Scientists achieve 19.6% single junction organic solar cells

On May 5, 2022, Beijing time, Professor Liu Feng of Shanghai Jiao Tong University, Professor Sun Yanming of Beihang University and Dr. Yan Jun of Imperial College of Technology published a paper entitled “Single-junction organic solar cells with over 19% efficiency enabled by a refined double-fibril network” on Nature Materials morphology”.

In this study, the authors achieved the matching of the morphological characteristic scale of the organic photovoltaic film with the core parameters of photophysical parameters by creating the morphology of the double fiber network, promoted the efficient utilization of excitons and carriers, reduced the composite loss constant, and achieved a breakthrough in device efficiency. The corresponding authors of the paper are Liu Feng, Sun Yanming and Yan Jun; the first authors are Zhu Lei, Zhang Ming and Xu Jinqiu.

In recent years, the field of organic photovoltaics has developed rapidly due to the development of new receptor materials, and the photoelectric conversion efficiency of devices has been continuously improved. Due to the intrinsic physical properties of excitons in organic semiconductors (coulomb binding, diffusion drift, interface dissociation), and the low mobility of organic semiconductor films, the morphology of the accept-blend heterojunction films has been a key factor affecting the performance of the device. On the basis of the special polymer-like accumulation morphology of banana-type Y6 receptors, The team of Liu Feng of Shanghai Jiao Tong University carried out a series of work on the optimization of complex multi-scale morphology and the preparation of efficient devices (Adv. Energy Mater. 10, 1904234, 2020; Adv. Mater. 33, 2007177, 2021; Nat. Commun. 12, 309, 2021)。

L8-BO is a novel class Y6 receptor, and the extra-arc branched side chains of molecules introduce stronger alkyl side-chain interactions, adjust the combined sequence of molecules in the lattice, and make the molecular stacks more tight and the space between the main chains smaller. This special multimolecular force allows multiple adjacent dimers to intertwine to form an assembly structure with a large aspect ratio, showing long-term symmetry and a higher tolerance to local arrangement defects (Figure 1). These properties make L8-BO easy to form a long needle-like single crystal structure, and retain this self-assembly property of large aspect ratio in the film, making it easier to form a polymer-like fibrous structure.

Figure 1: Large aspect ratio structure of L8-BO molecules in single crystals

Therefore, in the PM6:L8-BO blending system, the two-fiber network morphology was successfully constructed for the first time, and the device efficiency was outstanding (Nat. Energy 6, 605–613, 2021)。 It is found that there is still an imbalance in the diffusion capacity of excitons and carriers in ordinary binary blended films, and the characteristic size of the dual-fiber network in the active layer cannot be adapted to the optoelectronic physical parameters, so there is still room for improvement in device efficiency. The research team added D18 as a ternary component to the binary blending system, synergistically improving the crystallization properties to the receptor, balancing the diffusion length of the exciton and carrier at the receptor end (Figure 2), and significantly improving the multidimensional parameter adaptability of the device.

Figure 2: Exciton diffusion length in materials and systems

At the same time, the addition of D18 promotes the formation of a denser two-fiber network structure (Figure 3), and the characteristic size of the blended phase in the fiber gap is only 5 nm, which can better adapt to the transport properties of excitons and carriers and reduce the composite constant. Dense receptor-giving fibers intertwine to form high-speed channels for electron and hole transmission, and the small feature size of the blending phase can better match the diffusion capacity of exciton carriers, ensuring that the dissociated carriers can quickly diffuse to the crystal region for transmission, so that the efficiency is significantly improved.

Figure 3: Morphology of the dual fiber network in the active layer film

The photoelectric conversion efficiency of organic solar cells prepared by the dual-fiber network strategy reached 19.6%, and the filling factor was close to 82%, which was certified by the National Photovoltaic Industry Metrology and Testing Center (NPVM) of Fujian Institute of Metrology, which is the highest efficiency reported by single-junction organic solar cells at this stage. The research work was funded by the National Natural Science Foundation of China, the Ministry of Science and Technology, the Shanghai Municipal Science and Technology Commission, and the Shandong Provincial Science Foundation. (Source: Science Network)

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