Ultrafast cascaded energy transfer triggers highly luminescent Te ⁴⁺ -doped Cs ₂ ZrCl ₆ perovskites

The practical viability of lead-free perovskites is often constrained by their instability and low photoluminescence quantum yield (PLQY). Here, we achieved a thermally stable perovskite with a PLQY of 90.5% on Te 4+ -doped Cs 2 ZrCl 6 material through ionic-liquids (ILs)-mediated crystal growth. Temperature-dependent PL spectroscopy reveals that ILs treatment raises the thermal activation energy up to 109.9 meV, attributed to ILs-induced retarded crystal growth, which enables effective defect passivation and suppression of defect-assisted electron–phonon coupling process. Femtosecond transient-absorption spectroscopy further reveals ultrafast cascaded energy transfer from Cs 2 ZrCl 6 host to singlet state of doped Te 4+ and subsequent intersystem crossing (ISC) to its triplet state, with kinetic analysis demonstrating that ILs treatment drastically accelerates the route by shortening the host-to-dopant transfer time from 149.0 to 31.0 ps and the ISC time from 2115.5 to 732.9 ps compared to that of the untreated sample. These results reveal that ILs-mediated defect passivation can promote the cascaded energy-transfer process, thereby boosting perovskite PLQY.