ACS Energy Letters(IF:23.991) UNIST Chunhui Duan Near-infrared Electron Acceptance of Cyano-substituted 2-(3-Oxo-2,3-dihydroinden-1-ylidene)malononitrile End Groups for Organic Solar Cells Bulk efficiency>18.1%

Enlitech- Selection of Top Teams！

Highlights

Cyano-substituted 2-(3-oxo-2,3-dihydroinden-1-ylidene)acrylonitrile (CN-IC) end groups exhibit strong electron-withdrawing ability to reduce the bandgap of acceptor materials.
The A-DA’D-A type acceptor material BTPCN with CN-IC end groups achieved an optical bandgap of 1.29 eV, 0.12 eV lower than the Y5-BO reference material.
Organic solar cells employing PTTzF as the donor and BTPCN as the acceptor realized a high power conversion efficiency of 18.1%.

Results

Researchers at the Ulsan National Institute of Science and Technology (UNIST) in South Korea designed a series of A-DA’D-A type small molecule near-infrared electron acceptor materials BTPCN, where A represents two aromatic units and D/D’ thiophene derivatives. The key is the adoption of cyano-substituted 2-(3-oxo-2,3-dihydroinden-1-ylidene)acrylonitrile (CN-IC) as the end groups. Experiments demonstrate CN-IC end groups exhibit the strongest electron-withdrawing ability among all reported end groups so far.

Compared to the reference material Y5-BO, the BTPCN material with CN-IC end groups achieved an optical bandgap of 1.29 eV, 0.12 eV lower than the 1.41 eV of Y5-BO. This enables BTPCN to harvest more near-infrared light and broadens its absorption range. In addition, the CN-IC end groups lead to a relatively deeper highest occupied molecular orbital (HOMO) level distribution in BTPCN.

The team fabricated organic solar cells using BTPCN as the acceptor and the high-performance polymer PTTzF as the donor. Test results show significantly enhanced efficiency compared to solar cells using the Y5 end group acceptor.

Background

Organic solar cells feature high conversion efficiency and low production cost, making them a promising candidate for clean renewable energy conversion. Realizing the commercialization of organic solar cells requires designing highly efficient photoactive materials. Particularly, near-infrared absorbing electron acceptor materials are crucial to broaden the light harvesting range and increase the power conversion efficiency. A-D-A type small molecules and polymers are commonly used as near-infrared acceptor materials, and molecular end group optimization is considered an effective strategy to enhance their performance.

The Ulsan National Institute of Science and Technology (UNIST) is one of the four leading national science and technology institutes in South Korea, located in Ulsan Metropolitan City known as the industrial capital of Korea. Since its founding, UNIST has been devoted to developing itself into the most outstanding national university and a world-leading science and engineering institution. After installing Enlitech’s QE-R quantum efficiency measurement system in 2019, UNIST consecutively broke the world record for perovskite solar cells on NREL’s efficiency chart in August 2020 (25.5%), January 2022 (25.7%), and December 2022 (25.8%).

In January 2020, with the assistance of Enlitech’s REPS, UNIST strengthened the open-circuit voltage loss and achieved 24.8% conversion efficiency with perovskite solar cells, setting a new world record. UNIST is among the top photovoltaic research institutions in Asia.

Methods

Synthetic routes: The team purchased Br-IC and BTP-EH precursors from commercial suppliers, and synthesized the CN-IC end groups through a series of reactions. BTPCN-EH was obtained by condensing CN-IC with BTP-EH under acid catalysis. BTPCN-BO and BTPCN-HD were synthesized through similar routes.
Spectroscopic analysis: 1H NMR and 13C NMR were utilized to confirm the chemical structures of CN-IC and the three BTPCN acceptors. UV-vis absorption spectra were measured to characterize the optical properties.
Theoretical calculations: The geometries of end groups were optimized by DFT and their molecular orbital and electrostatic potential distributions were computed. Results reveal the stronger electron-withdrawing ability of CN-IC.
Cyclic voltammetry: The energy level distribution of materials was determined from CV curves. CN-IC helps narrow the bandgap while maintaining deep energy levels.
Device fabrication: Precise control over donor/acceptor ratio, solvent additives, thermal annealing etc. optimized the active layers.
Efficiency characterization: Under AM1.5G illumination, devices based on PTTzF polymer and three BTPCN acceptors were tested. The BTPCN-EH system reached 18.1% efficiency.

Instruments

In 2019, UNIST installed Enlitech’s QE-R quantum efficiency measurement system. Soon after, UNIST broke the world record for perovskite solar cells and was marked with the honor of certified 25.5% perovskite solar cell efficiency on NREL’s chart in 2020. Enlitech’s QE-R PV quantum efficiency measurement system is a highly popular and reliable QE/IPCE system capable of accurate EQE characterization of various solar cells. Customizable wavelength range (300-1100 nm, 300-1800 nm, 300-2500 nm etc.) and sample holders cater to diverse needs. The compact system features proprietary glovebox integration, dual-magnified image monitoring of light power and device signal, and repeatability exceeding 99.5%. It is the top choice of many first-class PV labs. Combined with Enlitech’s automated inspection software, QE-R enables rapid and precise measurements of IPCE, IQE and spectral response. Enlitech remains the only quantum efficiency system manufacturer certified to ISO 17025 standard for system calibration and testing. Its instruments comply with international standards ASTME 1021-15, ASTME948, IEC 60904-8/7/1. Exclusive EQE uncertainty assessments and quality control can be provided for journal publications with ISO/IEC 17025-accredited calibration.

Nearly 1000 QE-R systems have been installed worldwide, serving over 500 outstanding solar cell research labs. Over 1000 SCI papers have been published in top journals like Nature, Science, Joule and Advanced Materials.

The QE data from QE-R systems allow researchers to investigate device design, performance, process improvements, material bandgaps, defects and traps. The high reproducibility and accuracy also qualify QE-R systems for spectrum mismatch calculation and conversion efficiency uncertainty evaluation by metrologists.

Conclusion

This research designed a new type of A-DA’D-A near-infrared organic solar cell acceptor material bearing cyano-substituted indole acrylonitrile end groups. Owing to the strong electron-withdrawing ability, these materials realize narrow optical bandgaps while maintaining deep energy level distribution, which facilitates photoelectric conversion. Devices fabricated with these acceptor materials achieved an efficiency of 18.1%, breaking the current record. This work provides new insights into developing highly efficient organic solar cells.