Improving Energy Conversion Efficiency for Solar Cells

This is an excerpt from "Study of the Effect of the Firing Process to the Energy Conversion Efficiency of Solar Cells" by KIC and Top Solar...


Over the last few years, solar cell manufacturers have strived to improve energy conversion efficiency at a lower cost. Optimizing the metal electrodes firing process in furnaces is a common way to achieve this goal. This was overlooked in the past. Most of the time, such thermal process work was done mainly by experienced engineers without much in-depth engineering study and development.

In summary, the accurate measurment of the cell profiles and optimization of the furnace’s temperature settings during production play an important role in process control. Frequent tracking on the furnace’s temperature, collecting and analyzing measurement data can help optimize the firing process and find a suitable process window. The end result is the production of higher efficiency cells, stable production quality and reduced production cost.

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Wafers: Thermal Process Development

This is an excerpt from..

A Practical Guide for Improving Crystalline Solar Cell Efficiencies through
Optimization of the Firing Process

by Bjorn Dahle, KIC

As opposed to more mature industries such as semiconductor and electronics assembly, the solar industry currently does not have a clear understanding of the ideal wafer profile or process window for each unique application (wafer, silver paste, furnace and other variables). There may be numerous wafer properties and variables in the processes upstream from the metallization furnace that affect the ideal wafer profile. Therefore, each manufacturer must perform a design of experiment (DOE) to identify the best wafer profile or range of profiles (Process Window). Such a DOE typically involves changing the wafer profile while the cell efficiency, fill factor and other quality measurements are measured. The profile is changed numerous times (on identical wafers that have been processed up to the firing furnace) in a “trial and error” approach until the responsible engineer is satisfied with the cell efficiency.

The wafer profile itself is a result of the furnace settings and how the thermodynamic properties of the furnace heat and cool the wafer. A modern metallization (firing) furnace can be set up using tens of millions of alternative recipes (combination of zone temperatures and conveyor speed), making the DOE both difficult and time consuming. Even worse, when changing one profile parameter, e.g. the peak temperature, all the other parameters such as time above 500°C, time above 600°C, ramp rate, etc. also change. As a result, it becomes difficult to determine what caused the improvement in the cell efficiency.

Finally, the traditional methods to record the wafer profile suffer from inaccurate and non-repeatable measurements. This problem is caused by the method used to attach the thermocouples (TC) to the wafer. It is not uncommon to get a 50°C difference in peak temperature readings from one profile to the next when taken only minutes later. Clearly, a new TC attach method is required before any purposeful DOE can take place.

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Furnace for Thick Film Application

HSK series belt furnaces serve diverse industries such as microelectronics packaging, which include IC, SMT, HIC, MCM and MEMS applications, and advanced materials, including thick film, electronic component, photovoltaic cells, ceramics and metals processing applications. 

The HSK series fast fire furnace heats from ambient to 1050°C in approximately 40 minutes and is designed to sustain continuous on/off heating and cooling cycles resulting from alternating periods of production and non-use.  It features an ultra-clean low-mass refractory heating chamber equipped with FEC (Fully Enclosed Coil) heaters formed into ceramic insulation panels.  With the use of advanced insulation materials, lower thermal capacity enables the furnace to warm up and cool down very quickly and lose less heat to the environment. 

The furnace is monitored by type “K” thermocouples in the center of each heated zone.  Each temperature zone is control-led by its own SHIMADAN SR94 single loop intelligent temperature controller with full auto-tuning PID.  The single wave, zero trigger method enables precise and stable temperature control, avoiding damages to the peripheral equipment while prolonging the life of controlling devices at the same time. 

The furnace belt is balanced spiral Nichrome V mesh.  The belt speed is programmable in IPM with readout right on the monitor and deviation from set point alarm is also programmable.  Stepless speed regulation is controlled by FUJI frequency converter and is digitally displayed. 

The furnace is equipped with entrance/exit curtains and an exhauster to improve drying/firing temperature stability, as well as, to keep the firing chamber clean.  The 200mm (8”) diameter air powered Venturi exhauster supports full chamber width exhausting.  There are removable condensate collection traps and the exhaust flow can be adjusted by the flow meter.  

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Firing in the Thick Film Process

After the paste is screened onto the substrate, it is commissioned to level for 5 to 15 minutes at room temperature, followed by oven drying at 100°C to 150°C for another 10 to 15 minutes.  Firing takes place in continuous furnaces.     

Air-fired and nitrogen-fired systems are more commonly used.  In the initial firing phase (200°C to 500°C), organic resins are burned off in the burnout zone.  In air-fired systems, the polymer decomposes by simple oxidation.  The byproducts of the burnout zone are primarily CO₂ and H₂O, as well as, lightweight fly ash.  In the nitrogen-fired system, cleavage of the polymer at the ethyl groups and dehydrogenation take place.  This produces a complex mixture of polymers, monomers, and gases that burn off as gases, vapors, and droplets.     

As the climate rises to peak firing temperature, metal particles begin to sinter, glass frits begin to soften and flow, and a film is gradually formed.  The material is held at peak temperature for 10 minutes then cooled down.  After it cools down, pastes are formed firmly onto the substrate.  The entire cycle lasts for about 30 to 60 minutes.  


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Printing in the Thick Film Process

The selective deposition of thick-film materials onto a substrate essentially depends on forcing ink through a stencil, screen, or mask mounted under tension on a metal frame. Stainless steel mesh screens are the most commonly used for hybrid thick-film printing while etched metal masks are sometimes used for high precision purpose.

The circuit pattern is formed photographically on the mesh using ultraviolet (UV)-sensitive filler emulsion. The appropriated paste to be printed is placed on one side of the screen. A polyurethane squeegee is used to force the paste through the open meshes. The pressure applied to the squeegee must be sufficient to ensure complete filling of each aperture before the squeegee edge passes over the aperture, but not high enough to cause spreading of the paste between the screen and the substrate. 

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Thick Film Hybrids

Thick-film hybrids are generally used to substitute, interface or interconnect ICs. Hybrid constructions can be either single layer or multilayer. Single-layer circuits are conductor traces printed and fired onto a substrate with the utilization of crossover and pads of insulating dielectric. Three to five layers are common in multi-layer circuit. Connections between different layers are achieved by vias.

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Thick Film Pastes

Thick film technology is traditionally an additive process, that is the various components are produced on the substrate by applying ‘pastes’ (or ‘inks’) and are added sequentially to produce the required conductor patterns and resistor values.  Different formulations of paste are used to produce conductors, resistors and dielectrics (for crossovers or, occasionally, capacitors). 

Each paste contains:

- A binder, a glassy frit

- The carrier, organic solvent systems and plasticizers

- The materials to be deposited, typically pure metals, alloys, and metal oxides

The metal particles are bound together and to the substrate by the glassy phase, and this is particularly important at the substrate-ink interface. Fired surfaces are usually not even or homogeneous on a micro scale, a fact which can lead to problems when wire bonding.

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Thick Film Materials (Substrate)

The majority of substrates are ceramic for their mechanical strength, electrical resistivity over broad range, thermal conductivity and chemical inertness.  Alumina has become the most widely used because it to has an excellent combination of all these properties.  The 96% Al₂O₃ composition has become an industry standard and is used in 90% of all circuits.  The 4% impurities are selected to accomplish complete densification without grain growth and to maximize electrical properties.  The most common additives are magnesia and silica.  Magnesia inhibits grain growth of alumina crystals and silica is used to form a glassy matrix with alumina that binds the system together.


For high frequency application, ultralow-loss alumina substrate is made by eliminating the glassy phase. This is accomplished by sintering 99.5% pure Al₂O₃ at 1800-1900°C to form high-purity dense substrate.

New substrate materials continue to appear, including ‘porcelainised steel’ (vitreous-enameled steel), organic materials such as epoxies, flexible substrates, and even synthetic diamond.

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Introduction to Thick Film Technology

Thick film technology is an established method for efficiently manufacturing hybrid microelectronic circuitry. The thick film process can be defined as the sequential printing and firing of conductor, resistor, and dielectric paste formulations onto a substrate.

Thick films are typically in the range 5–50µm thick. This distinguishes a hybrid from thin film technology in which films are deposited by vacuum evaporation or sputtering processes and the thickness is usually less than 1µm. the range of resistivities is 10Ω/square to 10MΩ/square, there are considerable possibilities for building multi-layer structures. 

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Advantages and Disadvantages of Glass to Metal Seals

Advantages of Glass to Metal Seal:

- Extremely high durability
- Mechanical stress and strain resistance
- High shock and vibration resistance
- Chemical stability and resistance
- Absolute hermetic
- Inexpensive and easy to manufacture
- Electrical insulation

Disadvantages of Glass to Metal Seal:

- Unable to cope with large CTE mismatch
- Precise control of glass composition needed

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Copper-to-Glass Seals

Copper-to-glass seals: Although the CTE difference between glass and copper is large, a satisfactory vacuum seal can be made which utilizes the property of glass to “wet” copper oxide, some of which diffuses into the glass and forms a transition phase. For a satisfactory seal, the oxide layer should be no more than a few tenths of a micron thick and may consist of various oxides.

1.  The copper piece is tapered down to a very thin feather edge in order to follow the expansion and contraction of the glass. The feather edge must be smooth and polished.

2.  Degreasing and hydrogen-firing at 800°C.

3.  The copper piece is usually borated by heating to redness in air and quenching in a concentrated solution of sodium borate. The color of the copper at the seal site should have a uniform deep red to purple sheen.

4.  The copper and glass are heated to 1000°C in air and brought together. This is a process called beading

5.  Copper-glass seals must be carefully annealed in an oven immediately after glassing

6. The assembly is chemically cleaned to remove the excess copper oxides.

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Housekeeper Seals

Housekeeper seals can be made with any kind of glass (usually soft glass) if the materials used fulfill the following requirements:

1.     The expansion of the metal part must be larger than that of the glass in order to assure that the stresses normal to the glass-metal interface will always be compressive

2.     The metal used must be soft enough and thin enough to be able to deform plastically, in order to allow dimensional changes which take place in the glass as it cools

3.     The metal must form a strong bond with the glass

4.  The shape and dimensions of the seal must be designed so, as to provide a large contact area between metal and glass, avoiding at the same time any tensile stress perpendicular to the surface or too great stresses in any direction

The metal fulfilling all the requirements is copper, but in some such seals platinum, iron (steal) and molybdenum can also be used. Using one or the other of these metals, the seal can be made in various shapes: wire seal, ribbon seal, feather-edge seal or dics seal.

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Fabrication of Glass-to-Metal Seals

In most glass-to-metal seals, the procedure involves:

1.  

       1) Oxides formation on metal surface by heating to the proper temperature

2.        2)  “Wet” the oxides by fused glass to form a glass-oxide-metal transition

3.        3) Seal the metal-glass bond

It is worth to note that a good hermetic seal can be made without the presence of oxide, e.g., as in the case of platinum.

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Types of Glass-to-Metal Seals

There are many types of glass-to-metal seals. These seals can be categorized by their structures.

Internal seals are those in which the metal, in the form of wire, rod, ribbon, or tube, is surrounded by glass.

External seals are those in which a band, tube, or cup of metal adheres to the edge of a glass insert, which may be a round button or disc or it may be square or any other shape, in which ease the stresses are always greater and more complex than in a simple round button or disc.  Since glass is much stronger in compression than in tension, the differences in coefficients of expansion of the glass and metal components may be somewhat greater in external than in internal seals. Where expansion mismatch occurs, the ratio between the outside diameter of the tube in an external seal and its wall thickness should not be less than 15:1, with a maximum wall thickness of 0.01 inch. Where expansion coefficient differentials are small, as in Kovar-to-metal seals, these restrictions do not apply.

Tubular seal is “inside tubular” when the metal is surrounded by glass and “outside tubular” when the glass is surrounded by metal.  The tubular seals are so-called “Housekeeper” seals. The metal, such as copper or stainless steel, having a widely different expansion coefficient, is formed to a thin edge so that it is able to yield as it expands with heat relative to the glass, and the stresses in the glass do not exceed its tensile strength.

Edge seals are those in which the edge of the metal is embedded in glass. These resemble tubular seals in that they may be either normal edge or “Housekeeper” type, but the geometry and applications differ.

Butt seals are made by sealing a metal disc or ring to the end of a piece of glass tubing.

Window seals may be of any shape, although the round window is subject to less severe and complex stresses.

Combinations of different types of the seals is also possible.

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