TY - JOUR
T1 - Finding and Fixing Traps in II-VI and III-V Colloidal Quantum Dots
T2 - The Importance of Z-Type Ligand Passivation
AU - Kirkwood, Nicholas
AU - Monchen, Julius O.V.
AU - Crisp, Ryan W.
AU - Grimaldi, Gianluca
AU - Bergstein, Huub A.C.
AU - Du Fossé, Indy
AU - Van Der Stam, Ward
AU - Infante, Ivan
AU - Houtepen, Arjan J.
N1 - Funding Information:
A.J.H. acknowledges support from the European Research Council Horizon 2020 ERC Grant Agreement No. 678004 (Doping on Demand). This research is supported by the Dutch Technology Foundation STW (project No. 13903, Stable and Non-Toxic Nanocrystal Solar Cells, and project No. 12734, Cadmium-free All-Inorganic Quantum Dots as Down-Conversion LED Phosphors), which is part of The Netherlands Organization for Scientific Research (NWO) (Vidi grant, No. 723.013.002), and which is partly funded by Ministry of Economic Affairs.
Publisher Copyright:
© Copyright 2018 American Chemical Society.
PY - 2018/11/21
Y1 - 2018/11/21
N2 - Energy levels in the band gap arising from surface states can dominate the optical and electronic properties of semiconductor nanocrystal quantum dots (QDs). Recent theoretical work has predicted that such trap states in II-VI and III-V QDs arise only from two-coordinated anions on the QD surface, offering the hypothesis that Lewis acid (Z-type) ligands should be able to completely passivate these anionic trap states. In this work, we provide experimental support for this hypothesis by demonstrating that Z-type ligation is the primary cause of PL QY increase when passivating undercoordinated CdTe QDs with various metal salts. Optimized treatments with InCl 3 or CdCl 2 afford a near-unity (>90%) photoluminescence quantum yield (PL QY), whereas other metal halogen or carboxylate salts provide a smaller increase in PL QY as a result of weaker binding or steric repulsion. The addition of non-Lewis acidic ligands (amines, alkylammonium chlorides) systematically gives a much smaller but non-negligible increase in the PL QY. We discuss possible reasons for this result, which points toward a more complex and dynamic QD surface. Finally we show that Z-type metal halide ligand treatments also lead to a strong increase in the PL QY of CdSe, CdS, and InP QDs and can increase the efficiency of sintered CdTe solar cells. These results show that surface anions are the dominant source of trap states in II-VI and III-V QDs and that passivation with Lewis acidic Z-type ligands is a general strategy to fix those traps. Our work also provides a method to tune the PL QY of QD samples from nearly zero up to near-unity values, without the need to grow epitaxial shells.
AB - Energy levels in the band gap arising from surface states can dominate the optical and electronic properties of semiconductor nanocrystal quantum dots (QDs). Recent theoretical work has predicted that such trap states in II-VI and III-V QDs arise only from two-coordinated anions on the QD surface, offering the hypothesis that Lewis acid (Z-type) ligands should be able to completely passivate these anionic trap states. In this work, we provide experimental support for this hypothesis by demonstrating that Z-type ligation is the primary cause of PL QY increase when passivating undercoordinated CdTe QDs with various metal salts. Optimized treatments with InCl 3 or CdCl 2 afford a near-unity (>90%) photoluminescence quantum yield (PL QY), whereas other metal halogen or carboxylate salts provide a smaller increase in PL QY as a result of weaker binding or steric repulsion. The addition of non-Lewis acidic ligands (amines, alkylammonium chlorides) systematically gives a much smaller but non-negligible increase in the PL QY. We discuss possible reasons for this result, which points toward a more complex and dynamic QD surface. Finally we show that Z-type metal halide ligand treatments also lead to a strong increase in the PL QY of CdSe, CdS, and InP QDs and can increase the efficiency of sintered CdTe solar cells. These results show that surface anions are the dominant source of trap states in II-VI and III-V QDs and that passivation with Lewis acidic Z-type ligands is a general strategy to fix those traps. Our work also provides a method to tune the PL QY of QD samples from nearly zero up to near-unity values, without the need to grow epitaxial shells.
UR - http://www.scopus.com/inward/record.url?scp=85056731083&partnerID=8YFLogxK
U2 - 10.1021/jacs.8b07783
DO - 10.1021/jacs.8b07783
M3 - Article
C2 - 30375226
AN - SCOPUS:85056731083
SN - 0002-7863
VL - 140
SP - 15712
EP - 15723
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
IS - 46
ER -