CSS Coating Process

Compared to laboratory thin film deposition techniques, the most challenging aspect of large-scale and industrial thin film deposition lies in achieving uniformity and stability in the deposited films. The uniformity of thin film deposition depends largely on the substrate temperature, the control of the fluid atmosphere within the chamber, the transport control system, and the vacuum system.

The coating process utilizes the close-space sublimation method (CSS) to deposit the CdS and cadmium telluride semiconductor layers. The entire chamber consists of a series of vacuum chambers, with the substrate's transparent conductive layer facing downward. An oil-free vacuum system is employed to maintain a low vacuum of ≤1 mbar. Radiation heating from both the top and bottom ensures uniform heating of the substrate to 500°C. At this temperature, a 50nm CdS layer is first deposited, followed by a 3-4μm cadmium telluride layer.

The key equipment in the close-space sublimation method is a preheating chamber, a deposition chamber, and a cooling chamber. The deposition chamber primarily comprises a substrate transport system, an evaporation source system, and a vacuum system. The substrate transport system features substrate heating and insulation, employing PID control to maintain a substrate temperature of approximately 500°C during the coating process. The evaporation source system utilizes a graphite boat to store the evaporation material, with electric heating and a temperature controller maintaining the evaporation source temperature above 650°C. The vacuum system maintains a pressure between 0.9 and 3.5 kPa during deposition. The use of inert gas in the reaction chamber prevents Cd and Te2 from evaporating directly onto the substrate. Instead, they collide with inert gas molecules several times before reaching the substrate surface, resulting in a dense film with uniform thickness and appropriate grain size.

Our company imports advanced German CSS coating equipment, capable of continuous coating and large-scale production. We deposit CdTe films on glass substrates measuring 2.4 m x 1.2 m with controllable, uniform, and dense thickness, achieving films 2-5 μm thick, with adjustable thickness down to 0.1 μm, at a deposition rate exceeding 5 m²/min. Magnetron sputtering

Magnetron sputtering is a type of physical vapor deposition (PVD) process. This process uses DC magnetron sputtering to deposit a 350nm thick Mo back electrode layer in a vacuum chamber. As a critical component of CdTe thin-film solar cells, the choice of material for the back electrode is crucial to cell performance. Mo thin films are the preferred material for back electrodes in CdTe thin-film solar cells due to their low resistivity, excellent thermal stability, and excellent ohmic contact with the CdTe layer. Furthermore, their coefficient of thermal expansion is similar to that of glass and CdTe, making them a promising material.

In magnetron sputtering, a Mo target is placed in a vacuum chamber filled with an inert gas (such as argon). A DC voltage is applied between the Mo target and the substrate on which the film is to be deposited. During this process, the target collides with Ar atoms, ionizing them to produce Ar positive ions and new electrons. These electrons are accelerated by the electric field and collide with the Mo target, causing sputtering of the target. During sputtering, neutral Mo atoms are deposited on the substrate to form a thin film. The generated secondary electrons, influenced by the electron magnetic field, move in a circular motion, prolonging their residence time in the plasma near the substrate. In this region, they ionize a large amount of Ar, which collides with the Mo target, achieving a high deposition rate. As the number of collisions increases, the secondary electrons lose their energy and gradually move away from the target surface.

Photoresist Coating

After laser scribing, adjacent long unit cells must be insulated to achieve the final series connection requirements. Therefore, an insulating polymer must be filled into the P1 scribing lines. A photoresist mixture of polymer and photosensitizer is applied to the scribed film using a roll coating method. The coating thickness varies depending on the solid content of the photoresist, typically 10-20μm. The photoresist-coated film is then baked in an infrared and hot air oven to remove any water solvent. The dried film is then exposed to UV light from the glass surface. The exposure time and intensity are determined by the photoresist's specific properties. After exposure, the cell is rinsed with an aqueous developer containing 1% hydrogen peroxide to remove any excess unexposed photoresist. A gradient water wash is then used to remove any remaining developer.

The photoresist coating system utilizes a reverse-direction double-roller method for coating. The drying system uses hot air and infrared baking to rapidly remove solvents. The exposure system utilizes long-life UV lamps for uniform illumination over a wide area. The developer and cleaning system utilizes a metal-free design with a gradient spray.

Laser Etching

P1 Laser Etching (First Laser Etching)

The cleaned transparent conductive glass is placed on the laser marking machine with the conductive film facing down. A fundamental frequency YAG laser (45 J/cm², 3 kHz, 0.25 m/s) is used to separate the film into numerous thin strips. This allows for the production of multiple solar cells on a single glass plate. In subsequent processing, these cells can be connected in series and parallel to form finished panels. The separated SnO² films serve as the negative contact layer for each cell. P2 Laser Etching

The cleaned and dried substrate glass is then placed back into the laser scriber for a second etch using a YAG laser (5 J/cm², 40 kHz, 0.25 m/s).

The purpose of this second etch is to sever the deposited CdS and CdTe layers on the substrate glass without damaging the SnO² thin film. This paves the way for the next step, directly connecting the back contact layer (the positive electrode) to the TCO layer (the negative electrode), and forming a series connection between individual cells on the panel. The etcher is equipped with a vacuum cleaner to collect and concentrate dust for disposal.

P3 Laser Etching

After the back contact layer is completed, the substrate is placed into the laser scriber with the film side facing down for a third laser etch. This severing of the back contact layer creates the independent positive electrode for each cell. Along the short edge of the substrate, adjacent to the second scribe line, the metal back electrode and CdS/CdTe semiconductor layer are scribed into 155 parallel cells.