Blue skies open for solar innovation

The solar power industry has enjoyed an innovation boom over the last decade or so, with accelerating production capacity and matching price drops. Last year, the industry added some 133 gigawatts (GW) of capacity worldwide, which is almost 19 percent of the previously installed 710 GW of capacity. One GW of power equals the output of 3.125 million solar photovoltaic (PV) panels, according to the US Department of Energy.

Such leaps in global capacity reflect the fact that commercial solar panels are approaching 20 percent efficiency which is near the theoretical limits. What is more, manufacturers are consolidating production methods and creating standards that make it easier to share suppliers.

With such progress alight, it might seem like a time for blue-sky nanomaterials researchers to entrust their promising discoveries to tech transfer professionals and move on to the next theoretical challenge. But physicist Sajeev John of the University of Toronto, one of many materials scientists with a promising new solar cell trick up his sleeve, says, “I wouldn’t say I’m knocking on the door of existing industry. I want to keep the IP [intellectual property] and develop it, rather than just license it to existing solar cell companies.”

John and other researchers are betting that the nanomaterials they are developing, including reimagined silicon photonic crystals and organic photovoltaic materials, are different enough, and valuable enough, to merit new industrial production infrastructure.

Faster-than-predicted price drops in the solar electricity industry are opening the door for the industrialization of cutting-edge laboratory technologies. In 2019, physicists Iliya E. Kuznetsov and colleagues at the Institute for Problems of Chemical Physics of the Russian Academy of Sciences, wrote that crystalline silicon photovoltaic cells cost from US$1 to $3 per watt at peak capacity (Wp), meaning when the panel captures the most sunlight. They wrote that “to shift the paradigm on the world energy market, the cost of the solar-generated electricity should fall to 20 cents per Wp and below.” As of July 2022, European spot prices for low-cost solar modules are around 20 cents per Wp, while high-efficiency modules cost around 40 cents per Wp. The paradigm is shifting fast.

Last year, solar provided 3.72 percent of global electricity production, a share that has grown exponentially since 2005. Yet, thanks in part to pandemic-related supply chain snarls, the price went up last year for the first time in a decade. There is also a wide range between industry predictions of solar providing 14.5 terawatts-at-peak-capacity (TWp) by 2050 at the low end—about one-third of estimated global electricity generation—and up to 63.4 TWp at the high end.  Reaching the higher end of that range will require improved module efficiency at similar prices, more efficient production facilities, better use of raw materials, and more diversification of products to fit a wider variety of market niches, predicts VDMA, an industry group.

Nanomaterials contribute in each of those areas and hint at the potential for even larger efficiencies. But efficiency isn’t the only definition of improved solar power. Ideally, successful solar should take into account the environmental impact of manufacturing as well, since silicon PV production requires toxic materials like cadmium. Recycling solar panels and the political vagaries of access to raw materials are also part of the quest for sustainable and successful solar power.

“In the past, solar research has been mainly about efficiency, but in recent years it’s maybe looking at sustainability, polluting less,” says physicist David Maestre at the Complutense University in Madrid, Spain. National and industrial research roadmaps are also starting to consider how to avoid using materials mined in politically unstable countries, he says.

Those are recent considerations. Photovoltaic power’s so-called first generation consisted of crystalline silicon semiconducting cells and focused on efficiency for specialized uses, such as early space exploration. Those panels were perhaps 160 to 240 µm thick, with a charge-transporting network built onto the bottom of the cells.

By the 1970s, researchers had developed a more cost-conscious, second generation of solar PV cells using amorphous (rather than crystalline) silicon, and complementary materials like copper indium gallium diselenide. The cells used far less silicon, a costly material, but required a support structure.

Next emerged a third-generation solar cell technology in which researchers looked beyond silicon and inorganic materials to organic materials. “Organic polymers are cheap, light, and easy to produce,” Maestre says. They have a lower environmental footprint and could use already existing manufacturing processes such as inkjet printing, slot-die coating, and roll-to-roll processing. 

Organic components, such as polymers or small molecules in the sunlight-absorbing layers of solar cells, host electrons in the highest occupied molecular orbital (HOMO). Inbound light can excite electrons from the HOMO to the next most-energetic orbital, called the lowest unoccupied molecular orbital (LUMO). Early organic solar cells used fullerene-derived materials to grab those energetic electrons, but their overall efficiency seemed stuck for years below 13 percent, while first-generation crystalline-silicon cells were approaching double that capacity. They also suffered from reliability problems compared to silicon.

Then, in the mid-2010s, Xiaowei Zhan of Peking University and colleagues synthesized a nonfullerene electron acceptor whose bandgap, which determines the wavelength of light captured by the cell, was easier to tune. This breakthrough enabled efficiencies much closer to those of first-generation solar cells.

Since then, organic photovoltaics has come a long way, says Hafiz Sheriff at the University of Michigan. Sheriff and colleagues added a buffer layer to a nonfullerene acceptor solar cell, with the result being that the cell retained 94 percent of its original efficiency after a simulated 30 years of use. “We have proven that organic photovoltaics can be as efficient as traditional [crystalline silicon] solar cells with similar durability,” he says.

Thanks to its transparency, such a cell could produce power in windows, buildings, and even clothing. For transparent or flexible applications, “it’s hard for silicon to compete,” Sheriff says. Academic researchers, including at the Instituto de Ciencias Fotónicas in Barcelona, Spain, are starting spinoff companies like Vitsolc to do so. Other spinoff companies, such as Ambient Photonics, are playing up the low-light potential of organic dye-sensitized solar cells for use in indoor settings.

Beyond the organic-focused third generation of solar PV, a growing number of approaches involve changing the shape of inorganic materials, or mixing them in new ways, to capture more light. Many of these approaches take place at the nanoscale, such as adding gold and silver nanoparticles to silicon, improving infrared absorption.

Another approach is to build more surface area per unit volume into the cell at the nanoscale, via wires for example, which can capture more photons than a flat surface. Silicon nanowires may be able to achieve a whopping 42 percent efficiency. The wire’s tiny size allows engineers to use lower-quality silicon, which might also lower manufacturing costs. 

Still more approaches to higher-efficiency PVs include using hydrogenated graphene and carbon nanotubes in place of silicon. But these remain laboratory efforts.

“Anyone who has bet against silicon has lost,” says electrical engineer Santosh Kurinec of the Rochester Institute of Technology. Silicon’s head start means that researchers working on nanoscale projects have an incentive to work with, rather than replace, existing silicon PV technologies.

Indeed, not all nanoscale research for photovoltaics is about replacing silicon. One approach is to use nanotechnology to modify the optical transmission properties of the glass on top of a silicon cell, delivering more of the incident light at wavelengths the silicon can absorb.

The silicon layer itself is up for reinvention, too. At a small enough scale, around 10 µm, it is possible to do things with light that might give crystalline silicon a new lease on its place in the photovoltaic universe.

Along with Sajeev John’s group in Toronto, research groups at Swinburne University of Technology in Australia, and the Institute for Solar Energy Research Hamelin in Germany, are pioneering ways to capture light with silicon photonic crystals built into inverted pyramids or tepees. The approach involves redirecting some of the inbound light horizontally, rather than letting uncaptured light pass through the cell. The pyramidal shape provides additional surface area, relative to a strict horizontal film, and, at the right orientations, can capture incoming light from both vertical and horizontal directions.

If such architectures will work at commercial scales, they would allow ultrathin silicon cells to compete with today’s crystalline silicon in terms of energy efficiency, and with organic solar cells in terms of environmental impact and transparency. The silicon would be so thin that you could see through it, enabling some of the applications, such as solar-harvesting windows, that proponents of organic photovoltaics are targeting.

“I think it’s valuable, but my word of caution is the following: This is silicon, so it’s brittle, not flexible,” Sheriff says. For solar harvesting on bendy surfaces, organic will still retain an advantage.

“The biggest single challenge [for silicon photonic crystals] is finding a cost-effective scalable manufacturing technique,” John says. “In the lab we use direct laser writing, e-beam, silicon stepper technology. Maybe one day we could do roll-to-roll printing. We could also try polycrystalline silicon, which would maybe lower efficiency a little but cost considerably less.”

If the group can take this nano-know-how from the lab to commercial-scale production, they may just get the jump on the existing industry. “At this point,” John says, “it’s all the optical and electronic groups getting their part together without messing up the other part.” They will also have big competition from the many small makers of new nanomaterials up and down the PV supply chain.

First published by Photonics Focus: [html] [pdf].