I have described before how technology is going to play a critical role for manufacturers to reduce a cost of production but also how it will increase conversion to influence customer’s demand. In general terms, new material or structure will address the conversion, while process introduction will reduce the cost. In some cases, like quasi mono, they will result in both.
Saw-damaged layer removal
Saw-damaged and contaminated surface layer is removed from both sides of a wafer. Alkaline etches are commonly used, with subsequent rinsing in deionised water.
Texturing
Subsequent to etching, the silicon wafer surface reflects more than 35% of incident light. The wafer surface is textured to reduce reflectance. Mono-crystalline silicon wafers are textured in chemical reaction. Due to the nature of anisotropy in multi-crystalline silicon wafers undergo mechanical texturing instead. The optical reflection can be decreased to less than 10% in this processing step.
Emitter diffusion (junction formation)
Silicon wafers are usually boron-doped to be p-type. Phosphorus, an n-type impurity, is introduced to form the p-n junction. The process is carried out in a furnace at a temperature of approximately 900oC. Other dopant deposition methods include screen printing, or chemical vapour deposition. N- type wafer is phosphorus-doped and p-type impurity, boron, is introduced to form n-p junction.
Edge isolation/phosphor silicate glass etch (PSG etch)
Edge isolation techniques are applied to isolate the front and the rear side emitter. The techniques for edge isolation include mechanical, laser cutting, or plasma etching. Plasma etching is particularly synonymous with edge isolation of screen-printed silicon solar cells.
Anti-reflection coating/passivation
The reduction in the front surface reflectance of a crystalline silicon solar cell presents a possibility for improved cell efficiency. An anti-reflection coating is applied to minimize surface reflectance. Several materials with refractive indices can be used as anti-reflective coating. Silicon nitride deposited by plasma-enhanced chemical vapour deposition (PECVD) is the dominant anti-reflection coating for silicon solar cells.
Metal contact formation (metallisation)
The process of contact formation is vital because it affects various properties of the cell. The front-side metallization technique determines the shadowing and series resistance losses. For high-efficiency solar cells it is desired that front electrodes have low series resistance and low area coverage. The most widely used metal contacting technique for silicon solar cells is screen-printing. The front side contacting is achieved by a screen-printed silver paste, while the rear side electrode formation and surface passivation are achieved by alloying a screen-printed aluminium paste with silicon. The pastes are subsequently dried in an oven at a temperature of approximately 300oC.
Contact firing
The screen-printed contacts initially lie on top of the insulating anti-reflection coating. The silicon solar cells undergo a short heat treatment at temperatures up to 900oC, using a belt-driven furnace. During the firing process, the antireflection coating layer experiences selective dissolution such that the contacts penetrate through the anti-reflection coating onto the emitter while avoiding deep penetration into bulk silicon. Moreover, the back surface field formation with aluminium is achieved.
Selective Emitter
In this technology, the conductance of the emitter, the n-type phosphorous-containing region on the cell front surface, is selectively increased under the front metal grid contacts. Doping required for an efficient selective emitter can be formed by various methods, including deposited dopants, deposited etchants, patterned wet etch masks, spatially-defined ion implantation, etc. Those techniques are introduced at different stages of the cell processing described above. Printed SE is introduced before emitter diffusion, and it is found to be the most attractive for cost perspective and familiarity. In order to optimize results for SE technology the SE metallization is changed to reduce the space between the fingers (sub-electrode grid lines).). This process of fine metallization includes ink-jet printing and has been used commercially by Chinese manufactures: Hanwha and Ja Solar.
Another optimization is called double printing. Double printing is printing a metal grid pattern, and then over-printing another layer of metal exactly on top of the first to achieve a tall, narrow grid to produce less shadow.
More advanced improvements in cell efficiency are being pursued through the use of new cell structures that place ultimately all the contacts on the back surface of the wafer, eliminating the shadowing effect. They are metal wrap through, emitter wrap through and various integrated back contact structures. (MWT, EWT, IBC)
Metallization
In order to achieve high solar cell efficiency values, front contacts must conduct the electricity created upon illumination without a loss and cover as little cell surface area. In order to accomplish this, silver, material with highest conductivity is used. Copper has similar conductive qualities and with the cost difference between silver and copper, having the same efficiency, is best suited to replace it. Challenge for solar cell metallization with copper however is the creation of a homogenous and qualitatively high-value layer between silicon and copper. Such a layer must prevent diffusion into silicon. Effective prevention of copper diffusion is decisive in order to ensure the loss-free operation of the solar cell. Through laser ablation, anti-reflection coating (ARC) is removed and galvanic nickel-copper system with direct deposition to silicon create such a layer by nickel deposition without printed silver contact layer.
Overview IBC, MWT, EWT
Compared to IBC or EWT, the processing of MWT cells is relatively simple, and can be applied on both low and high material quality (purity). For IBC cells the emitter is only applied at the rear side. IBC technology is used mostly with monocrystalline and with n –type wafers. For EWT cells the emitter is applied on the front side, and led to the rear via a large number of holes in the wafer. This means in principle lower material quality can be used compared to IBC cells. The metallization in both (IBC and EWT) cases is only applied at the rear side. For MWT cells the emitter is applied on the front side as well as in the holes, thus also lower material quality can be used. Compared to EWT, the conduction in the holes of MWT cells is high because of the application of metal inside the holes.
The MWT cells require only a small number of through-holes to direct photo-generated electrons to the back surface through the metal electrodes and n-doped emitters, and produce higher collection photo currents due to absence of a bus bar (main electrode) on the front surface as in conventional cells.
The EWT cells have a larger number of close-spaced through-holes, which direct photo-generated electrons to the back surface solely through n-doped emitters. The EWT cells produce even higher photocurrents by eliminating the both bus bar (main electrode) and gridline (sub-electrode) shading on the front surface. From production perspective MWT is considered the least costly to introduce to conventional cell process.
In the next article poly and quasi mono will be explored.
