Novel flexible, lightweight and low cost “plastic” electronics, including OLEDs and organic solar cells, rely on semiconducting small molecules or polymers that have highly anisotropic optoelectronic properties. Therefore, the molecular orientation and degree of aggregation strongly affects the device performance, but it cannot be determined easily using conventional tools such as X-ray diffraction since most organics have poor crystallinty and are fragile.
To address this issue, a team of LCN researchers has developed a new method to determine the molecular orientation and clustering in thin films containing one of the most popular small molecules for organic solar cells, namely copper phthalocyanine (CuPc). The method is based on electron paramagnetic resonance (EPR), a common lab-based technique that relies on the interaction of a microwave field with the spins, i.e. atomic magnetic moments, of a sample. Because the interaction depends on the relative orientation of the sample and external fields, the position of the molecules can be accurately determined. Furthermore, the shape of the EPR signal depends on the proximity of the spins, and clustering can therefore also be elucidated.
The methodology was used to solve one of the puzzles of plastic electronics, i.e. why solar cells based on blends of electron donor CuPc and acceptor C60 show improved performance compared to parent bilayer cells. One obvious explanation is that the interface, and therefore the opportunities for separating photoexcited electron-hole pairs into useful charge, is increased. However, an additional contribution could be a change of CuPc molecular orientation, since it is non-ideal in bilayers, and was so far not determined in the amorphous blend. After proving that the methodology is reliable with a series of crystalline films which were engineered to adopt specific orientations, and developing theoretical algorithms to rationalise the EPR spectra, the team found that the CuPc molecular orientation is not modified by blending. Therefore, the current recipes could be significantly improved to further boost the performance in molecular solar cells.
The team believes that the procedure offers an easy way to assess the molecular thin film quality during roll-to-roll manufacturing, cheaply and in real time.
Schematic of a real-time spin-based diagnostic during organic device manufacturing.
Spin-Based Diagnostic of Nanostructure in Copper Phthalocyanine–C60 Solar Cell Blends
Marc Warner, Soumaya Mauthoor, Solveig Felton, Wei Wu, Jules A. Gardener, Salahud Din, Daniel Klose, Gavin W. Morley, A. Marshall Stoneham, Andrew J. Fisher, Gabriel Aeppli, Christopher W. M. Kay, and Sandrine Heutz
Publication Date (Web): November 27, 2012 (Article)