Citation

Anwang He, Yuancheng Qin, Weili Dai, Dan Zhou, Jianping Zou. Effective design of A-D-A small molecules for high performance organic solar cells via F atom substitution and thiophene bridge[J]. Chin. Chem. Lett., 2019, 30(12): 2263-2265  

Anwang He, Yuancheng Qin^*, Weili Dai, Dan Zhou, Jianping Zou     

In the past few decades, organic photovoltaic cells (OPVs) have driven an intense research effort to acquire new materials that increase power conversion efficiency and ultimately reach values comparable to inorganic silicon solar cells [1-6]. Although the power conversion efficiency of OPVs has not reached the ideal value yet and material stability are still a challenge, photovoltaic devices have powerful advantages, such as simple preparation, light weight, higher flexibility and low cost manufacturing [7-10].

In OPVs, the design of donor-acceptor (D-A) structure is a common method to construct narrow band system [11-17]. As one of the D-A structures, there are two acceptor units in the A-D-A structure, which facilitates the charge transfer force in the molecule and enhances the ability of the material to adjust the visible light absorption range. Meanwhile, the introduction of fluorine atoms into D-A type donor materials has attracted increasing attention due to its excellent properties [18-24]. For example, Hou et al. designed and synthesized a series of D-A polymers. All of these fluorinated polymers showed lower LUMO and HOMO levels than the non-fluorinated polymer, so OPV devices based on these polymers showed higher V_oc values [25]. Liu et al. designed and synthesized a series of polymers to adjust the planarity of the molecule by introducing fluorine atoms [26].

In this work, we design and synthesize three novel A-D-A materials based on benzothiadiazole for organic small molecule solar cell. These small molecules consist of donor 4, 8-bis(octyloxy) benzo[1, 2-b:4, 5-b']dithiophene (BDT) unit and benzothiadiazole (BT) acceptor units substituted with different fluorine atoms. BDT and BT unit have a larger conjugate plane, higher carrier mobility and easy modification of the structure, and both of them are excellent materials for constructing A-D-A structure. The introduction of a fluorine atom can further adjust the energy level of the molecule. As expected, these three small molecules displayed fine thermal stability, excellent absorption, as well as flat surfaces with PC₇₁BM. Among them, SBDT2 and SBDT3 with fluorine-substituted BT possess a relatively deeper the highest occupied molecular orbital (HOMO). Meanwhile, an encouraging PCE value up to 5.06% with a J_sc of 10.56 mA/cm², a V_oc of 0.85 V and a FF of 56.4% was obtained for the device based SBDT2/PC₇₁BM.

All the starting materials were obtained from Meryer Chemical Technology Co., Ltd. The synthetic routes of small molecules were shown in Scheme 1, and the detailed synthetic procedures in the Supporting information. Suzuki cross-coupling reaction was employed for the synthesis of SBDT1-3.

Scheme 1

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Scheme 1. The synthetic routes of SBDT1-3.

The TGA curves of all small molecules investigated are presented in Fig. 1a. The decomposition temperatures acquired from TGA curves, observed at 5% weightlessness, are 392 ℃, 411 ℃ and 418 ℃, respectively. These results confirm that all small molecules exhibit good thermal stability. As the fluorine atom increases, the thermal stability continues to increase. The reason is that the F element in fluorine-containing materials is highly electronegative and the polarity of the F—C bond is very strong, which makes it have some excellent properties. The normalized absorption spectra of small molecules (SBDT1-3) in solution or film are depicted in Figs. 1b and c. We can see that SBDT1-3 showed two distinct absorption bands both in chloroform. The bands range from 350 nm to 450 nm originate from the π-π^* transition of the benzothiadiazole core. The second region ranges from 450 nm to 600 nm originate from a mixture of π-π^* transitions and intramolecular charge transfer (ICT) transitions between BDT donor and the benzothiadiazole acceptor [27]. In film, SBDT1-3 have obvious red shift in absorption spectrum compared with solution. This can be interpreted by the stronger molecular aggregation and interchain interaction of SBDT1-3 in film. The optical bandgaps (E_g^opts) of SBDT1, SBDT2 and SBDT3 were determined to be 1.62, 1.69 and 1.74 respectively, based on the onsets (λ_onset) of the absorption spectra in film. The cyclic voltammetry (CV) curves were depicted in Fig. 1d. As shown in Fig. 1d, SBDT1-3 exhibited different initial oxidation peaks of 0.76, 0.87 and 1.01 V, respectively, indicating that benzothiadiazole acceptor unit substituted with different fluorine atoms have a significant effect on E_ox^onset. The E_HOMO and E_LUMO values are estimated to be -5.08 and -3.46 eV, -5.19 and -3.50 eV, and -5.33 and -3.59 eV for SBDT1, SBDT2 and SBDT3, respectively. The HOMO-LUMO energy diagrams of the polymers and PC₇₁BM were shown in Fig. 2b. It can be seen from the results that the more fluorine atoms are introduced, the HOMO level of small molecules become deeper.

Figure 1

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Fig. 1. (a) TGA thermograms of SBDT1-3, (b) Normalized absorption spectra of SBDT1-3 in chloroform solution and (c) in film, (d) cyclic voltammogram curve of the SBDT1-3.

Figure 2

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Fig. 2. (a) SMOSCs device, (b) energy levels of SBDTs, (c) J-V characteristic curve of SBDTs, (d) external quantum efficiency.

We prepared OPVs with an inverted configuration of glass/ indium tin oxide (ITO)/ZnO/SBDTs: PC₇₁BM/MoO₃/Ag (Fig. 2a). The weight ratio of electron SBDTs to PC₇₁BM was optimized to be 1:2. Fig. 2c shows the current density-voltage (J-V) characteristics of these devices using SBDTs and PC₇₁BM and the detailed device parameters are summarized in Table 1. It is found that device based on SBDT2 exhibits the highest PCE of 5.06% with a J_sc of 10.56 mA/cm², a V_oc of 0.85 V and a FF of 56.4% in the optimized conditions. It is obvious that device based on SBDT2 have higher PCE values than devices based on SBDT1 (3.99%) and SBDT3 (4.45%). This is due to the fact that device based on SBDT2 present the higher J_sc and FF values. Such results imply that proper introduction of F atoms can induce a good microscopic morphology. On the other hand, devices based on SBDT2 and SBDT3 show higher V_oc values than device based on SBDT1, which is consistent with their deeper lying HOMO levels. The external quantum efficiency (EQE) spectra of SBDTs were shown in Fig. 2d. SBDT2: PC₇₁BM device exhibits a maximum EQE value of 60% at 701 nm.

Table 1

Table 1 Photovoltaic properties of polymer solar cells based SBDTs:PC71BM (1:2) blended.

Table 1 Photovoltaic properties of polymer solar cells based SBDTs:PC71BM (1:2) blended.

The morphology of the blend of SBDTs/PC₇₁BM was measured by atomic force microscopy. As shown in Fig. 3, the AFM height images of SBDT1, SBDT2 and SBDT3 are a, b, and c, respectively. All small molecule films present very flat surfaces. The root mean squared (rms) roughness of the films of SBDT1, SBDT2 and SBDT3 mixed with PC₇₁BM was 0.855, 0.718 and 2.34 nm, respectively. The SBDT2 blend film with single fluorine atoms has relatively better flatness, indicating proper introduction of F atoms can modulate the morphology of the blended membrane and shape a good microstructure between the donor and PC₇₁BM.

Figure 3

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Fig. 3. AFM height images of the blend films of SBDTs/PC71BM.

In conclusion, we designed and synthesized three symmetrical A-D-A structured small molecules, SBDT1-3, consist of donor 4, 8-bis (octyloxy)benzo[1, 2-b:4, 5-b']dithiophene (BDT) unit and benzothiadiazole (BT) acceptor units substituted with different numbers of fluorine atoms. These small molecules show good thermal stability, excellent absorption and flat surfaces with PC₇₁BM.Among them, SBDT1 and SBDT2 with fluorine atoms exhibit deeper HOMO levels. Consequently, an encouraging PCE value upto 5.06% with a J_sc of 10.56 mA/cm², V_oc of 0.85 V and FF of 56.4% was obtained for the device based on SBDT2/PC₇₁BM. The result implies that proper incorporation of fluorine atoms is an effective way to increase the efficiency of organic solar cells.

The work was financially supported by the National Natural Science Foundation of China (Nos. 51663018, 51703091), Outstanding Youth Funds of Jiangxi Province (No. 20171BCB23056).

Supplementary material related to this article canbefound, inthe online version, at doi:https://doi.org/10.1016/j.cclet.2019.07.018.