Effects of a site Cation on Perovskite Quantum Dot for Emission Wavelength and Photoluminescence Quantum Yield

<i>ABX</i>₃ type (<i>A </i>site: Cs, Methylammonium (MA), Formamidinium (FA) etc., <i>B</i> site: Pb, and <i>X</i> site: halogen such as Cl, Br, I) perovskite quantum dot (PeQD) has attracted attention as a next generation material for the optoelectrical device such as light emitting diode (LED) due to its excellent optical properties. In general, it is known that the optical properties, in other words, the energy band gap of PeQD are composed of the hybrid orbital of the <i>B</i> and <i>X</i> site (<i>B</i>-<i>X</i> hybrid orbital). The state of the energy band gap directly depends on the PeQD optical properties such as the emission wavelength and photoluminescence quantum yield (PLQY). For instance, the emission wavelength of PeQD is controlled by inserting another <i>X</i> site materials (= <i>X</i>’ site). In detail, the new hybrid orbital composed of the <i>B</i> site, <i>X</i> site, and <i>X</i>’ site (<i>B</i>-<i>X</i>/<i>X</i>’ hybrid orbital) was generated by inserting the <i>X</i>’ site material, resulting in controlling the energy band gap. Therefore, controlling the emission wavelength in the entire visible range by only adjusting the halide components. On the other hand, surface defects of PeQD generate new trap levels that prevent the electron transition within the energy band gap, so excess <i>B</i> and <i>X </i>site materials are used to passivate the surface defects. As a result, it is possible to achieve a high PLQY. Based on these facts, the state of the energy band gap on PeQD was controlled by mainly using the <i>B</i> and <i>X</i> site materials composed of the energy band gap in the previous research.<br/>In this work, we proposed the strategies for “Controlling the emission wavelength” and “Improving the PLQY” by using the <i>A</i> site mixture on PeQD. The <i>A</i> site does not directly affect the <i>B</i>-<i>X</i> hybrid orbital. On the other hand, we also found the indirect effects of the <i>A</i> site against the emission wavelength and PLQY of PeQD. About the emission wavelength, it was successfully achieved to control the emission wavelength by inserting another <i>A</i> site material into the PeQD structure. In detail, the PeQD structure was distorted by using two type of the <i>A</i> site materials with different ionic radii. This distortion indirectly changed the distance between the <i>B</i> and <i>X</i> site. This distance is strongly related to the <i>B</i>-<i>X</i> hybrid orbital. Finally, successful control over the emission wavelength was accomplished through the indirect manipulation of the <i>B</i>-<i>X</i> hybridized orbital linked to the energy bandgap. From the results such as FT-IR, XRD, and TEM measurement, it was revealed that the PeQD structure is distorted. In addition, by effectively regulating the energy band gap, it was successfully precise controlled of the emission wavelength with a precision of 1 nm.<br/>On the other hand, we successfully achieved to prepare PeQD with high PLQY over 80% by inserting the alkali metal on the surface of PeQD. In detail, by inserting the alkali metal with a smaller ionic radius than the <i>A</i> site material composed of PeQD on the <i>A</i> site position of the PeQD surface, it was successfully suppressed the detachment of the <i>X</i> site adjacent to alkali metals. This means the generation of the trap level in the energy band structure was suppressed. In fact, PL lifetime measurement supported the proof of decreasing the trap level. In addition, PLQY values of PeQD with alkali metal was improved about 2 - 3 times compared with untreated PeQD.<br/>In summary, we proved the effect of the <i>A</i> site on the PeQD optical properties, notably, the emission wavelength and PLQY. By using this finding, it is possible to expand the application range not only to LED, but also to color conversion material, quantum dot laser, and so on.