Perovskite solar cells (PSCs) have shown exceptional photoelectric properties and ease of fabrication, reaching certified power conversion efficiencies (PCEs) of 26.1%, rivaling commercial silicon solar cells at lower costs. Mixed tin (Sn)-lead (Pb) perovskites, with their ideal bandgap of around 1.25 eV, are increasingly important for boosting PCEs and are crucial components in all-perovskite tandem solar cells. However, enhancing their efficiency and stability is essential for commercialization. Heat generation in solar cells, from phonon dissipation and Joule heating, is detrimental to stability and efficiency. Perovskites' poor thermal conductivity compared to silicon leads to heat accumulation and hotspots, causing thermal degradation. While thermal regulation improves performance and durability in other technologies, it has received limited attention in mixed Sn-Pb PSCs and tandems. The inferior thermal stability and oxidation of Sn²⁺ in mixed Sn-Pb perovskites create surface defects, further generating heat during recombination. Therefore, enhancing thermal conductivity and heat transfer is crucial. Carboranes, electron-delocalized carbon-boron molecules, offer high thermal conductivity and stability. This study uses *ortho*-carborane (*o*-CB) due to its low thermal hysteresis, aiming to improve heat dissipation and stability in mixed Sn-Pb perovskite solar cells and tandems.

The literature extensively covers the advancements and challenges in perovskite solar cell technology. Studies highlight the record-breaking efficiencies achieved and the ongoing efforts to improve stability and scalability. Research focuses on addressing issues such as thermal degradation, moisture sensitivity, and ion migration. Several papers discuss the use of various additives and materials to enhance perovskite film quality and device performance, including the exploration of different perovskite compositions and architectures. The use of thermal management techniques in other semiconductor devices has been thoroughly investigated, underscoring the importance of efficient heat dissipation for long-term stability. Existing literature on Sn-Pb perovskites reveals their potential for achieving higher efficiencies but also emphasizes their susceptibility to degradation due to factors such as Sn oxidation. The use of carboranes in other applications has been explored, showing promise in enhancing thermal and electrical properties.

The study employed *ortho*-carborane (*o*-CB) as a thermal regulation additive in mixed Sn-Pb perovskite solar cells. The *o*-CB was incorporated into the precursor solutions of mixed Sn-Pb perovskites at varying concentrations (0, 1, and 2 mg/mL). The resulting perovskite films were characterized using various techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM), and thermal conductivity measurements. The thermal conductivity was measured using the hot-disk technique. Infrared thermal imaging was used to visualize heat dissipation. The photothermal response was investigated using T-type thermocouples. Density functional theory (DFT) calculations were performed to understand the interactions between *o*-CB and the perovskite. Ultraviolet photoelectron spectroscopy (UPS) was employed to analyze the interfacial energy levels. Photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements were used to study the charge recombination dynamics. Inverted p-i-n PSCs were fabricated with the modified perovskite layers and their photovoltaic performance was evaluated by measuring J-V characteristics, external quantum efficiency (EQE), and stabilized power output (SPO). Long-term thermal stability tests were conducted at 85 °C, and operational stability was assessed under continuous one-sun illumination. All-perovskite tandem solar cells were fabricated using the optimized *o*-CB-treated low-bandgap mixed Sn-Pb subcells. The fabrication process involved multiple steps including substrate cleaning, layer deposition (PEDOT:PSS, perovskite, C₆₀, SnO₂, Ag), and annealing. Specific details of the precursor solutions and deposition methods are provided in the paper's methods section. Detailed characterization techniques used included XRD, SEM, XPS, UPS, UV-Vis, PL, TRPL, TPV, and J-V measurements, as well as Ansys Icepak simulation for heat dissipation analysis.

XRD and SEM analyses showed that the introduction of *o*-CB improved the thermal stability of the perovskite films. The PbI₂ peaks, indicative of perovskite degradation, were significantly lower in the *o*-CB treated films after thermal treatment. The SEM images revealed a uniform grain structure in the *o*-CB treated films, contrasting with the visible decomposition and second phase flakes observed in the pristine films. Thermal conductivity measurements confirmed that the *o*-CB treatment increased the through-plane thermal conductivity of the perovskite films. Infrared thermal imaging demonstrated faster cooling and slower heating in the *o*-CB treated films compared to pristine films. Temperature profiling under illumination showed that *o*-CB treatment lowered the surface temperature of the perovskite layer by approximately 5 °C. DFT calculations showed that *o*-CB interacts with the perovskite surface, reducing electron density in the upper layer and facilitating charge transfer. UPS measurements confirmed a shift in the valence band maximum and an upshift in the Fermi level after *o*-CB treatment, indicating reduced interfacial recombination. PL and TRPL measurements indicated a longer lifetime and reduced defect density in the *o*-CB-treated films. The *o*-CB treated PSCs showed significantly enhanced photovoltaic performance. The champion device exhibited a PCE of 23.4% with improved Jsc, Voc, and FF compared to the pristine device. Long-term thermal stability tests demonstrated that *o*-CB-treated devices retained 80% of their initial PCE after 1080 h at 85 °C, while pristine devices retained only 40%. The operational stability under continuous illumination also showed significant improvement. All-perovskite tandem solar cells incorporating the *o*-CB treated low-bandgap subcells achieved a champion PCE of 27.2%, maintaining 87% of its initial PCE after 704 h of continuous operation. The improved performance was attributed to enhanced thermal conductivity, reduced interfacial recombination, and improved perovskite film quality.

The findings demonstrate that incorporating *o*-CB effectively addresses the thermal instability challenges in mixed Sn-Pb perovskite solar cells and all-perovskite tandem devices. The improved thermal conductivity and reduced interfacial recombination contribute significantly to enhanced performance and long-term stability. The 5 °C reduction in surface temperature under illumination directly impacts the thermal degradation pathways, leading to the observed improvements in operational and thermal stability. The DFT calculations and UPS results provide mechanistic insights into the interactions between *o*-CB and the perovskite, confirming the role of *o*-CB in facilitating charge transfer and suppressing interfacial recombination. The results are highly significant for the field of perovskite solar cells as they provide a simple yet effective strategy to overcome a major limitation hindering the widespread adoption of this promising technology. The success in integrating the *o*-CB treated subcells into highly efficient tandem solar cells further validates the approach's potential for commercial applications.

This study presents a sustainable thermal regulation strategy for enhancing the stability and efficiency of mixed Sn-Pb and all-perovskite tandem solar cells using *ortho*-carborane (*o*-CB). *o*-CB treatment significantly improves thermal stability, reduces recombination, and enhances device performance. High PCEs of 23.4% (single-junction) and 27.2% (tandem) were achieved, with excellent long-term stability. Further research could explore other carborane isomers or investigate the integration of this strategy with other passivation techniques to further optimize the performance and stability of perovskite solar cells.

The study focuses on specific perovskite compositions and device architectures. The generalizability of the findings to other perovskite systems needs further investigation. While the study demonstrates significant improvements in long-term stability, long-term testing under various environmental conditions (e.g., high humidity, UV exposure) is necessary to fully assess the long-term durability. The encapsulation method used could affect the stability results; other encapsulation methods should be explored.