Abstract
In contrast to conventional crystalline homojunction cells, heterojunction cells (HJT cells) work with passivated contacts on both sides. This chapter explains the functioning of such passivated contacts; it discusses the tunnel effect: an effect, which is important for these contacts. The role of the various layers within HJT cells is described. The advantages and disadvantages of the various cell architectures for HJT cells are explained. Since high-efficiency HJT cells usually consist of n-type material, the difference between n-type and p-type material is described in more detail: the term “capture cross-section” is introduced. Capture cross-sections play a decisive role in the recombination mechanisms studied with the goal of differentiating n-type and p-type silicon. Thereafter, the fabrication procedures for HJT cells are discussed; in particular, the advantages of texturing in the crystal plane (100) for monocrystalline cells. The authors also describe how the depth of microcracks can be measured with the Bevel method—and how much wafer material has to be etched off, so as to obtain a surface free of microcracks. The last section shows the favourably low TC-values of HJT cells.
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Notes
- 1.
Diffusion length L = (τ * D)1/2. D is the diffusion constant and τ is the lifetime in seconds. Diffusion length describes the average length a carrier moves between generation and recombination.
- 2.
The term «eutectic» designates a homogeneous mixture of substances that melts or solidifies at a single temperature that is lower than the melting point of either of the constituents.
- 3.
The term «tunneling» comes from quantum physics: If an electron encounters a potential barrier, it can pass through the barrier with a certain probability, even if its energy is lower than that of the barrier. That would not be possible according to classical physics. This probability is called the «residence probability»—it is given by the wave function representing the electron. The electron is described in quantum mechanics by a wave: as a wave it can also extend to the other side of the barrier. OR, in other words, it can just «tunnel» through the barrier.
- 4.
Hetero derives from the Greek word Heteros meaning «different» or «other».
- 5.
Homo-, Greek prefix expressing the notion of “same, identical”.
- 6.
Configurations in which the bandgap of one of the materials is lying within the bandgap of the other material are called «heterostructures of Type 1». However, if the bandgaps are «staggered» in the sense that the conduction band of the second material is lower than the conduction band of the first material, and at the same time, the valence band of the second material is also lower than the valence band of the first material, then we have a «heterostructure of Type 2».
- 7.
The term «Group» here refers to the numbering of the columns in the periodic system.
- 8.
The idea that we start with individual layers, which we afterwards join together is, of course, just a “Gedankenexperiment”, an artifice we imagine in our minds, so as to understand better what happens. In reality the two individual layers are joined together right from the beginning—as the HJT cell is fabricated.
- 9.
Those majority carriers that accumulate now on opposite sides constitute a concentration imbalance which is a chemical potential. The process is self-regulating with electrical and chemical potential neutralising each other at every point in space. In fact, this is the requirement which is postulated in the beginning, the combined electrochemical potential is identified with the Fermi level.
- 10.
The probability for charge carriers to tunnel through a peak depends on the height and width of the peak, and also on the density of charge carriers present just before the peak; now, in ② there is a high density of holes, so there will be many holes tunnelling through the peak from left to right, whereas in ① there is a very low density of holes (most holes there are siphoned off towards the contact)—thus, there will be very few holes tunneling through the peak from left to right.
- 11.
Also called dark or leakage currents.
- 12.
In photovoltaics, the term emitter is defined as the region where the minority carriers leave the solar cell. In n-type material, the minority carriers are the holes and the holes leave the solar cell on the p side (positive electrode). The reverse applies to p-type material. In p-type material the minority carriers are the electrons and these leave the solar cell on the n-side (negative electrode).
- 13.
n+ means more doped than n and correspondingly p+ means more doped than p.
- 14.
In a more detailed view, the charge carriers flow through the peaks forward and backwards. However, the difference between forwards rates and backwards rates is very large, so that the backward flow can be neglected. See also footnote 10.
- 15.
The sheet resistivity of the TCO layer close to the amorphous layer is ~100 Ω/sq. Close to the glass of the module the sheet resistivity is much lower ~40 Ω/sq. Thus, with the thickness of the TCO layer, the resistivity can be optimized.
- 16.
“depleted” means here: it loses a part of the holes which are otherwise present in a p-type amorphous layer.
- 17.
Silicon has 1023 atoms per cm3.
- 18.
This means that we have n = ND + ni ≈ ND.
- 19.
Actually the term «capture cross-section» is used not only in the case of impurities, but in the case of all recombination centres or defects—as an example it is also used in the case of “dangling bonds”, which appear in amorphous silicon layers (see Chap. 6).
- 20.
Diffusion length L = (τ * D)1/2.
- 21.
Explanation of capture cross-sections: Low capture cross-section means that activity radius around impurities is small and recombination activity is low. High capture cross-section means that activity radius around impurities is large, and recombination activity is high.
- 22.
In addition to boron, iron and copper in combination with oxygen can also produce interference effects.
- 23.
Oxygen penetrates the silicon during crystallization and cannot be completely avoided.
- 24.
This is because it is very difficult to obtain homogeneous doping of silicon with Gallium (see segredation coefficient Chap. 5).
- 25.
When silicon is cooled after the crystal pulling process, thermal donors (TD) may be formed by oxygen clusters. These are negatively charged. Thermal donors influence the resistivity. In the n-type material the resistivity decreases, in the p-type material it increases. TD dissolve at over 500 °C during a gettering process. However, the influence on the improvement of the cell efficiency is not economically meaningful (0.2% abs.); this is the reason why gettering processes are not carried out with n-type material.
- 26.
The term crystal structure describes the arrangement of atoms, molecules or ions in a crystalline material. In the silicon crystal, the silicon atoms are located in an ordered three-dimensional structure. In x, y, z direction the structures look different, but are always periodical. Therefore, the material properties are also different in x, y, z directions. This fact is exploited, for example, in sawing by selecting the direction {100}.
- 27.
The crystal structure of silicon is cubic-surface-centred. The silicon crystal can be imagined as a cube. At the 8 corners sit the atoms (cubic) and in the middle of the 6 surfaces sit one atom each (surface-centred).
- 28.
The etching effect here is isotropic. Isotropic etching is independent of the crystal direction. The same amount is etched away everywhere on the wafer surface. So-called saw marks can therefore not be completely removed because regardless of the crystal structure, the same amount is removed everywhere. The saw marks are also etched and remain visually intact.
- 29.
“ASTM” means “American Society for Testing and Materials” but is now, an international standard organization that develops and publishes based on voluntary consensus technical standards for a wide range of materials, products, systems and services.
- 30.
- 31.
These values are applicable in the case where no object obstructs the incoming sunlight. In a PV system values of albedo are considerably lower and depend on the layout of the system.
- 32.
This means that the conversion efficiency for the light coming in from the back side is 92% of the conversion efficiency of light coming in, from the front side.
- 33.
which is equivalent to a world-record monofacial module of 20% module efficiency.
- 34.
The Busbar technology is often realized with high temperature pastes, at which temperatures of approx. 800 °C are used to melt the silver flakes in the paste. This is how the fingers and the busbars are applied. High temperature pastes are also used in multiwire applications. Instead of the busbars, pads are printed to fix the wires. The conductivity of such pastes is high. The five ribbons (busbar technology) or the 2 × 7 wires (multi wire technology) are soldered crosswise to the fingers at approx. 240 °C.
- 35.
Standard cells use process temperatures of 800–1000 °C.
- 36.
A common method is to calculate the energy price using the Levelized Cost of Electricity (LCOE). This allows different technologies to be compared on the basis of a standardized calculation basis, both within the PV and with other energy production sources.
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Leu, S., Sontag, D. (2020). Crystalline Silicon Solar Cells: Heterojunction Cells. In: Shah, A. (eds) Solar Cells and Modules. Springer Series in Materials Science, vol 301. Springer, Cham. https://doi.org/10.1007/978-3-030-46487-5_7
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