With tandem solar cells (TSC) gaining increased attention in the solar community, the development of wide bandgap perovskites (WBPs) is becoming a key aspect of research activities. In particular, perovskites with a bandgap of 1.7eV have shown to be optimal candidates to be integrated with silicon cells and achieve the highest efficiency in TSCs. However, WBP still face critical challenges, primarily related to their poor thermal and light stability. Increasing the bandgap typically requires higher bromine (Br) content relative to iodine (I), which under illumination induces halide segregation into Br-rich and I-rich domains, causing efficiency losses and severely limiting device lifetime [1]. To mitigate halide segregation, studies have explored various approaches, including additive, composition and interface engineering [2]. However, the operational stability of WBP remains largely underexplored. Moreover, even though studies have reported some stability characteristics for WBP, from 1.63eV to 1.90eV [2], the majority emphasize performance metrics rather than identifying the fundamental degradation mechanisms. Furthermore, most of these studies have been conducted on perovskite films fabricated using lab-scale, non-scalable processes, limiting their relevance for real-world applications. Addressing these gaps requires a more systematic and comprehensive investigation into the stability of WBPs fabricated using scalable processes and device architectures.

In this context, the present work aims to systematically assess the stability of 1.68 eV WBPs fabricated using scalable deposition methods. Two wide-bandgap compositions are considered, one deposited by blade-coating (Cs_0.15(MA_0.2FA_0.8)_.85Pb(I_0.77Br_0.20)₃) and the second via a two-step hybrid process (CsFAPb(I_1-xBr_x)₃) and compared with a reference 1.6eV device processed by blade coating. Stability testing is conducted under two stress conditions as defined by ISOS protocols[3]. Maximum power point tracking (MPPT), following the ISOS-L2 protocol, is performed under operational conditions at 60°C. The time required for a device efficiency to reduce to 80% of its initial value (T80) is used as a key metric for comparing stability across different samples. Additionally, thermal stability is evaluated using the ISOS-D3 protocol, where devices are aged at 85°C in the dark under a nitrogen atmosphere. To monitor degradation, current-voltage (I-V) measurements are systematically performed in both forward and reverse scan directions, before and after each stress condition. Moreover, to gain deeper insights into degradation mechanisms, a comprehensive characterization toolbox is employed. Capacitance-frequency (C-F) measurements are carried out under applied bias voltage in both dark and illuminated conditions, enabling to correlate the performance degradation with intrinsic device parameters, such as interface trap densities and ion diffusion processes. Additionally, photoluminescence (PL) spectroscopy, including both steady-state and time-resolved photoluminescence (TRPL), is employed to evaluate changes in bandgap energy and other optoelectronic properties.

In a preliminary study, MPPT at 60°C was applied on these 3 different architectures. The best WBP sample maintains more than 95% of its initial efficiency after 12h, a remarkable result for such devices. However, this perovskite shows to be relatively sensitive to process variation, with another sample dropping to 75% of its initial performance in that period, with a T80 of 254min, lower than the 720min obtained for the hybrid WBP device. As expected, the reference 1.6eV device shows very good stability, reaching only 90% of its initial efficiency after 30h. PL steady-state measurements show an increase in the bandgap and FWHM enlargements in some regions of the cell for both low stability WBP devices after MPPT. Additionally, C-F measurements show an increase of capacitance after degradation, highlighting the increase in mobile ions concentration. Collectively, these characterization techniques provide a comprehensive assessment of the stability of wide-bandgap perovskite solar cells under thermal and operational stress conditions.