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
- Long-term stability for perovskite solar cells: a big step forwardPerovskite solar cells are inexpensive to produce and generate a high amount of electric power per surface area. However, they are not yet stable enough, losing efficiency more rapidly than the silicon market standard. Now, an international team led by Prof. Dr. Antonio Abate has dramatically increased their stability by applying a novel coating to the interface between the surface of the perovskite and the top contact layer. This has even boosted efficiency to almost 27%, which represents the state-of-the-art. After 1,200 hours of continuous operation under standard illumination, no decrease in efficiency was observed. The study involved research teams from China, Italy, Switzerland and Germany and has been published in Nature Photonics.
- Successful master's degree in IR thermography on solar facadesWe are delighted to congratulate our student employee Luca Raschke on successfully completing her Master's degree in Renewable Energies at the Hochschule für Technik und Wirtschaft Berlin - and with distinction!
- TT-Award 2025: Perovskite solar cells from GermanyPhotovoltaics is the leading technology in the transition to clean energy. However, traditional silicon-based solar technology has reached its efficiency limit. Therefore, a HZB-team has developed a perovskite-based multi-junction cell architecture. For this, Kevin J. Prince and Siddhartha Garud received the Helmholtz-Zentrum Berlin's (HZB) Technology Transfer Prize of 5,000 euros.