Ultra‑Uniform White Backlight Using 180° Mini‑CSPLEDs and Quantum‑Dot Films

Abstract

We present a high‑uniformity, direct‑lit mini‑chip‑scale packaged LED (mini‑CSPLED) backlight unit (BLU) that integrates a quantum‑dot (QD) film, a diffusion plate, and dual prism films. Three mini‑CSPLEDs with emission angles of 120°, 150°, and 180° were fabricated via a chip‑scale package (CSP) process. The 180° device, despite a modest 4% power loss at 10 mA compared to its 150° counterpart, delivers a substantially wider emission cone that improves planar light uniformity and reduces LED count per area. By varying the QD film thickness (60, 90, 150 µm) on the blue mini‑CSPLEDs, we achieved CIE chromaticity conversion to the white region and increased BLU brightness. The 180° mini‑CSPLED BLU with a 150‑µm QD film attained an 86 % brightness uniformity, markedly superior to the 120° and 150° designs.

Background

Liquid‑crystal displays (LCDs) dominate modern visual technology, yet their performance is limited by phosphor‑based backlights, which suffer from low efficiency, broad spectra, rapid decay, and poor uniformity. Blue LEDs driving YAG yellow phosphors have become the standard BLU source, but they emit cool white light with sub‑optimal color rendering (CRI < 75). Enhancing luminous efficiency and color quality remains a key research goal.

Quantum dots (QDs) offer exceptional optical properties—high photoluminescence quantum yield, narrow emission, and spectral tunability—making them attractive as solid‑state color converters. QDs have already shown promise in solar cells, LEDs, photodetectors, and even electrocatalysis. In display backlights, QD films can replace conventional phosphors to expand the CIE gamut and improve color fidelity.

Backlight architectures are broadly classified as edge‑lit or direct‑lit. Direct‑lit BLUs, which place LEDs directly beneath the panel, provide superior brightness uniformity and optical efficiency, especially for thin‑panel TVs. However, achieving uniform illumination across the panel remains challenging, particularly as device thickness decreases. This study explores how emission angle engineering and QD film thickness optimization can enhance BLU uniformity.

Methods

The GaN LED epiwafer (λ = 460 nm) was grown by MOCVD on a c‑plane sapphire substrate. The structure comprised a 2‑µm undoped GaN buffer, a 2‑µm Si‑doped n‑GaN cladding, six InGaN/GaN MQWs, a 25‑nm Mg‑doped p‑AlGaN blocking layer, and a 0.2‑µm Mg‑doped p‑GaN cladding. Ni/Ag/Ni/Pt contacts were deposited via electron‑beam evaporation. Three mini‑CSPLEDs were fabricated with 120°, 150°, and 180° emission cones using a film‑transfer and CSP molding technique. The 180° design incorporates a transparent TiO₂/silicone resin nanocomposite sidewall and a top diffusion‑reflective layer.

CdSe/ZnS core–shell QDs (green ≈ 525 nm, red ≈ 617 nm) were mixed with PMMA to form QD films of 60, 90, and 150 µm thickness. These films served as color converters on 450‑nm blue mini‑CSPLED chips. The BLU comprised an 18 mm × 18 mm 3 × 3 mini‑LED array, a diffusion plate, the QD film, and two prism films. The optical distance between chip and diffusion plate was 2.5 mm.

Electrical performance was measured with a Keithley 2400 source meter and a calibrated power meter (CAS 140B). Spatial radiation patterns were captured using a goniophotometer (LEDGON‑100). Luminance and electroluminescence (EL) spectra were recorded with a spectral luminance meter (SRI‑RL‑5000). Brightness uniformity was evaluated at five points (L1–L5) using the formula (1).

Results and Discussion

All three mini‑CSPLEDs exhibited identical I‑V characteristics, with a forward voltage of ~2.72 V at 20 mA and ~3.12 V at 200 mA, confirming that the CSP process preserves electrical integrity. Light output increased linearly with current, reaching ~250 mW at 200 mA. The 180° device shows a slight (~5 %) drop in output, attributed to the diffusion‑reflective layer and broader emission.

Radiation patterns confirmed the engineered emission angles: 120° mini‑CSPLED (110.6°), 150° mini‑CSPLED (148.7°), and 180° mini‑CSPLED (180°). The 180° design produced a butterfly‑shaped distribution, ideal for planar illumination, while the 120° device displayed a Lambertian pattern due to full sidewall coverage.

With a 10 mA forward voltage of 24 V, all BLUs achieved CIE coordinates of (x ≈ 0.15, y ≈ 0.027) and blue output powers between 147–153 mW. Increasing QD film thickness shifted chromaticity toward the white region and boosted brightness proportionally: 60 µm → 90 µm → 150 µm. The 180° BLU with a 150‑µm QD film delivered the highest brightness and the most uniform light distribution, as evidenced by the reduced stripe pattern in the diffusion‑plate images.

Brightness uniformity, measured over five points, reached 86 % for the 180° BLU with a 150‑µm QD film, compared to 35 % and 39 % for the 120° and 150° designs. This represents a 1.47‑fold and 1.19‑fold improvement, respectively.

Adding an LCD panel further shifted CIE coordinates into the white region for all BLUs, demonstrating that the QD‑converted backlight can effectively support high‑color‑accuracy displays.

Conclusions

By combining 180° mini‑CSPLEDs with 150‑µm QD films, we achieved an ultra‑uniform planar white backlight with 86 % brightness uniformity, suitable for ultra‑thin display modules. The CSP technology protects the LED chip, expands the emission angle, and reduces LED density, paving the way for next‑generation high‑performance displays.

Abbreviations

BLU

Backlight unit

CCFL

Cold cathode fluorescent lamp

CdSe

Cadmium selenide

GaN

Gallium nitride

LCD

Liquid crystal display

mini‑CSPLED

Mini chip‑scale packaged light‑emitting diode

OLED

Organic light‑emitting diode

PLED

Polymer light‑emitting diode

QDs

Quantum dots

YAG

Yttrium aluminum garnet