Tailoring kesterite solar cells for indoor power
Many of today’s commercial solar technologies work well outdoors, but struggle inside. Despite falling costs overall, indoor solar can still be expensive, may contain toxic materials and often degrade over time. These limitations mean that the widespread use of indoor photovoltaic (IPV) cells is still limited, when it is needed to power the increasing number of devices in the ‘internet of things’ (IoT). Kesterite materials offer a promising solution: they are widely abundant, comparatively non-toxic and stable. Yet kesterites face challenges due to inherent difficulties with indoor lighting. “Indoor LED light is much weaker than sunlight and concentrated in the visible range, so outdoor-optimised kesterites are not ideal,” explains Edgardo Saucedo(opens in new window), senior scientist at the Polytechnic University of Catalonia(opens in new window) (UPC) – BarcelonaTech, and LEKPV project coordinator. “LEKPV therefore aimed to tailor kesterite materials and devices specifically for indoor energy harvesting in IoT and low-power electronics.” Through the LEKPV project, which was funded by the Marie Skłodowska-Curie Actions(opens in new window) programme, Saucedo and his colleagues combined advanced simulations with eco-friendly processes and novel designs for kesterites, aiming to improve their efficiency and usher in a new wave of IPV technologies. “Because kesterites can be processed by low-cost, scalable solution routes and use abundant elements, they have promising potential for future large-scale indoor energy-harvesting applications,” remarks Yuancai Gong, a postdoc researcher at the UPC and LEKPV lead scientist.
Targeted engineering for indoor light
The researchers combined advanced device simulations, solution-based absorber fabrication and targeted device engineering to understand and improve kesterite solar cells for indoor light harvesting. They ran complex simulations to identify the most suitable bandgap range and key loss mechanisms under LED illumination. Through a series of experiments, the researchers developed molecular ink and specific solution-processing routes so that kesterite ‘absorbers’ could be coated onto surfaces. They then altered the compositional engineering of kesterites, by for example adding germanium alloys to improve the overall properties of the materials. They also optimised the interface of the absorbers, where the light-absorbing layer meets other layers of a device, to improve performance in low-light settings.
World-beating efficiency
A key published result was a specific kesterite indoor solar cell achieving 15.1 % efficiency, while alloyed devices showed indoor efficiencies of up to around 18 % under LED conditions. “That is so far the highest efficiency reported at worldwide level for this technology,” says Saucedo. “LEKPV demonstrated that kesterite solar cells are a promising technology for indoor photovoltaics, especially when their bandgap and defect properties are tailored to LED illumination,” notes Saucedo. While the technology is not yet ready for immediate market deployment, the project outcomes are directly relevant for self-powered IoT sensors, smart-building devices and low-power electronics, where replacing or reducing the number of disposable batteries is an important market driver. Saucedo notes the project shows that IPVs should not simply reuse materials and concepts developed for outdoor solar. “Indoor light harvesting requires dedicated material design, testing protocols and device optimisation,” he adds. “Kesterite is particularly interesting in this context.”
Towards validation in real-world environments
The researchers plan to improve device efficiency, reproducibility and operational stability further, while moving from small-area cells towards mini-modules and demonstrators. “Further work will focus on refining the absorber composition, reducing defect-related losses, improving interfaces and validating the devices under real indoor environments,” says Saucedo. “We also aim to strengthen collaboration with researchers and potential end users in IPVs, IoT and smart-building applications, so that the scientific results can move closer to practical use.”
