Chinese Chemical Letters  2018, Vol. 29 Issue (9): 1343-1346   PDF    
A homopolymer based on double B ←N bridged bipyridine as electron acceptor for all-polymer solar cells
Xiaojing Long, Chuandong Dou, Jun Liu, Lixiang Wang    
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China
Abstract: Polymer electron acceptors for all-polymer solar cells (all-PSCs) are usually conjugated copolymers, which contain alternating electron-rich units and electron-deficient units. In this manuscript, we report a conjugated homopolymer (P-BNBP) based on an electron-deficient unit of double B← N bridged bipyridine, which can be used as electron acceptor for all-polymer solar cells. P-BNBP shows low-lying LUMO energy level of -3.59 eV, high absorption coefficient of 1.6×105 Lmol-1 cm-1 at 626 nm and moderate electron mobility of 4.37×10-6 cm2 V-1 s-1. All-PSC devices exhibit power conversion efficiencies of 2.44%-3.04%. These results demonstrate that conjugated homopolymers are promising as electron acceptor materials for all-PSCs.
Keywords: All-polymer solar cells     Homopolymer     Electron acceptor     B-N coordination bond     Yamamoto polymerization    

All-polymer solar cells (all-PSCs), utilizing conjugated polymers as both electron donor and electron acceptor in the active layer, have recently received great attention. In comparison to conventional polymer solar cells based on polymer/fullerene, all-PSCs exhibit many advantages, including improved light absorption and tunable energy levels of polymer electron acceptors, as well as superior thermal and mechanical stability of the blends [1-5]. In the past five years, the device performance of all-PSCs has been rapidly improved [6-11], and a champion power conversion efficiency (PCE) of 10.1% was recently reported by Huang and coworkers [12]. The rapid development of all-PSCs is mainly attributed to the exploration of new polymer donors, as well as optimizations on processing conditions and polymer/polymer blend morphologies [13-16]. In view of materials for all-PSCs, there are much less polymer electron acceptors than polymer electron donors. Polymer acceptors are usually based on D-A type conjugated copolymers, which are composed of alternating electron-rich (D) units and electron-deficient (A) units. Moreover, these copolymers are mainly using the strong electron-deficient naphthalene diimide (NDI) or perylene diimide (PDI) unit as the A unit [17-23]. As reported by Reynolds and co-workers, a homopolymer of isoindigo unit (P-IID) could be used as polymer electron acceptor [24]. However, the PCE of the resulting all-PSC device is only 0.5%. Therefore, it remains questionable whether homopolymers can be used as polymer acceptors for efficient allPSCs.

We have recently reported using boron-nitrogen coordination bond (B←N) to develop n-type conjugated polymers [25, 26] and n-type acenes [27]. We have designed a new kind of electrondeficient building block based on B←N unit, double B←N bridged bipyridine (BNBP) [28-30]. A family of n-type conjugated polymers have been developed by copolymerizing BNBP with some electronrich units. These BNBP-based copolymers have been successfully used as electron acceptors for all-PSCs and as electron-transporting semiconductors for organic field-effect transistors [31-35].

In this manuscript, we report a BNBP-based homopolymer (PBNBP) and its application as electron acceptor in all-PSCs. The homopolymer shows low-lying LUMO energy level (ELUMO) of -3.59 eV, high absorption coefficient of 1.6×105 L mol-1 cm-1 and moderate electron mobility of 4.37 ×10-6 cm2 V-1 s-1. We selected three widely-used polymer donors, poly[(ethylhexyl-thiophenyl)-benzodithiophene-(ethylhexyl)-thienothiophene] (PTB7-Th), poly [(2, 6-(4, 8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo)[1, 2-b:4, 5-b0] dithiophene)-co-(1, 3-di(5-thiophene-2-yl)-5, 7-bis(2-ethylhexyl) benzo[1, 2-c:4, 5-c0]dithiophene-4, 8-dione)] (PBDB-T) and 2D-conjugated benzodithiophenealt-fluorobenzotriazole copolymer (J61) [36-38] to blend with P-BNBP to fabricate all-PSCs. The resulting all-PSC devices exhibit PCEs of 2.44%-3.04%, which are higher than that of all-PSCs with homopolymer electron acceptors [24].

Scheme 1 shows the synthetic route of homopolymer P-BNBP. The starting material 1 was prepared according to our previous reference [39]. P-BNBP was synthesized by Yamamoto polycondensation of 1 with bis(1, 5-cyclooctadiene)nickel(0) as the catalyst. 1 has high reactivity in this polymerization condition and the reaction was finished within 20 min. The chemical structure of P-BNBP was verified by 1H NMR and elemental analysis. According to gel permeation chromatography (GPC) measurement, the number-average molecular weight (Mn) and the polydispersity index (PDI) are 248.0 kDa and 1.30, respectively. According to thermogravimetric analysis (TGA), P-BNBP shows good thermal stability with thermal decomposition temperature (Td) at 5% weight loss of 385 ℃ (Fig. S1 in Supporting information). P-BNBP can be soluble in chloroform and o-dichlorobenzene (o-DCB), indicating its potential for solution-processed organic electronic devices.

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Scheme 1. Synthesis of homopolymer P-BNBP.

Density functional theory (DFT) calculations at the B3LYP/6- 31G(d, p) level of theory were performed to investigate the configuration and electronic structure of P-BNBP. The model compound containing four BNBP units with long alkyl chains replaced by methyl groups was used (Fig. S2 in Supporting information). Its optimized structure exhibits a straight configuration with a twisted angle of 35° between the adjacent BNBP units. While the HOMO of the model compound is localized on the two terminal BNBP units, its LUMO is well delocalized over the conjugated backbone. This delocalized LUMO has been observed in BNBP-containing conjugated copolymers [26, 40, 41]. As shown in Fig. S2, from BNBP monomer to this model compound, the calculated ELUMO is decreased greatly by 0.71 eV and the EHOMO is decreased slightly by 0.18 eV. This is consistent with the more delocalization of its LUMO than its HOMO.

As shown in Fig. 1a, P-BNBP in o-DCB solution at 30 ℃ and 90 ℃ exhibits the similar UV/vis absorption spectra with a main peak at ca. 626 nm (Table 1), which are comparable to that of P-BNBP in thin film (Fig. S4 in Supporting information). It suggests that PBNBP has very strong tendency to aggregate in solution, which is probably due to its straight conjugated backbone. P-BNBP has the obviously narrow absorption spectrum compared with other conjugated polymers [15], such as P-IID homopolymer that exhibits the absorption band between 400-730 nm [24]. Moreover, the absorption spectrum of the solution at 30 ℃ exhibits a high absorption coefficient of 1.6× 105 L mol-1 cm-1. The temperaturedependent fluorescence spectra of P-BNBP were measured. The solution at 90 ℃ exhibits the fluorescence peak at 631 nm, which is gradually red-shifted with the decrease of the temperature (Fig. S4). These changes confirm the strong aggregation of P-BNBP in solution.

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Fig. 1. a) UV/vis absorption spectra of P-BNBP in o-DCB solution and in thin film; b) Cyclic voltammogram of P-BNBP in thin film. Fc = ferrocene.

Table 1
The photophysical and electrochemical properties, as well as π-π stacking distance and electron mobility of P-BNBP.

Cyclic voltammetry (CV) was employed to estimate the LUMO/HOMO energy levels of P-BNBP. As shown in Fig. 1b, P-BNBP shows irreversible reduction and oxidationprocesses with the onset potentials of Eonsetred =-1.21 V and Eonsetox = +0.94 V, versus Fc/Fc+. Accordingly, its ELUMO/EHOMO values are estimated to be -3.59 eV/-5.74 eV, respectively. The low-lying LUMO/HOMO energy levels indicate that P-BNBP can be used as electron acceptor.

Grazing-incidence X-ray diffraction (GI-XRD) and in-plane Xray diffraction (IP-XRD) measurements were performed on the drop-cast film of P-BNBP (see Supporting Information). The polymer shows a (100) diffraction peak of 2θ = 5.79°, corresponding to the lamellar packing distance of 15.26 Å. A broad (010) diffraction at 2θ = 20.34° was observed, corresponding to the π-π stacking distance of 4.36 Å. The electron mobility (μe) of P-BNBP was estimated to be 4.37 × 10-6 cm2 V-1 s-1 by using the spacecharge-limited current (SCLC) method with the current density/voltage curve of the electron-only device [42]. This moderate electron mobility is ascribed to its large π-π stacking distance (Table 1).

The low-lying LUMO/HOMO levels and moderate electron mobility of P-BNBP prompted us to use it as electron acceptor for all-PSCs. We selected three widely-used polymer electron donors, PTB7-Th, PBDB-T and J61 (Fig. 2a). All of these polymer donors have higher LUMO/HOMO levels than that of P-BNBP, indicating that the driving force is enough for the photo-induced charge separation in these three systems (Fig. 2b).

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Fig. 2. (a) Chemical structures of three polymer donors and (b) the energy level alignments of polymer donors and P-BNBP.

The all-PSC device configuration is ITO/PEDOT:PSS/PTB7-Th or PBDB-T or J61:P-BNBP/Ca/Al. The active layers are spin-coated from their CHCl3 solutions. Fig. 3 shows the J-V curves under AM 1.5G illumination (100 mW/cm2) and external quantum efficiency (EQE) spectra of the devices. The device data are summarized in Table 2. The device based on the J61:P-BNBP blend exhibits a PCE of 3.04% accompanying with an open-circuit voltage (Voc) of 1.01 V, a shortcircuit current (Jsc) of 6.42 mA/cm2 and a fill factor (FF) of 47.3%. Moreover, the devices exhibit the PCE of 2.44% for the PTB7-Th:PBNBP blend and 2.63% for the PBDB-T:P-BNBP blend, These PCE values are higher than that of all-PSCs with homopolymer electron acceptors [24]. However, they are still lower than that of polymer solar cells with small molecules or copolymers or fullerene-based derivatives as electron acceptors [11, 12, 43-45]. In addition, the Jsc calculated from the integration of EQE spectrum and AM1.5G spectrum agrees well with the Jsc value obtained from the J-V scan within 5% error.

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Fig. 3. a) JV curves and b) EQE spectra of the all-PSC devices based on the PTB7-Th: P-BNBP, PBDB-T:P-BNBP and J61:P-BNBP blends, respectively.

Table 2
Summary of all-PSC device performance.

The electron/hole mobilities of the blend films were measured by SCLC method with the electron-only and hole-only devices (Fig. S9). While the electron mobilities of the PTB7-Th:P-BNBP, PBDB-T:P-BNBP and J61:P-BNBP blends are estimated to be 4.99 ×10-6 cm2 V-1 s-1, 9.09×10-6 cm2 V-1 s-1, and 2.19 × 10-5 cm2 V-1 s-1, their hole mobilities are 3.96 ×10-5 cm2 V-1 s-1, 4.45 ×10-5 cm2 V-1 s-1, and 4.31 ×10-5 cm2 V-1 s-1, respectively. The J61:P-BNBP blend film exhibits the high and balanced electron/hole mobilities. The charge recombinations in the all-PSC devices were investigated by measuring the dependence of J-V curves on the light intensity. As reported, the Jsc follows a power-law dependence on the illumination intensity (JscPlightα, where Plight is light intensity and α is the calculated power-law exponent) [46]. As shown in Fig. S10 in Supporting information, the α value is 0.92 for the PTB7-Th:P-BNBP device, 0.93 for the PBDB-T:P-BNBP device and 0.94 for the J61:P-BNBP device. These α values are very close to unity, indicating that bimolecular charge recombinations in the allPSC devices are weak at short circuit condition. Both the suppressed charge recombination and the high and balanced electron/hole mobilities of the J61:P-BNBP device are consistent with its best device performance.

In summary, we synthesized a homopolymer based on BNBP unit. This homopolymer possesses low-lying LUMO/HOMO energy levels, high absorption coefficient and moderate electron mobility, which are very desirable for electron acceptors. The all-PSC devices based on P-BNBP exhibit the PCEs of 2.44%-3.04%. These results demonstrate that conjugated homopolymers are promising as electron acceptor materials for all-PSCs.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Nos. 21625403, 21574129), Strategic Priority Research Program of Chinese Academy of Sciences (No. XDB12010200), Jilin Scientific and Technological Development Program (No. 20170519003JH), Youth Innovation Promotion Association of Chinese Academy of Sciences (No. 2017265) and Open Project (No. sklssm201803) of the State Key Laboratory of Supramolecular Structure and Materials in Jilin University of China.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.cclet.2018.01.052.

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