Three‐Dimensional Concentration Mapping of Organic Blends

Abstract

The three‐dimensional morphology of mixed organic layers are quantitatively measured using high‐angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM) with electron tomography for the first time. The mixed organic layers used for organic photovoltaic applications have not been previously imaged using STEM tomography as there is insufficient contrast between donor and acceptor components. Contrast is generated by substituting fullerenes with endohedral fullerenes that contain a Lu3N cluster within the fullerene cage. The high contrast and signal‐to‐noise ratio, in combination with use of the discrete algebraic reconstruction technique (DART), allows generation of the most detailed and accurate three‐dimensional map of BHJ morphology to date. From the STEM‐tomography reconstructions it is determined that three distinct material phases are present within the BHJs. By observing changes to morphology and mixing ratio during thermal and solvent annealing, the effects of mutual solubility and fullerene crystallization on morphology and long term stability are determined. This material/technique combination shows itself as a powerful tool for examining morphology in detail and allows for observation of nanoscopic changes in local concentration.

P3HT:PCBM Bulk-Heterojunctions: Observing Interfacial and Charge Transfer States with Surface Photovoltage Spectroscopy

“…Concentration-specific 3D morphology of a BHJ film measured using HAADF-ET. 62 Assignment of photovoltage features F1−5 is shown with respect to the location within the films. HAADF-ET does obtain contrast from density changes only, so no assignment of crystallinity is made here.…”

“…Figure 5 maps the observed SPV features to distinct spatial regions in the polymer film. According to electron tomography data, 62,63 thermally annealed BHJ films contain three different phases, a fullerene-rich amorphous phase (∼10% P3HT by volume), a P3HT−fullerene mixed amorphous phase, and pure P3HT domains. The substrate surface has a higher density of the fullerene-rich and mixed phases, while the pure-P3HT domains are near the center of the film.…”

Mixed interlayers at the interface between PEDOT:PSS and conjugated polymers provide charge transport control

“…The layer formed at 180 C shows higher series resistance at higher forward potentials demonstrating that the number of doped paths through the P3HT/PSS interlayer is lower than for the interlayer annealed at 210 C. A normal annealed BHJ device will have some doped P3HT mixed with PSS, but vertical concentration measurements have shown that in fact, crystalline domains do not extend to the PEDOT:PSS interface and the PCBM concentration at the interface is still near 50%. [61][62][63][64] This comparison shows that P3HT/PCBM BHJ mixtures have high PCE under a variety of morphological conditions.…”

Modeling organic electronic materials: bridging length and time scales

“…P3HT is a semicrystalline polymer and the crystalline domains at immiscible with PCBM while the amorphous domains mix with PCBM. So the mixed morphology is either two or three phased depending on whether there is enough PCBM to fill the amorphous P3HT volume [26,80,81]. The cited DPD model generates a two phase morphology at all mixing ratios and thus produces morphologies that are bi-continuous but not physically relevant to P3HT.…”

“…Neutron scattering methods measure the structure and dynamics of hydrogen atoms in organic electronics through coherent scattering (small angle neutron scattering) and/or energy loss measurement (inelastic neutron scattering) [24]. Electron microscopy and tomography create 2D and 3D images of OPV device layers with nanometer resolution [25,26]. Similar to neutron methods, electromagnetic radiation techniques study both the structure and dynamics of non-hydrogen atoms in organic electronics, as the atomic electromagnetic cross-section increases with atomic number.…”