Quantitative Phase Fraction Detection in Organic Photovoltaic Materials through EELS Imaging

Abstract

Organic photovoltaic materials have recently seen intense interest from the research community. Improvements in device performance are occurring at an impressive rate; however, visualization of the active layer phase separation still remains a challenge. This paper outlines the application of two electron energy-loss spectroscopic (EELS) imaging techniques that can complement and enhance current phase detection techniques. Specifically, the bulk plasmon peak position, often used to produce contrast between phases in energy filtered transmission electron microscopy (EFTEM), is quantitatively mapped across a sample cross section. A complementary spectrum image capturing the carbon and sulfur core loss edges is compared with the plasmon peak map and found to agree quite well, indicating that carbon and sulfur density differences between the two phases also allows phase discrimination. Additionally, an analytical technique for determining absolute atomic areal density is used to produce an absolute carbon and sulfur areal density map. We show how these maps may be re-interpreted as a phase ratio map, giving quantitative information about the purity of the phases within the junction.

Water‐Processable Amphiphilic Low Band Gap Block Copolymer:Fullerene Blend Nanoparticles as Alternative Sustainable Approach for Organic Solar Cells

“…In fact, BF-TEM may not be the most suitable technique to visualize domains with small compositional differences. [19,41] From the comparison between the BF-TEM and EFTEM images, we can qualitatively confirm that this high contrast is not due to differences in sample thicknesses, as there are also some bright spots in the EFTEM image that remain dark in the BF-TEM ones (see yellow arrows in Figure 2b,c). In this kind of images the contrast is influenced by the local material electronic distribution, not only by the material density.…”

“…It is well‐known that the carbonaceous materials have plasmon energy loss around 20 eV. In particular, plasmon peaks between 19 and 22 eV and between 25 and 30 eV have been reported for p‐type polymers such as poly[ N ‐9′‐heptadecanyl‐2,7‐carbazole‐ alt ‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole) (PCDTBT) and poly(3‐hexylthiophene) (P3HT), and n‐type materials such as PC 61 BM, respectively . In Figure b,c, a comparison between BF‐TEM and EFTEM collected at 17 ± 6 eV is reported.…”

“…These larger nanostructures are often surrounded by the smaller NPs located on their edge, sometimes leading to the formation of tail-like structures composed of two or three nanoelements. [19,41] In Figure 2b,c, a comparison between BF-TEM and EFTEM collected at 17 ± 6 eV is reported. In fact, BF-TEM may not be the most suitable technique to visualize domains with small compositional differences.…”

Deconstructing the behavior of donor–acceptor copolymers in solution & the melt: the case of PTB7

“…The final PTB7 density from this simulation protocol (see the Computational Methods and SI for more details) is 1.13 g/cm 3 , which compares very well to experimental density for PTB7 films of 1.17 g/cm 3 . 95 To draw comparisons with the vacuum and solvent simulations, the radii of gyration and persistence lengths for each of the 338 polymer chains in the simulation box ( Figure 6 oligomers. 106 On average, the PTB7 persistence length in the glass is 10 nm, while the radius of gyration is 6.7…”

High‐Throughput Multiparametric Screening of Solution Processed Bulk Heterojunction Solar Cells

“…Similarly, for the PTB7-Th:PC70BM blend (Figure 3b) there is an apparent optimum thickness of 100 ± 10 nm and a polymer volume fraction of around 50 ± 10 vol%, which corresponds to a D:A ratio of 1:1.5 (w:w) (considering ρ PC70BM = 1.72 g cm −3 , ρ PTB7-Th = 1.17 g cm −3 and ρ PCDTBT = 1.15 g cm −3 ). [41][42][43] The D:A ratio can be obtained assuming that the density of both materials is homogeneous and invariant upon the blending process. Both D:A ratios found with this methodology agree very well with the reported values in the literature, thus giving validity to the methodology.…”