Perovskite solar cells: Interfacial loss mechanisms revealed
The SAM layer between the perovskite semiconductor and the ITO contact consists of a single layer of organic molecules. The mechanisms by which this SAM layer reduces losses can be quantified by measuring the surface photovoltage and photoluminescence. © HZB
Metal-organic perovskite materials promise low-cost and high-performance solar cells. Now a group at HZB managed to de-couple the different effects of self-assembled monolayers of organic molecules (SAMs) that reduce losses at the interfaces. Their results help to optimise such functional interlayers.
Losses occur in all solar cells. One cause is the recombination of charge carriers at the interfaces. Intermediate layers at such interfaces can reduce these losses through so-called passivation. Self-assembled monolayers (SAMs) with a carbazole core are particularly well suited for the passivation of semiconductor surfaces made of perovskite materials. A team led by HZB physicist Prof. Steve Albrecht together with a group from Kaunas Technical University in Lithuania demonstrated this some time ago, developing a silicon-perovskite-based tandem solar cell with a record efficiency of over 29 %.
Now, for the first time, a team at HZB has analysed the charge carrier dynamics at the perovskite/SAM-modified ITO interface in more detail. From time-resolved surface photovoltage measurements, they were able to extract the density of "electron traps" at the interface as well as the hole transfer rates using a minimalist kinetic model. Complementary information was provided by measuring the time-resolved photoluminescence.
"We were able to determine differences in passivation quality, selectivity and hole transfer rates depending on the structure of the SAM, and demonstrate how the time-resolved surface photovoltage and photoluminescence techniques are complementary," explains Dr. Igal Levine, postdoc at HZB and first author of the paper. Time-resolved surface photovoltage proves to be a relatively simple technique for quantifying charge extraction at buried interfaces that could significantly facilitate the design of ideal charge-selective contacts.
- Green hydrogen with PEC electrolysers: New insights into transport processesOne method of storing solar energy is to use PEC electrolysers to produce hydrogen. However, scaling up this technology remains challenging. Now, a team at the HZB Institute for Solar Fuels has used 2D fluorescence imaging and particle velocimetry to observe the movement of ions and dissolved gases within the electrolyte during electrolysis. These new insights may prove useful in the development of larger PEC electrolysers.
- New contact material boosts the efficiency of perovskite solar cellsA newly developed material for the electron contact improves the efficiency of single perovskite solar cells and perovskite/silicon tandem solar cells. The new material is based on a carborane molecule. It offers several advantages over the standard material C60, as shown by the study led by Steve Albrecht’s team. The new material has since been patented and is already commercially available.
- BESSY II: New sample environment allows glimpse into thermocatalytic processesA novel measurement cell allows, for the first time, soft and hard X-ray investigations under high pressures of up to 20 bar and temperatures of up to 400°C. This provides new insights into thermocatalytic processes, such as the Fischer-Tropsch synthesis for producing synthetic fuels. The development of the measurement cell is considered a significant achievement within the Care-O-Sene project.