Functional Materials

Over the past 20 years, organic compounds have increasingly found applications as active materials in devices such as computer displays or solar cells. Our group is mainly interested in synthesizing molecular materials for use in new-generation solar cells (e.g. bulk heterojunction solar cells, perovskite solar cells). Some important goals in this field include the preparation of fullerene or non-fullerene electron acceptors that can surpass the efficiency of the gold standard fullerene PC₇₀BM, the synthesis of hole transporting materials that boost the efficiency of perovskite solar cells, and a supramolecular approach for achieving an interdigitated heterojunction solar cell. 

Supramolecular Chemistry of Curved Pi-Systems

Highly strained π-systems such as the macrocyclic cycloparaphenylenes ([n]CPPs) have recently attracted much attention due to their challenging syntheses and unusual optoelectronic properties. We are interested in the strong, non-covalent interactions between these shape-persistent macrocycles and synthetic carbon allotropes such as C₆₀, C₇₀ or single-walled carbon allotropes.

In our first contributions in this area, we made use of the supramolecular chemistry of nanohoop [10]CPP to prepare and investigate a very unusual [2]rotaxane and to study the photophysics of electron transfer with a range of fullerenes, including the previsouly poorly understood dimeric compound (C₆₀)₂. 

Fullerenes in Organic Solar Cells

Due to a rapid decline of their production cost, fullerenes and their derivatives continue to be priviledged electron acceptors in organic photovoltaic devices.

In a first project, we wondered why prior to 2014, there were no reports of azafullerenes as electron acceptors in organic solar cells and we could demonstrate (in collaboration with Prof. Palomares, ICIQ Tarragona) that our new compound DPC₅₉N indeed outperformed PC₆₀BM in respect to short circuit current (J_SC) and external quantum efficiency (EQE) in blends with polymeric donor P3HT. 

In a second project, we teamed up with Profs. Brabec (Erlangen) and McGehee (Stanford) to investigate in great detail how the dimerization of fullerenes in the photovoltaic device affects the power conversion efficiency and how these losses can be minimized by a suitable annealing techniques.