Finding and Fixing Traps in II–VI and III–V Colloidal Quantum Dots: The Importance of Z-Type Ligand Passivation

Abstract

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 InCl3 or CdCl2 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.

A Chemically Orthogonal Hole Transport Layer for Efficient Colloidal Quantum Dot Solar Cells

“…Colloidal quantum dots (CQDs) are of interest in light‐emitting diodes,1,2 lasers,3 photodetectors,4 and solar cells5–14 owing to their tunable optical and electrical properties and their solution processing 15–18. Research efforts on surface passivation and device architecture have led to improvements in CQD solar cell performance,19–21 and these recently enabled power conversion efficiencies (PCE) above 12% for lead sulfide (PbS) CQDs 19…”

Semiconductor Quantum Dots as Components of Photoactive Supramolecular Architectures

“…The ability of Z-type ligands to fully saturate the anionic surface trap states in QDs made of II-VI and III-V semiconductors was also reported. [34] Although significant progress has been made in the aqueous phase synthesis of hydrophilic nanocrystals, [35] most synthetic routes that offer an excellent size and shape control of QDs involve the reaction of inorganic precursors in apolar or amphiphilic organic solvents, in the presence of the capping ligands. [8] These methods yield highly hydrophobic QDs, soluble only in apolar organic solvents such as toluene, hexane or chloroform, whose surface is typically coated with tris(n-octyl) phosphine oxide (TOPO, see Figure 4), alkylamine and/or alkanethiol ligands.…”

“…Recently, the incorporation of X‐type ligands, such as chloride anions, during the synthesis of PbS and CdS QDs was found to afford an efficient passivation of the surface trap sites, impossible to achieve with larger organic ligands. The ability of Z‐type ligands to fully saturate the anionic surface trap states in QDs made of II–VI and III–V semiconductors was also reported …”

Atomic Layer Deposition of ZnO on InP Quantum Dot Films for Charge Separation, Stabilization, and Solar Cell Formation

“…Colloidal semiconductor quantum dot (QD) thin films have various applications in optoelectronic devices because of an easily adjustable bandgap, solution‐based processing, and the potential to overcome the Shockley–Queisser limit in solar cells by exploiting carrier multiplication . Due to their high surface‐to‐volume ratio many QD materials suffer from oxidative and photothermal degradation; this is detrimental to the material properties .…”