Our current research topic are organic solar cells. Previously, we have also been working on nanocrystal solar cells. We are excited to learn how the operation of these kinds of devices is related to fundamental processes such as the generation and recombination of charge carriers. The following examples give you an idea of our latest work.
Morphology versus recombination
The non-geminate recombination in bulk-heterojunction organic solar cells is still not fully understood. During the last years, it has become increasingly clear that the blend morphology plays an important role in the underlying mechanisms. We have studied how the competition between extraction and recombination of photogenerated charge carriers is affected by the nanoscale phase separation. The results indicate a correlation between the size of the phase-separated domains and the fraction of carriers lost prior to extraction. Notably, the observed trend is at variance with current theoretical descriptions. Our aim is to contribute to a more complete physical model of the recombination behaviour down to a molecular level.
Loss mechanisms in nanocrysal solar cells
Colloidal nanocrystals have great potential for the application in thin-film solar cells. However, the mechanisms of charge transport and collection are fundamentally different from bulk materials. In solar cells made from a novel kind of copper-based nanocrystals, we found that charge carriers were only harvested from a narrow region within the active layer. The remaining part of the absorber can be considered as “dead zone” for charge collection. A combination of transient and stationary measurements revealed that this deficiency was due to unsaturated trap states. We would like to learn more about the formation of these traps and how to improve the charge collection.
Charge extraction at metal oxide interlayers
Nanostructured metal oxides are widely used as interfacial layers in organic and other thin-film solar cells. However, they are known to cause severe issues with the extraction of charge carriers. One prominent example are S-shaped current-voltage curves. We have investigated the reasons for this behavior using the example of zinc oxide nanoparticles. We could identify a surface-related oxygen adsorption mechanism to cause a kinetic barrier for charge extraction. The nature of this barrier is strongly entangled with the surface-to-volume ratio of the particles.
Optical properties of thin-film solar cells
Transfer-matrix simulations can be utilized to study the non-trivial photogeneration profiles in thin-film solar cells. We have used this method to study the optical characteristics of various photovoltaic systems having the structure of an optical cavity. A good example are semitransparent solar cells. In these devices, the carrier profile depends strongly on the side the light is incident from. Our goal is to use semitransparent solar cells as a tool to study the relationship between photogeneration and charge collection in different material systems.