Inorganic Interface Layer Inks for Organic Electronic Applications

Introduction

The commercialisation of organic electronic devices such as organic photovoltaic cells (OPV) and organic light-emitting diode (OLED) lighting continues to accelerate. To make these large-area, high volume applications fully accepted by the mass market, production costs need to be significantly lowered. This goal can be achieved if devices are fabricated by solution processable materials, such as printable inks. The challenge for materials manufacturers is to meet the following requirements:

• printable with industrial processing techniques
  
• excellent device performance
  
• low-cost through industrial scale-up process
  

While the active layers are transforming light into electrical charges (OPV) or transforming electricity into light (OLED), the interface layers in both OPV and OLED ensures device efficiency. For example, hole selective interface layers ensure good transport of holes between the active layer and the electrode, blocking electrons, whereas the electron selective interface layers ensure an efficient electron transport, blocking holes. State-of-the-art solution processable materials include PEDOT:PSS, a hole selective organic material and sol-gel deposited zinc oxide (ZnO) as an electron selective material.

The major drawbacks of polymers like PEDOT:PSS are its acidity which negativly impacts device stability, as well as its organic nature (e.g., relatively poor chemical stability). The main drawbacks for sol-gel ZnO are short shelf life, lack of reproducibility and high annealing temperatures. Therefore, there is a strong need to replace such materials by industrially applicable materials.

A range of inorganic charge selective inks enables cost-efficient, solution processed OPV or OLED manufacturing on flexible or rigid substrates, using printing technologies such as slot-dye coating or spin coating. The next generation of these inks is currently being optimized for inkjet or screen printing. Prod. Nos. 793361 (zinc oxide nanoparticle ink, N-10), 793388 (aluminum-doped zinc oxide nanoparticle ink, N-10X) and 793353 (tungsten oxide nanoparticle ink, P-10) are nanoparticle-based printing inks allowing processing temperatures of 80 °C (Table 1).

Nanoparticle Based Printing Inks

Tungsten oxide nanoparticle ink (P-10, Prod. No. 793353) is a hole selective interface layer ink based on a colloidal suspension of tungsten oxide (WO₃) nanoparticles in isopropanol. Zinc oxide nanoparticle ink (N-10, Prod. No. 793361) is an electron selective interface layer ink containing zinc oxide (ZnO) nanoparticles in isopropanol whereas Prod. No. 793388 (N-10X) contains aluminum doped zinc oxide (AZO) for enhanced electrical conductivity of the inorganic interface layers.

These inorganic inks are stable and optimized for OLED and OPV coating applications. These inks are free of any unwanted surfactants/dispersants which can negatively interfer with the electronic properties of organic electronic devices. This means the inks can be deposited and post-treated at temperatures below 100 °C without a surfactant removal step (e.g. plasma or ozone treatment).

The average particle size of the ZnO, AZO and WO₃ particles is optimized at around 12-16 nm. The inorganic solid concentration yields dry films of between 15 and 200 nm by conventional coating methods such as slot-dye coating, doctor blading or spin-coating.

Application in Regular and Inverted OPV cells

OPV cells are comprised of layered structures, where the photoactive layer is sandwiched between two electrodes. The hole extraction layer (HEL) and the electron extraction layer (EEL) ensure an efficient current between the active layer and the electrodes.

Single cell modules can have two different OPV geometries: normal and inverted (Figure 1). In normal cells typically indium tin oxide (ITO) is used as the anode and a metal with a lower work function than ITO (e.g. aluminum) is employeed as the cathode. In inverted cells, the cathode is usually ITO and the anode is a metal with a work function higher than ITO (e.g. silver). Inverted OPV modules tend to be more stable and generally show higher efficiencies.

WO₃ nanoparticle ink (P-10, Prod. No. 793353) can be applied in OPV cells as HEL materials due to their high work function, excellent processability and layer formation on both hydrophilic and hydrophobic substates. ZnO and AZO nanoparticle inks (N-10 and N-10X, Prod. Nos. 793361 and 793388) function well as EEL materials for the similar reasons. All these inks can be applied in the normal as well as in inverted OPV cells. Leading scientists have tested the inks for use as hole and electron selective interface layers in OPV cells. The results show that the performance is similar or better than state-of-the-art materials like PEDOT:PSS, and being perfectly applicable in fully solution processed modules.^1,2

Advantages of Inorganic Interface Layer Inks

The main advantages of the interface layer inks are:

• Solution processable by various coating techniques (e.g. slot-dye, doctor blading, spin-coating)
• Low layer annealing temperatures (below 100 °C)
• Good wettability, even on hydrophobic substrates (e.g. on top of the photoactive layer)
• Extended shelf-life
• 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)
• Degradation resistant (inorganic)
• Tungsten oxide nanoparticle ink can be mixed with PEDOT:PSS formulations in order to fine tune electronic and morphological dry layer properties (e.g. conductivity, surface roughness or layer porosity)