Using Solar Cells indoors
By: Per Edstrom
Tags: IoT, Solar, Nanotechnology, Plasmonics, Solar Energy

Indoor ”solar cells” — why and when it makes sense

Transparent solar cell from Peafowl Solar Power

For many people, the idea of using solar cells indoors seem counter-intuitive — the sunlight is outdoors, so how can we place a solar cell indoors? And in a way, they are right — the devices that convert indoor light to electricity are not strictly solar cells but fall under the more general term photovoltaics, or PVs for short. Photovoltaics simply refers to a device that can turn light into electricity. Solar cells are thus a type of PVs that are made to work with sunlight.

Light is the most abundant energy source

There are many options for renewable energy sources: light, wind, waves, streams, motion and radio-frequency waves, just to mention a few. Some of those, such as wind energy, is suited for production in specific locations. Others, such as light energy, gain traction by the trend towards distributed energy production. Distributed energy production means producing what is needed where it is needed, and it requires us to look for the energy source that is available in the place and time where we want to use the supply.

Since we humans depend so much on our vision for all kinds of tasks and activities, we have adapted our environment to ensure that light is available wherever we spend time awake. Light availability is even regulated in building standards and work environment regulations, and even though times and technology can change quickly it seems a safe prediction that the illumination requirements of our homes and work places will not significantly decrease in any foreseeable future. This makes light an optimal energy source for distributed energy production.

More resilience, less waste

The logic of producing what we need where we need it is both one of resilience and one of reducing energy waste. Distributed power production and self-powered devices make us less vulnerable both to large-scale power outages and to the politics of oil and gas. Meanwhile, a lot of energy waste can be eliminated by producing and delivering electricity at the voltage level on which it is consumed. To illustrate, imagine a regular outdoor solar cell that typically produces electricity at a voltage of 12 or 24 volt DC, which is then converted to 220V/110V AC or even higher to be phased into the grid. If it is to be transported over a larger distance this will happen at an even higher voltage. Once it reaches the point of consumption, the voltage for many applications such as charging a phone needs to be converted all the way down to 5–12V DC, even lower than the original voltage produced by the cell. Understandably, all these conversions result in energy losses, that would be avoided if we were instead able to produce the electricity we need, where we need it, and at the right voltage.

Self-powered devices save rare materials

The issue of local power supply is not only a matter of energy savings, but also a matter of material savings. In practice, a lot of low-power devices are never connected to the grid for charging — they instead rely on non-rechargeable batteries. In the short term, this seems like a simple and inexpensive solution, but these batteries contain rare and often toxic materials. A recent episode of the CleanTech Talk podcast argued that if you estimate the demand for lithium just by adding up automobile manufacturers projections for sale of electric vehicles, only 15% of the lithium needed for 2023 is available on Earth. This estimate does not even account for any demand outside of the electric vehicle industry. The calculation is rough, but the message is clear; rare battery materials seriously limit the possibilities to scale battery use. In addition to this, there are the often gravely underestimated costs for battery changes and recycling — the cost of changing batteries is often much greater than the cost of the batteries themselves, and recycling batteries cost as much as four times the cost of manufacturing. There is hardly any need to also make the argument why deploying toxic materials in great numbers is a bad idea.

Technology options for light harvesting

The most well-known photovoltaic technology, which is what most people think of when they hear the word “solar cell”, is the conventional black silicon solar cells. This technology is mature and cheap but has drawbacks: the surface must be flat and black, and to function well they should be placed perpendicular to an intense light source (e.g. the sun). This works well for rooftop installations but is not a great fit for indoor applications.

Newer organic photovoltaic cells are better suited for indoor use, with relatively good performance under low or diffuse light and the added feature of being bendable. They can also be made semi-transparent, which highlights another requirement that arises when we take photovoltaics indoors: aesthetics. In most indoor environments, we care not only about that our stuff should work, but we also want it to look good.

Transparency is an attractive design characteristic since ultra-high transparency allows for maximum flexibility in design. It is also a challenge to achieve when the aim is to capture light for energy production — if more light is captured, transparency decreases. A prominent approach to achieve high transparency has been to capture light that is outside of the visible range, such as infrared and ultraviolet light. For indoor applications though, this becomes prohibitive; in the indoor environment, lighting is designed to fit our visible range and there is not much of either infrared or ultraviolet light available.

Capture just a little bit

We are exploring a different angle which stems from the fact that surfaces that we in everyday life perceive as “fully” transparent, such as windows, often block as much as 50% of the incoming light. This means we can still afford to capture a bit of light in the cell before it becomes noticeable — we just have to be really good at converting (almost) all of that light into electricity. This is what our direct plasmonic solar cells are aiming for; by utilizing the extreme absorption properties of plasmonic nanoparticles, electricity can be produced by capturing only a tiny fraction of the incoming light.

Direct plasmonic solar cells can circumvent the Shockley–Queisser limit, a theoretical upper limit for the efficiency photovoltaics, by using resonance rather than excitation of electrons as energy conversion mechanisms. This means that capturing as little as 5–10% of the incoming light — much less than a window glass — would be sufficient to power a wide range of applications. With such technology, the desirable combination of ultra-high transparency and indoor functionality is no longer an impossible dream for photovoltaic technology.