Pusan National University and University of California, San Diego Researchers Image Real-Time Structural Changes in Perovskite Cells
Researchers use microscopy to successfully observe the structural changes that cause early degradation of perovskite cells
Photovoltaics convert the energy from sunlight directly into electricity and the efficiency of these devices is key to effectively harness solar energy. In this regard, perovskites are potential alternatives to conventional silicon-based ones as high-performance photovoltaics. However, instabilities due to ion migration limit their commercial viability. Now researchers from Korea and the USA have used an imaging technique to observe structural changes at the atomic level suggesting strategies to reduce perovskite solar cell degradation.
Perovskite solar cells (PSCs) represent the next generation of solar cells owing to their low costs and high-power conversion efficiencies. However, they tend to degrade quickly. As it turns out, when the PCSs are exposed to sunlight, freely moving ion vacancies form in the structure and migrate towards the electrodes. In dark conditions, the effect is reversed, and the ions are once again redistributed in the perovskite structure. Repeated cycles of this ion transport during the operation of the solar cell permanently degrade the cell and result in short lifetimes. However, degradation at the atomic level due to ion migration has not been directly observed.
Now researchers from Korea and the USA have successfully observed changes in the perovskite structure using an in-situ transmission electron microscopy (TEM) imaging technique. “This work offers vital insights on understanding ion migration and addressing instability challenges of perovskite optoelectronics”, explains Assistant Professor Min-cheol Kim of the Department of Mechanical Engineering, Pusan University, who led the study that was published in the journal ACS Energy Letters on September 14, 2021.
While electron microscopy has the scope to image structures at the atomic scale, the instability of the perovskite structure towards an external electrical bias (voltage) makes it challenging to monitor it for structural changes. The researchers circumvented the problem by imaging perovskites under a stable voltage. They first prepared a thin layer (or lamella) of a nano perovskite solar cell. The lamella was placed between the electrodes of an electronic chip and a stable voltage was applied across the sample. This setup enabled the researchers to stimulate the photovoltage experienced by the solar cell under working conditions. A constant voltage of 1 V was applied across the lamella and the structure was monitored by TEM.
The researchers observed a gradual disappearance of the lattice fringes of perovskite crystals, which indicated the “amorphization” of the structure. As is the case with perovskite crystals, amorphization was observed to be reversible under dark conditions. However, using TEM, the researchers noticed that recrystallization was faster when the sample was subjected to mild heating at 50°C. “This work not only demonstrates the ability to achieve real-time imaging of perovskite amorphization but also provides an effective method to recover the degraded performance of PSCs related to ion instability,” comments Prof. Kim
Elucidating the changes in perovskite structure could enable researchers detect points of failure and build more resilient solar cells. In addition, observing the rate of recrystallization can help determine the effectiveness of strategies used to restore the performance of solar cells. The future of photovoltaics looks bright!
Reference
Authors: Min-cheol Kim1,2, Namyoung Ahn3, Diyi Cheng2, Mingjie Xu4, So-Yeon Ham2, Xiaoqing Pan4, Suk Jun Kim5, Yanqi Luo2, David P. Fenning2, Darren H. S. Tan2, Minghao Zhang2, Guomin Zhu2, Kiwan Jeong3, Mansoo Choi3*, and Ying Shirley Meng2
Title of original paper: Imaging Real-Time Amorphization of Hybrid Perovskite Solar Cells under Electrical Biasing
Journal: ACS Energy Letters
DOI: https://doi.org/10.1021/acsenergylett.1c01707
Affiliations:
1Pusan National University, Korea
2 University of California, San Diego, USA
3 Seoul National University, Korea
4 University of California, Irvine, USA
5 Korea University of Technology and Education, Korea
Lab address: https://sites.google.com/view/pnu-aned/home
ORCID id: 0000–0001–8292–8117
About Pusan National University
Pusan National University, located in Busan, South Korea, was founded in 1946, and is now the no. 1 national university of South Korea in research and educational competency. The multi-campus university also has other smaller campuses in Yangsan, Miryang, and Ami. The university prides itself on the principles of truth, freedom, and service, and has approximately 30,000 students, 1200 professors, and 750 faculty members. The university is composed of 14 colleges (schools) and one independent division, with 103 departments in all.
Website: https://www.pusan.ac.kr/eng/Main.do
About the author
Min-cheol Kim is an assistant professor at School of Mechanical Engineering of Pusan National University. He received a B.S. and Ph.D. in Department of Mechanical Engineering from Seoul National University (SNU), Korea, in 2012 and 2018, respectively under supervision of Prof. Mansoo Choi. He went to University of California San Diego (UCSD) for postdoctoral research in Prof. Shirley Meng’s group (2019–2021). Dr. Kim’s research has been focused on the development of perovskite photovoltaics by investigating degradation mechanisms and demonstrating highly efficient and flexible perovskite solar cells. Right before joining Pusan National University, he worked as a staff engineer at Samsung Electronics, performing research about device reliability for NAND devices. Now, his group is focusing on the development of functional nano devices including perovskite solar cells, Li-ion batteries, advanced nano characterization, and memristors. His work has been published in multiple high impact scientific journals such as Energy & Environmental Science, Advanced Energy Materials, ACS Energy Letters, Nano Letters, etc.
He heads the Advanced Nano Energy Device Laboratory at the University and his research can be found at https://sites.google.com/view/pnu-aned/home
