Homojunction Cells
5.1 Production of Silicon Wafers and Solar Cells .1 Production of Silicon Ingots
Crystalline solar cells used for large-scale terrestrial applications consist almost exclusively of silicon as base material. There are good reasons for this:
S. Leu·D. Sontag (
B
)Meyer Burger Technology A.G, Gwatt, Switzerland e-mail:dsontag@web.de
S. Leu
e-mail:sylvere.leu@ciptec.ch
© Springer Nature Switzerland AG 2020
A. Shah (ed.),Solar Cells and Modules, Springer Series in Materials Science 301, https://doi.org/10.1007/978-3-030-46487-5_5
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98 S. Leu and D. Sontag
• Silicon is the second most abundant element of our Earth’s crust after oxygen.
Weighted by atomic per cent, the earth’s crust contains1: – 60.4% oxygen
– 20.4% silicon – 6.3% aluminium – 2.9% hydrogen.
• The bandgap of silicon is 1.12 eV and is, thus, not too far from the optimal value for converting—in solar cells—sunlight into electricity.
• Silicon melts easily, has good mechanical properties and is easy to machine (sawing, polishing, etching).
• Silicon can be made into a highly pure form, as a single crystal. This means that silicon has a homogeneous crystal lattice, which is very regular and has few unwanted foreign atoms.
• With doping, the type and conductivity of silicon can be easily changed.
• Silicon is non-toxic.
• Silicon forms a native oxide layer, which serves as a high-quality insulator, whereupon different layers can be deposited.
• Silicon can be thoroughly cleaned. Ultrapure silicon for the semiconductor indus-try is manufactured in purity levels of up to 11N2 in mass production. For low-efficiency solar cells, a purity of 7N is sufficient but, as the demand for higher efficiency solar cells increases, the specifications on silicon feedstock are also requiring lower concentrations of impurities.
• The vast amount of knowledge gained during the last 50 years by semiconductor industry in how to process silicon and how to make industrial tools for silicon-based devices. A lot of this knowledge has been used for Si PV as well.
Jöns Jacob Berzelius a Swedish chemist discovered silicon in 1823. Other semi-conductors such as germanium and gallium arsenide have higher charge carrier mobilities and, thus, allow for higher switching speeds in integrated circuits. But Germanium has two disadvantages in connection with solar cells:
(a) Its bandgap is too small to form single-junction solar cells
(b) Germanium oxide is unstable and not suitable for surface passivation and insulation layers.
Germanium and Gallium Arsenide (GaAs) are both too expensive to be suitable for the production of solar cells for terrestrial applications
1The earth’s crust forms the outer part of the earth and extends about 35–40 km into the earth’s interior. Its composition is very diverse. If one arranges the elements of the earth’s crust according to weight percent, a shift of the portions is noticeable in comparison to the distribution according to atomic percent: Oxygen (46.6%), silicon (27%) and aluminum (8%) are still the three most frequent elements.
211N means: 11 nines. 11N is 99.999999999% pure.
5 Crystalline Silicon Solar Cells: Homojunction Cells 99 The production of pure silicon ingots3takes place in four stages [1]:
(a) Production of raw silicon (b) Preparation of trichlorosilane (c) Production of high-purity silicon
(d) Production of ingots, either multicrystalline or monocrystalline.
(a) Production of raw silicon
Starting material is silica sand (quartz sand) SiO2, which is mined above all in Brazil and China where the quartz sand is very pure. At ~1800 °C, this silica sand is reduced with coal (i.e. with carbon C) to metallurgical grade silicon with a purity of 98–99%:
SiO2+C→SiO+CO (5.1)
SiO+C→SiC+CO (5.2)
2SiC+SiO2→3Si+2CO (5.3)
We now have “technical silicon”, e.g. raw or metallurgical silicon with a purity of 98–99%.
The next three steps (b) to (d) are made with the so-called Siemens process which was patented by the company Siemens A.G., in 1954.
(b) Preparation of trichlorosilane
Raw silicon is now finely ground; then it is converted in a fluidized bed reactor, with the aid of hydrochloric acid, to gaseous trichlorosilane. The process temperature for this step is approximately 300 °C:
Si+3HCl→H2+SiHCl3 (5.4)
(c) Destillation of Trichlosilane
Trichlorosilane boils at 31.8 °C; it is distilled in tall stainless steel columns. Thereby a lot of impurities are filtered out. With repeated distillation, the degree of purity can be increased.
(d) Production of ultrapure silicon
Trichlorosilane is broken down in a reducing atmosphere at around 1000 °C via the reaction
SiHCl3+H2→2Si+SiCl4+SiCl2+6HCl (5.5)
3This process step is very important, as it accounts for more than half of the total energy invested in the production of crystalline silicon solar modules.
100 S. Leu and D. Sontag
Fig. 5.1 The Siemens reactor process, which extracts pure silicon from trichlorosilane. The pure thin silicon rods, with a diameter of about 8 mm are electrically heated to about 1150 °C. Pure trichlorosilane flows around these hot silicon rods and polycrystalline silicon is deposited until the rod has a diameter of about 300 mm
so that Si atoms from the vapour are deposited on a silicon starting ‘seed’, usually consisting of a cylindrically arranged array of thin Si rods. It is the most widespread cleaning process for silicon—it results in pure silicon with a purity of 9 N–11 N, depending on how pure the trichlorosilane is. Figure5.1shows, in a schematic way, the process flow.
In the next step, the polycrystalline silicon rods grown in the Siemens reactor process are broken down in pieces of different sizes (chunks and chips) so that the various crucibles can be filled to produce either monocrystalline silicon or multicrys-talline silicon. The aim of this process step is to produce, from the ultra-pure silicon, a silicon crystal (ingot) with, on the one hand few dislocations, and, on the other hand a material in which the desired concentration of the doping material is contained.
(e) Production of monocrystalline ingots Czochralski Method
In the widely usedCzochralski4method (CZ), which is shown in Fig.5.2, ultrafine silicon chunks are filled into a crucible, which consists of pure quartz glass SiO2
and is coated with Si3N4. The quartz glass crucible is in its turn embedded in a graphite crucible, which supports it. The temperature resistance of these crucibles is over 1600 °C. The structure and coating of the crucible are crucial for the quality of the silicon [2]. After the quartz glass crucible has been filled with ultra-pure silicon
4Jan Czochralski was a Polish Chemist (1885–1953). He developed 1916 the Czochralski method for pulling single crystals from the melt.
5 Crystalline Silicon Solar Cells: Homojunction Cells 101
Fig. 5.2 Schematic representation of a puller for monocrystalline crystals according to the Czochralski method
(chunks and chips), it is electrically heated to approximately 1420 °C.5A shielding gas, usually argon, prevents impurities from entering the chamber; it also stops the oxygen from escaping out of the quartz glass crucible, by transport with the outward flow of argon.
Once the silicon has melted, a rotating and height-adjustable silicon seed crystal of 3–5 mm size is slowly approached to the melt without touching it; the goal is to bring the seed to the same temperature as the molten silicon. When the seed itself starts to melt, it is gently put into contact with the molten silicon in the crucible (Dipping Step). The crucible rotates in the opposite direction to that of the seed crystal. The rotation is important so that the heat distribution remains homogeneous and does not create thermal stress. Immediately before dipping, the melt is slightly cooled to a temperature just below the melting point.6The silicon atoms will now dock on the colder seed crystal, solidifying and adopting the orientation and structure of the seed crystal.
During theNecking Step, the seed is pulled out of the melt faster to reduce the diameter of the single crystal ingot to a minimum value (much smaller than the
5This is just above the melting point: silicon melts at 1412 °C.
6The Ostwald–Miers range (according to Wilhelm Ostwald and Henry Alexander Miers) is the temperature range in which the melting point is undershot during cooling in a liquid to a temperature, where crystallization does not take place as yet. Crystallization can thus take place on a seed crystal.
This is just below the melting point: silicon melts at 1412 °C.
102 S. Leu and D. Sontag original diameter of the seed)—this prevents the propagation of dislocations. Then, in the next step, the pulling speed is reduced to form the shoulder.
The crystal is gently pulled out of the melt; the pulling speed (~0.5 mm/min) is automatically controlled and adjusted to grow the ingot diameter to the desired value (typically 200–300 mm). Despite this limitation the shoulder (see Fig.5.2) can grow quickly and the ingot reaches rapidly the desired diameter. The necking process must be carried out carefully, because the situation must be avoided in which the thin inner rod breaks or dislocations occur in the crystal lattice. A flat shoulder is advantageous for production reasons. There is less waste and the entire drawing process is about 15–25% faster. However, there is a risk that, with flat shoulders, dislocations occur.
Dislocations can occur because the temperature difference between the inside of the ingot and the edge area is too high, due to impurities or due to external vibrations.
Usually, crystals for photovoltaic solar cells are pulled in the {100} plane. If dislo-cations occur, they propagate by sliding over the four sides of the {111} planes and, thus, slip outwards. Dislocations extend over a length approximately equal to the diameter of the ingot.Afterwardsthey disappear.
As soon as the diameter of the ingot is reached (200–300 mm), the drawing speed is increased to approximately 8 mm/min. The drawing speeds and the temperatures are continuously controlled and monitored, so that a constant diameter is formed.
After the ingot has been pulled, it must be mechanically secured (using a locking system) and slowly cooled, so that no cracks are created due to stress. The end of the drawing process must not be abrupt—because this can trigger thermal shocks, which can lead to dislocations in the crystal lattice. Therefore, the end of the drawing process is initiated via a tapered tail. Incidentally, this is also where most of the impurities are found, so the act of pulling the crystal can also purify it. Using the Czochralski method, round ingots of up to 4 m in length, typically 2.5 m, and up to 300 mm (12 in.) in diameter are drawn. The filling of the quartz glass crucible is about 150 kg and can be increased with subsequent recharging to about 200 kg.
The disadvantages of the method are: (1) the wall of the quartz glass crucible can react with the silicon, which limits the resistivity by the penetration of impurities;
(2) even if the crucible is coated with Si3N4, impurities from the coating layer can penetrate the silicon melt. This coating acts as a barrier and the impurities entering the silicon melt from the crucible are greatly reduced. Nevertheless the coating itself is a source of contamination. However, the total contamination (crucible and coating) is reduced; (3) finally oxygen from the quartz glass crucible wall penetrates into the silicon, forming SiO2according to the simplified reaction:
SiO2quartz glass crucible+2Simelt→Si+2SiO (5.6) This is, in fact, inevitable. In a typical ingot, the concentration of interstitial oxygen is between 1017 and 1018 cm−3. Because silicon has about 1023atoms per cubic centimetre, oxygen contamination is typically between 0.1 and 1 ppm.7
7«ppm» means «partspermillion».
5 Crystalline Silicon Solar Cells: Homojunction Cells 103 The oxygen atoms are originally randomly distributed in the silicon; during crys-tal growth, various complicated morphological processes take place and as a con-sequence, the oxygen atoms can join together and form clusters, so-called “precipi-tates”. Precipitates have various positive and negative effects, depending on how the process is conducted.
1. Precipitates lead to local disturbances in the crystal structure.
2. In a heat treatment process, this precipitation process can be partially controlled, in such a way that precipitates can be placed where the semiconductor is not active.
3. On the other hand, oxygen precipitates can serve as trap sites for metallic foreign atoms (gettering process)8[3].
4. Above a concentration of 1018cm−3, the solubility limit of oxygen in the silicon is reached and no further oxygen precipitates can be formed. This is also the reason that the oxygen concentration should be less than 1018cm−3.
5. If one has higher oxygen content, one will also have a more pronounced Light Induced Degradation (LID) inp-type material (see Chap.10) because of the B–O (Boron–Oxygen) complexes.
The oxygen accumulates mainly at the top of the ingot while the impurities tend to be at the tail (bottom).
Typical lifetimes of passivated wafers are in the range of 1.5–10 ms. The specific resistance is typically between 0.5 and 7cm. To obtain high cell efficiencies, the rule of thumb is that the quality factorτ/ρshould be greater than 1 ms/cm:
τ/ρ >1 ms/Ωcm (5.7) τ lifetime in ms
ρ specific resistivity incm.
The crucibles can be recharged two to three times in the hot state. After that, the impurities in the silicon become too large due to oxygen and carbon; the target value τ/ρ> 1 ms/cm (passivated wafer) can no longer be met. Once the Crucible has cooled to room temperature, it cannot be reused. Thanks to the use of a magnetic field (see Fig.5.2), the oxygen content can be reduced and the ingress of impurities from the Crucible can be prevented. This is called the Magnetic Czochralski (MCz) process.
Forp-type material boron is added, and forn-type material phosphorus is added to the silicon melt. Phosphorus has a segregation coefficient9of 0.35; boron has a
8Gettering process means a controlled modification of the silicon crystal by thermal processes to draw impurities far from the active part of the semiconductor, in order to reduce their potential degrading effects.
9The “segregation coefficient” is defined as theratioKof the impurity concentrationCsolidin the solid state (here: in silicon)tothe impurity concentrationCmeltin the melt:K=Csolid/Cmelt. The segregation coefficient defines how well impurities are separated from the rest of the material. If the segregation coefficient is 0.1, this means that the impurity concentrationCmeltin the melt is 10
104 S. Leu and D. Sontag
Fig. 5.3 Schematic representation of a monocrystalline float-zone puller
segregation coefficient of 0.7. Phosphorus is therefore less well absorbed by silicon and lingers longer in the liquid phase. It disperses less homogeneously than boron.
Therefore, forn-type material the resistivity varies in a wider range (0.5–7cm) than forp-type material (1–3cm).
Float-Zone Method
If lower impurity concentrations are required, one employs the relatively expensive10 float-zone method (FZ)according to Fig.5.3—here, no crucibles are used. A poly-crystalline silicon rod is clamped vertically and heated locally through an induction coil right up to the melting point. A seed crystal at the lower end of the rod initializes the drawing process.
The silicon rod now moves slowly down (or: the coil moves slowly upwards).
The polysilicon rod melts in the melting zone and crystallization begins. Since the impurities have segregation coefficients of <1, they remain in the melt and migrate with the melt upward. Repeated zone melting (which is not a process used in the photovoltaic industry, due to cost) allows the production of extremely pure silicon rods. Actually, this technique is employed to produce the rods used in the Siemens process. Because no crucible is used in the float zone process, virtually no impurities
times higher than the impurity concentrationCsolidin the solid state (here: in the solidified silicon).
The closer the segregation coefficient is to 1, the more homogeneous will be the resulting “mixture”
of the two materials. This is the reason that Phosphorous (P) (KP=0.3) and Boron (B) (KB=0.7) can be used for doping. Impurities like Oxygen and Iron have segregation coefficients of:KOxygen
> 1 andKiron1. Oxygen accumulates mainly at the top of the ingot while impurities like iron tend to be at the tail (bottom).
10Basically, the FZ process should be cheaper than the CZ process, because of faster rates of crystallization, higher throughput per puller and less energy consumption. However, the lower yield and the requirement of a machined polysilicon rod render this technique more expensive than the CZ process, when all cost factors are accounted for.
5 Crystalline Silicon Solar Cells: Homojunction Cells 105 are present in the resulting crystal: resistivity up to 1000cm is obtained! That is a very high value. In the Cz process (see previous section) we obtain forn-type material resistivity from 0.5 to 7cm, and forp-type material resistivity from 1cm to 3. FZ silicon contains much less impurities, which is why the resistivity is so high. In photovoltaics, float-zone wafers are mainly used in research to compare solar cells, which are produced with different process parameters and to show the limits of these processes with regards to cell efficiencies. The advantage is that material-related differences can be ruled out in this comparison, because float-zone material is very pure.
A comparison between the Czochralski method and the float-zone method is given in Table5.1.
Crystal growing is very energy intensive. Therefore, we will make here a rough estimate of the energy balance sheet. To produce 1 kg silicon, an equivalent energy investment of approximately 100 kWh has to be made. With 1 kg of silicon, 74 wafers of 180μm thickness can be sawed and a solar module with 450 Wp (Watt peak) can be produced; this module can generate, in the Central European Climate, during a period of 25 years, 12,000 kWh Electricity. This rough calculation shows the positive energy balance of crystalline silicon solar cells.
Production of Multicrystalline Ingots
In the production ofconventional multicrystallineingots either the Bridgman pro-cess (or less widespread in PV the block casting propro-cess) is used. In Fig.5.4, both methods are schematically shown.
A square quartz crucible ideally coated with Si3N4is filled with polycrystalline silicon chunks, heated and melted. Thereafter, the melt is slowly cooled from bottom to top and the crystals begin to grow from bottom to top. The growth process is not directed as in monocrystalline crystal growth. Crystals grow very randomly and form larger areas with different microstructures. The goal is to grow large crystals with a vertical columnar structure, so that the number of grain boundaries within the wafer
Table 5.1 Comparison of
Relative production costs (%) 100 >200 Cell efficiency for PERC (%) ~22.5 25a Cell efficiencyn-type TopCon
(ISE)
~23%
Cell efficiency for HJT (M2) (%) ~24 26.6b
aWorld record for PERC on FZ by USNW, small size
bKaneka, based on a IBC-HJT cell structure, M2
106 S. Leu and D. Sontag
Fig. 5.4 aBridgman Process: the melting process and the crystallization are carried out in the same Crucible;bblock casting process: after the silicon has been melted in a first Crucible, it is crystallized in a second Crucible. In contrast to the Bridgman process, the crystallization time and the cooling time can be reduced. Also, different heating systems can be used, which allows for process optimization
remains low and that the grain boundaries are always perpendicular to the surface of the wafer. Typically, the crucible has a size of G6 or G8. G6 means that the ingot has a size of 6×6 bricks, with a brick having an area that corresponds to the area of a solar cell. The height of the crucible is about 30 cm. In this way, ingots can be produced relatively quickly and inexpensively. However, recombination centres are formed at the grain boundaries, dislocations and clusters of precipitates, so that the cell efficiency of multicrystalline solar cells is a bit lower than that of monocrystalline solar cells.
High Performance Multi Crystalline Technology (HP-mc-Si)
It was discovered later (~2011) that large grains also create large clusters of dislo-cations. Therefore, the HP mc-Si process was developed. With the introduction of High Performance Multi Crystalline Technology (HP-mc-Si) in 2011, the
It was discovered later (~2011) that large grains also create large clusters of dislo-cations. Therefore, the HP mc-Si process was developed. With the introduction of High Performance Multi Crystalline Technology (HP-mc-Si) in 2011, the