Researchers are constantly exploring new and efficient energy-harvesting technologies, with solar energy and photovoltaic (PV) cells being among the most widely used. However, the Shockley-Queisser limit, which states that the maximum conversion efficiency of single-junction solar cells is about 33 %, has prompted scientists to seek alternative solutions.
Optical rectennas have emerged as a highly efficient option for energy harvesting. These devices combine an antenna and a rectifying diode, allowing them to capture and convert electromagnetic energy into usable electrical power.1
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How Does an Optical Rectenna Work?
An optical rectenna is a device that integrates an antenna and a diode to convert electromagnetic energy, particularly from light, into electricity. Unlike conventional solar cells that operate on quantum principles treating light as particles, optical rectennas utilize the wave nature of light. These devices feature a submicron antenna and an ultra-high-speed diode that work together to absorb and rectify optical frequencies.2
Traditional solar cells treat light as a collection of particles (photons), while an optical rectenna operates by capturing and converting incoming electromagnetic waves into electrical energy.
The antenna receives electromagnetic waves, generating an alternating current (AC) within the circuit. A low-pass filter ensures impedance matching between the antenna and rectifier, allowing only signals within the desired frequency range to pass while blocking unwanted frequencies.
The rectifier, typically a diode, converts the AC signal into direct current (DC), which can then be supplied to a load. Many optical rectennas are based on Brown’s design, which uses a single-diode clamping circuit to ensure the output voltage remains positive.3
The antenna’s design varies depending on the targeted frequency range—larger antennas are used for microwave frequencies, while compact antennas are suitable for infrared and optical waves.
Modern rectennas often eliminate the low-pass filter, enabling broadband frequency harvesting up to the device’s cutoff frequency. However, this modification complicates impedance matching between the antenna and rectifier, potentially reducing efficiency.
The efficiency of transferring the rectified DC signal to the load largely depends on the diode’s performance. Nevertheless, a well-designed DC-pass filter helps integrate the rectenna into contemporary energy systems.
Optical Rectennas for Light Harvesting
Optical rectennas use solar energy through photon quantization. The device operates at high frequencies and low photon flux, following classical rectification principles. In this process, an alternating voltage applied to a nonlinear diode results in rectified current through continuous interaction with the diode’s characteristic curve. Photon-assisted tunneling occurs at higher frequencies, where photon energy matches the diode’s nonlinearity.
When a high-frequency AC voltage is applied across a tunneling device, electronic states near the Fermi level become separated by corresponding photon energies. In this process, electrons absorb and emit individual photons as they tunnel through the insulating barrier. The diode’s AC resistance and responsivity are influenced by both photon energy and the diode’s intrinsic properties.4
High Efficiency and Compact Design
Optical rectennas are significantly more efficient than traditional energy-harvesting devices. In 2021, researchers at the University of Colorado at Boulder demonstrated resonant tunneling in metal-double-insulator-metal (MI2M) diodes used in optical rectennas. This advancement improved efficiency by nearly 100 times compared to previous designs.
By varying the NiO thickness in MI2M diodes, researchers optimized the current-voltage characteristics of the optical rectenna. Modifications to the quantum well’s depth and width allowed alignment between the metal Fermi level and quasi-bound states within the well. Additionally, by applying a higher potential difference across the diode and modifying oxide voltage distribution, scientists improved capacitive voltage division at high frequencies.
Deep wells are essential for accessing quasi-bound states at low voltages, which enhances energy-harvesting capabilities. The NiO/Al2O3 MI2M diode rectennas have achieved a remarkable increase in total conversion efficiency, surpassing previous benchmarks by a factor of 100.5
Additionally, these devices are highly compact and can be integrated into wearable devices for industrial applications. Millions of people use smartwatches, fitness bands, and other wearable electronics that require continuous power sources. Energy harvesting through flexible rectennas is an attractive solution, as these devices provide a reliable power supply without compromising performance due to bending or mechanical stress.
Experimental studies on flexible rectennas in wearable electronics have demonstrated outstanding conversion efficiencies of approximately 78 %, positioning optical rectennas as a key technology for modern electronics.6
Challenges and Emerging Solutions for Optical Rectennas
Regarding commercialization and large-scale implementation, optical rectennas face several challenges. Designing and modeling metal-insulator-metal (MIM) diodes for rectennas is a complex and time-intensive process, requiring precise material selection to achieve the desired voltage-current characteristics. Maintaining precise insulation thickness is necessary for quantum-well tunneling, which adds to fabrication complexity and costs.
Another significant challenge is integrating the diode with the antenna. Low coupling efficiency between these components can substantially reduce the overall performance of the rectenna system.7 Researchers are actively working on optimizing these aspects to improve efficiency and reduce manufacturing costs.
Artificial intelligence (AI) is being used to optimize antenna and rectifier performance. Advanced simulation tools are helping refine rectenna designs, making them increasingly viable for next-generation applications, including 5G and the Internet of Things (IoT).
With ongoing advancements, optical rectennas could achieve over 85 % conversion efficiency within the next decade, creating new opportunities for commercial applications in the energy-harvesting sector.
As research progresses, optical rectennas are expected to contribute significantly to sustainable energy solutions. To learn more about energy-harvesting technologies, consider exploring advancements in thermoelectric generators, piezoelectric materials, and wireless power transfer systems. Additionally, you may find these articles insightful:
References and Further Reading
- Bhateja, Y. (2024). Optical Rectennas-A Review. [Online] Preprints.org. Available at: https://www.preprints.org/manuscript/202409.1932/v1
- Yadav, D. (2021). Solar energy harvesting by carbon nanotube optical rectenna: A review. IEEE International Symposium on Sustainable Energy, Signal Processing and Cyber Security (iSSSC). https://doi.org/10.1109/iSSSC50941.2020.9358863
- Donchev, E., et al. (2014). The rectenna device: From theory to practice (a review). MRS Energy & Sustainability. https://doi.org/10.1557/mre.2014.6
- Zhu, Z., et al. (2013). Overview of optical rectennas for solar energy harvesting. Next Generation (Nano) Photonic and Cell Technologies for Solar Energy Conversion IV. https://doi.org/10.1117/12.2024700
- Belkadi, A., et al. (2021). Demonstration of resonant tunneling effects in metal-double-insulator-metal (MI2M) diodes. Nat Commun. https://doi.org/10.1038/s41467-021-23182-0
- Singh, N., et al. (2024). Ultra-thin flexible rectenna integrated with power management unit for wireless power harvester/charging of smartwatch/wristband. Sci Rep. https://doi.org/10.1038/s41598-024-57639-1
- Bhatt, K., et. al. (2017). Potential challenges and issues in implementation of MIM diodes for rectenna application. 2017 International Conference on Inventive Communication and Computational Technologies IEEE. https://doi.org/10.1109/ICICCT.2017.7975164
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