Photovoltaic cell technologies with passivation

Crystalline silicon (c-Si), in poly or multicrystalline (also known as polycrystalline) forms, dominates the photovoltaic cell market, with more than 90% of the market share compared to other technologies, including thin films and other semiconductor materials.

Much of the cost of manufacturing crystalline photovoltaic cells and modules lies in the process of obtaining wafers of silicon.

Costs associated with the quantity of raw materials and the use of energy in the manufacture of wafers They are the great villains of the crystalline silicon industry.

Recently, advances have been made in the manufacture of wafers with the introduction of the diamond wire sawing method. This method makes it possible to obtain 50% more wafers than traditional methods, with the same amount of raw material.

This means that wafers thinner materials can be obtained, typically with thicknesses less than 200 micrometers (1 micrometer is equal to 1 thousandth of a millimeter). The immediate consequence of this is the reduction in the prices of photovoltaic cells and modules for the consumer market.

But not everything is perfect. Reducing the thickness of silicon cells reduces conversion efficiency, as electron and hole recombination rates become higher. Why is this a problem?

To understand this we need to remember the working principle of photovoltaic cells. When light (or solar radiation) falls on silicon, it transfers energy to electrons located in the valence band (outermost layer of the atom).

Excited electrons with more energy jump from the valence band of silicon to the conduction band. In other words, the electrons are released from the atoms and gain freedom to move, being able to participate in the formation of the electric current.

We must not forget that electrons that jump to the conduction band leave behind empty spaces (corresponding to the absence of electrons), which we know in the literature as holes.

The generation of electron-hole pairs by the incidence of solar radiation on the silicon cell is one of the key phenomena of the photovoltaic effect.

The more light, the more free electrons and holes there are in silicon. So, the greater the intensity of incident light, the more electrical current a photovoltaic cell can provide.

This would be perfect if the phenomenon of carrier recombination did not exist. The excited electrons that gain energy and jump out of the atom, after some time naturally return to their initial state and recombine with the holes, reducing the cell's ability to supply electrical current.

Carrier recombination is an obstacle to increasing the efficiency of photovoltaic cells. Higher recombination rates reduce the availability of electrons and consequently the electrical current that the cell can provide, limiting the energy produced – in other words, the cell's efficiency is reduced.

Normally recombination is not such a big problem in conventional silicon cells. Carrier recombination is lowest in the center of the cell and highest at the surfaces.

At the ends of the cells there are impurities and defects in the crystalline structures that favor the recombination of carriers, which reduces the number of electrons (and holes) available for the generation of electrical currents.

Impurities and defects in the structures function as electron traps or recombination centers that facilitate the transition of electrons from the conduction band to the valence band, reducing the lifetime of the carriers (increasing the recombination speed).

The recombination speed and efficiency of photovoltaic cells can be controlled by adjusting the thickness of the wafer. Thicker cells prevent electrons from having a chance to reach surfaces, where recombination is easier.

Thinner cells provide higher rates of recombination, as electrons reach the ends of the cells more easily.

Monocrystalline silicon cells (produced by the Czochralski process) or polycrystalline cells typically have thicknesses of around 300 to 400 micrometers, which reduces the occurrence of recombination of electrons and holes on the surfaces, as the greater the thickness of the cell, the smaller the amount of carriers that can reach surfaces.

However, when we produce thinner cells (with thicknesses of 100 to 200 micrometers) the recombination rates are higher and this causes unacceptable reductions in efficiency, which in principle would make the use of wafers so fine.

Silicon passivation

The advancement of manufacturing technology wafers thinner materials must be accompanied by silicon passivation techniques. Passivation is the name we give to a process that consists of adding layers of other materials to cell surfaces.

They are very thin layers, with thicknesses of 7 to 10 nanometers (1 nanometer is equal to 1 thousandth of a micrometer or 1 millionth of a millimeter).

Silicon passivation can be carried out using different processes and with different materials. The result of passivation is the reduction of electron recombination at the ends of the silicon, providing significant efficiency increases over the efficiency of unpassivated silicon.

Basically, passivation works through two phenomena:

1. Reduction of the effect of defects on structures and regularization of surfaces or;
2. Creation of electric fields that repel electrons (and also holes) from cell surfaces, making recombination difficult.

The first technique is called chemical passivation and the second field-made passivation (FEP – Field Effect Passivation). Passivation techniques work through one of these two phenomena.

The objective of all passivation techniques is to reduce the rate of carrier recombination on surfaces, known in the literature as SRV (Surface Recombination Velocity).

In addition to increasing cell efficiency by reducing recombination speed, passivation techniques reduce the PID effect (Potential Induced Degradation), largely responsible for the degradation of cells over time. The PID effect is not covered in this article.

While passivation allows the use of wafers thinner, passivation can also be considered one of the techniques used by manufacturers to achieve increasingly higher efficiencies, along with other existing techniques.

The main existing passivation technologies that are already beginning to be used on a large scale by photovoltaic cell manufacturers are known as HJT, HIT It is PERC.

The industry has not yet decided which technique it will prefer and for now manufacturers are divided. There is a trend towards PERC technology, according to some surveys carried out at the end of 2017, although some manufacturers argue that HIT technology requires less energy and fewer manufacturing steps than PERC technology.

The following figure displays a projection of the market shares of the main crystalline technologies by 2026, considering the current market pace.

Commercial cells with passivation

HIT and HJT technologies

HJT and HIT are different names for the same thing. The HIT technique was developed about 20 years ago by the Japanese company Sanyo.

The patent's expiration in 2010 opened a window of opportunity for other manufacturers to turn their attention to the technique. HIT means Heterojunction Intrinsic-layer Technology and it is still a registered trademark of Panasonic (which acquired Sanyo), although the patent has already fallen.

Generally speaking, the industry refers to the technique as HJT (HeteroJunction Technology). It has already been proven possible to obtain HIT photovoltaic cells with efficiencies of more than 25% on a laboratory scale.

On a commercial scale, with HIT it is possible to manufacture 60-cell modules with powers of almost 310 W. For comparison, with conventional technology (non-passivated silicon measuring 300 to 400 micrometers) 60-cell modules do not exceed the 270 W barrier.

Even with the reduction in cell thickness, the HIT technique increases the efficiency of the modules by up to 15%.

PERC technology

This technology is a not-too-distant relative of HIT technology. The meaning of the acronym PERC varies depending on the source consulted. Authors refer to it as Passivated Emitter Rear Cell, Passivated Emitter Rear Contact or Passivated Emitter and Rear Cell.

All these denominations have more or less the same meaning. Regardless of the name used, let's try to understand how it works.

Like HIT, PERC technology represents a major advancement beyond traditional crystalline cells. Like HIT technology, PERC technology is not new – it was developed more than 30 years ago at the University of South Wales, in Australia, but has only now begun to be used commercially.

For a long time, manufacturers were focused on improving the manufacturing processes of traditional crystalline cells and PERC and HIT technologies, for one reason or another, remained in the refrigerator.

The recent interest in the PERC technique was largely motivated by aggressive competition among cell and module manufacturers. Manufacturers continually try to improve their products, using all possible devices and technologies to achieve greater efficiencies.

In addition to the passivation technologies that we are covering in this article, there are several techniques that can be combined and allow the efficiency of photovoltaic modules to be increased – we will cover these techniques in another publication.