Cation Composition Engineering of Halide Perovskite Semiconductor Nanostructures

Lead-based halide perovskites ABX3 (A= Cs, CH3NH3, CH(NH2)2; B=Pb; X = Cl, Br, I) have emerged as a promising class of semiconducting materials for optoelectronic applications because of their long carrier diffusion length, high absorption coefficient, bright photoluminescence property and the wide band-gap tunability. Lead halide perovskites are comparable in optoelectronic performance to conventional semiconductor technology, yet, they can be synthesized via solution-based methods, making their fabrication versatile and low-cost. However, due to the intrinsic toxicity of Pb, partially or completely replacing lead with other metal cations has been an important topic in recent years./nThis dissertation explores three approaches, substitution, alloying and doping, to replace or decrease the amount of Pb in halide perovskite, not only for reducing the toxicity of halide perovskite, but also for tuning the materials’ properties and enabling new applications. Chapter 1 provides an overview of the composition engineering in semiconductors, followed by an introduction of halide perovskites. In the subsequent chapters, the applications of the three approaches would be specifically discussed. Chapter 2 examines the optical and electrical tunability in all-inorganic halide perovskites by alloying CsPbI3 with CsSnI3. The first synthesis of single crystalline CsPbxSn1-xI3 nanowires (NWs) has been developed. The electronic band gaps of CsPbxSn1-xI3 NWs can be tuned from 1.3 eV to 1.78 eV by varying the Pb/Sn ratio, which leads to the tunable photoluminescence (PL) in the near infrared range. More importantly, the electrical conductivity increases as more Sn2+ is alloyed with Pb2+, possibly due to the increase of charge carrier concentration when more Sn2+ is introduced. The wide tunability of the optical and electronic properties makes CsPbxSn1-xI3 alloy NWs promising candidates for future optoelectronic device applications. The substitution method is discussed in Chapter 3. Due to the toxicity of lead, searching for a lead-free halide perovskite semiconducting material with comparable optical and electronic properties is of great interest. Rare-earth based halide perovskite in which Pb is replaced by rare earth metal represents a promising class of materials for this purpose. The first solution phase synthesis of single-crystalline CsEuCl3 nanocrystals with a uniform size distribution centered around 15 nm is discussed in this chapter. The CsEuCl3 nanocrystals have photoluminescence emission centered at 435 nm, with a full width at half maximum (FWHM) of 19 nm. Furthermore, CsEuCl3 nanocrystals can be embedded in a polymer matrix which provides enhanced stability under continuous laser irradiation. Lead-free rare earth cesium europium halide perovskite nanocrystals represent a promising candidate to replace lead halide perovskites. In Chapter 4, doping, the intentional addition of impurities into semiconductor crystals is discussed. A new colloidal method has been used for the synthesis of non-emissive large bandgap transparent halide perovskite Cs2SnCl6 nanocrystals with a diameter of 15 nm. As the transparent media is doped with bismuth ions, it displays strong blue emission with a photoluminescence quantum yield of 22.7 ± 3.9 %. The Density Functional Theory reveals that doping is influenced by the (111) facets of the transparent nanocrystals, which have the lowest surface energy, and that dopants tend to form on the surface of the nanocrystals, acting as luminescent centers. This study not only offers a new perspective to understand the doping mechanisms at nanoscale, but also offers a promising alternative to lead-based halide perovskites nanocrystals including promising optical performance and decreased toxicity.

Advisor: Yang, Peidong