Researchers at the Energy Department's National Renewable Energy Laboratory (NREL), in collaboration with researchers at Shanghai Jiao Tong University (SJTU), devised a method to improve perovskite solar cells, making them more efficient and reliable with higher reproducibility.
The research, funded by the U.S. Department of Energy SunShot Initiative, involved hybrid halide perovskite solar cells and revealed treating them with a specific solution of methyl ammonium bromide (MABr) would repair defects, improving efficiency. The scientists converted a low-quality perovskite film with pinholes and small grains into a high-quality film without pinholes and with large grains. Doing so boosted the efficiency of the perovskite film in converting sunlight to 19%.
The efficiency of perovskites in converting sunlight into electricity has jumped from slightly less than 4 percent in 2009, when the first tests were done, to more than 22% today. However, the efficiency can fluctuate according to the skills of the researchers making perovskites at different laboratories, to somewhere between 15% and 20%.
Perovskite films are typically grown using a solution of precursor chemicals that form the crystals, which are then exposed to a second anti-solvent that removes the precursor solvent. The fast-crystallization process is almost an art. NREL researchers found that, because of the narrow time window for properly adding the anti-solvent, it is easy to miss that window and perovskite crystals with defects could form. Defects, like noncontinuous crystals and nonuniform crystals with relatively small crystallite sizes and pinholes, can significantly reduce the effectiveness of a perovskite cell.
The scientists from NREL and SJTU came up with a better method, using what's called the Ostwald ripening process. The process involves small crystals dissolving and then redepositing onto larger crystals. The researchers were able to induce the Ostwald ripening process by treating the perovskite with a MABr solution. The amount of the solution proved key, as the ideal was proven to be about 2mg/ml.
The improved film quality made the cells more stable. The perovskite cells treated with MABr were shown to be more efficient than those without the treatment. Untreated cells had an efficiency of about 14% to 17%, while cells treated with the MABr solution had an efficiency of more than 19%.
Figure: NREL - Hybrid perovskite solar cells
The researchers noted, "With the Ostwald ripening process, different-sized nanocrystals formed with different film qualities could then grow into pinhole-free perovskite films with similar large crystal sizes." The researchers added, "Thus, this new chemical approach enhances processing tolerance to the initial perovskite film quality and improves the reproducibility of device fabrication."
Facile fabrication of large-grain CH3NH3PbI3−xBrx films for high-efficiency solar cells via CH3NH3Br-selective Ostwald ripening
Mengjin Yang | Taiyang Zhang | Philip Schulz | Zhen Li | Ge Li | Dong Hoe Kim | Nanjie Guo | Joseph J. Berry | Kai Zhu | Yixin Zhao
Nature Communications 7 | Article number: 12305 | doi:10.1038/ncomms12305
Received 06 May 2016 | Accepted 20 June 2016 | Published 01 August 2016
Organometallic halide perovskite solar cells (PSCs) have shown great promise as a low-cost, high-efficiency photovoltaic technology. Structural and electro-optical properties of the perovskite absorber layer are most critical to device operation characteristics. Here we present a facile fabrication of high-efficiency PSCs based on compact, large-grain, pinhole-free CH3NH3PbI3−xBrx (MAPbI3−xBrx) thin films with high reproducibility. A simple methylammonium bromide (MABr) treatment via spin-coating with a proper MABr concentration converts MAPbI3 thin films with different initial film qualities (for example, grain size and pinholes) to high-quality MAPbI3−xBrx thin films following an Ostwald ripening process, which is strongly affected by MABr concentration and is ineffective when replacing MABr with methylammonium iodide. A higher MABr concentration enhances I–Br anion exchange reaction, yielding poorer device performance. This MABr-selective Ostwald ripening process improves cell efficiency but also enhances device stability and thus represents a simple, promising strategy for further improving PSC performance with higher reproducibility and reliability.