The BrBACz-based binary OSC can achieve a power conversion efficiency of 19.70&. (Image Credit: NIMTE)
Researchers at the Ningbo Institute of Materials Technology and Engineering (NIMTE) of the Chinese Academy of Sciences developed two self-assembled molecules (SAMs) with large dipole moments. These served as hole-transporting layers in binary organic solar cells (OSCs), which achieved 19.70% power conversion efficiency, a record-setter.
OSCs are lightweight, translucent, and have good mechanical flexibility. Developing new materials and optimizing the fabrication process leads to a power conversion efficiency improvement of over 19%. However, it’s still difficult to strike a balance between stability and efficiency. That’s because the PEDOT:PSS hole transporting material is hydrophilic and very acidic, contributing to OSC’s low stability.
So, the team designed and synthesized two asymmetric SAMs that have a BrCz and BrBACz asymmetric backbone. Both the SAMs served as hole-transporting layers in binary OSCs based on the PM6:Y6, PM6:eC9, PM6:L8-BO, and D18:eC9 systems.
Compared to the PEDOT:PSS-controlled OSC, the BrCz-controlled and BrBACz-controlled OSC demonstrated lower surface energy, higher transmittance, and deeper work function. This boosted hole mobility, enhanced hole extraction, reduced interface resistance, and decreased carrier recombination. The BrBACz OSC’s power conversion efficiency reached 19.70% with a 0.856 circuit voltage and 29.20 mA cm-2 current density.
The BrBACz OSC maintains 95% of its efficiency when subjected to ambient air (40 ± 5% humidity) and continuous illumination for 1,036 hours. The team says this is the best air density for OSCs. Additionally, the BrBACz is practical in other binary systems like PM6:Y6, PM6:L8-BO, and D18:eC9, exhibiting excellent generality.
Overall, the study could pave the way toward the development and application of highly efficient and stable OSCs through molecular structure design.
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