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Improving materials for perovskite solar cells

Scientists at the Institut de Ciència de Materials de Barcelona (ICMAB) of the CSIC have participated in a research that compares perovskite solar cells containing different materials, in order to understand the differences in their performance.

Perovskite solar cells with different materials as HTMs also present different colours. Credit: ICIQA collaboration led by the Institut Català d’Investigació Química (ICIQ), with the Institut de Ciencia de Materials de Barcelona (ICMAB-CSIC) and IMDEA Nanociencia participation, reveals new data about the impact of changing materials in perovskite solar cell performance. The results of the work, led by Emilio Palomares group at the ICIQ, have been published in the peer-reviewed journal Energy & Environmental Science, and will help to rationalize the design of the components of cells, thus increasing their commercial appeal.

Among the current technologies, the perovskite-based solar cells are the fastest advancing solar technology to date. Since they were first used in 2009, perovskite solar cells have achieved high efficiencies (over 22% under standard solar irradiation) at low production costs. But there’s still room for improving their components. One of the improvements is based on adding more efficient Hole Transport Materials (HTMs), to obtain an optimal extraction of the electrical charge photogenerated in the perovskite.

The collaboration between researchers from ICIQ, the scientists Carmen Ocal and Esther Barrena from the Physical Chemistry of Surfaces and Interfaces group at the ICMAB-CSIC and the scientists at the IMDEA Nanociencia, sheds light on the reasons that explain the differences observed in perovskite solar cells performances when they contain HTMs of different molecules but similar chemical and physical properties.

When materials come into contact, properties change

The most widely used HTM, the spiro-OMeTAD, is prone to degradation. Therefore, current research is focused on finding alternatives. “For years, scientists have been designing new molecules to replace spiro-OMeTAD, looking for new molecules with similar electrical and optic characteristics which, hopefully, would have similar performance but would degrade less. But when the new HTM are tested in the cells, the performance is very poor. We decided to investigate why this happens”, explains Núria F. Montcada, a postdoctoral researcher at the ICIQ and and one of the first authors.

The researchers realized that new molecules with the potential to replace spiro-OMeTAD as HTM were selected on the basis of their properties in solution. However, in functional solar cells, these molecules are prepared in the form of thin films, whose surfaces in turn are placed in contact with other materials, forming interfaces. The created interfaces may confer changes in the properties of the molecules.

In the study, the ICMAB scientists measured the surface work function of each HTM layer on perovskites solar cells and they found out that “Spiro-OMeTAD energy levels align perfectly with respect to the other components of the cell, while the energetic landscape is less favorable for layers of the new HTM molecules tested. Surfaces and interfaces created in the solar cell stack have a crucial role in the functional device performances” say Carmen Ocal and Esther Barrena, scientists at the ICMAB. “Molecules have to be characterised under the same conditions as those in the solar cells”, they say.

The research has been funded by the Spanish Ministry of Science, Innovation and Universities, the European Research Council, the Alexander von Humboldt Foundation, the Generalitat de Catalunya and the Severo Ochoa Excellence Programme.

Reference article:

Ilario Gelmetti, Núria F. Montcada, Ana Pérez-Rodríguez, Esther Barrena, Carmen Ocal, Inés García-Benito, Agustín Molina-Ontoria, Nazario Martín, Anton Vidal-Ferran and  Emilio Palomares. Energy alignment and recombination in perovskite solar cells: weighted influence on the open circuit voltage. Energy & Environmental Science. 2019,12, 1309-1316. DOI: 10.1039/C9EE00528E