3D Nanoscale Characterization of Thin-Film Organic Photovoltaic Device Structures via Spectroscopic Contrast in the TEM 1

Abstract

The three-dimensional characterization of third generation photovoltaic device structures at the nanometer scale is essential to the development of efficient, reliable, and inexpensive solar cell technologies. Electron tomography is a powerful method for three-dimensional characterization; however, the application of this method to the organic materials systems that comprise typical high-efficiency devices is complicated by the difficulty in generating contrast from the compositionally similar materials. Herein we report the application of low-loss energy-filtered transmission electron microscopy as a method of generating spectroscopic contrast from a common organic bulk-heterojunction thin film consisting of a polymer donor and a fullerene-derivative acceptor. Spectral imaging methods combined with principal component analysis are used to characterize the contrast generation mechanism and to determine the optimum data acquisition parameters for this particular combination of organic phases. A proof of method for using the low-loss spectral signal as a basis for electron tomography is presented, and the advantages and drawbacks of the technique as applied to multiphase organic systems relative to the more commonly employed bright-field imaging approach are outlined.

Plasmon-loss imaging of polymer-methanofullerene bulk heterojunction solar cells

“…The high contrast of the plasmon-loss image enables a 3D electron tomography reconstruction of the P3HT-PCBM thin-film as first reported by Herzing et al 7 However, the plasmon intensity follows a non-linear relationship with respect to specimen thickness (t) and is therefore not a valid signal for electron tomography. The probability for n plasmon scattering events P n is given by Poisson statistics as (1/n!…”

“…Herzing et al 7 have shown that for poly(3-hexylthiophene) (P3HT)-phenyl-C61-butyric acid methyl ester (PCBM) bulk heterojunction solar cells the contrast can be improved by imaging with electrons that have lost energy to the material via plasmon excitations. The plasmon-loss images at different specimen tilts were used to reconstruct the 3D volume as in a conventional tomography experiment.…”

“…The plasmon peak from region B is asymmetric and shows higher intensity at energies below the peak maximum. Comparison of peak shapes for pure P3HT and PCBM published in the literature 7,13 suggests that region B is PCBM-rich, the higher intensity of the plasmon peak at low energies being due to interband transitions in the constituent C 60 molecule. 14 From Figure 2(c) the FWHM is largest within the channel regions, suggesting a higher PCBM concentration.…”

“…7,13 Several potential reasons can be put forward to explain the red shift in the PCBM plasmon peak. First the signal measured could be dominated by surface plasmons, especially if the PCBM is present as largely isolated molecules.…”

Quantitative Phase Fraction Detection in Organic Photovoltaic Materials through EELS Imaging

“…Energy filtered TEM (EFTEM) is an increasingly common method for enhancing contrast in organic materials. This is accomplished by introducing a post-specimen energy window that only allows electrons having a specific range of energies form the image [4][5][6][7][8][9][10][11]. The motivation behind the use of this technique is that the different materials within the bulk heterojunction (BHJ) have plasmon losses at different energies.…”