In a paper recently published in the journal ACS Applied Energy Materials, researchers carried out the color implementation of perovskite solar cells (PSCs) using multilayered transparent AZO/Ag/AZO bottom layer electrodes. The top optoelectronic controlling layer’s (OCL’s) thickness was also tuned via optical interference in this implementation.
Study: Color Implementation of High-Efficiency Perovskite Solar Cells by Using Transparent Multilayered Electrodes. Image Credit: Lek Changply/Shutterstock.com
Due to advancements in material chemistry, structural design, device physics, and process engineering, the power conversion efficiency (PCE) of PSCs has increased to 25.7% from 3.8%. Commercialization of PSCs continues to be a challenge due to the lack of large-area substrate-based reproducible fabrication, thermal stability, color application, and moisture resistance. Color implementation of PSCs is accomplished by two processes: (1) perovskite framework engineering and (2) thickness adjustment of specific layers like hole transport layers, perovskite absorption layers, and electrodes.
Owing to the complex framework and difficulties related to effective tuning of the thickness, the PCE values continue to be low. For color implementation, indium tin oxide (ITO) employed in PSCs would require tuning in a range greater than 250 nm. When compared to traditional ITO electrodes, oxide/metal/oxide (OMO) electrodes display superior flexibility.
Furthermore, since each OMO layer is allocated to a top OCL, a bottom layer, and a metal layer to permit sheet resistance control and independent transmittance. Consequently, OMO electrodes with differing reflectance, low resistance, and transmittance were created by tweaking the OCL thickness while maintaining fixed metal layer and bottom layer thicknesses.
About the Study
In this study, magnetron sputtering was used to deposit transparent AZO/Ag/AZO multilayered electrodes, while a four-point probe was utilized to test the OMO electrodes' sheet resistance lined up at intervals of 1 mm. The optical characteristics of the OMO electrodes, such as reflectance and film transmittance, were analyzed using an ultraviolet (UV)-vis spectrometer.
The chemicals used included dimethyl sulfoxide (DMSO), anhydrous N,N-dimethylformamide (DMF), lead(II) iodide (PbI2), toluene, and methylammonium iodide (MAI). OMO electrodes were trimmed to 25 x 25 mm and processed for 5 minutes with UV-ozone to increase wettability with no extra cleaning. By dissolving PbI2 and MAI in a DMF/DMSO combination, MAPbI3 was created. Spin coating of the poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) solution was carried out to deposit the PTAA layer on the OMO electrodes. A source meter was used to record the current density-voltage (J-V) curves.
The OCL thickness controlled the OMO electrodes' transmittance. The decrease or increase in the top OCL thickness in comparison to the 50 nm top OCL thickness decreased the transmittance. Due to the consistent metal layer thickness, the OMO electrodes' sheet resistance was maintained independently of the OCL thickness.
Furthermore, the resistance of the OMO electrodes was lower than that of the traditional single-layer ITO electrodes. The reflectance peaks were efficiently adjusted by tweaking the thickness of the top OCL. Even a slight change in the OCL thickness resulted in considerable reflectance differences. Furthermore, the optical interference caused by a minor difference in the OCL thicknesses can help in the development of the OMO electrodes/perovskite in a variety of colors.
The electrical and optical losses were due to the surface reflection qualities of OMO electrodes as well as interfacial defects between the hole transport layer (HTL) and OMO electrodes. However, the fabricated PSCs significantly reduced the losses due to the structural refinement of OMO electrodes in relation to the OCL thickness.
Furthermore, the HTL/OMO electrode defects had a negligible influence, as evidenced by the steady fill factor (FF) and open circuit voltage (VOC). The shunt resistance values obtained from the JV curve's slopes around the short-circuit current density (JSC) point were high in all PSCs that utilized OMO electrodes. This result demonstrated that PSCs using OMO electrodes were unaffected by unsuspected defects and were PSC-compatible.
The external quantum efficiency (EQE) curves of all PSCs utilizing OMO electrodes showed that PSCs using OMO (50/13/50) had better charge collection and generation over the whole wavelength range. Furthermore, the low reflectance promotes light transmission leading to an increased EQE depending on the wavelength. The findings validated the suitability of OMO electrodes for color implementation in PSCs.
To summarize, the researchers used OMO electrodes to propose a unique method for color implementation of PSCs. It was determined that PSCs that employed OMO (50/13/50) had the greatest PCE of 18.3% due to high JSC due to low sheet resistance and high transmittance of OMO electrodes.
According to the authors, JSC could be enhanced in subsequent research to decrease optical reflection loss with an increase in the incident light entering the OMO electrodes. The PCEs of PSCs that employ OMO electrodes could be improved by further optimizing the top OCL.
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Park, C., Lim, J. Wook, Heo, J. Hyuck, Im, S. Hyuk, Color Implementation of High-Efficiency Perovskite Solar Cells by Using Transparent Multilayered Electrodes, ACS Applied Energy Materials, 2022, DOI: https://doi.org/10.1021/acsaem.2c01658.
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