9:15 AM - ES18.03.04
Designing Highly Efficient Non-Fullerene Acceptors via Tuning the Intramolecular Charge Transfer Effect
Huifeng Yao1,Jianhui Hou1
Institute of Chemistry, Chinese Academy of Sciences1
Over the last few years, the development of fullerene-free organic photovoltaic (OPV) cells have achieved great attention, and the power conversion efficiencies (PCEs) of small molecular acceptor-based OPV devices have surpassed 14% recently. The non-fullerene small molecules have the advantage of easily tuned absorption spectra and molecular energy levels, contributing greatly to the rapid increasing PCEs of the resulting OPV devices. Among the varied kinds of non-fullerene acceptors, the ones with acceptor-donor-acceptor structures like ITIC show the outstanding performance. In our work, we tuned the intramolecular charge transfer (ICT) effect in this kind of molecules and designed a series of highly efficient materials.
First, we weakened the ICT effect of ITIC via replacing its benzene terminal groups with thiophene units to synthesize the ITCC, then we further added methyl groups on thiophene rings to decrease the ICT effect to prepare the ITCC-m. As thiophene rings and methyl groups have relatively electron-donating properties than benzene rings, the ICT effect in ITIC is weakened gradually in ITCC and ITCC-m. When compared to ITIC, the ITCC and ITCC-m show upshifted the lowest unoccupied molecular orbital (LUMO) levels and blue-shifted absorption spectra. In the OPV cells fabricated with a polymer PBDB-T as the donor material, the ITCC- and ITCC-m yielded much higher open-circuit voltages. Then, we adopted the contrary molecular design strategy to enhance the ICT effect in the small molecular acceptors. Taking IEIC as an example, we introducing the electron-donating alkoxy groups and electron-withdrawing fluorine atoms on the donor and acceptor parts of IEIC, respectively, to synthesize the IEICO and IEICO-4F.[2-3] Both of the acceptors show much-redshifted absorption spectra and thus obtained considerable enhancement of short-circuit current densities in the resulting OPV cells. Recently, we used the chlorination to replace the fluorination to further enhance the ICT effect and synthesized the IEICO-4Cl, which has very weak absorption in the visible range. When blending different donors, the colors of the blend films were changed accordingly. We selected three polymer donor with varied colors and fabricated the semi-transparent OPV devices, which showed very good photovoltaic performance. We also integrating the advantages of the wide bandgap and narrow bandgap acceptors to fabricate the double-junction OPV cells. For example, we used the ITCC-m and IEICO to fabricate the front and rear devices in the tandem cells, respectively, a high PCE of 13.8% was recorded. After that, we replaced the IEICO with IEICO-4F with broader absorption spectrum, the PCE approached to 15%.
In our recent work, in order to understand the efficient charge transfer mechanism in these non-fullerene-systems, we studied the distributions of the molecular electrostatic potential (ESP) on these molecules. As there is much difference in electronegative on the different part of the non-fullerene molecules, the theoretical calculations suggest that most parts of this kind of acceptor have positive ESP values. By contrast, the polymer donors usually have negative ESP on the backbones. Therefore, we think the ESP difference will induce an intermolecular filed to assist the charge transfer between the donors and acceptors in the active layers.
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