Bulky Molecules Keep Rival Defect-Fighters From Cancelling Each Other Out in Perovskite Solar Cells

Perovskite solar cells have spent a decade shattering efficiency records, but their Achilles heel has always been chemistry at the atomic scale. Tiny charged imperfections, known as point defects, litter the surface of the light-absorbing crystal. These defects act as traps that snatch up the energetic charge carriers the cell is trying to harvest, bleeding away efficiency and accelerating the material’s notorious tendency to degrade. Now a team of chemists and photovoltaic researchers at Northwestern University, working with collaborators in South Korea, has reported a deceptively simple fix for a long-standing paradox in how those defects are neutralized, and the result is a device that reaches a power conversion efficiency of 27.4 percent, with a certified steady-state value of 25.8 percent.

The paradox centers on two classic tools of the synthetic chemist: Lewis acids and Lewis bases. A Lewis base is a molecule that donates an electron pair, and it naturally seeks out positively charged, or cationic, defects on the perovskite surface. A Lewis acid, which accepts an electron pair, does the complementary job on negatively charged, or anionic, defects. In principle, adding both to a solar cell should heal both classes of defect at once. In practice, the strategy has repeatedly failed for the most fundamental of reasons: acids and bases react with each other. When mixed, they form stable adducts, quenching one another’s reactivity before either molecule ever reaches the defect it was designed to neutralize. The result is a passivation cocktail that neutralizes itself.

The new study, published in Nature Chemistry on 23 September 2026 and led by Donghoon Shin and Shuta Kitade under the supervision of Bin Chen and Edward H. Sargent, with Chad A. Mirkin and Mercouri G. Kanatzidis among the senior co-authors, sidesteps this self-defeating chemistry by making the two passivators chemically orthogonal. Orthogonality here means that each molecule reacts with its intended target on the perovskite surface but essentially ignores its partner in the solution. The key insight is geometric rather than electronic: the researchers exploited steric hindrance, the physical bulk of molecular groups, to keep the acid and base from getting close enough to share electrons.

The molecular design hinges on a family of bulky sp3-phosphine Lewis bases. Phosphines, compounds built around a trivalent phosphorus atom, are workhorse Lewis bases in coordination chemistry. By decorating the phosphorus center with bulky substituents arranged in tetrahedral, sp3-hybridized geometry, the team created bases whose lone electron pair is shielded by a thicket of atoms. When paired with fluorinated aromatic Lewis acids, these sterically congested phosphines simply cannot achieve the orbital overlap needed to form a conventional acid-base adduct. The fluorinated aromatic acids, for their part, are relatively weak and geometrically restrained acceptors, so the mutual attraction that normally dooms mixed acid-base formulations is suppressed at its source.

Crucially, the strategy works because of an asymmetry in accessibility. The defect sites on the perovskite surface, such as undercoordinated lead ions and halide vacancies, protrude from the crystal lattice and are more sterically exposed than the crowded acid-base pair is to itself. In other words, each passivator can still reach its target defect even though the two passivators cannot reach each other. The researchers describe this as a sterically gated arrangement, in which molecular bulk acts as a selective filter: small, exposed defect sites pass through the gate, while the bulky acid-base encounter does not. Spectroscopic measurements, including nuclear magnetic resonance and infrared techniques, along with defect-profiling methods such as drive-level capacitance profiling, support the picture that the two additives suppress trap states additively rather than destructively.

The photophysical consequences are exactly what defect passivation should deliver. Photoluminescence quantum yield measurements, which track how many absorbed photons are re-emitted rather than lost as heat, improve markedly when the orthogonal pair is applied, indicating that fewer photoexcited carriers are falling into trap states. Time-resolved photoluminescence shows longer carrier lifetimes, and transient absorption spectroscopy corroborates the reduction in non-radiative recombination pathways. Density functional theory calculations, performed with the VASP and ORCA packages, provide a molecular-level account of the binding energies involved, showing that each passivator binds effectively to its intended ionic defect while the acid-base interaction between the two molecules remains weak.

Translated into devices, the dual-passivation strategy produced impressive numbers. Solar cells fabricated with the sterically gated acid-base pair achieved power conversion efficiencies up to 27.4 percent, with an independently certified steady-state efficiency of 25.8 percent, placing the devices among the leading perovskite cells reported to date. Beyond raw efficiency, the team reported gains in operational stability, an equally important metric for a technology hoping to displace silicon in commercial panels. Because the passivation suppresses the charged surface defects that also serve as nucleation points for ion migration and chemical degradation, the molecular repair kit addresses both efficiency loss and instability with a single intervention.

The work also showcases an unusually broad collaboration across chemistry subdisciplines. Mirkin’s group contributed high-throughput spray-based screening methods for rapidly evaluating candidate passivators, developed with Jaewon Lee, while Kanatzidis brought deep expertise in halide perovskite materials chemistry. The team’s computational and spectroscopic characterization spanned X-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectrometry, and microscopy performed at Northwestern’s shared facilities. The underlying datasets have been deposited openly on Zenodo, and the authors have filed an invention disclosure with Northwestern University covering the molecular mutual passivation strategy, signaling interest in translating the chemistry toward commercial fabrication.

For the perovskite field, the significance of the result may lie less in the headline efficiency number than in the design principle behind it. Researchers have long known that single-function passivators leave half the defect population untouched, and that simply mixing acids and bases fails. The demonstration that steric gating can decouple two mutually reactive passivators gives the community a generalizable rule for building multi-component surface treatments: choose partners whose mutual reaction geometry is blocked but whose targets remain accessible. As perovskite technology pushes toward the detailed-balance efficiency limit set by Shockley and Queisser in 1961, every remaining percentage point must be wrested from non-radiative losses at surfaces and grain boundaries. Orthogonal, sterically engineered passivation offers a rational route to squeeze out those losses, and it suggests that the next generation of record-breaking cells may be designed as much by molecular architects thinking about shape and crowding as by device engineers optimizing layer thicknesses.

Subject of Research: Orthogonal Lewis acid-base defect passivation in perovskite solar cells

Article Title: Sterically gated Lewis acid and base pairs enable orthogonal defect passivation in perovskite solar cells

Article References: Sterically gated Lewis acid and base pairs enable orthogonal defect passivation in perovskite solar cells. (n.d.). https://doi.org/10.1038/s41557-026-02261-z

Image Credits: AI Generated

DOI: 10.1038/s41557-026-02261-z

Keywords: perovskite solar cells, Lewis acids, Lewis bases, defect passivation, steric hindrance, phosphines, power conversion efficiency, non-radiative recombination, surface chemistry, Nature Chemistry, photovoltaics, trap states