Enhancing Spectral Absorption of Organic Dye Sensitizers through ZnSO₄‑Mediated Coordination and Protonation for DSSCs Applications

Technical Summary: Enhancing Spectral Absorption of Organic Dye Sensitizers through ZnSO₄‑Mediated Coordination and Protonation

Problem Statement
Dye-Sensitized Solar Cells (DSSCs) utilizing natural anthocyanin dyes face significant performance limitations compared to silicon-based or synthetic alternatives. The primary challenges include weak anchoring of dye molecules to the TiO₂ semiconductor surface, poor chemical stability, and inefficient light harvesting. While previous strategies have employed metal ions (such as Al³⁺, Cu²⁺, and Fe²⁺) to improve dye adsorption and light harvesting, the specific role of Zinc (Zn²⁺) in conjunction with its counterion has been underexplored. Furthermore, common zinc salts like ZnCl₂ and Zn(NO₃)₃ present drawbacks: their counterions (Cl⁻ and NO₃⁻) can induce adverse reactions (e.g., radical formation or photoreduction), cause osmotic stress, or form soluble complexes that reduce the availability of free Zn²⁺ ions for effective chelation.

Methodology
The study investigates the dual-functional mechanism of Zinc Sulfate (ZnSO₄) as an additive in anthocyanin-based DSSCs. The experimental approach involved:

• Sample Preparation: Anthocyanin extracts from blackberry fruits were treated with varying concentrations of ZnSO₄ (0.2 g to 1.0 g per 10 mL), creating samples labeled 2-ZnTD through 10-ZnTD. An untreated control (UTD) was also prepared.
• Device Fabrication: Porous TiO₂ photoelectrodes were sensitized with the respective dye solutions. Counter electrodes were fabricated using graphite, and a standard iodine-based electrolyte was employed.
• Characterization: The study utilized a comprehensive suite of analytical techniques:
  • FESEM and EDX: To analyze surface morphology, particle aggregation, and elemental composition (specifically Zn and S presence).
  • pH Measurement: To monitor acidity shifts induced by ZnSO₄ dissolution.
  • UV-Vis Spectroscopy: To evaluate optical absorption spectra and molar absorptivity.
  • FTIR: To identify functional group interactions and confirm metal-ligand chelation.
  • Cyclic Voltammetry (CV): To probe redox behavior, electron transfer kinetics, and recombination rates.
  • IPCE and J-V Measurements: To determine Incident Photon-to-Current Efficiency and overall photovoltaic performance (Voc, Jsc, FF, and efficiency).

Key Findings and Results
The research demonstrates that ZnSO₄ acts through a synergistic dual mechanism: Zn²⁺ coordination and SO₄²⁻ mediated protonation.

1. Chemical and Structural Changes:

   • pH Shift: The addition of ZnSO₄ lowered the solution pH from 3.76 (UTD) to 2.61 (10-ZnTD). This acidity shift promotes the formation of the photoactive flavylium cation, deepening the solution's red color.
   • Chelation: FTIR analysis confirmed that Zn²⁺ forms stable bidentate complexes with the hydroxyl (A-ring) and carbonyl (C-ring) groups of anthocyanin. This coordination stabilizes the dye structure and disrupts hydrogen bonding, evidenced by shifts in O–H and C=O stretching regions.
   • Morphology: FESEM revealed that Zn²⁺ treatment altered the TiO₂ surface, creating inhomogeneous aggregates of rounded structures and native nanoparticles. This suggests Zn²⁺ acts as a shape modulator and facilitates multilayered dye aggregation, improving light scattering and dye loading compared to the uniform but less aggregated UTD surface. EDX confirmed the successful incorporation of Zn and S into the film.

2. Optical and Electrochemical Improvements:

   • Spectral Absorption: UV-Vis spectra showed a significant increase in optical density and a bathochromic shift (red shift) from ~520 nm to ~540 nm. This indicates a reduced HOMO–LUMO gap and extended orbital conjugation, enhancing visible light capture.
   • Recombination Suppression: CV analysis showed an increase in anodic peak currents (indicating improved electron transfer) and a suppression of cathodic peak currents in treated samples (specifically 8-ZnTD). This suggests that Zn²⁺ effectively passivates surface traps on TiO₂, reducing back-electron transfer and recombination losses.
   • Photovoltaic Performance:
     • The UTD cell exhibited low efficiency (η = 0.11%) with Voc = 0.46 V and Jsc = 0.52 mA/cm².
     • The optimized 8-ZnTD cell achieved the highest performance with η = 0.38%, Voc = 0.53 V, and Jsc = 1.74 mA/cm².
     • Overloading Effect: The 10-ZnTD sample, despite having the highest UV absorbance, showed a decline in efficiency (η = 0.35%) and Fill Factor (FF = 0.30). The authors attribute this to ionic crowding, increased series resistance, and potential dye aggregation that hinders charge extraction and electrolyte diffusion.

Significance and Claims
The paper claims that ZnSO₄ serves as a unique dual-functional additive that outperforms single-ion doping or other zinc salts. Its significance lies in:

• Synergistic Mechanism: It simultaneously stabilizes the flavylium cation through metal coordination (Zn²⁺) and optimizes the spectral integrity through controlled protonation (SO₄²⁻), avoiding the degradation issues associated with strong acids or reactive counterions like Cl⁻.
• Interfacial Engineering: The study validates that Zn²⁺ acts not only as a chelating agent but also as a structural modulator that improves the dye-TiO₂ interface, leading to better charge transfer and reduced recombination.
• Bio-inspired Photovoltaics: The work provides a foundational approach for future additive design in natural dye-based DSSCs, demonstrating that careful tuning of salt concentration (specifically 0.8 g/10 mL in this context) is critical to balancing spectral enhancement with interfacial stability.

The authors conclude that while the absolute efficiency remains modest (0.38%), the mechanistic insights into ZnSO₄'s dual role offer a viable pathway to revert and enhance the performance of natural dye sensitizers, laying the groundwork for more stable and efficient bio-inspired solar cells.