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. Here we present a previously unknown pseudohalide, cyanate (OCN-), with a comparable effective ionic radius (1.97 angstrom) to bromide (1.95 angstrom) as a bromide substitute. Electron microscopy and X-ray scattering confirm OCN incorporation into the perovskite lattice. This contributes to notable lattice distortion, ranging from 90.5 degrees to 96.6 degrees, a uniform iodide-bromide distribution and consistent microstrain. Owing to these effects, OCN-based perovskite exhibits enhanced defect formation energy and substantially decreased non-radiative recombination. We achieved an inverted perovskite (1.93 eV) single-junction device with an open-circuit voltage (VOC) of 1.422 V, a VOC x 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. Triple-junction solar cells with cyanate in ultrawide-bandgap perovskites exhibit enhanced defect formation energy and substantially decreased non-radiative recombination.