Research on organic photovoltaic published on Nature Materials

On May 5, 2022, the renowned academic journal Nature Materials published an article of Prof. Feng LIU from the School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, entitled “Single-junction organic solar cells with over 19% efficiency enabled by a refined double-fibril network morphology”. Dr. Lei ZHU, Ming ZHANG and Jinqiu XU are the co-first authors. Prof. Feng LIU from Shanghai Jiao Tong University, Prof. Yanming SUN from Beihang University and Dr. Jun YAN from Imperial College London are the co-corresponding authors. The School of Chemistry and Chemical Engineering, the Frontiers Science Center for Transformative Molecules, the In-situ Center for Physical Science and the Center of Hydrogen Science at Shanghai Jiao Tong University are the first affiliation.

Organic solar cells have important application prospects in fields such as photovoltaic buildings and flexible electronic devices. In recent years, due to the development of new materials, the photoelectric conversion efficiency of the devices has been improving and is gradually catching up with silicon-based solar cells and chalcogenide solar cells. Due to the intrinsic physical properties of excitons in organic semiconductors (Coulomb binding, diffusive drift, interfacial dissociation), as well as the low mobility of organic semiconductor films, donor-acceptor heterojunction film morphology has been a key factor affecting device performance. The organic photovoltaic film morphology is affected by both thermodynamic factors and kinetic processes, thus difficult to regulate. Prof. Feng LIU’s group discovered the special polymer-like stacking morphology of the highly efficient Y6 acceptor, and has carried out a series of researches on molecular assembly morphology and multiscale morphology optimization (Adv. Energy Mater. 10, 1904234, 2020; Adv. Mater. 33, 2007177, 2021; Nat. Commun. 12, 309, 2021). At the same time, Prof. Feng LIU’s group and collaborators optimized the structure of Y6 so as to design and synthesiz the L8-BO acceptor with better performance: the interactions between alkyl side chains were enhanced by introducing extra-arc long branched side chains, and the combinatorial sequence of molecules in the lattice was adjusted to make the molecules stack more tightly. This special multiple intermolecular force allows multiple neighboring dimers to intertwine into an assembled structure with high aspect ratio, presenting long-period symmetry and higher tolerance to local alignment defects (Fig. 1). These properties make it easier for L8-BO to form long needle-like single-crystal structures while the high aspect ratio self-assembly property is retained in the films, making it easier to form polymer-like fibrous structures.

Fig. 1: The high aspect ratio of single-crystal structure of L8-BO.

Organic thin-film photovoltaic devices constructed by co-blending L8-BO with PM6 exhibited outstanding photovoltaic conversion efficiency (Nat. Energy 6, 605-613, 2021). The team added D18 as a third component to the double-blend system to synergistically improve the crystalline property of the acceptor fibers and to balance the diffusion length of excitons and carriers at the acceptor end (Fig. 2), resulting in a significant improvement in the multidimensional parameter fitness of the devices. At the same time, the addition of D18 promotes the formation of a more condensed double-fiber network structure with the characteristic size of only 5 nm, which can better adapt the exciton’s and carrier’s transport property and reduce the complex constants. The dense donor-acceptor fibers interweave to form a high-speed channel for electron and cavity transport. The smaller characteristic size of the double-blend system can better match the diffusion ability of excitons and carriers to ensure the rapid diffusion of dissociated carriers to the crystal region, thus obtaining significantly increased efficiency.

Fig. 2: Exciton diffusion length in different materials and systems.

Fig. 3: The bicontinuous double-fibril network morphology of the PM6:D18:L8-BO blended film.

The photovoltaic conversion efficiency of organic solar cells prepared by the double-fibril network strategy reached 19.6% with a fill factor close to 82% (Fig. 4) and was certified by the National Photovoltaic Industry Metrology Test Center of Fujian Metrology Institute, a third-party independent institution (NPVM: 19.2%), which is the highest efficiency reported for single-junction devices of organic solar cells. Based on the formation mechanism of the double-fibril network morphology, the research team summarized the material selection principles for constructing the double-fibril morphology structure and verified its universality. The work bridged the gap between organic photovoltaic thin film morphology characteristic scale with the core photophysical parameters, which provides a new perspective for the preparation of subsequent high-efficiency organic solar cells.

Fig. 4: The photovoltaic conversion efficiency of organic solar cells prepared by the double-fibril network strategy.

This work was supported by Prof. Yanming SUN from Beihang University, Prof. Jenny Nelson and Dr. Jun YAN from Imperial College London, Prof. Haiming ZHU from Zhejiang University and Associate Prof. Junchao CHEN from Shanghai Jiao Tong University. This work was financially supported by the National Natural Science Foundation of China, the National Key R&D Program of China, the Program of Shanghai Science and Technology Commission’s Science and Technology Innovation Action Plan, and the Natural Science Foundation of Shandong Province.

Link to the article: https://www.nature.com/articles/s41563-022-01244-y

Translator: Chenyun SUN

Revisers: Feng LIU, Xiaoke HU