Selection Guide for Polysilicon Powder Dispersants in Photovoltaics
First, clarify the powder characteristics:
The surface of polysilicon powder is covered with a natural silicon dioxide oxide layer (containing silanol groups, with an isoelectric point of approximately 2 to 3), and due to its high powder density, it is highly prone to sedimentation. During the preparation process, it is strictly prohibited to introduce sodium, potassium, or heavy metal ions to prevent impurities from compromising the purity of the silicon material and interfering with subsequent smelting and battery manufacturing processes.
It is divided into two major scenarios: water-based slurries and solvent-based slurries (ethanol/isopropanol system), while also distinguishing between two applications:
① A wet grinding and purification process based on silicon wafer cutting waste liquid to achieve high-value regeneration of recycled silicon powder suspension
② Silicon-based conductive pastes and inks suitable for high-temperature sintering processes with low residual carbon characteristics
I、 Water based system (pure water/PEG aqueous solution, silicon powder suspension, wet ball milling, cutting fluid)
The surface of silicon powder is rich in acidic silicon hydroxyl groups, which undergo deprotonation reactions in alkaline media, resulting in a significant negative charge and a preference for anionic polymer dispersants
1) Preferred: Ammonium polyacrylate
✅ Recommended model: RK-4008Y, domestic ceramic grade ammonium polyacrylate (ammonium salt! Not sodium salt)
-Principle: Adsorb silicon hydroxyl groups through electrostatic interactions to maintain system stability, and the components do not contain alkali metal ions;
-Features: Low residual carbon rate and high cost-effectiveness;
-Use pH: 8.5-10.0 (adjust pH with ammonia water, do not use NaOH);
-Addition amount: The mass fraction of silicon powder is 0.5% to 1.7%;
- ⚠️ Disadvantage: A high pH environment can intensify the oxidation reaction of silicon powder and lead to continuous hydrogen evolution, so long-term sealed storage is strictly prohibited.
❌ 【 Strictly prohibited 】 Sodium ion impurities introduced by sodium polyacrylate can cause contamination of silicon materials!
2) Research/Ultra fine Silicon Powder Alternative: Polyvinylpyrrolidone (PVP)
Suitable for nano silicon powder and low solid content slurry systems;
Disadvantages: Due to the high organic content and high-temperature residual carbon content of the material, it cannot meet the process requirements for subsequent high-temperature sintering of silicon slurry. Therefore, its application is limited to temporary dispersion during silicon powder purification and wet grinding processes.
3) Photovoltaic cutting fluid circulation system (suspended silicon powder, anti deposition)
Selection: Low foaming polyether nonionic dispersant;
Representative: RK-4017E,RK-4061ED modified polyether dispersant;
Features: It has low foaming characteristics and good cycling stability, perfectly adapts to PEG system, and can effectively inhibit the agglomeration of ultrafine silicon powder; Prevent it from adhering to the surface of silicon wafers and equipment pipelines;
⚠️ Non ionic materials are only used as suspension aids, and high solid content grinding cannot be used as the main dispersant alone.
4) Not recommended: Small molecule dispersants (SDS, sodium hexametaphosphate)
Due to the interference of impurity ions, the double layer is compressed, and the electrostatic repulsion is weakened, resulting in particle aggregation, manifested as insufficient dispersion stability and rapid settling under long-term storage.
II、 Solvent based system (ethanol/IPA, silicone ink, tape casting, spray granulation, preferred)
The use of oxygen free sealing process and surface coating technology under inert atmosphere protection is the mainstream solution for high-end silicon slurry to solve the problems of silicon powder oxidation and hydrogen evolution when encountering water.
1) Preferred solution: Phosphate ester dispersant
Representative: RK-4011, RK-4013A, RK-4013C modified alkyl phosphate ester
-The phosphate group serves as the anchoring end and forms a strong binding with the hydroxyl group on the silicon surface, while the solvation chain maintains system stability by constructing a steric hindrance layer;
-Addition amount: 0.8-1.8 wt% powder;
-Advantages: It has good dispersibility and long-term suspension stability of the slurry, and can achieve good compatibility with binders such as PVB and ethyl cellulose;
Application: silicon based slurry tape casting, spray drying granulation and conductive silicon ink preparation.
2) Alternative solution: Low residual carbon polyester type super dispersant
For the application requirements of submicron polycrystalline silicon, priority should be given to selecting models with low molecular weight and low carbon residue characteristics;
⚠️ Attention: Long chain polymers are prone to carbon residue and sintering defects due to difficulties in rubber extrusion.
III、Silicon conductive paste (requiring high temperature sintering) with hard requirements
1. Prioritize low residual carbon dispersants;
2. It is necessary to avoid the problem of high residual carbon rate caused by high-temperature cracking of polyethyleneimine (PEI);
3. It is strictly prohibited to use silicon containing additives and prevent the introduction of additional silicon elements;
4. Any dispersant containing metal ions is prohibited.
IV、 Selection Priority Quick Reference Table
When preparing solvent based silicon powder slurry (such as sintered silicon ink or slurry for casting process), the application effect of phosphate dispersant (RK-4011) is better than that of low residual carbon polyester super dispersant
2. In the water-based high solid content silicon powder ball milling purification process (without sintering), ammonium polyacrylate RK-4008Y is used as a dispersant.
3. In the circulation system of diamond wire cutting fluid, low foaming modified polyether dispersants such as RK-4017E and RK-4061ED are used to achieve efficient suspension and stable dispersion of abrasives
4. In the short-term testing of nano silicon slurry in the laboratory, polyvinylpyrrolidone (PVP) was added as a dispersing stabilizer, and this formula does not involve sintering process
V、 Key practical points
1. Feeding sequence
Solvent/water → dispersant thoroughly stirred and dissolved → slowly added silicon powder, gradually increasing the rotational speed;
To avoid agglomeration and clumping of the powder caused by direct dry feeding, it is recommended to use pre wetting or gradual addition for dispersion.
2. Ideal addition amount determination: By drawing a curve of the viscosity of the slurry changing with the amount of dispersant added, the amount of addition at which the viscosity drops to the lowest point can be determined as the ideal addition amount; If the dispersant is excessive, it will not only cause viscosity to rise, but also lead to problems such as increased bubbles.
3. Water based system risk reminder:
Under the mechanochemical action of long-term ball milling, silicon powder reacts with water to produce silicon dioxide and release hydrogen gas;
To alleviate material swelling and inhibit powder oxidation, it is recommended to gradually transition to alcohol solvent systems in long-term production processes.
4. Compound approach: By synergistically combining the main dispersant with a small amount of non-ionic wetting agent, the wetting performance is increased, while strictly controlling the amount of organic silicon defoamer used.
VI、 Common fault prediction
-The rapid settling phenomenon of the slurry is mainly due to the dosage of dispersant not reaching the standard or the pH value of the system exceeding the ideal stable range;
-The viscosity of water-based silicon powder slurry increases abnormally during the grinding process due to surface oxidation of silicon powder, which is a common technical problem in this system;
-The appearance of black spots after sintering is mainly attributed to the excessive amount of dispersant or the use of models with high residual carbon values.
