Detailed process flow of preparing CIGS thin films by powder coating method

Step 1: Preparation of precursor powder

This is the foundation of the entire process. There are usually two paths:

1. Directly use commercially purchased elemental or alloy powders of Cu, In, Ga, Se. These powders need to have high purity (usually>99.99%) and suitable particle size (usually in the micrometer or sub micrometer range).

2. Self synthesis of CIGS precursor powder: (CuInGa) Se ₂ powder with specific stoichiometric ratios was synthesized in advance through methods such as co precipitation and ball milling alloying. This method can better control the compositional uniformity of the final film.

Step 2: Slurry preparation

This is one of the most critical technical steps. Mix the precursor powder with specific solvents and additives, grind it into a stable, uniform, and suitable coating slurry.

• Solvent: Common solvents include:

• Deionized water: the lowest cost and most environmentally friendly.

• Organic solvents, such as ethanol, isopropanol, etc., have a fast evaporation rate and are easy to dry.

• Adhesive: Add a small amount (such as 1-5 wt%) of organic or inorganic adhesive (such as ethyl cellulose, acrylic resin, etc.) to increase the adhesion of the slurry and prevent the dried film from falling off or cracking from the substrate.

• Dispersant: Add dispersants (such as fish oil, castor oil, etc.) to prevent powder particles from aggregating in the slurry and ensure uniform composition.

• Leveling agent: improves the surface smoothness of the coated film layer.

• Process: Mix the above components in a certain proportion and place them in a ball mill or planetary ball mill for several hours to tens of hours of grinding until a highly uniform and stable slurry is obtained.

Step 3: Base preparation and cleaning

• Substrate selection: The most commonly used substrate is sodium calcium glass covered with a molybdenum (Mo) back electrode. The Mo layer is usually prepared by magnetron sputtering as the back contact and electrode of CIGS thin films.

• Substrate cleaning: Use acetone, ethanol, deionized water, etc. to sequentially clean the substrate in an ultrasonic cleaning machine to remove surface contaminants, grease, and particles, ensuring good wetting and adhesion of the slurry.

Step 4: Coating/Printing of Slurry

Deposit the prepared slurry onto a Mo substrate using specific coating techniques. Common methods include:

• Screen printing: the most commonly used method. By scraping the slurry onto the substrate through a screen printing, the pattern and thickness of the film can be precisely controlled.

• Scraper coating: The equipment is simple and suitable for large-area, continuous coating.

• Inkjet printing: a non-contact, digital technology that can achieve fine patterns, but requires high efficiency and ink density.

After coating, a wet precursor film containing a large amount of organic matter is formed.

Step 5: Drying and Pre firing

1. Drying: Place the coated sample in a hot plate or oven at a relatively low temperature (such as 100-200 ° C) to allow the solvent in the slurry to slowly evaporate. This process requires controlling the heating rate to avoid cracking or "peeling" of the film due to rapid drying.

2. Pre burning/glue removal: In an inert atmosphere (such as nitrogen or argon) or a reducing atmosphere (such as a nitrogen hydrogen mixture), the temperature is raised to about 300-400 ° C to thermally decompose and completely evaporate the organic binders, dispersants, and other additives in the slurry. This step is crucial as residual organic matter can seriously affect the quality of subsequent selenization and the performance of the final device.

Step 6: Selenization and Heat Treatment

This is the most crucial step in the entire process, aimed at inducing chemical reactions and grain growth in the precursor film to form a well crystallized CIGS absorber layer with chalcopyrite structure.

• Selenization environment: Place the pre burned sample in an atmosphere containing selenium (Se) vapor for heat treatment.

• Selenium source:

• Solid selenium source: Place the sample together with elemental selenium powder in a sealed quartz tube, vacuum it, seal it or continuously introduce inert gas, and then heat it.

• Gaseous selenium source: such as H ₂ Se gas, but this gas is highly toxic and requires high equipment and safety requirements, so it is now rarely used.

• Selenium vapor: In a tube furnace, elemental selenium is heated separately using an evaporation source to produce selenium vapor, which is then transported to the sample surface using an inert gas (such as N ₂) as a carrier gas.

• Heat treatment system:

• Heating up: Heat up at a certain rate to the selenization temperature, usually between 500 ° C and 600 ° C.

• Insulation: Keep at the selenization temperature for a period of time (usually 10-60 minutes) to allow the elements to fully diffuse, react, and grain grow.

• Cooling: After the reaction is complete, cool the furnace or control the cooling rate to room temperature.

During this process, Cu, In, Ga in the precursor react with Se vapor to generate CIGS crystals with photoelectric activity. Accurate control of selenization temperature, time, and selenium partial pressure is key to obtaining high-performance CIGS thin films.

Step 7: Post processing and Device Completion

After selenization, a CIGS absorption layer is obtained. Subsequently, the entire device needs to be prepared according to the standard CIGS solar cell process:

1. Buffer layer deposition: Typically, a thin layer of CdS (about 50nm) is deposited by chemical water bath deposition (CBD) or using cadmium free alternatives (such as ZnS, In ₂ S3).

2. Window layer deposition: Deposition of intrinsic ZnO (i-ZnO) and aluminum doped zinc oxide (AZO) as transparent conductive window layers using methods such as sputtering or metal organic chemical vapor deposition (MOCVD).

3. Electrode preparation: Ni/Al collector gate wires are fabricated on AZO by thermal evaporation or sputtering.

4. Slicing and packaging: Divide large area solar cells into individual cells and laminate them for packaging to form a complete solar cell module.

Technological advantages and challenges

• Advantage:

• Low cost: Equipment investment and operating costs are much lower than those of vacuum evaporation method.

• High material utilization rate: The material utilization rate of slurry coating method can exceed 90%.

• Easy to scale up: Screen printing and other technologies are very suitable for large-scale, large-area production.

• Flexible composition: It is easy to precisely control the chemical stoichiometry of the film by adjusting the slurry formula.

• Challenge:

• Organic residue: If the pre firing process is not properly controlled, organic residue can form carbon impurities, leading to increased series resistance and decreased performance of the device.

• Thin film density: Compared with vacuum method, thin films prepared by powder method may be more prone to porosity, which affects the stability and efficiency of the device.

• Grain size and quality: Usually, the grain size is smaller than that of the vacuum method, and there are more grain boundaries, which may lead to increased recombination.

• Reproducibility: The uniformity of the slurry and the stability of the coating process require high reproducibility between batches.

summary

Powder coating method is a promising low-cost CIGS thin film preparation technology. The process flow is clear, and the core lies in the uniformity of the slurry, defect control during the drying and pre firing process, and optimization of crystal quality during the selenization process. Although its conversion efficiency is usually slightly lower than top vacuum evaporation methods, it remains an important direction for the industrialization of CIGS photovoltaic technology due to its significant advantages in cost control and large-scale production