Research

High-performance organic solar cells: the next generation photovoltaic technology

Professor Yanming Sun and his research team in Heeger Beijing Research and Development Center of Beihang University have recently made a series of significant progress in design of novel organic photovoltaic materials and fabrication of highly efficient organic solar cells. The results have been published in top peer-reviewed journals, such as Advanced Materials and Journal of American Chemical Society.
It is well-known that the current energy regimen, which relies on non-renewable and polluting energy sources, has unsustainable consequences for societal, economical, geopolitical, and environmental issues. For many years, scientists and engineers have been keen to find the most efficient and inexpensive photovoltaic devices for the utilization of solar energy. Solar cells, which directly convert sunlight into electricity, are one of the most promising and efficient technologies to harvest this energy. Currently, inorganic solar cells based on materials such as crystalline silicon, cadmium telluride, or copper indium germanium selenide (CIGS) are mature devices that exhibit relatively high solar energy conversion efficiency, and thus, dominate photovoltaic technologies available commercially. However, the high cost of inorganic solar cells and related environmental issues have partially impeded their pace to widespread deployment, which spurs research effort to explore alternative approaches given the growing demand for power worldwide.
Polymeric solar cells (PSCs) are a promising alternative because of their unique advantages, such as low-cost, light weight, solution processibility, and the potential of fabricating large area of flexible devices via roll-to-roll process. In virtue of these advantages, extensive research efforts around the world have been devoted to understanding and improving the performance of PSCs in the last two decades. Recently, Professor Yanming Sun and his coworkers made a series of significant progress in design of novel organic photovoltaic materials and fabrication of highly efficient organic solar cells.
The mobility of materials plays a key role in determining the performance of organic solar cells. It is revealed that the development of high-mobility large π-conjugated planar structure or unit is an effective way to improve the performance of photovoltaic material. By enlarging π-conjugated system of donor unit to increase the carrier mobility, several novel polymer donors have been designed and synthesized, which yielded high power conversion efficiencies (PCEs) over 10%. The results have been published in Advanced Materials and Advanced Energy Materials (Adv. Mater. 2015, 27, 2938; Adv. Mater. 2015, 27, 6969; Adv. Energy Mater. 2016, 6, 15021209), and highlighted by MaterialsViewsChina.com. Moreover, Sun et al. reported encouraging results in the field of PSCs (Adv. Mater. 2017, 29, 1604251). Through the alkyl side-chain engineering, a record PCE of 10.3% has been achieved for wide bandgap polymer-based solar cells. The results provided new insights into the structural and photovoltaic properties of copolymers and are helpful for rationally designing high-performance organic photovoltaic materials via alkyl side-chain engineering.
On the other hand, although fullerenes and their derivatives, such as PCBM, have been the dominant electron-acceptor materials in organic solar cells, they suffer from some disadvantages, such as weak absorption in the visible spectrum, inefficacy of tuning frontier energy levels, high cost and poor stability. It is necessary to explore non-fullerene electron acceptors that will not only retain the favorable electron-accepting and transporting properties of fullerenes, but also overcome their insufficiencies. After a decade of mediocrity, non-fullerene acceptors are undergoing rapid development and are emerging as a hot area of focus in the field of organic semiconductors. Professor Yanming Sun and his coworkers combined high mobility wide-bandgap material with different non-fullerene acceptors, resulting in PCEs more than 9%, with high FF over 70%, which presented a major breakthrough in non-fullerene organic solar cells. These results have been published in Journal of American Chemical Society (J. Am. Chem. Soc. 2016, 138, 10184; J. Am. Chem. Soc. 2016, 138, 37;J. Am. Chem. Soc. 2016, 138, 2973;J. Am. Chem. Soc. 2015, 137, 11156).



Fig. (a)-(d) Chemical structures of PBT1-EH and ITIC-Th, device structure of organic solar cells, J-V curve and IPCE spectrum of organic solar cells under simulated AM 1.5G irradiation.

Yanming Sun, professor, school of chemistry and environmental science, Heeger Beijing Research and Development Center, Beihang University, E-mail: sunym@buaa.edu.cn

References

[1] T. Liu, X. Pan, X. Meng, Y. Liu, D. Wei, W. Ma, L. Huo,* X. Sun, T. H. Lee, M. Huang, H. Choi, J. Y. Kim, W. C. H. Choy, Y. Sun*, Adv. Mater. 2017, 29, 1604251.
[2] T Liu, L. Huo, X. Sun, B. Fan, Y. Cai, T. Kim, J. Y. Kim, H. Choi*, Y. Sun*, Adv. Energy Mater. 2016, 6, 1502109.
[3] L Huo, T. Liu, X. Sun, Y. Cai, A. J. Heeger, Y. Sun*, Adv. Mater. 2015, 27, 2938.
[4] L Huo, T. Liu, B. Fan, Z. Zhao, X. Sun, D. Wei, M. Yu, Y. Liu, Y. Sun*, Adv. Mater. 2015, 27, 6969.
[5] D Meng, D. Sun, C. Zhong, T. Liu, B. Fan, L. Huo, Y. Li, W. Jiang, H. Choi, T. Kim, J. Y. Kim, Y. Sun,* Z. Wang,* Alan J. Heeger, J. Am. Chem. Soc., 2016, 138, 375.
[6] Y Lin, Q. He, F. Zhao, L. Huo, J. Mai, X. Lu, C.-J. Su, T. Li, J. Wang, J. Zhu, Y. Sun*, C. Wang, X. Zhan*, J. Am. Chem. Soc., 2016, 138, 2973.
[7] D. Meng, H. Fu, C. Xiao, X. Meng, T. Winands, W. Ma, W. Wei, B. Fan, L. Huo, N. L. Doltsinis, Y. Li,*, Y. Sun,* Z. Wang, J. Am. Chem. Soc. 2016, 138, 10184.
[8] D Sun, D. Meng, Y. Cai, B. Fan, Y. Li, W. Jiang, L. Huo*, Y. Sun*, Z. Wang*, J. Am. Chem. Soc., 2015, 137, 111561.