Perovskites for solar cells have an ABX₃ structure where the cation A is MA, FA, or Cs; the metal B is Pb or Sn; and the halide X is Cl, Br or I. Unfortunately, single-cation perovskites often suffer from phase, temperature or humidity instabilities.
Recently, double-cation perovskites (using MA, FA or Cs, FA) were shown to have a stable “black phase” at room temperature.(1,2) These perovskites also exhibit unexpected, novel properties. For example, Cs/FA mixtures suppress halide segregation enabling band gaps for perovskite/silicon or perovskite/perovskite tandems.(3) In general, adding more components increases entropy that can stabilize unstable materials (such as the “yellow phase” of FAPbI₃ that can be avoided using the also unstable CsPbI₃). Here, we take the mixing approach further to investigate triple cation (with Cs, MA, FA) perovskites resulting in significantly improved reproducibality and stability.(4) We then use multiple cation engineering as a strategy to integrate the seemingly too small rubidium (Rb) (that never shows a black phase as a single-cation perovskite) to study novel multication perovskites.(5)
One composition containing Rb, Cs, MA and FA resulted in a stabilized efficiency of 21.6% and an electroluminescence of 3.8%. The V_oc of 1.24 V at a band gap of 1.63 eV leads to a very small loss-in-potential of 0.39 V.

Lastly, to explore the theme of multicomponent perovskites further, molecular cations were revaluated using a globularity factor. With this, we calculated that EA has been misclassified as too large. Using the multication strategy, we studied an EA-containing compound that yielded an open-circuit voltage of 1.59 V, one of the highest to date. Moreover, using EA, we demonstrate a continuous fine-tuning for perovskites in the "green gap" which is highly relevant for lasers and display technology.

The last part elaborates on a roadmap on how to extend the multication to multicomponent engineering providing a series of new compounds that are highly relevant candidates for the coming years.(6)

(1) Jeon et al. Nature (2015)

(2) Lee et al. Advanced Energy Materials (2015)

(3) McMeekin et al. Science (2016)

(4) Saliba et al., Cesium-containing triple cation perovskite solar cells: improved stability, reproducibility and high efficiency. Energy & Environmental Science (2016)

(5) Saliba et al., Incorporation of rubidium cations into perovskite solar cells improves photovoltaic performance. Science (2016).

(6) Turren-Cruz et al. Methylammonium-free, high-performance and stable perovskite solar cells on a planar architecture Science (2018)