Latest technology in photovoltaic cell construction

Kamil Andruszkiewicz, M.Sc.

President of EcoABM company

Prof. Ryszard Tytko, M.Sc.

president of Eco Investment Sp. z o.o.

Construction and operation of PERC-type cells

If the photovoltaic cell has a silicon base of the p – positive type, it means that the PV cell is built positively (P – TYPE). Silicon is additionally enriched with Boron, which has one less electron than it, while the top of the wafer contains Phosphorus, whose number of electrons is adequately higher. This procedure allows the formation of a p – n (positive – negative) junction that enables energy flow.

This very type of technology was used in PERC (Passivated Emitter and Rear Cell) type cells, now widely used. As the first, the PERC method was used by the Norwegian company REC, and the passive polycrystalline modules of their production were introduced to the mass market in 2015. The uniqueness of PERC panels lies in the inclusion in the device structure of an additional layer of dielectric (electrical insulator) between the top of the electrode and the bottom of the p – n junction. The task of the insulator is to reduce the attraction of electrons to the aluminum bottom electrode. Contact between the electrode and the p – n junction is provided by holes cut with a laser. The back layer of the cell with “small holes” allows an electrical connection between opposite sides. The innovation of the insulator cell’s mode of operation is based on the use of light reflection, namely: the insulator, which is also a power reflector, reflects sunlight passing through the panel structure in the wavelength range of about 1,000 to about 1,200 nm from the rear aluminum reflection layer, thus directing photons back to the cell to be absorbed by the silicon coating.

In addition to optimizing the use of the wavelength passing through the cell, the reflection of light allows for a reduction in the temperature of the working panel, which would not be the case with a traditional device design, and it should be pointed out that silicon shows a decrease in efficiency as temperature increases. In the case of conventional cells, light that has not been absorbed for a chemical reaction is concentrated at the bottom of the panel, causing electrons to be released or light to pass through the cell, emitting heat. Because (in simple terms) the sun’s rays in PERC-type panels pass through them twice, their operation is much more efficient, which is influenced by their ability to use light of longer wavelengths and which translates especially into increased yields in the morning, evening and on cloudy days. In addition, PERC-type cells enable manufacturers to achieve lower module production costs, which significantly contributes to their success and popularization.

The dimensions of a PERC-type module are usually about 1770×999×35 mm. Modules with PERC technology can be purchased from photovoltaic wholesaler ecoABM through the website www.b2b.ecoabm.com.

Construction and operation of PERT type cells

If a photovoltaic cell has an n – negative silicon base, it means that the PV cell is built on the negative (N – TYPE). The formation of the n – p opposite junction is made possible by the reverse addition of boron and phosphorus.

This is the structure of N-Peak PV cells made with PERT (Passivated Emitter Rear Cell Totally Diffused) technology. They too, like PERC-type cells, use the passivation method for the rear wall of the cell, but without cutting holes in it. This is because in PERT technology, the passivation layer provides a barrier to prevent the escape of free electrons. Although the process occurs using the identical method of reflecting light off the back layer of the panel, depriving PERT cells of “small holes” results in multiplied energy production through more efficient radiation recovery compared to using a non-uniform passivation layer. The technology also makes PV cells much more resistant to degradation caused by light (LID) and light and heat (anti-LeTID).

Trends in the PV module manufacturing industry leave no doubt: N-TYPE modules are the future of photovoltaic installations, and this is particularly influenced by:

  • higher efficiency than P-TYPE modules;
  • longer lifetime of N-TYPE modules compared to P-TYPE modules;
  • the way the cells are arranged “overlapping”, which increases the surface area and efficiency of the module;
  • multi-wire cell connections (9 busbar) to reduce shadowing effect;
  • elimination of the so-called boron-oxygen defect;
  • resistant to LID (Light Induced Degradation) – the phenomenon of rapid but short-lived degradation under the influence of incident solar radiation on the cell;
  • very low annual power loss of about 1% in the first year and a very low decline of about 0.4% per year for 30 years;
  • the cost of the manufacturing process for P-type and N-type solar cells, which is comparable;
  • extremely durable and lightweight PVF TEDLAR film;
  • N-TYPE modules come in Monofacial and Bifacial variants (glass-to-glass, uses incident light from both the front and back sides of the cell, provide higher power);
  • greater return on investment.

Combining PERC-type and PERT-type cells into modules

Typical silicon cells, especially the older generation, have front electrodes made of thin horizontal paths (fingers) that collect charge from the entire wafer and pass it on to vertical interconnecting paths (known as busbars). The number of vertical and horizontal paths affects two parameters of the cell’s operation: the FF fill factor and the cell’s resistance. In practice, the resistance of the cell is affected by the length of the path that the electrical charge must travel in the wafer. In older cell solutions, the number of busbars was usually 2. In the latest ones, their number reaches up to 12.

Increasing the number of busbars affects not only the increase in cell efficiency, but also improves its operation in shaded conditions and in the case of microcracks or mechanical damage excluding in this case a smaller cell area. An increase in the number of vertical paths (busbars), on the other hand, changes the distance between the outermost point of the collecting path and the connecting path. In cells of the 2BB type, this path is 38 mm, 3BB – 25 mm, and 5BB already only 12.5 mm (for a 6-inch cell). The number of paths and the way the charge flows here depends on the manufacturing method used.

Technology for making busbars (link connections)

a) Merlin technology – uses a specially formed copper mesh instead of silver busbars on and under the photovoltaic cell due to the material’s lightness, lower price and better physical properties of copper, particularly its durability and charge conductivity from the cell. The use of Merlin technology increases module efficiency by about 8% and results in a reduction of module production costs by about 10%.

b) Mutli Busbar Connector technology – is based on a grid of copper wire conductors with a diameter of about 360 μm (10-6 m) coated with a tin-lead-silver alloy with a thickness of 15 microns (μm). The number of busbars on each cell is 12. The manufacturing technology and microscopic thickness provide a higher fill factor than in cells with five busbars. The circular wires also cause direct incident sunlight to be reflected at an angle and return to the cell. The average energy gain of the panel compared to traditional solutions is estimated at about 6-9 watts.

Half-cut modules – half-cut cells

A new development in the construction of silicon PV cells is the use of half-cut cells instead of full-size square cells (156 × 156 mm) and moving the junction box to the center of the module, which makes it possible to fit a doubled number of cells “cut” in half on the same panel area. This results in half the current generation of a smaller PV cell. A standard module consists of 60 cells, while a half cut cell consists of 120. In addition, the solution uses thin copper wires instead of aluminum cell connections. This reduced the resistance of the wires connecting the half-cut cells and lowered their operating temperature, which has a positive effect on the life of the panels. Splitting the cell in half reduced the internal electrical resistance, thus providing higher power output, higher efficiency and reliability.

Half modules are also more resistant to the negative effects of shading and PID (Potential Induced Degradation) phenomenon, as well as the risk of permanent damage (hot spot) caused by dynamic temperature changes. A significant benefit of this technical solution is the higher energy yield (about 2%) of the module per Wp, without increasing its surface area. The cell’s efficiency was also improved slightly by using PERC technology.

Despite the fundamental differences in the design of the presented cells, the front and rear electrodes are common to them. In addition, in the case of the front electrode, its design and manufacturing method is the same for the entire family of PERC and PERT type cells and does not differ in the way electrodes are made in typical crystalline photovoltaic cells.