Roll-to-roll screen printing machine is a high-efficiency production equipment, especially suitable for the production of photovoltaic cell backsheets. It can carry out continuous screen printing, thus greatly improving production efficiency. In the manufacturing process of photovoltaic cells, electrode printing is a key step that directly affects the performance and efficiency of the cells. Below, we will explore in detail how the roll-to-roll screen printing machine prints photovoltaic cell electrodes.
 
Basic Principle of Screen Printing
 
Screen printing is a process in which a squeegee extrudes printing ink, and the ink presents a reverse rolling state under the obstruction of the squeegee surface and the mesh knots of the screen plate. When the ink reaches the electrode pattern area of the screen plate that is not blocked by the emulsion film, it penetrates downward through the mesh holes to contact the printing substrate (silicon wafer). As the squeegee moves forward continuously, the ink detaches from the screen plate and adheres to the printing substrate due to the screen tension and the off-contact distance, achieving the purpose of printing. This process is crucial for the formation of photovoltaic cell electrodes, as it is related to the collection and extraction of electric current, as well as the series welding of individual cells.
 
Structural Characteristics of Roll-to-Roll Screen Printing Machine
 
The roll-to-roll screen printing machine features a more rational design and higher production efficiency. It consists of multiple components including an unwinding unit, a feeding unit, an unwinding film splicing storage rack, a corona dust removal device, an unwinding deviation correction device, a feeding storage rack, a screen printing unit, an auxiliary oven heating unit, a microwave curing unit, a corona device, a discharging unit, a winding deviation correction device and a winding unit. These units work collaboratively to ensure that the film material is stably and uniformly delivered to the auxiliary oven heating unit for heating.
 
Unwinding Unit and Winding Unit
 
The unwinding unit and winding unit are equipped with semi-automatic lifting devices for loading and unloading roll materials. This enables faster loading, unloading and roll changing of roll materials, improving production efficiency. The roll material rack includes a supporting base, an I-shaped rotating frame, a guide roller assembly and an air expansion chuck. Both ends of the I-shaped rotating frame are installed on the supporting base through rotating shafts and driven to rotate by a motor drive device. This design ensures the stability of the roll material during the unwinding and winding processes.
 
Splicing of New and Old Film Materials
 
The feeding unit is equipped with a film splicing device that realizes zero-speed splicing of new and old film materials. This splicing device can reduce waste generated during the splicing of new and old materials, improving the utilization rate of film materials. The film splicing device is composed of a frame, a first suction cup, a waste removal suction cup, a second suction cup, a working area, a pneumatic lever, a base and a linear guide rail. Through this structure, the pressing and cutting processes of the film material are more stable, and the operation time is shorter.
 
Unwinding Deviation Correction Device
 
The unwinding deviation correction device adopts a three-point double-roller deviation correction device with a three-point supporting deviation correction mechanism. This deviation correction device can improve the precision of unwinding deviation correction and has better applicability, while the costs of processing, assembly and maintenance are relatively low. The three-point double-roller deviation correction device includes a fixed frame, a floating frame, a drive device, a guide roller assembly and movable fulcrums. The movable fulcrums are arranged on the fixed frame in an isosceles triangle distribution, ensuring the precise alignment of the film material during the unwinding process.
 
Specific Process of Photovoltaic Cell Electrode Printing
 
In the manufacturing of photovoltaic cells, electrode printing usually includes steps such as back electrode printing, back electric field printing and front grid printing.
 
Back Electrode Printing
 
The function of back electrode printing is to form excellent ohmic contact characteristics, welding performance and adhesion. In terms of silver paste composition, silver-aluminum paste is composed of silver powder, aluminum powder, inorganic additives and organic carriers. When the printed pattern is intact, the pressure of the printing head should be as small as possible within the allowable range.
 
Back Electric Field Printing
 
The function of back electric field printing is to form a P+ junction area, so that the photogenerated carrier pairs excited by long-wave photons reaching the P/P+ junction are separated by the P/P+ junction, increasing the minority carrier diffusion capacity and improving the open-circuit voltage. The P/P+ junction can prevent the photogenerated electrons in the P region from recombining on the back surface. Compared with the structure without the P+ region, this high-low junction P/P+ structure can greatly reduce the recombination rate of the back surface. In terms of paste composition, aluminum paste is composed of aluminum powder, inorganic additives and organic carriers. The factors affecting the thickness of the printed aluminum paste include screen mesh count, mesh wire diameter, open area ratio, emulsion layer thickness, printing head pressure, printing head hardness, printing speed and paste viscosity.
 
Front Grid Printing
 
The function of front grid printing is to form excellent ohmic contact characteristics, welding performance and adhesion, as well as to collect and extract electric current. The front grid silver paste is composed of silver powder, inorganic additives and organic carriers. The adjustment of printing parameters is based on the criteria of complete patterns, full lines and appropriate weight of the printed paste.
 
Sintering Process After Printing
 
After the printing process is completed, a sintering process is required. Sintering can be regarded as a process in which atoms migrate from unstable high-energy positions in the system to positions with the lowest free energy. The purposes of sintering include burning out the organic components in the metal paste, burning through the insulating silicon nitride film, melting and alloying the metal in the paste with silicon to form ohmic contact, as well as annealing the silicon wafer subjected to plasma bombardment, activating the doped atoms and eliminating lattice damage.