Ion Motion Determines Multiphase Performance Dynamics of Perovskite LEDs

Abstract

them, inherent to all perovskite devices, is the short-term and long-term stability. PeLEDs also suffer from efficiency rolloff limiting their brightness. [7][8][9][10] Another issue, particularly important for the highspeed device operation, is the occurrence of hysteresis [1,11] and variations of the electroluminescence (EL) EQE and photoluminescence (PL) efficiency [3,12] under electrical stress.The performance hysteresis in perovskite solar cells was widely investigated and was attributed to the changes of perovskite-transport layer interfaces or to the motion of mobile ions. [13][14][15][16] Ion motion in metal-halide perovskites is a very complex and still poorly understood phenomenon. Generally, all constituents of metal halide perovskites-halides, lead, and organic cations-may form vacancies, interstitials, and anti-sites, which may be mobile and act as carrier traps. [16][17][18] On the other hand, theoretical evaluations show that only few of them form deep traps acting as non-radiative recombination centers, [17] while shallow traps have a negligible effect on a trap-assisted recombination. [19] In lifetime measurements of PeLEDs, it is constantly observed that different PeLED stacks improve over a timeframe of minutes to hours, then they start to degrade. [3,[20][21][22] Persistent enhancement of the EQE of PeLED, under applied positive voltage (so-called stressing regime), has been explained by the motion of the mobile excess iodide ions that fill vacancies and reduce Perovskite light-emitting diodes (PeLEDs) currently reach up to about 20% external quantum efficiency (EQE) and are becoming a promising technology for display and lighting applications. Still, many issues regarding their performance remain unresolved, particularly those related to stability, operation in non-stationary regimes, and efficiency roll-off at high current densities. Here, some of those issues in PeLEDs based on MAPbI 3 perovskite are addressed. The authors analyze the electroluminescence (EL) and current dynamics after the first-time voltage application and after application of sequences of voltage pulses, at different temperatures. Analysis of the results suggests that the complex dynamics observed on time scales from sub-seconds to minutes and hours can be explained by the spatial redistribution of mobile species, most likely iodine interstitials, characterized by ≈175 meV activation energy. This redistribution alters the carrier injection, spatial electric field, and charge carrier density distributions as well as density of nonradiative recombination centers within the perovskite layer. Mathematical modeling of the ion motion and related processes enabled to reproduce the EL and current dynamics and to disentangle complex sequence of processes governing the PeLED operation.

Intense Electrical Pulsing of Perovskite Light Emitting Diodes under Cryogenic Conditions

“…Not only does it form an energy barrier for carrier injection, but it also concentrates the injected carriers primarily at the different interfaces, which is detrimental to the radiative recombination. [ 13,20 ] When a forward DC bias is applied, these ions previously accumulated at the interfaces are drifted toward the opposite side, reversing the equilibrium ion distribution (Figure 4b). This will significantly facilitate charge injection and make radiative recombination more efficient.…”

“…The results discussed above can be summarized by a unified model shown in Figure 4, which extends the PeLED models previously reported in the literature. [ 13,20,29 ] In our previous work, we have shown through activation energy measurements that the dynamic performance of our PeLEDs is likely dominated by iodine negative ions and positive vacancies. [ 20 ] Initially, at RT in equilibrium (Figure 4a), due to the built‐in electric field, negative and positive ions are accumulated at electron and hole transport layer interfaces, respectively, leading to band bending at the interfaces.…”

“…[ 13,20,29 ] In our previous work, we have shown through activation energy measurements that the dynamic performance of our PeLEDs is likely dominated by iodine negative ions and positive vacancies. [ 20 ] Initially, at RT in equilibrium (Figure 4a), due to the built‐in electric field, negative and positive ions are accumulated at electron and hole transport layer interfaces, respectively, leading to band bending at the interfaces. Not only does it form an energy barrier for carrier injection, but it also concentrates the injected carriers primarily at the different interfaces, which is detrimental to the radiative recombination.…”

“…In our previous work, we have shown that ion motions in PeLEDs result in slow transients for current and EL on a subsecond to second or even longer timescale, which remarkably modifies the carrier injection behavior and recombination dynamics. [20,24] Thus, Ion migration and the final ion distribution determine the device characteristics. [20,21] In the current case, considering that even the most mobile ionic species have relatively high activation energies in the range of 100-300 meV, [25,26] ion migration in the PeLED is expected to be extensively slowed down or even frozen at low temperature.…”

“…[17] Moreover, cryogenic cooling of perovskite films and devices has also been shown to provide other benefits relevant for lasing, such as the exponential decrease in ASE threshold, [4,8,18] freezing of nonradiative recombination traps, higher device operational stability, etc. [19] Furthermore, considering that ion migration/distribution plays a crucial role in determining the optoelectronic characteristics of PeLEDs, [13,20,21] cryogenic cooling offers an opportunity to manipulate and freeze ion motions in the PeLED for an optimal device operation. However, despite the numerous advantages, intense electrical pulsing of PeLEDs under cryogenic conditions has not been studied yet, partially due to the scarcity of device stacks that can be properly operated at cryogenic temperatures.…”