May 19 2016
The cause and a solution as to why perovskite solar cells tend to degrade in sunlight was revealed by a breakthrough study conducted at Los Alamos National Laboratory. This study helps to eliminate one hurdle towards commercialization of this potential technology.
From left, Aditya Mohite, Jean-Christophe Blancon and Wanyi Nie are researchers at Los Alamos National Laboratory studying both the cause and a solution for the tendency of perovskite solar cells to degrade in sunlight. (Photo credit: Los Alamos National Laboratory)
The researchers discovered that the degraded devices displayed self-healing tendencies when placed for a short time in the dark. The team revealed that photo-degradation seen in perovskite cells is a complete electronic process that occurs due to charge buildup without causing chemical deterioration to the crystal structure, and can be decreased while the self-healing properties of the cells allowed them to bounce back in the dark.
We can stabilize the device performance by controlling the environmental temperature. The degradation of the devices can be suppressed by simply lowering the temperature by few degrees, that is, from 25 degrees Celsius to 0 degrees Celsius.
Wanyi Nie, Los Alamos National Laboratory
The team, headed by Aditya Mohite from the Los Alamos “Light to Energy” team in the Material Synthesis and Integrated Devices group, is studying organometallic halide semiconducting perovskite solar cells.
These cells have potential as they contain high power conversion efficiency (PCE) of more than 20% and require less fabrication costs. The perovskite material is synthesized using a low-temperature solution method. While accomplishing high PCE is vital, the successful evolution from a proof-of-concept experiment to real world commercially-viable photovoltaic technology needs the device to function with stability under constant sunlight coupled with humidity and air of outdoor conditions.
The issue of stability against ambient humidity and air can be circumvented through encapsulation methods, but the photo-stability of the perovskite-based gadgets was still an open question.
These solar cells will experience deformation with steady light soaking even when the gadget is in vacuum. Such deformation over time with solar illumination could weaken the market value of perovskite-based solar cells.
The new paper, “Light-activated photocurrent degradation and self-healing in perovskite solar cells” (DOI: 10.1038/ncomms11574), co-authored by Wanyi Nie and Jean-Christophe Blancon, elucidates the photo-degradation technique.
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. But what’s interesting is that the devices can self-heal when sitting in the dark for a short while.
Wanyi Nie, Los Alamos National Laboratory
By conducting widespread spectroscopy and device characterization, the team discovered that sunlight activates the meta-stable trap states at comparatively low energy deep in the perovskite bandgap, which caused the photo-generated charge carriers to be trapped and caught. Trapped carriers can further build up in the gadget over a period, minimizing the photocurrent. Alternatively, when the solar cell devices were placed in the dark for several minutes, “evacuation” of these trapped charges was possible, promoting the recovery of the performances of the pristine device upon the subsequent operation cycle.
The team also discovered that these methods were strongly dependent on temperature, and that temperature control over a range of a few tens of degrees could prevent the initiation of the photo-degradation mechanisms or accelerate the self-healing process.
After studying many probable physical mechanisms to clarify the microscopic origin of the configuration of these trap states, joint experimental and hypothetical investigations wrapped up that the most probable scenario is the development of tiny polaronic states involving lattice strain and molecular re-orientations of the organic cation found in the perovskite lattice.
Although several theoretical works have predicted the important role of the organic cation (CH3NH3) 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. 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.
Jean-Christophe Blancon, Los Alamos National Laboratory
Significantly, this research will give researchers around the world the first solution to the photo-stability problem in perovskite gadgets. Going forward, research will go in the direction of improvements and the lasting technological feasibility of perovskite-based photovoltaics.
Hybrid perovskite materials, which are crystalline semiconductors processed from solution at low temperature, tend to possess superior opto-electronic properties that have facilitated a wide range of device applications. Los Alamos is one of the leaders in the race among hybrid perovskite photovoltaic research group. By unraveling the stability issue, the team is set to use the material in other applications concerned with US energy security.
The paper’s Los Alamos authors are Wanyi Nie, Jean-Christophe Blancon, Amanda Neukirk, Hsinhan Tsai, Sergei Tretiak, Jared Crochet, Gautam Gupta and Aditya Mohite. From Brookhaven National Laboratory are Kannatassen Appavoo and Matthew Sfeir; from Rutgers University is Mannish Chhowalla; from Purdue University is Mohammad Alam; from Université de Rennes 1, France is Claudine Katan; and from INSA de Rennes, France is Jackie Even.
The research conducted at Los Alamos National Laboratory was supported by DOE Office of Basic Energy Sciences and by the Los Alamos Laboratory Directed Research and Development program. The research was performed partially at the Center for Integrated Nanotechnologies, a DOE Office of Science User Facility at Los Alamos.
The computational and DFT calculations conducted made use of the resources provided by the Los Alamos Institutional Computing Program, supported by the US Department of Energy National Nuclear Security Administration. Bay Area Photovoltaic Consortium (BAPVC) supported the study conducted at Purdue University. The study based in France was supported by Cellule Energie du CNRS (SOLHYB- TRANS Project) and the University of Rennes 1 (Action Incitative, Défis Scientifique Emergents 2015). This research applied resources of the Center for Functional Nanomaterials, which is a US DOE Office of Science Facility, at Brookhaven National Laboratory.