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Analytical Electron Microscopy Study to Resolve the Phase Morphology of Organic Solar Cell Blends
Sedighi, M.; Löffler, M.; Röder, F.; Zschech, E.;
To increase the efficiency of bulk heterojunctions for organic photovoltaic devices, the complicated photon-to-electron conversion process has to be understood in detail. To this aim, one challenge is to resolve the correlation between processing parameters of organic solar cells (OSC), the resulting nanoscale morphology of the absorber layer, and efficiency of the completed device. Here, we present the effect of substrate heating on the morphology of the OSC where the active layer is a blend of two small molecules; ZnPc (ZnC32H18N8) as donor and C60 as acceptor.

Obtaining insights into the morphology of the active layer requires the spatial resolution and a contrast mechanism to discriminate two phases with similar average atomic number. To tackle this challenge, we combine electron microscopy imaging with different analytical techniques; energy dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS) in TEM, as well as Energy selective Backscattered (EsB) imaging in SEM.

We imaged different phases of the donor and acceptor, forming ordered and non-ordered regions, depending on the way the heterojunction is fabricated. To this aim, we fabricated samples at substrate temperatures of 110°C and 150°C, each in two different configurations:

1) Focused ion beam prepared ultrathin lamella of a complete solar cell stack with glass substrate, Indium tine oxide (ITO) electrode, ZnPc:C60 blend of the active layer between electron and hole transport layers and aluminum top electrode.
2) Plane view sample as ZnPc:C60 blend deposited on a TEM grid coated with ITO layer.

It was shown that at a substrate temperature of 110°C, the solar cell device has high efficiency [1]., so we consider this as the optimum substrate temperature.

Since identification of the composition of each phase in the plane view sample is more straight forward, we use the plane view sample to attribute each structure to one component of the blend. SEM images recorded by using secondary electron detector show that, the high temperature sample consists of rod-like features and cube-shaped material in between the rods. By combination of analytical microscopy techniques, we can attribute the rod-like structures to ZnPc.

An energy-selective backscatter (EsB) electron detector in a SEM is used to obtain backscattered contrast of the plane view sample. Due to the atomic number difference between the donor phase and the acceptor phase, (the average atomic number of 8.6 for ZnPc and 6 for the C60) sufficient contrast can be achieved [2].

A clear confirmation for the attribution of the ZnPc phase to the rod-like features and the attribution of C60 to the granular structure in between can be drawn from the investigation of the sample in SEM using EDX and with even higher precision in TEM using an improved EDX system. The chemical mapping of zinc and carbon, proves the correct phase assignment, in both plane view and ultrathin samples, prepared at high temperature. For the sample produced at optimum temperature, a much smaller roughness was observed because of the absence of large ordered regions. Even in the plane view sample, the imaging contrast is low due to less separated phases and smaller domains.

To conclude, we clearly resolved the phase morphology of the interpenetrating network of ZnPc:C60 blends for high and optimum temperature samples in plane view and furthermore we were even able to reveal the morphology from TEM images of cross-section lamellas of the real solar cell stacks.

Since the ideal active layer should have domain sizes at the range of the exciton diffusion length (10-20nm), the sample produced at high temperature does not show the desired microstructure. Due to the domain sizes of ~100 nm there is no closed path for exciton dissociation. In addition to that, the sample shows a quite high roughness.

Acknowledgement:
Authors thank A. G. Cid for SEM images. This work was supported by the German Science Council Center of Advancing Electronics Dresden (cfaed). TEM-EDX results were achieved by funding (support code 03SF0451) through the Helmholtz Energy Materials Characterization Platform (HEMCP) initiated by the Helmholtz Association and the German Federal Ministry of Education and Research (BMBF).

References:
[1] S. Pfuetzner, C. Mickel, J. Jankowski, M. Hein, J. Meiss, C. Schuenemann, C. Elschner, A.A. Levin, B. Rellinghaus, K. Leo and M. Riede, “The influence of substrate heating on morphology and layer growth in C60:ZnPc bulk heterojunction solar cells” Org. Electron., 12 (2010), pp. 435–441
[2] A. G. Cid, M. Sedighi, M. Löffler, W. F. van Dorp and E. Zschech, “Energy-Filtered Backscattered Imaging Using Low-Voltage Scanning Electron Microscopy: Characterizing Blends of C60:ZnPc for Organic Solar Cells”. (DOI: 10.1002/adem.201600063)
  • Poster
    EMRS Fall Meeting 2016, 19.-22.09.2016, Warschau, Polen

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