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Doping on Demand: precise and permanent control of the Fermi level in nanocrystal assemblies

Periodic Reporting for period 3 - DOPING-ON-DEMAND (Doping on Demand: precise and permanent control of the Fermi level in nanocrystal assemblies)

Reporting period: 2019-01-01 to 2020-06-30

The aim of the project is to develop a completely new method to electronically dope assemblies of semiconductor nanocrystals (a.k.a quantum dots, QDs), and porous semiconductors in general. External dopants are added on demand in the form of electrolyte ions in the voids between QDs. These ions are introduced via electrochemical charge injection, and are subsequently immobilized by (1) freezing the electrolyte solvent at room temperature or (2) chemically immobilising the ions, or by a combination of both. The goal is to form stable doped porous semiconductors with a constant Fermi level that is controlled by the potential set during electrochemical charging. These doped films will subsequently be used to form pn junction diodes, low threshold lasers and solar cells.

This new technique to dope new classes of semiconductor materials will lead to new and improved possibilities to produce optoelectronic devices such as displays, lamps, lasers and solar cells. In addition to providing technological advances, this may also lead to new possibilities reduced energy use (especially in the case of lamps) and efficient generation of renewable energy with solar cells.
We have set up infrastructure to determine the in situ changes in the optical absorption and photoluminescence of thin films of semiconductor nanocrystal, polymers, fullerenes etc. while changing the electrochemical potential and, hence, while controlling the doping density.
With the combination of these techniques we have been able to investigate a wide range of materials (CIS nanocrystal, CdTe nanocrystal, CdSe/CdS core/shell nanocrystals, polythiophene conducting polymers, films of C60 and PCBM) and we have been able to study the existence of electron and hole traps in these materials. This has resulted in a large number of manuscripts, 3 submitted, 3 more in preparation.

We have learned to understand the key aspects that determine the kinetics and efficiency of electrochemical doping in various nanocrystal systems. We found that diffusion of counter ions into the pores in nanocrystal films, and or via intercalation in the crystal lattices, is the rate limiting factor and that tuning the nature and size of the counter ions allows to control this process. The results have been published as Van der Stam et al. (J. Am. Chem. Soc 2017) and Gudjonsdottir et al (J. Am. Chem. Soc 2018).
Expanding on the work listed above we have established that arresting the motion of counter ions after charge injection can be achieved in various ways, as also envisioned in the original proposal. To achieve this we can make use of phase transitions in the supporting electrolyte solvent (injecting charges above the melting point and then cooling down) or we can polymerise the solvent and ions after charge injection. This latter approach is an extension of the original proposal that works much better and affords the possibility of combining electrochemical doping with photolithography.

We plan to further demonstrate the broad applicability of these new approaches to doping of porous semiconductors.
The next steps involve:
*spatial patterning of doping densities via photolithography
*forming junction between p-doped and n-doped regions
*Demonstrating the reduction of the losing threshold in permanently doped films
*Attempting to form light emitting diodes, laser diodes and pn junction solar cells based on electrochemically doped quantum dots films.