3D nanoscale quantitative mapping in high-efficiency organic tandem solar cells using artefact-free spectral electron tomography

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Oral, Martin Pfannmöller, presentation 180
Publication date: 5th February 2015

One crucial target for organic photovoltaics using organic bulk heterojunctions is still to enhance device lifetime, which depends on the stability of the nanoscale morphology. Characterization of structure-property relationships has led to several paradigm shifts. It is now well-established that a particular phase distribution including mixed regions is required to improve charge separation [1]. This has important implications regarding lifetime as high-performance cells using amorphous polymers can feature increased stabilities [2]. However, we show that under certain conditions the initially high efficiency of a tandem solar cell decreases upon heating at 70 °C for 2 min. Fabricated devices with initial efficiencies of 6-7.2 % are comprised of a back cell of PDPP5T:PC₇₀BM and a front cell containing a novel amorphous high band-gap polymer (HBG1) and PC₆₀BM. Detecting morphological correlations for this architecture requires nanoscale imaging of the photoactive layers within the whole device. Earlier studies applying analytical transmission electron microscopy enabled valuable insights into structure-property relations based on floated BHJ layers [3]. Spatially resolved energy-loss spectra allow imaging of donors and acceptors at high resolution based on differences in optical excitations. Furthermore, visualization of the three-dimensional (3D) domain distribution was shown to be possible using analytical electron tomography [4]. However, in this conventional tomography approach samples can only be tilted to a maximum of ca. 70°. This creates artefacts in the reconstruction of the 3D morphology, called “missing-wedge” artefact, preventing reliable quantitative interpretation. In this work, we use rod-shaped specimens prepared by focussed ion-beam milling. With a dedicated tomography holder this allows recording of projections over the whole tilt range of ±90°. Combined with scanning transmission electron microscopy, low-energy-loss spectroscopy, and advanced data processing we provide 3D reconstructions of high-resolution spectra. This allows fitting of optical excitations in 3D by an adapted model function to yield 3D morphology maps. Renderings of these maps for a non-heated and heated sample are shown in Figure 1a. Detailed inspection reveals that PCBM content is strongly diminished near the recombination layer in the heated front cell (Figure 1b). Consequently, transport of electrons for recombination is reduced, which explains the drop in efficiency. As this method allows analysis of spectra for each voxel the superposition of reference spectra from pure regions can be computed by multiple least-squares fitting. In Figure 1c we show that this enables chemical composition mapping in 3D, approaching a resolution of <7 nm in all dimensions.
Figure 1: 3D correlation and quantification of nanomorphologies within organic tandem cell devices. a: 3D renderings of segmentations due to fitted low-energy-loss spectra (peak position) in reconstructed four-dimensional data sets. This was performed for a non-heated and a heated tandem cell with the following architecture: Back cell (PDPP5T:PCBM), recombination layer (PEDOT:PSS/ZnO, light blue/dark blue), front cell (HBG1:PCBM), and PEDOT:PSS (light blue). Polymer-rich domains are shown in green and PCBM-rich domains in semi-transparent red. b: Mapping of the minimal distances of PCBM-enriched voxels for electron transport to ZnO from the front cell to the recombination layer. Upon heating at 70 °C for 2 min the fraction of PCBM strongly decreases near the recombination layer. c: Average spectra of pure polymer and PCBM regions from both cells of the heated sample that can serve as reference spectra for a 3D compositional quantification. Orthoslices in XY, XZ, and YZ directions show enriched and mixed areas by means of the mapped polymer percentage at a resolution of <7 nm. Minimal and maximal contrasts were set to 20 and 64 %, which mark peak values of the histogram of compositions, i.e. preferential compositions for PCBM- and polymer-enriched domains, respectively. Scale bars represent 50 nm.
[1] Collins, B.A.; Tumbleston, J.R.; Ade, H. Miscibility, Crystallinity, and Phase Development in P3HT/PCBM Solar Cells: Toward an Enlightened Understanding of Device Morphology and Stability. The Journal of Physical Chemistry Letters 2011, 2, 3135-3145. [2] Lu, L.; Yu, L. Understanding Low Bandgap Polymer PTB7 and Optimizing Polymer Solar Cells Based on It. Advanced Materials 2014, 26, 4413-4430. [3] Pfannmöller, M.; Flügge, H.; Benner, G.; Wacker, I.; Sommer, C.; Hanselmann, M.; Schmale, S.; Schmidt, H.; Hamprecht, F.A.; Rabe, T.; Kowalsky, W.; Schröder, R.R. Visualizing a Homogeneous Blend in Bulk Heterojunction Polymer Solar Cells by Analytical Electron Microscopy. Nano Letters 2011, 11, 3099-3107. [4] Herzing, A.A.; Richter, L.J.; Anderson, I.M. 3D Nanoscale Characterization of Thin-Film Organic Photovoltaic Device Structures via Spectroscopic Contrast in the TEM. The Journal of Physical Chemistry C 2010, 114, 17501-17508. [5] We acknowledge financial support from the European Commission (FP7 European collaborative project SUNFLOWER (FP7-ICT-2011-7-contract num. 287594)).

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