Posted Nov 11, 2015, 9:35 PM
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Perovskite Solar Cell Boost From Brown
October 10th, 2015 by Tina Casey
Originally published on Solar Love.
We’ve been spilling a lot of ink over perovskite solar cells recently, and here comes Brown University with yet another breakthrough. Along with researchers from the National Renewable Energy Laboratory, the Brown team has figured out a way to grow larger perovskite solar cells while keeping the conversion efficiency at a fairly high level. (h/t James Wimberley)
The Promise Of Perovskites
Solar researchers are all over perovskites, a class of synthetic crystals that mimic the unique structure of naturally occurring perovskite. Perovskites could become a low-cost, easy-to-manufacture substitute for silicon in solar cells, and researchers have been slowly breaking down the barriers to achieving that.
One problem, according to the Brown team, is that while relatively high solar conversion efficiencies of more than 20% have been reported in the laboratory, those are based on very small samples, typically 1/10 of a square centimeter.
Conventional processes for producing larger perovskite solar cells have fallen short in the quality department, with defects that interfere with efficiency.
“Glue” For Bigger, Better Perovskite Solar Cells
The new study can be found in the journal Advanced Materials under the title “Square-Centimeter Solution-Processed Planar CH3NH3PbI3 Perovskite Solar Cells with Efficiency Exceeding 15%.”
It’s based on a perovskite fabrication process previously developed at Brown, in which a solvent containing perovskite precursors is spread out on a substrate. An “anti-solvent” is then applied to get rid of the first solvent, leaving a nice smooth film of perovskite crystals.
The beauty of this method, aside from yielding a high-quality product, is its potential for scaling up to a commercial rate while keeping costs down, one factor being its ability to produce perovskite solar cells under room temperature conditions.
Scaling that process up is the challenge tackled by the new perovskite study. Here’s the rundown from Brown:
“The trick is to add excess organic precursor that initially “glues” the small perovskite crystals and helps them merge into larger ones during a heat-treatment, which then bakes away the excess precursor.”
Initial results for the new perovskite solar cells show an efficiency of 15%, which is not bad considering the potential to churn them out quickly and cheaply.
The team is pretty confident that this method, or one closely resembling it, could eventually get up to the 20% range and higher.
Another Boost For Hot-Carrier Perovskite Solar Cells
October 31st, 2015 by Tina Casey
Originally published on Solar Love.
Perovskite solar cells have been getting a lot of attention for their potential to reach new heights of efficiency and new lows of cost. In the latest development, our friends over at the National Renewable Energy Laboratory have revealed a critical “bottleneck” in perovskite solar conversion that has huge implications for third-generation, super-efficient, hot-carrier solar cells.
Hot-Carrier Solar Cells
For those of you new to the topic of hot-carrier solar cells, you can get the lowdown in plain language from SPIE, the International Society for Optics and Phonics. The gist of it is as follows: With conventional solar cells, you only get two choices: simple materials, low cost, and low efficiency (such as single-junction solar cells); or multiple materials, high cost, and high efficiency (that would be multi-junction solar cells). In contrast, with hot-carrier solar cells, you could get simplicity of design, low cost, and high efficiency all in one, kind of like having your cake and eating it too. Here’s the explanation from SPIE:
“Heat production is detrimental to the output of solar cells. It occurs when a material absorbs photons with energies larger than its bandgap. To circumvent this problem, photogenerated charge carriers must be collected through specially designed contacts that are energy-selective. Carriers with large kinetic energies—‘hot carriers’—reach these contacts before losing most of their energy as heat. In principle, efficiencies as high as 86% could be achieved.”
If that sounds too good to be true, it is. Hot-carrier solar cells are a long way from the marketplace, one big challenge is how to rev up the kinetic energy transfer to top speed (as in, subpicoseconds), in order to prevent energy loss.
Nevertheless, the high return on efficiency makes the hot-carrier field full of temptation. When SPIE covered hot-carrier solar cells back in 2011, the high mark for solar conversion was 40%, achieved by multi-junction solar cells. In December 2014, CleanTechnica took note of a new solar conversion record of 46%. Considering that the theoretical limit is about 87%, that still leaves plenty of room for alternative, “third-generation” technologies to elbow in.
The Perovskite Solution For Hot-Carrier Solar Cells
So, finally, that brings us back around to the National Renewable Energy Laboratory (NREL). Just today, the lab announced that researchers have figured out a pathway for tackling the heat loss problem by deploying hot-carrier technology in perovskite solar cells.
Perovskite refers to a class of easily synthesized crystals with a structure based on a naturally occurring mineral, perovskite, that was discovered in the Ural mountains in 1839. Within recent years, researchers have been pursuing perovskites for improving solar conversion efficiency.
The commercial deployment of perovskite in solar cells faces some obstacles, with the use of lead being a particular thorny issue, but according to NREL, successive versions of perovskite solar cells have been “shooting up the efficiency charts faster than almost anything researchers have seen before,” making them juicy targets for further research, lead or no lead (for the record, tin is being explored as one alternative to lead).
When NREL began studying perovskites in 2009, it achieved a 3.8% efficiency rate. Within just a few years, the latest attempts have jumped up to more than 20%.
The new study, published in Nature Photonics under the title “Observation of a hot-phonon bottleneck in lead-iodide perovskites,” provides a pathway for pushing perovskite levels into the stratosphere, possibly as high as 66%. Here’s the rundown from the NREL media team:
“The NREL research determined that charge carriers created by absorbing sunlight by the perovskite cells encounter a bottleneck where phonons (heat carrying particles) that are emitted while the charge carriers cool cannot decay quickly enough. Instead, the phonons re-heat the charge carriers, thereby drastically slowing the cooling process and allowing the carriers to retain much more of their initial energy for much longer periods of time. This potentially allows this extra energy to be tapped off in a hot-carrier solar cell.”
Perovskite Power, Solar Cell Style
November 3rd, 2015 by Tina Casey
Originally published on Solar Love.
Where were we? Oh right, perovskite solar cells. For those of you new to the topic, perovskite actually refers to a class of synthetic crystalline materials based on the unique properties of the naturally occurring mineral perovskite.
Perovskite offer enormous promise in terms of solar cell conversion efficiency, but they also have an enormous Achilles heel, which is their tendency to fall apart when exposed to air, especially humid air.
Just this week we took note of a futuristic new development in the field involving so called hot-carrier perovskite solar cells, but the new perovskite solar cell research from the University of California – Los Angeles (UCLA) is a bit more down to earth in terms of solving the air-phobia problem.
A team from UCLA’s California Nanosystems Institute has figured out a new way to stabilize perovskite, basically by creating a perovskite solar cell sandwich.
The conventional approach is to use an organic material as the top buffer layer, in order to get the solar-generated electricity out of the cell. However, the materials typically in use aren’t all that stable themselves, and provide relatively poor cover for the perovskite layer.
By replacing those layers with metal oxides, the UCLA team achieved a “dramatic” difference in longevity. Specifically, the new perovskite solar cells are composed of layers in this order: glass, indium tin oxide, NiOx (oxidized nickel oxide), perovskite, zinc oxide, aluminum, with the ZnO layer providing protection by keeping the aluminum from the perovskite.
In a 60-day open air test at room temperature, the metal oxide perovskite solar cells retained 90 percent of their original conversion efficiency.
Okay, so that doesn’t solve the entire problem, but it indicates that the way forward is to engineer metal oxide layers that are more dense, providing better protection for the perovskite.
You can read all about it in the journal Nature.com under the title “Improved air stability of perovskite solar cells via solution-processed metal oxide transport layer.” The study includes a relatively plain-language description of the challenges involved in engineering a long-lived perovskite solar cell.
UCLA’s press materials also provide a good snapshot of just how fast the rate of perovskite R&D is accelerating. The team started less than two years ago with a conversion efficiency of less than one percent, and it’s already closing in on the 20 percent mark. In the new study, the relatively stable version clocked in at approximately 14.6 percent plus or minus 1.5 percent, for a maximum of 16.1 percent.
Trina Solar Breaks Solar Record With 21.25% Efficient Cells
November 11th, 2015 by Tina Casey
Originally published on Solar Love.
A new solar cell efficiency record has been set by China’s Trina Solar Limited, which announced that its new p-type multi-crystalline silicon solar cell has achieved a solar conversion efficiency of 21.25 percent according to the results of third-party testing. Greater efficiency does not necessarily translate into lower costs, but the manufacturing method is based on Trina’s existing technology and the company anticipates that its new solar cell will provide an extra push to the steep downward trend for the cost of solar-sourced electricity.
Solar Cell Efficiency Records
Where were we? Oh right, the new solar cell efficiency record. For those of you new to the topic, while complex multi-layer solar cells can already achieve solar conversion efficiencies far greater than 21 percent, relatively low cost and simplicity still provides silicon solar cells with an edge in the marketplace.
Trina Solar had its solar cell efficiency rated by the Fraunhofer ISE in Germany, which just a few days ago issued a report on the current state of solar technology and markets. Here’s what Fraunhofer had to say about solar cell efficiency records for technologies on the market today:
The record lab cell efficiency is 25.6 % for mono-crystalline and 20.8 % for multi-crystalline silicon wafer-based technology.
The highest lab efficiency in thin film technology is 21.0 % for CdTe and 20.5 % for CIGS solar cells.
In the last 10 years, the efficiency of average commercial wafer-based silicon modules increased from about 12 % to 16 %. At the same time, CdTe module efficiency increased from 9 % to 13 %.
In the laboratory, best performing modules are based on mono- crystalline silicon with about 23 % efficiency. Record efficiencies demonstrate the potential for further efficiency increases at the production level.
In the laboratory, high concentration multi-junction solar cells achieve an efficiency of up to 46.0 % today. With concentrator technology, module efficiencies of up to 38.9 % have been reached.
Did you catch that first item where it said the record for multi-crystalline solar cells was *only* 21 percent? Fortunately, the researchers who put together the report prefaced the whole thing by noting that in the fast-paced world of solar cell efficiency development, their information would probably be out of date sooner rather than later.
And, they were right.
The Trina Solar Cell Efficiency Record
That mark of 20.8 percent was probably rounded up from Trina’s earlier record-setting entry for solar cell efficiency, which clocked in at 20.76 percent about one year ago.
If you’re thinking that the difference between 20.76 and 21.25 doesn’t sound all that impressive, consider that even just breaking the 20 percent barrier has been a long, hard slog. Solar cell efficiency for multi-crystalline silicon technology was still hovering around the 19.8 percent mark back in 1999. As described in the solar cell efficiency study we came across, the researchers zeroed in on a “honeycomb” surface, echoing Trina’s Honey Plus brand:
The improved multicrystalline cell performance results [partly] from isotropic etching to form a hexagonally-symmetric “honeycomb” surface texture. This texture, largely of inverted hemispheres, reduces reflection loss and improves absorption of infrared light by effectively acting as a randomizer.
In its solar cell efficiency announcement, Trina notes that it purposefully focused on low-cost, scalable processes to manufacture the record breaking solar cell, leading to high-volume commercial production. Here’s the rundown:
The record-breaking p-type multi-crystalline silicon solar cell was fabricated on a high-quality mc-Si substrate with a process that integrates advanced Honey Plus technologies including back surface passivation and local back surface field. The 156×156 mm2 solar cell reached a total-area efficiency of 21.25%.
Solar Costs Continue To Plunge Globally
November 11th, 2015 by Giles Parkinson
Originally published on RenewEconomy.
Two stunning auction results in India and Chile in the last week have underscored the extraordinary gains that large-scale solar has made against its fossil fuel competitors.
In both countries, solar is now clearly the cheapest option compared to new coal-fired power stations. In Chile, where the auction was open to all technologies, fossil fuel projects did not win a single megawatt of capacity. And the auction produced the lowest ever price for unsubsidised solar – US6.5c/kWh.
In India, US firm SunEdison won the entire 500MW of solar capacity on auction in the state of Andhra Pradesh, quoting a record low tariff for India of INR 4.63/kWh (US7.1c/kWh). Again, this was unsubsidised. And again, it beats new coal generation, particularly generation using imported coal.
These bids follow an auction in the US last month by the Texas city of Austin, which contracted to build 300MW of large-scale solar PV at a price of less than US4c/kWh. Even after backing out a tax credit, this is still less than US6c/kWh, and still beats gas and new coal plants, if anyone was planning to build one.