Making silicon obsolete: thoughts on developments in perovskite research

“I don’t know any group that works on photovoltaics that isn’t looking at perovskites,” stated Dr Henry Snaith, leader of the perovskite research team of the Oxford University Department of Physics. With developments in perovskite research appearing to commence a materials revolution in the pv industry, Solar Bankers hopes to take a step back and humbly offer its opinion.

It seems the main obstacle hampering pv technology’s advancement to the forefront of energy production is the difficulty of simultaneously optimizing its cost and performance. The past tendency has been one of high efficiency necessitating increased costs, one of reduced costs being coupled with reduced performance. Very efficient commercial solar cells are based on thick silicon sheets produced in costly processes involving high temperatures. Conversely, thin-film models using copper indium gallium selenide (CIGS) save material costs but have poor efficiency. And those based on gallium arsenide currently produce the highest efficiencies among solar cells but are too expensive to attract mass investment.

This trade-off may in the future be resolved by developments in materials research. The perovskite class of materials could replace silicon as the main pv semiconductor by combining inexpensive production methods with competitive power-conversion efficiencies. Dr Snaith has stated that this could cut the cost of a watt of solar generating-capacity by three-quarters.

Perovskite is a class or species of minerals composed of, or similar in structure to, calcium titanium oxide. The intricacies of the material’s physical or chemical properties are not of interest here, but it is important to note that perovskite can describe either calcium titanium oxide itself, or a variety of other elemental combinations with the same crystal structure as calcium titanium oxide. This is an important distinction because, thus, many elemental concoctions can adopt the same semi-conducting properties and behavior as the calcium titanate base. Manipulating the proportions and quantities of the substances involved allows the semi-conductor’s behavior and properties to be calibrated, such as the frequency of electromagnetic radiation it absorbs best. 

For instance, Oxford Unviersity’s research team has produced a rather sophisticated organic-inorganic combination (what they call an organometal). The organic component of this perovskite cell acts as a dye which increases the number of frequencies of light the crystal is sensitive to, optimizing the crystal’s absorption capacity. The inorganic component then serves to conduct the additional electrons (current) thus released.

Perovskites are themselves naturally occurring materials. They consist of cheap bulk minerals and metals often contained in disposed electrical items, blended and purified at room temperature, in low-cost and comparatively primitive processes. Laboratory prototypes have emerged with a per-watt cost of 40 dollar cents (half the standard commercial value for silicon-based cells). Dr Snaith predicts that this may halve again at industrial scale. With respect to performance, perovskites have a comparatively high conversion potential because of their ability to absorb light on a very large spectrum. Researchers at UCLA reported in 2014 the development of a cell with an efficiency of 19.3% while the scientific community predicts efficiencies will continue to rise in excess of 20% by the end of the year.

Nonetheless, several concerns about the reliability of perovskite cells have arisen. And as the wonder of the perovskite phenomenon, with all its positive facets and revolutionary prospects, has been exhaustively discussed in a great variety of media (and we are here presenting ourselves as mere regurgitators of common wisdom), playing the devil’s advocate and practicing skepticism should prove a far more productive and interesting mental exercise.

Especially environmental concerns have hitherto deterred investors from taking a closer look at perovskite. Those perovskite concoctions which until now have produced the highest efficiencies were predominantly lead-based. Yet perovskite arrangements are also quite unstable – they are saltlike minerals easily dissolved in water – likely release lead if exposed to humid conditions for longer periods of time. Large amounts of lead possibly being carried into the environment by natural processes at utility-scale production would rather defeat renewable energy’s purpose of eco-friendliness, and is unattractive for commercialization. Furthermore, the toxic by-products and wastes associated with lead mining and production could pose a significant environmental hazard, especially if lead production expands to meet increasing demand from a growing perovskite industry.

Perovskite enthusiasts and researchers advance a number of arguments against the concern. According to an article from IEEE Spectrum, Dr Snaith argues that annual lead emissions from coal combustion are still 10 times the amount that would be needed for a terawatt of perovskite cells. It may be the case here that the article’s author misinterpreted Snaith’s statement, for the argument is weakened by a fallacy of commensurability. The argument would be stronger if phrased in a way that compared the lead emissions from coal and perovskite generating capacities of equal size. In this way, Dr Snaith quite rightly points out that concerns are slightly exaggerated if one terawatt of electricity from coal combustion required 10 times as many lead emissions as one terawatt of perovskite energy would.

Concern among investors is nevertheless understandable, considering the difficulty of marketing an apparently green technology which could carry toxic substances into the environment – even if only by negligible amounts. Researchers Dr Snaith, the current leader in the field, and his UCLA colleague Yang Yang (the current efficiency record holder) have both admitted that an ideal solution should dispense with lead.

Perovskite optimists have argued that concerns about lead emissions are inappropriate because, as in the case of lead glass, proper chemical binding can prevent lead emission all together. A valid point, but it neglects the sensitivity of the perovskite concoction – and thus its behavior as a semiconductor – to the chemical additions it interacts with. Thus an entirely new branch of perovskite research has emerged, aiming to replace the cells’ lead elements without significant losses in performance. This branch has been developing rigorously, with lead foundations rendered nearly obsolete even though significant losses in performance had to be conceded (a drop from the record 19.3% to around 3% according to initial trials). A skeptic might argue that while resolving the difficult trade-off between performance and cost, perovskite has merely produced a new trade-off between performance and environmental friendliness or reliability. It is unclear whether this may be the case: in 2013-14 several labs independently developing tin-based cells with efficiencies of around 7%. And researchers predict progress will accelerate. Yang Yang of UCLA has stated that tin might even contribute much more favorably to the material’s conductive behavior than lead, raising the material’s efficiency potential.

(Another issue surrounding the developments is the accuracy of the reported efficiency values. The scientific community has noticed that perovskite solar cells demonstrate rather pronounced hysteretic behavior (when the value of a physical quantity lags behind the effect causing it). This has led to concern whether claimed power-conversion efficiencies are accurate. Nature Materials magazine reckons the community should focus more on seeking confirmation from independent certification laboratories.)

Yet the replacement of its toxic elements does not resolve the more fundamental weakness indicated by the material’s previous tendency to release lead. We reckon that the main issue still remains proving the durability of the perovskite-based cell model. A perovskite cell would be required to maintain the high performance it promises throughout an effective lifetime of up to 30 years (roughly the standard for pv products). Needless to say, natural forces like weather and erosion must be taken into account. Perovskite has to prove its durability especially in a European setting, facing long-term exposure to the humidity it is so vulnerable to. For perovskite-instability in humid conditions does not only mean any toxic substances contained therein could be emitted, but more importantly that the general durability and reliability of the generating medium is yet unproven. Many regions in Africa and the Middle East as well as India – considered ideal locations for large-scale, yet non-intrusive solar harvesting – would demand a lot from cells. There, high temperatures combine with unusually strong air-particle corrosion due to the abundance of sand and strong winds. It just happens to be the case that especially humidity renders perovskite cells unstable, but a broader point is made here: it is now durability and reliability, rather than cost efficiency and generating power, that determine whether perovskite-based technology is fit for utility-scale and mass-market deployment. As a corollary, it is also these current concerns surrounding utility fitness, rather than efficiency potential, that pro tem slow the industrial deployment of perovskite cells – and keep large-scale investors reluctant.


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