Light-Induced Degradation of Metal-Free Organic Perovskites

Perovskites have attracted great interest in optoelectronics and photonics. As a new class of perovskites, metal-free perovskites have drawn growing attention due to the absence of toxic metal elements in them and their wide chemical diversity. Taking MDABCO-NH₄I₃ (MDBACO = N-methyl-N'-diazabicyclo[2.2.2]octonium) and MDABCO-NH₄Br₃ as examples, our first-principles calculations discover two fundamental features of metal-free perovskites that should be crucial for their applications. First, their photoluminescence emission originates from halogen vacancies, instead of the self-trapped exciton generally suggested. Second, in the vicinity of a halogen vacancy, optical excitation from the valence bands to the empty defect bands may cause release of H₂ and NH₃ molecules, which will not only lead to degradation of the perovskite but also quench its photoluminescence. To prevent the degradation and protect the optoelectronic and photonic performances of metal-free perovskites, short-wavelength illumination needs to be shielded.