SILICON SOLAR CELL FABRICATION TECHNOLOGY

Nguyen Thi Hai

Content

1. Produce Poly silicon

2. Making silicon wafer

3. Making a cell

1. Produce Polysilicon

Polysilicon is produced from rock.

There is two type of polysilicon. First is chunk polysilicon which is produce by Siemens Reactor. The second is granular polysilicon which is produce by Fluid Bed Reactor.

SiO2 + C = Si + CO2

Si + 3HCl =H2 + SiHCl3

Si + 4HCl = SiCl4 + 2H2

SiCl4 + H2 = SiHCl3 + HCl

4 SiHCl3 = SiH4 + 3SiCl4

HSiCl3 + H2 = Si + 3HCl

SiH4= Si + 2H2

Reductionby C

Silica

(SiO2)

Impure Si

(Si)

Ultrapure

SiCl4 / SiHCl3

UltrapurePolycrystalline

Si

Chlorination/distillation

Reductionby H2

Reductionby C

Silica

(SiO2)

Impure Si

(Si)

Ultrapure

SiCl4 / SiHCl3

UltrapurePolycrystalline

Si

Chlorination/distillation

Reductionby H2

Chunk polysilicon _ Siemens Reactor

-Generally, most of poly-Si produced by Siemens reactor are chemical vapor deposition (CVD) systems with mixed gas of monosilane (SiH4, MS) or trichlorosilane (SiHCl3, TCS) and hydrogen (H2).

-TCS-Siemens reactor is preferred than MS-Siemens reactor because TCS-Siemens is safe and inexpensive process for poly-Si production than MS-Siemens reactor. Therefore, almost 77% of the poly-Si produced worldwide is currently obtained from TCS-Siemens reactor.

- MS-Siemens reactors are typically operated at temperatures in the range 800–900°C. Typical temperature range for TCS-Siemens reactor is around 1050–1100°C much higher than MS-Siemens reactor

Granular Polysilicon_ Fluid Bed Reactor

Fluidized bed reactors have excellent heat and mass transfer characteristics and can be utilized for Silane decomposition to overcome the energy waste problem in Siemens process.

In the fluidized bed reactor (Kunii & Levenspiel, 1991), the reactive gas is introduced into the reactor together with preheated fluidizing gases, such as hydrogen or helium. Heat for the thermal decomposition is supplied by external heating equipment.

Pyrolysis of silicon containing gas produces silicon deposition on seed particles, the subsequent particle growth is due to heterogeneous chemical vapor deposition as well as scavenging of homogeneous silicon nucleic. This results in a high deposition rate by a combination of heterogeneous and homogeneous decomposition reactions. As the silicon seed particles grow, the larger particles move to the lower part of the bed and are removed as a final product.

The continuous removal of silicon seed particles after they have grown to the desired size leads to depletion of particles and it is necessary to introduce additional silicon seed particles into the fluidized bed to replace those removed final product (Würfel, 2005).

1. Produce silicon wafer

Single Crystal wafer.

Single- crystal growth

There are two method to grow the single crystal

Floating-zone method: a polysilicon rod is converted into a single-crystal ingot by passing a molten zone heated by a needle-eye coil from the end of the rod to the orther.

Czochralski method: the seed crystal is dipped into the melt polysilicon and the growing crystal is pulled upward

Polycrystalline Crystal wafer

After having wafer, the wafer will be exposed some steps to make a solar cell. There is two method to make a solar cell. - Screen printed solar cell- Buried Contract Solar Cells

3. Making a cell

Screen Printed Solar Cells

Screen-printed solar cells were first developed in the 1970's. As such, they are the best established, most mature solar cell fabrication technology, and screen-printed solar cells currently dominate the market for terrestrial photovoltaic modules. The key advantage of screen-printing is the relative simplicity of the process.

Phosphorous DiffusionScreen-printed solar cells typically use a simple homogeneous diffusion to form the emitter where the doping is the same beneath the metal contacts and between the fingers. To maintain low contact resistance, a high surface concentration of phosphorous is required below the screen-printed contact. However, the high surface concentration of phosphorous produces a "dead layer" that reduces the cell blue response. Newer cell designs can contact shallower emitters, thus improving the cell blue response. Selective emitters with higher doping below the metal contacts have also been proposed - but none have yet been introduced into commercial production.

Surface Texturing to Reduce ReflectionWafers cut from a single crystal of silicon (monocrystalline material) are easily textured to reduce reflection by etching pyramids on the wafer surface with a chemical solution. While such etching is ideal for monocrystalline CZ wafers, it relies on the correct crystal orientation, and so is only marginally effective on the randomly orientated grains of multicrystalline material. Various schemes have been proposed to texture multicrystalline materials by using one of the following processes:

mechanical texturing of the wafer surface with cutting tools or lasers

isotropic chemical etching based on defects rather than crystal orientation

isotropic chemical etching in combination with a photolithographic mask 

plasma etching 

Antireflection Coatings and Fire Through ContactsAntireflection coatings are particularly beneficial for multicrystalline material that cannot be easily textured. Two common antireflection coatings are titanium dioxide (TiO2) and silicon nitride (SiNx). The coatings are applied through simple techniques like spraying or chemical vapour deposition. In addition to the optical benefits, dielectric coatings can also improve the electrical properties of the cell by surface passivation. By screen-printing over the antireflection coating with a paste containing cutting agents, the metal contacts can fire though the antireflection coating and bond to the underlying silicon. This process is very simple and has the added advantage of contacting shallower emitters 

Edge IsolationThere are various techniques for edge isolation such as plasma etching, laser cutting, or masking the border to prevent a diffusion from occurring around the edge in the first place.

Rear ContactA full aluminium layer printed on the rear on the cell, with subsequent alloying through firing, produces a back surface field (BSF) and improves the cell bulk through gettering. However, the aluminium is expensive and a second print of Al/Ag is required for solderable contact. In most production, the rear contact is simply made using a Al/Ag grid printed in a single step.

SubstrateScreen-printing has been used on a variety of substrates. The simplicity of the sequence makes screen-printing ideal for poorer quality substrates such as multicrystalline material as well as CZ. The general trend is to move to larger size substrates - up to 15 x 15 cm 2 for multicrystalline materials and wafers as thin as 200 µm

Buried contact solar cell

The buried contact solar cell is a high efficiency commercial solar cell technology based on a plated metal contact inside a laser-formed groove. The buried contact technology overcomes many of the disadvantages associated with screen-printed contacts and this allows buried contact solar cell to have performance up to 25% better than commercial screen-printed solar cells. A schematic of a buried contact solar cell is shown in the figure below.

Cross-section of Laser Grooved, Buried Contact Solar Cell.

A key high efficiency feature of the buried contact solar cell is that the metal is buried in a laser-formed groove inside the silicon solar cell. This allows for a large metal height-to-width aspect ratio. A large metal contact aspect ratio in turn allows a large volume of metal to be used in the contact finger, without having a wide strip of metal on the top surface. Therefore, a high metal aspect ratio allows a large number of closely spaced metal fingers, while still retaining a high transparency. For example, on a large area device, a screen printed solar cell may have shading losses as high as 10 to 15%, while in a buried contact structure, the shading losses will only be 2 to 3%. These lower shading losses allow low reflection and therefore higher short-circuit currents.

Cross section of a partially plated laser groove.

In addition to good reflection properties, the buried contact technology also allows low parasitic resistance losses due to its high metal aspect ratio, its fine finger spacing and its plated metal for the contacts. As shown in the Emitter Resistance page, the emitter resistance is reduced in a buried contact solar cell since a narrower finger spacing dramatically reduces the emitter resistance losses. The metal grid resistance is also low since the finger resistance is reduced by the large volume of metal in the grooves and by the use of copper, which has a lower resistivity than the metal paste used in screen printing. As well, the contact resistance of a buried contact solar cell is lower than that in screen printed solar cells due to the formation of a nickel silicide at the semiconductor-metal interface and the large metal-silicon contact area. Overall, these reduced resistive losses allow large area solar cells with high FFs.

When compared to a screen-printed cell, the metalization scheme of a buried contact solar cell also improves the cell's emitter. To minimise resistive losses, the emitter region of a screen-printed solar cell is very heavily doped and results in a "dead" layer at the surface of the solar cell. Since emitter losses are low in a buried contact structure, the emitter doping can be optimized for high open-circuit voltages and short-circuit currents. Furthermore, a buried contact structure includes a self-aligned, selective emitter, which thereby reduces the contact recombination and also contributes to high open-circuit voltages.

The efficiency advantages of buried contact technology provide significant cost and performance benefits. In terms of $/W, the cost of a buried contact solar cell is the same as a screen-printed solar cell . However, due to the inclusion of certain area-related costs as well as fixed costs in a PV system, a higher efficiency solar cell technology results in lower cost electricity. An additional advantage of buried contact technology is that it can be used for concentrator systems 

Cutting silicon into wafers leaves the surface covered with cutting slurry and the surface is damaged due to the action of the saw.

Wafers are cleaned in a hot solution of sodium hydroxide that removes the surface contamination and the first 10 µm of damaged silicon. The wafers are then textured in a more dilute solution of sodium hydroxide with isopropanol as a wetting agent.

For multicrystalline wafers, acidic texturing is often used as it gives a more uniform etch rate across gain boundaries.

Doping

Solid state diffusion is a straight forward process and the typical method for introducing dopant atoms into semiconductors. In silicon solar cell processing starting substrates are typically uniformly doped with boron giving a p-type base. The n-type emitter layer is formed through phosphorous doping 

Saw damage etching and texturing