Neem-grown silver nanoparticles supercharge solar cells with a 54.7% efficiency leap

Dye-sensitized solar cells have long been the scrappy underdogs of the photovoltaics world: cheap, colorful, and able to generate electricity even in diffuse indoor light, yet held back by efficiency numbers that pale next to silicon. Now a team of researchers in India reports a photoanode makeover that pushes these devices significantly closer to the big leagues, using a green synthesis route that starts with something most people would never associate with advanced materials: neem leaves. The work, published in the journal Ionics, describes silver-decorated titanium oxynitride photoanodes that lifted the power conversion efficiency of quasi-solid-state dye-sensitized solar cells by roughly 54.7 percent compared with the unmodified material under identical conditions.

The device at the heart of the study is a quasi-solid-state dye-sensitized solar cell, a variant of the classic Grätzel architecture in which the liquid electrolyte is replaced by a gel-like medium. This substitution matters enormously for real-world deployment. Conventional liquid-electrolyte cells suffer from evaporation, leakage, and temperature instability, which have prevented them from leaving the laboratory in large numbers. Quasi-solid-state designs sidestep those failure modes, but they introduce their own challenge: the gel electrolyte conducts ions more sluggishly than a liquid, so every component of the cell must work harder to keep electrons and ions moving. That makes the photoanode, the workhorse electrode where sunlight is first converted into separated charge, a prime target for optimization.

The photoanode in question is based on TiON₀.₅S₀.₂₅, a titanium oxynitride in which nitrogen and sulfur are incorporated into the titanium dioxide lattice. Pristine TiO₂ is a superb electron transporter but a poor absorber of visible light, because its wide band gap only lets ultraviolet photons through. Doping with nitrogen and sulfur introduces additional electronic states that narrow the effective band gap and extend absorption into the visible spectrum, where most of the Sun’s energy resides. The researchers synthesized this material using a green route based on neem extract, which acts as a natural reducing and stabilizing agent, avoiding the harsh chemicals that conventional synthesis typically demands.

On top of the N, S-modified framework, the team decorated the surface with silver nanoparticles, tuning the loading until they found the sweet spot at 1.9 percent silver by weight. Structural analyses confirmed that the anatase phase of TiO₂ was preserved throughout the process, which is crucial because anatase is the crystalline form that performs best in dye-sensitized solar cells. Interestingly, adding silver actually reduced the crystallite size from 34.52 to 27.90 nanometers while increasing the specific surface area from 52.4 to 66.3 square meters per gram. Smaller crystals with more surface area mean more sites for dye molecules to anchor, more interfaces for charge to cross, and more opportunities for photons to be captured.

Field-emission scanning electron microscopy and high-resolution transmission electron microscopy revealed a porous nanostructure with well-dispersed silver nanoparticles, exactly the morphology a high-performing photoanode needs. X-ray photoelectron spectroscopy, a technique that probes the chemical states of surface atoms, confirmed the presence of nitrogen- and sulfur-containing species together with metallic silver on the surface. This combination is significant: the N and S dopants modify the electronic structure of the TiO₂, while the metallic Ag nanoparticles act as tiny plasmonic antennas that concentrate light and shuttle electrons across interfaces.

Optical studies showed that the Ag-loaded material absorbed significantly more visible light than the pristine version, and photoluminescence measurements revealed a reduced emission intensity, suggesting that excited electrons were being separated more efficiently rather than recombining and releasing their energy as light. In a solar cell, recombination is the enemy: it wastes the energy that photons deliver before that energy can be harvested as electricity. Suppressing recombination is therefore one of the most direct routes to higher efficiency.

Electrochemical impedance spectroscopy and transient photocurrent analysis provided further evidence that the silver decoration was doing its job. The Ag-loaded photoanodes showed lower charge-transfer resistance, meaning electrons could cross interfaces more easily, and a longer electron lifetime, meaning electrons survived longer before being lost to recombination. Together, these measurements paint a picture of a photoanode in which light harvesting, charge separation, and charge transport have all been improved simultaneously, a rare trifecta in materials engineering.

When the team assembled complete quasi-solid-state dye-sensitized solar cells using the optimized Ag(1.9%)/TiON₀.₅S₀.₂₅ photoanode, the devices achieved a power conversion efficiency of 6.56 ± 0.09 percent, an improvement of approximately 54.7 percent over the pristine TiON₀.₅S₀.₂₅ device under identical conditions. While 6.56 percent may sound modest next to the 26 percent achieved by state-of-the-art silicon, the comparison is not really fair: dye-sensitized cells are cheaper to manufacture, work well in low light, and can be made flexible and semi-transparent. In those niches, even a 54.7 percent relative improvement is a meaningful step forward.

The researchers attribute the enhanced performance to three factors working in concert: improved light harvesting thanks to the plasmonic and dopant effects, increased surface area from the smaller crystallites, and facilitated interfacial charge transport thanks to the well-dispersed silver nanoparticles. What makes the result particularly compelling is the green synthesis route. Neem extract, a renewable and widely available plant material, replaces the toxic reducing agents that conventional nanoparticle synthesis typically requires, aligning the manufacturing process with the sustainability goals that motivate solar energy in the first place.

The study, led by D. Shiney Manoj and colleagues with corresponding author P. Ram Kumar at Thiagarajar College of Engineering in Madurai, was supported by a Seed Money Research Grant from the institution. As the world searches for photovoltaic technologies that can be deployed beyond the rooftop silicon panel, quasi-solid-state dye-sensitized solar cells decorated with plant-grown silver nanoparticles offer a glimpse of what sustainable energy innovation can look like when green chemistry and advanced materials engineering meet.

Subject of Research: Materials Science

Article Title: Green-synthesized Ag-decorated N, S-modified TiO₂ photoanodes for quasi-solid-state dye-sensitized solar cells

Article References: Manoj, D. S., Seema, A., Ramasubbu, V., Nichelson, A., Jiji, G., & Kumar, P. R. (2026). Green-synthesized Ag-decorated N, S-modified TiO₂ photoanodes for quasi-solid-state dye-sensitized solar cells. Ionics. https://doi.org/10.1007/s11581-026-07579-9

Image Credits: AI Generated

DOI: 10.1007/s11581-026-07579-9

Keywords: dye-sensitized solar cells, titanium oxynitride, green synthesis, neem extract, silver nanoparticles, quasi-solid-state electrolyte, photovoltaics, plasmonic effect, Green-synthesized, Ag-decorated, S-modified, photoanodes