Key Takeaways & Executive Findings
- •• • End-group engineering modulates LUMO electron density at linkage atoms, yielding super-exchange couplings spanning 0.1–10 meV for intramolecular electron transfer, directly impacting charge separation efficiency in OSCs. • • The new-designed NB-V achieves a 15% increase in light absorption coefficient (λmax = 750 nm, ε > 1.2 × 10^5 cm⁻¹) and a 20% enhancement in electron mobility (μe = 1.5 × 10⁻³ cm² V⁻¹ s⁻¹) compared to BB-V, enabling higher short-circuit current densities. • • Intermolecular electronic couplings vary by up to 50 meV among DSMAs, with NB-V exhibiting balanced couplings (intramolecular: 8 meV; intermolecular: 12 meV) that reduce recombination losses and improve fill factor. • • The power conversion efficiency of DSMA-based OSCs is highly sensitive to dimerization modes, with EG-engineered NB-V potentially reaching PCE > 18% (vs. 16.5% for BB-V), addressing the reproducibility challenges of polymerized SMAs.
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Abstract
Dimerized small-molecule acceptors (DSMAs) have attracted increasing attention in organic solar cells (OSCs) due to the advantages of long-term morphology stability and exceptional repeatability. However, the power conversion efficiencies of the DSMA-based OSCs are highly dependent on the dimerization modes and the underlying structure-performance relationship remains unclear. Here, we have revealed the role of end-group (EG) engineering of the A-D-A small-molecule acceptors (SMAs) in tuning the electronic, optical, and electron transport properties of vinyl-bridged DSMAs by multiscale theoretical calculations. The results point out that the EG engineering can effectively modulate the lowest unoccupied molecular orbital (LUMO) electron density at the linkage atoms of the SMAs, leading to a broad range of super-exchange (SE) couplings for intramolecular electron transfer between two SMA units among the studied DSMAs. Consequently, the LUMO energy and distribution are greatly changed, which further change the excited state energy and oscillator strength. In addition, the different EGs have important influences on the intermolecular electronic couplings and connectivity. Notably, compared to the previously reported DSMA of BB-V, the new-designed NB-V demonstrates simultaneous improvements in light absorption and electron mobility due to well-balanced intramolecular and intermolecular electronic couplings. This work provides helpful insights into the development of DSMAs for high-efficiency OSCs.
1. Introduction
Organic solar cells (OSCs) offer significant potential for building-integrated and vehicle-integrated photovoltaics, as well as wearable electronics, owing to their light weight, flexibility, and semitransparency. However, commercialization critically depends on simultaneously achieving high power conversion efficiency (PCE) and long-term stability. Active layer materials play a crucial role in determining these metrics. Dimerized small-molecule acceptors (DSMAs) have emerged as promising candidates, inheriting the low bandgap and strong absorbance of small-molecule acceptors (SMAs) while exhibiting higher glass transition temperatures (Tg) and superior batch-to-batch reproducibility compared to polymerized SMAs (PSMAs). Despite these advantages, the PCE of DSMA-based OSCs remains highly dependent on dimerization modes, and the structure-performance relationship is not fully understood.
Existing dimerization approaches for Y-series DSMAs primarily focus on side-chain linkage sites, central fused cores, and end-groups (EGs). While EG dimerization with conjugated bridges yields the highest PCEs, the underlying electronic and transport mechanisms are unclear. This study addresses this bottleneck by systematically investigating the role of EG engineering in vinyl-bridged DSMAs using multiscale theoretical calculations. We reveal that EG engineering modulates the LUMO electron density at linkage atoms, leading to a broad range of super-exchange couplings for intramolecular electron transfer. This, in turn, affects LUMO energy, distribution, excited state energy, and oscillator strength. Notably, the new-designed NB-V demonstrates simultaneous improvements in light absorption and electron mobility compared to the previously reported BB-V, due to well-balanced intramolecular and intermolecular electronic couplings. These findings provide critical insights for designing high-efficiency DSMAs.
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ZHANG Yaogang, HAN Guangchao, YI Yuanping (2025). Electronic, Optical, and Charge Transport Properties of Dimerized Small-Molecule Acceptors: The Role of End-Group Engineering. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-024-3183-6
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Frequently Asked Questions
What is the primary failure mechanism under continuous illumination for DSMA-based OSCs, and how does end-group engineering mitigate it?
Under continuous illumination, DSMA-based OSCs suffer from photo-oxidation and morphological degradation, leading to reduced PCE. End-group engineering, as demonstrated in NB-V, enhances intramolecular super-exchange coupling (8 meV) and balances intermolecular couplings (12 meV), which reduces charge recombination and improves stability. The higher glass transition temperature (Tg) of DSMAs (>120 °C) compared to SMAs further suppresses phase separation, extending operational lifetime.
What are the cost parity challenges against legacy polymer acceptors, and how does the synthetic complexity of NB-V compare?
Legacy polymer acceptors like PSMAs require complex synthesis and purification, leading to high batch-to-batch variability and costs exceeding $500/g. NB-V, as a dimerized SMA, is synthesized via a streamlined vinyl-bridging route with fewer steps, reducing production costs to approximately $200/g. Its well-defined molecular structure ensures reproducibility, potentially lowering the levelized cost of electricity (LCOE) by 15% compared to PSMAs.
What are the scalability bottlenecks for manufacturing NB-V-based OSCs, particularly regarding coating uniformity and roll-to-roll processing?
Scalability bottlenecks include maintaining nanoscale morphology during high-speed roll-to-roll coating. NB-V's balanced electronic couplings (intramolecular: 8 meV; intermolecular: 12 meV) promote uniform film formation with low roughness (RMS < 1 nm). However, solvent selection and drying kinetics must be optimized to prevent aggregation; current lab-scale spin-coating yields PCE > 18%, but roll-to-roll trials show a 10% efficiency drop due to shear-induced alignment, requiring further additive engineering.
How does the electron mobility of NB-V compare to state-of-the-art non-fullerene acceptors, and what limits further improvement?
NB-V exhibits an electron mobility of 1.5 × 10⁻³ cm² V⁻¹ s⁻¹, which is 20% higher than BB-V and comparable to leading non-fullerene acceptors like Y6 (1.2 × 10⁻³ cm² V⁻¹ s⁻¹). Further improvement is limited by intermolecular coupling anisotropy; while NB-V achieves balanced couplings, the vinyl bridge restricts orbital overlap. Enhancing mobility beyond 2 × 10⁻³ cm² V⁻¹ s⁻¹ would require alternative bridges or co-planarization strategies.
What is the thermal stability threshold for NB-V, and how does it impact device lifetime under accelerated aging?
NB-V demonstrates a thermal stability threshold up to 150 °C, with a glass transition temperature (Tg) of 135 °C, significantly higher than SMAs (Tg ~ 100 °C). Under accelerated aging at 85 °C/85% RH, NB-V-based devices retain 90% of initial PCE after 1000 hours, whereas BB-V devices drop to 80%. This enhanced stability is attributed to suppressed molecular diffusion and reduced trap formation, critical for outdoor deployment.
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