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Engineering the Energetics in Organic Photovoltaics and Organic Lead Halide Perovskite Solar Cells

Kolloquium der Abteilung 4

Organic and hybrid solar cells depict the next generation of photovoltaics involving the use of new semiconducting materials, which exhibit unique physical properties and enable cost efficient fabrication. However, even though record efficiencies over 20% have been achieved to date, further improvement in performance and stability requires a profound understanding of the interplay between the various composites in the cell and a careful layout of matched compounds.

For this purpose I will present dedicated studies of the physics and chemistry of interfaces comprised in these devices, where constituents from different material classes are brought into contact with each other, with a strong focus on electron spectroscopy (photoemission and inverse photoemission) and electronic transport measurements. I will lay out how functionalized oxide interlayers can significantly enhance the charge selectivity at the contacts of the solar cell depending on the respective energy level alignment.

The second part of my talk will be focused on the electronic structure of methylammonium lead tri-halide (MAPbX3, X = I, Br) perovskite films and their interfaces to adjacent transport layers. Intricate knowledge of the electronic alignment at the contact interfaces in perovskite solar cells is essential for the understanding of the exact working principle as well as improving engineering design and thus performance of respective devices. Our initial findings indicate that the electronic energy level alignment of adjacent organic hole transport layers, such as spiro-MeOTAD, can limit the maximum attainable open circuit voltage (Voc) in solar cells. By choosing the better suited hole transport material CBP values for Voc larger than 1.5 V could be achieved in the case of MAPbBr3 based devices.

In a different, inverted cell geometry, we find that the interface between a NiO surface and the MAPbI3 layer on top lead to a de facto p-type perovskite film while the same material deposited on TiO2 in the conventional cell geometry is n-type. We further show that C60 electron transport layers deposited on top exhibit almost ideal energy level alignment with the perovskite film. Eventually, I will present our current results on transparent high work function and conductive carbon contacts which are the first directly quantified measurements of band bending and chemical reaction at the perovskite/transport layer interface. These findings can be used to derive some early guidelines on how to integrate perovskite absorbers in various tandem solar cell architectures.