The research team developed a plasma interface engineering method to improve indium tin oxide layers, solving adhesion, contact resistance, and stability issues for copper electroplating. The optimized process enabled uniform copper metallization and boosted device efficiency to 25.2%, significantly outperforming untreated reference cells.
Image: Nankai University
Researchers from Nankai University in China have fabricated a copper (Cu)-metallized heterojunction (HJT) solar cell using a new interface engineering strategy aimed at addressing poor interfacial adhesion and high contact resistivity in the transparent conductive oxide (TCO) based on indium tin oxide (ITO).
The ITO layer is crucial for the performance of HJT solar cells, as it forms an ohmic contact between the thin amorphous silicon layers (a-Si:H) and the metal electrodes, enabling efficient carrier extraction. It also protects the delicate passivation layers from damage during the deposition of the metal grid, ensuring the integrity of these sensitive interfaces. In addition, ITO contributes to optical enhancement by acting as an anti-reflection layer; by carefully tuning its thickness and refractive index, it reduces reflection losses and improves light coupling into the silicon absorber.
“We developed and Argon-hydrogen (Ar/H2) plasma-induced interface engineering strategy for ITO, which effectively addresses the critical challenges in electroplated metallization of HJT solar cells, including poor adhesion, high contact resistance, and limited stability, thereby enabling ultrahigh-quality copper electroplating on ITO,” corresponding author Guofu Hou told pv magazine. “The synergy of physical sputtering and hydrogen reactive species introduces interstitial hydrogen into the ITO lattice and increases the oxygen-vacancy concentration, while concurrently hydroxylating the ITO surface to achieve superhydrophilicity.”
“We combined systematic density functional theory (DFT) calculations, finite element method (FEM) simulations, and Python/OpenCV-based quantitative analysis of nucleation to elucidate the underlying mechanism,” said co-author Taiqiang Wang. “DFT results revealed that ITO hydroxylation markedly strengthens nickel ion (Ni²⁺) adsorption, with the adsorption energy decreasing from −0.753 eV to −2.18 eV. FEM simulations also indicated that plasma-induced improvements in the electrical properties of ITO lead to a more uniform surface current distribution during the electroplating process, effectively suppressing local over-plating. Consistent with these findings, image-based statistical analysis confirmed a higher nucleation density and the formation of a denser, finer-grained, and more uniform nickel (Ni) seed layer.”
The scientists deposited the ITO films on pre-cleaned glass substrates by physical vapor deposition (PVD). Before plasma treatment, the samples were sequentially ultrasonically cleaned in acetone, ethanol, and deionized water for 20 minutes, followed by drying under nitrogen and in air. Plasma treatment was carried out in a plasma-enhanced chemical vapor deposition (PECVD) system equipped with a 4-inch radio frequency (RF) electrode, using RF power between 0 W and 200 W or 0–2.5 W/cm².
Chemical composition, surface chemistry, and electronic structure of ITO films were analyzed using X-ray photoelectron spectroscopy (XPS), electron spin resonance (ESR), Kelvin probe force microscopy (KPFM), ultraviolet photoelectron spectroscopy (UPS), and UV–Vis spectroscopy. For cell fabrication, n-type crystalline silicon wafers were textured in potassium hydroxide (KOH) solution, followed by deposition of a-Si:H layers via PECVD and sputtered ITO.
The group explained that optimized Ar/H₂ plasma treatment simultaneously tailors the chemical composition, electronic structure, and surface energy of ITO, enhancing electrical conductivity, reducing the work function, and improving interfacial properties, whereas excessive treatment leads to material reduction and performance degradation. In addition, it effectively removes surface carbon contamination and sharpens core-level peaks, improving surface cleanliness and electrolyte wettability, and enabling uniform electroplating.
Based on the optimized plasma interface engineering, the process was integrated into the fabrication of a HJT solar cell featuring bifacial copper-electroplated metallization. HJT precursors were photolithographically patterned and metallized via sequential nickel/copper/tin (Ni/Cu/Sn) electroplating. The nickel layer serves as both a seed layer and a diffusion barrier to suppress copper-induced defects, followed by a thick copper layer for charge transport and a tin capping layer for oxidation protection and improved solderability.
Tested under standard illumination conditions, the HJT cell achieved a power conversion efficiency of 25.2%, an open-circuit voltage of 742.1 mV, a short-circuit current density of 40.49 mA/cm2, and a fill factor of 83.86%. A reference device built without the plasma treatment achieved an efficiency of just 21.10%, an open-circuit voltage of 724.1 mV, and a fill factor of 71.5%, with no value for the short-circuit density being released. 
“Our results indicate the feasibility of applying Ar/H2 plasma-induced interface engineering to electroplated copper metallization for high-performance SHJ SCs, which provide a promising pathway to reduce reliance on low-temperature silver pastes and to alleviate the cost and supply risks associated with global silver scarcity,” said Hou, noting that the observed performance enhancements are scalable to large-area devices, with the first samples achieving efficiencies above 24%.
The new solar cell concept was introduced in “Plasma-induced interface engineering enables high-efficiency Ag-free silicon heterojunction solar cells with electroplated metallization,” which was recently published in the Journal of Energy Chemistry. 
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