Congratulations to Linqing Qin for her article published in Angewandte Chemie International Edition!

Organic solar cells (OSCs) have attracted extensive attention in recent years due to their flexibility, low cost, light weight, and solution processability. Since the development of the twentieth century, the power conversion efficiency (PCE) has been greatly improved, and it has now reached more than 17%. The improvement of PCE is the most important indicator in solar cells, and an effective means to improve the efficiency is to increase the lifetime and diffusion distance of excitons by constructing triplet materials.

Different from the previous large spin-orbit coupling value obtained by introducing heavy atoms, the team of Professor Huang Hui from the University of Chinese Academy of Sciences combined the twisted D-A structure and the large π-conjugated structure to obtain a triplet acceptor material, which was used in In organic solar cells, energy conversion efficiencies of over 15% have been obtained, providing a new way to design high-efficiency organic solar materials.

The team combined A-D-A'-D-A and twisted structure to synthesize two compounds H1 and H2 as shown in Figure 1, combined with Y6 material, after basic physicochemical properties such as HOMO/LOMO energy level and solution film absorption characterization , focusing on its triplet properties.

As shown in Fig. 2, the transient luminescence lifetimes of three materials H1, H2 and Y6 were investigated. It is found that the lifetimes in 2-methylfuran at 298 K are only 11.67ns, 13.36ns and 10.41ns, while at 77K they reach 6.07μs, 8.15μs and 7.66μs, respectively, indicating that a large number of triplet states are generated. . At the same time, the transient absorption in the solution of the three materials was also tested, and it was found that the photoabsorption lifetime at 550nm can reach 42μs, 55μs and 41μs, respectively, which proves the existence of a large number of triplet states. Experiments on the magneto-optical currents in the pure films of the three materials show that the magneto-optical currents decrease with the increase of the magnetic field, indicating that there is a triplet-exciton quenching reaction. Paramagnetic signals appeared in the electron resonance experiments on pure powders of the three materials, confirming the existence of triplet states. These evidences strongly suggest that the three acceptor materials constructed from twisted D-A structures are triplet materials

Figure 2. a) Fluorescence lifetime of H1, H2 and Y6 at 298K; b) Fluorescence lifetime of H1, H2 and Y6 at 77K; c) Transient absorption spectrum of H1 solution; d) Decay lifetime at 550 nm; e) Magnetophotocurrent curves of H1, H2 and Y6; f) EPR spectra of H1 under dark and light conditions.

  Then the three materials were used to prepare devices, and the highest efficiency of H2 was 15%, which was comparable to the star material Y6. Then the role of triplet state in the mixed membrane is discussed. In the magneto-optical current experiment, it is found that the H2 device has the largest change in magneto-optical current, indicating that the CT state exciton dissociation efficiency is the highest, which is also consistent with the phenomenon that the H2 device has the highest current in the J-V curve. PIA signals appeared at about 640 and 850 nm in the transient absorption spectrum of the H1 mixed film, indicating the phenomenon of CT states in the D-A mixed film. The decay lifetime of the strongest signal at 770nm is fitted, which can reach 65ns, which can be considered as the triplet excitons of 3CT. At the same time, the CT state energy in the H1 device was measured to be 1.35 eV with the help of EQE and EL curves, which was found to be close to the T1 state energy, so it can be considered that the close T1 state and CT state energy can promote the transition from T1 to CT state.

Figure 3. a) J-V curves of H1, H2 and Y6 devices; b) EQE curves of H1, H2 and Y6 devices; c) magneto-photocurrent curves of H1, H2 and Y6 devices; d) PBDB-T:H1 hybrid film The transient absorption spectrum of ; e) the decay curve at 770 nm in the transient absorption spectrum of the PBDB-T:H1 hybrid film; f) the EQE and EL curves of the H1 device

  In conclusion, the authors synthesized acceptor materials H1 and H2 with D-A twist and large π-conjugated conjugation, together with Y6 combined with transient fluorescence, transient absorption, TD-DFT, magneto-optical current, electron paramagnetic resonance and other means The intrinsic characterization of its triplet properties was carried out, and it was proved that it is a triplet material and can generate a large number of triplet states. At the same time, the device was prepared, and while achieving high efficiency (>15%), the role of its triplet state in the mixed film was discussed, and it was found that triplet excitons could be generated and dissociated, which promoted the efficiency of the promote. This work provides a new idea for designing high-efficiency triplet solar cell acceptor materials in the future.

This achievement was recently published in Angewandte Chemie. Master students Qin Linqing, Liu Xingzheng, and postdoctoral Zhang Xin of the University of Chinese Academy of Sciences are the co-first authors of the article, and Professor Gao Feng, Yi Yuanping and Huang Hui are the corresponding authors.

DOI: 10.1002/anie.202006081 and 10.1002/ange.202006081