Theoretical insight on PTB7:PC71BM, PTB7-th:PC71BM and Si-PCPDTBT:PC71BM interactions governing blend nanoscale morphology for efficient solar cells. (April 2021)

• Record Type: Journal Article
• Authors: Caddeo, Claudia; Filippetti, Alessio; Bosin, Andrea; Videlot-Ackermann, Christine; Ackermann, Jörg; Mattoni, Alessandro
• Is Part Of: Nano energy. Volume 82(2021)
• Publication Date: 2021-04
• Subjects: Thermodynamics of polymers -- Atomistic simulations -- Organic photovoltaics -- Fullerene -- Ternary solar cells -- Low band gap polymers; Nanoscience -- Periodicals; Nanotechnology -- Periodicals; Nanostructured materials -- Periodicals; Power resources -- Technological innovations -- Periodicals; Nanoscience; Nanostructured materials; Nanotechnology; Power resources -- Technological innovations; Periodicals; 621.042
• DOI: 10.1016/j.nanoen.2020.105708
• Languages: English
• ISSNs: 2211-2855

Abstract

Abstract: The nanoscale morphology of solution processed bulk heterojunctions is governed by miscibility of donor and acceptor in the selected solvent and drying of the layer during processing. Ternary blends are of great interest for high efficiency polymer solar cells, but prediction of their morphology is highly complex. Here we perform atomistic simulations to study the miscibility of three different polymers of interest for ternary organic photovoltaics (the small band gap polymers PTB7 and PTB7-th, and the sensitizer Si-PCPDTBT) with the fullerene acceptor PC71 BM. The free energy of mixing of fullerenes with polymers is calculated as a function of the relative concentration. The blend density is also varied to simulate out-of-equilibrium conditions occurring during layer processing. By analyzing the results within the Flory–Huggins theory we find that, for a specific range of fullerene weight ratios and densely packed blends the sensitizer is most likely located in the host polymer phase due to its low miscibility with the fullerene. This configuration is the preferred one for the solar cell in order to deactivate hole traps typically formed in the binary blends and reduce recombination. Notably, we find that these results can be different qualitatively at lower density and in out-of-equilibrium blends. This work shows that weight ratios and density can be in principle chosen to select specific morphologies in ternary organic blends. Graphical abstract: Highlights: The mixing free energy F governs the nanoscale morphology of bulk heterojunctions. Out-of-equilibrium conditions can alter qualitatively the morphology of the blends. We calculate F for a variety of weight ratios also in out-of-equilibrium conditions. Weight ratios and density can be chosen to select specific morphologies in blends.