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Market Opportunities For Indoor Photovoltaics

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Standards for measuring IPV efficiency and specially designed IPV architectures could help drive a $850 million market by 2023

Billions of wireless sensors are expected to be installed over the coming decade as part of the Internet of Things (IoT), with almost half to be located inside buildings.

Now Ian Mathews and colleagues in the MIT PVLab have quantified the opportunity this presents in their paper ‘Technology and Market Perspective for Indoor Photovoltaic Cells', recently published in Joule.

"This is an exciting time for indoor photovoltaics, a range of options now exist for sensor designers and the market is reaching a tipping point that provides a significant opportunity for the photovoltaics community as well," said Mathews.

In 2017, the global market for IPV cells was $140 million, insignificant compared to the global market for solar power modules, which was over $100 billion in the same year. Driven by the growth of the market for IoT hardware, the annual IPV market size has the potential to be significant in its own right, reaching a predicted $850 million by 2023 and will likely continue to grow to a billion dollar market the following year.

By 2023, the demand for PV energy harvesters is expected to reach 60 million devices per year. This represents a 70 percent compound annual growth from today's market size, making it the fastest growing of all non-traditional PV markets.

When located indoors with no access to solar irradiance, IPV cells harvest the energy emitted by artificial light sources, with the illumination intensity typically three orders of magnitude less than sunlight. Operating PV cells at such low illumination intensities, combined with the significant differences in incandescent, CFL, LED, halogen, and the solar spectra have a significant impact on cell performance under indoor lighting conditions.

Existing PV Technologies

Characterising IPV cells is a growing research field with the performance of a considerable number of different PV technologies having now been measured under artificial light sources such as incandescent, compact fluorescent (CFL), halogen, and LED bulbs. Figure 1 at the top of the page compares the average power requirement (averaged across sensing, communicating, and sleep modes) of these IoT communications protocols to the expected average power output of 10 cm2 PV panels under different illumination conditions both indoors and outside.


Figure 2 above, shows (A) Outline of the different light spectra under which photovoltaic device efficiency is evaluated including the standard solar spectrum (AM1.5G) and typical spectra from White LED, CFL, and Halogen sources. (B) The maximum efficiencies versus band gap measured for indoor photovoltaic devices to date where circles represent measurements under LED bulbs and diamonds under CFL illumination.

For the thin-film technologies, CZTSSe and CdTe, only measurements under CFL bulbs at light intensities much lower or higher than  500 lux exist, and the researchers provide these for completeness. Also shown are the maximum theoretical efficiencies calculated in Rühle et al. using the detailed balance limit of efficiency method considering an LED spectrum (black line) and a CFL spectrum (red line) of 1 W/m2.

Considering the performance of individual cell technologies in more detail, silicon, the dominant cell material in the solar market with record solar efficiencies over 26 percent, demonstrates ambient light harvesting efficiencies of  8 percent, because of its narrow band gap, the dominance of Shockley Read Hall (SRH) recombination at low light intensities and low shunt resistance in tested devices. There are a limited number of studies that look to adapt silicon PV cells to very low-light harvesting or CFL or LED spectra, presenting an interesting opportunity for the Si PV research community. To overcome the band gap limitation of crystalline silicon, amorphous-silicon (a-Si) has gained a foothold as one of the dominant indoor PV technologies. The wider 1.6 eV band gap is better matched to indoor light spectra and results in higher photovoltages than standard silicon cells with efficiencies closer to 10 percent.

The most commercially successful PV technologies besides silicon are thin-film materials, especially CdTe and CIGS. CIGS studies under low-light conditions have shown that the devices tested suffer from low shunt resistance that significantly reduces their efficiency as light intensity decreases. CdTe, however, has a band gap of 1.5 eV and is known to maintain high performance under diffuse radiation and low light. Over the years, the technology itself has established a strong foothold in the PV market and is well characterised.

There are, however, not many measurements available for CdTe cells illuminated by artificial spectra, with the only published result stating 10.9 percent power conversion efficiency (PCE) measured under 9.1 W/m2 CFL illumination. Significant progress has also been made using earth-abundant thin-film cells with efficiencies approaching 10 percent measured for Cu2ZnSn(S,Se)4 (CZTSSe) cells under low illuminance CFL and AM1.5G spectra.

III-V light harvesters are strong contenders to power indoor wireless sensors because of the wider band-gap compositions possible and their record efficiencies under solar irradiance.28 Under indoor light, GaAs cells have been shown to maintain their high performance with efficiencies over 20 percent measured with cells on flexible substrates. Given that the optimum band gap for indoor light harvesting is closer to 2 eV, it would be expected that single-junction III-V cells with band gaps in the 1.8-1.9 eV range, such as GaxIn1−xP and AlxGa1-xAs PV cells would perform better than GaAs. In fact, studies comparing GaAs, GaInP, and/or AlGaAs cells under the CFL or LED spectra have shown very similar performance across the three cell types.

An ideal jumping off point for perovskite?

IPV cells made from organic materials are emerging as contenders for commercialisation as their absorption properties and architectures are adapted for ambient lighting. A number of organic photovoltaic (OPV) cells with low-light conversion efficiencies over 16 percent have now been demonstrated,30 with the best cells in the literature demonstrating efficiencies of over 28 percent, achieved using a material with an optical band gap of  1.8 eV. Dye-sensitised IPV cells have also shown considerable efficiency progress of late, with values over 27 percent measured under 200-1,000 lux light intensity, with one measurement of 31.8 percent demonstrated under 1,000 lux CFL illuminance.

A very promising class of material with the potential to enter the PV market in the coming years is perovskite solar cells. These materials have exceptional defect tolerance and photoluminescence quantum yields that are similar to the leading inorganic GaAs PV devices. These cells have recently been tested at low light levels and exhibit performance similar to the best III-V and organic devices with multiple cell efficiencies exceeding 25 percent.

But while a number of perovskite PV companies have recently launched, none are developing products for the IoT market and instead focus on, for example, silicon-based tandem cells (Oxford PV) or building-integrated PVs (Saule Technologies). "Our market analysis in this paper makes it clear that the rapid growth of the indoor IoT market could provide an ideal jumping-off point for perovskite products, allowing a new PV company to establish customers, revenue, and credibility before establishing larger-scale solar-panel-manufacturing facilities," say the researchers.


Commercial Challenges

In the literature to date, techno-economic studies for PV cell technologies concentrate on manufacturing scales compatible with the production of large solar power modules (>1 m2) for the utility scale or residential markets. Figure 3 above summarises relevant studies on the cost of manufacturing various solar power modules or cells, $/cm2, versus the volume of product produced per year, m2/year Also, on this plot, the researchers show in the shaded region, the expected volume of this market over the next five years versus the price per unit as predicted by BCC Research where, in summary, the market demand is expected to grow from  100 m2/year to 100,000 m2/year with an almost order-of-magnitude drop in market price over the same period.

Economically, no clear technology emerges as the winner, say the researchers. For the low costs predicted to translate to smaller production volumes, manufacturing processes with low factory capital expenditure will be required. Otherwise, the $/m2 of IPV technologies will be dominated by capex, as these fixed costs are recuperated across a relatively small volume of module sales. The impact of capex warrants further attention from researchers to ensure their materials and methods used will result in low-cost IPV modules at low volumes.

Stability

Another consideration for indoor PV cells is their ability to provide power over the multi-year lifetime of the wireless sensor. The environment within which the system will be deployed can be expected to include office spaces, unheated warehouses, cold rooms, etc. Wireless sensing products deployed in these environments can expect to witness high and/or low temperatures and relative humidity throughout their deployment.

For any technology that has been commercialised in the solar market, the degradation rate of the technology has been well established according to the standards of that industry with over 20-year lifetimes expected.

It can be taken, therefore, that silicon, CdTe, and CIGS PV cells fabricated for indoor applications will maintain high performance throughout their life powering a wireless sensor indoors.

Similarly, III-V solar cells have been used to power satellites in space for decades and can be considered to degrade at very low rates indoors. a-Si cells degrade because of the Staebler-Wronski effect losing up to 20 percent of their rated power output in the first few weeks of operation.44 PV cells made from organic and perovskite materials are known to have higher degradation rates.

Despite their low cost, organic cells have failed to achieve significant market penetration, as their low stability inhibits their widespread use in the large utility and residential-scale PV markets.

Perovskites, as a more recent breakthrough, have already achieved impressive efficiency results in a short space of time. Perovskite solar cell stability remains a concern, however, that could prevent successful commercialisation if not addressed,46 although indoor IoT requirements are less stringent than for outdoor power production.

Recommendations

The researchers' have two main recommendations to accelerate the development of IPV for IoT:

(1) To develop universally accepted standards for measuring IPV efficiency and also the energy requirements of wireless devices and communications protocols; that way, both technical communities can work independently today, and intersect down the road.

(2) To continue to develop IPV device and system architectures better suited for indoor energy yield. To date, the majority of measured IPV cells have exhibited efficiencies far below the theoretical maximum of 52 percent under 1 W/m2 CFL or LED illumination, with leading results around 30 percent. One avenue to higher efficiency is to design and fabricate cells with wider band gaps, closer to the  2 eV optimum.

'Technology and Market Perspective for Indoor Photovoltaic Cells' by Ian Mathews et al; Joule, Volume 3, Issue 6, 19 June 2019.


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