Cool carriers: triplet diffusion dominates upconversion yield

Perovskites have gained popularity both as the active material in photovoltaics and as bulk triplet sensitizers for solid-state triplet-triplet annihilation upconversion (TTA-UC). Prior to widespread implementation into commercial photovoltaics, an in-depth understanding of the environmental influences on device performance is required. To this point, the temperature-dependent structure-function properties of TTA-UC within methylammonium formamidinium lead triiodide (MAFA)/rubrene UC devices are explored. A strong temperature dependence of the underlying UC dynamics is observed, where the maximum UC efficiency is achieved at 170 K, reflecting the competition between triplet diffusion length, diffusion rate, and triplet-triplet encounter events. A combination of spectroscopic and structural methods and theoretical modelling illustrates that despite the significantly increased carrier lifetime of the perovskite at low temperatures, the TTA-UC dynamics are not governed by the underlying sensitizer properties but rather limited by the underlying triplet diffusion.