Ecological agriculture’s rise defines the current era, where physical agriculture enables sustainable agricultural systems [1]. Within this framework, optical technology holds significant importance, as light is a critical component of agricultural ecosystems. Light quality, intensity, and photoperiod critically regulate plant growth and development. Optimal light exposure enhances photosynthetic activity, leading to improved crop quality and yield. Historically, traditional light sources like fluorescent lamps, incandescent lamps, and high-pressure sodium lamps (HPS) have been utilized in agriculture [2]. However, they suffer from low efficiency, high energy consumption, and ineffective wavelengths for plant growth. Furthermore, excessive heat generation from some sources restricts their ability to provide close-range illumination. Recent advancements in optoelectronic technology have enabled the adoption of light-emitting diodes (LEDs) in agriculture, owing to their superior light efficiency, tunable spectra, and long operational lifespan. These features position LEDs as a sustainable and energy-efficient lighting solution for modern agricultural practices.

The 1990s witnessed significant progress in artificial lighting technology. Groundbreaking research by the University of Wisconsin, Purdue University, and NASA’s Kennedy Space Center pioneered the application of LED technology in plant science, specifically examining its impact on plant growth [3]. As shown in Figure 1 LEDs enable precise alignment of light wavelengths with plant photoreceptors, thereby optimizing metabolic processes and improving productivity. From the 1990s to the early 2020s, extensive studies investigated the effects of monochromatic and polychromatic LED light spectra on diverse plant species, uncovering notable changes in morphology and physiology [4]. For example, in 2005 von Wettberg reported that red light could regulate branch elongation in woody plants [5]. In 2013, studies further elucidated the effects of red, blue, and white light of different spectral qualities on plant biomass and leaf chlorophyll content [6].

The use of LED fixtures in plant lighting has seen significant expansion. Through advanced packaging technologies, LED chips are strategically arranged on substrates and can be driven either regionally or individually. Advanced controls permit precise adjustments for plant growth stages and external light, meeting diverse lighting demandss [7]. Today, LED fixtures are widely employed in areas such as greenhouse supplemental lighting [8], field supplemental lighting [9], and transplant production [10].

Packaging materials are pivotal in influencing LED performance. Materials with high refractive indices, UV/thermal resistance, and low stress enhance LED light output efficiency and lifespan. The inception of LEDs dates back to the 1950s, when British scientists created the first LED using gallium arsenide semiconductors in electroluminescence experiments. By the 1960s, LEDs had entered the commercial market, although the initial versions were limited to visible light and could not emit infrared. Despite this, they found widespread applications. Early packaging methods relied on a free dispensing coating process, which frequently caused yellow rings in the phosphor mixed adhesive layer, with only incremental improvements available [11]. In 2005, Fujita’s research group made a groundbreaking advancement by studying Ce 3+:YAG glass-ceramics, which brought ceramic packaging technology to the forefront [12]. This innovation catalyzed extensive research into diverse glass-ceramic materials. Today, red light chips in LED plant lighting modules predominantly employ ceramic packaging, while the Plastic Leaded Chip Carrier (PLCC) method is less frequently used. PLCC packaging, an evolution of COB technology, is also known as the cover module packaging mode. However, ceramic packaging involves complex manufacturing processes with inherently higher material costs. In contrast, PLCC’s plastic-based architecture enables simpler production, projecting potential cost advantages based on structural and material analysis. Final cost-effectiveness requires further validation through scalable manufacturing. Ceramic packaging typically requires the integration of a dome structure on the chip [13]. When red light chips are encapsulated using ceramic packaging, the emitted light is less susceptible to total reflection upon reaching the dome surface. In contrast, if the same red light chip is packaged using the PLCC method, although the cost is lower, the light extraction efficiency of the PLCC device is significantly compromised. This is due to the presence of the optical encapsulant on the front side of the PLCC package, which has a flat surface. Light from red chips strikes flat PLCC surfaces with high total reflection risk, reducing Photosynthetic Photon Efficacy (PPE) and compromising plant-growth enhancement. This study implements filler material between lens and light-emitting component in PLCC-packaged red LEDs, eliminating air gaps to suppress total reflection. The synergistic optimization of filler, encapsulant, and lens enhances light distribution, improving extraction efficiency by 22%. This maintains high PPE with cost-effective PLCC packaging, enabling broader horticultural applications.

The designed light-emitting module consists of a substrate part, a lens part, and a light-emitting component. As shown in Figure 2a, the light-emitting component is mounted on the substrate, with the lens plate positioned above it and connected to the substrate. The lens part is designed with lenses corresponding to the light-emitting component. The light-emitting component is a PLCC-packaged red light device, which includes a bracket and a red light chip encapsulated with optical encapsulant, where the side facing the lens plate is flat. Notably, optical encapsulant has been added between the lens and the light-emitting component. The filler material integrates the device and the lens into a cohesive unit, minimizing internal light deflection and loss, thereby achieving efficient light distribution in a single step. Additionally, it enhances the extraction efficiency of red light photons emitted by the light-emitting component.

As illustrated in Figure 2b, within the light-emitting component, the red light chip is connected to the positive and negative conductive metals via die attach adhesive and wires. The conductive metal, made of silver-plated copper sheets, provides both electrical conductivity and thermal dissipation. Figures 2c, d show the other side of the design.

The red LED chip employs vertical packaging. Horizontal packaging positions both p/n electrodes on the same chip side, causing current crowding [14]. Furthermore, the low thermal conductivity of the sapphire substrate in horizontal structures significantly hinders heat dissipation from the chip. Prolonged operation under high temperatures due to poor heat dissipation can degrade the performance and transmittance of the filler material, leading to substantial light output power loss. Studies have demonstrated that vertical structures significantly reduce ideality factors and series resistance with better heat dissipation performance [15, 16]. The vertical packaging structure achieves excellent current diffusion by replacing the sapphire substrate with a high thermal conductivity substrate (such as Si, Ge or Cu), thereby significantly improving the heat dissipation efficiency. Additionally, in the vertical structure, the two electrodes are positioned on opposite sides of the LED epitaxial layer. This configuration enables near-vertical current flow with negligible lateral currents, eliminating localized heating while boosting heat dissipation efficiency [17]. Consequently, vertical packaging delivers superior thermal management, improved luminous efficiency, and extended operational lifespan.

A reflective coating has been applied to the inner surface of the recessed portion of the bracket to enhance light reflection. In this design, a diffuse reflection coating is utilized to significantly improve luminous flux. The coating ensures that light emitted by the red light chip towards the bracket side is either reflected or totally reflected, then redirected towards the lens side as effective light, thereby substantially enhancing light extraction efficiency. Additionally, a high-reflectivity silver layer or other coatings, including mixed layers achieving both specular and diffuse reflection, can be employed to achieve specific optical effects.

The selected commercial filler material possesses a refractive index of 1.42 and demonstrates stable optical transmittance across its operational range of −20 °C–120 °C. Crucially, it exhibits no detectable thermal degradation at 65 °C, confirming robust stability under operational conditions [18]. The filler material comprises two layers, with silicon dioxide doped into the lower layer to increase reflectivity while maintaining the refractive index, further improving light extraction efficiency. Notably, current filler stability verification relies on short-term tests within defined temperature ranges. Actual applications involve prolonged thermal cycling, where sustained stress may cause gradual degradation of optical and thermal properties even after initial stability confirmation. This degradation impacts long-term device reliability, including luminous efficiency and lifespan.

We will implement systematic long-term thermal reliability validation for filler materials in subsequent phases. This work will track performance evolution under extended thermal exposure, providing practical data for material optimization. Our ongoing focus will monitor how these materials maintain stability during sustained operation, forming a key consideration for future industrial applications.

As shown in Figure 3, three types of light-emitting modules are currently prevalent in the industry. The PLCC-packaged light source device without a dome incorporates a PLCC-packaged red light chip and a lens. The main optical path is: red light emitted by the chip → planar optical encapsulant → air → lens part. At the interface between the optical encapsulant and air, total reflection and Fresnel reflection result in an estimated loss exceeding 18%. An additional 4% loss occurs due to Fresnel reflection at the lens interface. Consequently, such devices exhibit low light extraction efficiency and limited applicability.

In the PLCC dispensing light source device without a dome, white light chips are used instead of red light chips to improve light extraction efficiency, as the latter demonstrated poor performance. Filler material is added between the lens and the chip. The main optical path is: white light emitted by the chip → optical encapsulant → filler material → lens part. Due to the similar refractive indices of the optical encapsulant, filler material, and lens part, optical losses are reduced, but the improvement in light extraction efficiency is only approximately 8%. This indicates that the effects of white light and filler material are marginal.

In the dispensing light source device with a dome, a dome structure is introduced. The main optical path is: light emitted by the chip → dome → filler material → lens part. Although the dome significantly increases costs, the improvement in light extraction efficiency remains modest, at around 8%.

From these observations, it is evident that the addition of filler material provides only limited improvements in light extraction efficiency, regardless of whether white or red light sources are used or whether a dome is present. Given that filler material has a minimal impact on white light chips, its effect on red light chips, which inherently exhibit inferior light emission, is even less pronounced. Our design overcomes technical biases by strategically integrating both planar optical encapsulant and filler material in PLCC-packaged red light chips without a dome. This approach reduces or eliminates total reflection and optimizes light distribution through the combined use of planar optical encapsulant, filler material, and lens, thereby significantly enhancing light extraction efficiency. In summary, we have developed a cost-effective light-emitting module with superior light extraction performance.

This study proposes a novel assembly approach for red light chips, incorporating both planar optical encapsulant and filler material within a dome-less structure. The filler material is utilized to fully occupy the gap between the lens and the planar optical encapsulant, enabling optimized light distribution through the synergistic interaction of the planar optical encapsulant, filler material, and lens. This design significantly enhances light extraction efficiency. Furthermore, by addressing technical biases, the adoption of PLCC packaging in this design effectively reduces manufacturing costs. The resulting light-emitting module demonstrates a cost-effective solution with high light extraction efficiency. Experimental data indicate that this design improves the light efficiency of PLCC red light devices by approximately 22%, representing a substantial enhancement. Additionally, this paper explores further improvements through the use of dual-core red light devices, providing preliminary evidence that PPE can be further increased. These findings lay the groundwork for future design optimizations and control strategies. Overall, this design approach exhibits significant potential for practical applications.