The study addresses the challenge of thermal degradation in mixed Sn-Pb perovskite solar cells by incorporating carboranes, specifically ortho-carborane (o-CB), into the perovskite layer. O-CB, known for its efficient heat transfer capability, is selected due to its low thermal hysteresis. The introduction of o-CB improves thermal conductivity, reducing heat accumulation and lowering surface temperatures by about 5 °C under illumination. This results in enhanced thermal stability, with mixed Sn-Pb cells retaining 80% of their initial efficiency after aging at 85 °C for 1080 hours. When integrated into monolithic all-perovskite tandems, the efficiency exceeds 27%, and the tandem cell maintains 87% of its initial power conversion efficiency (PCE) after 704 hours of continuous operation under illumination. The improved thermal stability is attributed to better heat dissipation, reduced charge recombination, and enhanced film quality. The study also demonstrates the universality of this approach in improving the thermal stability and performance of pure lead perovskite solar cells.The study addresses the challenge of thermal degradation in mixed Sn-Pb perovskite solar cells by incorporating carboranes, specifically ortho-carborane (o-CB), into the perovskite layer. O-CB, known for its efficient heat transfer capability, is selected due to its low thermal hysteresis. The introduction of o-CB improves thermal conductivity, reducing heat accumulation and lowering surface temperatures by about 5 °C under illumination. This results in enhanced thermal stability, with mixed Sn-Pb cells retaining 80% of their initial efficiency after aging at 85 °C for 1080 hours. When integrated into monolithic all-perovskite tandems, the efficiency exceeds 27%, and the tandem cell maintains 87% of its initial power conversion efficiency (PCE) after 704 hours of continuous operation under illumination. The improved thermal stability is attributed to better heat dissipation, reduced charge recombination, and enhanced film quality. The study also demonstrates the universality of this approach in improving the thermal stability and performance of pure lead perovskite solar cells.