In an article recently published in the journal ACS Energy Letters, researchers discussed negative transient spikes in the halide perovskites.
Study: Negative Transient Spikes in Halide Perovskites. Image Credit: Audio und werbung/Shutterstock.com
Even while halide perovskite solar cells and a wide range of related devices have made great strides, critical features of the kinetic characteristics of these electronic and photovoltaic systems are still not fully understood. Analysis of the transient dynamics of photovoltaic perovskites has shown some fairly peculiar and exciting properties throughout the years as they have become the new star material for solar power.
The fact that the current transient dynamics frequently display a photoinduced negative spike, where the response drops to a minimum value before rising to the final equilibrium state, is one of the most pertinent and unanswered questions in this regard and is still the subject of active discussion.
This peculiar characteristic has been primarily described by an effect in which the applied stimuli lead to the formation of a transient ionic configuration, where an inverted polarization results in an electrical field that generates the first negative current. A time-dependent drift-diffusion model does a good job of describing the general form of transients, but it is unable to account for the negative overshoots that occur immediately following step changes and over extended periods of time.
In this way, it has been suggested that ionically gated transistors could record the apparent capacitive and inductive behavior of perovskite solar cells. The problem is far from being well-formulated by an analytical representation of the photocurrent dynamics in the time domain, despite the physical interpretation of the negative time transients.
About the Study
In this study, the authors presented a comprehensive analysis of the reported chemical mechanisms via independent measurement techniques in the frequency and time domain to address a persistent problem in the research and development of perovskite solar cells for more than ten years. The proposed investigation of the light-dependent negative overshoot photocurrent events in the time-domain discharge of the chemical inductor, a transversal mechanism present in a wide range of chemical, biological, and material systems, was based on an operational approach.
The team discussed the discharge of the inductive component isolated from the impedance in terms of the negative overshoots in the time photocurrent transients. They showed how the interpretation framework of the chemical inductor helped independent techniques correspond.
The researchers examined devices with almost ideal electrical behavior and no discernible dispersive effects under a range of voltage/illumination circumstances in order to determine the strengths of the proposed model for the description of the unusual transient occurrences. The proposed layer structure had an active layer of FTO/c-TiO2/m-TiO2/perovskite/spiro-OMeTAD/Au which exhibited a composition based on Rb/Cs/MA/FA/Pb/I/Br. These quadruplication perovskite solar cells were created with the nominal formula Rb0.05Cs0.05MA0.15FA0.75Pb1.05(I0.95Br0.05)3.
The transient spike occurred for samples with good equilibration, and the parameters from impedance measurements and photocurrent transient decays were the same. This match made it possible to rationally characterize perovskite devices in a consistent and repeatable manner since the equilibrium parameters derived from the spectroscopic response could be studied individually. The winning device had a fill factor (FF) of 78%, a short-circuit photocurrent density Jsc of 22 mA/cm2, an open-circuit voltage Voc of 1.11 V, and a power conversion efficiency (PCE) of roughly 19.1%.
An FTO/PEDOT:PSS/MAPI/Au memristor exhibited symmetrical photocurrent while devices with TiO2 layers exhibited directional photocurrent. Each time, a change from capacitive hysteresis at low voltage to inductive control at high forward bias was shown as a strong inverted hysteresis in the region past Voc that corresponded to the intense forward bias. This demonstrated that the inductor was a universal phenomenon that could arise in nearly symmetrical systems without selected contact layers and for diverse transport layers. The underlying physicochemical causes of these impacts, however, were unclear.
In conclusion, this study elucidated the normalized current-voltage curves for a typical photovoltaic device that was measured under a range of light illuminations using a sweep rate of 10 mV/s. The authors examined the proportional voltage- and light-dependent photocurrent undershoot.
They mentioned that the current study offers a solid resolution to the interpretation issue around this anomalous phenomenon that has been seen in perovskite devices for more than ten years. They stated that the findings of this study provide a broad foundation for comprehending the inductive transitory effects seen in novel and significant uses of halide perovskites, which can mimic the electrical activity of synapses and neurons when used as memristors.
The team believes that the current study can also serve as a foundation for a suitable explanation of unexpected events seen using techniques that are extremely beneficial. They also emphasized that for the proper operation of novel halide perovskite applications, such as power electronic components photodetectors, it is crucial to comprehend the frequent negative transient spikes.
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Fu, R., Lu, W., Guo, Y., et al. Negative Transient Spikes in Halide Perovskites. ACS Energy Letters (7) 2602-2610 (2022). https://pubs.acs.org/doi/10.1021/acsenergylett.2c01252