'Superstructured' solar cells achieve record efficiency of 10.9%

October 11, 2012 by Lisa Zyga, Phys.org feature
'Superstructured' solar cells achieve record efficiency of 10.9%
A comparison of the charge transfer and transport in (left) a solar cell with a titanium dioxide electrode (where excitons travel through the titanium dioxide) and (right) a solar cell with an alumina electrode (where excitons travel more quickly through the thin perovskite layer). Credit: Michael M. Lee, et al. ©2012 AAAS

(Phys.org)—It may sound counterintuitive that replacing one of the most photosensitive solar cell materials with a material with less desirable photosensitive properties can improve the solar cell's efficiency, but that's what scientists have shown in a new study. By replacing the highly photosensitive titanium dioxide (TiO2) with alumina (Al2O3) in a solution-processable solar cell, the researchers have achieved a record power conversion efficiency of 10.9%. They attribute this high efficiency to the Al2O3 acting as an inert scaffold, forcing the electrons to remain within and be transported through an extremely thin absorber (ETA) layer.

The researchers, led by Henry J. Snaith at the University of Oxford in the UK, with coauthors from the University of Oxford, Toin University of Yokohama in Japan, and the National Institute of Advanced Industrial Science and Technology in Ibaraki, Japan, have published their study on the highly efficient solar cells in a recent issue of Science.

"This is a new technology, so in essence a new record," Snaith told Phys.org. "All solar cell technologies have different efficiencies, with GaAs being the highest at over 28%. This is not an absolute world record, but probably the highest for a solution-processable solid-state solar cell. And the real excitement is where it may reach over the next few years; it should have a steep improvement curve."

The choice of in a solar cell is one of the most important factors contributing to solar cell , and TiO2 is often used as an electrode material in solution-processable solar cells due to its good ability for photoexcitation, or converting into electrons, as well as its strong electron-accepting properties when photosensitized with a dye or .

But in order to improve , the scientists here addressed the fundamental that arise throughout the photovoltaic process of absorbing photons and generating electrons. As they explain, energy is lost during the photogeneration of electron-hole pairs (excitons), the separation of tightly bound excitons, and the extraction of free electrons from highly disordered networks.

In attempts to overcome these losses, previous research has investigated the use of coating an ETA layer, 2 to 10 nm in thickness, on the internal surface of the TiO2 electrode in order to increase the current density and voltage. So far, solar cells with ETA layers have achieved power conversion efficiencies of up to 6.3%.

Here, the researchers have investigated the possibility that TiO2 may be hindering the effectiveness of the ETA layer due to its electronic disorder and low mobility. Because Al2O3 is a wide band gap insulator, the researchers found that, when it's used as the electrode, the photoexcited electrons remain in the ETA layer and do not drop to lower energy levels in the oxide as they do in the TiO2 electrode.

This difference offers several advantages. For instance, the researchers found that using Al2O3 significantly speeds up the electron transport process, forcing electrons to quickly travel through a perovskite ETA layer, and also increases the voltage. These improvements increased the from 8% with the TiO2 electrode to 10.9% with the Al2O3 electrode. Because the Al2O3 is mainly acting as a meso-scale scaffold, and does not play a role in photoexcitation, the researchers call this device a "meso-superstructured solar cell" (MSSC).

"The is acting as a scaffold for the perovskite layer, and subsequently the hole-conductor which is coated on top of the perovskite layer," Snaith said. "It is not electronically active, but purely acting as a physical support.

"It is very surprising and would not have been predicted," he added. "However, in hindsight we can see where the efficiency gains come from. The real surprise is that the perovskite layer is so effective at transporting charge and generating high photovoltage in the solar cell."

The scientists expect that the efficiency can be further improved in the future by various means, such as experimenting with new perovskites, using other semiconductors, and extending the absorption range.

"This work moves low-cost solution-processable significantly closer to the performance of perfectly crystalline semiconductors, while at the same time opening extensive possibilities for future research and development," Snaith said.

Explore further: Dye-sensitized solar cells with carbon nanotube transparent electrodes offer significant cost savings

More information: Michael M. Lee, et al. "Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites." Science Express. DOI: 10.1126/science.1228604

Journal reference: Science search and more info website Science Express search and more info website


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1 / 5 (1) Oct 11, 2012
"TiO2 is often used as an electrode material in solution-processable solar cells due to its good ability for photoexcitation, or converting photons into electrons,"

Na, you can't do that without 1022kev, stick to "generate" ;-)
1.8 / 5 (5) Oct 11, 2012
If they increase solar cell 300% they will be close to a coal power plant.
not rated yet Oct 11, 2012
I wonder how high they will get their efficiency levels i know 100% is unlikely i wonder what we will get to in the next few decades!
5 / 5 (3) Oct 12, 2012
If they increase solar cell 300% they will be close to a coal power plant.

Why are you booing a green technology? Job security?
5 / 5 (1) Oct 12, 2012
If they increase solar cell 300% they will be close to a coal power plant.

And an MSR is more efficient still, without the yearly deaths and pollution caused by coal.
not rated yet Oct 12, 2012
Why are you booing a green technology? Job security?

I don't understand people who boo green technology. From what I can tell, even though it cannot be used as a reliable constant baseload energy source, it works on a domestic level for individual homes. To boo it altogether seems as little Luddite-ish. I would have edited my previous comment but I ran out of time.
1 / 5 (1) Oct 12, 2012
If they increase solar cell 300% they will be close to a coal power plant.

Comparing this efficiency to burning a lump of coal does no good for the point you are trying to make. That is everyone should want energy to be relatively cheap. Like many "green" sources, solar PV is more expensive than conventional existing electricity production for most cases, especially in dealing with the nagging problems of night and shortened winter or cloudy days. Yet better & cheaper solar is a needed part of an overall energy plan, even if this one finds it's home primarily in our electronic "toys".
1 / 5 (6) Oct 13, 2012
Not to worry - in a few years, there will be so much demand for graphene that there will be a carbon shortage. Coal-fired power plants will be selling their exhaust to the highest bidder, the climate will change back(??), and the AGW crowd will need to find a new God to worship.
1 / 5 (1) Oct 14, 2012
Scooter...do you know what's the fourth most common element in the universe?

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