Manufacturing of crystalline Si solar cells

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First Photovoltaic Devices

Edmond Becquerel in the year of1839. Generated electricity by illuminating an electrode with

different types of light, including sunlight.

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Early Silicon Solar Cells

In 1941, before even this limited understanding of dopants, silicon photovoltaic devices based on the natural junctions were described.

(a) Cast ingot showing natural junction formed by impurity segregation during melting; (b) photovoltaic device cut perpendicular to junction; (c) device cut parallel to junction; (d) top surface of device cut parallel to junction.

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First modern silicon cell

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Dual rear contact structure. 1954 by Chapin, Fuller and Pearson. Efficiency of ~ 6%.

Silicon Wafers and Substrates

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Refining Silicon Metallugical grade silicon (MG-Si ~98% pure): The silica is

reduced (oxygen removed) through a reaction with carbon and heating to 1500-2000 C in an electrode arc furnace.

Powdered MG-Si is reacted with anhydrous HCl at 300 C in a fluidised bed reactor to form SiHCl3

Siemens process: the pure SiHCl3 is reacted with hydrogen at 1100C for ~200 300 hours to produce a very pure form of silicon

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Design of p/n junctionsSolar cells can tolerate higher levels of impurity than integrated circuits fabrication and there are proposals for alternative processes to create a "solar-grade" silicon.

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Single Crystalline Silicon

Single-crystalline wafers typically have better material parameters but are also more expensive.

In solar cells the preferred orientation is as this can be easily textured to produce pyramids that reduce the surface reflectivity. However, some crystal growth processes such as dendritic web produce material with other orientations.

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Single Crystalline Silicon

Single crystalline silicon is usually grown as a large cylindrical ingot producing circular or semi-square solar cells.

The semi-square cell started out circular but has had the edges cut off so that a number of cells can be more efficiently packed into a rectangular module.

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Czochralski (CZ) Silicon

Schematic of CZ Si growth

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Czochralski (CZ) Silicon

Top of Czochralski ingot. The bottom cylindrical

section has been cut off to make wafers.

Such "tops and tails" left over from growing the semiconductor industry are a large source of silicon supply for the photovoltaic industry.

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Preparation of multicrystalline Si wafer

Preparation of multicrystalline Si wafer

Techniques for the production of multicrystalline silicon are much simple, and therefore cheaper, compromising with materials quality.

Grain boundaries introduce high localized regions of recombination. Grain boundaries reduce solar cell performance by blocking carrier

flows and providing shunting paths for current flow across the p-njunction. 13

Wafer Slicing

Large multicrystalline silicon block being sliced up into smaller bricks.

Brick of multicrystalline silicon cut from slab and before being cut up into wafers.

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Wafer Slicing

Silicon brick being sliced up into wafers.

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Solid State Diffusion for emitter

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Solid state diffusion: Heating the wafer at a high temperature in an atmosphere containing dopant atoms causes some of the atoms to be incorporated into the top surface of the wafer.

In silicon solar cell: formation of n-type emitter layer on the p-type base.

Cell Fabrication Technologies

Screen-printed Solar Cells

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Screen-printed Solar Cells

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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.

Schematic process flow

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Schematic process flow

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Schematic process flow

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Schematic process flow

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Some Key Process Steps Phosphorous Diffusion (n+ emitter)

Shallower emitters (reduce dead-layer), thus improving the cell blue response.

Selective emitters with higher doping below the metal contacts no commercial production yet.

Surface Texturing to Reduce Reflection Single crystalline: by etching pyramids. Texture multicrystalline materials:

mechanical texturing of the wafer surface with cutting tools or lasers;

isotropic chemical etching based on defects; isotropic chemical etching with a photolithographic mask; plasma etching. 23

Some Key Process Steps Antireflection Coatings (ARC) and Fire Through Contacts

Commonly used: titanium dioxide (TiO2) or silicon nitride (SiNx). ARC are particularly beneficial for multicrystalline material that cannot

be easily textured. Improve the electrical properties of the cell by surface passivation. Metal contacts can fire though ARC and bond to the underlying silicon.

Edge Isolation Plasma etching, laser cutting, or masking the border to prevent a

diffusion from occurring around the edge in the first place.

Rear Contact In most production, the rear contact is simply made using a Al/Ag grid

printed in a single step.24

Front view of a completed screen-printed solar cell. As the cell is manufactured from a multicrystalline substrate,

the different grain orientations can be clearly seen. The square shape of a multicrystalline substrate simplifies the

packing of cells into a module. 25

Buried Contact Solar Cells

Metal contact is buried in a laser-formed groove inside the silicon solar cell.

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Advantages of Buried Contact Solar Cells

High transparency: Shading loss improves to 2-3% from screen printed solar cell of 10 to 15% improve Jsc.

A large metal height-to-width aspect ratio low parasitic resistance losses (emitter resistance loss).

No need very high emitter doping (or dead layer) improve Voc.

A higher efficiency solar cell technology results in lower cost electricity.

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PERL Solar Cells

The passivated emitter refers to the high quality oxide at the front surface that significantly lowers surface recombining.

The rear is locally diffused only at the metal contacts to minimize recombination at the rear while maintaining good electrical contact. 28

Passivated emitter with rear locally diffused.

efficiencies approaching 25% under the standard AM1.5 spectrum.

Rear Contact Solar Cells Rear contact solar

cells achieve potentially higher efficiency by moving all or part of the front contact grids to the rear of the device.

The higher efficiency potentially results from the reduced shading on the front of the cell.

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Making Solar Panel

A case study of bulk Si solar cell production line

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Source Material

One source is off cuts and scrap material from the semiconductor industry.

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Wafer scraps from the production line are recycled for growing new ingot.

Growing ingots

The tub has to withstand the melting point of silicon at 1415 C.

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Loading the growth tub into the furnace.

Sawing the Ingot into Bricks

The large silicon ingot is sawn into more managable bricks.

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Wafer Slicing

The wires are covered with slurry and are wound around the drums. When running the drums spin at high speed and the silicon bricks are pushed down from the top.

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Texturing

Wafers in cassettes for texturing in sodium hydroxide.

Spin dry is applied.35

Emitter Diffusion

The wafers are then put in a belt furnace to diffuse a small amount of phosphorous.

The wafers travels in diffusion furnace for roughly an hour.

Clean and dried. 36

Edge Isolation

Removes the phosphorous diffusion around the edge of the cell so that the front emitter is electrically isolated from the cell rear.

A common way to achieve this is to stack the wafers on top of each other then plasma etch using CF4 and O2.

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Anti-Reflection Coating

An antireflection of silicon nitride is typically deposited using chemical vapor deposition process (CVD).

large amounts of hydrogen (SixNy:H).

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3SiH4 + 4NH3 -> Si3N4 + 12H2

Before coating After coating

Screen-Print Front

Silver paste is forced through a patterned screen. Screen printer in operation: wafer are under the printer. After printing, heating @200 C to evaporate off the

organic binders in the paste. 39

Screen-Print Rear Al

The rear is printed in two parts. A thick layer of aluminum past covers most of the cell and

provides a back surface field. Drying the aluminum paste.

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Screen-Print Rear Silver

Screen printing the silver on the rear. Unloading the final dry process.

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Metal Firing

A firing process at high temperature bonds the Al / silver to the silicon.

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Testing

Testing the cells and putting them into modules. After the cells are tested they are sorted into

bins.43

Module Manufacture After the cell is finished it

strips are added. Individual solar cells are

protected from the weather by encapsulating into a module.

Each cell is only around 0.5 volts, to obtain sufficient voltage the cell are connected together in series using flat wires called tabs.

44Solar Cell Manufacture

Manufacturing of crystalline Si solar cellsFirst Photovoltaic Devices Early Silicon Solar Cells First modern silicon cellSilicon Wafers and SubstratesRefining SiliconDesign of p/n junctionsSingle Crystalline SiliconSingle Crystalline SiliconCzochralski (CZ) Silicon Czochralski (CZ) Silicon Preparation of multicrystalline Si waferPreparation of multicrystalline Si waferWafer Slicing Wafer Slicing Solid State Diffusion for emitterCell Fabrication TechnologiesScreen-printed Solar CellsScreen-printed Solar CellsSchematic process flowSchematic process flowSchematic process flowSchematic process flowSome Key Process Steps Some Key Process Steps Slide Number 25Buried Contact Solar CellsAdvantages of Buried Contact Solar CellsPERL Solar CellsRear Contact Solar Cells A case study of bulk Si solar cell production lineSource Material Growing ingots Sawing the Ingot into Bricks Wafer Slicing Texturing Emitter DiffusionEdge Isolation Anti-Reflection CoatingScreen-Print FrontScreen-Print Rear Al Screen-Print Rear Silver Metal FiringTesting Module Manufacture