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!

Why Settle for One?- The New HSH3003-0406 Hybrid Drying and Firing Furnace

      After years and years of sleepless nights and fruitless days, you finally hit the jackpot! You figured it out! You finally have devised the perfect way to increase dye sensitized solar cell efficiency while keeping the costs at all all time low. Now all you have to try it out.

      But how? You know exactly which differing types of furnaces you need to fulfill each step. Where? Your lab doesn't have too much extra space, but that is something you are going to have to work with. Being a small lab based research and development group, you also do not have the money to burn on costly machinery. So what are you going to do? The answer: the compact HSH3003-0406 Hybrid Drying and Firing Furnace.
 

HSH3003-0406 Hybrid Drying and Firing Furnace Design

     This new hybrid furnace was design especially for the firing and drying of nano-sized titanium oxide particles in DSSC to fit the needs of smaller scale research and development. It can operate in temperatures varying from room temperature up to 650°C from which it acts as a dryer below 200°C and functions as a firing furnace at higher temperatures. It also comes equipped with an industrial computer system in order to switch between drying and firing modes. Its compact size optimizes its minimal space requirements as well increases cost efficiency in order to please any budget. In addition, its unique technology has customizable options in order to fit precise needs.

      This hybrid, produced exclusively by Torrey Hills Technologies, has been manufactured in the hopes that innovative researchers are not limited by potentially restricted resources, whether it be space or money. Various universities and research companies worldwide have already taken advantage of what this furnace can offer.

     Torrey Hills technology strongly supports research in DSSC applications in order to promote a greener way of life encompassing more efficient, as well as affordable, eco-friendly practices.

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.

Hyundai Goes Green With Their New Hybrid Blue-Will

    Hyundai debuted their latest creation, Blue-Will, at the Detroit Auto Show earlier this week. This new hybrid sports a sleek body with unbelievable performance capabilities. It is stocked with features such as roof mounted Dye Sensitized Solar Cells, drive-by-wire steering and LED displays with touch screen control. It is also has the ability to be powered by either a Lithium Ion Polymer battery pack or a  four cylinder, 1.6 liter engine.
    In addition to its fuel saving techniques, it is also green friendly in the materials it uses. The cars makers used recycled PET soft drink bottles to make the headlamp bezels and bio-plastics for the interior and engine cover.
    Hyundai still has to decide on a few different options for Blue-Will. One interesting option that remains open is whether the car will be a plug-in hybrid or a regular hybrid. The plug in version is believed to be able to go as far as 40 miles on a single charge without using any gas. The down side is that this option would be more expensive than the regular hybrid.
    This new creation brings us one step closer to creating a transportation system that is fuel free by utilizing the beauty of DSSC's and solar energy.


(Information provided by iAfrica.com, Jalopnik, and USAToday)

DSSC Technology Has Reached New Depths- Within the Body

    A new type of technology that is being developed today revolves around creating various biological nanodevices that can be used for attaining a diagnosis and for varying therapeutic interventions. One huge problem that occurs when designing these types of biotechnologies is that they need to be wireless to ensure unrestricted access to parts of the body. In order to accomplish this, there needs to be a continual source of electrical energy that can be utilized in unusual environments such as the human body.
    Previous efforts to accomplish this includes the development of a direct current nano generator that uses ultrasonic waves. This breakthrough was made by Dr. Zhong Lin Wang, who is a COE distinguished professor and director of the Center for Nanostructure Characterization at Georgia Tech. Dr. Wang's research resulted in the creating of a nanogenerator that was able to take hydraulic energy from within the human body, such as a heart beat or blood flow, and convert it into electrical energy. Although this was an amazing find, improvements needed to be made in order to make it more practical for real life applications.
    Recent developments from researchers from the State Key Laboratory for Modification of Chemical Fibers and Polymer Materials at Donghua University located in in Shanghai, China and the Max Planck Institute for Colloids and Interfaces in Potsdam, Germany include building a 980 nm laser driven Dye Sensitized Solar Cell that can function even when it is covered by a thick layer of biological tissue. This is possible because these tissues have a high transparency to this specific frequency of light. This  photovoltaic cell is created by utilizing rare nanophosphors that absorb light and then send out a glow that, in turn, excites solar cells to make electricity. It is capable of providing a maximum output of  0.28 to 0.02 mW of energy, even after being covered with 1 to 6 layers of pig intestine that averaged about 1 mm thick each. This amount of energy is enough to power a large variety of biotechnology.
    This development is in its primary stages of research and scientists are still looking for ways to improve. Some of these desired improvements include making all of the components more biologically compatible, improving conversion efficiency, and making the cell even smaller in order to increase it application.


(Information was Provided by Nanowerk-Nanoscale Power Plants and Nanowerk- Photovoltaic Cells to Power Biological Nanorobots Inside the Body)

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)

Autonomy from the Outlet- Living a Chord-Free Life

     After years and years of being imprisoned indoors to charge all of the electronics we really so heavily on, break free from the confines of the chords and the order of the outlet! A light shines on an opportunity to get out and to not be constrained by limited battery life- a solar light. Recently, a new line of backpacks and bags have been released that have Dye Sensitized Solar Cells, or DSSC's,  built right into them that can be used to charge electronics. These cells are not only cheap to make but are also flexible and can be utilized in a wide variety of products including clothing, tents, awnings and even windows in the future.
     This product has shown over a 12% efficiency rate and can store up to 0.5W of power. What is even more appealing about the DSSC 's in this product is that they can also absorb other forms of artificial light in order to charge itself.
     As of right now, further studies are being done to increase the efficiency of these cells by attempting to incorporate nonvolatile electrolytes and organic dyes. These alterations could further reduce costs and increase efficiency.
     Even though this is a huge breakthrough, since product like these have not been made available before, many people are skeptical of the actual efficiency and effectiveness that these products will be able to provide for the general public.
     The first shipment of these bags has already been sent to Hong Kong. It is expected that these bags will be available for commercial sale by December 2009.
     With these new innovations in mind, we can strive to live in a world where there will be no carbon foot print and runs purely off renewable solar energy. Now it is easy to see what a huge impact these developments can make in our lives. It is safe to say that these Dye Sensitized Solar Cells are here to stay!

(Information provided by Gizmag)

Return of the Rainbow- Who Said Solar Cells Had to Look Boring?

     The Korean Institute of Technology has introduced a new method that allows a solar cell to absorb more light, making it more efficient than its previous amount of 11%. This development was lead by researcher Park Nam-Gyu who claims that this new discovery will improve power consumption by at least 50% making it more efficient and even more cost effective than it was before.
     Typically, a dye sensitized solar cell (DSSC) is a semiconductor that has been created from a photosensitized anode and an electrolyte. The cell is made of porous TiO2 particles that are covered with a specific dye that interacts with its respective electrolyte.
     Nam-Gyu's team was able to improve this design by finding a way to have the TiO2 particles take in different colors of dyes that allow the cell to absorb a wider spectrum of light, which will, in turn, increase efficiency.
     This was achieved by copying a scientific method of chromatography that involves separating chemical compound from mixtures. This process works in two phases, including the stationary phase and the mobile phase. In order to form the different layers, the team was able to control the release and settling of the dyes. As a result they were able to vertically align yellow, red and green dyes within the TiO2 film. This alignment was validated by an electron probe micro-analyzer.
     It is expected that when the DSSC reaches a higher efficiency, they will become commercialized. This will cause a huge shift in the solar market from silicon based thick film solar cells into lighter dye sensitized solar cells that are expected to reach equivalent efficiencies at a significantly lower cost of production.
     Next up is a more homely approach to solar cell usage.


(Information provided by  PV-Tech)

Aeronautic Achievements- Keeping Your Feet on the Ground, But Eyes to the Skies

     At the University of Washington, the Multidisciplinary Research Initiative (MURI) team has succeeded in creating a type of airborne solar cell that can potentially be used for the Air Force's unmanned aerial vehicles. They were able to create a new type of Dye Sensitized Solar Cell (DSSC) that can harvest energy better and therefore, fuel flights for longer.
     Dr. Minoru Taya, the group's lead researcher, was able to justify this by explaining that “These kinds of solar cells have a more specific power of convergence efficiency (PCE), very clean energy and easy scalability to a larger skin area of the craft, as well as low-temperature processing, which leads to lower costs overall”.
     Usually, a DSSC is made through the use of differing types of semiconductors, electrolytes and photo sensitive dyes. The DSSC that Dr. Taya's team made varies in the fact that it uses organic materials and moth-eye film in order to capture photons. Once the photons are captured, they become a synthesized electron that can store energy.
     This team has been trying to take on this huge feat for several years. In a previous attempt to test solar cells, they had attached a DSSC to the wings of a toy airplane. Although the propeller started, it was unable to take off due to the glass based DSSC being too heavy.
     In the future, the MURI team looks forward to improving their design to increase possible flight times and further suit the strict requirements of the Air Force's unmanned aerial vehicles they one day hope to power.
     Up next is a more colorful approach to solar cells production.


(Information provided by CleanTechnica)

Marine Mania- Looking to Ancient Life to Solve Today's Problems

     Engineers at Oregon State University and Portland State University have found a new way to potentially triple the electrical output of a conventional dye sensitized solar cell through the utilization of single celled marine life forms known as diatoms. Diatoms are a type of common unicellular algae that appear in a large variety of forms and have existed from as early as the Jurassic Period. They are well known for their unique cell wall made of silica and are readily available. Normally, they are used as a tool to monitor environmental condition and have never really been employed for electronic applications.
     This process is initiated by allowing the diatoms to settle on a conductive glass surface. The organic material is then removed, leaving their shells on the glass surface in order to create a template. Various biological agents are combined with titanium to create titanium dioxide particles that are used to create a thin film that becomes a semiconductor for the solar device.
     Although the exact physics of this device has not been understood yet, it has been deduced that that the holes in the shells increase the interactions between the dyes and the photons and therefore convert light into energy at a better rate. This is accomplished by allowing the photons bounce around inside of the shell, resulting in more energy.
     This new process for creating dye sensitized solar cells has many advantages as well as disadvantages. One main disadvantage is that the process is a little more expensive than the current process for making dye sensitive solar cells. This disadvantage is almost negligible considering that the output nearly triples. Some of the advantages of utilizing diatoms also include a decrease the cost of production and a simpler process to create the cell.
     At this point in time, this process is still being developed and has not been put into large scale use yet.
     After exploring the depths of the oceans, look out for what comes next from the skies above us.


(Information was provided by StumbleUpon)

Today's Fads or Tomorrow's Future?- 5 New trends from the Dye Sensitized Solar Cell World

     Environmentalists everywhere are rejoicing at the many breakthroughs that have lead to a significant reduction in the carbon footprint that is being left. Everywhere one looks, they see a new way to be even more environmentally friendly or to recycle and reuse the products they have already purchased in order to preserve Mother Nature. Scientists and engineers today are looking for new and affordable ways to use clean energy to replace the environmentally hazardous products. Here we will discuss 5 new trends that have emerged to create a cleaner and safer environment through the use of Dye Sensitized Solar Cells.
     Dye Sensitized Solar Cells, also known as DSSC's, are essentially a method used to capture sunlight and convert it into electricity, also known as solar energy. This is achieved by using specific dye molecules that absorb light and push charged particles through different layers of a cell, creating electricity. When compared to older methods of obtaining electricity, such as fossil fuels, DSSC's are a cleaner and a renewable source of energy. In comparison to older solar cell models, a DSSC is a cheaper, lighter and a more durable alternative.
     Stay tuned for the a look at the first trend- Marine Mania.