How low can solar go? We keep asking that question, and solar researchers keep coming up with new answers. In the latest cost-cutting development, researchers at the Lawrence Berkeley National Laboratory have come up with a relatively inexpensive, energy efficient process for manufacturing standard silicon solar cells without the use of an atomic “dopant.”
To Dope Or Not To Dope
Dopants are atomic-scale impurities added to the contacts of an electronic device, such as a silicon solar cell, to juice up their conductivity. An archived article from Berkeley lab explains:
“Doping materials is a fundamental component of the entire modern electronics industry,” Crommie says, referring to the process of adding impurities like phosphorus or boron to semiconductors like silicon to give the doped material an excess of negatively charged electrons or positively charged holes.
Fundamental or not, doping also adds complexity to electronic devices, which leads to increased manufacturing costs and can limit the lifespan of optimal performance.
Considering the razor-thin margins of the global solar industry, every tiny increment of cost savings can result in a significant competitive edge, which accounts for Berkeley Lab’s interest in dopant-free solar cells.
Not To Dope
The Berkeley Lab DASH (dopant free asymmetric heterocontact) solar cell is the result of a collaboration with scientists from Australian National University and the Swiss Federal Institute of Technology of Lausanne.
The issue that the solar industry face is one of balance. By not doping, you reduce costs but you also reduce efficiency. Doped silicon solar cells have achieved a range of 20 percent efficiency and slightly higher, but until now dopant-free versions have only gotten up to 14 percent.
The new DASH solar cell cranked that up to 19 percent. It consists of a crystalline silicon wafer covered in thin layers of amorphous silicon (amorphous is fancyspeak for dopant-free).
The moly oxide coatings serve as a dopant-free contact to absorb solar energy. On the back side is a thin layer of lithium fluoride, also dopant-free, that acts as a contact for electrons.
The two materials can be layered on at room temperature using a standard thermal evaporation process, which helps to keep manufacturing costs down. From end to end, the entire process only takes seven steps (following is from the study, see link below):
…the simplified architectures inherent to this approach allow cell fabrication in only seven low-temperature (≤200 ∘C), lithography-free steps. This is a marked improvement on conventional doped-silicon high-efficiency processes, and highlights potential improvements on both sides of the cost-to-performance ratio for c-Si photovoltaics.
The two materials were selected because they have a successful track record when used in other types of devices, though not specifically solar cells. They are both transparent, and their electronic structures complement each other. In addition, the naturally occurring defects in thin films of moly oxide are actually a positive feature that enhances the material’s electronic properties.
According to the lab, that’s just one combination so the next phase of the research will involve trying out other materials in different combinations.
If you want more details you can find them in the journal Nature Energy under the title “Efficient silicon solar cells with dopant-free asymmetric heterocontacts.”
All (Solar) Politics Is Local…
The cost of solar cells accounts for only the “hard” part of a solar installation. Other costs include labor, permitting and other “soft” factors. While the soft costs of an installation are not dropping as fast as the hard costs, both are falling, and that brings us to the political background.
In tandem with energy storage — which is also becoming more accessible and affordable — low cost solar enables more individual property owners to drift off the grid either partly or fully. That trend is already roiling the established utility marketplace, with a consequent pushback from the utility industry and its political supporters.
One hotspot that we’re watching in the US (along with our sister site PlanetSave) is Florida, where a battle royal has erupted over a new solar initiative that would enable property owners to buy solar arrays from independent companies. Florida is currently one of only four states in the US that prohibit such deals.
As a high heat state, Florida’s electricity consumption is tied to air conditioning, which is a ripe condition for distributed solar even without storage.
We’re also watching the state of play in Nevada, where a recent net metering decision by the state’s Public Utility Commission has resulted in a “black hole” for independent solar customers.
The independent solar industry has already pulled hundreds of jobs out of Nevada as result, with SolarCity leading the pack. We’re thinking that pushback from the solar industry won’t change any minds on the Nevada Public Service Commission, but it could serve as a warning to other states that are considering changing the rules in midstream.
Image: via Berkeley Lab:
…the top images show a cross-section of a solar cell design that uses a combination of moly oxide and lithium fluoride. These materials allow the device to achieve high efficiency in converting sunlight to energy without the need for a process known as doping. The bottom images shows the dimensions of the DASH solar cell components.