Optimizing Gold Nanoparticle Placement in TiO₂ Enhances Dye‑Sensitized Solar Cell Efficiency

Abstract

We fabricated TiO₂ photoanodes that incorporate gold nanoparticles (AuNPs) in a controlled, stacked architecture by sequentially depositing TiO₂ paste and AuNP solutions on indium tin oxide (ITO) glass. By varying the AuNP concentration at each layer, we explored how the spatial distribution of AuNPs within the TiO₂ matrix influences optical absorption and device performance. The most pronounced plasmon‑enhanced absorption and photovoltaic output were observed when a dense AuNP layer (≈1.3 µg cm⁻²) was positioned near the TiO₂ penetration depth that matches the peak absorption of the N719 dye (~520 nm). In this configuration, the short‑circuit current density (Jsc) rose to 10.8 mA cm⁻² and the energy‑conversion efficiency (η) reached 5.0 %—an improvement of 15 % and 11 % relative to reference cells lacking AuNPs. These findings confirm that deliberate AuNP placement within TiO₂ is a critical strategy for maximizing DSSC performance.

Background

Dye‑sensitized solar cells (DSSCs) have long been recognized for their low‑cost, scalable fabrication and environmental friendliness. Nonetheless, their power‑conversion efficiencies still lag behind those of perovskite, thin‑film, and crystalline silicon technologies. Traditional methods to boost absorption, such as thickening the TiO₂ layer, inadvertently increase electron recombination. Nanophotonics offers an alternative: metal nanoparticles generate localized surface plasmon resonances that amplify the local electromagnetic field and scatter light, thereby extending the optical path within the cell. Gold and silver nanoparticles are favored because their plasmonic resonances lie within the visible spectrum where most dyes absorb. While numerous studies have demonstrated plasmonic enhancements, the optimal depth distribution of AuNPs in the TiO₂ scaffold remains unexplored. Because Au and Ag are costly, judicious placement could reduce material usage while maximizing performance.

Materials and Methods

AuNP Synthesis

AuNPs of 20–90 nm were synthesized by the Turkevich protocol. A 0.01 wt % HAuCl₄ solution was boiled, and trisodium citrate was added to nucleate particles. Subsequent seed‑mediated growth yielded 40 nm, 60 nm, and 90 nm particles. After centrifugation, the AuNPs were dispersed in a 1:10 (water:ethanol) solvent mixture for deposition. For 20 nm particles, a 20 nm SiO₂ shell was grown via the Stöber method to suppress charge recombination.

Photoanode Fabrication

ITO glass (sheet resistance ≈10 Ω sq⁻¹) was cleaned and coated with TiO₂ paste using screen printing. Each layer (~1.1 µm thick) was fired at 450 °C for 15 min. AuNPs were drop‑cast onto the annealed TiO₂ surface and air‑dried; the weight of AuNPs in the solution was measured to control density. By repeating TiO₂ and AuNP deposition, we created stacked structures with AuNP layers at predetermined depths. A final anneal at 500 °C for 30 min consolidated the layers. Dye loading involved soaking the photoanode in 0.3 mM N719 (ethanol) for 20 h at 25 °C. Counter electrodes were prepared by drop‑casting 2 mg mL⁻¹ Pt precursor on TCO glass, followed by 400 °C annealing for 30 min. Cells were assembled with a 50 µm Himilan spacer and sealed with a cover glass. The electrolyte (0.05 M I₂, 0.05 M LiI, 0.6 M DMPII, 0.5 M TBP in acetonitrile) was introduced under vacuum to fill the cavity.

Characterization

UV–Vis absorption spectra were recorded (100 mW cm⁻², AM 1.5). TEM verified monodisperse, spherical AuNPs. SEM examined surface morphology. The TiO₂ thickness was measured by a surface profiler. J–V curves and IPCE spectra were obtained using a calibrated spectroscopic system with a 0.05 cm² active area.

Results and Discussion

AuNP Size Effects

Absorption spectra confirmed the expected red shift with increasing AuNP size, while TEM images verified monodispersity up to 60 nm. DSSCs incorporating 40 nm AuNPs exhibited the largest enhancements (≈45 % increase in Jsc) without compromising V_oc or FF. Smaller 20 nm particles required SiO₂ passivation to avoid V_oc losses, likely due to recombination at the metal surface.

Optimal AuNP Density and Position

In a 6 µm TiO₂ film, AuNP density per layer was varied from 0 to 5.4 µg cm⁻². Jsc and η peaked at 1.3 or 2.7 µg cm⁻²; beyond this, aggregation reduced plasmonic benefits and blocked incident light. Absorbance measurements revealed that positioning the AuNP layer near 4.0 µm from the ITO surface—coincident with the 520 nm absorption depth of N719—maximizes light‑field enhancement and IPCE. IPCE increments spanned 350–750 nm, peaking near 520 nm, directly mirroring the absorption enhancement.

Multiple AuNP Layers

By inserting AuNP layers at 1.1, 2.2, and 3.3 µm (P1–P3), we explored cumulative effects. The configuration with a high‑density layer (1.3 µg cm⁻²) at P3 and lower densities (0.65 µg cm⁻²) at P1 and P2 delivered the best performance: Jsc = 10.8 mA cm⁻², η = 5.0 %—a 15 % and 11 % gain over the reference. This suggests that a high‑density AuNP layer should reside at the penetration depth of the dye’s peak absorption, while modest densities near the front of the cell mitigate scattering losses.

Conclusions

Our systematic study demonstrates that strategic positioning of AuNPs within TiO₂ layers—specifically concentrating them at the depth where 520 nm light is fully absorbed—yields the greatest plasmonic enhancement in DSSCs. Low‑density AuNP layers near the illuminated surface further improve performance by enhancing light coupling without inducing significant recombination. These insights underscore the importance of nanoparticle distribution control for cost‑effective, high‑efficiency dye‑sensitized solar cells.