Synthesis and Photophysics of Mn- and Yb-doped CsPbX3 (X = Cl, Br, I) Perovskite and Analogous TlX Semiconductor Nanocrystals

Abstract

Recent breakthrough in lead halide perovskite in terms of getting high quality semiconductor through solution processability has changed our way of thinking. These lead halide perovskite are truly defect tolerant wherein presence of plethora of defects do not interfere with optical and transport properties. As a result, all inorganic CsPbX3 (X = Cl, Br, I) perovskite nanocrystals (NCs) show extraordinary photoluminescence quantum yield (PLQY) ~90 % without any surface modification and exhibit good performance in optoelectronic devices like solar cell and light emitting diodes. This success of the host NCs motivated us to dope them with Mn and Yb to add new optical functionality and also improve the stability of the host.

A larger part of the present thesis involves synthesis of Mn- and Yb-doped CsPbX3 perovskite NCs of various sizes and shapes to elucidate their optical properties. Quantum confinement in CsPbX3 NCs is expected to significantly enhance dopant-carrier interactions. As a result, doping of Mn in strongly confined CsPbCl3 nanoplatelets is carried out through one-pot synthesis at room temperature. These Mn-doped CsPbCl3 NPLs show intense optical emission at 586 nm (PLQY ~20 %) and is characteristic of spin-forbidden 4T1 → 6A1 Mn d-d transition. However, our one pot synthesis is unsuccessful to dope Mn in CsPbBr3 NCs. Then we design a post synthesis strategy to dope Mn and Yb in lower band-gap CsPbBr3 and CsPbI3 NCs of various shapes from strong to weak confinement regime. We discuss the origin of Mn-emission in CsPbBr3 NCs using temperature (300 to 10 K) dependent PL. Yb-doping yielded intense near infrared PL, which can be useful for infrared sensing. In a different perspective, post synthesis Mn-doping in CsPbI3 NCs improved structural stability of black perovskite phase from a few days to a month in ambient condition, extending the scope real life applications of this materials. Finally, we thought to completely replace Pb2+ with isoelectronic Tl+. Electronic band structures of TlX and CsPbX3 are very similar, which motivated us to check the optoelectronic properties of TlX NCs. Our TlX NCs showed photoluminescence at room temperature unlike bulk TlX with reasonable PLQY ~10 % in the blue-UV region. These TlX NCs showed high THz carrier mobilities (~220 to 329 cm2 V−1 s−1) and long diffusion lengths (~0.77 to 0.98 μm) suggesting prospective defect tolerant nature of our TlX NCs.