Understanding a new type of solar cell
Scanning electron microscopy of a Perovskite-solar cell: on a glass substrate (glass and FTO) highly porous titanium dioxide is deposited, which is impregnated with perovskite. This film is covered by an organic hole transporting material (HTM) and gold contact. © EPFL
Perovskite based solar cells are a hot topic in energy research and Science Magazine has put it on the list of Breakthroughs in 2013. In only a few years their efficiency has increased from 3 % to more than 16 %. However, a detailed explanation of the mechanisms of operation within this photovoltaic system is still lacking. Scientists from Ecole polytechnique fédérale in Lausanne (EPFL) and of HZB-Institute for Solar Fuels have now uncovered the mechanism by which these novel light-absorbing semiconductors transfer electrons along their surface. They examined perovskite based solar cells with different architectures with time resolved spectroscopy techniques. Their results, which are now published online in Nature photonics, open the way to the design of photovoltaic converters with improved efficiency.
The groups of Michael Gratzel and Jaques E. Moser at EPFL, working with the team of Roel van de Krol at HZB-Institute for Solar Fuels, have used time-resolved spectroscopy techniques to determine how charges move across perovskite surfaces.
The researchers worked on various cell architectures, using either semiconducting titanium dioxide or insulating aluminum trioxide films. Both porous films were impregnated with lead iodide perovskite (CH3NH3PbI3) and an organic “hole-transporting material”, which helps extracting positive charges following light absorption. The time-resolved techniques included ultrafast laser spectroscopy and microwave photoconductivity.
The results showed two main dynamics. First, that charge separation, the flow of electrical charges after sunlight reaches the perovskite light-absorber, takes place through electron transfer at both junctions with titanium dioxide and the hole-transporting material on a sub-picosecond timescale. “Secondly, we could measure by microwave photoconductivity that charge recombination was significantly slower for titanium oxide films rather than aluminum ones”, Dennis Friedrich from the van de Krol Team points out. Charge recombination is a detrimental process wasting the converted energy into heat and thus reducing the overall efficiency of the solar cell”.
The authors state that lead halide perovskites constitute unique semiconductor materials in solar cells, allowing ultrafast transfer of electrons and positive charges at two junctions simultaneously and transporting both types of charge carriers quite efficiently. In addition, their findings show a clear advantage of the architecture based on titanium dioxide films and hole-transporting materials.
More information:
Nature photonics 'Unraveling the mechanism of photoinduced charge transfer processes in lead iodide perovskite solar cells'
doi:10.1038/nphoton.2013.374
- Battery research with the HZB X-ray microscopeNew cathode materials are being developed to further increase the capacity of lithium batteries. Multilayer lithium-rich transition metal oxides (LRTMOs) offer particularly high energy density. However, their capacity decreases with each charging cycle due to structural and chemical changes. Using X-ray methods at BESSY II, teams from several Chinese research institutions have now investigated these changes for the first time with highest precision: at the unique X-ray microscope, they were able to observe morphological and structural developments on the nanometre scale and also clarify chemical changes.
- Rutger Schlatmann re-elected as ETIP PV ChairThe European Technology and Innovation Platform for Photovoltaics (ETIP PV) was created by the European Commission in order to promote photovoltaic technologies and industries in Europe. Now, the ETIP PV Steering Committee elected a new Chair, as well as two Vice-Chairs for the term 2024 – 2026. Rutger Schlatmann, head of the division Solar Energy at the HZB, and professor at HTW Berlin, was re-elected as the ETIP PV Chair.
- Perovskite solar cells: TEAM PV develops reproducibility and comparabilityTen teams at Helmholtz-Zentrum Berlin are building a long-term international alliance to converge practices and develop reproducibility and comparability in perovskite materials. The TEAM PV project is funded by the Federal Ministry of Education and Research (BMBF), Germany.