First users at FlexPES look ‘through’ solar cells

First users at FlexPES look ‘through’ solar cells

 

The FlexPES beamline has recently welcomed its first regular user, conducting experiments on a new type of semi-transparent solar cells. With its two branches and up to four focal points, FlexPES allows for research spanning over several scientific fields, from surface and material science, to studies on low-density matter.

Commissioning activities for the FlexPES beamline are almost over and the beamline recently welcomed its first regular user, James O’Shea from the University of Nottingham. James O’Shea, professor at the physics department and director of the University of Nottingham Energy Institute, came to MAX IV to study an innovative and fascinating technology: dye-sensitized solar cells.

A solar cell in the window

James O’Shea from the University of Nottingham controlling and calibrating one of the dye-sensitized solar cell samples mounted at branch A of FlexPES beamline.

Dye-sensitized solar cells are semi-transparent, coloured solar cells produced using a cheap material such as titanium dioxide (TiO2) and a dye molecule that is absorbed onto the surface of the oxide. “Titanium dioxide doesn’t absorb light, but the dye molecule does”, explains James O’Shea. “The dye molecule on the surface absorbs a photon and excites an electron, which then tunnels into the oxide allowing us to channel some of that energy in a circuit to make the solar cell.”

The fascinating thing about dye-sensitized solar cells is that the layer of TiO2 and dye molecules is semi-transparent. Mounting these solar cells on a glass with conductive surface we can retain the transparency of the glass material, and at the same time have a working solar cell that collects energy. “That’s the beauty of this technology. Because they let light through you can incorporate these solar cells in windows, partitions, or furniture pieces such as desk lamps. They work well indoors in dim light.” Another advantage of this technology is its sustainability. The dyes used in this study are purely organic, with no addition of rare or precious material. Furthermore, the production of dye-sensitized solar cells is less expensive and energy-intensive to create compared to traditional solar cells.

James O’Shea and his team came to FlexPES to deepen our knowledge in this promising new technology. Using techniques of High-resolution photoelectron spectroscopy (PES), X-ray absorption spectroscopy (XAS) and Resonant Photoemission (RPES), they analyzed numerous cells prototypes with different dye molecules. “We want to understand the chemical bonding between the dye molecule and oxide, and how the electronic structure of the two match up. Using RPES we also want to look at how fast the charge is transferred from the dye molecule to the oxide.” James used the photon energy to put the excited electron in the available state in the dye molecule and then investigate how the electron is transferred to the oxide.

The collected data will be crucial for understanding how to improve the technology. “We collaborate with Energy Materials Laboratory at Newcastle University, where they create solar cells and measure their performance in real working environment. So, we want to correlate our data about the electronic events with the performance data from Newcastle.”

Although at his first time at MAX IV, James O’Shea is an old acquaintance of MAX lab, and he says the unique mood he remembers is still intact. “MAX IV is great. I used to use a lot MAX II at the old MAX lab. Although on a completely different scale, I think MAX IV still has the same feeling given by the people that work here. It retains a sort of relaxed family atmosphere.”

A “Swiss army knife” beamline

The FlexPES beamline with its two branches in a view from above. Branch A, on the right, is the endstation for solid samples, and branch B, on the left, is the endstation for low density matter (LDM).

FlexPES is a new high-resolution beamline for soft x-ray absorption and photoemission at MAX IV 1.5 GeV ring. It has two branches, one for solid samples and one for low-density matter (LDM), which comprises anything not in a solid state. The beamline will serve different user communities ranging from LDM to surface and material science.

At each branch the beamline has two alternative focal points, where two different experimental stations can be placed. This allows researchers to have samples mounted in different environments and quickly select between them by moving the photon beam between the chambers. “In this way we are very flexible, and can easily adapt to the user’s needs in terms of sample environment and specific focusing conditions”, explains Alexei Preobrajenski, beamline manager for FlexPES.

The FlexPES team is finishing commissioning activities by verifying that expected design values in terms of flux, energy resolution, and spot profiles at all focal points are achieved. So far the results of these measurements match the calculated values quite well and the team is very pleased with the beamline performance. The equipment of both end stations is ready for user operation.

“In particular, we are now setting up an energy-dispersive fluorescent detector that enables us to study non-conducting samples, which are not possible to probe with electrons. This is a ‘photon in/photon out’ experiment as opposed to ‘photon in/electron out’” explains Alexei. This detector is already working, and the beamline team has tested it with electrically insulating samples. “Measuring x-ray absorption by energy-dispersive fluorescence is quite common at hard x-ray beamlines, but for soft x-rays it is rather rare. This is the only fluorescent detector in soft x-ray regime we have so far at MAX IV”.

A detailed view of the cell at one of FlexPES’s experimental stations. Both branches have two alternative focal points. This allows for a total of four experimental stations, two on branch A and two on branch B.