Monday, June 29, 2015

On your last post, you showed the NREL efficiency chart. What were all these technologies? Why so many?

NREL record solar cell efficiency chart

Good question! Different cells are useful for different applications. For example, where weight and size are a major issue and cost is less important, high-efficiency multijunction or GaAs cells are useful. On earth, the critical metric tends to be cost per watt of electricity generated, although in some cases, area or aesthetics (how it looks)/flexibility (what shapes does it come in, how can it be installed, how easy is it to add extra area) become important. Different companies use different techniques to make their solar modules, so actual module efficiencies often vary. So far, companies using a variety of different technologies, such as CdTe, GaAs, silicon, and CIGS, have been or are starting to be successful.

Multijunction: These cells are fairly expensive to produce. They have multiple p-n junctions (the basic structure of a solar cell) stacked on top of each other designed in such a way that you can get more energy from light produced by the solar spectrum than is possible using a single cell. The junctions will have different band gaps to make this possible. In many structures, cells are stacked in series with tunnel junctions in between to allow current to flow through the entire stack monolithically.

Concentrator: This can be applied to a variety of different technologies. Mirrors and lenses are used to concentrate light on a high-efficiency solar cell in order to increase the amount of light hitting the high-efficiency cell, thus allowing you to produce more energy in a smaller area. These cells, practically, would only be useful in areas with a lot of direct sunlight and are not practical for roof-top applications (heat dispersal is a major issue).

GaAs: Gallium arsenide (GaAs) is a direct band gap semiconductor that is used in high-efficiency solar cells. It historically has had the best performance and reliability, although it tends to be expensive. As such, it is often used in satellites, military, and space applications. Cells can be lightweight and small (even portable or, recently, flexible). Recently, however, thin-film GaAs production costs have come down, although they are still more expensive than silicon or other thin film technologies like CdTe.

Silicon (Si): Technologies based on the element Si have been able to piggyback off developments in the microelectronics industry (same material) to deliver somewhat low-cost energy with fairly high-efficiency. It is an indirect band gap material, which inherently lowers device efficiencies, but, for an indirect band gap material, it's efficiency is close to optimal. Good feedstock material is widely available and fairly inexpensive due to the semiconductor/microelectronics industry and the material is widely understood. The main drawback is the cost; it is very energy-intensive and expensive to purify the Si to a point good enough for high-efficiency cells and grow the Si into single crystalline wafers. Multicrystalline and thin-film crystal solar cell structures (and actions of international political leaders) have lowered the cost of this technology significantly to the point that it is the most common solar-cell material. However, these technologies do not have diminished, though still competitive performance. For electronics applications, it is widely stated in the industry, "never bet against silicon."

CdTe: CdTe solar cells are a thin-film solar cell technology (the width of the cell is smaller than a human hair). These cells are easily and inexpensively manufactured and have typical module efficiencies on par with to slightly less than silicon-based thin film and multicrystalline modules. The main benefit is the low cost and fairly forgiving process. The material is less effected by common defects, so it can be made inexpensively. Of minor concern is the toxicity of the cadmium, although the cadmium is securely bound to the tellurium and is further protected from accidental release without significant effects on cost. It is actually a fairly safe way of disposing of Cd waste from other processes. Because of these waste streams and the small amount needed per module, CdTe modules are among if not the most inexpensive cells on the market in terms of cost per watt. Of larger concern is the amount of Cd and Te present on the planet. There are currently not enough known deposits to fulfill the world's current energy needs, so this will eventually cause the price of raw materials to rise, raising module prices.

CIGS: Many people in our group research this material. It is just beginning to be commercialized and is projected to be another low-cost thin film technology with slightly more potential than CdTe. There are fewer abundance or toxicity concerns compared to CdTe, although competition with the microelectronics industry (think transparent touch screens on your cell phones) for In and Ga might raise prices in the future. They need less encapsulation and can be made on flexible substrates while retaining a high efficiency.

Amorphous Si:H: This is a technology in which thin films of silicon are deposited in such a way that there is little crystal ordering (hence, amorphous). This leads to a lot of dangling bonds in the silicon, so hydrogen is used to passivate these bonds. Instead of a true electronic band gap (energies where you can't have charge because of physics), you have a gap of energy states in which it is difficult for charge carriers to move (hence, a mobility gap). These materials have low efficiencies, although they are inexpensive to produce. They do out-perform all organic or dye-sensitized cells and are a baseline for comparing technologies. They also have some issues with degradation over time in sunlight.

Dye-sensitized cells: Dye-sensitized cells are solar cells in which electron and hole transport are localized into different spatial areas. These are an inexpensive, yet low-efficiency technology that, nonetheless, could eventually be competitive due to their low cost per watt. When light comes in, it excites carriers in a dye molecule. The electron is transported to TiO2 particles that form a scaffold. For charge neutrality, the electrolyte is then reduced. They can be somewhat flexible or semi-transparent. They perform well in low-light conditions with indirect sunlight. The Achilles heels of these are the inability of removing expensive catalyst materials like Pt or Ru, the liquid electrolyte, and the stability of the dye under UV light. The electrolyte contains toxic volatile organic compounds and can freeze or expand with temperature, making encapsulation difficult.

Perovskite cells: Check out the previous post to learn more! The new kids on the block, these cells can have extremely high efficiency and can be made inexpensively, although they have some major degradation and stability issues.

Organic cells: Organic solar cells use organic semiconducting materials instead of inorganic semiconducting materials. They have issues with UV degradation and charge separation (it is very easy for electron-hole pairs to recombine compared to separating the charges). Still, they would be a good low-cost option if their efficiencies could be improved. They also tend to degrade with water.

CZTSSe cells: This is the technology that this collaboration studies. All elements (Cu, Zn, Sn, and S, with optional Se) in this technology are widely available, inexpensive, and (excepting Se), non-toxic. It has been shown to tolerate inexpensive processing conditions with surprisingly high efficiencies (record cells have been made using solution-based processing, which is notoriously difficult to control compared to vacuum-based counterparts). The major issue with these seem to be the efficiency. These are also relatively new kids on the block, so only time will tell how CZTSSe cells will fair, but there seem to be some issues with defects and secondary phases that need to be addressed. Most of the advances to-date have been made by assuming that it is similar to CIGS, so a more thorough understanding of this materials system is required, including investigations into chemistries that may reduce the effects or abundance of problematic defects. If these efficiency/defect issues can be overcome, these cells might represent a lower, more-stable cost CIGS-like cell. However, module and cell efficiencies need to improve before this happens. They might be good on flexible substrates and for a wide variety of other applications, although GaAs will always out-perform it if cost were not an issue.

Quantum Dot cells: Tiny (nm-scale) blobs of material are deposited on a substrate. The size of the blob determines its band gap and, therefore, the energies of light that it would absorb. These cells are particularly interesting for multi-junction cells, since you can tune the band gap easily using a single concept/material. Quantum dots are easy to make and could be easily scaled up in a commercial operation. Thus, they could be inexpensive, depending on the material and process, but the efficiency would need to be improved for them to be competitive. They, historically, have had issues oxidizing or reacting with air.

Monday, June 22, 2015

What is all this hoopla around perovskite solar cells? Why aren't they on the market yet if they're so great?

NREL best research cell efficiency chart

Perovskite solar cells are a recent phenomenon. They've experienced an unprecendented rise in record cell efficiency and seem to naturally have fewer issues with defects than many other competing technologies. They also seem amenable to low-cost production techniques. That being said, many issues need to be overcome in order for them to be commercialized.
"Perovskite unit cell" by Sevhab - Own work. Licensed under CC BY-SA 4.0
via Wikimedia Commons -

Perovskites are based around the perovskite crystal structure, as shown above. This structure is repeated in all directions to form a crystal. Unfortunately, most of the high-performance cells contain lead but, since these are thin films, the amount of lead is pretty small and it is (relatively) safely stored beneath protective coatings. The word "halogen" refers to atoms in the second to last (17th, or 7A) column in the Periodic Table including F, Cl, Br, I, and At. In many of these cells, the halogen used is usually iodine (I), although chlorine (Cl) and bromine (Br) are also common. Methylammonium is a small organic ion.
This perovskite material is the part of the cell that absorbs the light. Electron and hole transport layers on either side of this material selectively remove charge carriers that have been created when light is absorbed.
Perovskite solar cells have a ways to go before becoming a marketable product. Solar cells on the market today have 25+ year warranties. These perovskite cells currently have issues with durability. Presently, they decompose when exposed to water, including water vapor in the air, so careful sealing is required (they actually turn yellow). Furthermore, when held under conditions similar to those of device operation (constant forward bias), the ions start to move and collect at either side of the device, causing performance degradation. The internal field cancels the field created by the light-induced field. Below is a plot of J-V curves taken with different voltage sweep rates. You can see that there are serious reproducibility issues under conditions useful for actual solar cell use.
W. Tress et al.,  Energy Environ. Sci., 2015, 8, 995-1004. The curve on the left represents actual J-V outputs, whereas the curve on the right shows J-V outputs normalized at -0.2 V. You can see the hysteresis in these curves. Hysteresis means that the curve made by varying the x-axis in one direction is different from the curve made by varying the x-axis in another direction.
These cells need to be more durable in order to be practical. While promising, more research is needed to get them to this point. They do represent a promising technology that definitely deserve the hype and excitement!

To learn more check out:
Will perovskite solar cells live up to their promise?
Perovskite solar cells could beat the efficiency of silicon
To succeed, solar perovskites need to escape the ivory tower

Monday, June 8, 2015

I'd heard the phrase "publish or perish." What does this mean? How do researchers tell people about their research?

Excellent question! Research is useless if it isn't be communicated. 

Researchers tell people about their research in a variety of ways. This blog is geared towards the general public and talks about how research is done. Group members also do a variety of other outreach and communication activities so you can all see what research is like in the real world.

We also communicate our research to the broader research community. We speak and present posters at conferences, discussing our work with other researchers. We also publish our work in academic journals. It is these publications that are the most important, since other researchers also review these works before they are published to make sure that everything is good. These journals are able to be referenced all over the world and are recognized as being trustworthy research that can be confidently referenced in the future.

The phrase "publish or perish" refers to life as an academic researcher, specifically those journal publications. Just as any worker is judged by his or her work, scientists are judged on their work. This performance is used to determine who will get funding to do more research and is used to determine which researchers gain tenure and advance, a position in which faculty have improved job security and more intellectual freedom. Researchers are judged by their publications, both the quality and quantity. Research institutions (such as universities) like to see that their researchers are leaders within their fields and driving thought forward.