Article pubs.acs.org/cm
Synthesis and Photovoltaic Properties of a Series of Narrow Bandgap Organic Semiconductor Acceptors with Their Absorption Edge Reaching 900 nm Xiaojun Li,†,‡ He Huang,‡ Haijun Bin,†,‡ Zhengxing Peng,∥ Chenhui Zhu,⊥ Lingwei Xue,† Zhi-Guo Zhang,† Zhanjun Zhang,*,‡ Harald Ade,*,∥ and Yongfang Li*,†,‡,§ †
CAS Research/Education Center for Excellence in Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ School of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences, Beijing 100049, China § Laboratory of Advanced Optoelectronic Materials, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, Jiangsu 215123, China ∥ Department of Physics and ORaCEL, North Carolina State University, Raleigh, North Carolina 27695, United States ⊥ Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States S Supporting Information *
ABSTRACT: Three n-OS acceptors with Eg values of 10%. The results indicate that ITVffIC is a promising narrow Eg acceptor for application in tandem and semitransparent PSCs.
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INTRODUCTION Polymer solar cells (PSCs) with a p-type conjugated polymer as a donor and a fullerene derivative (such as PCBM) or n-type organic semiconductor (n-OS) as an acceptor have attracted considerable attention as an emerging energy conversion technology in recent years,1−5 because of their advantages of simple device structure, light weight, and the ability to be fabricated into flexible and semitransparent devices. In the development of the PSCs, the design and synthesis of highperformance donor and acceptor photovoltaic materials play a crucial role.6−14 In particular, in the past two years, the development of narrow bandgap (Eg) A−D−A structured n-OS small molecule acceptors has promoted a rapid increase in the power conversion efficiency (PCE) of the PSCs to >12%.15−17 In comparison with traditional fullerene derivative acceptors, the advantages of the A−D−A structured n-OS acceptors include good morphology stability and the easy tuning of their © 2017 American Chemical Society
electronic energy levels and absorption spectra by using different electron-donating (D) and electron-accepting (A) units in the molecules. For example, the n-OS acceptor ITIC, with a fused-ring central donating unit and two INCN [2-(2,3dihydro-3-oxo-1H-inden-1-ylidene) propanedinitrile] accepting end groups, possesses strong and broad film absorption in the wavelength region from 560 to 780 nm with an optical Eg of 1.58 eV and suitable lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels for application as an acceptor.18 Via selection of medium or broad bandgap conjugated polymers as a donor, the PCE of the PSCs with ITIC as the acceptor reached 10− 12%.6,19−21 Nevertheless, there is still some room for the Received: September 16, 2017 Revised: November 7, 2017 Published: November 7, 2017 10130
DOI: 10.1021/acs.chemmater.7b03928 Chem. Mater. 2017, 29, 10130−10138
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Chemistry of Materials
Figure 1. (a) Chemical structures of the n-OS acceptors and polymer donor J71. (b) Synthetic routes for ITVIC, ITVfIC, and ITVffIC.
high Voc. Very interestingly, the recently reported highperformance PSCs with a narrow bandgap n-OS ITIC acceptor simultaneously displayed a high Jsc and a high Voc and benefitted from the high-efficiency charge separation even with a ΔEHOMO as small as 0.6 eV. Therefore, it is very important to tune the LUMO and HOMO energy levels of the photovoltaic materials when its Eg values are reduced to simultaneously obtain a high Jsc and a
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RESULTS AND DISCUSSION Synthesis and Thermal Stability. Figure 1 shows the molecular structures and synthetic routes of the three acceptors, ITVIC, ITVfIC, and ITVffIC. The introduction of the double bond into IT-CHO to obtain ITV-CHO was
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DOI: 10.1021/acs.chemmater.7b03928 Chem. Mater. 2017, 29, 10130−10138
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Figure 2. Ultraviolet−visible absorption spectra of ITVIC, ITVfIC, and ITVffIC in (a) a chloroform solution and (b) a film state. (c) Cyclic voltammograms of ITVIC, ITVfIC, and ITVffIC. The inset shows the cyclic voltammogram of the ferrocene/ferrocenium (Fc/Fc+) couple used as an internal reference. (d) Energy level diagram of the materials used in the PSCs.
red-shifted the film absorption edges to 900 nm (Eg = 1.37 eV) for ITVfIC and to 915 nm (Eg = 1.35 eV) for ITVffIC. The optical properties of the n-OS acceptors are listed in Table 1 for a clear comparison.
accomplished by using tributyl(1,3-dioxolan-2-ylmethyl)phosphonium bromide and NaH at room temperature, and the reaction was quenched with 10% HCl, with a high yield of 87%. Then Knoevenagel condensation of ITV-CHO with compounds 1−3 in chloroform afforded the acceptors ITVIC, ITVfIC, and ITVffIC, respectively, in high yield. Details of the synthesis are described in the Experimental Section. These three compounds exhibit good solubility in organic solvents. The thermal stability of these compounds was investigated using thermogravimetric analysis (TGA), as shown in Figure S1. The TGA plots of ITVIC, ITVfIC, and ITVffIC show decomposition temperature at 5% weight loss (Td) of 335, 324, and 324 °C, respectively, which indicates that the thermal stabilities of these three compounds are good enough for their application in PSCs. Absorption Spectra, Electronic Energy Levels, and Electron Mobilities. Panels a and b of Figure 2 show absorption spectra of ITVIC, ITVfIC, and ITVffIC in chloroform solutions and in thin films, respectively. The solution absorption spectra of the three acceptors (see Figure 2a) show similar absorption bands in the wavelength range of 550−800 nm with a slight red-shift from ITVIC to ITVfIC with monofluorine substitution and to ITVffIC with bifluorine substitution on its end groups. In the films, the three n-OS acceptors all show greatly red-shifted and broadened absorption spectra compared to those of their solutions, with a broad absorption band covering the wavelength range of 550−900 nm (see Figure 2b). The absorption spectrum of ITVIC film is significantly red-shifted with an absorption edge at 885 nm corresponding to an Eg of 1.40 eV in comparison with the absorption edge at 780 nm and an Eg of 1.58 eV for the ITIC film, which should be attributed to the introduction of a double-bond π-bridge in the molecule. The fluorination further
Table 1. Physicochemical Properties of the J71 Polymer Donor and the n-OS Acceptors
J71 ITVIC ITVfIC ITVffIC
λmaxa (nm)
λedgea (nm)
Egb (eV)
EHOMOc (eV)
ELUMOc (eV)
EHOMOd (eV)
580 755 780 780
632 885 900 915
1.96 1.40 1.37 1.35
−5.40 −5.46 −5.56 −5.58
−3.24 −3.97 −4.01 −4.04
−5.56 −5.60 −5.65
Absorption of the films. bCalculated from the absorption edge of the polymer films: Eg = 1240/λedge. cCalculated according to the equation ELUMO/HOMO = −e(Ered/ox + 4.36). dMeasured via ultraviolet photoelectron spectroscopy (UPS). a
The LUMO and HOMO energy levels of ITVIC, ITVfIC, and ITVffIC were measured by electrochemical cyclic voltammetry with Ag/AgCl as the reference electrode and the Fc/Fc+ couple used as the internal standard, and their cyclic voltammograms are shown in Figure 2c. The HOMO and LUMO energy levels were estimated from the onsets of oxidation and reduction potentials (Eox/red), respectively, according to the equation EHOMO/LUMO = −e(Eox/red + 4.36) (eV). (The redox potential of Fc/Fc+ is 0.44 V vs Ag/AgCl in our measurement system, and we take the energy level of Fc/ Fc+ to be 4.8 eV below vacuum.) The EHOMO values of ITVIC, ITVfIC, and ITVffIC were calculated to be −5.46, −5.56, and −5.58 eV, respectively, with ELUMO values of −3.97, −4.01, and −4.04 eV, respectively. Obviously, the ELUMO and EHOMO values of the compounds are downshifted by the fluorine substitution. 10132
DOI: 10.1021/acs.chemmater.7b03928 Chem. Mater. 2017, 29, 10130−10138
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Chemistry of Materials
Figure 3. (a) J−V curves of the PSCs based on a 1:1.2 (w/w) J71:acceptor ratio with thermal annealing at 160 °C for 2 min, under AM 1.5G illumination, 100 mW cm−2. (b) IPCE spectra of the corresponding PSCs.
Table 2. Photovoltaic Performance Parameters of the PSCs Based on a 1:1.2 (w/w) J71:Acceptor Ratio under Different Treatment Conditions, under AM 1.5G Illumination, 100 mW/cm2 acceptor (treatment) ITVIC (TAa) ITVfIC (TAa) ITVffIC (TAa) ITVffIC (As castb) ITVffIC (TA and additivec)
Voc (V) 0.89 (0.892 0.84 (0.834 0.80 (0.791 0.78 (0.782 0.81 (0.798
± 0.006)f ± 0.003) ± 0.002) ± 0.001) ± 0.004)
Jsc (mA cm−2) 14.47 (14.13 19.73 (19.66 22.83 (22.33 19.81 (19.04 20.60 (19.71
± 0.23) ± 0.54) ± 0.37) ± 0.30) ± 0.60)
fill factor (%) 57.64 (55.36 58.67 (56.98 52.66 (53.84 49.48 (50.06 63.18 (62.28
± 0.86) ± 1.12) ± 1.02) ± 0.58) ± 1.01)
PCE (%)
calculated Jsc (mA cm−2)
ΔEHOMOd (eV)
Elosse (eV)
7.34 (7.12 ± 0.16)
14.36
0.06
0.51
9.72 (9.48 ± 0.21)
19.12
0.16
0.53
9.61 (9.57 ± 0.21)
22.00
0.18
0.55
7.64 (7.45 ± 0.12)
19.15
0.18
0.57
10.54 (10.24 ± 0.24)
19.77
0.18
0.54
TA indicates thermal annealing at 160 °C for 2 min. bAs cast indicates without post-treatment. cTA and additive indicates with a 0.5% CN solvent additive treatment and thermal annealing at 160 °C for 2 min. dΔΕHOMO = EHOMOdonor − EHOMOacceptor. eEloss = Eg − eVoc, where Eg is the lowestenergy bandgap of the donor and acceptor components. a
In comparison with the EHOMO of −5.48 eV and the ELUMO of −3.83 eV for ITIC,15 the EHOMO of ITVIC is upshifted slightly and its ELUMO is downshifted by 0.14 eV because of the insertion of the double-bond π-bridges. Figure 2d displays the LUMO and HOMO energy levels of the three acceptors together with those values of the polymer donor J71 and electrode buffer layer materials. We also used ultraviolet photoelectron spectroscopy (UPS) to verify the EHOMO values obtained from CV measurements, as shown in Figure S2. The EHOMO values from the UPS measurements are −5.56, −5.60, and −5.65 eV for ITVIC, ITVfIC, and ITVffIC, respectively (see Table S1), which are close to the values measured by cyclic voltammetry and show the same downshifting tendency from ITVIC to ITVfIC and to ITVffIC. Electron mobility is another important property for the acceptors used in PSCs. We measured the electron mobilities of the acceptors by the space charge-limited current (SCLC) method with the ITO/ZnO/active layer/PDINO/Al electron only device structure. The measurement results are shown in Figure S3, and the calculated electron mobilities of ITVIC, ITVfIC, and ITVffIC are 2.02 × 10−4, 1.80 × 10−4, and 1.13 × 10−4 cm2 V−1 s−1, respectively, which are close (a little lower) to the electron mobility of ITIC,15 and similar phenomena of a slight decrease in electron mobility upon fluorization can be observed in other systems.25,28 Photovoltaic Performance. To investigate the photovoltaic performance of the acceptors, we fabricated the PSCs with the medium Eg polymer J71 as a donor, the three narrow
Eg n-OS compounds as the acceptor, ITO/PEDOT:PSS as the positive electrode, and PDINO/Al as the negative electrode. The donor:acceptor weight ratio was optimized to be 1:1.2, and thermal annealing at 160 °C for 2 min was performed for improving the photovoltaic performance of the PSCs. Figure 3a shows the current density−voltage (J−V) curves of the PSCs based on a 1:1.2 (w/w) J71:n-OS ratio with thermal annealing at 160 °C for 2 min under air mass (AM) 1.5G illumination, 100 mW/cm2, and the input photon to converted current efficiency (IPCE) spectra of the corresponding devices. The photovoltaic performance data of the devices are listed in Table 2 for a clear comparison. The J71:ITVIC-based PSC demonstrated a high Voc of 0.88 V and a moderate PCE of 7.34% with a relatively low short circuit current density (Jsc = 14.47 mA/cm2), which may be due to an overly low ΔEHOMO of 0.06 eV (see Table 2) for the donor and acceptor materials in the active layer. For the PSCs based on J71:ITVfIC, the Voc was decreased to 0.84 V because of the downshifted LUMO level of the ITVfIC acceptor, while Jsc and PCE were increased significantly to 19.73 mA/cm2 and 9.72%, respectively, which could benefit from the increase in ΔEHOMO to a reasonable value of 0.16 eV. With a further downshift of the LUMO and HOMO energy levels of the acceptor ITVffIC, the Voc was further decreased to 0.80 V and the Jsc was further increased to 22.83 mA/cm2 for the PSCs based on J71:ITVffIC with a ΔEHOMO of 0.18 eV. Interestingly, the Jsc values of these devices increase with the increase in ΔEHOMO of their donor and acceptor materials, indicating that a larger ΔEHOMO in the range of 0.16−0.18 eV (see Table 2) 10133
DOI: 10.1021/acs.chemmater.7b03928 Chem. Mater. 2017, 29, 10130−10138
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Chemistry of Materials
Figure 4. (a) J−V curves of the devices with a donor:acceptor weight ratio of 1:1.2 without extra treatment (As cast), with thermal annealing (TA), and with a solvent additive and TA (SA+TA) under AM 1.5G illumination, 100 mW cm−2. (b) IPCE spectra of the devices with a donor:acceptor weight ratio of 1:1.2. Plots of (c) Jsc vs Voc, (d) PCE vs Eg, and (e) PCE vs Eloss of the PSCs based on the NIR photovoltaic material with an Eg of
