Transient Photoluminescence Reveals the Dynamics of Injected Charge Carriers in Perovskite Light-Emitting Diodes.

Understanding the dynamics of injected charge carriers is crucial for the analysis of the perovskite light-emitting diode (PeLED) operation. The behavior of the injected carriers largely dictates the external quantum efficiency (EQE) roll-off at high current densities and the temperature dependence of the EQE in PeLEDs. However, limitations such as sample capacitance and external circuitry hinder precise control of carrier injection rates, making it challenging to directly track the dynamics of individual carriers. Here, we explore the recombination dynamics of injected charge carriers in a small-grain methylammonium lead iodide (MAPI) PeLED pumped at high current densities by investigating the dynamics of additional carriers photogenerated by ultrashort optical pulses. We show that photogenerated charge carriers predominantly recombine in a geminate fashion within a single perovskite grain. Conversely, recombination between photogenerated and injected carriers is rare, even at current densities up to 100 A/cm, due to the spatial separation caused by the internal electric field, which confines injected carriers near opposite electrodes. This spatial separation is a key mechanism behind the EQE roll-off in PeLEDs, with reduced carrier mobility at lower temperatures, mitigating this effect by weakening carrier localization and electron-hole separation.