Why Edge Passivation Has Become Critical: Rebuilding the Crystalline Interface

•5 min read

The main purpose of technology Edge Passivation (EPD) It is the reconstruction of the electronic surface of the edge that resulted from the laser cutting.

When a TOPCon cell is cut, the new edge becomes coarse and filled with microscopic defects and suspended bonds, and it turns into very efficient centers to reconnect electrons and gaps according to the mechanism of Shockley–Read–Hall (SRH)This results in direct electrical losses.

Therefore, the core mission of EPD technology is to encase these edges with a high-density nano-buffer layer that has specific electrical properties, so that as many deep energy states as possible are closed to the surface, preventing the re-union of charge carriers.

Although there are solutions based on amorphous silicon (a-Si), the industry has moved almost entirely to use Aluminium oxide (Al2O3) As the most effective material for passivating edges, thanks to two complementary mechanisms that work at the same time.

Mechanism 1: Chemical Passivation

During the deposition of the aluminum oxide layer, the chemical precursors react directly with the suspended bonds located on the silicon surface.

This results in a very thin, oxygen-rich layer at the interface, often in the form of:

SiOx Interface Layer

This layer is characterized by:

  • High density.
  • Few defects.
  • It closes a very large number of pending links.

Thus, the concentration of Interface State Density (DIT) cases drops to very low levels, significantly reducing the reconsolidation centers of the load carriers.

Mechanism 2: Field-Effect Passivation

It is this feature that has made aluminum oxide the world’s favorite material.

Unlike silicon nitride (SiNx) membranes, which usually have a constant positive charge, Al₂O₃ It naturally has a very high density of Fixed Negative Charges (Fixed Negative Charges).

These charges create a strong electric field near the silicon surface.

This area results in:

  • Clear curvature in energy levels (Band Bending).
  • Configure an electrical drain area near the edge.
  • The expulsion of electrons, which are the minority charge carriers in P-type silicon, or the modification of the distribution of carriers in N-type silicon by cell design.

The system does not need to chemically remove all defects, but effectively blocks electrons and gaps from reaching those defects, reducing the likelihood of recombination significantly.

That is, the electric field itself turns into a barrier that prevents electrical loss.

Hydrogen Passivation

The role of the aluminum oxide layer is not limited to the two previous mechanisms.

During subsequent heat treatment, such as:

  • Thermal annealing.
  • or light processing (light soaking).

The hydrogen atoms trapped inside the membranes are released, and begin to spread inside the silicon.

These atoms bind to deep crystalline defects, and chemically shut them down.

This process is known as:

Hydrogen Passivation

It significantly increases the lifespan of minority cargo carriers.

Multilayer passivation… the search for the last bit of efficiency.

As TOPCon cells approach their theoretical limits, a single layer of aluminum oxide is no longer enough.

This is why manufacturers are starting to use the so-called:

Stack Engineering

Any multi-layer membrane engineering.

One patent for TOPCon cells demonstrates an insulating structure consisting of three main layers, each with a different function.

First class.

Aluminium oxide (Al2O3)

represents the base passivation layer.

They are deposited using:

  • ALD
  • or CVD

Their thickness ranges from:

130 nm

Common thicknesses range from:

  • 5 Nm
  • 8 Nm
  • 10 Nm

Its function is to:

  • Reduce suspended links.
  • Providing the negative charges necessary for passivation in the electric field.

The second layer.

Hydrogen-containing silicon oxynitride (SiON)

It is deposited directly above the aluminum oxide layer.

Its thickness is usually less than:

10 Nm

Often between:

17 nm

This layer has two basic functions:

First:

It acts as a rich source of hydrogen atoms that circulate during subsequent combustion processes, increasing the efficiency of chemical passivation.

Two:

It forms an electrical barrier that prevents the positive charges in the outer layer from negatively affecting the electric field generated by the aluminum oxide layer.

That is, it maintains the passivation efficiency of the electric field.

Third layer.

Silicon Nitride (SiNx)

Represents the outer layer.

Their thickness ranges from:

150 nm

Their refractive index is adjusted between:

1.5 and 2.4

It performs three main functions:

  • Reduce the reflection of light.
  • Increased photon absorption.
  • Protect the interior layers from moisture and environmental factors.

In some advanced designs, a fourth layer is added over them to enhance resistance to environmental factors and chemical corrosion.

Why are non-destructive pieces not enough?

TLS technology may seem to have removed the problem.

But the fact that TLS did not prevent the emergence of suspended links, only reduced the depth of the affected area.

Even the best cutting processes cannot maintain the perfect continuity of the crystalline network after silicon separation.

That’s why the new rim remains a key source for reuniting cargo carriers.

That’s where it became a technique. EPD The loop that complements the TLS technique.

Non-destructive cutting reduces the size of the damage, while the passivation of the edges removes its electrical effect.

That’s why I became a technologist. TLS + EPD Today, they are a key criterion for the production of TOPCon high-efficiency quad-division cells, especially with the global trend towards 264-piece panels and technologies. 0BB and high-density packaging.