, DIO concentration of 0.5%, and solvent of carbon disulfide for 20 s vapor annealing are achieved. Figure 3 depicts the optimal J−V curves of the (IDO-TiC-T)2-DFBT- and (Rh-TiC-T)2-DFBTbased solar cells, and Table 2 summarizes the corresponding photovoltaic data. It is found that, by further SVA process, the JSC, FF, and PCE values are significantly advanced for both
introducing an asymmetric TiC unit can tune the molecular planarity toward reducing intermolecular self-aggregation, which is a benefit for its molecules for forming suitable phase separation and maintaining strong intermolecular π−π stacking. Meanwhile, the electronic distributions of HOMO and LUMO are similar for both SMs. The electron density is delocalized over the whole molecular backbone for the HOMO wave function. In contrast, the electron density is mainly localized on the central acceptor unit of DFBT for the LUMO wave function. This indicates that both SMs also have a strong intramolecular charge transfer (ICT) effect, which is available for obtaining medium-bandgap absorption. 3.2. Optical and Electrochemical Properties. The ultraviolet−visible (UV−vis) absorption spectra of (IDOTiC-T)2-DFBT and (Rh-TiC-T)2-DFBT are recorded in Figure 2 in their neat thin films and CHCl3 solutions. The relevant data are summarized as optical properties in Table 1. It is noted that both SMs show similar medium-broad absorption spectra in the range 300−800 nm. In the CHCl3 solutions, the maximum molar extinction coefficients are 1.78 × 105 mol−1 L−1 for (IDO-TiC-T)2-DFBT and 1.34 × 105 mol−1 L−1 for (Rh-TiC-T)2-DFBT at about 575 nm. (IDOTiC-T)2-DFBT exhibits a little enhanced absorption in comparison with (Rh-TiC-T)2-DFBT. This is consistent with their external quantum efficiency (EQE) values under optical radiation, recorded in Figure 3. In their neat films, both SMs display red-shifted and expanded spectra in comparison with 4733
DOI: 10.1021/acsaem.9b00303 ACS Appl. Energy Mater. 2019, 2, 4730−4736
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Figure 4. TEM images of (IDO-TiC-T)2-DFBT/PC71BM without (a)/with (b) SVA treatment, and of (Rh-TiC-T)2-DFBT/PC71BM without (c)/ with (d) SVA treatment.
SM-based fullerene OSCs although the VOC value has no change. The maximum PCE of 7.50% is obtained in the (IDOTiC-T)2-DFBT/PC71BM-based cell with a high VOC value of 0.9 V. This PCE value is about 1.38 times higher than that in the (Rh-TiC-T)2-DFBT/PC71BM-based cell. Therefore, tuning the electron-accepting end group can remarkably improve the photovoltaic properties of these (A′−D)2A-type small molecules. 3.4. Morphological Characterization. The morphologies of active layers were recorded through transmission electron microscopy (TEM) and atomic force microscopy (AFM). Figure 4 and Figure S6 show the recorded TEM and AFM images, respectively. It is found that both active layers exist with an increasing domain with SVA treatment, which is a benefit for carrier transportation. Nanofibrils are also observed in the (IDO-TiC-T)2-DFBT/PC71BM active layer with SVA treatment, implying that the excitions are more efficiently separated in the active layer. The formed root-mean-square (RMS) surface roughness is 1.35 nm for the (IDO-TiC-T)2-
DFBT blend and 1.56 nm for the (Rh-TiC-T)2-DFBT blend under SVA treatment, which are consistent with the corresponding TEM images. There are small increases of 0.30 and 0.43 nm in comparison with the corresponding values for the (IDO-TiC-T)2-DFBT blend and the (Rh-TiC-T)2DFBT blend without SVA treatment, respectively. Although the RMS roughness has a small increase for both molecular blends after SVA treatment, the well-organized nanofibril morphology is observed in the blending films. It is available to form the transportation channels for holes and electrons and results in an increasing FF value in the devices.36 Therefore, the (IDO-TiC-T)2-DFBT/PC71BM solar cell exhibits higher PCE under SVA treatment. 3.5. Carrier Mobilities. To investigate the carrier transport performance of both SMs in active layers, the hole-only and the electron-only devices were made. Figure S5 depicts the J1/2−V curves of these devices based on SM/PC71BM active layers using (IDO-TiC-T)2-DFBT and (Rh-TiC-T)2-DFBT as the donor under the optimized processing conditions, 4734
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ACS Applied Energy Materials separately. The hole and electron mobilities (μh and μe) in these devices are summarized in Table S7 and calculated according to the Mott−Gurney equation. It is found that all carrier mobilities are significantly increased and the μe/μh values are more balanced for both devices after SVA treatment. The μh value of the (IDO-TiC-T)2-DFBT:PC71BM device is increased from 3.68 × 10−5 to 1.30 × 10−4 cm2 V−1 s−1 after SVA treatment. It is higher than that of the (Rh-TiC-T)2DFBT:PC71BM device. Furthermore, the lower μe/μh value of 2.58 is exhibited in the (IDO-TiC-T)2-DFBT:PC71BM device rather than in the (Rh-TiC-T)2-DFBT:PC71BM device with SVA treatment. Therefore, a higher carrier mobility and more balanced μe/μh value improve the photovoltaic performance in the (IDO-TiC-T)2-DFBT-based PSCs after SVA treatment.37
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4. CONCLUSION In summary, two medium-bandgap small-molecule donors of (IDO-TiC-T)2-DFBT and (Rh-TiC-T)2-DFBT with a novel electron-rich TiC unit were obtained. An advanced PCE of 7.50% with a VOC value of 0.9 V was achieved in the (IDOTiC-T)2-DFBT/PC71BM-based solar cells. This work presents a potential strategy to improve photovoltaic properties via design of an asymmetric electron-rich TiC unit in SMs. We believe that the TiC is an outstanding electron-rich building block to construct high-performance organic small-molecule donor materials.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.9b00303.
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Details about synthetic procedures, device fabrication, and other important characterization data (TGA, AFM, DSC, CV, DFT, SCLC) (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Wenhong Peng: 0000-0002-0536-2313 Weiguo Zhu: 0000-0002-4244-2638 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge the financial support from the National Natural Science Foundation of China (51673031, 51573154), the Major Program of the Natural Science Research of Jiangsu Higher Education Institutions (18KJA480001), the Top-Notch Academic Programs Project (TAPP) for Polymeric Materials Science and Technology & the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions, Jiangsu Provincial Talents Project of High-Level Innovation and Entrepreneurship, and the Talent Project of Jiangsu Specially-Appointed Professor.
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ACS Applied Energy Materials
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DOI: 10.1021/acsaem.9b00303 ACS Appl. Energy Mater. 2019, 2, 4730−4736
