The Power of Light: Ambient Energy for Batteryless IoT

Portable electronic devices powered by small solar panels are certainly not a brand new thing: many of us will remember solar-powered calculators and watches, which were a hit in the 1980s. Yet, that was a trend that did not pick up, and batteries have taken over the business of powering small electronics almost entirely. But the era of batteries might be coming to an end. The growth of IoT together with technical progress in ultra low power electronics and photovoltaic cells for indoor environments are fueling a new opportunity for light-powered electronics. In this article, we review the latest tech trends within light-harvesting photovoltaics for indoor environments, the business opportunity within IoT and small electronics, and the companies leading this growing market space. 

The photoelectric effect and photovoltaics

The photoelectric effect is nature’s phenomenon that allows the production of electricity from light. Simply put, it consists of the emission of electrons when light (electromagnetic radiation) hits a material, for instance, a metal; these electrons (called photoelectrons) in turn can generate an electric current. The phenomenon was first observed experimentally by Heinrich Hertz in 1887 and later explained by Albert Einstein - a work that granted Einstein the Nobel Prize in Physics in 1921.

Photovoltaic cells take advantage of the photoelectric effect to produce electricity. In fact, photovoltaics can be more accurately explained by a closely related concept called the photovoltaic effect - which can be defined as the generation of voltage and electric current in a material upon exposure to light. The difference between the photoelectric and the photovoltaic effect is that the former usually refers to photoelectrons knocked out of the material completely, while the latter refers to photoelectrons able to flow but still confined within the material where light strikes. In all practical sense, photovoltaic cells - the building blocks of solar panels - have the wonderful ability to transform light into electricity!

Solar panels all around

The much-needed rush towards renewable energy and decarbonization created a growing interest in solar panels. In the past few decades, technology evolved to very high levels of efficiency while reducing costs at the same pace. This means that nowadays solar panels have become almost a commodity, which anyone can deploy at an affordable cost on their household rooftop. In many parts of Europe and other regions, governments provide incentives for families and enterprises to invest in decentralized solar power installations to save on energy bills and help decarbonise electric grids. Utility-scale centralized solar power plants are also being commissioned in many parts of the world.

Hence, solar power has become a major part of the renewable energy transition across the globe. According to data from the International Energy Agency, in 2022 solar energy generation reached almost 1300 TWh, representing 12.8% of the total installed power capacity on a global scale. It is the highest-growing energy source in the electricity generation mix, and is poised to dominate power grids by 2027 with a share of 22%, finally surpassing coal as the dominant source. Obviously, this means that solar power is a blossoming business too! Estimates point to the global market for solar panels being worth $152 billion in 2022, growing at a CAGR of 8.2% to reach $264 billion by 2030. 

Not all light shines the same

So, outdoor solar panels are a technical and commercial success, while championing the fight against climate change - that’s a wrap! But what happens indoors?

Direct sunlight is commonly considered to shine at 32.000 to 100.000 Lux - a measure of illuminance or light intensity as perceived by the human eye. For comparison, an overcast day represents 1000 Lux, roughly the same as the lighting in a typical TV studio, while office lighting is usually in the ballpark of 500 Lux and a comfy living room goes down to 100-150 Lux. To no surprise, light levels indoors are way lower than typical outdoor daylight. Artificial light sources used in indoor ambients include incandescent, halogen and fluorescent lamps, and, most frequently nowadays, LED (light-emitting diode) lights. Their emission spectra are significantly different from each other and quite different from solar light. In particular, most artificial sources emit only in the visible wavelengths, while the terrestrial solar spectrum extends towards the near-infrared wavelength ranges. And, notably, the power density of indoor light sources is within 60 to 300 microWatt per cm2 - which is approximately three orders of magnitude lower than that of terrestrial outdoor solar light. In sum, indoor light is different and holds less power than sunlight, which means that photovoltaics must be tweaked and tailored for higher efficiency in order to thrive under indoor lighting conditions. But, is it worth the effort?

Ultra low power taking over the IoT

The Internet of Things (IoT) is growing at a fast pace. The number of connected IoT devices is set to reach 16.7 billion by the end of 2023, growing at a CAGR of 16% to reach close to 30 billion by 2027. Still, the expectations have cooled off a bit lately - and the prophecy of 1 trillion IoT devices by 2035 feels now harder to fulfil. But why is that? If we look at the big picture, the pivotal question here might simply be: how do we possibly power one trillion electronic devices?

 Indeed, the growth of the Internet of Things (IoT) is driving a push for ultra low power electronics. IoT nodes (sensors, interfaces, tags and trackers) are typically powered by small batteries (coin cells), but there’s growing evidence that batteries are not a good solution for small electronics and are ultimately preventing the IoT industry from reaching its full growth potential. The problem has several angles, from maintenance of IoT networks and logistics of batteries recharging/replacement, to scarcity of key raw materials to make batteries and how these are mined, and further to end-of-life of the batteries themselves - as we have repeatedly mentioned in several articles. With all these factors in play, it’s hard to believe that the supply chain for batteries will be able to keep up with the demand for billions and billions (eventually trillions) of IoT devices. All things considered, the industry is pushing for more and more power efficiency, reducing size and extending lifetime of batteries as a mid-term solution, while looking for better alternatives. 

Speaking of alternatives, harvesting readily available ambient energy is certainly at the top of the list! The sources of such ambient energy might be radio frequency waves (from Wi-Fi routers, cellular networks, TV or radio transmission towers), thermal gradients (from industrial operations, the human body, etc.), mechanical power (movements, vibrations), or… light! 

IoT devices are spread over all kinds of environments. For instance, smart agriculture sensors or smart parking systems installed outdoors can be powered by sunlight. But, undoubtedly, a large portion of IoT devices are meant for indoor or hybrid (indoor and outdoor) environments. Hence, tailoring photovoltaic cells for indoor conditions is definitely worth the effort! In the following, we will focus on indoor photovoltaics and how harvesting indoor light is leading the way in powering IoT devices and small electronics. 

But before we move into that, let us briefly introduce some concepts that are important to compare the different technologies in play.

Key concepts in brief

The first concept is bandgap - a crucial parameter when it comes to active materials for photovoltaics. Technically, the bandgap refers to the energy required to promote an electron from the valence band to the conductive band in a certain solid material - expressed in electronvolts (eV). In photovoltaics, the optical bandgap determines the portion of the light spectrum that is absorbed by the cell. 

The second concept is the efficiency of photovoltaic cells or panels (i.e. a combination/grid of several cells), which is essentially how much of the incident energy from the light source is converted into electricity by the cell or panel - expressed in percentage.

A few other key parameters can be better understood from the so-called I-V and P-V curves - which are basically graphical representations of how a particular photovoltaic system (cell/panel) behaves. Here, the open circuit voltage (Voc) corresponds to the maximum voltage that the system can deliver, i.e. the voltage when there is no load/current drain. The short circuit current (Isc) is the current through the system when the voltage is zero. The P-V curve allows the identification of the maximum power point (mpp) for the system operation, which is where the respective voltage (Vmmp, voltage at maximum power point) and current (Impp, current at maximum power point) create the highest power output. A photovoltaic system can work without mpp tracking, but there are several advantages in implementing mpp tracking with the assistance of a power management integrated circuit (PMIC).

Finally, a note on measuring and comparing the efficiency of different photovoltaic cells under different conditions. There is a well-established standard to test the performance of photovoltaics under outdoor/sunlight conditions - based on the AM1.5G reference spectra. However, there is no equivalent standard for testing under indoor light conditions. Hence, it is extremely challenging to compare performance data of indoor photovoltaics from different sources. Most often, manufacturers report the performance of their indoor photovoltaic panels using tables or graphics showing the power density obtained (microWatts per cm2) at given Lux levels (typically 50 to 1000 Lux). Yet, the lack of standardized test conditions hampers fair and square benchmarking of the manufacturers’ claims.

Photovoltaics for indoors

As explained above, small electronics and IoT devices are progressing towards extreme power efficiency, opening the opportunity for energy harvesting technologies - where photovoltaics are leading the pack. Indoor photovoltaics benefit from a number of advantages, in particular: the common availability of lighting inside buildings; higher power density compared to other indoor ambient energy sources; and technology scalability, where panels can be made with areas from sub-mm2 to more than 100 cm2 - so that indoor photovoltaics can be used to power a wide range of IoT and small electronic devices. Not least, a transition to photovoltaics can foster environmental sustainability - replacing batteries with small photovoltaic panels based on eco-friendlier raw materials, while extending the lifetime of electronic devices.

The recent years have witnessed unprecedented technical progress within new materials and architectures for photovoltaic technology for indoor use. 

Crystalline silicon (c-Si) has become the standard active material for outdoor solar panels, providing the best match with the properties of sunlight. c-Si has a bandgap of 1.2 eV, within the optimum range for absorbing sunlight radiation, which is 1.1-1.4 eV. However, given that typical indoor light sources emit only in the visible range, the optimum bandgap shifts to 1.9-2.0 eV. This means that c-Si does not work well with indoor light. Hence, researchers within academia and the industry have been developing and testing alternatives. The leading light-harvesting technologies for indoors are based on amorphous silicon (a-Si), III-V semiconductor materials, dye-sensitized solar cells (DSSCs), organic photovoltaic cells, and perovskite solar cells. Let’s take a brief look at the current status and the pros and cons of each of these technologies.

Amorphous silicon has been in use for many years: for instance, it was the preferred choice for the photovoltaic panels popular in electronic calculators back in the 1980s and 90s. The bandgap of a-Si materials is around 1.6 eV, closer to the optimum for indoor light conditions than c-Si. Compared to to c-Si, a-Si cells typically show higher efficiency under indoor light conditions (up to 21% for a-Si vs. 4-6% for c-Si under LED lighting), but lower efficiency under standard sunlight conditions (8% for a-Si Vs. 17-18% for c-Si). This behaviour is attributed to a higher Voc under low light together with the larger bandgap in a-Si. a-Si can also be deployed in thin-film form using low-cost flexible substrates - benefiting from cost-effective manufacturing processes. Still, it is challenging to get a consistently high performance from a-Si as active material for indoor photovoltaics.

III-V photovoltaic cells are based on so-called III-V semiconductor materials - such as gallium arsenide (GaAs), aluminum gallium arsenide (AlxGa1-xAs) or other alloys - as the active layer. These are very efficient under sunlight, excelling indoors as well due to the wider bandgap compositions possible and the potential for high power densities. III-V photovoltaics have been used in high-end applications, like powering satellites in space, but the high fabrication costs are a strong barrier to their wide commercial adoption.

DSSCs are based on light-absorbing dyes deposited on a scaffold (e.g. TiO2) and interfaced with a redox mediator. Their mechanism of operation is somewhat similar to the photosynthesis processes that occur in plants. Dyes can be sensitive to various wavelengths and intensities in the visible spectrum, so DSSCs can be made suitable for both outdoor and indoor conditions. Notably, some dyes have bandgaps close to optimum for indoors, making DSSCs a particularly efficient solution for low illumination levels - with reported efficiencies up to 30%. However, there are several areas of improvement towards more sustainable and reliable DSSC technology - including new dyes, more stable alternatives to liquid electrolytes (which are commonly used and become a source of instability), innovation in device architecture and cost-effective fabrication techniques (such as large area thin film deposition and roll-to-roll processing).

Organic photovoltaic cells have carbon-based organic semiconductors in the form of small molecules or polymers as the active material. Similarly to DSSCs, they have a good spectral/bandgap match with indoor light, hence showing better performance indoors and under relatively low light conditions - with reported efficiencies also approaching the 30% mark. Organic photovoltaics have shown a number of advantages, such as: processability using solution-based techniques at low temperatures, possibility to form flexible structures and fabrication using flexible substrates while keeping good performance, compatibility with roll-to-roll manufacturing processes, and low toxicity of the organic materials used (although there are environmental concerns with the solvents used during manufacturing). On the downside, they show higher degradation rates and low stability - limiting their lifetime. Research avenues on organic photovoltaics have been focusing, for instance, on trying various organic semiconducting materials, techniques for increasing the optical bandgap of the active materials, as well as reducing production costs.

Perovskites have emerged as a promising class of materials for photovoltaics. These refer to a family of ternary materials with a general chemical formula represented by ABX3. Perovskites have been exploited both for solar/outdoor and indoor applications - exhibiting promising performance under both conditions. They’ve shown great photophysics properties (such as high absorption coefficients and high charge mobility), as well as unique intrinsic defect tolerance. Besides, they’re prone to simplified and cost-effective manufacturing: for instance, they can be solution-processed at low temperatures and deposited on thin-film flexible substrates with roll-to-roll or sheet-to-sheet processes. Efficiency levels for indoor perovskite photovoltaics are at least on par with those in DSSCs or organic cells, with reports reaching 30% and higher. The main drawbacks with perovskite-based cells are poor ambient stability, toxicity (mainly because lead is part of their composition and there are concerns with its leakage) and electronic band structure issues. Yet, researchers are on the move to mitigate these issues, including attempts to develop non-toxic perovskite compounds by replacing lead with benign elements. 

For a deeper dive into the technical details of indoor light harvesting technologies, please refer to the recent review articles on the matter - e.g., Pecunia et al. 2021, Li et al. 2021, Li et al. 2020, Biswas and Kim 2020, Mathews et al. 2019. 

At this stage, it seems too early to draw a conclusion on which technology(ies) will win this market. Yet, it seems that so-called third-generation technologies are taking the front - with DSSCs and organic photovoltaics seeing most of the close-to-market activity at the moment, while perovskites show great potential and the fastest acceleration in efficiency ever seen in this space. These third-generation options are prone to simpler, cheaper and more sustainable manufacturing - namely thin film deposition, with printing and roll-to-roll processes at relatively low temperatures and without vacuum - which provides a clear edge over silicon-based products.

Summing up, there are several contender technologies but no clear winner yet on the indoor photovoltaics playing field. Some are better in performance and efficiency, others in cost and manufacturability. Regardless of the intrinsic pros and cons of each of the technologies, it becomes clear that the ways in which these are tweaked and tuned by developers are crucial for real-world applications. Besides, there will likely be space for more than one technology type in this market - at least in the coming period before the market matures and consolidates. For instance, some applications might depend on high efficiency, others on durability, and yet others might be more sensitive to cost. Also, there will be applications requiring cells printed in flexible substrates, while others live better with rigid modules. Next, we will drive along the companies at the leading edge of the indoor photovoltaics market.

Market players within indoor photovoltaics

Currently, there are companies developing and/or commercializing indoor photovoltaic products based on all the technology categories described above. Amorphous silicon is considered second-generation solar technology - following first-generation crystalline silicon - and panels are available from several sources, such as Panasonic (Japan), Solems (France) and Powerfilm Solar (US). As for third-generation technologies, the emerging players include: Lightricity (III-V semiconductor materials); Ambient Photonics, Exeger and GCell (DSSC); ASCA, Dracula Technologies, Epishine and Ribes Tech (organic photovoltaics); Perovskia Solar, Rayleigh Solar Tech and Saule Technologies (perovskites); Ricoh and others (working on DSSCs, organic and perovskites).

Lightricity is a UK-based start-up that explores light harvesting technology developed at Sharp Laboratories in Oxford using III-V materials, with a focus on efficiency. The modules are rigid, but size, shape and thickness can be customized. Their lead application areas include asset trackers, building automation devices and watches/wearables.

Ambient Photonics, headquartered in California, US, provides DSSC technology based on novel, proprietary molecules. The company claims light harvesting efficiencies 3-5 times better than conventional Ruthenium DSSC dyes. Their dyes have tunable properties enabling to optimize cells for different indoor light sources. They produce rigid panels on glass substrate, which can have any size and shape, using scalable industrial printing processes. The primary target applications include remote controls, computer keyboards, electronic shelf labels (ESLs) and smart building/home devices.

Exeger is a scale-up company developing and producing photovoltaic products from Stockholm, Sweden. They provide indoor and hybrid (indoor and outdoor) versions, based on DSSC technology. Their modules are flexible and customizable not only with regards to size and shape, but also in terms of textures and graphics that each customer can choose to print, e.g. to sustain brand aesthetics. The company is targeting several application segments, namely consumer electronics (e.g. headphones, speakers, computer accessories), IoT (sensors, trackers, ESLs, remote controls) and smart workplace devices (helmets, hearing protection, vests).

GCell is another company providing DSSCs, with headquarters in the UK. Their modules are manufactured through roll-to-roll processes, being flexible, prone to customization and easy integration into final products. Their lead product cases are computer keyboards and i-beacons. 

ASCA is developing organic photovoltaic films capable of making virtually any surface active. The films are highly flexible, enabling curved or wavy surfaces, transparent (up to 50%) and offered in different colors - with a focus on design freedom. The company has production facilities in France and Germany, implementing high-speed roll-to-roll processes for printing on flexible PET films. The range of application targets includes outdoors (building-integrated photovoltaics, urban furniture, e-mobility, smart cities and agriculture) and indoor products (smart homes, computer keyboards, clocks, clothing and accessories, etc.).

Dracula Technologies is a French company developing organic photovoltaics, using inkjet printing for manufacturing thin, flexible, free-shape and lightweight modules that excel under indoor low light conditions. Lead areas of application are within devices for smart buildings and homes, remote controls and trackers.

Epishine (Linköping, Sweden) is another fast-growing player working on organic photovoltaic cells and modules. They offer thin-film, flexible and lightweight products, which can be customized by using surface overlays, e.g. to provide texture. The company has established production using roll-to-roll processes and is scaling up capacity. Their primary application targets are smart homes and building sensors, asset tracking, remote controls and ESLs. 

Ribes Tech (Milan, Italy) is a fourth player implementing printed organic photovoltaics, offering extremely flexible, lightweight and custom-shaped modules. Lead use cases focus on smart labels, bracelets/wearables, IoT sensors and smart home devices.

Perosvskia Solar is a Swiss company combining core expertise within printed electronics and perovskite photovoltaic technology. Their modules are rigid (glass substrate), customizable in size and shape, highly efficient indoors and easy to integrate into end products. They propose cost-effective inkjet printing technology using novel green nanoparticle inks based on perovskites. Targeted applications span across smartwatches, trackers, smart homes and consumer electronics.

Rayleigh Solar Tech is a Canadian start-up developing thin and flexible perovskite photovoltaic cells. They’re exploiting roll-to-roll printing technology to reach scalable and low-cost manufacturing of cells of any size and shape. The perovskite-based composition of the cells is tunable to both indoor and outdoor conditions. The company is looking at several outdoor application areas, such as e-mobility, building-integrated solar cells and agriculture, but also at indoor applications given the technology’s potential under low light ambients.

Saule Technologies (Poland) is another company providing perovskite-based photovoltaics printed on thin and flexible foils, and promising great performance across different light environments. They’re also addressing both outdoor markets (namely building-integrated and building-attached photovoltaics as well as e-mobility) and indoor IoT product categories (headphones, computer keyboards, ESLs, smart home devices).

The Japanese multinational Ricoh is working on all three third-generation solar technologies, having announced DSSC and organic photovoltaic prototypes and sample products. Other large industrial groups have also research and innovation activities on this front, such as Toshiba, Panasonic and Sharp. 

A market ecosystem for energy harvesting

Powering an electronic system with energy from light requires more than just a light-harvesting photovoltaic panel. As a minimum, the system will need a so-called power management integrated circuit (PMIC) and, for most applications, some kind of electronic component to store energy. 

The same goes for any other type of energy harvester - which will require a similar electronic setup. This calls for the development of a market ecosystem around energy harvesting, where product developers can pick and choose from multiple suppliers of electronic components - from different types of energy harvesters (solar/light, thermal, kinetic, radio frequencies, etc.), to power management electronics (microcontrollers/PMIC), and to energy storage elements (conventional or new forms of capacitors, supercapacitors or rechargeable batteries).

ONiO contributes to such an ecosystem with a unique microcontroller, custom-designed for energy harvesting applications - ONiO.zero. ONiO.zero is breaking records on ultra low power for IoT and small electronics, providing the best-in-class power management platform for energy harvesting from multiple sources. This obviously includes light-harvesting photovoltaic devices, with ONiO.zero featuring advanced maximum power point (mpp) tracking capabilities. On top of this, ONiO.zero presents a playground for developers - offering full microcontroller capabilities, wireless communication, a rich suite of interfaces, advanced security features, memory, etc. 

In conclusion, the time is ripe for energy harvesting to definitely take off. The latest technical advances within ultra-low-power electronic and energy harvesters (photovoltaics and others), in parallel with an IoT market in clear expansion, are laying the foundation for a blossoming energy harvesting ecosystem - which is set to end the rule of coin cells in small electronics.