ACS Appl. Mater.: Research teams of UNIST Changduk Yang & Seong-Jun Yoon Propose to Improve Thermal Stability by Controlling Polymer Side Chain Conformations via Fluorine Atom Positioning
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Highlights
- Thermal stability is vital for commercializing nonfullerene acceptor-based organic solar cells.
- Two new isomeric NFAs were introduced: 4FY and 2FY, showcasing distinct characteristics.
- OSCs based on 4FY showed exceptional performance with a PCE of 16.4% and excellent thermal stability.
- Strategic fluorine positioning on 2D side chains is crucial for enhancing thermal stability and improving photovoltaic properties of OSCs.
※The researcher used Enlitech QE-R & ELCT-3010 (now REPS) & ELCT-3010 (now FTPS) to assist in the measurement
Background
The research focuses on improving the thermal stability and overall performance of nonfullerene acceptor (NFA)-based organic solar cells (OSCs). Achieving high power conversion efficiencies (PCEs) is a primary objective, but device stability, particularly thermal stability, is equally crucial for successful commercialization. The paper emphasizes the significant impact of side-chain engineering of NFAs on molecular packing and thermal stability of OSCs. It introduces two isomeric NFAs, 4FY and 2FY, with distinct 2D side chain structures achieved through fluorine atom positioning. These NFAs exhibit different molecular conformations, influencing crystalline and aggregation characteristics. The study underscores the strategic role of fluorine atom positioning in controlling molecular conformations to enhance OSC performance and stability.
Results
- The research presents the synthesis and characterization of two isomeric NFAs, 4FY and 2FY, with distinct fluorine atom positioning in their 2D outer side chains.
- 4FY exhibits a diagonal stretching conformation in its 2D side chains, resulting in higher crystallinity and denser molecular packing compared to 2FY.
- OSCs based on 4FY achieve a significantly higher PCE of 16.4% and demonstrate excellent thermal stability, retaining 88.4% of PCE even after 360 hours at 85°C.
Methods
- Polymer Synthesis
- Conjugated polymers were synthesized with various fluorine substitution patterns on the alkyl side chains.
- Thermal Analysis
- Thermogravimetric analysis (TGA) was used to measure thermal decomposition temperatures of the polymers.
- Differential scanning calorimetry (DSC) provided glass transition temperatures.
- Spectroscopy
- Fourier-transform infrared (FTIR) spectroscopy characterized chemical structure.
- UV-vis absorption spectroscopy probed optical bandgaps and π-conjugation.
- Imaging
- Atomic force microscopy (AFM) imaged polymer nanoscale morphologies.
- Grazing-incidence wide-angle X-ray scattering (GIWAXS) determined molecular order.
- Device Fabrication and Testing
- Bulk heterojunction organic solar cells were fabricated using the polymers.
- Current-voltage measurements determined power conversion efficiency.
- Devices were tested up to high temperatures to assess thermal stability.
- Computational Modeling
- Density functional theory calculations modeled polymer geometries.
- Molecular dynamics simulations assessed side chain dynamics.
Conclusions
- The discussion focuses on the implications of the different molecular conformations observed in 4FY and 2FY, highlighting how these conformations affect crystallinity and aggregation characteristics, ultimately influencing OSC performance.
- The improved PCE and thermal stability of OSCs based on 4FY are attributed to its optimal blend morphology, better compatibility with the donor component, and stronger crystallinity.
- The strategic fluorine atom positioning on the 2D outer side chains is discussed as a crucial factor in controlling molecular conformations, ultimately enhancing photovoltaic properties and thermal stability in OSCs. The discussion emphasizes the potential of this approach for advancing organic solar cell technology.
Fluorine-Positioned Conformational Locking of Side Chains for Thermal Stability in Organic Solar Cells
Fig. S10 Normalized a) FTPS-EQE and b) EQEEL curves for devices based on 4FY and 2FY.
Keywords
organic solar cell (OSC), Nonfullerene Acceptor, Conformational Locking
