Perovskite Solar Cell Stability in Humid Air: Partially Reversible Phase Transitions in the PbI2-CH3NH3I-H2O System

Significance Statement

Organic-inorganic perovskite solar cells have shown dramatic developmental progress in recent years. Perovskite solar cells are easy to process and have high power conversion efficiency as well as the potential for low manufacturing cost.  Perovskite solar cells could become a viable alternative to crystalline silicon photovoltaics. While the latter technology has been commercialized for large production, the deployment of perovskite solar cells has been limited by performance degradation.

Instabilities in devices as well as materials, particularly due to reactions with water, limit the lifespan of perovskite solar cells. A 25-30-year lifespan may be required for perovskite solar cells to effectively compete with other photovoltaics currently in the marketplace.

Researchers led by Professor Michael Heben from the University of Toledo in collaboration with Professor Michael Gräetzel at Ecole Polytechnique Fédérale de Lausanne and Professor Ullrich Steiner at Adolphe Merkle Institute in Switzerland performed in-situ laser beam induced current mapping on perovskite solar cells in a moist environment. The work was done to analyze and understand the degradation modes for the cells in humid air. Their effort was the first to spatially- and time-resolve perovskite degradation by laser beam induced current analysis. Their work is published in the journal Advanced Energy Materials.

The authors conducted the experiment in four stages where they observed interesting aspects as the degradation proceeded. By analyzing the findings thoroughly, they were able to understand the processes at every stage of development. At the initial stage, the external quantum efficiency map was not smooth over the entire sample area. Despite taking great care during the process, the authors recorded a large variation in the external quantum efficiency from about 60%-80% in regions they thought would defect free. Therefore, they proposed that there was still great potential of improving the functionality of the perovskite solar cells by enhancing the uniformity of the current collection.

The authors discovered a peak in quantum efficiency that appeared 5-10 minutes after the films were exposed to moisture. This was explained by passivation of recombination centers by water molecules at the interface between the perovskite and spiro-OMeTAD layers in the device.  Water led to the improvement of the morphology and crystallinity of the films during the course of deposition, and led to improved device performance.

This study successfully investigated the evolution of the external quantum efficiency during the exposure of solar cells to water vapor. The outcomes of the tests proceeded through two stages that were controlled by water-induced carrier extraction and transport changes in the spiro-OMeTAD layer. During two other stages, there were changes in the perovskite material itself. The authors proposed phase equilibria within the PbI2-CH3NH3I-H2O system, consistent with their previous work (Song et al., Chemistry of Materials, 2015, 27, 4612), in order to understand the water-induced degradation and recommend a dehydration process prior to encapsulation to enhance device stability.

Perovskite Solar Cell Stability in Humid Air Partially Reversible Phase Transitions in the PbI2-CH3NH3I-H2O System - advances in engineering

About the author

Zhaoning Song is a Postdoc researcher in Dr. Michael Heben’s group at the University of Toledo. He received his BS degree in physics from Xiamen University, China, in 2009 and his PhD in physics from the University of Toledo in 2016. His research interests include solution processing of thin-film photovoltaics and nanomaterials for optoelectronic applications. The goal of his research is to develop more cost-effective alternatives to conventional PV technologies and to explore underlying physical processes of preparing the materials.

About the author

Antonio Abate is a research group leader at the Helmholtz-Centrum Berlin. He is an expert in hybrid organic-inorganic materials for optoelectronics. His group is currently researching active materials and interfaces to make stable perovskite solar cells. Before to move to the Helmholtz-Centrum Berlin, Antonio was leading the solar cell research at the Adolphe Merkle Institute and he was a Marie Skłodowska-Curie Fellow at École Polytechnique Fédérale in Switzerland. After getting his PhD at Politecnico di Milano in 2011, he worked for 4 years as a postdoctoral researcher at the University of Oxford and the University of Cambridge.

About the author

Suneth C. Watthage is a PhD candidate in the Department of Physics of the University of Toledo in the concentration of material science. He received his BS degree in engineering physics from the University of Colombo, Sri Lanka, in 2008 and his MS degree in engineering technology from Middle Tennessee State University in 2012. His research interests mainly focus on fabrication and characterization of nanomaterials for thin film photovoltaics and opto-electronic devices.

About the author

Geethika K Liyanage is a PhD candidate in the department of Physics and Astronomy, University of Toledo, Ohio, United States of America. He received his B.S. degree in Engineering Physics from University of Colombo, Colombo, Sri Lanka, in 2011. And he received his M.S. degree from Bowling Green State University, Bowling Green, Ohio, United States, in 2014. His current research is focused on vacuum based fabrication and characterization of photovoltaic devices on flexible substrates under the supervision of Dr. Michael Heben.

About the author

Adam B. Phillips received his BS degree in physics from Case Western Reserve University in 1999 and his PhD in physics from the University of Virginia in 2007. He joined the University of Toledo in 2008 and is currently a research associate professor at UT’s Wright Center for Photovoltaic Innovation and Commercialization. His research includes investigating low cost materials and methods for carbon-free energy sources.

About the author

Ullrich Steiner is a professor and a group leader of Soft Matter Physics at Adolphe Merkle Institute (AMI), Switzerland.

About the author

Michael Gräetzel is a professor of physical chemistry at the Ecole Polytechnique Fédérale de Lausanne (EPFL). Michael Gräetzel directs there the Laboratory of Photonics and Interfaces. He pioneered research in the field of energy and electron transfer reactions in mesoscopic systems and their use in energy conversion systems, in particular photovoltaic cells and photo-electrochemical devices for the splitting of water into hydrogen and oxygen and the reduction of carbon dioxide by sunlight as well as the storage of electric power in lithium ion batteries.

About the author

Michael J. Heben is a professor and the Wright Center Endowed chair for Photovoltaics in the Department of Physics and Astronomy at the University of Toledo (UT). He earned his MS degree in materials science and engineering from Stanford and his PhD in chemistry from Caltech under the guidance of N.S. Lewis. He became a postdoc with A.J. Nozik at SERI/NREL in 1990, and was a principal scientist and group leader when he left NREL for UT in 2008.

Reference

Zhaoning Song, Antonio Abate, Suneth C.  Watthage,  Geethika K. Liyanage, Adam B.  Phillips, Ullrich Steiner, Michael Graetzel, and Michael J.  Heben. Perovskite Solar Cell Stability in Humid Air: Partially Reversible Phase Transitions in the PbI2-CH3NH3I-H2O System. Advanced Energy Materials 2016, 6, 1600846.

Go To Advanced Energy Materials 

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