Cheaper solar cells could be on the way thanks to ...

New solar cell devices that are cheaper and easier to make could soon make their way to market thanks to materials made at Imperial College London.

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Traditional solar cells are made from silicon, which has good efficiency and stability, but is relatively expensive to make and can only be manufactured in stiff panels.

Perovskite solar cells offer an intriguing alternative; they can be printed from inks, making them low cost, high efficiency, thin, lightweight and flexible. However, they have trailed behind silicon solar cells in efficiency and, importantly, stability, breaking down under normal environmental conditions.

Stable and efficient perovskite cells could ultimately allow solar energy to be used in more applications.

Professor Nicholas Long

New metal-containing materials called ferrocenes could help with these problems. Researchers from City University of Hong Kong (CityU) have added Imperial-made ferrocenes into perovskite solar cells, vastly improving their efficiency and stability. The results are published today in the journal Science.

Co-lead author Professor Nicholas Long, from the Department of Chemistry at Imperial, said: &#;Silicon cells are efficient but expensive, and we urgently need new solar energy devices to accelerate the transition to renewable energy. Stable and efficient perovskite cells could ultimately allow solar energy to be used in more applications &#; from powering the developing world to charging a new generation of wearable devices.

&#;Our collaboration with colleagues in Hong Kong was beautifully serendipitous, arising after I gave a talk about new ferrocene compounds and met Dr Zonglong Zhu from CityU, who asked me to send over some samples. Within a few months, the CityU team told us the results were exciting, and asked us to send more samples, beginning a research program that has resulted in perovskite devices that are both more efficient and more stable.&#;

The power of ferrocenes

Perovskite forms the &#;light-harvesting&#; layer of solar cell devices. However, these devices have been less efficient at converting solar energy into electricity than silicon-based solar cells, primarily because the electrons are less &#;mobile&#; &#; they are less able to move from the harvesting layer to the electricity conversion layers.

Ferrocenes are compounds with iron at their centre, surrounded by sandwiching rings of carbon. The unique structure of ferrocene was first recognised by Imperial&#;s own Nobel Prize-winner Professor Geoffrey Wilkinson in , and ferrocenes are still being researched around the world today for their unique properties.

One property their structure gives them is excellent electron richness, which in this case allows electrons to move more easily from the perovskite layer to subsequent layers, improving the efficiency of converting solar energy to electricity.

Tests performed by the team CityU and in commercial labs show that the efficiency of perovskite devices with an added ferrocene layer can reach 25%, approaching the efficiency of traditional silicon cells.

Two birds with one stone

But this isn&#;t the only problem the ferrocene-based materials solved. The team at Imperial have been experimenting with attaching different chemical groups to the carbon rings of ferrocene, and after sending the Hong Kong team several versions of these, made by PhD student Stephanie Sheppard, the collaborators discovered a version that significantly improves the attachment of the perovskite layers to the rest of the device.

This added attachment power improved the stability of the devices, meaning they maintained more than 98% of their initial efficiency after continuously operating at maximum power for 1,500 hours. The efficiency and stability gained thanks to the addition of a ferrocene layer brings these perovskite devices close to current international standards for traditional silicon cells.

Lead researcher Dr Zonglong Zhu from CityU said: &#;We are the first team to successfully boost the inverted perovskite solar cell to a record-high efficiency of 25% and pass the stability test set by the International Electrotechnical Commission.&#;

The team have patented their design and hope to licence it, eventually bringing their perovskite devices to the market. In the meantime, they are experimenting with different ferrocene designs to further improve the performance and stability of the devices.

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&#;Organometallic-functionalized interfaces for highly efficient inverted perovskite solar cells&#; by Zhen Li, Bo Li, Xin Wu, Stephanie A. Sheppard, Shoufeng Zhang, Danpeng Gao, Nicholas J. Long, and Zonglong Zhu is published in Science.

Costs of solar panels have plummeted over the last several years, leading to rates of solar installations far greater than most analysts had expected. But with most of the potential areas for cost savings already pushed to the extreme, further cost reductions are becoming more challenging to find.

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Now, researchers at MIT and at the National Renewable Energy Laboratory (NREL) have outlined a pathway to slashing costs further, this time by slimming down the silicon cells themselves.

Thinner silicon cells have been explored before, especially around a dozen years ago when the cost of silicon peaked because of supply shortages. But this approach suffered from some difficulties: The thin silicon wafers were too brittle and fragile, leading to unacceptable levels of losses during the manufacturing process, and they had lower efficiency. The researchers say there are now ways to begin addressing these challenges through the use of better handling equipment and some recent developments in solar cell architecture.

The new findings are detailed in a paper in the journal Energy and Environmental Science, co-authored by MIT postdoc Zhe Liu, professor of mechanical engineering Tonio Buonassisi, and five others at MIT and NREL.

The researchers describe their approach as &#;technoeconomic,&#; stressing that at this point economic considerations are as crucial as the technological ones in achieving further improvements in affordability of solar panels.

Currently, 90 percent of the world&#;s solar panels are made from crystalline silicon, and the industry continues to grow at a rate of about 30 percent per year, the researchers say. Today&#;s silicon photovoltaic cells, the heart of these solar panels, are made from wafers of silicon that are 160 micrometers thick, but with improved handling methods, the researchers propose this could be shaved down to 100 micrometers &#;  and eventually as little as 40 micrometers or less, which would only require one-fourth as much silicon for a given size of panel.

That could not only reduce the cost of the individual panels, they say, but even more importantly it could allow for rapid expansion of solar panel manufacturing capacity. That&#;s because the expansion can be constrained by limits on how fast new plants can be built to produce the silicon crystal ingots that are then sliced like salami to make the wafers. These plants, which are generally separate from the solar cell manufacturing plants themselves, tend to be capital-intensive and time-consuming to build, which could lead to a bottleneck in the rate of expansion of solar panel production. Reducing wafer thickness could potentially alleviate that problem, the researchers say.

The study looked at the efficiency levels of four variations of solar cell architecture, including PERC (passivated emitter and rear contact) cells and other advanced high-efficiency technologies, comparing their outputs at different thickness levels. The team found there was in fact little decline in performance down to thicknesses as low as 40 micrometers, using today&#;s improved manufacturing processes.

&#;We see that there&#;s this area (of the graphs of efficiency versus thickness) where the efficiency is flat,&#; Liu says, &#;and so that&#;s the region where you could potentially save some money.&#; Because of these advances in cell architecture, he says, &#;we really started to see that it was time to revisit the cost benefits.&#;

Changing over the huge panel-manufacturing plants to adapt to the thinner wafers will be a time-consuming and expensive process, but the analysis shows the benefits can far outweigh the costs, Liu says. It will take time to develop the necessary equipment and procedures to allow for the thinner material, but with existing technology, he says, &#;it should be relatively simple to go down to 100 micrometers,&#; which would already provide some significant savings. Further improvements in technology such as better detection of microcracks before they grow could help reduce thicknesses further.

In the future, the thickness could potentially be reduced to as little as 15 micrometers, he says. New technologies that grow thin wafers of silicon crystal directly rather than slicing them from a larger cylinder could help enable such further thinning, he says.

Development of thin silicon has received little attention in recent years because the price of silicon has declined from its earlier peak. But, because of cost reductions that have already taken place in solar cell efficiency and other parts of the solar panel manufacturing process and supply chain, the cost of the silicon is once again a factor that can make a difference, he says.

&#;Efficiency can only go up by a few percent. So if you want to get further improvements, thickness is the way to go,&#; Buonassisi says. But the conversion will require large capital investments for full-scale deployment.

The purpose of this study, he says, is to provide a roadmap for those who may be planning expansion in solar manufacturing technologies. By making the path &#;concrete and tangible,&#; he says, it may help companies incorporate this in their planning. &#;There is a path,&#; he says. &#;It&#;s not easy, but there is a path. And for the first movers, the advantage is significant.&#;

What may be required, he says, is for the different key players in the industry to get together and lay out a specific set of steps forward and agreed-upon standards, as the integrated circuit industry did early on to enable the explosive growth of that industry. &#;That would be truly transformative,&#; he says.

Andre Augusto, an associate research scientist at Arizona State University who was not connected with this research, says &#;refining silicon and wafer manufacturing is the most capital-expense (capex) demanding part of the process of manufacturing solar panels. So in a scenario of fast expansion, the wafer supply can become an issue. Going thin solves this problem in part as you can manufacture more wafers per machine without increasing significantly the capex.&#; He adds that &#;thinner wafers may deliver performance advantages in certain climates,&#; performing better in warmer conditions.

Renewable energy analyst Gregory Wilson of Gregory Wilson Consulting, who was not associated with this work, says &#;The impact of reducing the amount of silicon used in mainstream cells would be very significant, as the paper points out. The most obvious gain is in the total amount of capital required to scale the PV industry to the multi-terawatt scale required by the climate change problem. Another benefit is in the amount of energy required to produce silicon PV panels. This is because the polysilicon production and ingot growth processes that are required for the production of high efficiency cells are very energy intensive.&#;

Wilson adds &#;Major PV cell and module manufacturers need to hear from credible groups like Prof. Buonassisi&#;s at MIT, since they will make this shift when they can clearly see the economic benefits.&#;

The team also included Sarah Sofia, Hannu Lane, Sarah Wieghold and Marius Peters at MIT and Michael Woodhouse at NREL. The work was partly supported by the U.S. Department of Energy, the Singapore-MIT Alliance for Research and Technology (SMART), and by a Total Energy Fellowship through the MIT Energy Initiative.

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