2D/LOW DIMENSIONAL METAL HALIDE PEROVSKITE Semiconductor Superlattice Single Crystal

On August 10, 2022, Beijing time, professor Xu Sheng’s team at the University of California, San Diego published a new study titled “Perovskite superlattices with efficient carrier dynamics” in the journal Nature.

Through the epitaxial technology of crystal solution based on micro-nano integration, the research group has realized the preparation of two-dimensional/low-dimensional metal halide perovskite semiconductor superlattice single crystals, and realized its efficient carrier dynamics at the three-dimensional scale. The corresponding author of the paper is Xu Sheng; The first author is Thunderstorm, Li Yuheng.

Three-dimensional metal halide perovskite semiconductors have excellent photoelectric effect and low-cost solution processing process, in recent years in solar cells, light-emitting diodes, and light sensors and other fields have made rapid development, especially in the field of solar cells, its photoelectric conversion efficiency has surpassed the traditional silicon solar cells, has great application prospects. However, its long-intractable stability problems have become the key to its further development.

Different from the traditional three-dimensional metal halide perovskite, its two-dimensional and quasi-two-dimensional structure has excellent structural stability and electrical stability, which meets the conditions for preparing long-term stable optoelectronic devices. However, two-dimensional and quasi-two-dimensional metal halide perovskites contain inorganic perovskite traps and organic barrier layers in crystal structure, with a naturally formed multi-quantum well structure. For optoelectronic devices, this structure has extremely high requirements for the arrangement of multi-quantum well orientation: when the carrier passes through the barrier layer, due to the large band gap of the barrier layer, a serious non-radiative composite will occur at the junction of the trap/barrier layer, which greatly affects the kinetic characteristics of the carrier and the corresponding device performance; Therefore, if the control carrier is transported only in the well, the impact on its dynamics can be negligible. However, based on the great challenge of achieving accurate alignment of quantum well orientation, the current research on 2D and quasi-2D perovskite thin film devices is the device prepared by its polycrystalline thin film. Polycrystalline thin films are composed of countless randomly arranged crystal cells oriented, and these disordered multi-quantum wells create a huge energy barrier, making the current performance of two-dimensional and quasi-two-dimensional perovskite thin-film devices much lower than that of traditional three-dimensional metal halide perovskite devices.

Recently, Professor Xu Sheng’s team at the University of California, San Diego, based on the epitaxial technology of crystal solution based on micro-nano integration, realized the accurate arrangement of two-dimensional/quasi-two-dimensional multi-quantum well structure at the atomic scale by constructing a three-dimensional/two-dimensional/quasi-two-dimensional all-perovskite heterojunction, and constructed the first case of perovskite superlattice single crystal. This superlattice single crystal is formed by a cross-arranged two-dimensional multi-quantum well single crystal structure, which realizes the continuous arrangement of the well layer in three dimensions, and all exhibit excellent carrier dynamics. In solar cells prepared from these superlattice single-crystal thin films, lattice stress can lead to local aggregation of doping at the atomic scale, creating a dual-band structure. Based on this structure, the open circuit voltage of its solar cells has been greatly improved, and it has shown a possible exciton band of multi-relaxation.

Figure 1: Formation and structural characterization of perovskite superlattice single crystals.

Through crystal epitaxial technology, 2D/quasi-2D perovskites can form countless cross-growing sheet monocrystallines on a three-dimensional perovskite single crystal substrate. These flaky single crystals can be further fused by coplanar fusion to form flat superlattice single crystals. In the superlattice single crystal, the continuous arrangement of the trap layer is achieved in the three dimensions of a, b, and c.

Figure 2: Carrier kinetic characterization at the three-dimensional scale of perovskite superlattices single crystals.

Figure 3: Lattice stress and its effects of perovskite superlattice single crystals.

Through the regulation of heterogeneous crystalline lattice stress, the compressive stress of up to 14.6% on the organic barrier layer in superlattice single crystals is realized, which effectively reduces the energy band and exciton binding energy of the crystal, and further improves the generation efficiency of carriers.

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Figure 4: Dual-energy band structure of perovskite superlattice single crystal and possible exciton band multi-relaxation phenomenon in superlattice solar cells.

Based on the high open-circuit voltage of superlattice single-element dual-band solar cells, the research team proposed a possible exciton band multi-relaxation phenomenon. In the atomic-scale dual-energy band network structure, superlattice single-crystal dual-energy band solar cells show obvious excitation energy correlation, and there may be excitons in the low-energy band doped trap layer relaxing into the high-energy band unoppoped trap, thus contributing quasi-Fermi energy level splitting higher than the low-energy band.

Figure 5: Ultrafast laser pump-probe transient absorption characterization of excitons in perovskite superlattices single crystals.

Through the ultrafast laser pump-probe transient absorption characterization of excitons in superlattice single-element dual-band solar cells, the research group found that its built-in potential can significantly affect the relaxation behavior of excitons across regions in the bi-energy band structure.

In this study, the two-dimensional and quasi-two-dimensional structures of metal halide-type perovskites were realized by constructing superlattice single crystals, and the efficient carrier dynamics of metal halide-type perovskite structures at the three-dimensional scale were realized. By regulating the lattice stress to achieve the doping of superlattice single crystals, the internal dual-energy band network structure is constructed, and a potential exciton band multi-relaxation phenomenon is observed and proposed. (Source: Science Network)

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