Glass frit optimization in silver paste leads to measurable gains in solar cell efficiency

A research team led by China’s Sichuan University has investigated methods to tune the content of lead(II) oxide (PbO) and silicon dioxide (SiO₂) within silver paste, with the aim of reducing contact resistance at the silver-silicon (Ag–Si) interface in crystalline silicon solar cells and thereby improving overall device performance.

“We systematically investigated the effect of the PbO/SiO₂ molar ratio—the most fundamental and critical compositional parameter in glass frits—on the Ag–Si contact performance,” corresponding author Jingquan Zhang told pv magazine. “We also clarified the intrinsic ‘composition–structure–property’ relationship, providing a rational design guideline for high-performance silver-paste glass frits.”

The scientists explained that glass frits are key components in silver paste, as they enable sintering and interfacial reactions during processing. They lower the processing temperature, etch the silicon nitride (SiNx) anti-reflection layer, and enhance the dispersion and wettability of silver particles. Most importantly, they promote the formation of an interfacial layer that reduces contact resistance between silver electrodes and silicon. In this process, SiO₂ forms the backbone of the glass network structure, while PbO serves as a network modifier that tunes viscosity, glass transition temperature, and chemical reactivity.

For their experiments, the researchers used five lead-based glass frits with controlled PbO/SiO₂ molar ratios of 10.00, 8.00, 6.00, 4.00, and 2.00. They were synthesized via the melt-quenching method and labeled A1 to A5. To isolate the effect of the PbO/SiO₂ ratio, the total molar content of PbO and SiO₂ was kept constant at 65%, while the remaining 35% consisted of fixed auxiliary oxides, including boron oxide (B₂O₃) and bismuth(III) oxide (Bi₂O₃), along with other additives. High-purity raw materials were first weighed according to the designed formulations, thoroughly mixed, and then melted in an alumina crucible at 1,250 C for 30 minutes. The molten glasses were rapidly quenched in deionized water to form amorphous frits, which were subsequently ball-milled and sieved to obtain fine powders with particle sizes of approximately 1–3 μm.

The glass frits were then incorportated into silver pastes that were screen-printed onto 182 mm x 182 mm n-type monocrystalline silicon wafers to form front-side electrode grids, followed by drying at 160 C and sintering at a peak temperature of 730 C. After sintering, all samples underwent laser-enhanced contact optimization (LECO) to further improve electrical performance, yielding final solar cell samples labeled A1-C to A5-C.

The structure and phase composition of the glass frits were analyzed using X-ray diffraction, scanning electron microscopy, and Fourier transform infrared spectroscopy, while chemical states and elemental distribution were examined using X-ray photoelectron spectroscopy and energy-dispersive X-ray spectroscopy. Thermomechanical analysis was used to determine glass transition behavior. At the device level, interface microstructure between the silver grid and silicon wafer was studied using cross-sectional scanning electron microscopy after selective etching of silver and glass layers.

The analysis showed that glass frits are fully amorphous and consist of uniformly distributed particles, ensuring good dispersion and complete melting during sintering. An intermediate PbO/SiO₂ ratio was found to produce the most favorable interfacial structure, characterized by a continuous 100–200 nm glass layer and a dense, uniform distribution of Ag crystallites at the Ag–Si interface. In contrast, excessive or insufficient PbO content results in either over-fluidity and substrate over-etching or incomplete melting and poor interfacial contact. These structural variations directly translate into device performance, with the optimal composition achieving the lowest contact resistance of 0.79 mΩ·cm² and the highest conversion efficiency of 23.65%.

“Under optimal conditions, the specific contact resistance shows an approximately 8% reduction compared with off-ratio samples, while the cell conversion efficiency increases to 23.65%, representing an improvement of about 1 percentage point,” Zhang stated.

“An optimal molar ratio of 6 was identified, which gives a glass transition temperature of about 288  C and enables the formation of a continuous glass layer and uniformly distributed nano-sized Ag crystallites at the interface,” he concluded.

The new technique was presented in the study “Effect of PbO/SiO₂ molar ratio in silver paste glass frit on Ag-Si contact resistance and crystalline silicon solar cell performance,” published in Solar Energy Materials and Solar Cells. The research team included scientists from Chinese technology companies Sichuan Dongshu New Materials Co., Ltd and SunSync Solar Technology (Yibin) Co., Ltd.