In recent years, the power conversion efficiency of perovskite solar cells has rapidly achieved the highest value, over 25% as of December 2020. The perovskite solar cells are favorably recognized as next-generation solar cells, and interestingly, the world record efficiencies have been achieved mostly by researchers of South Korea. Perovskite solar cells have superior or similar efficiencies to existing solar cells but boast of extremely low manufacturing costs. Moreover, the low-temperature processability allows it to produce flexible or stretchable solar cells (Figure 1). Therefore, global attention is paid to halide perovskite materials which are light-absorber of perovskite solar cells.
Figure 1: Flexible perovskite solar cell (first row) and stretchable ultra-lightweight perovskite solar cell (second row)
1) [ACS Appl. Mater. Interfaces 2020, 12, 6, 7125] [Energy & Environmental Science 2015, 8, 916]
2) Nature Materials volume 14, pages1032-1039(2015)
Characteristics of halide perovskites
The term “perovskite” is named after Russian mineralogist Lev Perovski combined with the mineral calcium titanate (CaTiO3), which was discovered in the Ural Mountains of Russia in 1839. Since then, it has been widely used to refer to compounds with the same type of crystal structure as CaTiO3. Expressed with the general formula ABX3, the perovskite structure consists of eight corner-sharing BX6 octahedra surrounding the A cation, as seen in Figure 2(a).
Halide perovskite materials also have the same type of crystal structure as the perovskite structure. Different from existing perovskite materials, such materials feature the X site with monovalent halide anions, such as Cl-, Br-, and I-, while monovalent and divalent cations are located at the A and B sites, respectively. In addition, the A-site elements may include not only inorganic cations but also organic ones. The most typical hybrid organic-inorganic halide perovskite material is methylammonium lead iodide, CH3NH3PbI3. Figure 2(b) shows molecular structure of the methylammonium monovalent cation.
Figure 2(a): Crystal structure of the ABX3-type perovskite (left) Figure 2(b): Molecular structure of methylammonium ion (right)
Halide perovskite materials have excellent optical and electrical properties in terms of practical applications in photoelectric devices. Regarding light absorption, halide perovskites show 104/cm or higher absorption coefficients within the visible light spectrum, which is superior to most of conventional solar cell materials. In addition, because the band gaps can be easily changed by substituting ions at each site, the upper limit of the wavelength of light absorbed can be diversely designed as needed.
As for the electrical properties, the charge diffusion length in a thin film made of this material reaches several micrometers, especially in a single crystal, the charges can diffuse up to tens of micrometers, indicating a very high charge collection efficiency that is the fraction of electrons collected at the external device before they are recombined. One of the advantages is the formation of a high-quality thin film even by a low-temperature solution-based process because of low crystallization temperature; even, single crystals can be synthesized at near room temperature.
Applications of halide perovskites
Halide perovskites have been applied in various fields. As mentioned above, solar cells can be taken as an example. Halide perovskite materials were first used in solar cells in 2006, when the research team led by Professor Miyasaka at Toin University of Yokohama in Japan utilized the material as a photosensitizer for sensitized solar cells. However, the solar cell efficiency then was just about 3%, so it did not attract much attention.
In 2009, when Professor Nam-gyu Park's research team at Sungkyunkwan University in South Korea applied this material to the all-solid-state sensitized solar cell, the device efficiency began to increase dramatically. Meanwhile, the research team led by Dr. Sang Il Seok (currently serving as a professor at Ulsan National Institute of Science and Technology) of the Korea Research Institute of Chemical Technology, South Korea, succeeded in maximizing device efficiency through conversion into a thin film solar cell.
As of December 2020, the world's highest efficiency of perovskite solar cells stood at 25.5%. This is higher than that of a copper indium gallium selenide solar cell (CIGS) (23.4%), or a polycrystalline silicon solar cell (23.3%). This almost approaches the efficiency of a monocrystalline silicon solar cell (26.1%). Surprisingly, it took at least 40 years for other solar cells to achieve their current performance status, whereas perovskite solar cells showed that performance just in 7 years, as shown in the Best Research-Cell Efficiencies Chart by National Renewable Energy Laboratory (NREL) (Figure 3).
Figure 3: Yearly changes in Best Research-Cell Efficiencies Chart. The enlarged section shows the recent trend in perovskite solar cells.
In addition to solar cells, halide perovskites are applied in various fields such as light-emitting devices (LEDs), X-ray detectors, image sensors built in cameras, lasers, and next-generation semiconductor devices (Figure 4). For certain capabilities, it has been reported that halide perovskite-applied devices even outperform the existing ones in each field. The low-temperature processability leads to vigorous research activities for implementing future devices, such as flexible/stretchable devices or ultra-thin/ultra-lightweight attachable ones. Therefore, halide perovskite materials are expected to attract more global attention as next-generation materials.
Figure 4: Various uses of halide perovskites
Halide perovskite-based LED (Nature Nanotechnology, 2014, 9, 687)
Halide perovskite-based synaptic semiconductor device (Advanced Materials, 2016, 28, 5916)
Halide perovskite-based X-ray detector (Nature 2017, 550, 87)
Future research challenges
The commercialization of halide perovskite-based technologies has resulted in the emergence of challenges to be solved. Widely used in photovoltaic devices, including solar cells, halide perovskite materials have organic-inorganic hybrids. This composition is chemically vulnerable to external environments. In particular, the materials are extremely sensitive to moisture, heat, etc. at incident light, which causes deformation. Thus, studies need to be carried out to solve this problem, ensuring the practical use of the materials.
In addition, lead located at the B site must be replaced with another element. In reality, the relative amount of halide perovskites used to the total amount of materials constituting a device is very small, so no violation of the safety regulations has been reported until now. However, considering that the regulations will be stricter, we must develop lead-free or low-lead halide perovskite materials.
In response to the two challenges above, research should be conducted to develop a new halide perovskite material or control the defects in the material. For this objective, a large-scale, rapid computational simulation (or computer simulation) will be required in conjunction with a theoretical or experimental approach.
According to the recent research trend, two-dimensional (2D) materials are being actively studied in addition to 3D materials, such as A2B'B''X6, A2BX6 (double perovskite), or A3B2X9, all of which are slight variants of the ABX3-type perovskite structure (Figure 5). Therefore, halide perovskite materials and their applications are projected to have a bright future in both academic and industrial settings.
Figure 5: Various perovskite-like structures which are actively studied recently. (a) A2B'B''X6 (B' : monovalent cation - pink ball; B'' : trivalent cation - gray ball), (b) A2BX6 (B : quadravalent cation - pink ball), and (c) A3B2X9 (B : trivalent cation - pink ball). The green balls denote the A site monovalent cations, and the purple balls denote the X site halide ions. (d) Illustration of 2D structures of perovskite materials.
Associate Professor, School of Materials Science and Engineering Department, Kyungpook National University