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Letter
Rylene Annulated Subphthalocyanine: A Cone-Shaped Non-Fullerene Acceptor Promising for Organic Solar Cells Tengda Huang, Hui Chen, Jiajing Feng, Andong Zhang, Wei Jiang, Feng He, and Zhaohui Wang ACS Materials Lett., Just Accepted Manuscript • DOI: 10.1021/acsmaterialslett.9b00221 • Publication Date (Web): 28 Aug 2019 Downloaded from pubs.acs.org on August 28, 2019
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ACS Materials Letters
Rylene Annulated Subphthalocyanine: A Cone-Shaped Non-Fullerene Acceptor Promising for Organic Solar Cells Tengda Huang,†,# Hui Chen,§ Jiajing Feng,‡ Andong Zhang,‡ Wei Jiang,*,† Feng He,*,§ and Zhaohui Wang*,‡ CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China. ‡ Key Laboratory of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, China. § Department of Chemistry, Southern University of Science and Technology, Shenzhen 518055, PR China. # University of Chinese Academy of Sciences, Beijing 100049, PR China. †
ABSTRACT: The development of various non-fullerene acceptors further promoted efficiencies of organic solar cells in the past years. Subphthalocyanine is recently emerging as electron-acceptor that has unique cone geometry, multiple functionalization and broad absorption. Here, we designed and synthesized two fully conjugated rylene annulated subpthalocyanines (SubPcPDI3-Cl and SubPcPDI3-OC6H4tBu) bearing different axial substituents. Theoretical calculation revealed a cone-shaped and fully conjugated molecular geometry and the dihedral angle between perylene diimide (PDI) wings and cone ground is about 25o. Both acceptors showed red-shifted absorbance about 680 nm and optical bandgaps around 1.70 eV due to the π-electron conjugation of between the SubPc core and surrounded PDI wings. Thus, the broad absorption, aligned energy levels, and balanced hole/electron mobility ratio endow SubPcPDI3-Cl an excellent power conversion efficiency of 7.53% in non-fullerene solar cells, which presents the highest photovoltaic performance based on subphthalocyanines in solution-processed solar cells.
Bulk-heterojunction (BHJ) organic solar cell (OSC) is fastgrowing since it has been reported in the 1990s.1,2 The BHJ device is typically fabricated with conjugated polymers as electron-donor and fullerene derivatives as electronacceptor for active layer.3 However, the shortcomings of fullerene limited the variability of the blends.4 The major focus of BHJ OSCs has shifted to non-fullerene acceptors (NFAs) in the past few years.5-7 Compared to fullerene materials, NFAs have broad coverage in the visible spectrum region, synthesized flexibility including tunable optical bandgap and energy level, low-cost and large-area manufacturing.8,9 Dramatic progress has been made in order to further promote efficiencies, with the best power conversion efficiency (PCE) over 15% for single junction and 17.3% for tandem structure.10-12 Subphthalocyanine (SubPc) consists of three diiminoisoindole units that are around the boron core via nitrogen atoms to forming a 14 π-electron, cone-shaped aromatic macrocycle with C3 symmetry.13-15 The unique nonplanar geometry prevents molecular aggregation either in solution or solid state with high solubility. SubPc has a relatively higher absorption in the 460-560 nm of visible light region called Q band and a weaker absorption in the 260-370 nm named Soret band. The optical and chemical properties of SubPc are facile to tune due to their easy functionalization in axial and/or peripheral positions.16 In general, SubPcs are electron-donors typically used in the
Figure 1. Concept for the design of rylene annulated subphthalocyanine.
vacuum-deposited solar cells, in which fullerenes are acceptor component.17-20 Present researches aiming at functionalized SubPcs for seeking extensive applications are growing but less.21 Especially, SubPcs can be converted from electron-donors to electron-acceptors when electronwithdrawing groups are introduced to the peripheral region or fused with polyaromatic rings.22-25 Up to now, a PCE of 8.4% has only been achieved by vacuum-deposited bilayer solar cells using two SubPcs as co-acceptor.26 Solution-processed BHJ devices have been recently fabricated employing a non-conjugated SubPc-PDI joint as acceptor with optimized PCE of 4.53%.27
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Scheme 1. (a) Synthetic route to SubPcPDI3-Cl and SubPcPDI3-OC6H4tBu; (b) top-view and (c) side-view of SubPcPDI3-Cl; (d) Chemical structure of donor polymer PBDBT-2F.
Herein ， we designed and synthesized two fully conjugated rylene annulated subpthalocyanines named SubPcPDI3-Cl and SubPcPDI3-OC6H4tBu (Figure 1), in which three perylene diimides (PDIs) annulated the central SubPc core. Theoretical calculation revealed a cone-shaped and fully conjugated molecular geometry and the dihedral angle between PDI wings and cone ground is about 25o. The introduction of electron-withdrawing PDI units broadens the absorption spectrum with high absorption coefficient in the visible light region. More importantly, the fully conjugated and fused connection between the SubPc core and surrounded PDI wings finely degenerates the lowest unoccupied molecular orbital (LUMO) levels of the corresponding acceptors, which shows much better compatibility with common donors. Different axial substituents have slight influence on optoelectronic properties but greatly affect the molecular aggregation and crystallization behaviour to fine tune the device performance. A promising PCE of 7.53% has been reached from SubPcPDI3-Cl-based devices, which presents the highest PCE for solution-processed BHJ OSCs based on SubPcs. The synthetic route of two SubPc acceptors is showed in Scheme 1. The key precursor 1 was achieved by our newly reported procedure for constructing quasi-planar PDIannulated phthalocyanine.28 The two SubPcs bear different axial substituents which are chlorine and 4-t-butylphenol group. The targeted SubPcPDI3-Cl (2), as dark-green solid was obtained by the cyclotrimerization of compound 1 in the presence of BCl3 in p-xylene solution at 140 oC for 1 h. Then, reaction with 4-t-butylphenol in toluene and 1,8diazabicyclo[5.4.0]undec-7-ene (DBU) solution at 110 oC for 8 h further afforded a dark-green solid of SubPcPDI3OC6H4tBu (3). The structures of two SubPcs were
unambiguously characterized by MALDI-TOF mass spectrometry and NMR spectroscopy. The two SubPc compounds are well soluble in common solvents, such as CH2Cl2, CHCl3, tetrahydrofuran and chlorobenzene. According to the thermal gravimetric analysis (TGA), both molecules showed great chemical and thermal stability with decomposition temperature over 377 oC (Figure S4). Table 1. Optical properties and energy levels of SubPcPDI3-Cl (2) and SubPcPDI3-OC6H4tBu (3), together with SubPc-Cl and PDI as references εmax
Egopt
[nm]a
[M-1 cm1]a
[eV]
PDI
527
SubPc-Cl
ELUMO
EHOMO
b
[eV]c
[eV]c
87 600
2.30
-3.86
-6.16d
565
68 500
2.14
-3.54
-5.44
SubPcPDI3Cl
680
126 000
1.74
-3.81
-5.39
SubPcPDI3OC6H4tBu
682
88 100
1.70
-3.81
-5.34
compd
λmax
Measured in CHCl3 solution (1 × 10-5 mol L-1). b Calculated by the onset of absorption in CHCl3 solution according to Egopt (eV) = 1240//λonset). c Estimated from the onset of the first reduction or oxidation peaks and calculated according to ELUMO = - (4.8 + Eonsetre) eV or EHOMO = -(4.8 + Eonsetox) eV, and the Eonset values are versus Fc/Fc+. d Calculated according to EHOMO = (ELUMO - Egopt) eV. a
The optimized geometries of SubPcPDI3-Cl and SubPcPDI3-OC6H4tBu were obtained by density functional theory (DFT) at the B3LYP/6-31G(d) level. The alkyl chains were replaced by H atoms to simplify the calculations. In contrast to quasi-planar ZnPcPDI4 with four PDI wings,28 the SubPc-based molecules are cone-shaped structure and
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ACS Materials Letters the dihedral angle between PDI wings and cone ground is 24.5o for SubPcPDI3-Cl and 25.0o for SubPcPDI3OC6H4tBu (Scheme 1 and Figure S2), respectively. Compared with non-conjugated SubPc-PDI,27 the nonplanar and fully conjugated SubPcPDI3-Cl and SubPcPDI3OC6H4tBu can benefit more from the rich scientific heritage of both SubPc cone and rylene unit.
about 20 nm (Figure S3), indicating the weak intermolecular aggregation even in the solid state. To further investigate electrochemical properties, cyclic voltammetry (CV) method was used in CH2Cl2 solution with SubPc-Cl and PDI as comparison (Figure 2b). The HOMO and LUMO levels were estimated from the onset of the first oxidation and reduction peaks. The LUMO levels of SubPcPDI3-Cl and SubPcPDI3-OC6H4tBu are almost the same of -3.81 eV, very close to PDI, indicating that the LUMO is rather PDI-like. The HOMO levels of SubPcPDI3-Cl and SubPcPDI3-OC6H4tBu are -5.39 eV and -5.34 eV, respectively. In contrast, the HOMO and LUMO levels of ZnPcPDI4 are -4.73 eV and -3.66 eV, respectively.28 The two SubPcs decreased both HOMO and LUMO levels, which match well with most donors and are more suitable for fabrication of OSC devices. Furthermore, The HOMOs are located at the SubPc core for both acceptors, while the LUMOs are mainly located at the PDI wings calculated by DFT simulation. (Figure S1).
Figure 2. UV-vis absorption spectra in CHCl3 solution (1 × 10-5 M) (a) and cyclic voltammograms in CH2Cl2 solution containing Bu4NPF6 (0.1 M) (b) of SubPcPDI3-Cl (green trace) and SubPcPDI3-OC6H4tBu (blue trace) with SubPc-Cl (pink trace) and PDI (red trace) as references.
The UV-vis absorption measurement was employed in CHCl3 solution with SubPc-Cl and PDI as references (Figure 2a). The bands from 370 nm to 520 nm are mainly owing to the three annulated PDI wings. The characteristic Q-band of SubPc is red-shifted with the absorption peaks at 680 nm for SubPcPDI3-Cl and 682 nm for SubPcPDI3-OC6H4tBu. And SubPcPDI3-Cl shows a much higher extinction coefficient, which is up to 125000 M-1 cm-1 in the Q region, almost one order higher than SubPcPDI3-OC6H4tBu and two parent molecules. Compared to SubPc-Cl, the Q-band of both acceptors is red-shifted more than 115 nm, which originates from the extended π-electron conjugation of PDIannulated SubPc core through the fully conjugated molecular design. The optical band gap of both acceptors is 1.74 eV for SubPcPDI3-Cl and 1.70 eV for SubPcPDI3OC6H4tBu, which are narrowed than that of SubPc-Cl about 2.14 eV (Table 1). The Q-band of two SubPc compounds in film has a small red shift compared with the solution peak
Figure 3. J-V characteristics of the optimized inverted solar cells of PBDBT-2F:acceptor (a) and the corresponding EQE spectra (b).
To study photovoltaic performance of two acceptors, solution-processed BHJ OSCs were fabricated with an inverted structure of ITO/ZnO/PBDBT2F:acceptor/MoO3/Ag. PBDBT-2F has HOMO and LUMO levels of -5.45 eV and -3.65 eV, and absorption ranges from
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Table 2. Photovoltaic parameters of optimized inverted solar cells of PBDBT-2F:acceptor VOC
JSC
[V]
[mA/cm2]
SubPcPDI3-Cl
0.97±0.001
13.97±0.15
SubPcPDI3-OC6H4tBu
1.00±0.001
10.93±0.13
acceptor
a
PCEa
PCEmax
[%]
[%]
54.68±0.33
7.41±0.11
7.53
41.73±0.30
4.49±0.15
4.63
FF
Average value ± standard deviation was calculated from 10 independent devices.
300 to 680 nm which complements well with two acceptors to ensure full harvesting of light (Figure S3).29 We examined the weight ratios of donor-acceptor (D-A) and the content of additives in order to gain better performance (Table S1 and Figure S5). Finally, when the D-A ratio was 1:1 (w/w) and 1.0% 1,8-diiodooctane (DIO) additive was added, the photovoltaic properties are the best. The current densityvoltage (J-V) and external quantum efficiency (EQE) were depicted in Figure 3, while the detailed parameters were illustrated in Table 2. The optimized device of SubPcPDI3Cl obtained the maximum PCE of 7.53%, with an opencircuit voltage (VOC) of 0.97 V, a short-circuit current (JSC) of 14.10 mA cm-2 and a fill factor (FF) of 55.04%. However, the device results of SubPcPDI3-OC6H4tBu, not as good as SubPcPDI3-Cl, the PCE is only 4.63% with VOC of 1.00 V, JSC of 11.05 mA cm-2 and FF of 42.02%. The higher JSC of SubPcPDI3-Cl should result from the better harvest photocurrent of active layer. Both devices own broad EQE response from 300 to 800 nm, contributing from the absorption of both donor and acceptor. Noticeably, the EQE spectrum of OSCs based SubPcPDI3-Cl is much higher than that of SubPcPDI3-OC6H4tBu in the region from 350 to 750 nm, which indicating the better exciton separation in SubPcPDI3-Cl-based devices. Both the integrated current density from the EQE match well with the JSC from J-V test. The charge mobility was measured using the space charge limited current (SCLC) method based on single carrier devices. The device structure for hole-only was ITO/PEDOT:PSS/PBDBT-2F:acceptor/MoO3/Ag. And the electron mobility was determined by the structure of ITO/ZnO/PBDBT-2F:acceptor/PDINO/Al. For two blends, the values of both hole and electron mobility of SubPcPDI3Cl are much higher than that of SubPcPDI3-OC6H4tBu (Figure S6), and the hole/electron mobility ratios of SubPcPDI3-Cl are nearly closer to 1 (Table S2). The balanced charge transport is helpful to obtain higher JSC and FF for SubPcPDI3-Cl-based devices. The dependence of JSC and VOC on the light intensity (Plight) were also employed to understand the charge recombination in devices (Figure S7, S8). The results indicate that SubPcPDI3-Cl-based device has less bimolecular recombination, reduced charge accumulation and balanced mobility between hole and electron,30 which promotes carrier extraction and charge transport and also leads to a better device performance. The morphologies of PBDBT-2F:SubPcPDI3-Cl and PBDBT-2F: SubPcPDI3-OC6H4tBu were measured by atomic force microscopy (AFM) and grazing-incidence wide-angle X-ray scattering (GIWAX) in Figure 4. The AFM images of two blend films are very similar, exhibiting smooth surfaces, with root-mean-square roughness (RMS) of 1.58 nm for SubPcPDI3-Cl and 1.48 nm for SubPcPDI3OC6H4tBu, which are beneficial to the charge separation and
transport in devices. In both blends, distinct π-π stacking peaks (010) in the out-of-plane direction (Figure S9) were seen at qz = 1.67 Å-1 and 1.69 Å-1 for PBDBT-2F:SubPcPDI3Cl and PBDBT-2F:SubPcPDI3-OC6H4tBu blends, respectively. Meanwhile, lamellar diffraction peaks (100) were also observed in the in-plane direction for the two blend films. The peak shape of PBDBT-2F:SubPcPDI3-Cl blend is obvious sharped compared the that of PBDBT2F:SubPcPDI3-OC6H4tBu. The result indicates that blend with SubPcPDI3-Cl preferred face-on orientation, which is beneficial to vertical charge transport and promoting PCE values.
Figure 4. AFM images of optimized (a) PBDBT2F:SubPcPDI3-Cl blend film and (b) PBDBT-2F:SubPcPDI3OC6H4tBu blend film; GIWAX patterns of (c) PBDBT2F:SubPcPDI3-Cl blend film and (d) PBDBT-2F:SubPcPDI3OC6H4tBu blend film.
To summarize, we designed and synthesized two fully conjugated rylene annulated subpthalocyanines (SubPcPDI3-Cl and SubPcPDI3-OC6H4tBu) bearing different axial substituents as non-fullerene acceptors for the use of OSCs. Theoretical calculation revealed a coneshaped and fully conjugated molecular geometry and the dihedral angle between PDI wings and cone ground is about 25o. Both acceptors showed red-shifted absorbance about 680 nm and optical bandgaps around 1.70 eV due to the πelectron conjugation of between the SubPc core and surrounded PDI wings. The fully conjugated structure contributed to enhancing the photovoltaic performance achieving a PCE of 7.53% for PBDBT-2F:SubPcPDI3-Cl device, which presents the highest PCE values of solutionprocessed BHJ OSCs based on SubPcs. The enhanced photovoltaic characteristics indicate that SubPc is emerging
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ACS Materials Letters as high-performance acceptor in OSCs, especially with proper molecular engineering design.
ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Experimental details, synthesis and characterizations, TGA curves, UV absorption, CV curves, theoretical calculations, device details and characterizations for all new compounds (PDF)
AUTHOR INFORMATION Corresponding Author * E-mail:
[email protected] * E-mail:
[email protected] * E-mail:
[email protected] Author Contributions All authors have given approval to the final version of the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (NSFC) (No. 21790361, 21734009 and 51673202), the National Key R&D Program of China (2017YFA0204701), and the Youth Innovation Promotion Association of Chinese Academy of Sciences (No. 2017048). We also thank Dr. Joseph Strzalka and Dr. Zhang Jiang for the assistance with GIWAXS measurements. Use of the Advanced Photon Source (APS) at the Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract no. DE-AC0206CH11357.
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