Perovskite solar cells: Properties still remain enigmatic

The effective mass of the electrons can be derived from the curvature around the maxima of the ARPES measurement data (image, detail).

The effective mass of the electrons can be derived from the curvature around the maxima of the ARPES measurement data (image, detail). © HZB

ARPES data along a specific direction in the reciprocal lattice for various photon energies. The data show no dramatic changes in the effective mass.</p> <p>&nbsp;

ARPES data along a specific direction in the reciprocal lattice for various photon energies. The data show no dramatic changes in the effective mass.

  © HZB

In order to explain the particularly favourable properties of perovskite semiconductors for solar cells, various hypotheses are circulating. Polarons or a giant Rashba effect, for example, are thought to play a major role. A team at BESSY II has now experimentally disproved these hypotheses. In doing so, they further narrow down the possible causes for the transport properties and enable better approaches for the targeted optimisation of this class of materials.

 

Research on inorganic and hybrid organic lead halide perovskites has been booming for several years.  This class of materials has extremely interesting properties: for example, some perovskite semiconductors also convert the energy-rich blue spectrum of sunlight into electrical energy, so that solar cells based on perovskites in tandem with silicon sub-cells now achieve efficiencies of 30%. Perovskite semiconductors are also suitable for light-emitting diodes, as semiconductor lasers, and radiation detectors. Unlike conventional semiconductors, these materials can be produced cheaply and with little energy expenditure from solutions to yield thin films.

But even after years of intensive research, the microscopic processes in perovskite semiconductors that ensure superior charge transport are not understood in detail. The only thing that is clear is that the charge carriers that are released in the material by sunlight apparently have long lifetimes and are lost less frequently, for example at defects or through recombination.

Various hypotheses considered

In order to explain this behaviour, various hypotheses are circulating, which a team at BESSY II has now tested experimentally. The team led by Prof. Oliver Rader was advised by perovskite expert Prof. Eva Unger at HZB, who also provided the facilities in the HySPRINT laboratory for sample preparation.

Polarons:

One hypothesis is that polarons form in lead halide perovskites and contribute to charge transport. Such polarons are oscillations of ions in the crystal lattice which react to the movement of electrons because of their charge. Since perovskites consist of negative (here lead) and positive ions (here cesium), the assumption that polarons play a role was obvious. Measurements by another group also seemed to support this hypothesis.  

ARPES-Data: no large Polarons

At BESSY II, however, this hypothesis can be tested in detail experimentally. With angle-resolved photoemission spectroscopy (ARPES), it is possible to scan the electronic band structures. A weighty share of polarons in the charge transport would become apparent through a higher effective mass. ARPES measures the kinetic energy of the electrons, i.e. 1/2 m v2 with mass m and velocity v. The "tougher" the electron transport, the higher the so-called "effective" mass m. Since the momentum is p=mv, the formula corresponds to a parabola E= (p2)/(2m) which is measured directly in the experiment (see figure): the larger m, the smaller the curvature of the parabola.  However, the measurements Maryam Sajedi carried out on crystalline samples of CsPbBr3 did not show smaller curvatures, thus refuting the hypothesis of large polarons. "The effective mass we determined from the measurement is not larger than theoretically predicted," says Maryam Sajedi. And Oliver Rader explains: "To make sure that we took into account all possible effects other than polarons, for example the repulsion of the electrons from one another, we worked together with theoreticians from Forschungszentrum Jülich. However, there is no increased mass in the experiment for which one would have to postulate polarons."

No giant Rashba effect

The second hypothesis assumes a giant Rashba effect to limit the losses due to recombination of charge carriers. The Rashba effect is based on a strong spin-orbit coupling that could be produced in lead-halide perovskites by the heavy metal lead. Again, earlier work pointed to this effect as a possible explanation for the long lifetimes of the charge carriers. Maryam Sajedi examined samples of both inorganic CsPbBr3 and hybrid-organic MAPbBr3 with spin ARPES and analysed the measurement data. "This effect is at least a hundred times smaller than assumed," she comments on the result.

Falsification helps progress

"We have been able to experimentally disprove two common hypotheses about the transport properties in perovskites, which is an important result," says Rader. The elimination of invalid hypotheses is very helpful for the further optimisation of those materials.

arö

You might also be interested in

  • Green hydrogen: How photoelectrochemical water splitting may become competitive
    Science Highlight
    20.03.2023
    Green hydrogen: How photoelectrochemical water splitting may become competitive
    Sunlight can be used to produce green hydrogen directly from water in photoelectrochemical (PEC) cells. So far, systems based on this "direct approach" have not been energetically competitive. However, the balance changes as soon as some of the hydrogen in such PEC cells is used in-situ for a catalytic hydrogenation reaction, resulting in the co-production of chemicals used in the chemical and pharmaceutical industries. The energy payback time of photoelectrochemical "green" hydrogen production can be reduced dramatically, the study shows.
  • Soup & Science: Die solare Zukunft
    Nachricht
    20.03.2023
    Soup & Science: Die solare Zukunft
    Damit die Energiewende gelingt, sind erneuerbare Energiequellen von zentraler Bedeutung. Gerade in der Photovoltaik steckt ein enormes Potential. Strom aus der Fassade, bauwerkintegrierte Photovoltaik, neue Baumaterialien – sind dabei wichtige Themenschwerpunkte. Samira Aden ist zu Gast bei Lunchtalk Soup & Science, einer Kooperation von rbb24 Inforadio und der Technologiestiftung Berlin.

    Gespräch findet am Donnerstag 23. März 2023, 12:30 Uhr statt.

    Ort:
    Technologiestiftung Berlin
    Grunewaldstraße 61-62
 10825 Berlin

    Referentin
    Samira Jama Aden 
Helmholtz-Zentrum Berlin für Materialien und Energie, BAIP Beratungsstelle für bauwerkintegrierte Photovoltaik

    Moderation
    Axel Dorloff
 rbb24 Inforadio
Chef vom Dienst / Aktuelle Redaktion / Wissenschaft

    Das Gespräch wird voraussichtlich am Sonntag, 2. April 2023 um 9:33 Uhr und 14:33 Uhr sowie am Dienstag, den 4.April 2023 um 19:33 Uhr auf rbb24 Inforadio ausgestrahlt und ist anschließend auf der Inforadio Webseite  und in der ARD Audiothek als Podcast verfügbar. 

    Anmeldeschluss ist bis Mittwoch, 22. März 2023 möglich. Eine Teilnahme ist aufgrund der begrenzten Plätze nur mit Anmeldung möglich.

  • Perovskite solar cells from the slot die coater - a step towards industrial production
    Science Highlight
    16.03.2023
    Perovskite solar cells from the slot die coater - a step towards industrial production
    Solar cells made from metal halide perovskites achieve high efficiencies and their production from liquid inks requires only a small amount of energy. A team led by Prof. Dr. Eva Unger at Helmholtz-Zentrum Berlin is investigating the production process. At the X-ray source BESSY II, the group has analyzed the optimal composition of precursor inks for the production of high-quality FAPbI3 perovskite thin films by slot-die coating. The solar cells produced with these inks were tested under real life conditions in the field for a year and scaled up to mini-module size.