Solar cells that work indoors? They have been in digital watches and calculators. But, this category seems to be undergoing a revolution now. Epishine, a Swedish firm, claims to enable a whole new set of indoor IoT devices powered by their organic solar cells. Electronics For You’s Nidhi Agarwal spoke to Jonas Bergqvist, Chief Technology Officer at Epishine, and here is what she found out…
Q. How do you make indoor solar cells, and what makes them unique?
A. We use organic solar cells at Epishine. ‘Organic’ means that the semiconductor, or active layer, used for these solar cells is based on hydrocarbons. The technology we work with is polymer solar cells where we in the current version use a fine blend of long polymer chains and small molecules called fullerenes, in what can be compared to a very thin layer of spaghetti and meatballs. Our solar cells are like a sandwich, with the bread being protective plastic films with a number of thin layers in between, where the spaghetti and meatball layer is in the middle. Between the plastic film and the active layer, there is also an electrode that transports photo current to the contacts and other thin layers that help to direct the photo current within the solar cell.
We make these layers using printing and coating, mainly screen printing and slot-die coating, in a roll-to-roll process. What is special is our lamination method. We put electrode and polymer layers on two separate substrates, then join them with heat and pressure. This gives us good quality and works well in low light. Compared to older indoor solar cells, like in calculators, ours perform better and are more robust.
Q. Can you tell us what organic solar cells are?
A. Organic solar cells use special materials from nature to turn sunlight into electricity. We are using materials that are based on carbon. We use PET substrates; which are about 200 microns thick, and the solar cell itself is less than a micrometre. The cell material is mainly conjugated organic molecules, and the main component is the PET substrate. There is also a small silver print on the side for conducting current from the cell to the contacts.
Q. How does your technology turn indoor light into electricity?
A. It is about using the photovoltaic effect, like regular solar cells, but for indoor light from LEDs or fluorescent sources. Indoor light is much weaker than outdoor light, often 100 to 1000 times less intense. Our cells focus on absorbing visible light, unlike silicon cells that also absorb near-infrared for more power. A key for indoor cells is high parallel resistance or rather to prevent short circuits, especially important in low light. This prevents performance drops. So, similar to standard solar cells, but with a focus on eliminating also minor short circuits for indoor effectiveness.
Q. What steps have you taken to increase their efficiency in low light?
A. Our main goal has been to launch our product, focusing on organic solar cells. Currently, the active layer uses a conjugated polymer mixed with fullerenes, which are like small carbon spheres with a functional tail, allowing for decent performance. However, our R&D team is working to replace these fullerenes with different small molecules, which could potentially double our solar cells’ power conversion efficiency. This involves tweaking some components in the active layer, using what is known as non-fullerene acceptors in academic circles. Our aim is to improve power conversion efficiency through these changes.
Q. How are organic photovoltaics different from regular solar cells and how do they help IoT applications?
A. Our innovation benefits IoT applications because it offers high, consistent performance, especially important for indoor IoT products where light varies a lot. You can trust the solar cell’s minimum performance and only need to consider light intensity changes based on placement. Additionally, our solar cells are lightweight, only 0.2 millimetres thick, have low environmental impact, and are easy to integrate.
Q. Could you provide data or results from real-world indoor performance testing?
A. The indoor solar cell measurements typically yield around 20 to 22 microwatts per square centimetre at 500 lux (LED illumination) in an office setting. We collaborate with companies like ELCs for products like temperature and humidity sensors. We have data on light intensity expectations for various environments, including offices, homes, and retail spaces. Customers with products in the market provide additional data. It is essential to balance power needs based on expected light levels for IoT products using indoor light. We have deployed several products in real-life applications, demonstrating their effectiveness.
Q. How do you help in making ultra-low-power microcontrollers batteryless?
A. We have seen a convergence between our indoor solar cell technology that can harvest energy in very low light and the decreasing power draw of ultra-low-power microcontrollers. This convergence enables the deployment of low-power IoT sensors indoors. The current challenge is to make these ultra-low-power electronics compatible with solar cells, considering their different behaviour compared to batteries.
Q. Is there any energy storage system? How does the microcontroller operate in the absence of light?
A. We can deliver around 500 microwatts from our standard 50x50mm PV module in an indoor office environment (500 lux), Microcontrollers consume much less on average but have higher peak currents, so a reservoir is needed. For devices that work only when there is light, a small capacitor is sufficient. If you want the device to operate during the night, weekends, or extended dark periods, a larger storage element like a supercapacitor is needed. For longer durations or seasonal changes, a rechargeable battery, or hybrid capacitor is used. Emerging technologies like printed batteries and solid-state batteries are also being explored.
Q. Does your solution currently include an integrated energy storage system?
A. We specialise in solar cells and guide IoT companies on adopting energy harvesting, providing system proposals. While we do not sell complete systems, we offer valuable recommendations. In this collaboration, our contribution is the solar cell. The ultra-low-power microcontroller is specifically designed for energy harvesting, making it a better fit for our solar cells.
Q. What should manufacturers consider when using indoor solar cells in their devices?
A. Manufacturers in the IoT product space typically have experience with components that are suitable for SMT soldering, reflow soldering, wave soldering, and similar techniques, along with easily accessible connectors. Our product involves thin-film and printed electronics. We have collaborated with the industry to identify effective methods for soldering or gluing components onto a board, as well as integrating mechanical parts seamlessly. Currently, we provide support for these processes. Eventually, these processes may become standard practices, enabling the industry to handle them independently.
Q. How do you ensure the durability of indoor solar cells in varying light conditions?
A. We conduct extensive long-term accelerated aging tests on our production batches. This involves subjecting the modules to various conditions such as damp climate chambers, ovens, and intense light exposure, both continuously and intermittently. This is done to hasten the exposure of modules to high levels of photons, water molecules, and temperature fluctuations. We also have modules on office walls that have shown consistent and reliable performance over three years.
Q. What does the competitive landscape look like in the indoor solar cell industry?
A. For indoor solar cells, it is mainly driven by companies working on organic solar cells, like ours and DSSC, and perovskite solar cells, which show promise but are a bit behind. Some work is going on with gallium arsenide, a high-performance material, but it is costly. Amorphous silicon manufacturers also play a role.
Q. What sets Epishine apart from competitors in the market?
A. We differentiate through flexible, reliable, high-performance solar cells with a competitive cost structure. We offer both individual cells and series-connected cells, allowing for adaptable voltage output to meet electronic requirements.
Q. How are your indoor solar cells better, and how much more efficient are they?
A. Our indoor solar cells provide a power conversion efficiency of approximately 15% under 500 lux LED lighting. It is worth noting that there is a new measurement standard for indoor solar cells, and it may take time for the market to adjust. To make fair comparisons, indoor solar cells from various suppliers should be tested under the same lighting conditions.
Q. What other applications do you see for indoor solar cells besides IoT?
A. Indoor solar cells find use in remote controls, electronic shelf labels, and low-power consumer electronics such as earphones, keyboards, and mice. While we have not launched products in this market, we are aware of competitors who have. We are actively discussing partnerships and opportunities in various sectors. Currently, our main focus is on IoT connected devices, but we are also exploring other markets, with a keen interest in electronic shelf labels.
