Outlining Electron Transport Materials
Carrier mobility in semiconducting systems is determined by the device composition and architecture, which typically comprises several sequential layers of organometallic materials. These are formulated as inks or dispersions that can broadly be characterized as either electron-hole transport/electron blocking materials or electron transport/hole blocking materials. Solid-state optoelectronic and organic photovoltaic (OPV) architectures require layers of each type to transport a charge between an anode and a cathode in a thin film array.
In the case of OPVs and organic light-emitting diodes (OLEDs), hole and electron transport layers (HTL/ETL) are used to mitigate recombination close to the interface. This is a potential loss pathway that could quench the device’s emission properties or result in poor optical attenuation. Maximizing carrier generation is of critical importance in achieving high performance and efficiency in any semiconducting system. One ideal architecture for an OPV involves a stack of layered materials with buffers between the electrodes and photoactive layer:
- An underlying substrate, typically a polymer or silicate.
- An anode/cathode stacked on top of the substrate.
- A layer of electron transport material printed on the electrode.
- A photoactive layer above the ETL.
- An HTL printed on the photoactive component.
- A final anode/cathode to complete the circuit.
Sandwiching the photoactive component of an OPV system with hole and electron transport materials encourages the charge to move away from the interfaces before recombination occurs, which ensures optimal quantum efficiency. It is important to note that this is not the final word in OPV structures and that this prototypical architecture can also be inverted.
Functions of Electron Transport Materials
Electron transport materials encourage a flow of electrons to move from an interface while blocking holes. While a hole is an absence of an electron and is not considered a physical particle in and of itself, vacancies can be passed between the atoms in a semiconductor system, inhibiting carrier generation. Electron-hole pairs are the typical unit of electronic recombination. By blocking one aspect of this unit from flowing to an interface, leak currents can be avoided, and a greater charge can be collected or converted into light.
This is a fairly rudimentary explanation of how electron-hole and electron transport materials function. You can read about this in more depth in our previous blog post Nanoparticle Inks: Buffer Layers for Printed Electronics.
Zinc oxide (ZnO) is an ideal electron transport material due to its inherent conductivity. It is particularly suited to OPV and OLED printing when solution-processing is available. However, this is not always the case. Alternative printing methods can be costly and can also risk damage to the photoactive layer.
Avantama has developed a series of electron transport materials based on aluminum-doped ZnO nanoparticles in a 2-propanol solvent. The concentration of particles and the particle size distribution can be tailored as required, and the solution is compatible with both Perovskite absorbers and silver anodes. This ensures high device applicability regardless of the architecture or deposition method, with available processing techniques including spin coating and slot die printing.
Avantama N-10 and Avantama N-20X are suitable electron transport materials for advanced semiconductor systems in a range of printed electronics markets. If you would like to learn more, simply contact a member of the team today.