Dye Cell Manufacturing 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. 

Third Generation Solar Cell

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 onto the nanocrystalline oxide semiconductors such as TiO2 to collect sunlight. Dye cells employ relatively inexpensive materials including 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. 

Metallization (2nd Generation Solar)

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.  go to www.beltfurnaces.com and go to the "Technical Data" section to learn more..

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. 

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. 

Second Generation Solar Cell

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.  Over the course of the next few blogs we will discuss Thin Film Deposition, Thin Film Annealing, and Metallization.

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. 

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. 

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 into 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. 

First Generation Solar Cells

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; Phosphorus Diffusion, Silicon Wafer Metallization, and Solar Module Assembly.  We will discuss these three standard manufacturing processes over the course of the next few blog entries!   

A brief history on solar cells and the generations of solar cell manufacturing today

Solar cells grew out of 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. Subsequently Russian physicist Aleksandr Stoletov built 

the first solar cell based on the outer photoelectric effect (discovered by Heinrich Hertz earlier in 1887). Albert Einstein explained the photoelectric effect in 1905 for which he received the Nobel Prize in Physics in 1921. Russell Ohl, working on the series of advances that would lead to the transistor, developed and patented the junction semiconductor solar cell in 1946.  Thin film solar cell annealing furnace. Today’s 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. 

Screen Printing and its impact on cell efficiency

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.

The ideal width of a belt furnace for CIGS processing

Standard belt size is usually from 350-650mm.  However, wider belts have been designed to accomodate wider glass substrates.  A 700mm wide belt has been successfully built, however, efforts are underway to build a 1000mm wide belt!  For more info on this check out our website at www.beltfurnaces.com

Furnaces by Torrey Hills Technologies

Check out this video and catch a glimpse of our furnaces!!

Temperature and Soaking time in CIGS 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. 

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.

A Typical Fabrication of a CIGS Solar Cell

A typical fabrication of a CIGS solar cell encompasses the sputtered deposition of molybdenum back contact material on a substrate.  The substrate material can be a rigid soda lime glass or a flexible polyimide.  The sputtering of the back contact is followed by the deposition of the CIGS absorber layer, CDS buffer layer and the final Zno contact layer.  Molybdenum is primarily used as a back contact due to its ability to form an ohmic contact, as well as, inert behavior to corrosive gasses.  Cadmium sulphide is used with CIGS material to form an n-type semiconductor material along with a p-type CIGS absorber material.  Some of the widely used methods for depositing CdS include chemical bath deposition (CBD), sputtering and closed space sublimation (CSS).  Zno is used as the front contact material due to its superior electrical and optical properties.  

Thin Film Photovoltaics; A cost effective alternative

The ever increasing demand and growing cost of silicon has made thin film photovoltaic’s a competing substitute to silicon solar.  Thin film photovoltaic (PV) modules are seen as a realistic alternative for a cost effective generation of electricity from sunlight.  They are often referred to as the second generation of photovoltaic technology.  Some promising materials for thin film solar cells include amorphous silicon, cadmium telluride, CuInSe2 and its alloys.  While all of these materials are low band gap, they are all polycrystalline as well leading to a loss in efficiency due to grain boundary recombination.   Amid the polycrystalline thin film solar, CIGS solar cells have been documented in various pieces of literature as being unrivalled in efficiency. 

Fireclay Tile Slideshow- A THT Furnace Customer

Check out this slideshow we did for our customer Fireclay Tile!!  They purchased a furnace from us recently!