Advantages of DSSC

The advantages of DSSC compared to other solar electrical generation are that DSSC has a high price/performance ratio, which means it is less expensive to make; it is also light and mechanically robust due to its materials’ properties. With its ultra thin profile, the cells can be made into different shapes to meet specific needs, such as curved or other creative designs. And due to its robustness, DSSCs maintain its efficiency at high temperatures as well. The DSSCs have efficiency between 10%-11%, higher than other thin-film solar panels having efficiency of 5%- 13% in average. What makes it even more attractive is that they work even in low-light conditions, such as during cloudy weather where there is non-direct sunlight.

Materials Selection

The transparent coating for the negative terminal is made of a thin layer of fluorine-doped tin oxide. It is a substrate that allows sunlight to pass through, meanwhile conducts electricity.
As for the semiconductor, either zinc oxide (ZnO) or titanium dioxide (TiO2) can be used. TiO2 is used and preferred because its surface is highly resistant to continued electron transfer. But it is not sensitive to visible light; it only absorbs small amount of solar photons, therefore, dye sensitizers are needed to attach to the titanium dioxide layer to harvest large portion of the sun light. ZnO has a higher electron mobility compared to TiO2, but has a limited selection of organic dyes, which makes it less desirable until better alternative sensitizers are discovered. Dye-sensitizers can be made of different types of materials. One option is the natural dye, which come from various resources such as blue berries, blackberries, and raspberries. They are easy to come by and good for school training courses or testing purposes. Another option is the synthetic dye, which gives better performance due to its optimized light collection property. The material for the positive terminal layer or the cathode layer can be from either platinum or carbon (graphite). The platinum has a higher efficiency; however, the carbon alternative is easier and less expensive, good for school works or testing purposes.

Electricity Generation Schematics

Initially, sunlight passes through the transparent conductive oxide (TCO) layer into the dye-sensitized layer causing the electrons within the molecules to excite. The electrons rapidly get injected into the TiO2 particles, which act as a semiconductor, transporting light induced electrons toward the negative terminal. The negative terminal layer, or, the TCO layer, is where all the electrons are collected. Then, the electrons get further transported toward the external circuit, generating electricity. Afterwards, the electrons get reintroduced into the solar cell through the positive terminal into the electrolyte. From here, The electrolyte transports the electrons back into the dye molecules, where the process is repeated. Make sure to Like us on Facebook www.facebook.com/torreyhillstechnologies

Description of a Dye Sensitized Solar Cell

A dye sensitized solar cell is mainly composed of three parts. The first part, The substrate, is the negative terminal. It has a layer of transparent glass outside, and a coating of transparent conductive oxide (TCO) inside that allows sun light to pass through. The middle part: a layer of dye sensitizers is attached to a layer of nano-structured titanium dioxide (TiO2), which is attached to the negative terminal to collect sunlight, all of which are immersed in an electrolyte solution to allow charge transportation. The top part is the positive terminal. It contains a coating of carbon (graphite) or platinum for electron transfer, and an outside layer made of transparent glass. The top and bottom parts are joined together to prevent the middle portion from leaking.

DSSC

Dye-sensitized solar cell is the most cost-effective third-generation solar technology available. It was invented in 1991 by Michael Gratzel and Brian O’Regan. Recently, it has attracted more interest due to its low material cost, ease of production, and high conversion efficiency compared to other thin-film solar technologies. However, due to its technical constraints, the actual maximum efficiency is only half the efficiency of crystalline silicon-based solar cells. In an effort to replace the current existing solar technologies, improving its efficiency is critical to widespread adoption of this technology. The process parameters have been reported as having a significant impact in determining the efficiency of the solar cell, as well as, the materials being used.

Crystalline Silicon Solar Cell vs. DSSC

In manufacturing crystalline silicon solar cells, 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. For crystalline silicon solar cells, Torrey Hills Technologies recommends our HSG Series Photovoltaic Drying Furnace or our HSH Series Photovoltaic Fast Response Furnace. In manufacturing DSSC, 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. For DSSC, Torrey Hills Technologies recommends our HSG Series Photovoltaic Drying Furnace, HSH Series Photovoltaic Fast Response Furnace, or our HSK Series Photovoltaic Fast Response Furnace. MAke sure to follow us on twitter www.twitter.com/torreyhillstech

Belt Furnace for CIGS Processing

A furnace that processes CIGS solar cells should be capable of operating to 650°C or higher. The aim is for a wider belt with superior cross belt uniformity so that larger substrates can be contained in the future. More over, a muffle is required to ensure a cleaner operating environment because various processes in CIGS processing, such as Sulpharization, involves the introduction of different gasses at different time periods. The muffle needs to have the capability to control the gas type and gas flow in each zone. With all of this in mind, Torrey Hills Technologies has designed a furnace that is well suited to match the requirements for thin film solar applications. Figure 4 illustrates a firing furnace that is capable of processing CIGS solar cells and Table 3 lists the technical specifications for the ideal CIGS furnace.

The designed HSA furnace uses ceramic heater boards in order to achieve elevated temperatures. Aside from the standard belt size of 350 and 650mm, wider belts have been designed as well to accommodate wider glass substrates. While a 700mm wide belt has been successfully engineered, efforts are underway to build a belt as wide as 1000mm. As a standard feature, this furnace is equipped with a steel brush for cleaning the conveyor belt, however, Ultrasonic belt cleaning is available as an extra option.

A microprocessor based PID controller is what controls the furnace. Type K thermo-couples are used in determining the zone temperatures and the controls are located on the right hand side of the furnace which can be viewed from the entrance. The central processing unit (CPU) is mounted under the exit table and the CPU is primed with a Windows operating system for easy computing. The computer system is pre-installed with a program for controlling the furnace parameters, including the belt speed and the zone temperatures. Temperature profiles can be stored and retrieved as well for future purposes. Thermocouple ports are located at the entrance table for connecting the profiling thermocouple directly into the microprocessor. This feature allows for the monitoring and recording of actual temperatures experienced by the part. Software is also included with the computer to capture, display, printout and store the furnace profile. Additionally, 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.

Effect of Selenization Profile on Solar Cell Efficiency

The temperature and soaking times play two very important roles in determining the final efficiency of the CIGS solar cell. Many researches have revealed significant evidences for how the different sintering temperatures and soaking times have a direct impact on the efficiency outcome. The results of the experiment by Kadam et al demonstrate that a sintering temperature above 500 oC and a soaking period between 30 and 60 minutes can improve the efficiency of the cell significantly.In his experiments, when the samples were selenized at 400 °C for 10 minutes with a temperature ramp of 6°C per minute, it resulted in smaller, non-faceted grains. This is an indication that either the temperature or the soaking time is inadequate. When the soaking time was increased from 10 to 15 minutes, no significant improvements were noted in the results. Therefore, the ramp rates can be construed as having a larger impact on efficiency. In addition, Kadam et al studied the impact of various heating rates. Changes in the parameters were made: heating rate was increased from 6 °C per minute to 20 °C per minute and the temperature was increased to 425 °C with a soak period of 30 minutes. The resulting solar cell film appeared to be more homogeneous. Furthermore, the temperature was increased to see the effects on the grain size and cell efficiency, which indicated that at 475 °C, the grain size ranged from 0.5 um to 1.5 um with an efficiency of 5.56%. At the same time, they detected that increasing the peak temperatures above 500 °C could also help with improving efficiency. However, at higher temperatures the grains become well faceted and less thick. It is fair to conclude that high quality CIGS solar cells should be prepared at temperatures above 500 °C with an ideal soaking time between 30 and 60 minutes. Understanding that the heat rate, temperature, and soaking times are important factors in determining the ultimate cell efficiency, a belt furnace with a wide range of firing temperatures and a fast heat rate design are desired for constructing highly efficient solar cells.

A little about Torrey Hills Technologies

Torrey Hills Technologies, LLC is the leader in developing and delivering innovative yet affordable industrial furnace equipment to diverse industries. Our firing and drying conveyor belt furnaces have been widely used in solar cell (photovoltaic's) manufacturing, semiconductor packaging, circuit board assembly, and advanced materials processing (thick film, metals, ceramics, and various electronic components). We have customers located around the world in North America, South America, Europe, Asia, and Australia.Today, we have become a significant player in the supply of fast fire furnaces to silicon, thin film, and 3rd generation solar cell manufacturers. Especially in the production of low-cost 3rd generation Dye Sensitized Solar Cells (DSSCs), our furnaces have secured a dominating position.

Processing CIGS in a Belt Furnace

The fabrication of CIGS solar cells within a production environment contains the deposition of copper, indium and gallium on the selected substrate material while annealing them at elevated temperatures in controlled atmospheres. First, the substrate material is cleaned and then heated to an elevated temperature. Afterwards, copper, indium, gallium and selenium are deposited through a sputtering process and then the deposited parts are selenized in an elevated temperature profile. The selenization process involves ramping up from room temperature to 450°C in about 4 minute’s. The samples are soaked at this temperature for 7 minutes and then elevated to 550°C for approximately 4 minutes. The samples are then held at 550°C for another 7 minutes and then cooled down. Finally, while held at an elevated temperature, hydrogen sulphide gas is introduced to sulphurization.  Make sure to follow us on twitter  www.twitter.com/torreyhillstech

Furnace for Firing Operation

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.

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Belt Furnace Parameters

Apart from the firing conditions, studies have reported a correlation between the furnace parameters and the efficiency of the solar cell. Edward Bruce has done studies on the influence of belt furnace parameters and solar cell efficiency. He observed that the cleanliness of the furnace has a significant impact on cell efficiency. The common source for furnace impurities include carbon residue, impurities from air inlet, foreign materials picked up by the conveyor due to contact, etc. Solar cells that were fired immediately after cleaning the furnace have been reported as being more efficient than their counterparts that had been fired on a furnace with impurities. The thickness of the oxide layer has also been reported as having a significant impact on cell efficiency. The oxide layer thickness can be controlled, as well, by controlling the atmosphere inside of the muffle. Hilai et al has reported an increase in the conductivity of the silicon solar cell fired within a reduced atmosphere with a small percentage of Hydrogen. The substrate carrier boat has been reported as having very little impact in determining cell efficiency. Results from Bruce et al studies have shown no significant impact in efficiency for directly placing the silicon substrate onto the nichrome belt and firing them on a quart carrier.  Check out the full whitepaper http://www.beltfurnaces.com/efficiency_of_silicon_cells.html  


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Effect of Soak Time

Soak time refers to the time at which the substrates with metallization printed on them are held at peak temperature. The soak time is an important parameter in determining the cell efficiency because the diffusion of silicon and silver is a function of time and temperature. Research done by Lee et al, Cooper et al, and Hilai et al have demonstrated the effects of soak time on the efficiency of the solar cell. From the results in the published literature, it is understood that there is a significant interaction between soak time and peak temperature. The highest efficiencies have always been reported for shorter soak time. It has been reported that increasing the soak time will increase the contact resistance. A study, done by Cooper et al, of the interface and bulk metallurgies of the contact, has demonstrated an increase in the thickness of the interfacial glass layer with an increase in soak time. The thicker contact layer results in an increase of contact resistance.  Make sure to "like" us on Facebook www.facebook.com/torreyhillstechnologies

Effect of Peak Temperature

The peak temperature of the sintering process is observed as having a significant impact in determining the efficiency of the silicon solar cell. Studies by Carroll et al have reported a direct correlation between peak temperature and cell efficiency. The results from this study have shown that the ideal peak temperature range for metallization firing is 780-800 oC. Cheng et al has also studied the effects of various peak temperatures on the efficiency of solar cells and has confirmed the same results. The study observed that when the silicon solar cells are under fire, one half of the cells display high contact resistance while the other half displays low contact resistance. At the same time, when the solar cells are fired within the optimum range of 780-800 oC, the contact resistance has been reported as being uniform throughout the entire cell at higher efficiency. However, when the solar cells were over fired, there was an overall decrease in efficiency due to shunting. An investigation by Hilai et al has demonstrated similar phenomena. At lower temperatures, the distribution of Ag was irregular and the contact formed was very small with high specific contact resistance. However, at high temperatures, the excessive shunting resulted in a decrease in efficiency. Thus, from the results of the above studies, it can be observed that the peak firing temperature has an important role in determining the efficiency of the silicon solar cells.

The Firing Process (Sintering) in Silicon Solar Cell Manufacturing

The firing process, also referred to as sintering, is one of the key steps with which the front-metal contact is formed in a silicon solar cell. In this process, the thick film paste is dried at about 150oC to remove much of the solvents. The presence of solvents can cause excessive out gassing which can lead to cracks and voids. The dried substrates are then fired inside a firing furnace. The firing process consists of four primaries. The first step is the initial temperature ramp up where the paste solvents are volatilized. The second step is the burn out. The objective of the burn out phase is to remove all of the organic binder that was used in paste formation. The burn out phase is carried out at 300-4000C. The third step is the sintering, or the firing process, which is done between the ranges of 700-800C. During this process, the Ag metal forms a bond with the underlying silicon substrate to form metal contact. The final step in the firing process is the wafer cool down phase. The mechanism behind contact formation in a fire through contact is very complex and not fully understood [4]. According to Mohammed Et al, the process starts by evaporating the solvent between 100-200C and then burning out the polymer between 200 and 400C. Later, from 400-600C, the glass frit melts and the sintering of the Ag particles take place. Furthermore, from 600-800C, molten glass with some amount of dissolved Ag etches the silicon nitride anti reflection coating and then reaches the Si surface. Here, it reacts and etches a very thin layer of Si. Ag in the glass will then precipitate onto the Si surface in the form of crystallites. The quality of the contact influences the shunt resistance, series resistance and junction leakage current, which have a significant impact on the efficiency of the solar cell. Hence, it is very important to understand various aspects of the firing process to be able to achieve higher efficiencies. Some of them include; peak temperature, dwell time, and the temperature at which Si –Ag alloy formation happens.  Over the next few blogs I will discuss these!  Make sure to follow us on twitter  www.twitter.com/torreyhillstech

Factors Affecting Screen-Printing Process: Snap Off Distanc, 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.

Screen Printing

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.

Torrey Hills Technologies And The Solar Cell Market

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.   Like us on facebook and join our discussions.. www.facebook.com/torreyhillstechnologies