Journal
ACS APPLIED MATERIALS & INTERFACES
Volume 13, Issue 15, Pages 17869-17881Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acsami.1c02613
Keywords
organic solar cells; ZnO electron transporting layer; pH condition; surface passivation; long-term stability
Funding
- National Natural Science Foundation of China [51773224, 22075315]
- Youth Innovation Promotion Association, CAS [2019317]
- Ministry of Science and Technology of China [2016 YFA0200700]
- Vacuum Interconnected Nanotech Workstation, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences (CAS)
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The surface pH conditions of solution-processed ZnO films were found to be closely related to the performance and stability of NFA OSCs. Treating ZnO films with acid solutions improved device performance, while removal of surface -OH groups alleviated performance issues and improved stability. Acid solutions, particularly PET treatment, were effective in enhancing performance, leading to long-term stability of the heterojunction solar cell.
For the inverted organic solar cells (OSCs), the interface contacts between the ZnO electron transporting layer and the organic active layer play an important role in the device performance and stability. Since the solution-processed ZnO surface always contains some base or zinc salt contaminants, we explored how the surface pH conditions influence the performance and stability of the nonfullerene acceptor (NFA) cells. A tight relationship between the surface pH condition and the device performance and stability was established. Specifically, device performance and stability were improved by treating the ZnO films with acid solutions but worsened after base treatment. The large number of hydroxyl groups on the surface of the solution-processed ZnO films was proved to be the main reason for the surface pH condition-related performance, which caused oxygen-deficient defects and unfavorable vertical phase separation in the blend films, hindered the photogenerated charge transfer and collection, and consequently resulted in low short-circuit current density and power conversion efficiency (PCE). The surface -OH groups also boosted the photocatalytic activity and led to fast degradation of the nonfullerene acceptor. Removal of the surface -OH groups can alleviate such problems. Different acid solutions, ZrAcac, 2-phenylethylmercaptan (PET), and glutamic acid (GC), were used to treat the ZnO films, and PET treatment was the most effective treatment for performance improvement. An efficiency of 16.46% was achieved for the PM6:Y6 cells and the long-term stability under continuous illumination conditions was significantly improved with a T-80 lifetime of over 4000 h (4410 h), showing the excellent long-term stability of this heterojunction solar cell. Our understanding of the surface pH condition-related device performance and stability would guide the development of a feasible method for solving the interface problems in OSCs. We also provide a practical strategy to modify ZnO with acid solutions for high-performance and stable NFA OSCs.
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