Additive-Induced Stabilization of the Energetic Landscape of PM6:Y12 Organic Solar Cells

This study demonstrates that incorporating the solvent additive 1-chloronaphthalene (1-CN) significantly enhances the photostability of PM6:Y12 organic solar cells by stabilizing the donor's HOMO energy level and preserving nanostructural integrity, thereby preventing the driving force loss and efficiency degradation observed in additive-free devices.

The Gist

The Big Picture: A Solar Cell Race

Imagine an Organic Solar Cell (OPV) as a high-speed relay race. The goal is to catch sunlight and turn it into electricity.

• The Runners: The race involves two main characters: PM6 (the Donor) and Y12 (the Acceptor).
• The Track: They run together in a mixed-up layer called a "Bulk Heterojunction."
• The Goal: When sunlight hits them, they need to pass a "baton" (an electron or a hole) to each other efficiently to generate power.

For years, scientists have figured out how to make these runners fast (high efficiency). But there's a problem: they get tired and slow down very quickly when left in the sun. This is the "stability" problem.

The Mystery: Why Do They Fail?

The researchers wanted to know: Why do these solar cells stop working after a few hours of sunlight?

They suspected that the "energetic landscape" (the terrain the runners are on) was changing. Think of it like a hill. To pass the baton, the runner needs a specific slope to slide down. If the hill flattens out or shifts, the baton gets stuck, and the race stops.

The Investigation: Looking Deep Inside

Usually, scientists only look at the surface of the solar cell, like looking at the paint on a car. But this paper used a special technique called UPS Depth Profiling.

• The Analogy: Imagine peeling an onion layer by layer. Instead of just looking at the skin, they peeled back the solar cell to see what was happening deep inside the "meat" of the device.
• The Discovery: They found that the PM6 runner was the weak link. After being in the sun, the PM6's "energy hill" shifted dramatically. It became a steep cliff instead of a gentle slope. This meant the baton (the charge) couldn't get passed to the Y12 runner anymore. The Y12 runner was fine, but the PM6 runner had changed its shape and energy levels, breaking the team's coordination.

The Hero: The "1-CN" Additive

The researchers tested a secret ingredient: 1-Chloronaphthalene (1-CN). In solar cell manufacturing, this is a "solvent additive"—a liquid added to the mixture before it dries.

• The Analogy: Think of 1-CN as a construction foreman or a traffic controller.
  • Without the Foreman (No Additive): When the solar cell dries in the sun, the runners (PM6 and Y12) get disorganized. They trip over each other, their energy levels shift, and the "track" becomes a mess. The PM6 runner gets exhausted and changes its energy, causing the whole race to fail.
  • With the Foreman (With 1-CN): The 1-CN additive slows down the drying process. It gives the runners time to get into perfect formation. It acts like a stabilizing glue. When the sun hits them later, the 1-CN helps them hold their shape. The "energy hill" stays exactly where it needs to be.

The Results: What Happened?

1. Without the Additive: After 15 hours of sun, the PM6 runner's energy level dropped by a huge amount (200 meV). The "slope" needed to pass the baton disappeared. The solar cell lost most of its power.
2. With the Additive: The PM6 runner stayed stable. The energy levels didn't shift. The "slope" remained perfect. The solar cell kept working efficiently, retaining its power much longer.

The "Why" Behind the Magic

Why did the additive work?

• Structure: The additive helped the molecules pack together more neatly (like soldiers standing in perfect rows).
• Stability: Because they were packed so neatly, the sunlight couldn't easily scramble their arrangement or change their energy levels.
• The Verdict: The study proved that the PM6 material is the one that degrades, not the Y12. But by using the 1-CN additive, we can "lock" the PM6 in place so it doesn't degrade.

The Takeaway

This paper is a breakthrough because it solves a puzzle that has been confusing scientists for a while.

• Old Thinking: "Maybe the whole mix is unstable."
• New Finding: "No, it's specifically the PM6 material that changes energy levels, but we can stop it from changing if we use the right additive."

In simple terms: If you want a solar panel that lasts a long time, you don't just need fast runners; you need a foreman (the additive) to keep them organized and stable so they don't get tired and confused by the sun. This research shows exactly how that foreman works, paving the way for cheaper, longer-lasting solar energy for everyone.

Technical Summary

1. Problem Statement

Organic Photovoltaics (OPVs), particularly those based on non-fullerene acceptors (NFAs) like PM6:Y12, have achieved high power conversion efficiencies (PCE > 20%). However, their commercial viability is hindered by a lack of simultaneous high efficiency and long-term operational stability.

• The Gap: While solvent additives (e.g., 1-chloronaphthalene, 1-CN) are known to optimize film morphology and boost initial performance, their impact on photostability and the evolution of the energetic landscape (energy levels) during photoaging remains unclear.
• Specific Challenge: In low-bandgap NFA systems, efficient charge generation relies heavily on hole transfer from the acceptor to the donor, driven by the Highest Occupied Molecular Orbital (HOMO) offset. It is unknown how photodegradation alters these energy levels in the bulk of the active layer and whether additives can mitigate such shifts.
• Measurement Limitation: Traditional surface-sensitive techniques (like standard UPS) cannot accurately represent the bulk energetics of the active layer due to vertical compositional gradients and surface enrichment.

2. Methodology

The study employed a multi-modal characterization approach to investigate PM6:Y12 bulk heterojunction (BHJ) solar cells with and without 0.5% v/v 1-CN additive.

• Device Fabrication: Conventional structure: ITO/PEDOT:PSS/PM6:Y12/PDINN/Ag. Devices were aged under continuous 1-sun illumination (AM 1.5G, 100 mW cm⁻²) in ambient air for 15 hours.
• Depth-Resolved Energetics (Key Innovation):
  • UPS-GCIB-DP: Ultraviolet Photoemission Spectroscopy combined with Argon Gas Cluster Ion Beam Depth Profiling. This technique gently etches the film to acquire UPS spectra at defined depths, mapping the HOMO/LUMO levels throughout the ~100 nm bulk layer without damaging the organic semiconductor.
  • Complementary Techniques: Low-Energy Inverse Photoemission Spectroscopy (LEIPES) for electron affinity; X-ray Photoemission Spectroscopy (XPS) for chemical composition.
• Structural & Optical Analysis:
  • GIWAXS: Grazing-Incidence Wide-Angle X-ray Scattering to analyze nanostructural ordering and crystallinity.
  • Optical Spectroscopy: UV-Vis absorption and Photothermal Deflection Spectroscopy (PDS) to assess optical bandgaps and Urbach energy (energetic disorder).
  • Transient Absorption (TA) Spectroscopy: Femtosecond TA to probe charge generation dynamics and recombination kinetics.
• Device Performance: Current density-voltage (J-V) characteristics and External Quantum Efficiency (EQE) measurements.

3. Key Findings & Results

A. Identification of the Degradation Pathway

• Donor Instability: Photodegradation is primarily driven by the polymer donor (PM6), not the acceptor (Y12).
  • PM6: Shows significant photobleaching, loss of vibronic structure (indicating reduced conjugation length), and a massive increase in Urbach energy (disorder).
  • Y12: Remains remarkably stable optically and structurally.
• Chemical Stability: XPS analysis showed no significant new chemical species or large-scale oxidation/decomposition, suggesting degradation is morphological/structural rather than purely chemical.

B. Evolution of the Energetic Landscape

• Without Additive (1-CN):
  • Upon aging, the PM6 HOMO level undergoes a critical downward shift of ~200 meV (from -5.3 eV to -5.5 eV).
  • The Y12 HOMO remains stable at -5.50 eV.
  • Consequence: The HOMO offset (driving force for hole transfer from Y12 to PM6) collapses from ~200 meV to nearly zero. This eliminates the thermodynamic driving force for exciton dissociation, leading to severe efficiency loss.
• With Additive (1-CN):
  • The 1-CN additive stabilizes the PM6 HOMO level, preventing the downward shift.
  • The HOMO offset is preserved, maintaining the driving force for efficient hole transfer and charge generation.

C. Structural Correlations (GIWAXS)

• Without Additive: Neat PM6 and blends show a drastic loss of lamellar ordering (PM6 (100) peak intensity drops by ~85% in neat films and ~62% in blends).
• With Additive: 1-CN promotes stronger initial molecular ordering. Upon aging, the degradation of PM6 ordering is significantly mitigated (only ~28% reduction in peak intensity).
• Conclusion: The preservation of nanostructural integrity by 1-CN is directly linked to the stabilization of the energetic landscape.

D. Charge Dynamics and Device Performance

• Transient Absorption: Aged devices without 1-CN show a quenching of the long-lived charge carrier signal, indicating inhibited free-charge generation due to the loss of driving force. Devices with 1-CN retain better charge generation kinetics.
• Performance Metrics:
  • Fresh Devices: 1-CN improved PCE from 14.25% to 17.3% (due to better morphology).
  • Aged Devices: Additive-free devices suffered severe degradation in $J_{SC}$ and Fill Factor (FF). Devices with 1-CN retained significantly higher PCE, demonstrating enhanced operational stability.

4. Key Contributions

1. Methodological Advance: Demonstrated the efficacy of UPS-GCIB-DP in revealing the bulk energetic evolution of OPV blends, proving that surface measurements alone are insufficient to predict device degradation mechanisms.
2. Mechanistic Insight: Identified that the primary degradation mechanism in PM6:Y12 is the energetic misalignment of the donor (PM6) caused by photoaging, which specifically destroys the hole-transfer driving force.
3. Additive Role Redefined: Established that solvent additives like 1-CN do more than just optimize initial morphology; they act as stabilizers that lock the energetic landscape and nanostructure against photo-degradation.
4. Design Guidelines: Highlighted that future molecular design for OPVs must prioritize the energetic stability of the donor polymer under operational stress, not just its initial energy levels.

5. Significance

This work bridges the gap between morphological stability and electronic stability in organic solar cells. It provides a critical explanation for why certain additive-engineered devices outperform others in long-term stability tests. By proving that the collapse of the HOMO offset is the "Achilles' heel" of PM6:Y12 devices, the study offers a clear target for material scientists: stabilizing the donor's HOMO level is essential for achieving commercially viable, high-efficiency, and stable organic photovoltaics. The findings suggest that processing strategies must be optimized not just for initial efficiency, but specifically for the preservation of the energetic landscape during operation.