Perovskites come in all shapes and sizes and perovskite quantum dots (QDs) are one particular form with great potential. Their unique combination of characteristics leads to improved phase stability and increased theoretical photoluminescence quantum yield (PLQY) compared to their 3D counterparts. However, most perovskite QDs are based on toxic Pb, so replacing it, even partly, with non-toxic Sn, is highly wanted.
Unfortunately, even a slight incorporation of Sn in Pb QDs causes a dramatic degradation of their radiative recombination, resulting in extremely low PLQYs. The reason behind this is believed to be the oxidation of Sn2+ to Sn4+, which follows the breaking of Sn-halogen bonds. In one of our previous works we identified ultra-low Na doping as an effective strategy for improving the PLQY of Sn-Pb QDs, however the improved PLQY degrades rapidly. X-ray photoelectron spectroscopy analyses showed that Na potentially enhances the Sn-I bonds, leading to better properties, but the atomistic origin of the improved PLQY and the subsequent degradation remain unknown.
In this most recent work, Junke employed DFT calculations and AIMD simulations to investigate the effect of Na doping on Sn-Pb perovskite QDs. He found that Na strengthens the Sn-I covalent bonding and therefore hinders the formation of catastrophic defects. Moreover, the binding of the passivation ligands to the surface of the QDs is enhanced with the incorporation of Na, further improving the QDs. Nevertheless, the effect of Na doping varies depending on the surface. Specifically, Na suppresses the diffusion of the passivating ligands on CsI-terminated surfaces, but promotes their detachment from MI2-terminated ones, leading to degradation.
The insights gained from this work are essential for the design of efficient, non-toxic perovskite QDs. The whole article can be found in the Journal of Materials Chemistry A.
Other publications by Junke on the topic with Prof. Qing Shen (The University of Electro-Communications, Japan) can be found here:
F. Liu, J. Jiang, T. Toyoda, M. K. Kamarudin, S. Hayase, R. Wang, S. Tao, Q. Shen, K. Jan, Ultra-Halide-Rich Synthesis of Stable Pure Tin-Based Halide Perovskite Quantum Dots: Implications for Photovoltaics, accepted by ACS Applied Nano Materials.
F. Liu, J. Jiang, Y. Zhang, C. Ding, T. Toyoda, S. Hayase, R. Wang, S. Tao,and Q. Shen, Near‐Infrared Emission from Tin–Lead (Sn–Pb) Alloyed Perovskite Quantum Dots by Sodium Doping, Angewandte Chemie, 10. 1002 (2020).
F. Liu, C. Ding, Y. Zhang, T. Kamisaka, Q. Zhao, J. M. Luther, T. Toyoda, S. Hayase, T. Minemoto, K. Yoshino, B. Zhang, S. Dai, J. Jiang, S. Tao, and Qing Shen, GeI2 Additive for High Optoelectronic Quality CsPbI3 Quantum Dots and Their Application in Photovoltaic Devices. Chemistry of Materials, 2019, 3, 31, 798-807.