QLEDs for Household AC: Efficient Lighting Solutions
03-06-2024 | By Liam Critchley
Key Things to Know
- Challenges in LED Systems: Traditional LEDs face issues with energy losses and dependency on driver circuits, leading to increased costs and reduced lifespan.
- Promising QLED Innovations: Quantum dot LEDs (QLEDs) are being developed to operate directly from household AC currents, potentially revolutionising lighting efficiency and longevity.
- Plug-and-Play QLED Advantages: Recent developments in plug-and-play QLED systems allow them to be directly connected to household AC sockets, providing a cost-effective and efficient lighting alternative.
- Enhanced Durability of QLEDs: QLED devices show reduced joule heating and charge accumulation, resulting in longer lifetimes and more reliable performance in residential and commercial applications.
Challenges and Innovations in LED Technology for Household Applications
Light emitting diodes (LEDs) have become a mainstream lighting technology due to possessing a range of desirable characteristics—including a high efficiency and longevity, being a solid-state material, and being environmentally safe. LEDs are p-n semiconducting diodes, so can only be operated using a low-voltage direct current (DC) source.
Most LEDs are used in the home, which runs off high-voltage alternating current (AC)—as does the whole electrical grid, and energy is distributed around the grid as an AC. This means that many household devices—such as LED lamps— cannot be used directly with household power sockets. To change the high-voltage AC current into a low-voltage DC current, an AC-DC (not the rock band) converter is required—which is usually a transformer and a rectification circuit.
While many modern-day converters are very efficient, with conversion efficiencies of over 90%, there are still energy losses that occur during the conversion process. Additionally, driver circuits are required to regulate the DC power, which can lower the durability of the LED. Overall, the AC-DC converters account for about 17% of the overall cost of LED lamps while increasing power consumption and reducing the durability of the lamp.
Impact of Conversion and Driver Circuits on LED Efficiency and Durability
Research shows that traditional LEDs require not only conversion but also sophisticated driver circuits to maintain optimal operation. This dependency is a primary contributor to both the increased cost and reduced lifespan of LED systems. It's essential for innovations in LED technology to address these inefficiencies to enhance both performance and economic viability.
These efficiencies are still high, yet challenges remain, so the next step is to develop LEDs and electroluminescence (EL) devices that can be driven by household AC currents—110 V/220 V at 50 Hz/60 Hz—without the need for complicated back-end electronics. One way that is being investigated is using quantum dot LEDs (QLEDs)
According to recent studies, the transition to quantum dot LEDs (QLEDs) holds significant promise. These advanced LEDs can potentially operate directly from household AC currents without the need for complex conversion systems, thereby addressing current limitations and paving the way for more efficient and longer-lasting lighting solutions.
Current AC Electroluminescence Developments
Developments in AC-driven electroluminescence (EL) devices have been extensively documented. Innovations include utilising phosphor emissive layers that function without direct current, thereby reducing energy loss and operational costs. Such advancements are critical for the future of efficient, cost-effective lighting solutions.
There have been various AC-driven EL devices developed over the years. These devices use a phosphor emissive layer sandwiched between two insulating layers to block the injection of external charge carriers. This prevents a DC current from flowing through the device. These devices behave like capacitors, where the electrons (generated from trapped sites) can tunnel into the emissive layer in the presence of a high AC electric field. Once the electrons have gained enough energy, they impact the luminescent centres to generate excitons and emit light. However, these devices have low levels of brightness and efficiency, making their application scope limited, and attempts to improve the performance have resulted in an incompatibility with household AC electricity.
Advancements and Limitations of AC-Driven Electroluminescent Devices
A recent development that has come to the fore has been an AC-EL unit that can be operated at 50 Hz/60 Hz and consists of two side-by-side DC devices. These two devices can be turned on alternately by shorting the top electrodes and connecting the bottom electrodes to an AC source. The AC-EL unit is created by serially connecting a forward device and an inverted device, so when the forward device is on, the inverted device is off and behaves like a resistor. This type of architecture has shown promise, but because it only operates at low voltages, it cannot be integrated into 110 V/220 V standard household electricity.
Notably, the AC-EL unit represents a significant advancement. By serially connecting forward and inverted devices, researchers have developed a mechanism where these units can operate alternatively, significantly enhancing energy efficiency and operational stability. However, further refinement is required to adapt these units for higher voltage applications typical in household environments.
The Tandem QLED Device can Operate in Positive and Negative AC Cycles
Until recently, no AC-EL device could be directly driven by 110 V/220 V at 50 Hz/60 Hz household electricity with a noteworthy efficiency or any semblance of longevity. Researchers have now developed a tandem QLED device that can be operated at both positive and negative AC current cycles. QLEDs have a range of beneficial features—which is why we’re seeing them being utilised in more displays nowadays—such as tuneable emission colour, high colour saturation, and high brightness levels.
Advantages of QLED Technology for Modern Applications
Recent advancements in QLED technology now allow for direct operation under standard household AC conditions, achieving unprecedented efficiency and longevity. These QLEDs are not only highly efficient but also exhibit superior colour accuracy and brightness, making them ideal for modern display and lighting applications.
The high external quantum efficiency (EQE) achieved in both positive and negative AC cycles highlights the potential of QLEDs for practical applications. By eliminating the need for AC to DC conversion, these devices offer a streamlined solution for household lighting, significantly reducing costs and improving energy efficiency.
Streamlined Household Lighting Solutions with QLEDs
The tandem devices are composed of two vertically stacked QLED that are connected by a transparent and conductive indium-zinc-oxide (IZO) intermediate electrode. The top and bottom electrodes become shorted as a common electrode in this design and the IZO becomes a counter electrode, and this allows two QLEDs with opposite polarities to be connected in parallel. These are the basic building blocks that can be used to build complete lighting devices that work on AC.
These tandem devices offer a unique approach to achieving high efficiency and continuous light emission. By utilising indium-zinc-oxide (IZO) as an intermediate electrode, researchers have managed to create a setup where QLEDs can operate efficiently in both AC and DC modes. This design not only simplifies the overall architecture but also enhances the performance and durability of the lighting systems.
Advantages of Plug-and-Play QLED Systems
When multiple QLED building blocks are connected in series, a household AC electricity plug-and-play QLED light source is obtained—which can be directly plugged into a household AC socket. Under 220V/50 HZ, the plug-in and play QLED system obtained a power efficiency of 15.7 lm W-1 and a tuneable brightness from 1000 cd m-2 up to 25,834 cd m-2 when varying the AC voltage.
The plug-and-play nature of these QLED systems marks a significant advancement in lighting technology. With the ability to adjust brightness levels efficiently and operate directly from standard household AC sockets, these systems present a cost-effective and energy-efficient alternative to traditional lighting solutions. This innovation is set to revolutionise both residential and commercial lighting applications.
The QLED devices were found to exhibit a continuous light emission during the entire drive cycle and could be operated over a wide frequency range—from several Hz to MHz range). Additionally, many LED suffer from the accumulation of charges and joule heating due to the continuous unidirectional flow of electrons through the device. However, in the QLED device, the accumulation of charges and joule heating is significantly reduced under an AC driving current, meaning that the QLEDs have longer lifetimes.
The developed plug-and-play system can be directly used with household AC plug sockets, removing the need for AC-DC convertors or complex back-end circuitry. LEDs and QLEDs are becoming increasingly important lighting technologies because they are more energy-efficient than standard lighting. Developing QLED systems that can directly work with AC currents could help develop a range of low-cost, compact, efficient, and stable solid-state light sources.
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