Hybrid perovskite solar cells (PSCs) have shown ever-increasing power conversion efficiency (PCE), yet their commercialization is hindered by long-term operational instability. Ion diffusion and chemical reactions between metal electrodes (Ag, Al, or Cu) and perovskites contribute significantly to the degradation of inverted (p-i-n) PSCs under built-in electric fields or illumination. Irreversible changes like metal migration and electrode corrosion cause more damage than the intrinsic degradation of the perovskite material itself. Current high-efficiency inverted PSCs have operational stability of less than 2000 hours at room temperature, dropping to <1500 hours above 60 °C under maximum power point tracking (MPPT) and continuous illumination. Ag electrode-induced degradation involves three main mechanisms: halide anion diffusion into the Ag electrode causing corrosion; redox couple formation between the metal contact and Pb²⁺ ions, accelerating halide loss and Pb⁰ formation; and metal diffusion into the perovskite layer, creating insulating metal halide or defect states. Inert physical barriers (graphene, chromium or bismuth interlayers) are used to impede ion/metal diffusion, but iodine can still permeate under heat/light. Chemical anticorrosion methods using materials that coordinate with metals (TTTS, BTA, C[4]P) mitigate electrode corrosion but offer limited PCE improvement, sometimes even reducing it. This study proposes a novel strategy to address these limitations.

The literature extensively documents the instability issues in perovskite solar cells, primarily focusing on ion migration and the resulting degradation of the perovskite layer and electrodes. Various strategies, including the use of physical and chemical barriers, have been explored to mitigate these issues. However, these methods often suffer from limitations, such as incomplete prevention of ion migration or reduction in overall device efficiency. The use of chemical anticorrosion methods employing molecules capable of coordinating with metals has shown some promise, but these strategies have yielded limited improvement in overall device performance and longevity. Therefore, there is a need for a new approach to simultaneously enhance both the efficiency and stability of perovskite solar cells. This study builds on the existing knowledge of these limitations and proposes a novel strategy.

This research introduces a silver coordination-induced n-doping (CIN) strategy for PCBM using 4,4'-dicyano-2,2'-bipyridine (DCBP). DCBP's heterocyclic structure and pre-organized nitrogen coordination allow it to chelate Ag and release free electrons, which are then absorbed by PCBM, resulting in n-doping. DCBP also suppresses Ag and iodide migration and AgI formation. Density functional theory (DFT) calculations were performed to investigate the interaction between DCBP and Ag, and the interaction between DCBP and iodine. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) was used to analyze the distribution of Ag and iodine in devices before and after aging. X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) were employed to determine the chemical state of Ag. Electron spin resonance (ESR), ultraviolet photoelectron spectroscopy (UPS), and time-of-flight mass spectrometry were used to confirm n-doping in PCBM. The photovoltaic performance and stability of the devices were evaluated using J-V curves, external quantum efficiency (EQE) measurements, and accelerated aging tests under one-sun illumination and high temperature/humidity conditions. Additionally, other characterization methods were used, including optical microscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), and space charge limited current (SCLC). The detailed device fabrication process is described in the methods section, outlining the specific materials and steps involved in creating the perovskite solar cells.

The study demonstrates that DCBP effectively inhibits the bidirectional migration of Ag and iodine, protecting the Ag electrode and perovskite layer. TOF-SIMS analysis revealed significantly less Ag and iodine migration in devices with DCBP compared to control devices after 800 hours of aging. Optical microscopy images showed a uniform Ag electrode surface in the target devices, while the control devices exhibited large holes. XRD confirmed less AgI and PbI₂ formation in the target devices. XPS and AES analysis confirmed the change in Ag valence state from 0 to +1 after coordination with DCBP, indicating electron release and subsequent n-doping of PCBM. ESR measurements showed a paramagnetic signal confirming the generation of fullerene radical anions. UPS measurements showed a more favorable energy band alignment between the n-doped PCBM and the perovskite layer. The electron mobility and conductivity of the PCBM film were enhanced by the CIN strategy, particularly under illumination. KPFM analysis revealed a stronger electric field in the perovskite/ETL interface of the target devices, facilitating charge collection. The target devices exhibited a champion efficiency of 26.03% (certified 25.51%) with a low non-radiative voltage loss of 126 mV, compared to the control devices. The devices showed excellent long-term stability, retaining 95% of their initial efficiency after 2500 hours under one-sun illumination and >90% after 1500 hours under accelerated aging conditions (85 °C, 85% RH). TRPL and PL measurements confirmed faster carrier transport and extraction in the target devices. SCLC measurements showed a lower trap density in the perovskite film with DCBP. The improved performance was attributed to the passivation effect of DCBP and the n-doping of PCBM, leading to improved charge transport and reduced non-radiative recombination.

The findings demonstrate the effectiveness of the CIN strategy in improving both the efficiency and long-term stability of inverted perovskite solar cells. The simultaneous improvement in V_oc, J_sc, and FF is attributed to the combined effects of passivation and n-doping. The passivation of surface defects by DCBP enhances the quality of the perovskite film, resulting in higher V_oc and FF. The n-doping effect enhances electron extraction and transport, leading to higher J_sc and FF. The significant improvement in long-term stability is a direct consequence of the suppression of Ag and iodine migration. This approach is a significant advancement in the field of perovskite solar cells, offering a practical solution to address the key challenges limiting their commercial viability. The observed results directly address the limitations of previous approaches by achieving a substantial enhancement in both efficiency and stability.

This study successfully demonstrated a novel silver coordination-induced n-doping (CIN) strategy for enhancing the efficiency and stability of inverted perovskite solar cells. The use of DCBP in the PCBM layer resulted in a champion efficiency of 26.03% (certified 25.51%) and excellent long-term stability under various aging conditions. This approach offers a promising avenue for improving the performance and reliability of perovskite solar cells for practical applications. Future research could explore other suitable organic ligands for different metal electrodes and investigate the scalability of this approach for large-area device fabrication.

While the study demonstrates significant improvements in device efficiency and stability, some limitations exist. The long-term stability tests were performed under specific accelerated aging conditions, and the results may not perfectly predict the lifetime under all operating conditions. The study focused on a specific perovskite composition and device architecture, and further investigation is needed to determine the generalizability of the CIN strategy to other systems. The mechanism of the CIN process could be further explored through detailed theoretical and experimental studies. Finally, the cost and scalability of the DCBP synthesis need to be considered for large-scale commercial production.