Researchers at Brown University and University of Nebraska - Lincoln (UNL) has come up with a new titanium-based material for making lead-free, inorganic perovskite solar cells. In a recently published paper the researchers showed that the material can be a good candidate, especially for making tandem solar cells -- arrangements in which a perovskite cells are placed on top of silicon or another established material to boost the overall efficiency.
According to the researches interest in perovskites, a class of materials with a particular crystalline structure, for clean energy emerged in 2009, when they were shown to be able to convert sunlight into electricity.
The first perovskite solar cells had a conversion efficiency of only about 4 percent, but that has quickly skyrocketed to near 23 percent, which rivals traditional silicon cells. And perovskites offer some intriguing advantages. They're potentially cheaper to make than silicon cells, and they can be partially transparent, enabling new technologies like windows that generate electricity.
The team made semi-transparent perovskite films that had bandgap -- a measure of the energy level of photons the material can absorb -- of 1.8 electron volts, which is considered to be ideal for tandem solar applications. The material had a conversion efficiency of 3.3 percent, which is well below that of lead-based cells, but a good start for an all-new material, the researchers say.
The researchers used a high-temperature evaporation method to prepare the films, but says the team is investigating alternative methods.
The research showed the material has several advantages over other lead-free perovskite alternatives. One contender for a lead-free perovskite is a material made largely from tin, which rusts easily when exposed to the environment. Titanium, on the hand, is rust-resistant. The titanium-perovskite also has an open-circuit voltage -- a measure of the total voltage available from a solar cell -- of over one volt. Other lead-free perovskites generally produce voltage smaller than 0.6 volts.
The researchers say that material's relatively large bandgap compared to silicon makes it a prime candidate to serve as the top layer in a tandem solar cell. The titanium-perovskite upper layer would absorb the higher-energy photons from the sun that the lower silicon layer can't absorb because of its smaller bandgap. Meanwhile, lower energy photons would pass through the semi-transparent upper layer to be absorbed by the silicon, thereby increasing the cell's total absorption capacity.
The research was supported by the National Science Foundation (OIA-1538893, DMR-1420645).
"Titanium is an abundant, robust and biocompatible element that, until now, has been largely overlooked in perovskite research."
"We showed that it's possible to use titanium-based material to make thin-film perovskites and that the material has favorable properties for solar applications which can be tuned."
"One of the big thrusts in perovskite research is to get away from lead-based materials and find new materials that are non-toxic and more stable."
"Using computer simulations, our theoretician collaborators at UNL predicted that a class of perovskites with cesium, titanium and a halogen component (bromine or/and iodine) was a good candidate. The next step was to actually make a solar cell using that material and test its properties, and that's what we've done here."
"Open-circuit voltage is a key property that we can use to evaluate the potential of a solar cell material."
"So, having such a high value at the outset is very promising."
"Tandem cells are the low-hanging fruit when it comes to perovskites."
"We're not looking to replace existing silicon technology just yet, but instead we're looking to boost it. So if you can make a lead-free tandem cell that's stable, then that's a winner. This new material looks like a good candidate."
Nitin Padture, Senior author of the new paper, the Otis E. Randall University Professor in Brown's School of Engineering and director of Institute for Molecular and Nanoscale Innovation
"There's a lot of engineering you can do to improve efficiency."
"We think this material has a lot of room to improve."
Yuanyuan Zhou, An assistant professor (research) of engineering at Brown and a study co-author
"We are also looking for new low-temperature and solvent-based methods to reduce the potential cost of cell fabrication."
Min Chen, Ph.D. student of materials science at Brown and the first author of the paper
perovskites has emerged as a promising alternative to silicon
A class of materials called perovskites has emerged as a promising alternative to silicon for making inexpensive and efficient solar cells. But for all their promise, perovskites are not without their downsides. Most contain lead, which is highly toxic, and include organic materials that are not particularly stable when exposed to the environment.
Source: Authors of the paper
Cesium Titanium(IV) Bromide thin films based stable lead-free perovskite solar cells
Min Chen | Ming-Gang Ju | Alexander D. Carl | Yingxia Zong | Ronald L. Grimm | Jiajun Gu | Xiao Cheng Zeng | Yuanyuan Zhou | Nitin P. Padture
doi.org/10.1016/j.joule.2018.01.009
Highlights
- Fabrication of Cs2TiBr6 halide perovskite thin films is demonstrated for the first time
- Cs2TiBr6 thin films have ∼1.8 eV bandgap and balanced carrier-diffusion lengths >100 nm
- Cs2TiBr6 thin films are highly stable under environmental (humidity/heat/light) stresses
- First ever solar cells using Cs2TiBr6 thin films show a stable efficiency of up to 3.3
Context & Scale
Halide perovskites (HPs) have attracted a great deal of attention in the field of photovoltaics (PVs) in recent years. The efficiency of perovskite solar cells (PSCs) has seen an unprecedentedly rapid rise within a short period of time. However, the toxicity of Pb and the volatility of the organic ions in the state-of-the-art HP light absorbers have been recognized as the two major obstacles in the path of PSC development and commercialization. Thus, the search is on for all-inorganic, Pb-free HPs for use in PSCs. In this study, we demonstrate experimentally the promise of a new candidate HP: cesium titanium(IV) bromide, a member of a larger family of Ti-based HPs that are all-inorganic, non-toxic, and contain only earth-abundant elements. The exploration of PSCs based on this new family of HPs is likely to have a lasting impact on the global landscape of environmentally friendly PVs, and these materials may also find use in other promising (opto)electronic applications.
Summary
Significant effort is being devoted to the search for all-inorganic Pb-free halide perovskites (HPs) for photovoltaic applications. However, candidate HPs that combine all the desirable attributes — ease of synthesis, favorable bandgaps, outstanding optoelectronic properties, high stability, no toxicity — are extremely rare. Here, we demonstrate experimentally the promise of cesium titanium(IV) bromide (Cs2TiBr6), a part of the Ti-based vacancy-ordered double-perovskite halides family, in perovskite solar cells (PSCs). We show, for the first time, that high-quality Cs2TiBr6 thin films can be prepared through a facile low-temperature vapor-based method. These films exhibit a favorable bandgap of ∼1.8 eV, long and balanced carrier-diffusion lengths exceeding 100 nm, suitable energy levels, and superior intrinsic and environmental stability. The first demonstration of Cs2TiBr6 thin films-based PSCs shows stable efficiency of up to 3.3%. Insights into the Cs2TiBr6 film-formation mechanisms and the PSC device operation are provided, pointing to directions for improving Ti-based PSCs.