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
- Porous Radical Organic framework improves lithium-sulphur batteriesA team led by Prof. Yan Lu, HZB, and Prof. Arne Thomas, Technical University of Berlin, has developed a material that enhances the capacity and stability of lithium-sulphur batteries. The material is based on polymers that form a framework with open pores (known as radical-cationic covalent organic frameworks or COFs). Catalytically accelerated reactions take place in these pores, firmly trapping polysulphides, which would shorten the battery life. Some of the experimental analyses were conducted at the BAMline at BESSY II.
- Shedding light on insulators: how light pulses unfreeze electronsMetal oxides are abundant in nature and central to technologies such as photocatalysis and photovoltaics. Yet, many suffer from poor electrical conduction, caused by strong repulsion between electrons in neighboring metal atoms. Researchers at HZB and partner institutions have shown that light pulses can temporarily weaken these repulsive forces, lowering the energy required for electrons mobility, inducing a metal-like behavior. This discovery offers a new way to manipulate material properties with light, with high potential to more efficient light-based devices.
- Lithium-sulphur batteries with lean electrolyte: problem areas clarifiedUsing a non-destructive method, a team at HZB investigated practical lithium-sulphur pouch cells with lean electrolyte for the first time. With operando neutron tomography, they could visualise in real-time how the liquid electrolyte distributes and wets the electrodes across multilayers during charging and discharging. These findings offer valuable insights into the cell failure mechanisms and are helpful to design compact Li-S batteries with a high energy density in formats relevant to industrial applications.