What are the structures of photovoltaic cells?
Photovoltaic cells can be roughly categorized into aluminum back surface (BSF) cells, PERC cells, heterojunction (HJT) cells, TOPCon cells, and IBC cells based on their structure.
Aluminum Back Surface (BSF) Cells
Aluminum back surface (BSF) cells are a common type of solar cell structure. In this structure, aluminum is used as the back electrode material. The aluminum back electrode is coated on the back of the cell and provides an electric field at the rear. This helps some of the electrons excited by the photovoltaic material to reach the back electrode, thus enhancing the photoelectric conversion efficiency.
The manufacturing process of BSF cells involves doping the silicon surface with phosphorus to form an N-type region. A chemical or physical etching technique is then used to form the aluminum back electrode on the N-type region. A suitable thin film or coating is applied on the front to form the P-type region, creating a p-n junction. Finally, metal grids are applied on both the front and back electrodes to collect the current.
PERC Cells
PERC cells, also known as Passivated Emitter and Rear Cell, enhance the conversion efficiency by adding rear surface passivation and laser processing to the traditional BSF cell design. This results in a significant performance improvement.
The main process of PERC cell fabrication includes cleaning and texturing the silicon wafer, diffusing to form the PN junction, laser doping to prepare the surface emitter, rear passivation, laser drilling/slitting, screen printing, sintering, and testing. PERC cells are known for their simple structure, short process flow, and mature equipment.
TOPCon Cells
TOPCon (Tunnel Oxide Passivated Contact) is a solar cell technology based on selective carrier principles using a tunneling oxide layer for passivated contact, achieving better passivation effects.
The TOPCon cell structure involves an N-type silicon substrate, where an ultra-thin silicon oxide layer is deposited on the back, followed by a thin layer of doped silicon. Together, these form a passivated contact structure, effectively reducing surface recombination and metal contact recombination, which provides greater potential for improving the efficiency of N-PERT cells.
TOPCon cells retain and utilize traditional P-type cell equipment and processing, requiring only additional boron diffusion and thin-film deposition equipment, without the need for rear-side hole formation or alignment, significantly simplifying the manufacturing process. It also offers high compatibility with production lines for PERC and N-PERT bifacial cells. TOPCon cells are known for low degradation, high bifaciality, and low temperature coefficients, leading to significant energy gain in final power stations.
Heterojunction (HJT) Cells
Heterojunction cells, abbreviated as HJT cells, are a type of solar cell made from a combination of crystalline silicon substrates and amorphous silicon thin films.
These cells are made by creating a PN junction using different semiconductor materials or different crystallographic states of the same material, with an intrinsic amorphous silicon layer inserted at the heterojunction interface for passivation of the front and back surfaces, achieving better passivation effects.
The HJT cell structure includes an N-type monocrystalline substrate, a P-i-type hydrogenated amorphous silicon layer on the illuminated side (with thickness of 5-10 nm), an i-N-type hydrogenated amorphous silicon layer on the rear side (also with thickness of 5-10 nm), along with transparent electrodes and collector electrodes, forming a symmetric structure as a bifacial cell.
HJT cells offer high efficiency, low degradation, low temperature coefficient, high bifaciality, simple processing, and thin film advantages.
IBC Cells
IBC (Interdigitated Back Contact) cells, also known as back-contact cells, move all of the front electrode grid lines to the back of the cell, with the PN junction and metal contacts located on the back, arranged in a finger-like pattern. This reduces shading of the cell's front by the metal grid lines, improving the conversion efficiency.
IBC cells are relatively pure monofacial cells and primarily increase efficiency through structural changes. Because the front metal contacts do not block sunlight, the photon conversion area is greater.
The structure of IBC cells can be combined with various technologies such as PERC, TOPCon, HJT, and perovskite, and it is expected to become a next-generation platform technology. The combination of IBC and TOPCon is called TBC, and when combined with HJT, it is known as HBC.
IBC cells have an aesthetically pleasing appearance, making them particularly suitable for building-integrated photovoltaics, and they show good commercial prospects.