Manufacturing Solar Cells - Assembly & Packaging

By Ken Kuang, Joyce Zhang and Bill Ishii, Torrey Hills Technologies, LLC, San Diego, CA 

The trend in packaging has shown a serious shift: attendance at assembly and packaging conferences has been dwindling over the past few years whereas solar power shows have increased in popularity. More and more electronics assembly and packaging companies are appearing at solar expos. There are significant opportunities for electronics engineers in the rapidly expanding solar business.

Solar cells are derived from the 1839 discovery of the photovoltaic effect by French physicist A. E. Becquerel. However, it was not until 1883 that the first solar cell was built by Charles Fritts, who coated the semiconductor selenium with an extremely thin layer of gold to form the junctions. The device was only about 1 percent efficient. A Russian physicist named Aleksandr Stoletov then built the first solar cell based on the outer photoelectric effect (discovered by Heinrich Hertz earlier in 1887). Also, Albert Einstein explained the photoelectric effect in 1905 for which he received the Nobel Prize in Physics in 1921. Finally, Russell Ohl, who worked on the series of advances that would lead to the transistor, developed and patented the junction semiconductor solar cell in 1946. 

Modern solar cells can be described as the co-existence of three different generations: crystalline silicon, thin film, and  dye. Along with the development of solar cells, there has also been a parallel development of solar cell manufacturing technologies. Assembly and packaging engineers have played a significant role in developing these manufacturing techniques, creating incredible potentials in every generation of the solar business.


First Generation

Elemental or crystalline silicon is the principal component of most semiconductor devices, most importantly integrated circuits or microchips. Silicon's ability to remain a semiconductor at higher temperatures has made it a highly attractive raw material for solar panels. Silicon's abundance, however, does not ease the challenges of harvesting and processing it into a usable material for microchips and silicon panels. At least three standard manufacturing processes mean that there are technical opportunities for assembly and packaging engineers.

1. Phosphorus diffusion. There are two main layers that are essential to the solar cell's function. One is a p-type layer, which means that the wafers are boron doped, and an n-type layer created by introducing phosphorus. The silicon wafer usually already starts off by already being doped with boron. In order to form the n-type layer, phosphorus has to be introduced to the wafer at high temperatures of around 870°C for 15-30 minutes in order for it to penetrate the wafer. The excess n-type material is then chemically removed.

These diffusion processes are usually performed through the use of a batch tube furnace or an in-line continuous furnace. According to BTU, detailed cost of ownership models have shown that in-line diffusion can deliver per wafer costs of as low as one third the cost of a batch diffusion furnace. The basic furnace construction and process are very similar to the process steps used by packaging engineers.

2. Silicon wafer metallization.Electrical contacts are formed through squeezing a metal paste through mesh screens to create a metal grid. This metal paste (usually Ag or Al) needs to be dried so that subsequent layers can be screen-printed using the same method. As a last step, the wafer is heated in a continuous firing furnace at temperatures ranging from 780 to 900°C. This completes the metallization process, removes solvent and binder, and forms electrical contacts. Metallization is the most critical step. The challenge of reducing wafer thickness for higher efficiency has created stringent requirements for both the equipment and the process itself.

3. Solar module assembly. Solar module assembly usually involves soldering cells together to produce a 36-cell string (or longer) and laminating it between toughened glass on the top and a polymeric backing sheet on the bottom. Frames are usually applied to allow for mounting in the field, or the laminates may be separately integrated into a mounting system for a specific application such as integration into a building. The basic process is very similar to the SMT process assembly that packaging engineers are already familiar with, albeit on a larger scale. The packaging industry's lean manufacturing methodology can be applied directly to solar module assembly.

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.

The electrochemical dye solar cell was invented in 1988 by Professor Graetzel of Lausanne Polytechnique, in Switzerland. The "Graetzel" dye cell uses dye molecules adsorbed in nanocrystalline oxide semiconductors, such as TiO_2, to collect sunlight. Dye cells employ relatively inexpensive materials such as glass, Titania powder, and carbon powder.

Graetzel's cell is composed of a porous layer of titanium dioxide nanoparticles, covered with a molecular dye that absorbs sunlight, like the chlorophyll does in green leaves. The titanium dioxide is immersed in an electrolyte solution, above which is a platinum-based catalyst. As in a conventional alkaline battery, an anode (the titanium dioxide) and a cathode (the platinum) are placed on either side of a liquid conductor (the electrolyte). Sunlight passes through the cathode and the conductor, and then withdraws electrons from the anode, at the bottom of the cell. These electrons travel through a wire from the anode to the cathode, creating an electrical current.

 
Similar to Semiconductors

The basic dye cell manufacturing steps also resemble the approaches taken by the semiconductor and packaging industry. For example, a screen printer is typically used to apply titania and other layers to the Transparent Conductive Optical (TCG or TCO) glass. Nanocrystalline TiO₂ pastes are screen printed onto the TCO glass, then dried and fired in a continuous belt furnace. The sintering process allows the titanium dioxide nanocrystals to partially "melt" together, in order to ensure electrical contact and mechanical adhesion on the glass. All these furnaces are typically modified from standard thick film furnaces.

After dye staining and anode side application of proprietary current collectors, platinum catalyst is obtained by using the Pt-Catalyst T/SP product which can either be squeegee printed or screen-printed using a polyester mesh of 90. The solar cell needs to be dried at 100°C for 10 minutes before being fired at 400°C for 30 minutes. During the assembly, sealing and filling processes, TCO glass with the completed Titania layer is mated to the cathode current collector, protective glass plate, sealed, busbar attached to the cell and then the cell is filled with electrolyte. Custom designed, fully automated and efficient cell assembly, sealing and electrolyte filling machine sets are required for these production steps.

At one time, Torrey Hills Technologies sold in-line continuous furnaces mostly for thick film and brazing applications. Several years ago, in response to the growing demands of the solar manufacturing industry, the company's engineers reinvented the original technology and adjusted it to different types of solar cell processing. A critical step in solar cell manufacturing is metallization through screen printing. By changing the specifications of thick film drying and firing furnaces, the company stepped comfortably into the solar cell market.

Solar technologies have created compelling technical challenges and business opportunities for assembly and packaging engineers. The traditional thick film, thermal treatment and assembly techniques play key roles in solar cell manufacturing. Many skill sets possessed by electronics engineers can be easily reinvented and applied to the solar cell industry. 

 Contact: Torrey Hills Technologies, LLC

6370 Lusk Blvd., Suite F111, San Diego, CA 92121 

858-558-6666 fax: 858-630-3383

 E-mail: info@torreyhillstech.com 

Web: http://www.torreyhillstech.com

Create Your Own Dye Sensitized Solar Cell

     Understanding the production of a Dye Sensitized Solar Cell may seem seem far from a walk in the park for many people. Every description throws out seemingly cryptic technical names of components, such as a 'photoelectrode composed of a transparent conducting oxide', or various processes, such as utilizing a firing furnace, that will not make sense to or may not be available to anyone that is not an engineer. This may, in turn, intimidate people from learning more about this innovative invention.

     So how can one learn more about these cells in layman's terms? By creating one at home! The video below teaches people how to make a Dye Sensitized Solar Cell at home using readily available parts.

Video by Solar Blogger

      Now that you have seen how to make a cell, it will be easier to understand its components and production. So instead of looking confused the next time you hear someone use the phrase 'photoelectrode composed of a transparent conducting oxide', you will be able to smile back and know exactly what part they are referring to!

Three Common Problems With Your Furnace and How to Fix Them

Drying or firing a product in a belt furnace is often a very important step in manufacturing cells and packages. If there is a problem with the furnace, it can result in a huge set back in production as well as quality. It can create defective products and even compromise the name and credibility of the company you have worked so hard to build. Here are three common problems that one might face when using a furnace and how to quickly and easily fix them.


1. What do I do if the temperature in one zone increases very slowly or does not reach the set temperature for an extended period of time?
a. Check the temperature control in that zone to make sure if the indicator light on the controller is on or not. If the light is not on, reset the setting in the controller.
b. If the temperature controller is on, check the heating board to see if there are any open circuits. Disconnect the heating broad wiring and use a multi-meter to check if the coils are burnt out. You will easily be able to tell if your coils are burned out by using a multi-meter because it will show a infinite resistance. If the heating board is burn out, replace it. When a new heating board is installed, 1-2 days of drying is needed before the furnace can be set at higher temperatures.
c.If the heating board does not have an open circuit, check the solid state relay for that zone. Replace it with a spare or one from a working zone and see if the problem still exists.


2. What if the temperature in one of the zones climbs over 100C or goes higher than the temperature I had originally set?
a. Check the temperature controller in that zone to see if the controller indicator light is on or off. If the controller light is on, reset the setting in the controller.
b. If the controller light is off, the solid state relay is probably bad. Replace the solid state relay and test it again.


3. The products seem to have oxidized during the process. Why did this happen and how can I prevent this from happening again?
a. Oxidation is caused by high oxygen content in the muffle. This can be checked by the O2 analyzer. Double check for leaks on the nitrogen/hydrogen piping by turning on all of the gas flow meters and by using soapy water to check for bubbles.
b. Next, check for leaks in the gaskets in muffle connections. Tighten all the bolts that hold the muffles together to ensure they are not loose. Also, check the condition of the gasket itself and replace if necessary.


If these nifty tips do not seem to solve the problem, absolutely do not hesitate to call the furnace manufacturer in order to receive more model-specific instructions to fix your furnace. Good luck and merry manufacturing!

Patent 12/161,289 Review- Dye Sensitized Solar Cell and Dye Sensitized Solar Cell Module

      On January 21, 2010, a new patent was approved in the United States for a new type of dye sensitized solar cell and module that aims at improving performance, enhance short circuit current and reduce costs. This patent was filed by Ryohsuke Yamanaka, Nobuhiro Fuke and Atsushi Fukui on December 12, 2006.  

      Prior to filing for their patent, Yamanaka, Fuke and Fukui looked into the reasons why Dye Sensitized Solar Cells were inefficient. They were able to conclude that all of of the current solar cells consumed a lot of energy during their drawn out processes while not giving enough output, and were not able to be created on a larger scale. They also noticed that their heavy weight restricted its usage. Their new patent focuses on fixing all of these problems.

      Yamanaka et. al were able to pinpoint where the flaws in the current DSSC's were. In dealing with large scale replication, a solar cell that is created with a surface area as large as 1 m would highly reduce in efficiency due to an increase in solar resistance. This, in turn, reduces the fill factor and the short circuit current conversion. In order to counter this, the new patent proposes to have multiple DSSC's that can be connected into a series. Secondly, DSSC's were believed to be made in a very costly manner by utilizing a large amount of varying and expensive components, thereby having an intricate process in which to manufacture the cells. Yamanaka et. al were able to fix this problem by using by using less materials, such as thinner photoelectric conversion layers, as well as better manufacturing processes, such as firing furnaces, and therefore, use less energy to combine these materials.

      In doing this, the combined efforts of Yamanaka, Fuke and Fukui were able to create a new patent for a more cost effective and efficient dye sensitized solar cell. Although no official statistics have been claimed yet, we look forward to seeing how much of an improvement these changes can actually create.

What is a Dye Sensitized Solar Cell (DSSC)?

    The Dye Sensitized Solar Cells, also known as Grätzel cells,  are a type of third generation solar cell that is low in cost, easy to manufacture and simpler to adjust in order to suit a large variety of applications. This was accomplished by using lower costing materials than the previous silicon based product as well as a simpler production process. Recent studies have shown that the Dye Sensitized Solar Cells can  produce up to 11% efficiency and continued researched shows a possibility for it to become even more  effective.  


 What is a DSSC? 


    Essentially, DSSC's are based on a wide bandgap semiconductor such as TiO2, that has been made highly receptive to light through the use of a layer of dye. The cell consists of four main parts including a photoelectrode composed of a transparent conducting oxide with a layer of TiO2 film,  a counter electrode encompassing another transparent conducting oxide with a platinum catalyst deposited on it, a layer of dye that can be excited by light, and an electrolyte to fill in the voids of the inner cell.

How are they made?
    The first step to manufacture a DSSC on glass it to prepare the photoelectrode. This is accomplished by taking a glass substrate coated with a transparent conducting oxide, such as SnO2:F, and depositing a layer of  TiO2 through the process of screen printing using a firing furnace at about 500° C. This process will remove excess organic residue and create electrical contact. The photoelectrode is then submerged in a light sensitive dye. Simultaneously, a counterelectrode is prepared where another glass substrate coated in a transparent conducting oxide gets infused with a platinum catalyst.   Next, the dyed photoelectrode is sealed to the counterelectrode using a thermoplastic thin film that is placed in between. These three components are then heated at about 150° C and placed under pressure. Once sealed, the voids in the device are filled with an electrolyte through holes in the counterelectrode. To ensure longevity, the cell's holes are filled and the whole cell is covered with glass.


(Information Provided by The Energy Research Centre of the Netherlands, Wikipeida and Dyesol)

About Us

     FurnacePro is a blog run by Torrey Hills Technologies located in San Diego, CA. For the last six years, we have become a major supplier of infrared firing and drying furnaces for both electronic and solar cell industries. We have provided equipment and materials for companies all over the world in over 35 countries. Our in-house facilities have the ability to cater specifically to the needs of our customers. Torrey Hills Technologies was rated No. 188 on the Inc. 500 list by Inc. Magazine for being one of the nation's fastest growing private companies. It was also rated No. 5 in the Top 100 Inc. 500|5000 Companies by Industry.
     Although we provide a variety of products with high qualifications, this blog is solely meant for discussing different types of technological advancements related to our field as a method for people to be able to freely discuss and learn. We feel that it is important for everyone to be aware of the new technology around them, even if they are not engineers. Overall, we hope that this blog is not only an easy way for for people to understand what is going on, but also a way to spark interest in the thrilling world of engineering.