Abstract
Scanning transmission electron microscopy (STEM) at low energies (≤ 30 keV) in a scanning electron microscope is well suited to distinguish weakly scattering materials with similar materials properties and analyze their microstructure. The capabilities of the technique are illustrated in this work to resolve material domains in PTB7:PC71BM bulk-heterojunctions, which are commonly implemented for light-harvesting in organic solar cells. Bright-field (BF-) and high-angle annular dark-field (HAADF-) STEM contrast of pure PTB7 and PC71BM was first systematically analyzed using a wedge-shaped sample with well-known thickness profile. Monte-Carlo simulations are essential for the assignment of material contrast for materials with only slightly different scattering properties. Different scattering cross-sections were tested in Monte-Carlo simulations with screened Rutherford scattering cross-sections yielding best agreement with the experimental data. The STEM intensity also depends on the local specimen thickness, which can be dealt with by correlative STEM and scanning electron microscopy (SEM) imaging of the same specimen region yielding additional topography information. Correlative STEM/SEM was applied to determine the size of donor (PTB7) and acceptor (PC71BM) domains in PTB7:PC71BM absorber layers that were deposited from solution with different contents of the processing additive 1,8-diiodooctane (DIO).
Highlights
Lowering the electron energy from 80 keV and above in standard scanning transmission electron microscopy (STEM) experiments to 30 keV in scanning electron microscopes equipped with a STEM detector leads to the suppression of knock-on damage and contrast enhancement due to the increased scattering probability [1,2,3].The latter is favorable to distinguish weakly scattering materials with similar material densities and average atomic numbers
Summary and conclusions The benefits of correlative low-energy STEM/scanning electron microscopy (SEM) are demonstrated on PTB7:PC71BM bulk-heterojunction absorber layers, which are commonly employed in organic solar cells
When comparing PTB7 and PC71BM, Bright field (BF)–STEM allows unambiguous material identification because PTB7 always shows a higher intensity than P C71BM independent of the specimen thickness
Summary
Lowering the electron energy from 80 keV and above in standard scanning transmission electron microscopy (STEM) experiments to 30 keV in scanning electron microscopes equipped with a STEM detector leads to the suppression of knock-on damage and contrast enhancement due to the increased scattering probability [1,2,3].The latter is favorable to distinguish weakly scattering materials with similar material densities and average atomic numbers. Lowering the electron energy from 80 keV and above in standard scanning transmission electron microscopy (STEM) experiments to 30 keV in scanning electron microscopes equipped with a STEM detector leads to the suppression of knock-on damage and contrast enhancement due to the increased scattering probability [1,2,3]. STEM resolution in scanning electron microscopes is still lower than in transmission electron microscopes, best instruments provide a spatial resolution of ~ 0.34 nm which is sufficient to tackle numerous materials problems [4, 5] Another advantage of scanning electron microscopes is the inherent availability of surface topography imaging by secondary-electron scanning electron microscopy (SESEM), which can be correlatively applied in combination with STEM. This capability is important because STEM contrast can be influenced by topography effects and SESEM images support the interpretation of STEM images
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