1.Development trend of surface passivation

The top priority of crystalline silicon solar cells has always been their surface passivation. Early screen printing solar cells, limited by the technical means at the time, people simply introduced the TiO2 layer. However, the passivation function of TiO2 did not play an ideal role.

In the 1990s, with the development of process technology, silicon nitride (SiN x) films prepared by plasma enhanced chemical vapor deposition (PECVD) technology became the mainstream, and they were generally used as anti-reflection layers and passivation layers on the front of solar cells.

With the use of SiNx, the passivation optimization of the front of the battery has entered the bottleneck stage, so people began to turn the research direction to the back surface with serious compounding problems. UNSW introduced PERC and PERL constructs in the 1990s.

What these two structures have in common is that they rely on the silicon oxide layer to achieve passivation on the back of the solar cell. At the same time, the local open hole forming point contact process effectively reduces the area of the non-passivation area. The difference is that the latter forms a back electric field through local doping diffusion near the open hole. But it also makes the process dramatically more complicated.

Although PERC and Perl-structured solar cells have relatively good surface passivation effect, they limit the contact area on the back of the solar cell to the open hole range.

In addition to increasing the complexity of the preparation process, the process of opening holes will also cause damage to the silicon material in the contact range and increase the degree of composite between the metal and the semiconductor contact area. In addition, the existence of open holes also makes carriers unable to transmit from the shortest path perpendicular to the contact surface, which leads to the increase of string resistance and Fill Factor (FF) loss in the transmission process.

Passivated Contact technology is applied to solar cells to form passivated contact solar cells, which has become the focus of current research.

2. How TOPCon solar cells work

① Carrier separation process in TOPCon solar cells

It is generally believed that the internal power generated by current in solar cells is formed by the separation of photogenerated carriers by the built-in electric field of PN junction, because when light with an energy greater than the bandgap width of semiconductor materials irradiates on the surface of PN junction, the original dynamic balance generated by diffusion and drift motion will be destroyed, thus generating new electron hole pairs and separating under the action of the built-in electric field. So it creates a photoelectric current,

However, some researchers now believe that by disrupting the equilibrium Fermi level, creating a quasi-Fermi level gradient, an electric current can be generated. The conductivity of different types of carriers in the contact region on both sides of the absorption layer is different, which makes the outgoing photogenerated electrons and holes transported to different directions respectively.

The photoexcited electrons and holes in the absorption layer are transported along the conduction and valence bands respectively. Ideally, the electrons and holes in the diagram reach the outer circuit through the left electron contact and the right hole contact, respectively. The composite current density (J0c) and contact resistivity (ρc) are generally used to measure the carrier selective passivation contact performance.

ρc represents the output capacity of the passivated contact to the multiion, that is, the resistance loss of the electron contact region to the electron (multiion) current. The J0c is used to show that the blocking ability of passivation contact to minority electrons is generated by the recombination of some minority holes into the electron contact region and many minority electrons.

② Carrier transport process in TOPCon solar cells

TOPCon solar cells are based on the selective collection of passivated contact structures by carriers, which are formed by preparing a layer consisting of a tunnelled silicon oxide layer and a heavily doped silicon thin film layer on the back of the solar cell.

Due to the good passivation effect of ultra-thin silicon oxide and heavily doped silicon films, the energy band on the surface of the silicon wafer is curved (thus forming a field passivation effect), the probability of electron tunneling is greatly increased, and ρc is also greatly reduced. Due to the excellent carrier selective passivation contact properties (J0c< 10fA/cm2, ρc< 30 mΩ·cm 2), so that the efficiency of crystalline silicon solar cells prepared by TOPCon technology has reached more than 26%.

③ For ultra-thin tunnelling oxide layers, there are currently two carrier transport theories in academia:

The first is quantum tunneling

That is, microscopic particles such as electrons can still pass through the barrier probabilistically to the other side when the barrier height is greater than the particle energy. According to the "uncertainty principle", time and energy cannot have definite values at the same time, and the more certain one quantity is, the more uncertain the other quantity is.

That is, the energy of a particle in a very short period of time will be extremely uncertain, and the range of energy values will become larger. Therefore, although the energy of the particle (which should be the average of the particle over its energy range) is less than the barrier height, there is a certain probability that the high energy state in this range exceeds the barrier height for a very short time.

If the barrier space span is small, the particle with the high energy state can pass through the barrier in a very short time.

The second is the pinhole theory

When the oxide layer exceeds 2nm, the probability of carrier tunneling will be greatly reduced. At this time, carriers are mainly transmitted through pinholes, and when the number of pinholes in the oxide layer is too small, the carrier transmission will be limited.

The excessive number of pinholes indicates that there are too many defects in the oxide layer, which leads to the reduction of the chemical passivation effect of the oxide layer. Under this transport mechanism, the quality of silicon oxide is very high.