Indoor solar cells spur design-option reassessments

Conventional solar cells are just that: photovoltaic devices which, by their physics, extract and transform energy from the sun. Their sensitivity and efficiency are matched to the optical-energy spectrum of radiated and received power from the Sun to the extent possible, Figure 1.

Figure 1 The solar optical spectrum is complex and the available power per wavelength is a function of many factors. Source: Pennsylvania State University

In many small-scale applications, these same solar cells are used indoors and powered by ambient light from source’s overhead fixtures (which may be fluorescent or LEDs of various color temperatures), incandescent lamps (yes, some are still out there), diffuse or shaded natural light, and even specialized light such as halogen sources.

Given the indoor situation, two things are obvious:

The designation as “solar” is somewhat of a misnomer since the Sun is no longer the source and so “photovoltaic” (PV) would be more accurate—but that’s the widely used, colloquial way of describing these cells.
The energy spectrum of these indoor lighting sources is mismatched to the responsiveness of the solar cells, so efficiency is low.

While there have been some smaller, less-critical indoor products using solar power alone such as small calculators, such harvesting of ambient indoor optical energy is generally limited in its usefulness.

That situation may be changing as several companies have developed solar cells (we’ll stick with that misnomer) based on technologies which are very different from those used by conventional “real” solar cells. These indoor-optimized cells use complex layers of dyes along with specialized physical and chemical processes to achieve their indoor-optimized results.

Both Ambient Photonics (Scotts Valley, CA) and Exeger Operations AB (Stockholm) use variations of dye sensitized solar cell (shortened to DSC or DSSC) technology to produce light-sensitive cells which are optimized for indoor settings. The production process is a high-volume printing-like operation rather than the furnace-based process used for conventional solar cells.

Ambient says they have reinvented the chemistry of the dye sensitized solar cell (DSSC) with novel, proprietary molecules, using light-sensitive dyes to collect photons and convert them into electrons. In their electrochemical system, these light-sensitive dye molecules harvest and produce energy, with the dyes functioning similar to how chlorophyll behaves during photosynthesis in converting photons into energy.

They maintain that their energy-harvesting technology can harness photons across the light spectrum, yielding more than 90 percent conversion efficiency in low-light condition, even when compared to standard DSSC cells, Figure 2. They also function effectively despite the dynamic, changing indoor low-light conditions which are largely a function of the time of day.

Figure 2 Ambient says their DSSC process yields results which are superior to conventional film-based PV cells. Source: Ambient Photonics

Exeger’s dye sensitized solar cell uses a new architecture which they say improves real-life performance, provides greater flexibility, and offers seamless integration possibilities. In their approach, a unique conductive electrode material has replaced the traditional expensive and inefficient indium-tin-oxide (ITO) layer, Figure 3.

Figure 3 The Exeger’s process requires multiple layers of sophisticated materials and films and is compatible with mass production. Source: Exeger Operations AB

Dubbed Powerfoyle, it is flexible and durable and so can be integrated on curved surfaces such as headbands, Figure 4. It can be produced in sizes from 15 cm² to 500 cm², and therefore integrated into products ranging from small IoT sensors to speakers and larger accessories.

Figure 4 A bendable, flexible solar cell opens up new design-in and application opportunities. Source: Exeger Operations AB

For most design engineers, how these companies have achieved their indoor-friendly solar cells is not as important as what these innovations may do with respect to design options and degrees of freedom. Do power sources such as these enable increased consideration of IoT devices (sensors, trackers, shelf labels, and even remote controls) which do not need battery replacement, yet require more power other harvesting schemes (such as ambient RF-harvesting) support? For example, electronic door locks in hotels are an interesting possibility, as they are continually exposed to indoor lighting and used relatively infrequently; in theory, that’s a good combination of harvesting and use cycles.

Applications do not have to be limited to such small devices, either; Exeger has an agreement with a headphone manufacturer for ambient-powered units with the headband capturing ambient light. The same idea can be used for providing power to safety vests and alarm devices.

Of course, the energy source itself is only part of the harvesting chain. For designers, the dominant issue is not “how did they do it” but instead “what can it perhaps do for me?”; “what new opportunities does it provide?”; and “what do I need to do in my design to make use of this power source?”

For example, designers will have to decide on a suitable energy-storage and charging arrangement, whether using a rechargeable battery and the issues of limits on viable charge/discharge cycles, or a supercapacitor and the unique issues of using these non-chemical storage cells.

It will be interesting to see if these indoor-friendly solar cells become a standard part of the design-in possibilities, or if they have downsides which only become apparent when you get into the nitty-gritty design details of product design, manufacturing, use patterns, and long-term performance.

Do you see a viable energy-harvesting role for these non-Sun-driven solar cells? Would they allow you to create something you haven’t been able to do thus far? What possible design-in issues do you see?

Bill Schweber is an EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features.

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Indoor solar cells spur design-option reassessments

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