image: Figure 1: Simulated (a)-(c) and experimental (d)-(f) TrEL results for a 1 kHz signal with varying duty cycles. (a) First simulated pulse, starting from rest. (b) Simulated TrEL signals for converged-state pulse after 0.2 seconds (200th pulse). (c) Simulated overshoot with normalized plateau value for 200th pulse. (d) Experimental low temperature TrEL signal measured at 175 K and 3.5 V. (e) Experimental TrEL signal at 1.7 V measured at room temperature. (f) Experimental overshoot with normalized plateau value.
Credit: ©Science China Press
Researchers at Zurich University of Applied Sciences (ZHAW), in collaboration with international partners, have published a study to better understand the physics of metal halide perovskite light emitting diodes (PeLEDs). Perovskite semiconductor materials are highly promising to be used in energy-efficient and cost-effective lighting and displays, but maintaining stable light emission remains challenging due to the mixed ionic-electronic conductivity of perovskites. This work addresses the impact of mobile ionic defects on the light emission of PeLEDs under pulsed operation, using drift-diffusion simulations to explain the peculiar behaviour seen in experiments.
Transient electroluminescence signals (TrEL), i.e. light emission over time, were measured for a continuous train of microsecond-long voltage pulses. By varying the duty cycle, defined as the proportion of each cycle the voltage pulse is active (time-on) versus inactive (time-off), a rather constant TrEL was measured after turn-on, which unexpectedly showed higher intensity for the higher duty cycles. In addition, a large overshoot in light emission was observed upon turning off the pulse, which decreased and eventually disappeared with increasing duty cycle. By simulating the interaction between charge recombination mechanisms and mobile ions in a full PeLED device, the researchers were able to explain the experimental results.
The inclusion of mobile ions into the simulations was found to be crucial, as without them, the light emission signal would stay the same for all duty cycles. For a single microsecond-long voltage pulse, ionic transients are not observable, but the pulse train's cumulative effect leads to varying but approximately static ionic distributions after enough pulses. In turn, the distribution of ionic defects in the perovskite layer changes the energy landscape in the device in a way which greatly affects how the injected electronic charges recombine. For the low duty cycles, before ions can redistribute, charges in the perovskite were found to accumulate close to the transport layers – resulting in higher recombination through defect states at the interfaces, lower overall light emission, and a subsequent overshoot. For the high duty cycles, after ions redistribute, less accumulation of charges near the interfaces during the voltage pulse results in higher light emission from direct band-to-band recombination, and no overshoot when the voltage is turned off.
Moreover, the simulations highlight injection energy barriers at the perovskite/charge-transport layer interfaces, such as is the case for the hole transport layer (HTL) in their example, as a significant source of non-radiative charge recombination. The researchers suggest that the degree to which ionic redistribution affects recombination processes in PeLEDs can be understood from their pulsed operation and point out that reducing non-radiative recombination at interfaces, either by defect passivation or improved energy level matching, should result in less pronounced dependence of the TrEL on the duty cycle. The study contributes valuable insights to better understand the effects of the mixed ionic-electronic conductivity in PeLED devices.
Supported by the European Union's Horizon 2020 research and innovation program, this research effort highlights the importance of understanding the complexities of ionic dynamics in the advancement of perovskite-based devices.
Funding: European Union's Horizon 2020 research and innovation program under grant agreement 851676 (ERC StGrt).
Journal
Science Bulletin
Method of Research
Computational simulation/modeling