Achieving 3.31 V in Triple‐Junction Perovskite–Organic Tandem Solar Cells via Lattice Distortion–Induced Phase Stability

Tandem solar cells offer an approach to overcoming the Shockley–Queisser limit inherent to single‐junction solar devices. Perovskite‐based triple‐junction cells, particularly the perovskite‐perovskite‐organic (PPO) configuration, offer a balance of high theoretical power conversion efficiency (PCE), processability, and reduced manufacturing complexity. However, their PCE is often limited by the front wide‐bandgap perovskite sub‐cell due to halide segregation, non‐radiative recombination, and poor interfacial quality between stacked layers. Herein, we report the development of triple‐junction PPO solar cells that overcome these limitations, offering improved performance. We show that incorporating rubidium (Rb + ) ions into the A‐site of the wide‐bandgap Cs 0.40 FA 0.60 Pb(I 0.40 Br 0.60 ) 3 perovskite induces lattice distortion, suppressing halide segregation and reducing mobile‐ion concentration. The ensuing cells show an open‐circuit voltage ( V OC ) of 1.46 V, which is among the highest reported values for ultra‐wide‐bandgap PSCs. For the mid‐bandgap perovskite sub‐cell, urea addition promotes grain growth, resulting in a denser film that prevents crack formation during deposition atop the wide‐bandgap front sub‐cell. By combining the two perovskite sub‐cells with an organic bulk‐heterojunction sub‐cell, and guided by optical simulations, we developed triple‐junction tandem devices with record V OC (3.31 V) and PCE (23.03%) for the PPO configuration. Our work underscores the role of compositional engineering in advancing perovskite‐based multi‐junction photovoltaics.