In a recent article published in the journal ACS Applied Energetic Materials, researchers achieved the color implementation of perovskite solar cells (PSC) using AZO/Ag/AZO multilayer transparent bottom layer electrodes. The thickness of the upper optoelectronic control (OCL) layer was also tuned via optical interference in this implementation.
Study: Color implementation of high-efficiency perovskite solar cells using transparent multilayer electrodes. Image Credit: Lek Changply/Shutterstock.com
Due to advances in materials chemistry, structural design, device physics, and process engineering, the power conversion efficiency (PCE) of PSCs has increased from 3.8% to 25.7 %. Commercialization of PSCs continues to be a challenge due to the lack of reproducible fabrication based on a large surface area substrate, thermal stability, color application, and humidity resistance. The color implementation of PSCs is accomplished through two processes: (1) engineering the perovskite framework and (2) adjusting the thickness of specific layers such as hole transport layers, absorption of perovskite and the electrodes.
Due to the complexity of the frame and the difficulties in effectively adjusting the thickness, the PCE values remain low. For color implementation, the indium tin oxide (ITO) used in PSCs would require tuning in a range greater than 250nm. Compared to traditional ITO electrodes, oxide/metal/oxide (OMO) electrodes exhibit superior flexibility.
Additionally, since each OMO layer is assigned an upper OCL, a lower layer, and a metal layer to enable sheet resistance control and independent transmission. Therefore, OMO electrodes with different reflectance, low resistance, and transmission were created by adjusting the thickness of the OCL while maintaining the fixed thicknesses of the metal layer and bottom layer.
About the study
In this study, magnetron sputtering was used to deposit transparent AZO/Ag/AZO multilayer electrodes, while a four-point probe was used to test the sheet resistance of OMO electrodes aligned at 1 mm intervals. Optical characteristics of OMO electrodes, such as film reflectance and transmittance, were analyzed using an ultraviolet (UV)-vis spectrometer.
Chemicals used included dimethyl sulfoxide (DMSO), anhydrous N,N-dimethylformamide (DMF), lead(II) iodide (PbI2), toluene and methylammonium iodide (MAI). OMO electrodes were cut to 25 x 25 mm and treated for 5 minutes with UV ozone to increase wettability without additional cleaning. By dissolving the PbI2 and MAI in a DMF/DMSO combination, MAPbI3 has been created. Spin poly coating[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) was performed to deposit the PTAA layer on the OMO electrodes. A source meter was used to record current-voltage (JV) density curves.
The thickness of the OCL controlled the transmission of the OMO electrodes. Decreasing or increasing the thickness of the upper OCL compared to the thickness of the upper OCL by 50 nm decreased the transmittance. Due to the constant thickness of the metal layer, the sheet resistance of the OMO electrodes was maintained independent of the thickness of the OCL.
In addition, the resistance of OMO electrodes was lower than that of traditional single-layer ITO electrodes. Reflectance peaks were effectively tuned by adjusting the thickness of the upper OCL. Even a slight change in the thickness of the OCL resulted in considerable differences in reflectance. Additionally, the optical interference caused by a minor difference in OCL thicknesses can aid in the development of OMO/perovskite electrodes in a variety of colors.
The electrical and optical losses were due to the surface reflective qualities of the OMO electrodes as well as interface defects between the hole transport layer (HTL) and the OMO electrodes. However, the fabricated PSCs significantly reduced the losses due to the structural refinement of the OMO electrodes relative to the thickness of the OCL.
Additionally, HTL/OMO electrode defects had negligible influence as evidenced by constant fill factor (FF) and open circuit voltage (VCO). The shunt resistance values obtained from the slopes of the JV curve around the short-circuit current density (JCS) were elevated in all CBPs that used OMO electrodes. This result demonstrated that PSCs using OMO electrodes were not affected by unsuspected defects and were compatible with PSCs.
External quantum efficiency (EQE) curves of all PSCs using OMO electrodes showed that PSCs using OMO (50/13/50) had better charge collection and generation across the entire wavelength range. Additionally, the low reflectance promotes light transmission leading to increased EQE as a function of wavelength. The results validated the suitability of OMO electrodes for color implementation in PSCs.
In summary, the researchers used OMO electrodes to provide a unique method of implementing the color of PSCs. CSPs that employed OMO (50/13/50) were determined to have the largest PCE of 18.3% due to the high JCS due to low sheet resistance and high transmission of OMO electrodes.
According to the authors, JCS could be improved in further research to reduce optical reflection loss with increased incident light entering the OMO electrodes. PCEs of PSCs that use OMO electrodes could be improved by further optimizing the upper OCL.
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Park, C., Lim, J. Wook, Heo, J. Hyuck, Im, S. Hyuk, Color implementation of high-efficiency perovskite solar cells using transparent multilayer electrodes, ACS Applied Energetic Materials2022, DOI: https://doi.org/10.1021/acsaem.2c01658.