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A new approach to boost the efficiency of non-fused ring electron acceptor solar cells

Device performances. Credit: Nature Energy (2024). DOI: 10.1038/s41560-024-01564-0

The power-conversion efficiencies (PCEs) of organic solar cells based on compounds known as polymer donors and fused ring electron acceptors (FREAs) have recently exceeded 19%. In contrast, organic solar cells based on non-fused ring electron acceptors (NFREAs), more affordable compounds characterized by non-fused (i.e., separate) aromatic rings, have so far exhibited disappointing efficiencies of around 16%.

As synthesizing NFREAs is significantly less expensive than synthesizing FREAs, developing more efficient solar cells based on these materials could have important implications. Specifically, it could facilitate the widespread adoption of organic solar cells, thus potentially contributing to the reduction of emissions and the mitigation of environmental issues.

Researchers at Shanghai Jiao Tong University, Qingdao University and other institutes in China recently proposed a new approach to fabricate more efficient organic solar cells based on NFRAs. This approach, outlined in a paper published in Nature Energy, relies on the use of a solvent based on chloroform (CF) and o-xylene (OXY), as well as a solid-state additive that further enhances crystallization in NFRAs, thus enabling higher PCEs in solar cells based on these compounds.

“Non-fused ring electron acceptors (NFREAs) potentially have lower synthetic costs than their fused counterparts,” Rui Zeng, Ming Zhang and their colleagues wrote in their paper. “However, the low backbone planarity and the presence of bulky substituents adversely affect the crystallinity of NFREAs, impeding charge transport and the formation of bicontinuous morphology in organic solar cells. We show that a binary solvent system can individually control the crystallization and phase separation of the donor polymer (for example, D18) and the NFREA (for example, 2BTh-2F-C2).”

A new approach to boost the efficiency of non-fused ring electron acceptor solar cells
Materials and solvent selection. a, The chemical structures of D18, 2BTh-2F-C2 and relevant solvents. Solvents with low volatility and low solubility for the polymer donor are framed by a blue box; solvents with high volatility and low solubility for the polymer donor are framed by a violet box; solvents with low volatility and high solubility for the polymer donor are framed by a green box; solvents with high volatility and high solubility for the polymer donor are framed by a bluish-purple box. b, The solubility of D18 in various solvents in the δv–δh diagram (δh, molecular hydrogen bonding interactions; δv, δV = √δ2 D + δ2 P ). c, Normalized absorption of D18 in various solvents. The blue rectangle represents the absorption peak of D18 in good solvents; the violet rectangle represents the absorption peak of D18 in bad solvents. d, Solvent classification diagram based on vapor pressure and solubility. Good solvents show a RED index smaller than 1, which are inside the solubility sphere; bad solvents have a RED index larger than 1, and the larger the RED number, the worse the solubility. e, Vapor pressure as a function of volume fraction in the binary solvent of CF&OXY. The solid vertical line represents 12% volume fraction of OXY in solvent mixture, the upper dashed horizontal line represents the CF vapor pressure in 12%-OXY solvent mixture for 142.26 torr, and the lower dashed horizontal line represents the OXY vapor pressure in 12%-OXY solvent mixture for 0.41 torr. When OXY takes up the majority of the solvent mixture, the vapor pressure of CF and OXY is both 4.88 torr with the same evaporation rate. f–h, Time-dependent contour maps of in situ UV–vis absorption spectra for D18:2BTh-2F-C2 blend precursor solutions in CF condition (f), OXY condition (g) and CF&OXY condition (h). The dashed lines and dashed box represent the spectral change time for D18 and 2BTh-2F-C2 of the CF-, OXY- and CF&OXY-based blend precursor solution in the film-forming process. Credit: Zeng et al. (Nature Energy, 2024).

As part of their study, Zeng, Zhang and their collaborators first designed and synthesized a compound mixture containing CF and OXY. They then observed how a donor polymer and NFREA responded to this solvent mixture, focusing specifically on the formation of films on these compounds.

“We select solvents such as CF and OXY that evaporate at different temperatures and rates and have different solubility for the donor polymer D18,” the researchers wrote. “Upon evaporation of chloroform, D18 starts to assemble into fibrils. Then, the evaporation of o-xylene induces the rapid formation of a fibril network that phase segregates 2BTh-2F-C2 into pure domains and leads to a bicontinuous morphology.”

The researchers also introduced a solid-state additive, namely 1,4-diiodobenzene (DIB), to their sample. This additive was placed in the formed photoactive thin film, while it had almost dried, to further enhance the crystallization of the NFREA.

The researchers used their approach to develop new solar cells based on NFREAs, which they then assessed in a series of initial tests. Remarkably, they found that the morphology enabled by their solvent and additive enabled PCEs of 19.02% for small-area (0.052cm2) cells and 17.28% for 1 cm2 devices.

This recent study opens new possibilities for the fabrication of organic solar cells based on NFREAs, which could be significantly less expensive than their FREAs-based counterparts. The promising findings gathered by this research team could soon inspire further efforts in this direction, potentially contributing to the future commercialization of organic solar cells.

More information:
Rui Zeng et al, Achieving 19% efficiency in non-fused ring electron acceptor solar cells via solubility control of donor and acceptor crystallization, Nature Energy (2024). DOI: 10.1038/s41560-024-01564-0

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A new approach to boost the efficiency of non-fused ring electron acceptor solar cells (2024, July 12)
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