Chapter 7b Fabrication of Solar Cell

Different kind of methods for growth of silicon crystal

Electromagnetic Continuous Casting, not commercially available

Non-wafer Technologies

Specifications of PV Silicon Wafer

Cleaning and texturing A layer with thickness of about 10 um has to be etched from both sides of wafers cut by wire saw. The damage removal etch is based on 20~30 wt% aqueous solution of NaOH or KOH heated to 80~90 o C. The silicon surface after saw damage etching is shiny and reflects more than 35% of incident light. Random chemical texturing is used. Monocrystalline silicon substrate (100) can be textured by anisotropic etching at temperature of 70~80 o C in a weak, usually 2wt%, solution of NaOH or KOH with addition of isopropanol.

texturing This etch produces randomly distributed upside pyramids. However, It has problems of repeatability, lack of pyramid size control, and the presence of untextured regions. The important parameters are: adequate surface preparation, temperature control, mixing rate and isopropanol concentration. This requires the use of appropriate additives which enhance the pyramid nucleation process.

Junction formation, surface passivation and ARC Highly doped P in atms-cm -3 Typical emitter sheet resistance used in screen printing is between 40 and 60 Ohm/sq. Front surface passivation: SiO 2 Antireflection coating: ZnS and MgF 2, PECVD nitride (with n in 1.9~2.4) Or Coating SiO 2 thicker to 110 nm which serves as surface passivation and antireflection coating.

Front contact formation Front contact formation: should have a large height-to- width ratio of finger metallization. After contact opening, metallization is obtained by a self-aligned plating process of nickel, copper, and a thin layer of silver. In screen printing method, a stainless steel or polyester mesh screen stretched on a metal film frame is covered by a photo-emulsion layer. Openings- which define the front contact pattern- are photolithographically formed in the emulsion layer. Highly conductive silver paste is pushed by a squeegee through the openings in the screen onto substrates with well defined adjustable pressure.

Rear structure Alloying a screen printed aluminum paste with silicon. Aluminum can form a eutectic alloy with silicon at a temperature of 577 o C. During cooling down, the silicon recrystallizes and is doped with Al at its solubility limit creating a p + back surface field (BSF) layer. A sufficient thickness of Al is required to achieve a significant contribution of Si in the formation of the liquid phase. Very low back surface recombination velocity (200cm/s) have been reported for thick screen printed and evaporated Al layers (at least 20 μm and 10 μm thick, respectively) fired at a temperature above 800 o C.

Gettering – traps metal impurities by phosphorous diffusion By phosphorous diffusion, metallic Impurity migration towards gettering sites takes place as a consequence of a large emission of Si interstitials due to the formation of SiP particles by heavy P-diffusion. Enhanced solubility of metallic impurities in such heavily P-diffused regions, and impurity segregation at Si 3 P 4 precipitates lead to efficient gettering. P-diffusion can be performed prior to cell fabrication followed by the removal of the heavily diffused layers (pre-gettering), or a part of emitter formation depending on the optimal gettering conditions required for the material and on the costs involved.

Gettering by aluminum treatment Formation of a p+/p high/low junction at the rear side of the cell by re-growth from a fast alloyed Al-Si melt is the most commonly used process for creating the back surface field. This has an additional advantage of bulk gettering by prolonged firing or by a thermal anneal after the initial firing. Fast alloying of screen printed Al- paste by firing, followed by the removal of excess aluminum and a subsequent thermal anneal for up to 1 h, resulted in a considerable enhancement in the bulk diffusion length in large area mc Si wafers.

PV module

EFG: Edge-defined film fed growth, a continuous production of a thin foil or sheet directly from the silicon melt. MIS: metal oxide semiconductor

High Efficiency Silicon Solar Cell

Cell performance: 18% Reduce recombination velocity at the front contacts

Micro-groove: 20%

Both contacts are on the rear of the cell: 22%

Efficient light trapping

Screen printing has the problems of large line width and contact resistance

Improved contact resistance by screen printing

Sanyo announced 23% this year for HIT large solar cell

solar cell performance: 20%