Imaging of polymer:fullerene bulk-heterojunctions in a scanning electron microscope: methodology aspects and nanomorphology by correlative SEM and STEM

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:PC 71 BM bulk-heterojunctions, which are commonly implemented for light-harvesting in organic solar cells. Bright-field (BF-) and highangle annular dark-field (HAADF-) STEM contrast of pure PTB7 and PC 71 BM 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 (PC 71 BM) domains in PTB7:PC 71 BM absorber layers that were deposited from solution with different contents of the processing additive 1,8-diiodooctane (DIO).

Quantitative analysis of backscattered‐electron contrast in scanning electron microscopy

“…HAADF-STEM imaging was used to verify the arrangement of the PTB7 and PC 71 BM layers in the sample because the HAADF-STEM contrast of PTB7 and PC 71 BM is already well understood. 33 In Figure 7B, the wedge thickness increases from left to right. In the thin sample regions, PC 71 BM shows brighter contrast compared to PTB7.…”

“…However, materials with chemical compositions as close as Zn(O 0.5 S 0.5 ) and Zn(O 0.4 S 0.6 ) cannot be distinguished because the scattering properties of the two materials are almost identical. 33 From the bulk sample, a wedge-shaped lamella with a wedge angle of 25 • was prepared by FIB milling using the procedure described above.…”

Revisiting Solvent Additives for the Fabrication of Polymer:Fullerene Solar Cells: Exploring a Series of Benzaldehydes

“…Then, the BHJ layers were floated off in water and picked up by a silicon wafer. For better handling and crosssectional imaging, multiple blend layers were stacked as described in more detail in the study by Li et al [56] Two consecutive protective layers of 1 μm of electron-beam-deposited Pt and 1 μm of ion-beam-deposited Pt were used to avoid damage due to Ga-ion implantation during milling. A crosssectional lamella was extracted from this bulk sample and attached to a Cu support grid.…”

Highly Selective Cu Staining of Sulfur-Containing Polymers Facilitates 3D Nanomorphology Reconstruction of Polymer:Fullerene Blends in Organic Solar Cells by FIB-SEM Tomography

“…Energy-filtering (EF)­TEM ,− and energy-filtering SEM are also adequate techniques for phase-sensitive imaging. In another approach, STEM at comparably low electron energies was performed, which yields improved contrast due to a higher signal-to-noise ratio. , A 4D-STEM pair distribution function analysis was carried out to resolve domains in polymer:fullerene blends, but this technique requires laborious data analysis . All techniques mentioned above yield two-dimensional (projection) images of the 3D domain structure in BHJs.…”