Organic photovoltaic as future technology with cost-efficient production and printability is a promising research field. New materials and studying improved synthesis techniques over the past years lead to efficiencies of more than 13 % by organic solar cell [1]. In order to improve device efficiencies, the conversion rate of photo-generated excitons to electron-hole pairs should be increased. This critical aspect, depends on the complex morphology of distributed donor and acceptor materials.

In this work we resolve the morphology of an organic solar cell, where the active layer is a blend of two small molecules; ZnPc (ZnC₃₂H₁₈N₈) as donor and C₆₀ as acceptor_.  The investigation of nanoscale morphology and phase distribution is conducted using Energy Dispersive X-ray spectroscopy (EDX) and Energy selective Backscattered (EsB) imaging in SEM [2]. The results are confirmed using analytical TEM.

The unique aspect of this work is substituting complicated TEM method by SEM with these advantages:

• Possibility of resolving chemical composition in a real solar cell in contrast to commonly deposited blend layer on TEM grids, that allows to study the complete stack of glass substrate, Indium tine oxide (ITO) electrode, electron and hole transport layers and aluminum top electrode.

• No influence of thick (90 nm) layer of heavy ITO, which is under the donor acceptor blend layer, on the TEM study.

• Gaining insight into the height distribution and roughness beside the lateral distribution (as in TEM)

To correlate morphology and material contrast we combined EDX data of the pure materials (composition of specific structure) with EsB detector mapping (strong contrast, unknown corresponding material). EDX spectrum of two material phase is shown in Figure 1, the same features where imaged by EsB detector, as it can be seen in Figure 2. Taking advantage of this combined data, we overcome poor EDX lateral and depth resolution. Since exciting Zn-K using high energy electrons, deteriorates the spatial resolution and damage the material.

Furthermore, we manage to observe the phase morphology, despite the very close mass average and quite similar chemical composition of the two phases.

In principle the technique can be extended to 3D mapping by use of slice-and-view approaches. Finally, our analytical TEM (EELS and EDX) investigations proved the corresponding morphology of each phase.

Acknowledgement:

Authors thank A. Garitagoitia Cid for SEM supply of the images in figure 2. This work was supported by the German Science Council Center of Advancing Electronics Dresden (cfaed).

References:

[1] Heliatek record on February 8^thhttp://www.heliatek.com/en/press/press-releases

[2] “Energy-filtered backscattered imaging using Low Voltage SEM” A. G. Cid, M. Sedighi, M. Löffler, W. F. van Dorp and E. Zschech. (submitted)

Figures:

Figure 1: (a) electron image of ZnP:C60 blend and (b) EDX spectra of both phases. Spectrum 1 taken in region 1 (showing a rod-like structure in the electron image) is plotted in yellow and spectrum 2 taken from phase 2 of the blend is plotted by a solid line. The appearance of the Zn peak in spectrum 1 and its absence in spectrum 2 shows that the rod-like phase is Zn- rich which correlates the phase to ZnPc.

Figure 2: SEM images of ZnPc:C60 blend with (a) In-lens and (b) EsB detector showing the material contrast of the same region. Rod-like features, having bright contrast, corresponding to a higher average atomic number, thus containing more of the heavier element (zinc), can be seen. Also cubes, which appear darker, thus containing more of the lighter element carbon.

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EMC Abstracts - https://emc-proceedings.com/abstract/revealing-nanoscale-morphology-of-organic-solar-cell-blend-by-analytical-electron-microscopy/