Adding cesium to perovskite solar cells significantly increases their thermal and photostability, while maintaining high efficiency, a new study demonstrates. Metal halide perovskite photovoltaic cells are appealing because they have the potential to boost the efficiency of commercial silicon photovoltaic cells by 20 to 30%, when placed on top as a second layer. This boost occurs because perovskite cells can absorb a wider range of light, including higher-energy blue light; however, perovskite cells that reach the preferable optical band gap of ~1.75 electron volts (eV) include materials that have poor photostability and thermal stability. David McMeekin and colleagues were able to improve the stability of these cells by partially substituting the traditional formamidinium cations with some cesium cations. Whereas conventional perovskite cells undergo unstable phases, the addition of cesium created a single crystalline phase within the cell, making it much more stable. With an optical band gap of 1.74 eV, the cesium cell demonstrated 17% efficiency on its own. Layered upon silicon photovoltaics, the modified perovskite cell was able to boost the silicon cell efficiency by 7.3%. This boost, in theory, means it's feasible to achieve greater than 25% efficiency in perovskite/Si tandem cells, the authors say. As well, since perovskites are "tunable" to a spectrum of light, more stable perovskites hold important implications for light-emitting applications.