Improving QD Display Optics with High-Refractive-Index Particles

Challenges

Request for the development of light diffusion characteristics in QD displays. It is difficult to control light diffusion with inorganic fine particles...

C promptly heads to a meeting with a client. The topics of discussion are related to the design of QD displays that achieve further uniformity in brightness, energy efficiency, and long lifespan.

QD displays are a display technology that can achieve high color purity, wide color gamut, and high brightness due to the narrow-band emission characteristics of quantum dots, allowing for improved light utilization efficiency compared to conventional methods. To achieve high brightness while ensuring uniform light incidence on the QD layer and maintaining viewing angle characteristics and in-plane brightness uniformity, it is necessary to incorporate optical sheets with appropriate light diffusion functions.

C reflected on the meeting as follows: "Titanium dioxide has a high refractive index due to its fine particles and is generally used for light diffusion applications because of its high chemical and thermal stability."

"However, in QD displays, designs using high-scattering materials such as titanium oxide are effective for achieving uniform incident light. On the other hand, if scattering becomes excessive, optical loss increases, which in turn leads to a decrease in color conversion efficiency and color purity. Therefore, customers have requested proposals for materials that can solve these issues" (C said)

C is considering the development policy with the team. There were various discussions about whether to explore alternatives to titanium dioxide or to combine titanium dioxide with another fine particle.

Generally, inorganic particles may have low dispersibility due to their low affinity with binder resins, raising concerns about increased haze through particle aggregation and interfacial scattering.

Therefore, focusing on the dispersion and affinity to resin, we decided to examine organic microparticles. As candidates for organic microparticles, we considered acrylic resin microparticles and styrene-acrylic resin microparticles; however, since their refractive indices are approximately 1.5 and around 1.55 respectively, a significant difference in refractive index arises when combined with titanium oxide, resulting in a challenge of low light scattering effects.

As the deadline for submission to the customer approaches, C feels a strong sense of urgency.

Solutions

A high refractive index of 1.66 for organic microparticles. There is a prospect for their use as light diffusers!

C decided to consult a senior employee from the neighboring department. This was because he remembered that the senior employee had announced at a recent research presentation that he had succeeded in improving the performance of paint by examining fine particles with a high refractive index.

"I learned about a product called EPOSTAR from a senior employee at Nippon Shokubai. Recently, I found an introduction on a webpage stating that EPOSTAR can be used as a light diffuser, so I was grateful to be introduced to it" (C said)

The webpage introduced by my senior colleague states that EPOSTAR, an organic microparticle, achieves a high refractive index of 1.66 due to its aromatic structure containing nitrogen atoms, and exhibits excellent reflectivity in the visible light range. It also mentions that EPOSTAR has high light diffusion properties and can be utilized as a light diffuser.

Since they are organic microparticles, they have a high affinity with resins, and there seems to be little concern about aggregation. Person C will contact Nippon Shokubai to obtain samples and proceed with the evaluation.

"I received a proposal from the sales representative regarding the provision of samples of products with various particle sizes, as well as information on dispersion methods and recommended dispersants. I thought this was quite promising since organic fine particles have a significantly high refractive index" (C said)

As a result of the evaluation, the control of light diffusion has improved, enabling enhancements in brightness uniformity and the suppression of unwanted light scattering.

"The evaluation as a display is still ongoing, but it seems that we can achieve optical performance that meets customer demands, such as improved uniformity due to reduced brightness unevenness and enhanced visibility, and we can also expect a reduction in power consumption" ( C)

C is progressing with preparations for submitting samples to customers, focusing on incorporating EPOSTAR from Nippon Shokubai as an organic light diffuser.

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