Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
mA/cm2, Voc of 0.85 V, fill factor (FF) of 56.4%. These studies indicate that proper incorporation of fluorine atoms is an
effective way to increase the efficiency of organic solar cells.
Keywords:
A-D-A small molecule
Benzothiadiazole
Fluorination
Organic solar cells
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 Voc 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
Jsc of 10.56 mA/cm2, a Voc of 0.85V 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. 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 °C, 411 °C and 418 °C, 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 to 450 nm
originate from the π-π* transition of the benzothiadiazole core. The second region ranges from 450-600 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 (Egopts) of SBDT1, SBDT2 and SBDT3
were determined to be 1.62, 1.69 and 1.74 respectively, based on the onsets (λonset) 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 Eoxonset. The EHOMO and ELUMO 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
He, Anwang
