Stabilization of the Perovskite Phase
- Mixing A-site cations such as formamidinium (FA), methylammonium (MA), and cesium (Cs) creates an entropic stabilization effect. The configurational entropy gained from mixing reduces the tendency of the material to transition into undesirable non-perovskite phases (e.g., the yellow δ-phase of FAPbI3), thereby stabilizing the photoactive black perovskite phase at room temperature.
- For example, pure FAPbI3 and CsPbI3 tend to form unstable yellow phases under ambient conditions, but their mixture, such as Cs0.2FA0.8PbI2.84Br0.16, maintains a stable black perovskite structure, essential for photovoltaic activity.
Enhanced Optoelectronic Properties and Efficiency
- Mixed A-cations improve light harvesting especially in the near-infrared region, broadening the absorption spectrum and enhancing the power conversion efficiency (PCE) of the cells.
- Combinations like FA and MA have shown the best performance so far, delivering high efficiencies over 17% with minimal hysteresis effects.
Improved Long-Term Stability
- Mixed halides (e.g., partial substitution of iodide by bromide) combined with mixed cations contribute to enhanced thermal and ambient stability by reducing ion migration and structural degradation under environmental stresses like moisture, oxygen, light, and heat.
- Advanced fabrication techniques with mixed A-site cations yield perovskite films with large crystal grains, low trap densities (~1015 cm-3), and high crystal quality, which are critical factors for long-term operational stability.
- Solar cells with mixed-cation perovskites have demonstrated excellent stability in ambient air conditions for extended periods (over 7000 hours), retaining a large fraction of their initial efficiency even without encapsulation.
Underlying Physical Mechanisms
- First-principle calculations show that the small internal energy required to form solid solutions of mixed cations alongside the entropic gain outweighs the formation energy of undesired phases, thus promoting phase stability.
- The mixed A-cation and halide compositions optimize the optoelectronic landscape by tuning bandgaps (e.g., 1.48 eV for FAPbI3, 1.73 eV for CsPbI3), which helps to balance efficiency and stability.
In summary, the combined use of mixed A-site cations and halides stabilizes the desired perovskite phase through entropic effects and energy considerations, improves optoelectronic properties, and enhances environmental and thermal stability, all of which drive the improved performance and durability of perovskite solar cells.
Original article by NenPower, If reposted, please credit the source: https://nenpower.com/blog/what-role-do-mixed-a-cations-and-halides-play-in-improving-the-stability-of-perovskite-solar-cells/
