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Engineering Challenges in Quantum Dot Displays

Any great technology that pushes the boundaries of performance also has a set of challenges to overcome. Quantum dot displays are not an exception. Whether we consider QDCF or QD-EL there are a range of challenges, such as the requirements of in-cell polarizer development, air processing, and custom quantum dot-polymer mixtures. Addressing these issues will require quantum dots to become more stable during the manufacturing process.

Let's examine the obstacles of the most recent approaches to QD implementation – QD-PL on-pixel (QDPR) and QD-EL configurations – as well as the developments in the display space aimed to resolve these.

Challenges with Quantum Dot Color Filter (QDCF) panels

1. Low quantum efficiency

As QDCF approach requires high concentrations of the quantum dots, the issue of the low efficiency comes up because of the reabsorption of emitted protons. Quantum dots suffer from the overlap between absorption and emission resulting in the efficiency to drop.

As discussed in Quantum Dot Fundamentals, only a few materials are suitable for producing visible QD semiconductors with high quantum efficiency, with the most efficient one being cadmium selenide (CdSe). As the use of cadmium is restricted by regulations due to its toxic properties, manufacturers are still in search of Cd-free materials with high photoluminescence quantum yield.

2. Need for in-cell polarizer

Quantum dots depolarize light and non-polarized light cannot pass through the top polarizer without causing the screen to malfunction. Therefore, the second polarizer needs to precede the quantum dot layer in the optical path. This means that the in-cell polarizer needs to be developed for QDCF application.

Polarizer not for QDCF application

Polarizer for QDCF application
Source: Samsung Display Co.

3. Manufacturing process

The manufacturing of the quantum dot color filters is the massive challenge for the industry. Although we have seen substantial improvements in quantum dot strength and stability, in order to mass produce QDCFs, quantum dots need to become stable in air, water and heat resistant as manufacturing involve multiple curing, washing and baking steps. QDCF design also requires quantum dots to have ultra-high absorption in a thin layer to avoid the blue light leakage issue in the red and green sub-pixels.

There are a few possibilities for QDCF manufacturing:

  • Use photoresist embedded with quantum dots (QDPR) — because this is a subtractive process, this method is quite expensive

  • Use inkjet printing — this is an additive method and manufacturers are getting close to creating sub-pixel structures that meet size and performance requirements

Ink jet printing process
Source: Samsung Display Co.
  • Use nano-imprint lithography — this method allows for creating nano-scale patterns through mechanical deformation of imprint resist and curing them by heat or UV-light

Nano-imprint lithography process
Source: Samsung Display Co.

Challenges with Quantum Dot Electro-Luminescent (QD-EL) panels

1. Low external quantum efficiency

The main challenge in QD-EL display configuration is the low external quantum efficiency (EQE). Similar to the QD-PL displays, quantum efficiency issue is also relevant to the electro-emissive quantum dot application. It's a challenge to find new Cd-free materials with high external quantum yields for blue quantum dot emitters. Reduced EQE is also due to the imbalance in charge injection and structural issues causing non-radiative energy transfer (FRET).

There are some options being researched to improve on EQE through enhancements to quantum dot design or QD-EL structure:

  • Establishing core-shell interfacial control allowing for the formation of the alloyed interface and insertion of the intermediate shell
  • Creating larger-sized quantum dots with a thicker shell for improved stability
  • Using ZnMgO instead of ZnO as the electron-transmit layer, allowing for better charge balance, higher conduction band level, and suppression of exciton quenching
  • Using mixed or stacked hole transport layers to enable better hole transport
  • Introducing the insulating interlayer between the electron transfer layer and emission layer allowing for control over the electron injection and reducing leakage current to improve efficiency
Insulating interlayer for QD-EL structure
Source: Samsung Display Co.

A roadmap for the future of QD displays

As you can see, the future is bright for quantum dot technology in displays. While the size of the challenge of taking the advanced QD technologies to the mass production and making manufacturing process viable and cost-effective, is commensurate with the performance and efficiency benefits, it's not a small task for the leading display players.

Samsung Display is currently a leading manufacturer of quantum dot enabled displays globally. Here is how we see the QD display space will evolve.

Broadly speaking, the evolution will take place in three phases:

  1. Photo-enhanced displays – adoption of devices utilizing quantum dot enhancement film (QDEF)
  2. Photo-emissive displays – implementation of quantum dots in color filters (QDCF)
  3. Electro-emissive displays – using electroluminescent mechanisms of quantum dots (QD-EL)
Quantum dot technologies compared
Source: Samsung Display Co.

Quantum dot technology in displays is developing very quickly. As it can utilize existing supply chains, QD displays will scale quickly and move from the realm of ultra high-end market to the mid-high end.

While challenges in the commercialization of advanced technologies are massive, the display community knows precisely what problems need solving and have made a significant leap towards these advancements.

When QD-EL is commercialized, it will replace LCD and OLED in large format display market for both TV and digital signage applications.