Decoding High-performance | N-TOPCon Technical White Paper 01

In recent years, the photovoltaic industry has ushered in a period of revolution in cell technology. The former “King” PERC technology, the new forces TOPCon, HJT and xBC and other cell technologies are competing with each other. HY SOLAR has firmly selected the TOPCon technical route as early as 2022 and actively launched the upstream and downstream layout of the industry. Now,?a zero-carbon industrial chain with N-type coverage of 100% has been formed!

Why HY SOLAR chooses TOPCon technology in various technical routes. The next few issues of "Decoding High-performance" will lead you to find the answer in HY SOLAR N-TOPCon technology white paper. First, we will start with the?principle of TOPCon cell.

Before you can understand TOPCon solar cells, you need to understand what a "solar cell" is. The principle of solar cells is based on the photovoltaic effect of the PN junction, which is the origin of the term PV. The so-called photovoltaic effect is that after the object is irradiated, the charge distribution in the object changes, generating electric potential and current.?

A PN junction is a semiconductor structure, which is a junction with special electrical properties formed by tightly bonding a P-type semiconductor and an N-type semiconductor by using different doping processes, as shown in the figure below.

Since the concentration of free electrons of the N-type semiconductor is much greater than the hole concentration, and the hole concentration in the P-type semiconductor is much greater than the electron concentration, there will be a concentration difference between electrons and holes occurs at the junction of the PN junctions, and electrons and holes will spread from places with high concentrations to places with low concentrations. When electrons in the N-type semiconductor diffuse to the P-type side, the N-type semiconductor is positively charged due to the loss of electrons, and the P-type semiconductor is negatively charged. In this way, an electric field is established at the contact interface, which we call it the built-in electric field.

When sunlight or other light rays irradiate on the PN junction, a large number of electron-hole pairs are generated, the electron-hole pairs are separated by the built-in electric field, the electrons move to the N-type region, the holes move to the P-type region, the N region becomes a negative charge, the P region becomes a positive charge, and a stable potential difference is established between the two ends of the P-N. When the PN junction is connected to the external circuit and given light, a steady stream of current flows through the circuit.

The concept of TOPCon cell was first proposed by Fraunhofer Institute for Solar Energy Systems in Germany at the 28th PVSEC PV Conference in Europe in 2013. In 2017, the institute achieved a laboratory efficiency record of 25.8% on 4cm2 cells. According to the theoretical model developed by Jan Schmidt et al. 2018, the?theoretical extreme efficiency of TOPCon cell can reach 28.7%, which is much larger than the theoretical efficiency limit of PERC cell technology.

TOPCon is a Tunnel Oxide Passivated Contact solar cell technology based on the selective carrier principle. The cell structure is shown in the figure above.

The main principle of the technology is that in an ultra-thin tunnel silicon oxide SiOx/and highly doped polycrystalline silicon interface layer, electrons (majority carriers) are transmitted through tunneling and pinhole mechanisms by utilizing quantum tunneling effect, and holes (minority carriers) are blocked by a potential barrier, so that the recombination of minority carriers is greatly reduced, and meanwhile, the technology has good majority carriers transmission performance. So that the surface recombination and the metal contact recombination are effectively reduced, and the open-circuit voltage and the short-circuit current of the cell are improved.

Introduction of main film layers

■ Front SiNx thin film (about 75 nm): Since SiNx is rich in hydrogen atoms; it can chemically passivate defects on the surface and in the body during heat treatment, thereby reducing recombination of surface electrons. Meanwhile, due to the optical characteristics of the SiNx, the anti-reflection effect of the front side and the back side of the cell can be realized;?

■ Back SiNx thin film: In order to avoid the damage of the slurry to the film layer in the subsequent metallization sintering process, SiNx is mainly used for protecting the film layer on the back side by characteristic of the chemical stability of the SiNx film;?

■ Al2O3?(≤5nm): Due to its high negative charge density, it can provide good field effect passivation for P-type semiconductors such as the back of PERC cell and the front of TOPCon cell, i. e. a layer of dielectric film with highly stable charge is added near the surface, a gradient electric field is created near the surface to reduce the surface electron concentration and thus reduce the recombination rate of surface electron holes.?

■ The ultra-thin tunnel layer SiO2?and the polycrystalline silicon film: These two form a passivation contact structure together to serve as a passivation layer on the back surface of the cell, and the energy band bending at the interface caused by the work function difference between the highly doped polycrystalline silicon (Poly-Si) layer and the N-type silicon substrate ensures that sufficient energy levels can be occupied after the tunnel of electrons, so that the tunneling is easier.