Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Charge transport jamming in solar cells

24.09.2014

Polymer researchers from Mainz decipher the working mechanism of novel perovskite solar cells.

Researchers from the Max Planck Institute for Polymer Research in Mainz, jointly with scientists from Switzerland and Spain, have investigated the working mechanism of a new type of solar cells in which an organic-inorganic perovskite compound forms the light-absorbing layer. These cells can be produced in an inexpensive way with the simplest means.


Schematic of the working principle of a perovskite solar cell.

S. Weber


Measuring a solar cell: Rüdiger Berger (left) and Stefan Weber (right).

Photo: N. Bouvier

In comparison, the conventional silicon solar cells are energy-intensive and expensive to manufacture. Using Kelvin probe microscopy, the team of Mainz researchers around Rüdiger Berger and Stefan Weber observed the charge transport inside an illuminated solar cell. They found out that the positively charged carriers accumulated in a particular region in the solar cell.

This phenomenon could be compared to a bottleneck on a freeway: should many cars – or charge carriers in our case – want to pass the bottleneck at the same time, the traffic will inevitably slow down or come to a halt. According to these findings, the perovskite solar cells could soon achieve efficiencies comparable to those of commercial solar cells. Their results are published in the scientific journal “Nature Communications”.

The perovskite solar cells produced in a laboratory directed by the Swiss scientist Michael Grätzel contain a layer of an organic-inorganic compound which crystallizes in the cubic perovskite structure. "These structures absorb light very well", says Rüdiger Berger explaining the working mechanism of these solar cells.

"The light absorbed by the perovskite layer snatches an electron from an atom creating a positively charged electron vacancy - also known as "hole". Now the electrons just have to be brought to the electrode on the one side of the cell and the holes to the other side. That’s all we need for a working solar cell!" In the solar cell, the perovskite film therefore rests on a nanostructured layer of titanium dioxide that collects the electrons generated upon exposure to light and conducts them to the lower electrode.

The holes are conducted to the upper electrode by a layer of the organic hole conductor material spiro-OMeTAD situated on top of the perovskite film. "The many different layers in the solar cell are extremely important. They ensure an efficient sorting between the two charge carrier types" adds Berger's colleague Stefan Weber. "However, the charge carriers have to overcome a small barrier every time they jump from one material to the other. These barriers act like a construction site on a busy freeway where the vehicles clog. This charge transport jamming in the solar cell leads to losses and thus to a lower efficiency".

In order to observe the charge transport within the solar cell, the Mainz researchers have split the cell in two halves. They then polished the cross section with a finely focused ion beam. With the fine tip of a scanning force microscope, they were able to image the structure of the layer down to a resolution of a few nanometers. In addition, Kelvin probe microscopy was contemporaneously used to measure the local electrical potential underneath the tip. From the potential distribution, the researchers were then able to derive the field distribution and thus the charge transport occurring through the various layers of the cell.

In several measurement series, the researchers found that a strong accumulation of positive charges takes place in the perovskite layer upon exposure to light. They suppose that titanium dioxide, the electron conductor, does its job much more efficiently than the hole conductor. In other words, the holes do not reach their electrode as fast as the electrons do; they accumulate along the way. The excess of positive charges in the perovskite layer results in the creation of a reversed electric field which also contributes to the slow down of the hole transport.

“We could for the first time correlate the charge distribution with the individual material layers in the cell”, says Rüdiger Berger. "The charge transport jamming of positive charges in the illuminated perovskite layer tells us that the transport through the hole conductor currently constitutes the bottleneck for the efficiency of the solar cell". The observations of the Mainz researchers can help to increase the efficiency of the perovskite solar cells over the 20% mark and thus offer a genuine alternative to the conventional silicon solar cells.

Weitere Informationen:

http://www.mpip-mainz.mpg.de/4055851/PM9_14en - press release and original publication
http://www.mpip-mainz.mpg.de/home/en - Max Planck Institute for Polymer Research

Natacha Bouvier | Max-Planck-Institut

More articles from Physics and Astronomy:

nachricht First users at European XFEL
21.09.2017 | European XFEL GmbH

nachricht Tiny lasers from a gallery of whispers
20.09.2017 | American Institute of Physics

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Highly precise wiring in the Cerebral Cortex

Our brains house extremely complex neuronal circuits, whose detailed structures are still largely unknown. This is especially true for the so-called cerebral cortex of mammals, where among other things vision, thoughts or spatial orientation are being computed. Here the rules by which nerve cells are connected to each other are only partly understood. A team of scientists around Moritz Helmstaedter at the Frankfiurt Max Planck Institute for Brain Research and Helene Schmidt (Humboldt University in Berlin) have now discovered a surprisingly precise nerve cell connectivity pattern in the part of the cerebral cortex that is responsible for orienting the individual animal or human in space.

The researchers report online in Nature (Schmidt et al., 2017. Axonal synapse sorting in medial entorhinal cortex, DOI: 10.1038/nature24005) that synapses in...

Im Focus: Tiny lasers from a gallery of whispers

New technique promises tunable laser devices

Whispering gallery mode (WGM) resonators are used to make tiny micro-lasers, sensors, switches, routers and other devices. These tiny structures rely on a...

Im Focus: Ultrafast snapshots of relaxing electrons in solids

Using ultrafast flashes of laser and x-ray radiation, scientists at the Max Planck Institute of Quantum Optics (Garching, Germany) took snapshots of the briefest electron motion inside a solid material to date. The electron motion lasted only 750 billionths of the billionth of a second before it fainted, setting a new record of human capability to capture ultrafast processes inside solids!

When x-rays shine onto solid materials or large molecules, an electron is pushed away from its original place near the nucleus of the atom, leaving a hole...

Im Focus: Quantum Sensors Decipher Magnetic Ordering in a New Semiconducting Material

For the first time, physicists have successfully imaged spiral magnetic ordering in a multiferroic material. These materials are considered highly promising candidates for future data storage media. The researchers were able to prove their findings using unique quantum sensors that were developed at Basel University and that can analyze electromagnetic fields on the nanometer scale. The results – obtained by scientists from the University of Basel’s Department of Physics, the Swiss Nanoscience Institute, the University of Montpellier and several laboratories from University Paris-Saclay – were recently published in the journal Nature.

Multiferroics are materials that simultaneously react to electric and magnetic fields. These two properties are rarely found together, and their combined...

Im Focus: Fast, convenient & standardized: New lab innovation for automated tissue engineering & drug

MBM ScienceBridge GmbH successfully negotiated a license agreement between University Medical Center Göttingen (UMG) and the biotech company Tissue Systems Holding GmbH about commercial use of a multi-well tissue plate for automated and reliable tissue engineering & drug testing.

MBM ScienceBridge GmbH successfully negotiated a license agreement between University Medical Center Göttingen (UMG) and the biotech company Tissue Systems...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

“Lasers in Composites Symposium” in Aachen – from Science to Application

19.09.2017 | Event News

I-ESA 2018 – Call for Papers

12.09.2017 | Event News

EMBO at Basel Life, a new conference on current and emerging life science research

06.09.2017 | Event News

 
Latest News

Glycosylation: Mapping Uncharted Territory

21.09.2017 | Life Sciences

Highly precise wiring in the Cerebral Cortex

21.09.2017 | Health and Medicine

Did marine sponges trigger the ‘Cambrian explosion’ through ‘ecosystem engineering’?

21.09.2017 | Earth Sciences

VideoLinks
B2B-VideoLinks
More VideoLinks >>>