Aditya Mohite (left), Jean-Christophe Blancon (middle) and Wanyi Nie (right) from Los Alamos National Laboratory are part of a team of researchers that have discovered both the cause and a solution for the tendency of perovskite solar cells to degrade in sunlight. Photo: Los Alamos National Laboratory.A new study has found both the cause and a solution for the pesky tendency of perovskite solar cells to degrade in sunlight, potentially removing one of the roadblocks to the commercialization of this promising photovoltaic technology.
In a key finding, a team of researchers from the US and France has discovered that degraded perovskite solar cells exhibit self-healing powers when given a little time in the dark. The team determined that photo-degradation in perovskite cells is a purely electronic process caused by charge accumulation, which occurs without chemical damage to the perovskite crystal structure and so can be reduced. Furthermore, the cells' self-healing properties allow them to rebound in the dark.
"We can stabilize the device performance by controlling the environmental temperature," said Wanyi Nie at Los Alamos National Laboratory, lead author of a paper on this work published in Nature Communications. "The degradation of the devices can be suppressed by simply lowering the temperature by few degrees, that is from 25°C to 0°C."
The researchers, led by Aditya Mohite from the Los Alamos ‘Light to Energy’ team in the Material Synthesis and Integrated Devices group, have been exploring organometallic halide semiconducting perovskite solar cells. These are a promising solar cell technology because of a high power conversion efficiency (PCE) that exceeds 20% and low fabrication costs – the perovskite material is synthesized via a low-temperature solution process. While achieving a high PCE is important, the successful transition from a proof-of-concept experiment to a market-viable photovoltaic technology requires the solar cells to operate with stability under continuous sunlight in outdoor conditions.
Stabilizing perovskite solar cells against ambient air and humidity can be achieved with encapsulation, but ensuring the photo-stability of the cells remains a challenge. Perovskite solar cells will undergo degradation on exposure to sunlight even when the device is under vacuum. Such degradation over time with solar illumination could undermine the commercialization of perovskite solar cells.
In the new paper, the researchers describe investigating this photo-degradation process in detail. "What we found in this study is that under constant 1-sun illumination the large-grain perovskite solar cells degrade majorly in terms of the photocurrent," Nie said. "But what's interesting is that the devices can self-heal when sitting in the dark for a short while."
By performing extensive spectroscopy studies, the team found that sunlight triggers the activation of meta-stable trap states at relatively low energy deep in the perovskite bandgap, which results in the trapping and capturing of photo-generated charge carriers. Over time, these trapped carriers accumulate in the cell, reducing the photocurrent.
Placing the solar cell in the dark for several minutes, however, allows for ‘evacuation’ of these trapped charges, causing the cell to recover and for its performance to return to normal upon the next operating cycle. The team also found that these processes are strongly temperature dependent, and that varying the temperature over just a few tens of degrees can either circumvent the activation of the photo-degradation mechanisms or speed-up the self-healing process.
After exploring several possible physical mechanisms that could explain the microscopic origin of the formation of these trap states, joint experimental and theoretical investigations revealed that the most likely scenario involves the creation of small polaronic states by a combination of lattice strain and molecular re-orientations of the organic cation (CH3NH3) in the perovskite lattice.
"Although several theoretical works have predicted the important role of the organic cation in organometallic halide perovskite, it is one of the first joint experimental-theoretical reports on the observation of its impact on the properties of perovskite materials and devices," said co-author Jean-Christophe Blancon, also at Los Alamos National Laboratory. "Our understanding of the organic cation is still primitive, but our work demonstrates its utmost importance in the photo-stability of perovskite devices and calls for further investigations in the future."
Most importantly, this study provides researchers across the world with a solution to the photo-stability issue in perovskite cells, and further research is now underway toward improving the long term technological viability of perovskite-based photovoltaics.
This story is adapted from material from Los Alamos National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.