Proceedings of nanoGe Fall Meeting 2021 (NFM21)
DOI: https://doi.org/10.29363/nanoge.nfm.2021.113
Publication date: 23rd September 2021
Polymer solar cells (PSCs) are among the most promising approaches toward low-cost environmentally sustainable and renewable photovoltaic devices. The insufficient stability is still a limiting factor for commercialization of PSCs and one of the key challenges of PSC technology. In PSCs based on fullerene acceptors, the thermal stability at high temperatures is affected by the diffusion of fullerenes inside the donor and the phase separation via Ostwald ripening mechanisms. The nanoscale morphology and thermodynamic stability in polymer blends are ultimately governed by miscibility of donor and acceptor (in the selected solvents and processing conditions). Predicting miscibility and stability is a formidable challenge that requires deeper fundamental understanding on polymer blends thermodynamics.
In this talk we discuss the application of large-scale atomistic methods for the modeling of polymer blends and for the prediction of their stability through the application of the Flory-Huggins theory[1]. We focus on the interesting case of PTB7 small band gap polymers that have been used to synthesize ultrastable PTB7:PC70BM blends for efficient PSCs[2]. The observed thermodynamic stability with the suppression of the thermally activated fullerene diffusion, is explained in terms local minima of the mixing enthalpy versus fullerene concentration. [2].
We also discuss results related to ternary blends with the sensitizer Si-PCPDTBT added to blends of PTB7 (or PTB7-th) donor polymers and fullerene acceptor PC71BM[3]. In addition to the dependence on fullerene concentration we find that the mixing enthalpy is strongly affected by the blend density. This outcome is relevant to model out-of-equilibrium conditions during which density fluctuations are expected and provides information on the preferable morphology of the sensitizer in the blend. We finally discuss perspectives and theoretical challenges for the atomistic modeling of polymer blends and for the reliable prediction of stability and thermodynamic properties.
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