Electrochemical control over the charge carrier density in colloidal semiconductor nanomaterials: applications, challenges and limitations
Colloidal semiconductor nanomaterials, such as quantum dots (QDs) and nanoplatelets (NPLs), are important materials for opto-electronic applications. They are already commercialized as phosphors in displays, and intensely investigated as active materials for e.g. LEDs, lasers and photodetectors. A common element in these applications is that charging of the CsPbX3 materials is involved, either through charge injection, intentional electronic doping, or photoexcitation followed by charge separation. However, charging of colloidal nanomaterials is not always easy, nor is it innocent.
In this talk I will discuss how electrochemistry can be used to both control the charge density and study the effect of charging on colloidal nanomaterials. The porous nature of nanomaterials allows efficient electrochemical doping, accommodated by charge compensation with electrolyte ions. This can be used to control the Fermi-level and study its effect on the electronic and optical properties of the materials, using steady state or (ultrafast) time-resolved spectroscopy.
Control over the Fermi level is very useful to study the position of band edges, the occurrence of traps in the band gap and to controllable remove the threshold for optical gain via doping. I will briefly give examples of this. I will then show that it is possible to fix the Fermi level after electrochemical doping so that this method can be used to permanently dope films of nanocrystal. This allows the creation of e.g. pan junctions making electrochemical doping an interesting alternative technique for the formation of semiconductor devices.
However, changing the Fermi level is not always innocent and may induce structural changes that lead to trap formation and even decomposition. Understanding the nature of the electrochemical reactions that occur on the surface of colloidal nanomaterials is key in controlling their efficiency and stability. Subtle surface reactions, like the formation of dimers or the local reduction of surface ions on II-VI and III-V semiconductor nanocrystals, can already lead to quenching of the photoluminescence. For other materials, most notably lead based perovskites, progressive reduction of Pb ions results in the complete cathodic dissolution. I will discuss how this is governed by the solubility of surface Pb complexes, which form the weakest link in the system.