Solar Cell Manufacturing in the Second Generation

Second generation solar cell, also known as thin-film solar cell (TFSC) or thin-film photovoltaic cell (TFPV), is made by depositing one or more thin layers (thin films) of photovoltaic material on a substrate. The most advanced second-generation thin film materials in use today are amorphous silicon (aSi), cadmium telluride (CdTe), and copper indium gallium selenide (CigS). The thickness range of such a layer is wide and varies from a few nanometers to tens of micrometers. is thin-film now the way to go? There are certainly many good reasons for moving to thin films for the solar cell manufacturing process. 

1. Thin film deposition. 

Copper indium gallium selenide (CigS) is used for the thin film active layers in CigS solar cells, commonly formed using 

sputter deposition. During this vacuum-based process, a plasma of electrons and ions is created from inert argon gas. These ions dislodge atoms from the surface of a crystalline material which is then deposited to form an extremely thin coating on a substrate. Depositing thin film by sputtering is the same process used in semiconductor manufacture and in packaging. 

2. Thin film annealing. 

After sputtering, the thin film needs to be annealed to achieve optimum results. it is also possible to inject additional chemicals during the annealing process. An annealing furnace is similar to the brazing furnace commonly used in packaging industries. The muffle is typically made of SUS 316L material to ensure good corrosion resistance for the thin film solar panel’s corrosive environment. A typical belt furnace can anneal up to 600 x 1200mm (23.6 x 47.2-in.) thin film solar panels after thin film deposition. 

3. Metallization. 

Like its first generation cousin, the manufacture of thin film solar cells need Al or Ag screen printing metallization, originally invented for the thick film process. Such metallization pastes or inks can be used on both rigid (glass, silicon) and flexible (polyimide, polyester, stainless steel) substrates. The metallization can be accomplished through either thermal curing or firing. 

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The Screen-Printing Method

The screen-printing method consists of a thick film metal paste that is composed of metal powder, glass fritt, solvent and non-volatile polymers that are blended together in a three roll mill. A squeegee applies a downward force on the paste moving it across the screen that has a deposition pattern within. This action creates a reduction in viscosity of the paste so it can penetrate into the screen holes. This way, the metal paste is deposited in select patterns onto the substrate. The factors that affect the screen-printing process include snap off distance, squeegee pressure and squeegee speed.

The snap off distance is the distance between the screen and the wafer. During the printing process, when the paste is printed onto the screen, a downward force is applied to the screen. The screen, being elastic, restores its shape and this upward movement aids in the deposition of the paste. If the snap off distance was too high, then pressure will have to be applied to force the paste onto the wafer. If it is too low, the paste might not get released from the screen. The pressure applied also plays an important role in the deposition of the paste. When too much pressure is applied, excessive paste from the screen could be deposited which can break the wafer. On the contrary, when too little pressure is applied, the paste might not get released from the screen at all. The speed of the squeegee movement also determines the print quality. If the speed is too high, the paste can miss many holes and lead to a non-uniform deposition.

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Principles and Advantages of Ultrasonic Belt Cleaning

In an ultrasonic belt cleaner, the solvent vibrates at ultrasonic frequencies. These ultrasonic waves cause extremely rapid pressure decreases and increases in the fluid. The sudden decrease develops instants where the water can no longer exist in a fluid state and, as a result, gas bubbles form. In other words, water can only exist in a liquid state above a minimum pressure (the threshold of cavitation). When the pressure decreases below the critical pressure, the water becomes a gas and evaporates. Then, at the ensuing increase in pressure, the gas bubbles will collapse again. This process of creating minute bubbles in the liquid is known as cavitation and it is responsible for the scrubbing effect that propagates ultrasonic cleaning.


Simply put, the advantages of using ultrasonic belt cleaning technology have to do with precision, speed, and consistency. Ultrasonic energy infiltrates obscure regions of the belt, which means that all areas of the belt will be categorically cleansed. Ultrasonic cleaning also works faster than any other conventional cleaning procedure in the elimination of contaminants. Due to the efficiency of ultrasonic antisepsis, the labor saving advantages designate ultrasonic technology as the preeminent economical design in cleaning furnace belts. And, unlike manual cleaning (like using a steel brush) ultrasonic technology offers incomparable cleaning consistency throughout the entire belt.

Using a steel brush as a tool for cleaning the belt, on the other hand, can often be time-consuming, imprecise, and inconsistent. A steel brush uses steel wire bristles to clean the surface area around the belt, however, due to its abrasive nature and inability to reach abstruse regions of the belt, it does not obtain the cleaning capabilities that an ultrasonic belt cleaner can easily handle.

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What a DSSC is!

A dye-sensitized solar cell is composed primarily of three parts. The first part, the substrate, is the negative terminal. The substrate has a layer of transparent glass on the outside and a coating of transparent conductive oxide (TCO) on the inside. This warrants sunlight to pass through. In the center sector, a layer of dye sensitizers bind to a layer of nano-structured titanium dioxide (TiO2), where the TiO2 is connected to the negative terminal to collect sunlight. All of the layers are then immersed in an electrolyte solution to allow charge transportation. The top part is the positive terminal and it contains a coating of carbon (graphite) or platinum for the purpose of transferring electrons. The outside layer is made of transparent glass and the top and bottom divisions are joined together to prevent the centered portion from leaking.


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A Typical Schematic of a Belt Furnace

Solar Cell Process Applications for Belt Furnaces

Thick Film Processing

After a paste is screened onto a substrate and it settles for 5–15 minutes at room temperature, it undergoes oven drying at 100-150°C for 10–15 minutes to remove solvents. Firing is then completed in conveyor belt furnaces at temperatures between 500-1000°C.

Crystalline Silicon Solar Cell Manufacturing

Electrical contacts are usually formed by screen printing. The firing is done in conveyor belt furnaces at a temperature of about 700°C for a few minutes. Upon firing, the organic solvents evaporate and the metal powder becomes a conducting path for the electrical current.

Thin Film Solar Cells Manufacturing
A transparent conducting glass, coated with doped SnO2 or ITO film, is used as a substrate. A thin film, such as CdS, is then deposited through CSS or CBD techniques. The CdS film is heat treated by a conveyor belt furnace in a reducing atmosphere or in the presence of CdCl2 at 400-500°C.

Dye Sensitized Solar Cells (DSSC) Manufacturing
TiO2 nanoparticles have been used extensively to increase the interfacial surface area in Dye Sensitized Solar Cells. Nanoparticle films are generally made by screen printing a paste of titania nanocrystals and then sintering the particles together at 450-500°C in a conveyor belt furnace.

From Wikipedia: http://en.wikipedia.org/wiki/Conveyor_belt_furnace

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What is a Conveyor Belt Furnace?

A conveyor belt furnace is a furnace that uses a conveyor or belt to carry process parts or materials through the primary heating chamber for rapid thermal processing. It is designed for fast drying and curing of products and is nowadays widely used in the firing process of thick film and metallization processes of solar cell manufacturing. Other names for conveyor belt furnace include metallization furnace, belt furnace, atmosphere furnace, infrared furnace and fast fire furnace, to just list a few.

Normally a conveyor furnace adopts tunnel structure and is composed of multiple controlled zones which include, but not limited to, preheating zone, binder burn out zone, heating zone, firing zone and cooling zone. A conveyor furnace also features fast thermal responses, uniform and stable temperature distribution; it can heat treated parts to 1050 deg. C. (may vary for different model). Belt speed of a conveyor furnace can be up to 6000mm/min. Products are heated efficiently by infrared radiation (it can also be ceramic heaters or IR lamps) and are dried and fired after passing through the controlled zones, followed by rapid cooling.^[1]

^{For more info on belt furnaces please go to www.beltfurnaces.com}

An Educational Video on THT Furnaces

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CdTe PV panel maker First Solar to supply Sempra's 150MW Nevada plant expansion for PG&E

This post is from: http://www.semiconductor-today.com/news_items/2011/AUG/FIRSTSOLAR_040811.html

Pacific Gas and Electric Company of San Francisco, CA, USA (a subsidiary of PG&E Corp) and Sempra Generation (a subsidiary of San Diego-based Sempra Energy) have entered into a 25-year contract for 150MW of renewable power from an expansion of Sempra Generation's Copper Mountain Solar complex in Boulder City, NV. 

First Solar Inc of Tempe, AZ, USA will provide the ground-mounted cadmium telluride (CdTe) thin-film photovoltaic (PV) panels and serve as the engineering, procurement and construction (EPC) contractor for the project.

Construction on the 1100-acre solar plant should begin in early 2012. The first 92MW of panels at Copper Mountain Solar 2 should be installed by January 2013, with the remaining 58MW due for completion by 2015. Under the terms of the contract, PG&E has the option to accelerate the commercial operation date of the second phase. When fully developed, Copper Mountain Solar 2 will produce enough electricity to power about 45,000 homes.

Copper Mountain Solar 2 is a step in Sempra’s plan to construct 1000MW of additional renewable capacity by 2015, says its president & CEO Jeffrey W. Martin. The plant is Sempra’s third and largest solar project in Nevada, and will supply power to California consumers. The power supply contract between PG&E and Sempra is subject to approval by the California Public Utilities Commission.

“The combination of First Solar’s advanced thin-film PV modules with our industry-leading EPC capabilities enables us to rapidly deploy utility-scale solutions like Copper Mountain Solar 2, bringing down the cost of renewable energy,” says Jim Lamon, First Solar's senior VP of EPC, operations & maintenance.

Sempra Generation and First Solar have previously teamed-up on the construction of two other large-scale solar projects in Nevada, including Copper Mountain Solar 1. The 48MW installation was completed in late 2010 and is currently the largest photovoltaic solar power plant in the USA. PG&E is currently delivering the power produced at the plant to its customers.

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Competitive Market Advantages for DSSC Manufacturing

The production of DSSCs incur relatively low cost: new patented technologies will result in less than $1.5 ppw for module manufacturing cost at initial production, dropping to about half this figure ($0.7 ppw) on the basis of economy of scale from multiple plants as opposed to more than $3 ppw today.

DSSCs have an additional advantage in that they are particularly suited to warmer climates. When hot, crystalline silicon modules lose efficiency far more than do dye cells.

DSSCs also work well in a wide range of lighting conditions and orientation, and they are less sensitive to partial shadowing and low-level illumination compared to silicon.

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Furnace for Firing Operation in Silicon Solar Cell Manufacturing

The HSH series furnace is a specially designed infrared furnace that caters to the needs of the photovoltaic metallization firing requirements. The heating in this furnace is achieved with the help of short wave infrared lamp heaters. The fast response of the IR lamps allow for quick heating. The furnace is rated at 1000 oC and can operate very well in the 750-800 oC range required for sintering of front contact metallization. The belt width comes in various standard sizes, including, 250mm, 300mm and 380mm to match with the requirements of the wafer size. Cooling can be achieved through forced air, as well as, water per requirement. The presence of a muffle helps to control the atmosphere within the furnace, as well as, prevent the external atmosphere from entering. In short, the muffle design helps maintain a cleaner furnace atmosphere. As a standard feature, this furnace is equipped with a steel brush and helps in the cleaning process of the conveyor belt. Ultrasonic belt cleaning is available as an extra option as well.

A microprocessor based PID controller controls the furnace. Type K thermo-couples are used for determining the zone temperatures. Controls are located on the right hand side and can be viewed from the entrance of the furnace. The central processing unit (CPU) is mounted under the exit table. The furnace is controlled by a microprocessor based controller system and the CPU is loaded with a Windows operating system that allows for easy computing. The computer system comes with a pre-installed program for controlling the Confurnace parameters including the belt speed and the zone temperatures. Temperature profiles can be stored and retrieved for future purposes. Thermocouple ports are located at the entrance table for connecting the profiling thermocouple directly to the microprocessor. This feature allows the monitoring and recording of the actual temperature experienced by the part. Software is also provided with the computer to capture, display, printout and store the furnace profile. The furnace is equipped with a redundant overheat safety protection system which incorporates an additional type “K” thermocouple in the center of each controlled zone and the multi-loop alarm.

For more information and specifications go to http://www.beltfurnaces.com/HSHsolar.html

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