Limiting phosphonic acid interlayer–perovskite reactivity to stabilize perovskite solar modules.

Phosphonic acid (PA)–based interlayers used in metal-halide perovskite solar cells (PSCs) can suffer from instability at elevated temperatures. We report that the acidic protons of PAs weakly bound to indium tin oxide (ITO) can accelerate the oxidation of iodide, decomposition of formamidinium, and reduction of lead ions and that these reactions accelerate at high temperature and on exposure to ultraviolet light. Also, some common PA molecules weakly bonded to ITO can desorb and react with perovskites. We synthesized a bis(diarylamino)biphenyl-based PA that binds more strongly to ITO and show that its use in PSCs led to an operational lifetime of nearly 3000 hours with 10% efficiency loss (T90) at 85°C under a metal halide lamp (including ultraviolet light) with maximum power point tracking. Minimodules had power conversion efficiencies >22% with an aperture area of >20 square centimeters and exhibited a T90 lifetime of ~2200 hours under similar testing conditions. Editor's summary: A bis(diarylamino)biphenyl-based phosphonic acid (PA) hole acceptor forms strong covalent bonds with the indium tin oxide electrodes that dramatically enhance perovskite solar cell durability. Fei et al. developed these molecules to circumvent the high-temperature instability and reactivity caused by weakly bound PA-based interlayers. Minimodules with an aperture area of more than 20 square centimeters had power conversion efficiencies greater than 22%. They lost only 10% of their efficiency at 85°C under maximum power point tracking with a solar light source that also emitted ultraviolet light. —Phil Szuromi INTRODUCTION: Metal halide perovskite solar cells combine high power conversion efficiencies with pathways to low-cost manufacturing. A critical element in perovskite solar cells is the hole-transport layer, which extracts positive charges generated by light. Phosphonic acid–based self-assembled monolayers (PA-SAMs) are widely used to enhance initial device performance. However, ensuring long-term module stability under realistic operating conditions—particularly under the combined stresses of elevated temperature and full-spectrum illumination, including ultraviolet (UV) light—remains a major challenge that limits commercialization. RATIONALE: Our investigation was motivated by the broader observation that perovskite solar cells incorporating most PA-SAMs often fail high-temperature (85°C) light-soaking stability tests, despite improving initial device efficiency. We also observed accelerated degradation when PA-SAMs were blended into the perovskite as a means to omit the hole-transport layer. We hypothesized that this instability arises from chemical reactions between the perovskite and the PA-SAMs. Specifically, if phosphonic acid groups are not firmly anchored to the transparent conductive oxide (TCO), they may react with constituents of the metal halide perovskite, initiating interfacial degradation. Accordingly, we reasoned that designing molecules that form exceptionally strong bonds to the substrate would suppress such reactivity and enhance device longevity, given that UV irradiation can cleave weak PA-SAM–TCO bonds. RESULTS: We first conducted a systematic study and verified detrimental chemical reactions between PA-SAMs and perovskites. Using solution-state nuclear magnetic resonance spectroscopy and x-ray diffraction, we found that PA-SAM molecules oxidized iodide, decomposed the formamidinium cation, and reduced Pb to metallic lead. These reactions accelerate markedly at elevated temperature and under UV illumination. To quantify weak anchoring of PA-SAMs to TCOs, we developed a characterization workflow that combined mild acid washing with electrochemical and surface analyses. This revealed that ~30% of molecules in commonly used PA-SAMs are bound to TCO primarily through weak hydrogen bonds, making them prone to detachment during operation. To address this, we designed and synthesized a triphenylamine-based phosphonic acid (1PA-TPD) engineered for robust, covalent attachment to a TCO. Perovskite solar cells incorporating an optimized mixture of this strongly TCO-binding PA-SAM and a complementary one with strong affinity to the perovskite exhibited enhanced film crystallinity and reduced defect density. Under continuous operation at 85°C with full-spectrum illumination, these devices retained 90% of their initial efficiency (T90) for nearly 3000 hours, substantially outperforming controls. For scale-up, we fabricated minimodules (aperture area >20 cm2) that achieved >22% power conversion efficiency and a T90 of ~2200 hours under the same harsh conditions. CONCLUSION: We identified and addressed a previously underappreciated degradation pathway in perovskite photovoltaics: reactions between PA-SAMs and the perovskite under simultaneous light and heat. By designing a molecule that forms a strongly anchored monolayer, we minimized the exposure of reactive acidic groups at the interface. This strategy delivers perovskite cells and modules with exceptional operational stability, marking a substantive step toward the reliability that is needed for commercial deployment. Reducing reaction at the PA-SAM–perovskite interface through stronger molecular anchoring.: Weakly bonded phosphonic acids (PAs; lower left) can readily detach from the TCO substrate and leave reactive anchoring groups exposed and prone to reacting with perovskite components. By contrast, 1PA-TPD (lower right) establishes robust anchoring to the substrate, suppressing detachment and interfacial reactions and thereby improving device stability under realistic operating conditions. Colors in the ball-and-stick models are as follows: white, hydrogen; gray, carbon; blue, nitrogen; red, oxygen; orange, phosphorus. [ABSTRACT FROM AUTHOR]