Strain coupling and Jahn-Teller effect in efficient and stable sky-blue germanium-lead perovskites

Substitution of lead (Pb) with environmentally friendly germanium (Ge) is a very important and promising method to reduce the toxicity and improve the efficiency and stability of perovskite films for practical applications in light-emitting diodes (LEDs) and solar cells. However, a previous study shows that the efficiency of LEDs based on the germanium-lead (Ge-Pb) perovskites will decrease as the Ge content exceeds 30%, due to trap-assisted recombination. Here we demonstrate highly luminescent sky-blue Ge0.3Pb0.7-based perovskite films with photoluminescence quantum efficiencies (PLQEs) of up to 48%, showing a relative improvement of 17% over similarly prepared Ge-free, Pb-based ones. The PL stability of the Ge0.3Pb0.7-based perovskite films is also relatively improved compared to that of the Pb-based ones. The reasons of the PL efficiency and stability improvement are discussed in this study. The symmetry-adapted shear strain variations related to the suppression of the Jahn-Teller distortion after Ge substituting could play an important role in increasing the stability of the Ge-inclusion perovskites.