BY ANTHONY KING
Outdoors, solar energy has hit boom time, accounting for up to three-quarters of extra renewable energy capacity in 2023 and surpassing wind for the first time. Next, indoor solar will be crucial for powering the sensors and controls soon to become ubiquitous in our offices and homes. Anthony King reports
Estimates suggest there will be around 5bn cellular Internet of Things (IoT) devices by 2030 – a number that could ultimately stretch to 200bn. But neither disposable nor mains rechargeable batteries will be convenient or sustainable power options.
‘We expect sensors to be distributed everywhere, with 70% in indoor settings,’ says Marina Freitag, a Professor of energy at Newcastle University, UK. ‘This network is supposed to make our life more sustainable and energy efficient, but we need to power them somehow,’ she adds.
Freitag and others are working on materials that harvest the light bathing our living and workspaces. The nature of this light is markedly different from the outdoors. A person reading beside a table lamp is exposed to around 200lux – the unit of illuminance – while someone working in a super-bright hospital will experience around 1000 to 5000lux. Light outside contains vastly more energy.
‘Typical indoor light levels are less than 1% of what you would have outdoors,’ says Matt Carnie, a materials scientist at Swansea University, UK, ‘with 100,000lux outdoors on a bright summer day. Also, indoor light is comprised almost wholly of visible light, while around 5% of solar energy outdoors is ultraviolet and 50% is infrared.’
The basic components of all solar cells are semiconductors designed to absorb light’s energy and transfer it to an electron, generating an electric current. An important factor in photovoltaic (PV) semiconductors is bandgap, which is the wavelength of light the material can absorb and convert into electrical energy. Silicon-wafer technology accounted for 97% of total production of PVs in 2023, notes a recent report from the Fraunhofer Institute in Germany. For indoor solar, however, it’s a different story.
‘Silicon is an indirect band gap material, so it needs more materials and thicker films to have very high light intensity,’ says Freitag. Amorphous silicon has been used to power desk calculators for decades, but only collects around 6% of available energy and produces a current and voltage unsuited to modern IoT electronics. Materials for indoor PVs need direct band gap materials, so that not much energy is needed to get their electrons moving. Three next-generation types of material are in competition: organic photovoltaics (OPV), dye-sensitised solar cells and perovskite solar cells. All have tunable bandgaps. Each has pros and cons, but consumers can now buy gadgets powered by them.
‘I was very surprised to find headphones powered by dye-sensitised solar cells at Newcastle airport recently,’ says Freitag. The cells are part of the headband and continually recharge the battery.
Efficient dyes
The three technologies for snatching indoor light have been around for decades. ‘The introduction of dye-sensitised solar cells would have been around 1991. In the mid-1990s, we had more efficient organic solar cells and then perovskite cells in the early 2000s,’ says Carnie, who works on electronic devices powered by such materials. With dye-sensitised solar cells favoured by Freitag in her Newcastle lab, the bandgap and absorption is tunable, meaning tweaks in composition can tailor them for specific light sources. ‘It is a very complex system, and everything can be manipulated on a chemical level, so it’s a nice playground for a chemist,’ she says.
These solar cells comprise two electrodes, usually made of conductive glass coated with a dye and metal oxide. The dye gets excited by picking up photons of light. This then injects an electron into particles of titanium dioxide. Tweaking the recipe has boosted how much indoor light can be converted into electrical energy. ‘They started at around 7% efficiency, then got up to 10% to 12% for almost 30 years at solar light intensities,’ says Freitag. ‘Now devices are 38% efficient in published results in indoor conditions, and up to 40% in my lab.’
Firms such as Exeger in Sweden, Ricoh in Japan and Ambient Photonics in California, US, are showing off consumer electronics powered by their own indoor dye-sensitised technologies. Ambient incorporates its glass solar cells into PCs and remote controls, while Ricoh’s glass-based products are in sensors, remote controls and desks. The best solar cells are likely to be unobtrusive and barely noticeable.
Exeger’s solar material is in nine products, with seven more on the way. ‘Our superconducting electrode allows us to produce flexible solar cells,’ says CEO and founder Giovanni Fili. Dispensing with glass, the company manufactures cells at two factories in Sweden that can look like leather, brushed steel or carbon fibre. ‘We can have any kind of texture, so imagine the endless design possibilities. Product integration is key,’ says Fili, who shows off a leather bag with seamless solar powered cells. He contrasts this with some competitor products with silver lines running through their solar cells to collect current.
Thin-film organics
Dye-sensitised solar is not the only game in town. Thin-film solar cells made from plastics with semiconducting properties and organic electronics are flexible and lightweight. Manufacture of these organic PVs can be on a roll-to-roll system, which is less expensive than many other processes. ‘Organics perform well under low-light conditions,’ says Karl Leo, a physicist at the Technical University of Dresden, Germany. ‘Also, the deposition technologies and the tools for fabrication are very well developed for OLEDs,’ referencing the displays in TVs, smartphones and PCs.
Leo’s company, Heliatek makes ultra-light, ultra-thin solar films that can be glued to various surfaces. These are destined only for the outdoor market, such as on buildings where lightweight structures are necessary, rather than indoors. The active layer on such OPVs is extremely thin, ca. 100nm, he notes, whereas a silicon solar cell is thousands of times thicker.
A company deploying printed OPVs to capture indoor light is Swedish firm Epishine, with 15 products on the market. It claims to make cables and disposable batteries unnecessary for electronics due to technology developed at Linköping University, Sweden. ‘The semiconductive active layer used in Epishine’s solar cells is composed of a binary heterojunction made from a semiconducting polymer and a fullerene derivative,’ notes co-founder and CTO Jonas Bergqvist. A heterojunction is where two semiconductor materials with different bandgaps come together – useful to boost energy conversion.
High-performing OPVs typically require two materials with slightly different energy levels to help photocurrent leave the solar cells. Back in Swansea, Carnie chose an Epishine solar cell to power an indoor air quality sensor moving toward commercialisation. Another company developing OPV technologies is Dracula Technologies, near Lyon in France. This involves printing 3-4nm thin layers by inkjet, with all-green solvents. The firm has some products on the market and is starting on prototypes with ST Microelectronics in October 2024 to create autonomous air and light sensors, according to Sadok Ben Dkhil, CTO of Dracula. ‘OPVs can be manufactured using very simple and less expensive techniques than those used for conventional photovoltaic technologies,’ he says.
Crystal hybrids
The third cell technology suited to indoor devices are perovskite solar cells, which have a crystal structure akin to perovskite mineral, calcium titanium oxide (CaTiO3). Perovskite structures are usually composed of a metal cation, an organic anion and oxygen, and can be deposited as thin films to make solar cells. The best examples are methylammonium tin halide and methylammonium lead halide. Variations can be stacked to get high current densities and high voltages. ‘What’s really exciting about perovskites is you can tune them to absorb visible, infrared or blue light and layer them,’ says Henry Snaith, a physicist and materials expert at the University of Oxford, UK. For example, the halide anion can be switched from iodine to bromine, which would influence the bandgap and light absorption of the material.
A recent review noted the power conversion efficiency of perovskites has vaulted 26% for single-junction solar cells and more when combined with silicon (Nature Rev. Mater., DOI: 10.1038/s41578-024-00678-x). In 2010, Snaith spun out the company Oxford PV to commercialise perovskites. These are placed on top of conventional silicon solar cells, stepping up efficiency by at least 20% for a combined efficiency above 30%, according to the company. Another pioneer is Saule Technologies in Poland, founded in 2014, which produces printed, ultra-thin and flexible perovskite solar cells. Saule showcased a transparent perovskite foil, sealed between glass sheets, on the front of buildings, and a charging station for electric cars and bikes.
Snaith believes the day is approaching when perovskite will compete indoors. ‘Perovskite cells can reach 30% efficiency. Indoors – if you tune them correctly in terms of band gaps and choose the right layers to put on top of each other – they can actually approach 40%,’ he says. ‘[Dye-sensitised solar cells] don’t produce as high voltage and perovskites are about 50% more efficient indoors, which gives them much more power output.’
Carnie also notes that perovskites are relatively easy to manufacture from earth abundant materials. ––The inclusion of lead in some perovskite materials is controversial, however. There is a belief by some that the EU is going to move against any indoor devices containing lead; others say this lead is in such small amounts and locked away so well in perovskite PVs that it poses no safety risks to consumers.
Solar cells produced by Epishine
Image: Epishine
Brighter outlook
Experts in indoor PV say demand is increasing in tandem with a proliferation of indoor sensors and electronics. Outdoor solar is an established market, with predictable demand, which is why Oxford PV focused on this segment rather than the nascent indoor market. ‘There’s a higher risk of trying to launch your technology directly into a new market, because you have to make predictions about its size,’ says Snaith. ‘But there’s also excitement and the opportunity to define what the market is.’
Improvements in materials are being made, such as reducing voltage or energy loss in OPVs. ‘By employing intricate molecular design, efficiency can be improved by minimising voltage loss,’ says Bergqvist at Epishine. ‘We also work to enhance photon capture and maximise the conversion efficiency of photons into photocurrent,’ which can be done via molecular design and even choice of solvents during manufacture. Meanwhile, Henrik Lindström, CTO at Exeger, says R&D are exacting better performances out of the system of dye, TiO2 and electrolyte: ‘often you choose one dye and play around with the electrolyte to maximise performance.’ Meanwhile, Freitag lauds dye-sensitised solar cells as closest to nature, with chlorophyll as the energy-capturing pigment of photosynthesis, and says: ‘we steal lots of ideas from nature and we use sustainable, earth-abundant elements.’
Recent progress with indoor PVs is stepped up by the focus on integration into electronics. Carnie says the key to his indoor air sensor is the efficient extraction of power from the solar cell, with advanced power management electronics and sophisticated machine learning to help predict when the device will harvest more or less energy. ‘I like to see companies that focus not just on PVs, but on integrating it directly with wireless devices,’ says Freitag.
There is a sense of dynamism pervading the indoor solar market, with companies showing off real products. ‘When I moved to the UK five years ago and told people to focus on indoor solar technology, they would look at me like I was crazy,’ recalls Freitag. ‘Even in the last year, more researchers have started moving in this direction.’
Visit Blog.ashx?h=650&w=1440&hash=89D64A69F15F997CBC5F8282FEF4BF07)
