Suppressing redox reactions at the nickel oxide interface to improve the efficiency and stability of perovskite solar cells

Professor Tonggang Jiu's team at Shandong University achieved a significant improvement in the efficiency and stability of perovskite solar cells by regulating the NiO_x/perovskite interface chemistry by introducing a novel self-assembling molecule, (4-(2,7-dibromo-9,9-dimethylacridin-10(9H)-yl)butylphosphonic acid (2Br-4PADMAc), into the NiO_x/perovskite interface. This effectively suppressed Ni^>3+ generation and interfacial redox reactions, optimized energy level matching, and promoted high-quality film crystallization. Compared to the traditional molecule 2PACz, 2Br-4PADMAc exhibits stronger interfacial anchoring and a larger molecular dipole moment, which can improve hole transport efficiency and reduce PbI₂ accumulation. The inverse perovskite solar cell modified with this molecule achieved a photoelectric conversion efficiency of 25.28% and excellent stability. This research provides a new strategy for optimizing the efficiency and stability of NiO_x-based perovskite solar cells through interfacial molecule design. The article was published as an open access research article in CCS Chemistry, the flagship journal of the Chinese Chemical Society.

Background information:

Perovskite solar cells (PSCs), as representatives of third-generation solar cells, have seen rapid advancements in efficiency and stability in recent years. Among numerous inorganic hole transport materials, nickel oxide (NiO_x) stands out due to its excellent hole extraction capability, low visible light absorption, simple synthesis process, and low cost. However, the undesirable interaction between the highly active Ni^>3+species in NiO_x films and perovskite, as well as charge carrier recombination at the NiO_x/perovskite interface, hinders the further development of NiO_x-based PSCs . For example, Ni^>3+ species in NiO_x films may lead to deprotonation of A-site cations and oxidation of X- site ions in perovskite, thereby increasing the PbI₂ content and initiating nonradiative recombination at the buried interface .

Highlights of this article:

This work introduced self-assembled monolayers (SAMs) with different structures at the NiO_x/perovskite interface and investigated in detail the mechanism by which these SAMs inhibit the formation of Ni^>3+ substances, thereby suppressing redox reaction byproducts at the interface. In this study, by comparing two SAM molecules, 4-(2,7-dibromo-9,9-dimethylacridin-10(9H)-yl) butyl)phosphonic acid (2Br-4PADMAc) and (2-(9H-carbazole-9-yl) ethyl)phosphonic acid (2PACz), the author's found that the 2Br-4PADMAc molecule had a stronger anchoring effect with NiO_x, effectively regulating the valence state distribution of nickel atoms and effectively isolating redox reactions at the interface.

The enhanced coordination between bromine and lead atoms in the 2Br-4PADMAc molecule effectively stabilizes the perovskite crystal and reduces PbI₂ accumulation at the interface. Simultaneously, the strong interaction between 2Br-4PADMAc and PbI₂ suppresses interfacial redox reactions, thereby promoting high-quality crystallization of the perovskite film and reducing nonradiative recombination. By improving interfacial contact, optimizing energy level arrangement, enhancing the quality of the perovskite film, and suppressing defects from byproducts, a photoelectric conversion efficiency of 25.28% and excellent stability were achieved. Furthermore, due to the suppression of interfacial redox reactions, perovskite degradation was effectively inhibited even at high temperatures, improving the long-term operational stability of the device.

Summary and Outlook:

This work utilized self-assembled molecules with different structures to modify the NiO_x/perovskite interface. The negatively charged phosphate groups enabled 2Br-4PADMAc to anchor at Ni^>3+ defects on the NiO_x surface. This not only optimized the Ni valence state distribution to improve the conductivity of the top charge transport layer but also prevented redox reactions at the interface. These findings provide a potential strategy for mitigating harmful byproducts in the fabrication of efficient and durable inverse perovskite solar cells .

Qinglin Du (Shandong University and China University of Petroleum) and Yilin Chang (Shandong University) are co-first authors on the paper.  Le Liu (Shandong University), Lianqing Yu (China University of Petroleum), and Tonggang Jiu (Shandong University) are the corresponding authors.

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About the journal: CCS Chemistry is the Chinese Chemical Society’s flagship publication, established to serve as the preeminent international chemistry journal published in China. It is an English language journal that covers all areas of chemistry and the chemical sciences, including groundbreaking concepts, mechanisms, methods, materials, reactions, and applications. All articles are diamond open access, with no fees for authors or readers. More information can be found at https://www.chinesechemsoc.org/journal/ccschem.

About the Chinese Chemical Society: The Chinese Chemical Society (CCS) is an academic organization formed by Chinese chemists of their own accord with the purpose of uniting Chinese chemists at home and abroad to promote the development of chemistry in China. The CCS was founded during a meeting of preeminent chemists in Nanjing on August 4, 1932. It currently has more than 120,000 individual members and 184 organizational members. There are 7 Divisions covering the major areas of chemistry: physical, inorganic, organic, polymer, analytical, applied and chemical education, as well as 31 Commissions, including catalysis, computational chemistry, photochemistry, electrochemistry, organic solid chemistry, environmental chemistry, and many other sub-fields of the chemical sciences. The CCS also has 10 committees, including the Woman’s Chemists Committee and Young Chemists Committee. More information can be found at https://www.chinesechemsoc.org/.