The researchers led by Liyuan Han, director of the Photovoltaic Materials Unit, formulated a method that enables the creation of perovskite materials which have a 40-nm wider optical absorption spectrum, a high short-circuit current and high open-circuit voltage. The method marks a new approach to enhance the efficiency of perovskite solar cells.
Currently available 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 two 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.
To resolve these issues, a new method was used to fabricate a new type of mixed cation-based perovskite material. A pure, single-crystalline precursor material,
(FAI)1-xPbI2, was fabricated under altering temperatures. A reaction was performed 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 observations indicated that electrons in the material rarely recombine and they have long lifetimes. The optical absorption spectrum of the solar cells employing this material covered up to 840 nm, which was 40 nm wider than the spectrum of conventional perovskite material (MA3PbI3). As a result, the solar cells that were developed obtained 1.4 mA/cm2 higher short-circuit current than the MAPbI3 solar cells that were manufactured under the same conditions.