Yongfang Li

Yongfang Li was born in 1948. He graduated from high school in 1966, but entered university (East China University of Science and Technology (ECUST)) more than 11 years later in February of 1978 due to the cultural revolution. He received his Master degree from ECUST in 1982 and his Ph. D. degree with the specialty of Physical Chemistry from Fudan University in 1986. Then he did his postdoctoral research with Prof. Renyuan Qian at Institute of Chemistry, Chinese Academy of Sciences (ICCAS) in the field of electrochemistry of conducting polymers from 1986 to 1988, and became a staff in 1988 and promoted to professor in 1993 in ICCAS. He did his visiting research in Institute for Molecular Science, Japan, from 1988 to 1991 and in University of California at Santa Barbara from 1997 to 1998. In 2012, he got the position of professor in Soochow University. He was elected as a member of Chinese Academy of Sciences in 2013.

His present research interests are photovoltaic materials and devices for polymer solar cells (PSCs), including the conjugated polymer and organic small molecule donor materials, fullerene or nonfullerene organic semiconductor acceptor materials and electrode buffer layer materials. His main achievements on PSCs include:

(1) Two-dimension-conjugated polymer donor materials. In 2004, Yongfang Li proposed the strategy to broaden the absorption and increase hole mobility of the conjugated polymers by attaching conjugated side chains. They synthesized a series of two-dimension (2D)-conjugated polythiophene (PTh) derivative with conjugated side chains which show broad absorption, higher hole mobility and down-shifted HOMO energy levels. A 2D-conjugated PTh derivative with thienylene-vinylene conjugated side chains demonstrated 38% higher power conversion efficiency (PCE) in the PSCs with the polymer as donor and PCBM as acceptor, in comparison with the devices with P3HT as donor. Later, in collaboration with Dr. Jianhui Hou's group, they extended the 2D-conjugation concept into the narrow bandgap copolymers based on benzodithiophene (BDT) donor unit by attaching thiophene conjugated side chains on the BDT unit, and synthesized a series of high performance bithienyl-BDT (BDTT)-based narrow bandgap polymers. The BDTT-based 2D-conjugated copolymers have been developed into a type of representative high performance polymer donor materials. Related 5 representative publications: (a) J. Am. Chem. Soc., 2006, 128, 4911-4916. (b) Adv. Mater., 2008, 20, 2952-2958. (c) Angew. Chem. Int. Ed., 2011, 50, 9697-9702. (d) Acc. Chem. Res. 2012, 45, 723–733. (e) Energy Environ. Sci. 2014, 7, 2276-2284.

(2) Indene-fullerene bisadduct acceptor materials. The soluble C60 derivative PCBM and C70 derivative PC70BM are the most representative acceptors for PSCs before 2015. But the LUMO level of PCBM is too low when blending with the most representative polymer donor P3HT, resulting in a low open circuit voltage (Voc) of ca. 0.6 V and the PCE of the P3HT-based PSCs is limited to ca. 4%. In considering that the value of Voc of PSCs is proportional to the difference between the LUMO of the acceptor and the HOMO of the polymer donor, Yongfang Li designed the new fullerene derivatives with the electron-rich bis-indene substituents to upshift the LUMO level of the fullerene derivatives. They synthesized indene-C60 bisadduct (ICBA) and indene-C70 bisadduct (IC70BA). The LUMO level of ICBA is ca. 0.17 eV up-shifted than that of PCBM. The PSCs with P3HT as donor and ICBA as acceptor showed a higher Voc of 0.84 V and a higher PCE of 6.48%. By using IC70BA as acceptor, the PCE of the PSCs with P3HT as donor was further increased to 7.4% which is the highest efficiency for the P3HT-based PSCs. ICBA has become another representative fullerene derivative acceptor except of PCBM. Related 5 representative publications: (a) J. Am. Chem. Soc., 2010, 132, 1377-1382. (b) J. Am. Chem. Soc. 2010, 132, 17381–17383. (c) Adv. Mater., 2010, 22, 4355-4358. (d) Adv. Funct. Mater., 2010, 20, 3383-3389.. (e) Energy Environ. Sci. 2012, 5, 7943-7949.

(3) Side chain engineering of the wide bandgap conjugated polymer donors for nonfullerene PSCs. On the basis of their 2D-conjugated BDTT-based polymer donor materials, Yongfang Li et al. developed a series of J-series broad bandgap D-A copolymers based on BDTT donor unit and bifluoro-benzothiazole (FBTA) acceptor unit for nonfullerene PSCs with narrow bandgap n-type organic semiconductor (n-OS) as acceptor. The copolymer J51 of BDTT and FBTA shows a PCE of 9.26% in the PSCs with a narrow bandgap n-OS ITIC as acceptor. By side chain engineering on the thiophene conjugated side chains with the alkylthio, alkylsilyl, alkyl-bifluoro substituents, they synthesized the new J-series polymers J61, J71 and J91 with HOMO energy level down-shifted gradually. The PCEs of the PSCs with ITIC as acceptor and J61, J71 or J91 as donor reached 9.53%, 11.41% and 11.63% respectively with gradually increased Voc from 0.89 V for J61 to 0.94 V for J71 and to 0.98 V for J91. By using a side chain isomerized new n-OS m-ITIC acceptor, the J71-based PSCs demonstrated a further increased PCE of 12.05% which is among the highest efficiency for the PSCs reported so far. Related 5 representative publications: (a) Adv. Mater., 2016, 28, 8288–8295. (b) J. Am. Chem. Soc., 2016, 138, 4657–4664. (c) Nature Commun, 2016, 7, 13651. (d) Adv. Mater., 2017, 1703344. (e) Adv. Energy Mater., 2017, 1702324.

(4) Organic small molecule photovoltaic materials. Yongfang Li started his research on the organic small molecule donor materials more than 10 years ago with the interests on the soluble triphenylene amine (TPA)-based star molecules. But at early time, the PCE of the organic solar cells (OSCs) with the TPA-based molecule as donor and PCBM as acceptor was low, only 1~2%. By introducing an electron-accepting (A) dicyanovinyl (DCN) as end group in the TPA-based star-shaped molecules, they synthesized the A-D-A structured star-shaped molecule and improved the PCE of the OSCs to ca. 3% in 2011. Then they used their 2D-conjugation strategy of polymer to the A-D-A structured small molecules and synthesized 2D organic molecule donor materials with BDTT as the central unit and further improved the PCE of the OSCs to 6.75% in 2013. Recently, they synthesized new medium bandgap organic small molecule donor materials for the nonfullerene OSCs with n-OS as acceptor. The all small molecule OSCs showed PCE of 9.73% and 10.11% which is the highest efficiency for the all small molecule OSCs reported till now. In addition, his group synthesized a new n-OS acceptor m-ITIC by side chain isomerization of ITIC, and the PCE of the J61-based PSCs with m-ITIC as acceptor reached 11.77%. Related 5 representative publications: (a) Chem. Mater., 2011, 23, 817-822. (b) Chem. Mater. 2013, 25, 2274-2281. (c) J. Am. Chem. Soc. 2017, 139, 5085-5094. (d) Chem Mater. 2017, 29, 7543–7553. (e) J. Am. Chem. Soc., 2016, 138, 15011–15018.

(5) Conjugated polymer acceptors and all polymer solar cells. All polymer solar cells (All-PSCs) with the p-type conjugated polymer as donor and n-type conjugated polymer as acceptor possess the unique advantages of good morphology stability and flexibility. 10 year ago, Yongfang Li's group started the research on the all-PSCs by collaboration with Xiaowei Zhan's group. The PCE of the all-PSCs with his 2D-conjugated polythiophene as donor and Zhan's n-type copolymer as acceptor reached ca. 1% in 2007. Last year, they used a wide bandgap polymer J51 as donor and a narrow bandgap n-type polymer N2200 as acceptor to fabricate the all-PSCs. Benefitted from the complementary absorption and matching electronic energy levels of the polymer donor and polymer acceptor, PCE of the all-PSCs reached 8.27%. Recently, they synthesized a new strong absorption low bandgap n-type conjugated polymer acceptor PZ1 by using the strong absorption n-OS small molecule as the building blocks of the polymer. The PZ1-based all-PSCs demonstrated a PCE of 9.19% which is the highest efficiency for the all-PSCs. Related 3 representative publications: (a) J. Am. Chem. Soc. 2007, 129: 7246-7247. (d) Adv. Mater. 2016, 28(9), 1884–1890. (c) Angew. Chem. Int. Ed., 2017, DOI: 10.1002/anie.201707678.

Yongfang Li has published more than 600 papers and the published papers were cited by others for more than 27900 times with h-index of 85. He was awarded several awards including (1) The Second-Class National Award of Natural Sciences in 1995 for the contribution in the "Studies on Conducting Polypyrrole", (2) The First-Class Award of Beijing City for Science and Technology in 2005 for the contribution in the "Studies on Electrochemistry of Conducting Polymers and Polymer Light-emitting electrochemical Cells", (3) 2012 Lecture Award from PMSE of ACS for the lecture of "Two-dimension-conjugated polymers for high efficiency polymer solar cells".