Interplay of Crystallization and Amorphous Spinodal Decomposition during Thermal Annealing of Organic Photoactive Layers

TL;DR

This work addresses how the nanomorphology of organic photoactive layers evolves during thermal annealing, a critical factor for device performance. It employs a phase-field framework to capture the coupled dynamics of amorphous spinodal decomposition and crystallization in the PCE11:PCBM bulk heterojunction, with predictions validated against SEM. The study demonstrates a demixing-assisted crystallization mechanism and shows that pre-existing PCE11 crystallites can seed demixing and arrest coarsening, steering the final morphology toward PCBM clusters surrounded by polymer. The results provide a physically grounded pathway to tailor processing-structure relationships in organic electronics and offer a framework applicable to other donor-acceptor blends for upscaling and manufacturing optimization.

Abstract

Tailoring the nanomorphology of organic photoactive layers through a specialized chain of processing steps is an imperative challenge on the path towards reliable and performant organic electronic manufacturing. This hurdle generally proves delicate to be overcome, as organic materials can be subject to many different phase transformation phenomena that are able to interfere with each other and produce a wide variety of morphological configurations with distinct structural, mechanical, and optoelectronic properties. A typical combination of such mechanisms, which the present systems are often prone to, and which is complex to investigate experimentally at the nanoscale, is the phase separation resulting from the interplay between amorphous demixing and crystallization. In this work, an in-house Phase-Field modeling framework is employed to simulate and, consequently, explain the phenomenological behavior of a photoactive bulk heterojunction during a thermal annealing treatment. The model predictions are validated against available electron microscopy imaging of the nanostructural evolution during the process. It is demonstrated that the simulations can successfully provide a detailed comprehension of crystal nucleation and growth shaped by amorphous spinodal decomposition, so as to yield valuable insights for physically-based morphology control. In addition, this study shows the relevance of extensive thermodynamic and kinetic characterizations of organic semiconductor mixtures (e.g., phase diagram assessments, surface tension measurements, composition-dependent molecular diffusivity evaluations) for the associated field of research.

Figures (7)

• Figure 0: Phase-Field simulations are carried out to elucidate nanomorphology formation mechanisms in organic photoactive layers under thermal annealing. The predicted bulk heterojunction structures are in excellent agreement with Scanning Electron Microscopy measurements. The study provides a detailed comprehension of key mechanistic drivers that need to be understood to devise physically-based strategies for optimized organic electronics fabrication.
• Figure 1: Melting point depression fit (lines) for the binary PCE11:PCBM system according to the Flory-Huggins theory (see Eq. \ref{['eq:MeltingPointDepression']}). The experimental data (dots) stems from Differential Scanning Calorimetry (DSC) measurements by Levitsky et al. levitsky_toward_2020. Parameters relevant for the fit are specified in SI-C.
• Figure 2: Evolution of PCE11:PCBM morphology upon thermal annealing at 130 $^\circ$C, as imaged with Scanning Electron Microscopy (SEM) by Levitsky et al. levitsky_bridging_2021. The staining agent selectively infiltrates PCE11 domains, which appear bright. PCBM-rich regions range from dark gray (amorphous) to black (crystals). The experiment shows that the mixture is subject to an initial amorphous demixing (see snapshots at 0 and 2 minutes of annealing time), consecutively followed by PCBM crystal nucleation, growth, and clustering (e.g., after 12 and 24 minutes of annealing time). Adapted from Ref. levitsky_bridging_2021 with permission from the Royal Society of Chemistry.
• Figure 3: Phase diagram calculated for the PCE11:PCBM system using the thermodynamic parameters calibrated with DSC experiments (see Sec. \ref{['Sec:ParamSetup']}). PCE11 crystallization is omitted according to the interpretation of the thermal annealing process by Levitsky et al. levitsky_bridging_2021. The evolution of the mixture composition at 130 $^\circ$C is specified by the white arrows. The as-cast blend first presents a single unstable amorphous phase with 55 v% of PCE11 (point A). Spontaneous amorphous demixing via spinodal decomposition generates two phases, respectively enriched in PCE11 and PCBM. The composition of the phases is driven towards the binodal (point B for the PCE11-rich one). Subsequently, PCBM crystallization induces further phase content purification until the crystalline-amorphous equilibrium is attained (point C denotes the corresponding liquidus composition).
• Figure 4: Simulated morphology evolution of the PCE11:PCBM mixture under thermal annealing at 130$^\circ$C. a) Early annealing stages with initial amorphous demixing. b) Intermediate to late annealing stages on which growth-dominated PCBM crystallization takes place. In both a) and b), the first row presents the volume fraction field of the PCE11 polymer ($\phi_{\mathrm{PCE11}}$). The second row then displays the PCBM order parameter field ($\psi_{\mathrm{PCBM}}$) which monitors the presence of PCBM crystals ($\psi_{\mathrm{PCBM}}=1$) in the system. The observed phase transformations arise according to the phase diagram represented in Fig. \ref{['fig:SimpleTADiagram']}. All relevant simulation parameters are specified in the SI (SI-C). For comparison with the experiments, the SEM acquisitions of Levitsky et al. levitsky_bridging_2021 are also reproduced in c) (Adapted from Ref. levitsky_bridging_2021 with permission from the Royal Society of Chemistry).