Buffer Layer Nanoparticle Inks for Printed Electronics

Printed electronic devices consist of multiple layers of functional materials. Each of these materials must be optimized to both perform well in the device, and to be printed as easily as possible. Other factors such as interactions between layers must also be taken into consideration.

Custom nanoparticle inks, such as those available from Avantama, are ideal materials for printed electronics. Avantama’s expertise in materials development can help with the creation of the ideal functional materials for printed devices.

The hole transport layer (HTL) must ensure good transport of holes between the active layer and the electrode whilst blocking electrons, whereas the electron transport layer (ETL) ensures an efficient electron transport whilst blocking holes. State-of-the art materials include organic PEDOT-PSS as HTL and e.g. vacuum-processed zinc oxide (ZnO) as ETL.

PEDOT-PSS is an effective HTL material because it can be solution processed, resulting in improved good device performance. The major drawbacks are its acidity (which can lead to decreased efficiency in inverted architectures) as well as poor long-term stability due to its organic nature.

ZnO, being an inorganic material, is well suited to be used for the ETL. The main drawbacks to the use of ZnO are to do with the difficulties in processing. Solution processing is not always possible, and vacuum deposition of ZnO could be too expensive.

If the ZnO is solution processed, by the sol-gel method, the active layer can be damaged. In addition, solution processed ZnO cannot form a sufficient electrical contact to the silver electrode.

Therefore, new inorganic, solution-processable inks yielding good device performance are sought after. Avantama is specialized in developing exactly such materials.

Layers for printed organic photovoltaic stacks.

Figure 1. Layers for printed organic photovoltaic stacks. Image credit:  © Avantama 2014

Hole Transport Layer Inks - Avantama P-10 WO3

Avantama has developed the first fully printable HTL ink for organic electronics applications. The WO3 ink is stable and optimized for printing applications, and is especially suited to use in organic light emitting diodes (OLED) or organic photovoltaics (OPV).

A major breakthrough that Avantama has achieved with this ink is in making this HTL ink 100% surfactant free. This means that the inks yield effective HTL layers at ambient temperature The average particle size in WO3 ink is around 10 nm. The solid concentration is optimal, yielding dry films between 20 and 200 nm in thickness.

The hole transport layer ink is optimized for a variety of different coating (e.g. slot-dye) and printing (e.g. ink-jet) techniques, for full flexibility in processing.

Electron Transport Layer Inks - Avantama N-10 and N-10X

Avantama has also successfully developed ETL inks based on ZnO or AZO nanoparticles. In the latter case, due to its unique technology, Avantama is able to vary the doping level of Aluminum between 100 ppm and 50 wt%, in order to optimize the electronic performance.

The development of the exact ZnO/AZO ink formulation in each case is guided by the successful development of the WO3 hole transport layer ink.

The electron transport layer inks are also optimized for different coating (e.g. slot-dye) and printing (e.g. ink-jet) techniques.

Comparison to State-of-the-Art

Avantama has proven that their HTL and ETL inks yield layers with a comparable or better performance to state-of-the-art material, e.g. PEDOT-PSS for HTL or vacuum processed ZnO as ETL layer in OPVs. The unique processing properties give Avantama buffer layers a distinct advantage.

Leading scientists have tested Avantama's electronic inks for the application as HTL as well as ETL. Some of the results can be seen in detail in the following publications:

  • High Fill Factor Polymer Solar Cells Incorporating a Low Temperature Solution Processed WO3 Hole extraction Layer, T. Stubhan et al., Advanced Energy Materials 2012, 2, 1433-1438.

  • Inverted structure organic photovoltaic devices employing a low temperature solution processed WO3 anode buffer layer, Ning Li et al., Organic Electronics, Vol. 13, Iss. 11, Nov. 2012, 2479-2484.

Advantages of Avantama Electronic Inks

The main advantages of Avantama’s offer are:

  • Fully solution processable by slot-dye coating, doctor blading, spin-coating, ink jet printing etc.
  • Low post-treatment temperatures of below 100°C
  • Good life time
  • Material flexibility: compositional adaptations (e.g. metal doping) possible
  • Extended shelf-life
  • Ink quality: Avantama dispersions are of highest quality, e.g. d99 < 50 nm, if required < 30 nm
  • High batch-to-batch reproducibility (narrow  specification ranges in viscosity, wettability, particle size)
  • No aggressive or harsh chemicals contained (deposition on organic photoactive materials possible without damage)
  • Inorganic interface layers (degradation resistant)
  • Avantama P-10 (WO3) can be mixed with commercial PEDOT-PSS formulations in order to fine tune electronic and morphological dry layer properties (e.g. conductivity, surface roughness or layer porosity)

About Avantama – Customized Solutions

Avantama is the worldwide leader in the tailor-made development of specialty nanoparticles and nanoparticle dispersions. Avantama develops innovative materials on request to perfectly meet the different industries’ needs.

Avantama's technology and knowhow enable ultra-fast proof-of-concepts and short development cycles. This makes Avantama the ideal partner to speed up tomorrow's material innovations.

This information has been sourced, reviewed and adapted from materials provided by Avantama Ltd - Nanoparticle Dispersions.

For more information on this source, please visit Avantama Ltd - Nanoparticle Dispersions.

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