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.
- 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.
- Metallic nanocatalysts: what really happens during catalysisUsing a combination of spectromicroscopy at BESSY II and microscopic analyses at DESY's NanoLab, a team has gained new insights into the chemical behaviour of nanocatalysts during catalysis. The nanoparticles consisted of a platinum core with a rhodium shell. This configuration allows a better understanding of structural changes in, for example, rhodium-platinum catalysts for emission control. The results show that under typical catalytic conditions, some of the rhodium in the shell can diffuse into the interior of the nanoparticles. However, most of it remains on the surface and oxidises. This process is strongly dependent on the surface orientation of the nanoparticle facets.
- 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.