Periodic Reporting for period 4 - DOPING-ON-DEMAND (Doping on Demand: precise and permanent control of the Fermi level in nanocrystal assemblies)
Reporting period: 2020-07-01 to 2020-12-31
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 can subsequently be used to form pn junction diodes, low threshold lasers and 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.
The work has been very successful overall.
We have shown the successful control over the doping density in films of semiconductor nanocrystals, conducting polymers and fullerenes. Moreover, we have shown that this charge density can be stabilized at room temperature in two different ways: 1) by using nitrile base high melting point solvents and performing the electrochemical charging at elevated temperatures. At RT the solvents are froze and the charge density is stable. 2) By using photopolymerizable solvents and electrolyte ions. After doping, exposure to UV light stabilizes the charge density. We have also shown for method 1, that it can be used to make pn junctions.
Finally we showed that electrochemical doping can indeed be used to lower, even remove, the threshold for optical gain in semiconductor nanocrystals, bringing the realization of solution process low-threshold nanocrystal lasers much closer.
The results show that electrochemical doping is a promising and viable way to control the charge density in films of semiconductor nanocrystals, enabling the design of semiconductor devices such as LEDs, lasers, photodiodes and solar cells based on these materials.
The work has resulted in over 20 journal publications and a patent. Is has also formed teh basis of an ERC proof-of-concept to explore the valorisation potential of the methods developed to stabilize the charge density after electrochemical doping.
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.
The work has been very successful overall.
We have shown the successful control over the doping density in films of semiconductor nanocrystals, conducting polymers and fullerenes. Moreover, we have shown that this charge density can be stabilized at room temperature in two different ways: 1) by using nitrile base high melting point solvents and performing the electrochemical charging at elevated temperatures. At RT the solvents are froze and the charge density is stable. 2) By using photopolymerizable solvents and electrolyte ions. After doping, exposure to UV light stabilizes the charge density. We have also shown for method 1, that it can be used to make pn junctions.
Finally we showed that electrochemical doping can indeed be used to lower, even remove, the threshold for optical gain in semiconductor nanocrystals, bringing the realization of solution process low-threshold nanocrystal lasers much closer.
The results show that electrochemical doping is a promising and viable way to control the charge density in films of semiconductor nanocrystals, enabling the design of semiconductor devices such as LEDs, lasers, photodiodes and solar cells based on these materials.
The work has resulted in over 20 journal publications and a patent. Is has also formed teh basis of an ERC proof-of-concept to explore the valorisation potential of the methods developed to stabilize the charge density after electrochemical doping.
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 polymerize 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 have demonstrated the broad applicability of these new approaches to doping of porous semiconductors by:
*spatial patterning of doping densities via photolithography.
*forming junction between p-doped and n-doped regions.
*Demonstrating the reduction of the threshold for optical gain in doped semiconductor nanocrystal films.
Future steps involve improve the control over spatial patterning of the doping density and attempting to form light emitting diodes, laser diodes and pn junction solar cells based on electrochemically doped nanocrystal films.
We have demonstrated the broad applicability of these new approaches to doping of porous semiconductors by:
*spatial patterning of doping densities via photolithography.
*forming junction between p-doped and n-doped regions.
*Demonstrating the reduction of the threshold for optical gain in doped semiconductor nanocrystal films.
Future steps involve improve the control over spatial patterning of the doping density and attempting to form light emitting diodes, laser diodes and pn junction solar cells based on electrochemically doped nanocrystal films.