Graphene as a front contact for silicon-perovskite tandem solar cells

The perovskite film (black, 200-300 nm) is covered by Spiro.OMeTAD, Graphene with gold contact at one edge, a glass substrate and an amorphous/crystalline silicon solar cell.

The perovskite film (black, 200-300 nm) is covered by Spiro.OMeTAD, Graphene with gold contact at one edge, a glass substrate and an amorphous/crystalline silicon solar cell. © F. Lang / HZB

HZB team develops elegant process for coating fragile perovskite layers with graphene for the first time. Subsequent measurements show that the graphene layer is an ideal front contact in several respects.

Silicon absorbers primarily convert the red portion of the solar spectrum very effectively into electrical energy, whereas the blue portions are partially lost as heat. To reduce this loss, the silicon cell can be combined with an additional solar cell that primarily converts the blue portions. Teams at HZB have already acquired extensive experience with these kinds of tandem cells. A particularly effective complement to conventional silicon is the hybrid material called perovskite. It has a band gap of 1.6 electron volts with organic as well as inorganic components. However, it is very difficult to provide the perovskite layer with a transparent front contact. While sputter deposition of indium tin oxide (ITO) is common practice for inorganic silicon solar cells, this technique destroys the organic components of a perovskite cell.

Graphene as transparent front contact:

Now a group headed by Prof. Norbert Nickel has introduced a new solution. Dr. Marc Gluba and PhD student Felix Lang have developed a process to cover the perovskite layer evenly with graphene. Graphene consists of carbon atoms that have arranged themselves into a two-dimensional honeycomb lattice forming an extremely thin film that is highly conductive and highly transparent.

Fishing for graphene:

As a first step, the scientists promote growth of the graphene onto copper foil from a methane atmosphere at about 1000 degrees Celsius. For the subsequent steps, they stabilise the fragile layer with a polymer that protects the graphene from cracking. In the following step, Felix Lang etches away the copper foil. This enables him to transfer the protected graphene film onto the perovskite. “This is normally carried out in water. The graphene film floats on the surface and is fished out by the solar cell, so to speak. However, in this case this technique does not work, because the performance of the perovskite degrades with moisture. Therefore we had to find another liquid that does not attack perovskite, yet is as similar to water as possible”, explains Gluba.

Ideal front contact:

Subsequent measurements showed that the graphene layer is an ideal front contact in several respects. Thanks to its high transparency, none of the sunlight’s energy is lost in this layer. But the main advantage is that there are no open-circuit voltage losses, that are commonly observed for sputtered ITO layers. This increases the overall conversion efficiency. “This solution is comparatively simple and inexpensive to implement”, says Nickel. “For the first time, we have succeeded in implementing graphene in a perovskite solar cell. This enabled us to build a high-efficiency tandem device.”

Journal of Physical Chemistry Letters: Perovskite Solar Cells with Large-Area CVD-Graphene for Tandem Solar Cells; Felix Lang, Marc A. Gluba, Steve Albrecht, Jörg Rappich, Lars Korte, Bernd Rech, and Norbert H. Nickel
J. Phys. Chem. Lett., 2015, 6 (14), pp 2745–2750
DOI: 10.1021/acs.jpclett.5b0117

arö

You might also be interested in

  • Stability of perovskite solar cells reaches next milestone
    Science Highlight
    27.01.2023
    Stability of perovskite solar cells reaches next milestone
    Perovskite semiconductors promise highly efficient and low-cost solar cells. However, the semi-organic material is very sensitive to temperature differences, which can quickly lead to fatigue damage in normal outdoor use. Adding a dipolar polymer compound to the precursor perovskite solution helps to counteract this. This has now been shown in a study published in the journal Science by an international team led by Antonio Abate, HZB. The solar cells produced in this way achieve efficiencies of well above 24 %, which hardly drop under rapid temperature fluctuations between -60 and +80 Celsius over one hundred cycles. That corresponds to about one year of outdoor use.
  • NETZWERKTAG der Allianz für Bauwerkintegrierte Photovoltaik
    Nachricht
    24.01.2023
    NETZWERKTAG der Allianz für Bauwerkintegrierte Photovoltaik
    Der 2. Netzwerktag der Allianz BIPV findet statt am

    14.02.2023
    10:00 - ca. 16:00 Uhr

    Das HZB, Mitglied in der Allianz BIPV, freut sich, Gastgeber des branchenweiten Austausches zu sein. Neben Praxiserfahrungen von Vertretenden aus Architektur, Fassadenbau und angewandter Forschung steht der direkte Austausch und die Diskussion im Vordergrund.

  • Webinar | Ausgezeichnete Solararchitektur: Ausgewählte Projekte aus dem Architekturpreis gebäudeintegrierte Solartechnik 2022
    Nachricht
    17.01.2023
    Webinar | Ausgezeichnete Solararchitektur: Ausgewählte Projekte aus dem Architekturpreis gebäudeintegrierte Solartechnik 2022
    Die Solarenergienutzung an Gebäuden ist ein zentrales Thema auf dem Weg zur Klimaneutralität. Solartechnische Systeme sollten selbstverständliche Bestandteile innovativer Gebäudehüllen wie auch Bausteine energetischer Sanierung sein.