Probing 3D reciprocal space of textured nanocrystalline thin films using electron diffraction tomography

Abstract text (incl. figure legends and references)

Texture in crystalline materials dictates their anisotropy properties, e.g. opto-electronic properties in light harvesting polycrystalline semiconductors. Texture is conventionally evaluated by probing the reciprocal space using X-ray and/or electron diffraction (ED) methods. In organic bulk heterojunctions of organic solar cells (OSCs) grazing incidence wide angle X-ray scattering (GIWAXS) is a well-established method. However, real space information is missing to correlate domain structure with crystal texture. TEM can deliver both real and reciprocal space as well as analytical signals. However, ED in OSCs is not well explored largely due to radiation damage challenges. In this work we establish a method to probe the 3D reciprocal space of textured nanocrystalline thin films by means of electron diffraction tomography (EDT), complemented by 4D-STEM analysis.

As sample systems a radiation robust AlN thin film (Fig. 1) and a radiation sensitive active layer of OSC blend (Fig. 2) are chosen. A tilt series of ED patterns over ±70° is acquired with a single-axis tilt holder. A 3D reciprocal space representation of the data is reconstructed using Process Electron Tilt Series (PETS) software and the corresponding 2D slices are extracted (cf. Fig. 1b and Fig. 2e) for comparison with GIWAXS. The texture is further studied using 4D-STEM and 4D-SCED [1].

The AlN thin film shows a pronounced fiber texture (columnar grains of ~50 nm diameter and ~500 nm length grown along direction, Fig. 1a) and its beam robustness qualifies it as reference for the described method. Figure 1b shows an overview of the EDT workflow. The 3D reciprocal lattice pattern confirms random in-plane rotation of grains and small out-of-plane tilt. The local grain orientation, mapped with ACOM [2] based on nano-beam ED 4D-STEM data (Fig. 1c), shows that the tilted grains are clustered in groups.

For the OSC samples, composed of small molecule donor DRCN5T and acceptor PC71BM, dominating texture of π-stacking face-on and edge-on were identified based on GIWAXS studies (Fig. 2c and ref. [3]). In TEM energy-filtering of SAED (EF-SAED) is necessary to suppress inelastic background scattering. Due to the limited dose budget, each ED pattern in the tilt series was acquired at fresh sample areas. The reconstructed 3D reciprocal lattice is compared with the GIWAXS data showing good agreement. A fully quantitative comparison is underway and will be presented together with pros and cons of the two methods.

[1] M. Wu, et. al., Nat. Commun. (2022) 13, 2911

[2] C. Ophus, et. al., Microsc. Microanal. (2022) 28, 390

[3] M. Berlinghof, et. al., Z. Kristallogr. (2020) 235, 15

The authors gratefully acknowledge financial support by the DFG via the Collaborative Research Centre SFB 953 "Synthetic Carbon Allotropes".

Figure 1: Characterization of highly textured columnar AlN thin film. (a) BF TEM images. (b) ED patterns from a tilt series, principle of 3D reciprocal space reconstruction and 3D view of the reciprocal space. (c) visualized texture maps based on 4D-STEM data.

Figure 2: Characterization of active layer DRCN5T:PC71BM after solvent vapour annealing. (a) molecule structure of donor DRCN5T (sulphur-rich) and acceptor PC71BM (carbon-rich) and scheme of donor crystal structure. (b) EF-SAED, (c) GIWAXS, (d) nano-morphology based on STEM-EELS data. (e) 3D reciprocal space from EF-SAED EDT. (f) face-on (grayscale) and edge-on (coloured) donor domains revealed in 4D-SCED data.