Luminescent light diffuser for diffuse lighting applications

Abstract

The lighting industry currently accounts for a significant proportion of all energy demand. Luminescent white lighting is often impure, inefficient, expensive, and detrimentally emits as a point source, meaning the light is emitted from a focused point. A luminescent light diffuser offers the potential to create a spatially broad lighting fixture. We developed a luminescent light diffuser consisting of three commercially available luminescent dye species (rhodamine 6G, fluorescein, 7‐diethylamino‐4‐methylcoumarin) dispersed within a polymer matrix (polyvinyl alcohol), or commercial paint, and coated on a planar waveguide. A Light‐emitting diode (LED) (385 nm) is directed into the waveguide which excites the luminescent species, coating the panel, creating a device that emits spatially broad pure white light. As the emission depends on escape cone emission from the waveguide, the device’s emission was found to depend highly on the coating film quality and components. We present two systems: a small 40 mm × 40 mm prototype, made using standard water‐soluble polymer (polyvinyl alcohol), to study the underlying operational principles, and a 100 mm $\times$ 100 mm device with optimized efficiency fabricated with a clear commercial paint. By doping the polymer matrix with scattering silica microparticles we achieved a maximum photon outcoupling efficiency of 78%, whilst maintaining colour purity with an increased device size of more than 300 times (compared with the input LED). This work shows that it is possible to construct an inexpensive and spatially broad lighting source, whilst maintaining colour purity at a low cost.

A luminescent light diffuser consisting of three commercially available luminescent dye species was dispersed within a polymer matrix and coated on a planar waveguide. A light‐emitting diode (LED) (385 nm) is directed into the waveguide which excites the luminescent species, coating the panel, creating a device that emits spatially broad pure white light.

1. INTRODUCTION

Global energy consumption has grown exponentially since the start of the industrial revolution and the energy demand created by lighting sources is one of the largest sectors of energy use.^{[ 1} , ^{2 ]} With the increasing global population and development, energy use for lighting is expected to rise. The ability to mitigate this increase in energy demand rests entirely on the development of more efficient lighting systems.^{[ 3 ]} Currently, the most significant and popular light‐based technologies are light‐emitting diodes (LEDs).^{[ 4 ]} Whilst LEDs are superior to older lighting technology,^{[ 5 ]} such as incandescent lamps (efficiency as low as 2.2%)^{[ 6 ]} and fluorescent lights (efficiency of ~25.0%),^{[ 7 ]} a major drawback of LEDs is that they are point sources, meaning that the light emitted is much harsher on our eyes and not distributed uniformly such as natural light.^{[ 8} , ^{9 ]} Transitioning to high‐quality LED lighting sources could save a significant amount of energy.^{[ 10 ]} Using efficient white LED lighting options can lead to substantial reductions in carbon dioxide emissions^{[ 11} , ^{12 ]} and research has shown that the energy offset of switching to efficient white lighting from LEDs globally would save more energy than all current renewable resources combined.^{[ 13} , ¹⁴ , ^{15 ]}

Whilst recent research has focused on using structural scattering elements to diffuse LED light,^{[ 16} , ¹⁷ , ^{18 ]} we propose to use a waveguide and luminescence to produce spatially broad light emission from an LED. Conventional diffusers rely on scattering layers (Figure 1b) and, as such, require multiple LEDs to achieve spatial broadness. Our proposed luminescent light diffuser (LLD), the first of its kind (Figure 1a) offers a low‐cost solution to light diffusion. In the LLD all but the top face of the light diffuser device is mirrored to maximize internal reflection and heighten the light output from the top face. The top face is coated with the white light‐emitting phosphor mix. Light inputted from the compact size LED through the side is then internally reflected within the device and exits through the top face through luminescence events. Due to the large difference in refractive index between the air and the polymer, only a tiny fraction of light can leave the polymer‐covered face directly; the rest must leave by emission events through an escape cone (Figure 1a). Although this concept is novel, it is similar in concept to a luminescent solar concentrator,^{[ 19 ]} but in reverse. The concept has also been hinted at previously, for example with paintable luminescent solar concentrator waveguides on cars.^{[ 20 ]}

FIGURE 1.

(a) Diagram of a luminescent light diffuser. Grey sides indicate externally mirrored surfaces. The light blue box on the left‐hand side indicates an LED directly attached to the device. Purple arrows depict light originating from the LED device being internally reflected within the device and exiting through the white polymer–dye mix that covers the top face (shown by a pink surface). Black arrows show the emission from the device. (b) A conventional light diffuser that works via scattering events

Tuning LEDs to emit specific wavelengths of light is also a challenge for synthetic chemists. Commercial lighting makes use of luminescent molecular phosphors to tune emission wavelengths. A combination of blue‐, green‐, and red‐emitting phosphors can be combined in such a way as to emit white light.^{[ 21 ]} Incorporating dye molecules into lighting devices can significantly reduce costs of colour tuning and potential power demand, leading to highly versatile lighting devices.^{[ 22 ]} Dye phosphors are usually combined within a polymer matrix, which improves stability and prevents dye aggregation.^{[ 23} , ²⁴ , ²⁵ , ²⁶ , ^{27 ]}

Here we show that it is possible to construct an inexpensive and spatially broad lighting source, whilst maintaining colour purity at a low cost, providing an alternative and fundamentally new device, to the current system of light diffusers that potentially offers a more uniform broad lighting source with fewer LED point sources required. We fabricated a series of luminescent light diffusers, achieving spatially large white light coverage, and characterized their light‐diffusing properties. We examined various methods to increase light outcoupling in 40 mm $\times$ 40 mm prototype devices that were made using a standard water‐soluble polymer, polyvinyl alcohol (PVA). We then maximized the efficiency and spatial broadness in 100 mm $\times$ 100 mm devices fabricated with commercial paint. We found that light emission from the waveguide can be maximized via modification of the polymer matrix. By doping the polymer matrix with scattering SiO₂ microparticles we achieved a maximum photon outcoupling efficiency of 78%, whilst maintaining colour purity at an increased device size 300 times larger than the input LED.

2. EXPERIMENTAL

All materials were purchased via Sigma‐Aldrich and used as supplied. ‘Concrete Clear’ and standard black commercial paint were supplied by Resene Paints NZ. Optical coupling gel (SS‐988 Non‐Curing Optical Coupling Gel – V‐788 Offset) was purchased from Silicone Solutions. Silica microparticles were 15 μm average diameter (Sigma‐Aldrich).

2.1. Mirroring

Glass slabs (40 mm × 40 mm × 5 mm and 100 mm × 100 mm × 5 mm) were cleaned with a solution of cerium oxide in deionized water (5.8 M). Ammonia in MeOH (7 M) was added to a solution of silver nitrate (0.1 M, 10 ml) dropwise until a colourless, transparent solution was formed. Then a solution of potassium hydroxide (0.8 M, 5 ml) was added to the ammonia and silver nitrate solution, forming a greyish‐brown precipitate. The precipitate was re‐dissolved using further dropwise addition of ammonia in MeOH (7 M). To this solution, variable amounts of glucose powder were added. Immediately, the solution was pipetted onto a face of a glass slab forming the mirror. This process was repeated on all sides of each glass slab, excluding a top face. The glass slabs were dried. The resultant mirror faces were painted with black paint (Resene testpot – All Black’ N25‐001‐264) for stability.

2.2. Polymer matrix

PVA (52 g) was dissolved in deionized water (500 ml). Each dye was dissolved in its own PVA water solution. The concentrations of the dyes and volume of the PVA water solution were: rhodamine 6G (0.01 M, 20 ml), fluorescein (1.15 × 10⁻⁴ M, 50 ml) and 7‐diethylamino‐4‐methylcoumarin (7.22 × 10⁻⁵ M, 200 ml). The resultant mixture was stirred at 40°C until the PVA dissolved and the mixture became homogenous. For white paint a molar ratio of red dye:green dye:blue dye:PVA of 1:7:0.8:9460 was combined and mixed until homogeneity was achieved.

2.3. Paint mix

Solutions of dyes in water: rhodamine 6G (0.017 M, 10 ml), fluorescein (0.013 M, 10 ml) and 7‐diethylamino‐4‐methylcoumarin (0.00055 M, 100 ml) were added to Concrete Clear commercial paint in a volume ratio of 1:10 (dye:paint). The resultant paint mixtures were mixed to produce white paint, in a volume ratio of 4:5:30 (rhodamine 6G:fluorescein:diethylamino‐4‐methylcoumarin).

2.4. Absorbance and photoluminescence experiments

Absorbance measurements were taken on a Cary50 ultraviolet–visible (UV–vis) spectrophotometer. Photoluminescence measurements were collected on an Edinburgh Photonics FLS1000 photoluminescence spectrometer.

2.5. Integrating sphere photoluminescence experiments

Measurements were taken in an 8‐in. Labsphere integrating sphere. An Energetic EQ‐99x laser‐driven light source (LDLS) coupled to a Spectral Products CM110 1/8 m monochromator was used as a 385 nm excitation source. The signal was detected using a Kymera 328i Andor Spectrograph with a DU420A‐BVF iDus detector. Results were calibrated against a known spectral source, Ocean Optics, HL‐3 plus, vis–NIR light source. The photoluminescence quantum efficiencies for all samples were measured in an integrating sphere as per De Mello et al.^{[ 28 ]}

2.6. Luminescent light diffuser device manufacture

Small‐scale prototypes made with the PVA matrix were on a 40 mm × 40 mm × 5 mm mirrored glass slab and the large‐scale devices made with commercial paint were on 100 mm × 100 mm × 5 mm mirrored glass slab.

• 40 mm × 40 mm × 5 mm prototypes: The polymer mix was pipetted onto the unmirrored glass face. A total amount of 6 ml was used for full coverage and to minimize LED light escaping the device. The mix was dried.

• 100 mm × 100 mm × 5 mm devices: The doped Concrete Clear paint was dropped onto the unmirrored glass, and a blade coated to form a thin film on an RK Printcoat Instruments – K Control Coater. Excess paint was used to ensure full coverage and speed control is set at the fastest speed that avoids creating empty spots.

An InGaN LED (LED385L – Thorlabs) was mounted to illuminate into the side of the resulting devices. The LED(s) were run at 4.0 V for all measurements.

Device efficiencies were measured in a 16‐in. integrating sphere connected as above and defined as the ratio of photons out the top face compared with the photons input from the LED.

2.7. Polymer matrix alterations

• 40 mm × 40 mm × 5 mm prototypes: Four alterations were made to the small‐scale prototypes or the polymer mix application before analysis.

  1. The polymer mix was spread uniformly over the glass slab face and dried slowly.
  2. The polymer mix was spread uniformly over the glass slab face and dried quickly.
  3. The glass slab face was treated with 100 grit sandpaper before the polymer mix was spread uniformly and dried slowly.
  4. The polymer mix was added variable amounts of silica microparticles before the resulting mix was spread uniformly over the glass slab face and dried slowly.
• 100 mm × 100 mm × 5 mm devices: Six alterations were made to the large‐scale devices or the polymer mix application before analysis.

  1. Standard two layers of blade‐coated film
  2. Two layers of blade‐coated film with optical coupling gel
  3. Standard one‐layer of blade‐coated film
  4. One‐layer of blade‐coated film with optical coupling gel
  5. 0.3 g SiO₂‐doped one‐layer of blade‐coated film
  6. 0.4 g SiO₂‐doped one‐layer of blade‐coated film

For the large‐scale device a series of silica microparticles concentrations were tested to maximize outcoupling. An optical coupling gel (SS‐988 Non‐Curing Optical Coupling Gel – V‐788 Offset) was also used.

2.8. Spatial light emission distribution measurements

An InGaN LED (LED385L – Thorlabs) was mounted to illuminate into the side of the resulting devices. The LED(s) were run at 4.0 V for all measurements. Using a custom‐built apparatus, a fibre optic cable (Ocean Optics QP600–2‐VIS–NIR) was positioned 0.5 mm above the device at various X–Y coordinates. Spectral measurements were taken on an Ocean Optics QE Pro high‐performance spectrometer.

2.9. Transmission electron microscopy (TEM) imaging

TEM pictures were recorded on a Jeol JEM‐2100F FE analytical electron microscope under 200 kV acceleration.

2.10. Dektak profilometry surface morphology scans

Surface morphology scans were performed on a Veecon Dektak 150 with 1.67 μm/sample resolution and 1.50 mg of force. Five scans of each film were taken with a scan length of 1000 μm. Average surface roughness is represented as the average standard deviation of each sample's five scans.

3. RESULTS AND DISCUSSION

3.1. Colour analysis – achieving spectrally pure white light

We first showed a small‐scale proof of concept, by fabricating ‘small’ devices (detailed below) and demonstrating colour tuneability.

We used red‐emitting rhodamine 6G (Figure 2a), green‐emitting fluorescein (Figure 2b) and blue‐emitting 7‐diethylamino‐4‐methylcoumarin (Figure 2c) for our devices. The absorbance and emission spectra of each dye are shown in Figure 2(e). All emission data were excited at 385 nm to replicate those used in the LED. The dyes' colour purity is shown in Figure 2(f). The combination of all three dyes in the correct ratio had the capability to emit white light and any colour within the triangle shown in Figure 2(f). This could be increased with the addition or substitution of different dyes or chromophores.^{[ 29} , ³⁰ , ³¹ , ³² , ³³ , ^{34 ]}

FIGURE 2.

(a) Rhodamine 6G. (b) Fluorescein. (c) 7‐Diethylamino‐4‐methylcoumarin. (d) Polyvinyl alcohol. (e) Absorbance and emission spectra of the three dyes used. (f) Commission Internationale de l'éclairage (CIE) plot of the three dyes, white light and reproducible white light films. The dashed triangle highlights the available colours our system could achieve

A mix of dyes in PVA (Figure 2d) was pipetted onto a glass slide to develop the white light‐emitting mixture, ensuring full coverage. The emission data collected were then plotted onto the chromaticity diagram for determination of the true colour. This allowed us to make alterations to achieve optimum white light. When white light emission was initially achieved (Figure 2f), triplicate mixtures were formed and showed good agreement. As the true colours presented were closely situated, it strongly indicates that white was achievable and reproducible. Moreover, it showed that the polymer–dye mix could be altered to suit the demands of the lighting industry. Although white light is required for this device, future devices may be more easily tuned to emit different colour lights to suit a range of requirements.

We measured the properties of the plain PVA polymer film by dropping a solution of PVA onto a glass slide and leaving it to air dry. Absorbance and emission data were collected (Figure 3(a)). Absorption data for the PVA film allowed us to conclude that the film did not absorb any wavelengths of light of interest: the only light absorption was ~325 nm, which is lower than our light input (385 nm) and below the wavelengths of any dye emission. Therefore, it was suitable to use in the device and would not alter emission data from the polymer–dye mix (Figure 3a). The data show that the PVA film emits a small amount of blue light, at ~450 nm. As such, its emission will play a small role in our device’s colour.

FIGURE 3.

(a) Absorption and emission spectra of undoped PVA film. The blue line notes the wavelength of the LED used for excitation. (b) Emission spectra of the white light‐emitting device and coloured dyes for comparison. Note the lack of 385 nm emission. (c)(i–iv) Samples of emission colours from doped PVA films, including white, and samples of working LLDs. Note: The non‐uniformity of film emission seen in (ci) is due to the non‐uniform nature of the PVA films. These images are purely to show spectral range and luminescence not actual device films

Emission analysis with the white light‐emitting polymer–dye mix showed three clear peaks corresponding to each of the dyes (Figure 3b,ci,cii). We first noted that the peak at 385 nm, corresponding to the LED emission had disappeared, implying that negligible ultraviolet (UV) LED light escapes the device. The peak corresponding to the blue dye looks to be a combination of the blue dye and the polymer matrix. The clear definition of all three peaks indicated that all dyes played a role in the final white light emission. Rhodamine 6G has the lowest molar value in the ratio for white light emission, yet it shows the most significant peak in the white emitting polymer–dye mix. Published data have claimed that rhodamine 6G has a photoluminescent quantum efficiency (PLQE) of 0.95,^{[ 35 ]} fluorescein has a PLQE of 0.97^{[ 36 ]} and 7‐diethylamino‐4‐methylcoumarin has a PLQE of 0.54.^{[ 37 ]} These data implied that Förster resonance energy transfer (FRET) events were most likely to occur within the device system, funnelling energy into the red dye.^{[ 26 ]} However, this was not necessarily a problem in our system, as we could easily tune the component dyes to enable the desired output colour. A picture of the working device is shown in Figure 3(cii, ciii), clearly showing spatial diffuse emission.

3.2. Enhancing photon outcoupling efficiency through polymer matrix alterations

Having demonstrated facile colour control, we then investigated a range of different alterations to the polymer matrix to improve photon outcoupling. This was done via small‐scale 40 mm $\times$ 40 mm $\times$ 5 mm prototype devices. Luminescence efficiencies, defined as (the number of visible photons emitted from the top surface divided by the number of imputed UV photons) $\times 100$, were calculated using emission data of each different device and PLQE data of each dye were used for the analysis and comparison of the devices. For optimal efficiency, an LLD must strike a balance between light waveguiding along the device, and light outcoupling through escape cone or scattering losses. We demonstrated control of this balance by investigating three different ways to increase light outcoupling. The four devices under investigation were silica microparticle doped (Figure 4a), an uneven polymer surface (Figure 4b), a natural polymer surface (Figure 4c) and a scratched glass surface (Figure 4d). The average surface roughness of the created polymer matrix films is presented in the Supporting Information Table S1. A TEM image of the silica microparticles used is presented in Figure S1. The theory behind doping the white light‐emitting polymer–dye mix with silica microparticles was to add a scattering component. The uneven polymer surface was made to ensure a rough top surface, encouraging the escape cone angles to sit at different angles to the surface, and be more accessible. The natural polymer surface acts as a control; the polymer–dye mix was left to dry evenly and smoothly, such that the surface was uniform across the face of the device. Finally, the scratched glass surface was the area where the glass surface was roughened with sandpaper to investigate if light could be transfer more efficiently into the polymer–dye mix. The absorbance, transmission, and emission spectra of the fabricated films are presented in Figure S2.

FIGURE 4.

Diagrams of different methods to improve light outcoupling. (a) Silica microparticle doped. (b) Uneven polymer surface. (c) Natural polymer surface. (d) Scratched glass surface

We found that the order of device efficiencies from highest to lowest were: silica microparticle doped (Figure 4a), uneven polymer surface (Figure 4b), natural polymer surface (Figure 4c) and then scratched glass surface (Figure 4d; Table 1). The highest efficiency achieved was 82%, which was assigned to the silica ball‐doped polymer device. This result demonstrated that 82% of the total amount of light (in terms of photons) inputted into the device from the attached LED were re‐emitted from the phosphors in the polymer–dye mix as white light. The efficiency of this device was high, especially considering a size increase of more than 48 times (0.33–16 cm²) was achieved (for all devices) and white light was successfully produced at this scale.

TABLE 1.

Prototype test efficiencies or different surface treatments

Device	Luminescence efficiency, %
Silica microparticle doped	82.4
Rough polymer surface	68.2
Natural polymer surface	64.3
Scratched glass surface	40.3

The scratched glass device was the least efficient, with an efficiency of 24%, lower than the natural polymer surface device. It is not entirely clear why this device had such low efficiency, but a plausible reason could be due to decreased light coupling into the polymer matrix. The scratched glass could have prevented light from entering the polymer as scratching may have added a detrimental sctaering effect. Therefore, when the polymer mix was dried on top, only a small fraction of the light was able to leave the waveguide and exit through the polymer–dye mix. Therefore, a smooth waveguide‐polymer surface is required for the device to have reasonable efficiency. A smooth surface allows efficient transfer of light from the glass slab waveguide into the polymer for good quality light emission. Once the light is inside the polymer, it is then important to scatter it, to create an efficient emission from the face of the device.

The device that had a rough surface at the polymer‐air interface was the second most efficient device. However, the visually rough surface device only showed an efficiency of 4% greater than the natural surface device. The silica microparticle‐doped device had the highest efficiency. The silica microparticles added a scattering component that caused light to effectively ‘bounce’ around within the waveguide. Leading to an increased chance of emission of light through the escape cone critical angle. The results obtained would suggest that the best way to increase the efficiency of the device was to target total emission angles by the addition of scattering components.

3.3. Large‐scale device analysis

Using what we discovered with the small‐scale prototypes above, we fabricated, characterized, and optimized large‐scale devices. These devices were identical to the small‐scale devices but were on 100 mm $\times$ 100 mm $\times$ 5 mm glass slabs and used a commercially available ‘Concrete Clear – Resene Paints’ paint as the polymer matrix. After fine tuning the dye composition we proceeded to optimize our surface coating layers and device components. We performed the following optimizations: standard two‐layer blade‐coated films (inset Figure 5a); standard one‐layer blade‐coated films (Figure 5c); the addition of an optical coupling gel (SS‐988 Non‐Curing Optical Coupling Gel – V‐788 Offset) between the LED, and the glass slab was used for a two‐layer blade‐coated film (Figure 5b); optical coupling gel with a one‐layer blade‐coated film (Figure 5d); and two one‐layer blade‐coated films differently doped with silica microparticles: either 0.3 g silica (Figure 5e) or 0.4 g silica (Figure 5f) per 2 ml of paint. Average surface roughness of the created large‐scale device films are presented in Table S1. Absorbance, transmission, and emission spectra of the fabricated films are presented in Figure S3.

FIGURE 5.

Light emission distribution of various large‐scale devices. (a) Standard two‐layer film. (b) Two‐layer film with optical coupling gel. (c) Standard one‐layer film. (d) 0.3 g SiO₂‐doped one‐layer film. (e) One‐layer film with optical coupling gel. (f) 0.4 g SiO₂‐doped standard one‐layer film. Note: Burgundy region between x = 0, y = 3–7 and x = 1, y = 4–6 is an artefact, as we were unable to measure the area where the LED clip was situated

We measured our large devices by systematically mapping the output light distribution using a spatially variable optical fibre setup (see Experimental). We choose a metric to measure the effectiveness of our devices, with outcoupling efficiency defined as the number of coordinates with photoluminescence (PL) intensities (at 400–800 nm) above 10% of the maximum intensity close to the exciting LED. The data in Figure 5 showed a trend of increasing efficiency and light diffusion (Table 2). We found that moving from a two‐layer to a single‐layer film improved the efficiency from 15.2% to 18.2% and showed similar improvements in diffusion (11.6–13.2%). The optical coupling gel had a dramatic effect, adding roughly 10–20% efficiency (two layer–single layer, respectively) and further increasing the diffusion. The final optimization of adding different amounts of silica microparticles was found to improve efficiency from the single‐layer device by roughly 30%. Although there was an increase, there were diminishing returns when adding the 0.4 g of silica microparticles compared with the 0.3 g. More than 0.4 g of silica microparticles resulted in unsuitable polymer surfaces. We report a maximum optimized efficiency of 78% which is a 185% increase in efficiency compared with our standard two‐layer films.

TABLE 2.

Device performance metrics

Treatment	Figure reference	PLQE (%)	Diffusion (% of data points above 10% relative light)
Standard two‐layer film	(a)	15.3	11.6
Two‐layer film with optical coupling gel	(b)	25.2	14.0
Standard one‐layer film	(c)	18.1	13.2
One‐layer film with optical coupling gel	(d)	42.3	17.3
One‐layer film with optical coupling gel and 0.2 g silica microparticles	(e)	69.8	19.8
One‐layer film with optical coupling gel and 0.4 g silica microparticles	(f)	78.1	21.5

As we desired to create diffuse light from a point source LED, it is worth mentioning the increase in light output dimensions achieved. The surface area of our starting LED, small‐scale prototype and large‐scale devices are shown in Table 3. As shown, we increased the light output area of our devices by more than 300‐fold and developed a method to tune their colour. This gives the proposed LLD an advantage over conventional diffusers.

TABLE 3.

Dimensions and surface area of light systems

Sample	Surface area (cm2)	Diffusion increase (as a multiple of LED surface area)
LED	0.33
Small‐scale prototypes	16	48
Large‐scale devices	100	303

3.4. Cost–effectiveness

There is no single key parameter that allows a clear comparison of this system with the current commercial alternative (illustrated in Figure 1b). A figure of merit, whilst there is no standard agreed upon in the industry, will be some combination of quality (i.e. colour) and quantity of diffused light multiplied by efficiency, divided by cost. A key comparison is the cost of extra dyes needed for the LLD vs the cost of the extra LEDs needed to supply traditional light diffusers. For this, we found that we could produce our ‘luminescent paint’ at US$0.02 per cm², which includes the cost of the dyes, polymers, and solvent, this totalled US$1.86 per fabricated device. The mirroring process to create 100 cm² devices was at a cost of US$3. However, we predict that this cost will decrease dramatically with scaling, as our methods to create a mirror were far from industry standards. The LED itself costs US$14, which represents ~75% of the total device price and does not include the cost of a conventional diffuser. As such, the cost of extra LEDs needed in a traditional diffuser is a major inhibition of the current technology. Overall, the LLD approach is promising because of its simplicity of processing, the potential for upscaling, and ease of colour tuning. We also note the potential of this technology to become a ‘paintable diffuser’ if the need for mirroring can be reduced. Note all cost were calulated in early 2021.

4. CONCLUSION

We fabricated proof‐of‐concept LLDs that displayed pure white light emission. Using three phosphors to produce white light, we ensured tuneability across the spectrum. The devices also produced spatially broad light, rather than point source emission, and we characterized the degree to which this spatial broadness was enhanced (although more development is needed to ensure even distribution with increased device size). We demonstrated the degree to which light outcoupling efficiency could be enhanced using different polymer matrix processing techniques. We achieved a maximum efficiency of 78%, found when doping the polymer–dye mix with silica balls, whilst reaching a size increase of 300‐fold (compared with the exciting LED) and maintaining white light emission. As this is the first device made of this kind, there is huge potential to increase efficiency. Our results indicate that the LLD is a potentially relevant technology to reduce the energy demand of lighting sources in both commercial and home applications.

Supporting information

Table S1 Average surface roughness, as measured using Dektak profilometry, of both the small‐scale polymer matrix systems and large‐scale paint devices

Figure S1 Transmission electron microscopy image of the SiO₂ microparticles

Figure S2 (a) Absorbance, (b) transmission and (c) emission spectra of the small‐scale polymer matrix fabricated films

Figure S3 (a) Absorbance, (b) transmission and (c) emission spectra of the large‐scale fabricated film

Witzmann A., Gordon C. K., Howarth J., Unsworth S., Rossi A., Hardy J., Price M. B., Davis N. J. L. K., Luminescence 2023, 38(1), 47. 10.1002/bio.4416