Triple-junction solar cells with cyanate in ultrawide bandgap perovskites.

Perovskite bandgap tuning without quality loss makes perovskites unique among solar absorbers, offering promising avenues for tandem solar cells1,2 . However, minimizing the voltage loss when their bandgap is increased to above 1.90 eV for triple-junction tandem use is challenging3-5 . We present a novel pseudohalide, cyanate (OCN- ), with a comparable effective ionic radius (1.97 Å) to bromide (1.95 Å) as a bromide substitute. Electron microscopy and X-ray scattering confirm OCN incorporation into the perovskite lattice. This contributes to significant lattice distortion, ranging from 90.5° to 96.5°, a uniform iodide/bromide distribution and consistent microstrain. Due to these effects, OCN-based perovskite exhibits an enhanced defect formation energy and significantly decreased nonradiative recombination. We achieved an inverted perovskite (1.93 eV) single-junction device with an open-circuit voltage (VOC ) of 1.422 V, a VOC × FF (fill factor) product exceeding 80% of the Shockley-Queisser limit and stable performance under maximum power point tracking, culminating in a 27.62% efficiency (27.10% certified efficiency) perovskite-perovskite-silicon triple-junction solar cell with 1 cm2 aperture area.