Artificial light could power a range of autonomous devices
Autonomous off-grid energy harvesters can tap energy from a variety of environmental sources – including vibrations, heat and light – to power low-consumption devices, such as sensors and dataloggers. “These innovations are of particular interest for internet of things devices, as they reduce dependency on batteries, minimising maintenance and waste,” says Mariano Campoy-Quiles, coordinator of the Marie Skłodowska-Curie Actions(opens in new window) supported HOPES project. HOPES tailored solar cells to harvest indoor artificial light from LEDs and bulbs, identifying the best light for a given absorber and fine-tuning this ‘harvester-emitter’ coupling to increase efficiency. “Many light capturing materials exist with an absorption spectrum which overlaps well with many light sources. But there are also many kinds of low-intensity artificial lights – with wavelengths of 400 to 700 nanometres, two to three times less intense than sunlight – that aren’t being harvested,” explains Campoy-Quiles from the Institute of Materials Science of Barcelona(opens in new window).
Optimising power conversion efficiency
HOPES was able to quickly screen for the most suitable absorption materials by combining very fast photovoltaic evaluations (around 50 times faster than conventional methods), with an on-demand light source – equipment generating different spectrum illumination conditions. Varying the application speed of customised coatings on the light harvester’s active layer, enabled the team to fabricate multiple thicknesses to be evaluated simultaneously, rather than preparing separate devices for each condition. “This created a much larger experimental dataset, as well as increasing measurement speed. By introducing controlled thickness gradients in the active solar cell layer, we were able to evaluate many device conditions within the same experiment,” notes Muhammad Ahsan Saeed, lead researcher. As the chemical structure of organic semiconductors(opens in new window) influences their optical properties, HOPES tweaked the organic materials to best match the absorption profile to specific artificial light emissions. High-throughput screening of organic materials and their blends was performed in solar cells of various thicknesses (50-450 nanometres), and with active layers of different gradients of wedge. The resulting platforms were then evaluated against a variety of lights with different colour temperatures and light intensities. “We demonstrated that different artificial lights should be harvested by different materials – reinforcing our idea about the importance of the harvester-emitter marriage. We also offered a fundamental explanation of what governs such behaviour(opens in new window),” says Saeed. The team achieved a conversion efficiency of up to 30 % for indoor lighting, compared to organic solar cells operating under the sun which typically returns between 15 and 20 %. “A key finding was that achieving high indoor efficiency wasn’t simply about using the highest-performing material under solar illumination. The type of material, active-layer thickness and illumination spectrum, all must be considered together for the optimal output,” explains Saeed.
Devices that are efficient, stable and reproducible
Powering autonomous sensors by indoor lighting could increase their adoption for applications such as gas sensors in smart buildings, shelf labelling in retail and fully wireless remote controls for consumer electronics. “Our findings can help companies reduce the experimental effort required to develop such devices. But while artificial and diffuse light is abundant, it is also diluted, so we need to further increase efficiency, even up to 40 %,” adds Campoy-Quiles. Meanwhile, the team are also considering combining optimisation processes with automated data analysis and machine learning, to inform game-changing advanced designs with market appeal.
