Chinese Chemical Letters  2018, Vol. 29 Issue (2): 325-327   PDF    
Ethynyl-linked perylene bisimide based electron acceptors for non-fullerene organic solar cells
Changshi Yua,b, Yunhua Xua, Shijie Lianga,b, Xudong Jianga,b, Guitao Fenga,b, Cheng Lib, Weiwei Lib    
a Department of Chemistry, School of Science, Beijing Jiaotong University, Beijing 100044, China;
b Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
Abstract: In this work, star-shaped perylene bisimide (PBI) derivatives with spiro-aromatic cores linked with ethynyl units were developed as electron acceptors for non-fullerene organic solar cells. The ethynyl linkers were found to enhance the planarity of the conjugated backbone, resulting in high electron mobilities and near-infrared absorption. The ethynyl-linked PBI acceptors showed high power conversion efficiencies (PCEs) up to 4.27% due to the high short-circuit current density (Jsc) of 8.52 mA/cm2 and fill factor (FF) of 0.59, while the PBI acceptor without ethynyl units provided a low PCE of 3.57% in nonfullerene solar cells. The results demonstrate that ethynyl units can be applied into designing new PBI electron acceptors with improved charge transport properties and photovoltaic performance.
Key words: Electron acceptor     Non-fullerene organic solar cells     Perylene bisimide     Charge transport     Ethynyl    

Large π-conjugated perylene bisimide (PBI)-based materials with electron transporting properties have been widely applied into organic electronics, such as high performance organic fieldeffect transistors [1] and non-fullerene organic solar cells (NFOSCs) [2-9]. The strong electron-withdrawing ability of PBI units enables the low-lying lowest unoccupied molecular orbital (LUMO) levels of PBI derivatives, which can be potentially used as electron acceptor for NFOSCs. However, single PBI compound has the strong self-aggregation tendency in thin films [10]. This will cause largedomain phase separation when blended with donor materials, which is detrimental to the exciton diffusion in organic solar cells [11]. In order to reduce the crystallinity, multi-dimensional PBIbased electron acceptors with twisted backbone were developed, exhibiting high power conversion efficiencies (PCEs) in NFOSCs due to the optimized micro-phase separation [12-16]. However, in this kind of designation, PBI compounds show poor charge transport properties, causing unbalanced hole and electron transport and enhanced charge recombination [17, 18]. Therefore, it is important to design PBI molecules with the high carrier mobility and meanwhile reduced aggregation tendency in order to simultaneously improve the efficiency of exciton diffusion and charge transport in NFOSCs.

In our previous work, we initiate a strategy to design PBI molecules, in which fused and twisted backbone was used to maintain the mobility and crystallinity [19]. The method was also reported in other groups, in which the PCEs above 9% have been realized [13, 16]. Herein, a new designation by incorporating ethynyl units into PBI electron acceptors was reported for achieving high mobility and good micro-phase separation in organic solar cells. The structures start from a star-shaped PBIbased electron acceptor PBI1 (Scheme 1), in which PBI units are connected with spiro-fluorene group via single bond [20]. PBI1 has a three-dimensional structure so as to prevent the aggregation of PBI units, but the charge transport of PBI1 is reduced. By using ethynyl linkers to replace single bond in the molecule PBI2 (Scheme 1), the molecular planarity can be improved. Meanwhile, the spiro-structure is helpful to prevent the self-aggregation of PBI2. We further use thiophene to improve the planarity of the conjugated backbone, as PBI3 (Scheme 1). The detailed physical and photovoltaic properties based on these PBI molecules as electron acceptor will be present, and the micro-phase separation will be investigated. The results show that ethynyl-linked PBI molecules can be potentially applied in high performance NFOSCs.

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Scheme 1. The chemical structures of (a–c) PBI-based electron acceptors and (d) the donor polymer PBDB-T.

The synthetic procedures of the PBI molecules were present in Scheme 2. Starting from tetrabromo-spiro compound 1 and 3, the tetra-ethynyl precursors 2 and 4 can be obtained via Sonagashira coupling reaction, which was used to react with Br-PBI to yield the acceptors PBI2 and PBI3. The molecule PBI1 was synthesized according to the literature procedures [20]. All the three PBI acceptors perform good solubility in CHCl3 and chlorobenzene (CB). The chemical structures are confirmed by NMR and MALDI spectra, as shown in the Supporting information.

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Scheme 2. (a) Synthetic procedures of the acceptor PBI2 and PBI3. (ⅰ) PdCl2(PPh3)2, PPh3, CuI, i-Pr2NH at 85 ℃, 12h. (ⅱ) CH2Cl2, NaOH, r.t., 2h. (ⅲ) Br-PBI, Pd(PPh3)4, CuI, Toluene/Et3N (2:1, v/v) at 80 ℃, 24h. (b) The chemical structure of the compound Br-PBI.

The optical absorption spectra of the PBI molecules in CHCl3 solution and thin films were shown in Fig. 1. PBI1 and PBI2 have similar absorption in solution with optical band gap (Eg) of 2.08eV and 2.05eV (Table 1), and PBI2 has a strong aggregation peak around 560nm. This indicates that PBI2 desires planar backbone compared to PBI1. The acceptor PBI3 shows a near-infrared absorption spectra with Eg of 1.73eV, in which two absorption regions of 300–600nm and 600–750nm can be observed. The region at 600–750nm can be attributed to the intramolecular interaction between PBI and spiro-thiophene central core. PBI1 and PBI3 show similar absorption spectra in thin films compared to that in solution, but PBI2 has different absorption spectra. The strong peak around 570nm is disappeared, indicating the possible H-aggregation in PBI2 thin film.

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Fig. 1. Absorption spectra of the PBI acceptors (a) in CHCl3 solution and (b) in thin films.

Table 1
Optical and electrochemical properties of the PBI based acceptors.

The frontier energy levels of the PBI acceptors were determined by cyclic voltammetry measurement, as shown in Fig. S1 in Supporting information and the data was summarized at Table 1. PBI2 has low-lying HOMO and LUMO levels compared to PBI1, which is similar to other ethynyl-based conjugated materials [21]. PBI3 shows similar LUMO level (-3.84eV) with PBI2, but HOMO level shifts to -5.55eV compared to that of PBI2 (-5.89eV). This HOMO variation is due to the electron-donating ability of thiophene units.

We then use density functional theory (DFT) calculations to study the molecular configuration of these PBI compounds, as shown in Fig. 2. Methyl units were used to replace the branched alkyl chains for simplicity. For PBI1, the dihedral angle between PBI and spiro core is 53.4°, while it is significantly reduced to 11.9° and 5.5° for PBI2 and PBI3. Therefore, two PBIs and biphenyl/thienyl units in the center are in one plane for PBI2 and PBI3, resulting in improved planarity. Together with the spiro-core, PBI2 and PBI3 perform two-dimensional structures, while PBI1 shows threedimensional configuration.

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Fig. 2. DFT calculations of the PBI-based polymer segments. (a) PBI1, (b) PBI2 and (c) PBI3.

The charge transport properties of the PBI acceptors were investigated byorganic field-effect transistorswith a bottom – gate bottom – contact configuration, as shown in Fig. S2, Table S1 in Supporting information and Table 2. PBI1 shows an electron mobility of 2.2 × 10-3 cm2 V-1 s-1, which enhances to 1.5 × 10-2 cm2 V-1 s-1 and 1.0 × 10-2 cm2 V-1 s-1 for PBI2 and PBI3. The improved charge transports agree well with the improved planarity of the PBI molecules.

Table 2
Field-effect electron mobilities of the PBI based acceptors in a BGBC configuration.

We then use the three PBI acceptors for application in nonfullerene electron acceptors by using a wide band gap polymer PBDB-T [22] as electron donor (Scheme 1). The device uses inverted configuration with ITO/ZnO and MoO3/Ag as electrode, and the sandwiched photoactive layers were prepared via solution-process. We provided detailed optimization, including the solvent, ratio of donor to acceptor and thickness of active layers. The optimized photovoltaic properties were summarized at Table 3 and their J-V characteristics were shown in Fig. 3a.

Table 3
Solar cell parameters of optimized solar cells of PBDB-T:PBI-based acceptors (1:1).

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Fig. 3. (a) J-V characteristics in the dark (dashed lines) and under white light illumination (solid lines). (b) EQE of the optimized solar cells of PBDB-T:PBI-based acceptors. Thin films were fabricated from CB/DIO (0.5%) with the thickness around 80 nm.

PBDB-T:PBI1 based solar cells show a PCE of 3.57% with a shortcircuit current density (Jsc) of 7.62 mA/cm2, Voc of 0.90 V and FF of 0.52. The PCEs were enhanced to 4.27% with an increasing Jsc of 8.52 mA/cm2 and FF of 0.59% based on PBI1 as electron acceptor. However, PBDB-T:PBI3 solar cells provided a reduced PCE of 2.73% due to the low Jsc of 6.73 mA/cm2. Vocs of PBI2 and PBI3 based cells were slightly low compared to that of PBI1 based cells, which was consistent with their low-lying LUMO levels. The different Jscs were also reflected by their external quantum efficiencies (EQEs), as shown in Fig. 3b. PBI2-based cells show high EQE spectra compared to PBI1-based cells, while EQE of PBI1-cells was reduced. It is worthy mentioned that although PBI3 shows near-infrared absorption in the region of 650–750 nm, the EQE in this region is relatively low. This is possibly due to the weak absorption coefficiency of PBI3 in this region.

We also use atom force microscopy (AFM) to study the morphology of blended thin films, as shown in Fig. 4. Finely phase separation of donor and acceptor can be observed in PBI1 and PBI2 based cells, while in PBDB-T:PBI3 blend thin films we can observe large phase separation with large domain. This may explain the relatively low photocurrent in PBI3-based cells.

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Fig. 4. AFM height image (3 × 3 μm2) of the photoactive layers. The fabrication condition is referred to Table 3. The root mean square (RMS) roughness is also included.

In conclusion, we developed two PBI electron acceptors by incorporating ethynyl linkers into the molecules in order to improve the planarity of the acceptors. The new acceptors show better charge transport properties and hence higher PCEs in nonfullerene solar cells. The results demonstrate that the design by using ethynyl linkers in PBI-based electron acceptors is an efficient route to enhance the charge transport properties, which can be potentially used for high performance non-fullerene solar cells.

Acknowledgments

This work was supported by the Recruitment Program of Global Youth Experts of China. The work was further supported by the National Natural Science Foundation of China (Nos. 21574138, 51603209 and 91633301) and the Strategic Priority Research Program (No. XDB12030200) of the Chinese Academy of Sciences.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cclet.2017.08.016.

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