A high-quality mixed-organic-cation perovskite (MA)x(FA)1−xPbI3 is prepared from a phase-pure non-stoichiometric intermediate complex (FAI)1−x−PbI2. The phase-pure (FAI)1−x−PbI2 probably facilitates homogenous nucleation and modulates the growth kinetics during the crystallization of (MA)x(FA)1−xPbI3. This strategy can be expected to pave the way for the development of mixed-organic-cation perovskite solar cells.
NIMS researchers have developed the world’s first method to fabricate high-quality perovskite materials capable of utilizing long-wavelength sunlight of 800 nm or longer. Compared to conventional methods, this method enables the creation of perovskite materials that have a 40-nm wider optical absorption spectrum, a high short-circuit current and high open-circuit voltage. Thus, this method is regarded as a new approach to enhance the efficiency of perovskite solar cells.
Current perovskite solar cells possess optical absorption spectra skewed toward shorter wavelengths. To improve the energy conversion efficiency of these cells, it is vital to develop perovskite materials with optical absorption spectra expanded to include longer wavelengths. Accordingly, several research institutes are developing perovskite materials, (MA)xFA1-xPbI3, which include 2 types of cations, MA and FA, capable of absorbing light in the longer wavelength region. However, these cations have demerits: their mixing ratio and crystallization temperature are difficult to control. Moreover, due to their tendency to form a mixed crystal phase, there had been no effective method established to fabricate high-purity, single-crystalline perovskite materials.
Solution? A method has been developed to fabricate a new mixed cation-based perovskite material. We first fabricated a pure, single-crystalline precursor material, (FAI)1-xPbI2, under altering temperatures. Then, we performed a reaction between the precursor and MAI (methylammonium iodide). The resulting perovskite material, (MA)xFA1-xPbI3, was a single crystalline phase and had a long fluorescence lifetime. The electrons in the material rarely recombine and they have long lifetimes. The optical absorption spectrum of the solar cells covered up to 840 nm, which was 40 nm wider than the spectrum of conventional perovskite material (MA3PbI3). As a result, the solar cells we developed obtained 1.4 mA/cm2 higher short-circuit current than the MAPbI3 solar cells that were manufactured under the same conditions.
In future studies, they intend to develop high-quality perovskite solar cells capable of utilizing a broader spectrum of sunlight by adjusting the ratio of the two cations. http://onlinelibrary.wiley.com/store/10.1002/adma.201501489/asset/supinfo/adma201501489-sup-0001-S1.pdf?v=1&s=dab121856dec938cda549fd492de6a20c5223a1a
http://www.nims.go.jp/eng/news/press/2015/11/201511020.html
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